COMPLEX MACROMOLECULAR ARCHITECTURES BY ATOM TRANSFER RADICAL

COMPLEX MACROMOLECULAR ARCHITECTURES BY ATOM TRANSFER

RADICAL POLYMERIZATION

Anna Carlmark

AKADEMISK AVHANDLING

Som med tillstånd av Kungl Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknisk doktorsexamen fredagen den 7 maj 2004, kl 10.00 i Kollegiesalen, Valhallavägen 79, KTH, Stockholm. Avhandlingen försvaras på engelska.

To my Family

ABSTRACT Controlled radical polymerization has proven to be a viable route to obtain polymers with narrow polydispersities (PDI’s) and controlled molecular weights under simple reaction conditions. It also offers control over the chain‐ends of the synthesized polymer. Atom transfer radical polymerization (ATRP) is the most studied and utilized of these techniques. In this study ATRP has been utilized as a tool to obtain different complex macromolecular structures. In order to elaborate a system for which a multitude of chains can polymerize in a controlled manner and in close proximity to one another, a multifunctional initiator based on poly(3‐ethyl‐3‐(hydroxymethyl)oxetane was synthesized. The macroinitiator was used to initiate ATRP of methyl acrylate (MA). The resulting dendritic‐linear copolymer hybrids had controlled molecular weights and low PDI’s. Essentially the same system was used for the grafting of MA from a solid substrate, cellulose. A filter paper was used as cellulose substrate and the hydroxyl groups on the cellulose were modified into bromo‐ester groups, known to initiate ATRP. Subsequent grafting of MA by ATRP on the cellulose made the surface hydrophobic. The amount of polymer that was attached to the cellulose could be tailored. In order to control that the surface polymerization was ‘living’ and hence that the chain‐end functionality was intact, a second layer of a hydrophilic monomer, 2‐hydroxyethyl methacrylate, was grafted onto the PMA‐grafted cellulose. This dramatically changed the hydrophilicity of the cellulose. Dendronized polymers of generation one, two and three were synthesized by ATRP of acrylic macromonomers based on 2,2‐bis(hydroxymethyl)propionic acid. In the macromonomer route, macromonomers of each generation were polymerized by ATRP. The polymerizations resulted in polymers with low PDI’s. The kinetics of the reactions were investigated, and the polymerizations followed first‐order kinetics when ethyl 2‐bromopropionate was used as the initiator. In the ‘graft‐onto’ route dendrons were divergently attached to a dendronized polymer of generation one, that had been obtained by ATRP.

SAMMANFATTNING Kontrollerad radikal polymerisation har visat sig vara en användbar teknik för att, under enkla reaktionsbetingelser, framställa polymerer med snäv molekylviktsfördelning (PDI) och kontrollerad molekylvikt. Den ger också kontroll över kedjeändar. Atom transfer radical polymerization (ATRP) är den mest studerade och använda av dessa tekniker. I denna studie har ATRP använts som ett verktyg för att erhålla komplexa makromolekylära strukturer. För att utveckla ett system där ett stort antal kedjor kan polymeriseras på ett kontrollerat sätt i närheten av varandra, syntetiserades en multifunktionell initiator baserad på poly(3‐etyl‐3‐(hydroximetyl)oxetan. Makroinitiatorn användes för att initiera ATRP av metyl akrylat (MA). Den resulterande dendritiska‐linjära sampolymerhybriden hade kontrollerad molekylvikt och ett lågt PDI. I stort sett samma system användes för att ympa MA från ett fast substrat, cellulosa. Ett filterpapper användes som cellulosa substrat och cellulosans hydroxylgrupper modifierades till bromester‐grupper, kända för att initiera ATRP. Efter ympningen av MA med ATRP blev cellulosan mycket hydrofob. Mängden polymer som ympats på cellulosan kunde styras. För att kontrollera at ytpolymerisationen var ’levande’ och att kedjeändarnas funktionalitet var intakt, ympades ett andra lager av en hydrofil monomer, 2‐hydroxietyl metakrylat, på den PMA‐ympade cellulosan. Detta förändrade dramatiskt hydrofiliciteten hos cellulosan. Dendroniserade polymerer av generation ett, två och tre syntetiserades genom ATRP av akryliska makromonomerer baserade på 2,2‐dimetylolpropansyra. Via makromonomer metoden polymeriserades makromonomerer av generation ett, two och tre. Polymerisationerna resulterade i polymerer med lågt PDI‐värde. Kinetiken hos reaktionerna undersöktes och polymerisationerna följde första‐ordningens kinetik när etyl 2‐bromopropionat användes som initiator. Via ”ympning‐till” metoden fästes dendronerna divergent till en dendroniserad polymer av generation ett, som framställts genom ATRP:

LIST OF PAPERS The thesis is a summary of the following papers:

I “Atom Transfer Radical Polymerization of Methyl Acrylate from a Multifunctional Initiator at Ambient Temperature”, A. Carlmark, R. Vestberg, E. Malmström, Polymer, 2002, 43, 4237‐4242

II “Atom Transfer Radical Polymerization from Cellulose Fibers at Ambient Temperature”, A. Carlmark, E. Malmström, Journal of the American Chemical Society, 2002, 124(6), 900‐901

III “ATRP Grafting from Cellulose Fibers to Create Block‐copolymer Grafts”, A. Carlmark, E. Malmström, Biomacromolecules, 2003, 4, 1740‐1745

IV “Dendronized Aliphatic Polymers by a Combination of ATRP and Divergent Growth”, M. Malkoch, A. Carlmark, A. Woldegiorgis, A. Hult, E. Malmström, Macromolecules, 2004, 37, 322‐329

V “ATRP of Dendronized Aliphatic Macromonomers of Generation One, Two and Three”, A. Carlmark, E. Malmström, submitted to Macromolecules

The thesis also contains unpublished results.

TABLE OF CONTENTS

1 PURPOSE OF THE STUDY .................................................................... 1

2 INTRODUCTION .................................................................................... 2

2.1 Background ........................................................................................................2

2.2 Controlled Radical Polymerization (CRP) .....................................................2 2.2.1 Atom Transfer Radical Polymerization (ATRP).............................................3

2.3 Macromolecular Architecture..........................................................................6 2.3.1 Dendritic polymers..........................................................................................7 2.3.2 Surface confined polymers.............................................................................10

2.4 Cellulose ...........................................................................................................12 2.4.1 Cellulose confined polymers ..........................................................................13

3 EXPERIMENTAL....................................................................................... 15

3.1 Materials ...........................................................................................................15

3.2 Characterization ..............................................................................................15

3.3 Polymerization from poly(3‐ethyl‐3‐(hydroxymethyl) oxetane) ..............17 3.3.1 Synthesis of macroinitiator............................................................................17 3.3.2 Polymerization of methyl acrylate from macroinitiator ................................18

3.4 Grafting from cellulose...................................................................................19 3.4.1 Immobilization of the initiator.......................................................................19 3.4.2 Grafting of the modified filter paper with methyl acrylate (MA)..................19 3.4.3 Grafting of the PMA‐grafted filter paper with 2‐hydroxymethyl methacrylate

(HEMA).............................................................................................................20 3.4.4 Grafting of a second layer of methyl acrylate from the PMA grafted cellulose .......................................................................................................................20 3.4.5 Composite preparation...................................................................................20

3.5 Synthesis of dendronized macromonomers ................................................21

3.5.1 Synthesis of acetonide‐protected‐[G1]‐bis‐MPA decane acrylate (G1)............. .......................................................................................................................21 3.5.2 Synthesis of acetonide‐protected‐[G2]‐bis‐MPA decane acrylate (G2) and

acetonide‐protected‐[G3]‐bis‐MPA decane acrylate (G3)..................................21

3.6 Synthesis of dendronized polymers .............................................................22 3.6.1 The macromonomer route..............................................................................22 3.6.2 The ‘graft‐onto’ route ....................................................................................24

4 RESULTS AND DISCUSSION............................................................ 25

4.1 Polymerization from poly(3‐ethyl‐3‐(hydroxymethyl) oxetane) ..............25 4.1.1 Synthesis of macroinitiator............................................................................26 4.1.2 Polymerization from macroinitiator..............................................................26

4.2 Polymerization from cellulose.......................................................................29 4.2.1 Immobilization of the initiator.......................................................................29 4.2.2 Grafting of the modified filter paper with methyl acrylate (MA)..................30 4.2.3 Grafting of the PMA‐grafted filter paper with 2‐hydroxyethyl methacrylate

(HEMA).............................................................................................................34 4.2.4 Cellulose Composites .....................................................................................37

4.3 Dendronized polymers...................................................................................38 4.3.1 Macromonomer Route...................................................................................39 4.3.2 ‘Graft‐onto’ Route .........................................................................................48

5 CONCLUSIONS ..................................................................................... 50

6 FUTURE WORK...................................................................................... 51

7 ACKNOWLEDGEMENTS.................................................................... 52

8 REFERENCES.......................................................................................... 54

1. Purpose of the Study

1

1 PURPOSE OF THE STUDY The prerequisite demand for polymers for advanced applications is polymers with well‐defined structures. However, controlled polymerization techniques are required to obtain precise architectures. Free radical polymerization is used to synthesize 50% of all commodity polymers. However, the technique offer poor control over structural features. Controlled radical polymerization techniques can fulfill the criteria of accurate control over chain ends, molecular weight and molecular weight distribution. The purpose of this study was to investigate atom transfer radical polymerization, ATRP, as a viable method to synthesize new, complex macromolecular architectures. A sub‐goal in this study was to develop an ATRP system for a macromolecular initiator, with a large number of initiating sites that would polymerize in a controlled manner to high conversions. The findings would then be explored on a solid substrate for surface polymerization, in order to obtain a well‐defined surface. The substrate of choice was cellulose fibers. Another sub‐goal was to utilize ATRP for the synthesis of aliphatic dendronized polymers based on bis‐MPA. Dendronized polymers based on bis‐MPA can be seen as potential scaffolds for a wide variety of functional groups

2. Introduction

2

2 INTRODUCTION

2.1 Background Polymers can be synthesized through several different methods, such as free radical polymerization, polycondensation, anionic‐ and cationic polymerization, ring‐opening polymerization and coordination polymerization. Of the above mentioned, free radical polymerization is the most widely used industrial polymerization method. The advantages with this technique are its low requirements on reactant purity and that a wide variety of monomers can be polymerized and co‐polymerized under simple conditions. However, this method offers poor control over the molecular weight and polydispersity of the resulting polymer. The relatively high concentration of reactive free radicals causes side reactions, such as termination and chain transfer, to take place to a great extent. Furthermore, it is desirable to accurately control the end‐group functionality of a polymer, which is difficult in free radical polymerization. In order to obtain complex, well‐defined macromolecular architectures more controlled polymerization methods are needed.

2.2 Controlled Radical Polymerization (CRP) Controlled/living polymerizations are defined as polymerizations where no termination takes place and where the polymerization proceeds in a controlled manner and continues until all the monomers are consumed. The molecular weight increases nearly proportionally to the monomer conversion and the resulting polymers have low polydispersity indexes (PDIs). Living cationic‐, anionic‐, ring‐opening‐ and group‐transfer polymerizations are examples of such techniques. However, these reactions are synthetically demanding, as they require high purity of the reactants as well as the absence of both oxygen and water, which limits their industrial use. More controlled radical polymerization techniques have therefore been developed such as atom transfer radical polymerization (ATRP),1‐3 reversible addition fragmentation chain transfer (RAFT)4,5 and the nitroxide mediated free radical process (NMP).6‐10 Controlled

2. Introduction

3

radical polymerization is based on the maintenance of a low, stationary concentration of the active species (free radicals) and the establishment of a fast, dynamic equilibrium between the active and dormant species.11,12

2.2.1 Atom Transfer Radical Polymerization (ATRP) One of the most widely studied controlled radical polymerization techniques is atom transfer radical polymerization (ATRP). ATRP has been thoroughly investigated since it was developed independently by Matyjaszewski and Wang1,2 and Sawamoto et al.3 in 1995. It was developed from redox catalyzed telomerization reactions13,14 as well as from atom transfer radical addition (ATRA),15 Figure 1. ATRA is a modification of the Kharasch addition reaction, in which a transition metal catalyst acts as a carrier of the halogen atom in a reversible redox process.16,17 ATRP has proven to be a powerful tool in the synthesis of polymers with narrow polydispersities and controlled molecular weights.18 The reaction is termed controlled/”living” since termination reactions are not completely avoided.2,19

R X

Y

Y

R

Y

Mtn

Mtn+1Xki

R.

R-X

.

Figure 1. Atom Transfer Radical Addition (ATRA). 2.2.1.1 Components in ATRP All ATRP systems are composed of monomer, initiator and catalyst (a transition metal and a suitable ligand). Various vinyl monomers, such as styrenes,2,20‐22 (meth)acrylates,2,23‐30 acrylonitriles,31,32 and (meth)acrylamides33‐35 can be homopolymerized with ATRP. The initiator is typically an alkyl halide (RX). The halide is usually bromide or chloride, although iodide based initiators have been reported.36 Examples of halogenated compounds that have been used as initiators in ATRP are carbon

2. Introduction

4

tetrachloride and chloroform, benzyl halides and α‐halo esters.37 The only requirement is that the initiator must have a halogen attached to an atom containing radical stabilizing substituents. Also, the initiation step must be faster than or equal to the propagation rate for a controlled polymerization.38 Several transition metals have been used in ATRP. Copper is by far the most common metal, due to its versatility in ATRP and relatively low cost. However, other metals such as iron,39,40 ruthenium,3,41 nickel,27,42 molybdenum,43,44 rhenium,36 rhodium45 and palladium46 have also proven successful for various monomers. The metals are used in conjunction with a large variety of ligands. The ligands are an important part of the ATRP system and its role is three‐fold. Firstly, the ligands solubilize the metal in the organic media. Secondly, they control selectivity by steric and electronic effects. Finally, by their electronic effects, they also affect the redox chemistry of the final metal complex.47 The ligands are usually nitrogen‐1,48‐50 or phosphine‐based.3,39,40 2.2.1.2 Mechanism of ATRP In ATRP, the active species is formed when the halogen in the alkyl halide is abstracted by the metal complex in a reversible redox process as illustrated in Figure 2. The bond between the alkyl and the halide is cleaved homolytically and a carbon‐centered radical is formed on the alkyl.2 In this process the deactivation rate must be higher than the activation rate in order to create a low concentration of propagating radicals. Thus, the equilibrium between active and dormant species must be greatly shifted towards the dormant species. If deactivation is very slow or non‐existent the polymerization becomes uncontrolled.47 The over‐all rate of the reaction is highly dependent on the redox potential of the metal complexes. The general opinion is that ATRP involves chain propagation via free radicals and that the homolytically cleaved halide is in no way associated with the formed free radical on the chain‐end. However, it has been shown that polar solvents, such as water, greatly affect the rate of the polymerization.51,52 Haddleton et al. have suggested that the mechanism of the propagation step in Cu(I) mediated ATRP is different to that of a free radical due to association between the chain‐end radical and the metal complex.53

2. Introduction

5

N

NN

N

NN N

Br

N

CuCu

+

+ R-Br

+ .

+ R.Br- Br-

Figure 2. The metal complex exemplified with Cu(I) as the catalyst and 2,2’‐bipyridine as ligand. The polymerization takes place in two steps: initiation and propagation. In Figure 3 the mechanism is exemplified with copper(I) as catalyst. Termination reactions also occur, but no more than a few percentages of the growing chains undergo termination in ATRP.37

R

R

+ Cu(I)X/ligand +.

X-Cu(II)X/ligand

Initiation

Propagation

Pn-X + Cu(I)X/ligand Pn + X-Cu(II)X/ligand Keq

Keq

R-X

(X=Cl, Br)

+ monomerkp

P1

Pn + monomerkp Pn+1

Termination

Pn + Pm

kt Pn+m

. .

. .

...

.

Figure 3. The mechanism of ATRP.19

2. Introduction

6

2.2.1.3 Kinetics of ATRP Based on the reaction schemes in Figure 3, Matyjaszewski has described the ATRP kinetics by the following equations:20

]PX][Cu(I)X[]Cu(II)X][P[ 2⋅

==deact

acteq k

kK (1)

]M[]Cu(II)X[

]Cu(I)X[]RX[]M][P[]M[2

eqppappp KkkkR =⋅== (2)

Equation (2) is based on the assumption that the termination step can be neglected and that a fast pre‐equilibrium is established and hence that the propagation rate constant, kp,, is constant throughout the reaction. The propagation rate (Rp) is first‐order with respect to monomer concentration [M], initiator concentration [RX], and activator concentration [Cu(I)X].20 However, the reaction is not simply negative first‐order with respect to deactivator concentration [Cu(II)X2]. This is due to the persistent radical effect, the irreversible formation of Cu(II)X2 in the initial stages of the polymerization.54,55

2.3 Macromolecular Architecture The search for new, polymeric materials has traditionally focused on the chemical composition of linear and crosslinked polymers.56 However, the architecture of a polymer will have a large influence on its final properties.57 Polymers with the same repeating unit can have very different physical properties depending on its structure.58 Therefore, the properties of a polymer can be tailored, not only by changing its chemical composition, but also by changing its architectural composition. Controlled polymerization techniques enable the synthesis of complex macromolecular architectures. Examples of complex macromolecular architectures are shown in Figure 4.

2. Introduction

7

Figure 4. Examples of macromolecular architectures: a) block‐copolymer b) comb‐polymer c) network d) star polymer e) micto‐arm star polymer f) hyperbranched polymer g) dendrimer h) dendritic‐linear hybrid

2.3.1 Dendritic polymers Polymerization of ABx‐type monomers yields polymers with a highly branched structure. This group of polymers is called dendritic polymers, comprised of hyperbranched polymers, dendrimers and dendritic‐linear copolymer hybrids, Figure 4f‐h. 2.3.1.1 Hyperbranched polymers Hyperbranched polymers, Figure 4f, are highly branched macromolecules. They can be synthesized with most known polymer‐forming reactions, such as condensation reactions,59‐61 cationic procedures,59 ring‐opening polymerizations62‐65 and free radical procedures, including controlled/”living” free radical polymerizations.66,67

2. Introduction

8

The globular structure and the large number of end‐groups give hyperbranched polymers unique properties.58,59,68 The polymers do not entangle to the same degree as linear polymers, which greatly alters their mechanical and rheological properties, compared to their linear analogs.58,69 The hyperbranched polymers have lower viscosity than their corresponding linear polymers with equal molecular weights.58,59 2.3.1.2 Dendrimers When dendritic polymers are perfectly branched they are called dendrimers, Figure 4g.70,71 Dendrimers can be synthesized either divergently or convergently. In the divergent growth the generations are grown generation by generation from a small core molecule. In convergent growth the individual dendrons are made first and then coupled to a small core molecule.72 2.3.1.3 Dendritic‐linear copolymer hybrids The large number of end‐groups of dendritic polymers, Figure 4f‐g, can be modified in several different ways. One way is to transform the end‐groups into initiating groups for further polymerization of linear chains. The resulting polymer will have a dendritic core with linear polymer chains growing out from it, resulting in a hybrid between a dendritic and a linear polymer, Figure 4h. The dendritic polymer then functions as a macroinitiator i.e. a multifunctional initiator. If the chain‐ends of the dendritic polymer are turned into initiating groups for ATRP, ATRP can be performed from the macroinitiator yielding linear chains with controlled length.73‐81 For example, Fréchet et al.74 used isophthalate ester‐functionalized dendrons, featuring benzylic halide groups at their focal points, for controlled radical polymerization of styrene, Haddleton et al.73 used carbosilane dendritic initiators for the polymerization of methyl methacrylate, Hedrick et al.76 used a polyester‐based dendrimer for the ATRP of methyl methacrylate and Frey et al.79 used a well‐defined hyperbranched polyglycerol core for methyl acrylate. This combination of dendrimer synthesis and controlled radical polymerization is an interesting field that yields new polymeric materials.73 2.3.1.4 Dendronized polymers If dendrons are attached to a linear polymer chain instead of a small core molecule the resulting polymer is called a dendronized polymer.82 This is actually a sub‐class of comb polymers, Figure 4b, where the comb’s teeth are

2. Introduction

9

dendrons instead of linear polymer chains. Depending on the dendrons size, shape and attachment to the backbone the dendronized polymer can attain a fully stretched conformation, instead of a random‐coil, due to steric repulsion between the dendrons.83 These rod‐like, cylindrical dendrimers are believed to have new and interesting properties. Due to the rigidity of dendronized polymers they are expected to have very high bending moduli, making them interesting for constructions on the nanometer scale.84 The surface and backbone are engineerable as well and other potential applications are as catalyst supports, light harvesting and/or conducting materials, and for variable applications as colloids.85,86

MacromonomerRoute (A)

‘Graft-onto’ Route (B)

Figure 5. Schematic presentation of the two main routes in the synthesis of dendronized polymers: the macromonomer route (A) and the ‘graft‐onto’ route (B). Dendronized polymers can be synthesized by either of two main routes, the macromonomer route or the ‘graft‐onto’ route (Figure 5).83 In the macromonomer route, a macromonomer containing a dendron and a polymerizable group (typically a styrene or (meth)acrylate) are polymerized. The macromonomer route is considered superior to the ‘graft‐onto’ route since it offers accurate control over the pendant dendrons.87 However, steric hindrance may become a problem. Once the dendrons become too large, the polymerizable group may be ‘shielded’ and inhibit polymerization.87,88 Low molar mass products or no polymers are not unusual.83 Consequently, it has been hypothesized that the incorporation of a spacer between the polymerizable group and the dendron might increase the availability of the polymerizable group.82,89,90 However, as Schlüter et al. have discovered, difficulties surrounding the polymerizations might depend solely of the monomer concentration in the

2. Introduction

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reaction mixture.87 For macromonomers of generation three or higher they have found that a monomer concentration of >45 % (by weight) is necessary to reach higher conversions.84 The macromonomer route has been explored using various protocols such as radical polymerization,91 ring‐opening metathesis polymerization (ROMP),92 Suzuki polycondensation,93 Heck coupling,94 and ATRP.95,96 In the ‘graft‐onto’ route dendrons are coupled to a pre‐made polymer. This approach offers some synthetic freedom in terms of the dendrons, which can be obtained by divergent or convergent growth. The purification is usually straightforward since the dendronized polymer is easily isolated by precipitation. However, this strategy suffers the drawback of incomplete coupling, due to the steric crowding of the dendrons.97,98 The molecular architecture, including back‐bone chemistry, size of the dendrons, the presence of a flexible spacer and the loading of dendritic wedges, will influence the conformation.84 Also, the complex chemical/physical interactions between dendron/back‐bone, dendron/dendron, and dendron/back‐bone/solvent will contribute.84 To elucidate the structure‐property relationship for dendronized polymers it is important to elaborate versatile synthetic procedures to access structurally different hybrids.

2.3.2 Surface confined polymers The interest to control surface properties has increased remarkably in recent years, with a growing demand from technologies ranging from microelectronics to biotechnology. Surface grafting of polymer chains is one of the most versatile methods to modify and control the properties of a surface.99 Polymer chains can be attached to a solid surface, and then form so‐called polymer brushes. The polymers can either be attached to the surface physically or covalently. Covalent attachment is usually preferred in order to avoid adhesion problems.100 This can be accomplished in two ways, by ‘grafting‐to’ (Figure 6) or ‘grafting‐from’ (Figure 7).101 In the “grafting to” technique the polymers are pre‐synthesized. They are end‐functionalized in such a way that they will react with groups on the surface, hence attaching the entire chain of the polymer. The advantages with this technique are that the properties of the polymer easily can be tailored and that the polymer can be analyzed prior to attachment. However, only small amounts

2. Introduction

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of polymer can be immobilized onto the surface with this technique.101 As more chains become attached to the surface it gets more difficult for free chains to diffuse through the already attached polymer and reach the reactive sites on the surface. Thus, only thin films can be formed (about 1‐5 nm) and the resulting surface has a low grafting density.100

Figure 6. Schematic presentation of the ‘grafting‐to’ technique. In the ‘grafting‐from’ technique initiating groups are immobilized on the surface, followed by polymerization from these groups, yielding polymer brushes. The attached polymer is more difficult to tailor this way but much thicker films and higher grafting density can be obtained.101

Figure 7. Schematic presentation of the ‘grafting‐from’ technique. One way to tailor the film thickness and the attached polymer in the ‘grafting‐from’ technique is to immobilize initiators for controlled radical polymerization. This way monomers can be grafted from the surface by ATRP. ATRP has previously been utilized for ‘grafting‐from’ processes from silicon,99,102‐105 gold106‐

108 and silica109 surfaces and from porous substrates.99,108 Organic substrates have also been used, such as polystyrene latexes.78,110

2. Introduction

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One of the greatest advantages with using ATRP for ‘grafting‐from’ is its ability to tailor the film thickness. It also offers control of the end‐groups of the polymer on the surface and the surface can thereby be further modified in different ways. Two main approaches have been used to control the growth of the polymer brushes by CRP. The first approach involves addition of sacrificial initiator to the reaction solution, thus forming a non‐attached bulk polymer.111 However, it has been shown that the kinetics from the surface are different from that of the bulk, thus the bulk polymer will only be a relative representation of that on the surface.108,109,112 The second approach is to add deactivator (for example, Cu(II)X2) to the reaction and not use a sacrificial initiator.104 The addition of deactivator is necessary in order to gain control over the polymerization from the surface. This way the only parameter that determines the film thickness is the reaction time. The biggest advantage with this approach is the absence of a free, linear polymer, facilitating work‐up procedures. Another great advantage with employing a ’living’ polymerization technique is the possibility to prepare block‐copolymer brushes from the surface (a schematic figure of a block‐copolymer is shown in Figure 4a). The first covalently attached surface block‐copolymer by ATRP was reported by Matyjaszewski et al. when synthesizing poly(styrene‐block‐t‐butyl acrylate) from silicon wafers.104

2.4 Cellulose Cellulose is the world’s most abundant renewable polymer resource. Cellulose is a natural polymer made up of β‐1‐4 D‐glucose units, Figure 8.

O

OO

OH OHO

OO OOH

OH OHOH

OHOH

OH

OH

OH

OH

n OH

OH Figure 8. Repeating β‐1‐4 D‐glucose units, structure of cellulose. Due to the large number of hydroxyl groups and to its linear structure, cellulose forms strong inter‐ and intra‐molecular hydrogen bonds. The intermolecular hydrogen bonds cause the natural cellulose to form a sheet‐like structure and the polymer is crystalline.113 The individual cellulose molecules bundle up together to form microfibrils, Figure 9. The microfibrils stack together and make up the

2. Introduction

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fibrils, which gives the cellulose fibers. The polymer is very hydrophilic due to its many hydroxyl groups and it adsorbs water easily.

Figure 9. Hierarchical structure of cellulose. Source: http//www.ill.fr/AR‐99/page/13polym.htm

2.4.1 Cellulose confined polymers Cellulose and its derivatives find use in a wide variety of applications, such as fibers, plastics, coatings, films, wood and paper products. One of the uses of plant fibers is in composites, since cellulose fibers exhibit excellent mechanical properties considering their low density and are easy to process.114,115 However, the fiber‐to‐matrix bonding needs to be optimized since cellulose is very polar and hydrophilic and hence has poor adhesion to the often hydrophobic polymer matrix.114 Improved adhesion can be obtained by chemical modification of the fiber, for example by graft copolymerization or by the introduction of a coupling agent.116 Property improvements include: heat resistance, improved elasticity, resistance to abrasion and wear, oil and water repellency, ion‐exchange capabilities and antibacterial activity.117‐119 Grafting of hydrophobic polymers, such as methyl methacrylate, styrene, acrylonitrile, and butadiene greatly enhance cellulose adhesion to hydrophobic fibers.120

2. Introduction

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Cellulose fibers have been graft copolymerized using various techniques in the past.121‐130 Most of them are based on a ‘grafting‐from’ process, where radicals are formed along the polymer backbone either by various chemical initiators or by irradiation, followed by a free radical polymerization of vinyl monomers. Disadvantages with these techniques are that the polymerization from the surface is highly uncontrolled and hence, molecular weight and molecular weight distribution cannot be controlled. In these methods chain scission can occur and does not necessarily render the cellulose backbone intact.131 One major drawback with this approach is that a bulk polymer is always formed and this homopolymerization is more dominant than the graft polymerization.119

3. Experimental

15

3 EXPERIMENTAL The following section briefly describes the performed reactions. Full details can be found in the papers.132‐135

3.1 Materials The hydroxy‐functional polyoxetane was prepared according to a literature procedure64 from 3‐ethyl‐3‐(hydroxymethyl)oxetane supplied from Perstorp AB. Tris(2‐(dimethylamino)ethyl)amine (Me6‐TREN) was prepared similar to Ciampolini and Nardi136 from tris(2‐aminoethyl)amine (98%, Aldrich). 2‐Hydroxyethylmethacrylate (HEMA, Aldrich, 97%) was purified immediately prior to use, according to a procedure described elsewhere.134 A filter paper, Whatman 1, was used as cellulose substrate. Acetonide‐protected 2,2‐bis(hydroxymethyl) propionic anhydride was prepared according to literature from 2,2–bis(hydroxymethyl) propionic acid (bis‐MPA)137, kindly supplied by Perstorp AB, Sweden. All other chemicals and solvents were used as received.

3.2 Characterization Infrared spectra were recorded on a Perkin‐Elmer Spectrum 2000 FTIR equipped with a MKII Golden Gate™, Single Reflection ATR System from Specac Ltd, London, UK. The ATR‐crystal was a MKII heated Diamond 45° ATR Top Plate. 1H‐NMR and 13C‐NMR spectra were recorded on a 400 MHz Bruker Aspect NMR, using CDCl3 and MeOD‐d4 as solvents. The solvent residual signal was used as internal standard. Molecular weights and polydispersities were determined by Size Exclusion Chromatography (SEC) on a Waters 717plus auto sampler and a Waters model 510 apparatus equipped with two PLgel 10 µm mixed‐B columns, 300*7.5 mm (Polymer Labs., U.K.), with CHCl3 as the mobile phase, 1 ml min‐1. Linear polystyrene standards ranging from 1 700 to 706 000 g mol‐1 were used for calibration.

3. Experimental

16

Size exclusion chromatography (SEC) analyses were also performed with a Waters 6000A pump, a PL‐EMD 960 light scattering evaporate detector, two PL gel 10 µm mixed‐B columns (300 x 7.5 mm) from Polymer Labs and one Ultrahydrogel linear column (300 x 7.8 mm) from Waters. DMF was used as solvent at a flow rate of 1.0 ml min‐1. The solvent was preheated to 70°C. Linear polyethylene oxides standards ranging from 620 to 78 000 g mol‐1 were used for calibration. Size exclusion chromatography (SEC) using THF (1.0 ml min‐1) as the mobile phase was performed at 35 °C using a Viscotek TDA Model 301 equipped with a GMHHR‐M column with TSK‐gel from Tosoh Biosep, a Viscotek VE 5200 GPC Autosampler, a Viscotek VE 1121 GPC solvent pump and a Viscotek VE 5710 GPC degasser. A universal calibration method was created using broad and narrow linear polystyrene standards. Corrections for the flow rate fluctuations were made by using THF as an internal standard. Viscotek Trisec 2000 version 1.0.2 software was used to process data. MALDI‐TOF experiments were conducted on a Bruker Reflex III MALDI‐MS instrument, equipped with a N2‐laser, 337 nm (Bruker Daltonik GmbH, Bremen, Germany). All mass spectra were obtained in linear mode. The matrices utilized were either trans‐3‐indoleacrylic acid (t3iA) or 2,5‐dihydroxybenzoic acid (DHB). Matrix solutions were prepared as 0.1 M solutions in THF. Sample preparation: the samples were dissolved in THF (2‐5 mM) and 1 µl of the sample solution was added to the matrix solution (10 µl). The samples were prepared both as sample‐matrix solutions and as sample‐matrix‐NaTFA solutions, employing a 0.1 M NaTFA solution in THF. The preparation protocol included mixing of 0.5‐1.0 μl of sample with 10 μl of matrix and/or 0.5‐1.0 μl of NaTFA (cationization agent). Then 0.2‐0.4 μl of the mixture was spotted on the MALDI target and was left to crystallize in room temperature. Normally 100 pulses were acquired for each sample. Thermal analysis was performed on a Mettler DSC 820 calibrated according to standard procedures. Glass transition temperatures were determined as the inflexion points in the heat‐flow curve from the second cooling. The analysis was carried out under nitrogen using a heating and cooling rate of 10 °C min‐1. Electron Spectroscopy for Chemical Analysis (ESCA) spectra were recorded using a Kratos AXIS HS x‐ray photoelectron spectrometer (Kratos Analytical,

3. Experimental

17

Manchester, UK). The samples were analyzed using a monochromator (Al x‐ray source). The analysis area was below 1 mm2. Contact angle measurements were performed on a Ramé Hart goniometer. Deionized water (Millipore, resistivity: 18.4 MΩ cm) was used. The advancing contact angles were obtained by keeping the needle in the water droplet after positioning it on the surface and by carefully adding more water until the advancing angle appeared to be maximal. The measurements were performed with the needle remaining in the droplet. Atomic Force Microscopy (AFM) was performed in tapping mode on a Nanoscope IIIa from Digital Instruments with silicon tips, supplied by Nanosensors, of resonance frequencies about 350 kHz. Scanning Elecron Microscopy (SEM) was conducted on a JEOL JSM‐5400 . The samples were sputtered two times with Au/Cd (60%/40%) for 30 seconds each time. The samples were fastened on aluminium carriers with carbon tape. The sputtering was performed in a Desk II from Denton Vacuum operating at 45 mA. In order to analyze the graft ratio the initiator modified papers were weighed before and after polymerization with methyl acrylate. The graft ratio (%) was calculated as follows: Graft ratio (G, %) = (weight of grafted PMA/weight of cellulose substrate)*100

3.3 Polymerization from poly(3‐ethyl‐3‐(hydroxymethyl) oxetane)

3.3.1 Synthesis of macroinitiator The hydroxyl groups in the polyoxetane were esterified by reaction with 2‐bromoisobutyryl bromide, yielding bromo‐ester end‐groups on the hyperbranched polymer, Figure 10.132

3. Experimental

18

Br

Br

O

O O

O

O

O

Br

O

OBrO

Br

O

O O

Br

O

O

Br

O

OO

O

Br

O

BrO

O

O

OO Br

O

Br

O

O

O

Br

O

OO

O

BrO

Br

O

O

OO

OBrBr

O

O

OH

O

OH

O OOH

OH

O

OH

OOH

OH

O

O

OHOH

O

OH

O

OH OH

O

OH OH

O

DMAPEt3N

THF

Figure 10. Reaction between poly(3‐ethyl‐3‐(hydroxymethyl)oxetane) and 2‐bromoisobutyryl bromide.

3.3.2 Polymerization of methyl acrylate from macroinitiator Each polymerization was performed by essentially the same procedure, Figure 11. The ratio of initiating sites‐to‐monomer was varied in order to obtain different degrees of polymerizations (DP’s).132

O

OO

O BrBr O

OO

OO

OBr

Br

O

OBr

O

Br

O

OOO

O

BrO

Br

O

OO

Br

OO

Br

OO

Br

O

O

BrO

OO

BrO

O

O

O

Br

O

O O

O

Br

OO

BrO

O

O

OO

OO

Br

OO

BrO

O

OO

OO

Br

OO

Br

OO

OOO

O

O

O

Br

OO

OO

Br

OO

O

O

BrO

O

OO

OO

Br

OO

OO

O

Br

O O

BrO

O

OO

OO

O

Br

OO

O

OO

O O

O

Br

O

O O

O

Me6-TRENMA, Cu(I)BrEtOAc

Figure 11. Polymerization of methyl acrylate from the macroinitiator.

3. Experimental

19

3.4 Grafting from cellulose

3.4.1 Immobilization of the initiator The hydroxyl groups on the filter paper surface were esterified by reaction with 2‐bromoisobutyryl bromide, yielding bromo‐ester groups on the surface, Figure 12.133

OH OH

Br O

O

Br O

OBrBr

O

Cellulose Cellulose DMAPEt3NCH2Cl2

Figure 12. Immobilization of initiator on the cellulose. The reaction was left to proceed at room temperature for 1‐24 hours. The filter paper was thereafter thoroughly washed with dichloromethane and ethanol under ultrasonication. It was dried under vacuum.

3.4.2 Grafting of the modified filter paper with methyl acrylate (MA) The initiator modified filter paper was grafted with MA according to Figure 13.

Br

O O

OO

Br

O O

OO

Br O

O

Br O

O OBr

O

O

O

Cellulose +

Cu(I)BrMe6-TRENEtOAc

Cellulose

Figure 13. Polymerization of MA from initiator‐immobilized cellulose. The grafting was accomplished by immersing the initiator‐modified paper into the reaction mixture containing MA, Cu(I)Br (0.1 equiv. to sacrificial initiator),

3. Experimental

20

Me6‐TREN (0.1 equiv.), sacrificial initiator (2‐ethyl isobutyrylbromide, EBiB), and ethyl acetate (EtOAc, 33 w/w%). All polymerizations were carried out at room temperature for 18 hours.133

3.4.3 Grafting of the PMA‐grafted filter paper with 2‐hydroxymethyl methacrylate (HEMA)

The polymerization conditions of HEMA with ATRP was adopted from Baker et al.138 Cellulose substrates grafted with PMA (targeted DP’s of 100 and 300) were immersed into a solution of HEMA, H2O (50 vol%), Cu(I)Cl, Cu(II)Cl2, and bipyridine (bipy), Figure 14. The reaction was left to proceed under continuous stirring for two hours in room temperature.134

O O

BrO

O

O O

BrO

O

O

O

O O

O

Br

OHO

O

O

O O

O

O

Br

OH

Cellulose Cellulose

HEMAH2OCu(I)ClCu(II)Cl2Bipy

Figure 14. Grafting of a block co‐polymer from cellulose.

3.4.4 Grafting of a second layer of methyl acrylate from the PMA grafted cellulose

Cellulose substrates grafted with PMA (targeted DPs of 100 and 300) were immersed into a reaction solution containing MA, EtOAc (33 w/w%), Me6‐TREN, sacrificial initiator (2‐ethyl bromoisobutyrate, EBiB) and Cu(I)Br. Polymerizations were performed as before, with a targeted DP of 200 for the second graft. The polymers were worked‐up and the papers were washed as described elsewhere.134

3.4.5 Composite preparation Laminate composites were fabricated for evaluation, one with unmodified filter paper as a control and one with PMA‐grafted filter paper. The matrix was based

3. Experimental

21

on a K2‐system of epoxy and diamine, commercially available. The filter papers were soaked in the matrix. The excess matrix resin and air bubbles were removed by the use of an applicator and the resulting laminate was cured.

3.5 Synthesis of dendronized macromonomers

3.5.1 Synthesis of acetonide‐protected‐[G1]‐bis‐MPA decane acrylate (G1)

G1 macromonomer was synthesized from 1,10‐decanediol. The diol was first monoacrylated and then a first‐generation dendron based on bis‐MPA was coupled using anhydride chemistry, Figure 15.135,139

O

O

O

O

O O

O

OH

OOH

OH

Cl

O

O

O

O

O O

O

O

Et3NDMAPTHF

DMAPPyridine

G1 Figure 15. Synthesis of G1 macromonomer.

3.5.2 Synthesis of acetonide‐protected‐[G2]‐bis‐MPA decane acrylate (G2) and acetonide‐protected‐[G3]‐bis‐MPA decane acrylate (G3)

G2 and G3 macromonomers were synthesized from G1 macromonomer by hydrolysis of the acetonide‐group, utilizing Dowex H+‐resin. This yielded hydroxyl groups that were reacted with the anhydride of bis‐MPA, Figure 16.135

3. Experimental

22

O

O

O

O

O O

O

O

O

O

OH OH

O

O

O

O O

O

O O

O

O

O

O OOO

O OO O OO

O

O

OO

OO

OO O

OO

O

OO

OO

O

O

O

O

OO

Dowex H+-resinMeOH

DMAPPyridine

G2

G3

Deprotection andEsterification

G1

Figure 16. Synthesis of G2 and G3 macromonomers.

3.6 Synthesis of dendronized polymers

3.6.1 The macromonomer route G1, G2 and G3 macromonomers were polymerized by ATRP, according to Figures 17, 18, and 19, yielding polymers P(G1), P(G2) and P(G3).

O(CH2)10

O

O OO

O

Brn

O

O

Br

OO

O O

OO

O O

(CH2)10DPECu(I)BrMe6-TRENRT

G1P(G1)

Figure 17. ATRP of G1 macromonomer.

3. Experimental

23

O

(CH2)10

O O

O

Br

OO

BrOO

n

(CH2)10OO

O OOO

O O O O

OO

O OOO

O O O O

DPECu(I)BrMe6-TREN50°C

G2P(G2)

Figure 18. ATRP of the G2 macromonomer.

OO

O

O

OO

OO

OO O

OO

O

OO

OO

OO

OO

OO

O

O

OO

OO

OO O

OO

O

OO

OO

OO

OO

O

O

BrO

(CH2)10

O

OOBrO

O

n

(CH2)10

P(G3)G3

DPECu(I)BrMe6-TREN50°C

Figure 19. ATRP of the G3 macromonomer.

3. Experimental

24

3.6.2 The ‘graft‐onto’ route Polymers P(G2) and P(G3) were also synthesized through the ‘graft‐onto’ route, by divergent growth from P(G1). The acetonide‐groups in P(G1) were hydrolyzed and the resulting OH‐groups were reacted with the anhydride of bis‐MPA, as in the synthesis of the macromonomers G2 and G3, resulting in P(G2). P(G3) were synthesized by the same procedure, however requiring longer reaction times for the coupling reaction of the anhydride.135

4. Results and Discussion

25

4 RESULTS AND DISCUSSION The purpose of this study was to investigate the use of atom transfer radical polymerization (ATRP) as a tool to synthesize new and interesting macromolecular architectures. One aim was the synthesis of polymer brushes from a solid substrate, i. e. cellulose. In the polymerization from a solid substrate the chains polymerize in close proximity to one another, wherefore side‐reactions such as recombination and termination may occur to a greater extent. The first goal was therefore to develop a multifunctional system that was highly controlled based on a soluble macroinitiator having a large number of initiating groups. This system was used in the polymerization of methyl acrylate (MA) from cellulose.

4.1 Polymerization from poly(3‐ethyl‐3‐(hydroxymethyl) oxetane) The soluble multifunctional system was developed by the use of poly(3‐ethyl‐3‐(hydroxymethyl)‐oxetane, which is a hydroxyl‐functional hyperbranched polymer, having approximately 25 end‐groups. The hydroxyl groups were turned into initiating groups for ATRP by reaction with 2‐bromoisobutyryl bromide. The resulting macroinitiator was used as an initiator for polymerization of MA. There are several difficulties in using a multifunctional initiator instead of a mono‐functional compound. Each molecule gives rise to a large number of growing chains and this causes an increased probability for intermolecular reactions. For example, Frey et al.79 observed gelation after 35% monomer conversion when a multifunctional initiator was used for ATRP of MA in bulk. In order to prevent a polymerizing system from forming a gel, the concentration of propagating radicals has to be kept low. This can be accomplished in different ways. Diluted systems give a lower concentration of radicals. The system can be diluted either by adding a solvent or by quenching the reaction at very low conversion of the monomer, thus allowing the excess monomer to dilute the


26

system. Another approach is to keep the amount of free radicals low by lowering the amount of catalyst, i.e. by lowering the ratio between the initiating sites and the catalyst.

4.1.1 Synthesis of macroinitiator The synthetic scheme for the synthesis of the macroinitiator is shown in Figure 10. As can be seen from the FT‐IR spectra of the macroinitiator and the polyether, Figure 20, the macroinitiator was successfully synthesized with complete conversion of the hydroxyl groups. The polyether shows a broad peak around 3300 cm‐1, originating from the OH‐end group, which disappears after the reaction with 2‐bromoisobutyryl bromide. The macroinitiator shows a peak around 1730 cm‐1 which is not present in the polyether. This peak originates from the carbonyl in the bromo‐ester group.

5001000150020002500300035004000

cm-1

Abs

orba

nce

Polyoxetane

Macroinitiator

Figure 20. FT‐IR of the polyoxetane and the macroinitiator.

4.1.2 ATRP of methyl acrylate from the macroinitiator The macroinitiator was used for the polymerization of methyl acrylate (MA), Figure 11. The polymerization was performed at room temperature, mediated by Cu(I)Br and Me6‐TREN. Matyjaszewski et al. have shown that MA can be polymerized by ATRP at room temperature when using Me6‐TREN as a ligand in conjunction with Cu(I).49 Polymerization proceeded quickly and the resulting PMA had very low polydispersity. Essentially the same reaction conditions were therefore chosen for this study. As can be seen in Table 1, methyl acrylate was successfully polymerized from the macroinitiator. The polymerization was fast at room temperature and yielded polymers with narrow polydispersities. The polymers were analyzed with SEC, 1H‐NMR and DSC.


27

Table 1. Results from SEC and the NMR analyses of the polymers.

No [M]:[I]:[Cu(I)]:[L] Time [min]

Mna

[g mol-1] PDI DP

(SEC)DP

(NMR) Mn

b

[g mol-1] 1 30:1:0.05:0.05 20 32 950 1.25 12 14 36 880 2 50:1:0.05:0.05 45 42 100 1.13 16 20 49 800 3 50:1:0.05:0.05 40 48 200 1.23 19 28 67 010 4 50:1:0.05:0.05 60 50 850 1.22 20 31 73 470 5 100:1:0.1:0.1 55 86 700 1.84 37 67 150 950 6 100:1:0.05:0.05 150 84 250 1.42 36 70 157 410 7 150:1:0.05:0.05 120 95 450 1.28 41 87 194 000

aMolecular weight measured by SEC in CHCl3 using conventional calibration bMolecular weight calculated from 1H‐NMR In this study the polymerization had to be performed in ethyl acetate (EtOAc) since the attempted bulk polymerizations resulted in polymers with high polydispersities. Molecular weights and polydispersities were determined by SEC, using linear polystyrene standards. It is hypothesized that the measured molecular weights were much lower than the actual values, since highly branched polymers are known to have smaller hydrodynamic volume than their linear analogues.76 The Mn has therefore also been calculated from 1H‐NMR. The determined polydispersity values might not agree completely with the real values since hybrid dendritic‐linear polymers are analyzed.88 When a ratio of initiating sites‐to‐catalyst of 1:0.05 was used the polymers from all the reactions had a low polydispersity, ranging from 1.1‐1.4, indicating a well‐controlled polymerization. The SEC chromatograms of the polymers were narrow and monomodal. When the ratio of initiating sites‐to‐catalyst was raised to 1:0.1 there was a sudden increase in the polydispersity, which shows a lack of control over the polymerization at this concentration of radicals. In order to obtain different degrees of polymerization (DP), the ratio of monomer‐to‐initiating sites was varied. The DP of the PMA arms was calculated both from SEC and from 1H‐NMR. When calculating the DP from SEC the assumption was made that the macroinitiator had 25 initiating sites and that each initiating site gave rise to one polymer chain. The assumed the molecular weight of the macroinitiator was 6750 g mol‐1, as determined by SEC.


28

The monomer conversion was calculated from 1H‐NMR of the crude product. Kinetic experiments showed that the reaction rate rapidly decreased around 65 % conversion. Longer reaction times did not give an increase in conversion. This is illustrated in Figure 21 where the conversion vs time plot is linear up to 65 % conversion and then there is a sudden change in the slope. This is suggested to be due to the increased viscosity at higher conversion, making the diffusion of monomers in the solution slower, in combination with the low concentration of radicals in the reaction media.

0

10

20

30

40

50

60

70

80

0 10 20 30 40Time [h]

Con

vers

ion

[%]

Figure 21. Conversion vs time plots of the polymerization of MA from the macroinitiator. The PMA‐grafted polymers exhibited two glass transitions, Tg’s, (Table 2) as analyzed by differential scanning calorimetry (DSC), indicating that the resulting block‐copolymer is phase‐separated. The lower glass transition (Tg1 at approx. ‐40°C) is unaffected by the length of the PMA grafts and therefore assumed to originate from the hyperbranched polyether core. The hyperbranched polyether could roughly be compared to poly(ethylene oxide) (PEO), which is a linear polyether that has a Tg of ‐60°C. In contrast, the higher glass transition (Tg2) is affected by the length of the grafts and therefore suggested to originate from the poly(methyl acrylate) grafts. According to literature data the Tg of free PMA should be around 6‐9°C.140


29

Table 2. Thermal analysis of the polymerized macroinitiator.

No. DPsec Tg1 [°C] Tg

2 [°C] 1 12 ≈ -40 3 3 19 ≈ -40 6 6 36 ≈ -40 12

4.2 Polymerization from cellulose ATRP has previously been performed from substrates such as gold106‐108 silicon,99,102‐105 and silica particles. 109 Only a few examples are found in the literature of grafting with ATRP from organic substrates.78,110 This study reports on the first grafting from a naturally occurring polymer, cellulose, with ATRP. Since our initial study other research groups have reported on the ATRP from cellulose as well.141,142 Herein, a filter paper was chosen for cellulose substrate. This was done in order to avoid problems with free fibers and simplify work‐up procedures. The filter paper of choice was Whatman 1, due to its high cellulose content and low amount of impurities.

4.2.1 Immobilization of the initiator In order to conduct ATRP from cellulose the hydroxyl groups on the filter paper was converted into initiating moieties for ATRP. This was accomplished as for poly(3‐ethyl‐3‐(hydroxymethyl)‐oxetane, by esterification of the hydroxyl groups with 2‐bromo isobutyrylbromide. Attempts were made to analyze the initiator modified cellulose fibers with FT‐IR, DSC and contact angle measurements. However, the content of bromo‐ester groups at the surface was too small to enable measurement with FT‐IR and DSC and no difference between virgin filter paper and modified filter paper could be detected. The water adsorption ability on the surface had noticeable decreased but no contact angle could be measured since the papers rather rapidly absorbed water. The bromo‐content on the surface was only detectable with ESCA. The cellulose fibers were subjected to the reaction solution containing 2‐bromoisobutyryl bromide for different time periods, 1, 2, 6, 18 and 24 hours. As can be seen in Table 3, the bromo‐content on the surface increased up to a reaction time of 6 hours, after which no real change in the bromide content could be seen with longer reaction times. Since halogens are difficult to analyze with ESCA, due to its deterioration from the surface under irradiation, the content of


30

the carbonyl carbon (denoted C4‐carbon in Table 3) was also analyzed. As can be seen the amount of carbonyl from the surface increases with the same trend as the bromide content. This confirms that the measured bromide content is consistent.

Table 3. ESCA of initiated filter paper

Reaction time [h]

C4-carbon [atom%]

C4-carbon increase [atom %]

Br [atom %]

Ref 0.7 - - 1 1.3 0.6 0.5 2 1.4 0.7 0.7 6 2.0 1.3 1.4

12 2.0 1.3 1.4 18 2.0 1.3 1.3 24 2.2 1.6 1.6

The initiator modified filter paper was also analyzed by AFM, Figure 22. In the figure the pictures to the left are topography images and the ones to the right are the phase contrast images. As can be seen, there is not a large difference between the virgin filter paper, 22a, and the initiator modified filter paper, 22b. The fiber structure appears intact, although slightly more closely packed in Figure 22b. a) b)

Figure 22. AFM of a) virgin filter paper b) initiator modified filter paper.

4.2.2 Grafting of the modified filter paper with methyl acrylate (MA) The grafting of MA was performed on papers that had been reacted with 2‐bromoisobutyryl bromide for 24 hours. The polymerization was conducted in ethyl acetate in order to decrease the viscosity in the reaction solution and thereby increase the diffusion of monomers to the surface as well as to simplify


31

the work up of the product. The reaction conditions were similar to the ones elaborated for the multifunctional polyether initiator. Also, comparable conditions were used as Baker et al.106 when they polymerized methyl methacrylate (MMA) from gold surfaces at room temperature. Since the number of initiating sites on the surfaces was unknown, sacrificial initiator, EBiB, was added in order to control the length of the PMA‐grafts, as the ratio between free initiator and monomer determines the degree of polymerization. In this study DPs of 100, 200 and 300 were aimed. In order to verify that the polymer on the surface was covalently attached and not just physiosorbed, a blind test was performed (denoted blank in Table 4 and Figure 24). This sample was treated identically to the samples subjected to polymerization with the exception that no initiator was immobilized on its surface prior to being subjected to the polymerization solution. The aimed DP of the bulk polymer for this sample was 200.

Table 4. Results from SEC and Contact Angle Measurements.

Sample (DP aim)

Mn theor.

Mn DP*) PDI Contact Angle, (θ)a

blank 17,212 14,830 172 1.05 - DP-100 8,609 6,430 75 1.10 - DP-200 17,212 12,810 149 1.06 128±10º DP-300 25,818 25,630 298 1.05 133±5º

*) The degree of polymerization was calculated from SEC data.

The non‐immobilized polymers were analyzed with SEC, Table 4. All the polymerizations proceeded in a controlled manner, yielding PMA with very low polydispersities, below 1.1. The targeted DP’s deviate somewhat from the measured ones indicating that the reaction conditions are not optimized. The roughness of the filter paper surface makes surface characterization of the modified cellulose fibers difficult. It renders ellipsometry impossible and contact angle measurements problematical. The values of the contact angle measurements in Table 4 should therefore only be considered a rough estimation. For that reason, only the advancing angle, (θ)a, has been measured. The blank sample quickly adsorbed water and behaved just like the virgin filter paper with respect to water adsorption. For sample DP‐100 no contact angle could be measured since the paper slowly adsorbed water. Samples DP‐200 and


32

DP‐300 were very hydrophobic and no adsorption of water could be detected. This indicates that the surface becomes increasingly hydrophobic with increasing DP of the grafted PMA arms. In Figure 23 the hydrophobicity of the PMA‐grafted filter paper is illustrated by a drop of water.

Figure 23. A drop of water on a PMA‐grafted filter paper with an aimed DP of 200. The grafted and un‐grafted filter papers were also analyzed by FT‐IR. Figure 24 shows the carbonyl absorption at 1730 cm‐1. A clear difference can be seen in the carbonyl content on papers with different DP’s. As can be seen in the curve denoted “filter paper”, that originates from virgin filter paper only subjected to solvents and MA, there are no carbonyls present in the cellulose fiber structure. Hence, this peak in the other samples originates only from the carbonyl in PMA. It is clear that the amount of PMA grafted on the surface increases almost proportionally with graft length of the bulk polymer. This indicates that the thickness of the grafted layer can be tailored by altering the monomer‐to‐sacrificial initiator ratio. Furthermore, this implies that the polymer graft‐lengths on the surface can be controlled by ATRP.

0,000

0,050

0,100

0,150

0,200

0,250

0,300

1550160016501700175018001850cm-1

Abs

orba

nce

DP-300

DP-200

DP-100

Blank

Filter paper

Figure 24. The carbonyl absorbance in FT‐IR of grafted and un‐grafted filter paper.


33

FT‐IR analysis of the blank filter paper revealed a small carbonyl peak, which is attributed to physiosorbed PMA. This implies that the washing procedure was not completely successful. Most likely, all grafted surfaces contain some physiosorbed PMA. However, the difference between the grafted samples and the blank is still evident. In order to verify that the polymerization from the surface is controlled, it is desirable to cleave off the PMA grafts and analyze them separately with SEC. This was attempted and the grafts were cleaved from the cellulose fiber with hydrolysis (KOH/MeOH). After the hydrolysis the filter paper had regain its initial hydrophilicity and quickly absorbed water. Unfortunately, the small amount of grafted PMA on the surface made work‐up procedures impossible and the polymers could not be detected after cleavage. Attempts were made to grow higher DP’s in order to increase the amount of PMA on the surface, but increasing the targeted DP(>600) broke down the cellulose structure. This could be explained by the fact that the cellulose structure is held together by hydrogen bonds. When a substantial amount of the hydroxyl groups are reacted, the intermolecular bonds weaken and this causes the cellulose structure to deteriorate at high DPs. However, Matyjaszewski et al.142 do report the detachment of poly(2‐(dimethylamino)ethyl methacrylate) from a Whatman 1 filter paper. The detachment was conducted by acidic hydrolysis. This should however, in both their case and our case, hydrolyze the ester bond in the acrylate, yielding poly(acrylic acid). In their findings they have analyzed polymer that has been polymerized from the surface employing both the method with added sacrificial initiator and the method with deactivator added to the bulk. They found that the surface confined polymer, synthesized with sacrificial initiator added to the bulk, showed better control than the one with deactivator added to the bulk. The grafted filter paper was also analyzed by ESCA. However, no bromo‐content could be detected on the surface. This has also been observed by others104 and is probably due to the ESCA method, which only measures depths of 2‐10 nm. The chains on the surface could exhibit an entangled behavior and the chain‐ends might therefore not be situated directly on the outermost layer. Another explanation is, of course, that termination reactions have taken place to a large extent on the surface, and the chain ends are “dead”. An AFM image of PMA‐grafted cellulose is shown in figure 25a. Compared to the images in Figure 22, a large difference can be seen in the structure. In Figure 25a the fibrillar structure is much less defined and the surface appears to be


34

covered. The PMA that is covering the cellulose has a low Tg which should be interpreted by AFM as a blurry surface. a) b)

Figure 25. AFM images of a) PMA‐grafted filter paper b) PMA‐block‐PHEMA‐grafted filter paper. The results of the gravimetric measurements are shown in Figure 26. Duplicate samples were analyzed for each targeted DP (100, 200 and 300). As can be seen the graft ratio increased linearly with increasing targeted molecular weight. However, at the highest targeted molecular weight there was a larger discrepancy between the two samples. This is hypothesized to be due to insufficient washing or drying of these samples.

02468

101214161820

0 50 100 150 200 250 300 350

DPaimed

G [%

]

Figure 26. Gravimetric measurements of PMA‐grafted filter paper.

4.2.3 Grafting of the PMA‐grafted filter paper with 2‐hydroxyethyl methacrylate (HEMA)

In order to verify that the chain‐ends of the grafted PMA layer were intact, a second layer of PHEMA was grafted on top of the PMA, creating a block‐


35

copolymer on the surface. The conditions for this reaction were adopted from Baker et al.143, who showed that their reaction conditions yielded a thick layer of PHEMA on the surface. In this approach, no sacrificial initiator was added, as in the case of PMA. The reaction time had to be carefully monitored, as this was the only parameter that determined the thickness of the layer. Control over the reaction was ensured by the addition of deactivator, in this case Cu(II)Cl2,. After the PMA‐grafted filter paper had been grafted with PHEMA, the paper was much stiffer than before. Its hydrophobic behavior had changed dramatically as it quickly absorbed water. This difference in hydrophobicity is shown in Figure 27, where one half of a PMA‐grafted filter paper was polymerized with PHEMA while the other half remained just PMA‐grafted. This paper was immersed in a solution of colored water. The difference in water absorption between the two sides was evident. Clearly the ATRP method can be used to tailor the hydrophilicity of cellulose.

Figure 27. Picture showing the difference in water absorption of the PMA‐grafted side and the P(MA‐b‐HEMA)‐grafted side.

The P(MA‐b‐PHEMA)‐grafted filter paper was also analyzed by AFM, Figure 25b. As can be seen, there is a large difference compared to the PMA‐grafted filter paper in Figure 25a. The fiber structure is no longer visible at all and the surface appears to be completely covered. It also seems that the grafted layer is much thicker than before. However, AFM revealed that the surface was very uneven, and images from different spots on the surface appeared to be quite different from each other.


36

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Figure 28. FT‐IR spectra of the grafted filter paper with DP=100 of the 1st PMA‐graft.

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Figure 29. FT‐IR spectra of the grafted filter paper with DP=300 of the 1st PMA‐graft. As can be seen from the FT‐IR spectra in Figures 28 and 29, the carbonyl peak is significantly increased after grafting the PMA‐grafted paper with HEMA. However, the increase is much larger for the paper with a targeted DP of 100 of the first PMA‐graft than for the paper with a targeted DP of 300, even though both papers had been subjected to the HEMA reaction solution for equal times. One reason for this could be that more side‐reactions had taken place when a higher DP was targeted for the PMA‐graft. This would result in less living chain‐ends. Another plausible explanation is that since the HEMA polymerization was performed in water, the chain‐ends might be less accessible in a thicker PMA layer, due to back‐folding. To investigate the latter hypothesis both PMA‐grafted papers (DP=100 and 300) were polymerized with a second layer of PMA. The targeted DP in both cases was 200 for the second graft. As can be seen in Figures


37

28 and 29, the peak area‐increase in this case is nearly equal for both polymerizations. This is an indication that the decrease in PHEMA‐grafting from a thicker layer of PMA originates from the water insoluble nature of the PMA layer, resulting in back‐folding of the chain‐ends.

4.2.4 Cellulose Composites One of the reasons to modify cellulose is for its use in composites. However, since cellulose in its native form is vastly hydrophilic a major reason for failing in composites has thus far been poor adhesion between the polar cellulose and the often un‐polar polymer matrix. In order to investigate if the PMA‐grafted paper had any effect on a layman composite of cellulose and a polymer, two different laminate composites were made; one with unmodified filter paper and one with PMA‐grafted filter paper. In these very preliminary investigations a matrix of epoxy and diamine was used. The composites were analysed by tensile testing. The test results were rather inconclusive but showed that the modified cellulose composite was less stiff and hence more ductile that the unmodified one. The fracture surface from the tensile test was analyzed by SEM, Figure 30. The choice of matrix was not ideal, since the epoxy ring may also react with hydroxyl groups on the cellulose. However, primary diamines react more easily with the oxirane ring. Also, the wetting of the surface should be much better in the case of the modified cellulose since the epoxy is very hydrophobic and thus creates a better bonding between matrix and cellulose. Even though the polymer matrix was poorly chosen, the effect of the PMA‐grafting was still clearly visible in the SEM images. When an unmodified filter paper was used there was a clear ‘pull out’ effect of the fiber, Figures 30a and 30b. However, when the PMA‐grafted filter paper was used instead, the ‘pull out’ was reduced, Figures 30c and 30d. This indicates that the grafted PMA had a positive effect on the interface between the cellulose and the thermoplastic matrix.


38

a) b)

c) d)

Figure 30. Composites of cellulose: a) and b) unmodified cellulose c) and d) PMA‐grafted cellulose.

4.3 Dendronized polymers Dendronized polymers are believed to have new and interesting properties. In order to obtain well‐defined dendronized polymers a controlled polymerization technique had to be utilized, in this case ATRP. It is also essential to use an efficient and versatile chemistry to obtain dendrons. In this study the pendant dendrons were based on acetonide‐protected bis‐MPA. They were obtained by employing anhydride chemistry with successive protection/deprotection steps.137,139 Dendronized polymers of generation one, two and three were obtained by both the macromonomer route and the ‘graft onto’ route by divergent growth (Figure 5). The acetonide‐group can easily be hydrolyzed, yielding highly hydroxy‐functional polymers. The macromonomers were synthesized according to Figures 15 and 16. A flexible spacer of ten carbons was incorporated into the structure to increase the availability of the polymerizable


39

group, the acrylate function. All the macromonomers were purified by MPLC prior to use.

4.3.1 Macromonomer Route ATRP of the macromonomers was performed in diphenyl ether (DPE). The concentration of PDE was kept at 50 w/w% since Schlüter et al. have shown that polymerization of macromonomers of generation three or higher is only successful if the monomer concentration is >45 w/w%.83 Two different initiators were investigated, ethyl 2‐bromoisobutyrate (EBiB) and ethyl 2‐bromopropionate (EBrP), Figure 31.

O

O

O

O

O Br

O

O Br

O

.

.

initiation

initiation

Ethyl 2-bromoisobutyrate (EBiB)

Ethyl 2-bromopropionate (EBrP)

tertiary radical

secondary radical Figure 31. Structures of the initiators and the subsequent radicals that are formed after homolytical cleavage of the H‐Br bond. To monitor the polymerizations, samples were withdrawn at time intervals throughout the reactions and analyzed by 1H NMR and SEC (in DMF using conventional calibration). The kinetics of the polymerizations were investigated by using equation (3), derived from equation (2).

tktktKk apppeqp =⋅== ]R[]Cu(II)X[

]Cu(I)X][RX[)]M[

]M[ln(

2

0 (3)

As can be seen from equation (3), there is a linear dependence of ln([M]0/[M]) on reaction time t. The slope of the line gives the apparent rate constant, kapp. The kinetics of the ATRP of dendronized polymers have to the author’s knowledge not been investigated before. At the end of each polymerization the reaction mixture was dissolved in THF, passed through a column of alumina in order to remove the copper and


40

precipitated into cold hexane. The final polymers were analyzed by 1H‐ and 13C NMR, SEC (in THF using universal calibration), MALDI‐TOF and DSC. 4.3.1.1 Polymerization of G1 macromonomer The first attempts to polymerize G1 with ATRP were conducted using EBiB as the initiator (Figure 31), N,N,N’,N’,N’’‐pentamethyldiethylenetriamine (PMDETA) as the ligand in conjunction with Cu(I)Br. EtOAc was used as the solvent. The reaction temperature was set to 70 °C. However, as can be seen in Table 5, the final polymer (denoted P(G1)‐1) had a rather high PDI from SEC using universal calibration. This polymer was also analyzed by MALDI‐TOF, Figure 32. The MALDI‐TOF gave a molecular weight of 13 000 g mol‐1 and a PDI of 1.08. However, this method is known to fractionate samples, thereby discriminating the higher molecular weight fractions, especially when the sample has a broad polydispersity, which will lower the PDI.

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Figure 32. MALDI‐TOF analysis of P(G1). To achieve better control and thereby reduce the PDI of the final polymer the ligand was changed to Me6‐TREN and the polymerization of G1 was performed at room temperature in DPE. The initiator‐to‐Cu(I) ratio was kept at 1:0.1. As can be seen in Figure 33, first‐order kinetic plots of the polymerization of G1 deviated slightly from linearity when EBiB was used as initiator. This deviation also occurred if the reaction temperature was raised (50 °C), if a small amount of deactivator was added at the beginning of the reaction (Cu(II)Br2) or if the catalyst was changed to Cu(I)Cl. However, the final polymer had low PDI, between 1.2‐1.4 (Table 5, entry P(G1)‐2 and P(G1)‐3), but the obtained molecular weights were larger than the targeted ones. This indicates that termination reactions had taken place at early stages of the reaction. However, the reaction


41

showed better control under these conditions compared the ones used to synthesize P(G1)‐1.

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] 0/[M

]

Figure 33. First order kinetic plots showing ln([M]0/[M]) vs time in the solution polymerization of G1 at RT, [M]:[I]:[Cu(I)]:[Me6‐TREN]=50:1:0.1:0.1 ( ) I=EBiB ( ) I=EBrP.

Table 5. SEC analysis of polymer P(G1)‐P(G3), the macromonomer route.

Sample Mna

[g/mol] PDI DPaimed DPcalc. Conv

[%] P(G1)-1b 27 000 1.7 50 70 - P(G1)-2b 30 240 1.4 50 79 81 P(G1)-3b 66 560 1.2 100 173 50 P(G1)-4c 25 540 1.1 50 67 81 P(G2)-1b 37 360 1.2 50 57 97 P(G2)-2c 46 670 1.1 50 71 83 P(G3)-1b 62 660 1.8 50 52 53 P(G3)-2b 31 690 1.3 50 26 45 P(G3)-3c 53 000 1.6 50 44 80

aMn obtained by SEC using universal calibration (UC) in THF. bInitiator: ethyl 2‐bromoisobutyrate (EBiB) cInitiator: ethyl 2‐bromopropinate (EBP) To further increase the control, the initiator was changed to EBrP (Figure 31). With this initiator the polymerization followed first‐order kinetics, Figure 33. The


42

difference between the two initiators efficiencies in ATRP has previously been described by Matyjaszewski et al.38 and van der Linde et al.144 When EBiB is employed as initiator the radical that is initially formed is tertiary, Figure 34. This radical is relatively stable. When one unit of acrylic macromonomer is added, the formed radical is secondary and hence more unstable and reactive, Figure 34. For that reason some chains may start to polymerize before all the chains have been initiated. Also, there might be a high concentration of radicals in the initial reaction solution, leading to termination in the beginning of the reaction. This will give the persistent radical effect. When EBrP is used as initiator instead, the radical initially formed is secondary, Figure 31, and it will be equally reactive to the one formed when one unit of macromonomer is added.

O

OO

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OO

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initiationreinitiation

M

propagationtertiary radical

secondary radical Figure 34. The initiating species in the polymerizations of acrylates using EBiB as initiator. When EBrP was utilized as initiator the polymerization proceeded to 81 % conversion and the final polymer had a low PDI value (Table 5, entry P(G1)‐4), indicating that the polymerization was controlled. The withdrawn samples were analyzed by SEC in DMF using conventional calibration against linear PEO standards. As can be seen in Figure 35, the PDI values are low throughout the reaction and the molecular weight increases linearly with conversion. However, the curve do not approach zero at zero conversion. This indicates that the initial reaction is fast. The values in Figure 35 are obtained by conventional calibration using linear PEO standards. It is a well‐known fact that this method underestimates the values of the molecular weights of branched polymers or polymers with large side‐groups.68 Therefore, the plotted values of the molecular weights in Figure 35 will deviate from the values of the final polymers in Table 5,


43

that have been analyzed in THF using universal calibration. The universal calibration gives a value of the molecular weight that is closer to the real one.145 Unfortunately, this method is sensitive to the concentration of the samples and could therefore only be employed when analyzing the worked‐up polymers. The withdrawn samples also contain solvents and monomer and the real concentration of the polymer is difficult to determine and for that reason only conventional calibration was possible for these samples.

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Figure 35. Dependence of molecular weight and molecular weight distribution on the monomer conversion for the polymerization of G1 using EBrP as initiator. 4.3.1.2 Polymerization of G2 macromonomer The G2 macromonomer has a molecular weight of 656 g mol‐1. When polymerizing the G2 macromonomer, the reaction temperature was raised to 50 °C and the initiator‐to‐Cu(I) ratio was increased to 1:0.2 for optimal results. As can be seen in Figure 36, the polymerization followed first‐order kinetics regardless of initiator. From the slope of the curves, it can be concluded that EBiB resulted in a higher polymerization rate. The final polymers had low PDI’s in both SEC (table 5, entry P(G2)‐1 and P(G2)‐2) and by MALDI‐TOF. MALDI‐TOF gave a molecular weight of 34 000 g mol‐1 and a PDI of 1.06. The results of the MALDI‐TOF analysis is shown in Figure 37.


44

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] 0/[M

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Figure 36. First‐order kinetic plots plots showing ln([M]0/[M]) vs time in the solution polymerization of G2 at 50 °C, [M]:[I]:[Cu(I)]:[Me6‐TREN]=50:1:0.2:0.2 ( ) I=EBiB ( ) I=EBrP.

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Figure 37. MALDI‐TOF analysis of P(G2). As can be seen in Figure 38, the PDI values for the polymerizations remained low throughout the reaction and decreased with increasing monomer conversion. The molecular weight increased linearly with increasing conversion.


45

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Figure 38. Dependence of molecular weight and molecular weight distribution on the monomer conversion for the polymerization of G2 using EBrP as initiator. 4.3.1.3 Polymerization of G3 macromonomer ATRP of G3 was performed similar to G2, but since this macromonomer has a molecular weight of 1200 g mol‐1, the concentration of active sites in the reaction solution is low. Due to the high molecular weight of the macromonomer itself, very small amounts of Cu(I)Br needed to be used in order to keep a 1:0.1 ratio of initiator‐to‐Cu(I) at a 1 g scale of macromonomer. Since it could be concluded from the polymerizations of G2 that a doubling of the catalyst did not result in higher polymerization rates, an even higher ratio of initiator‐to‐Cu(I) was used here. A ratio of 1:0.5 and 1:1 was investigated and it was concluded that a 1:1 ratio resulted in slightly higher conversions wherefore that ratio was the selected one in this study. The purification of the final polymer was difficult. The G3 macromonomer is not very soluble in hexane and a fractionated precipitation was necessary. Nonetheless, all the final polymers contained traces of macromonomer, as well as of DPE. When EBiB was employed as the initiator, the reaction did not follow first‐order kinetics, Figure 39. The polymerization would only proceed to 50 % conversion. Higher conversions could not be reached, even at longer reaction times (96 h) or by increasing the reaction temperature (100 °C). Nonetheless, as can be seen in Table 5, the PDI value of the final polymer was low (P(G3)‐1). It is speculated that the lower polymerization rate originates from side reactions, such as irreversible termination, at early stages of the polymerization, leading to the


46

persistent radical effect. This will accumulate Cu(II)Br2 in the system which functions as a deactivator and subsequently slows down the reaction. It is hypothesized that a slower reaction rate might enable the polymer to adopt a conformation that will inhibit further polymerization, either by shielding the polymerizable group or by micellization.146 It could also be that the decreasing concentration of monomers in the reaction solution together with a lower concentration of Cu(I) and increased concentration of Cu(II) makes propagation highly unfavorable.

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Figure 39. First‐order kinetic plots showing ln([M]0/[M]) vs time in the solution polymerization of the polymerization of G3 at 50 °C, [M]:[I]:[Cu(I)]:[I] = 50:1:1:1 ( ) I=EBiB ( ) I=EBrP. When EBrB was utilized instead, the reaction reached 80 % conversion in 24 hours. At that time the reaction mixture had become viscous and the magnetic bar could no longer stir the solution. The reaction followed first‐order kinetics, Figure 39. In this case, however, the final polymer had an increased PDI value of 1.6, Table 5 (P(G3)‐3). In the SEC‐traces, a high molecular weight shoulder was present. It is hypothesized that side‐reactions begin to take place at higher conversions. As is shown in Figure 40, the PDI value increases slightly with increasing conversion. This is unusual for ATRP, where typically most of the termination reactions take place in the beginning of the reaction, and the PDI value decreases with increasing conversion.38 In the polymerization of G3, the molecular weight increases linearly with increasing monomer conversion, Figure 40. As in the case for G1, the molecular weight does not approach zero at zero conversion. In this case, however, the molecular weight should approach 1 200 due to the high molecular weight of the macromonomer. P(G3) is a highly


47

branched polymer, wherefore the deviations in SEC using conventional calibration will be very large. For that reason it is difficult to estimate Mn vs conversion using this method. Also, it is possible that the solution behavior of this polymer is quite different from G1 and G2.

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Figure 40. Dependence of molecular weight and molecular weight distribution on the monomer conversion for the polymerization of G3. MALDI‐TOF analysis of P(G3) was not possible due to difficulties in the ionization step. ATRP of the G4 macromonomer was attempted but so far no polymer has been obtained. 4.3.1.4 Thermal analysis of P(G1), P(G2) and P(G3) The polymers of G1‐G3 were analyzed by DSC. The results are shown in Table 6. As expected, the glass transition temperature (Tg) increases with increasing generation of pendant dendrons. The results from the DSC measurements are somewhat inconclusive since P(G3) contained both residual macromonomer and solvent (DPE). This will lower the Tg.


48

Table 6. DSC analysis of P(G1)‐P(G3).

Sample Tg [°C] P(G1)-2 -43 P(G2)-1 -27 P(G3)-1 2

4.3.2 ‘Graft‐onto’ Route In parallel to the macromonomer route the ‘graft‐onto’ route was explored as a route to the same dendronized polymers. It was hypothesized that this route may lead to higher generation dendronized polymers since the problem of steric hindrance and shielding of the polymerizable group during polymerization is circumvented. In the ‘graft‐onto’ route, P(G2) was synthesized by divergent growth from P(G1). This was accomplished by hydrolysis of the acetonide‐groups in P(G1) yielding OH‐groups that were subsequently reacted with the anhydride of bis‐MPA. P(G3) was synthesized in the same way from P(G2). The deprotection and coupling of the anhydride were carefully monitored by 1H‐ and 13C NMR, as it is of outmost importance that the polymer is fully deprotected and subsequently, that all OH‐groups are coupled with bis‐MPA. In order to ensure this, a large excess of anhydride was used in the coupling step as well as long reaction times, both when deprotecting and coupling the polymer. In Table 7, the results of the SEC analysis is shown. As can be seen the original polymer, (P(G1)‐1), had rather broad PDI, since the reaction conditions used in the ATRP of this polymer were not optimized. However, it is peculiar that the PDI values increases with increasing generations. This is probably attributed to incomplete coupling when utilizing this method. Additionally, it could also be due to the dendronized polymers behavior in the solvent or its interaction with the columns in SEC.


49

Table 7. SEC analysis of dendronized polymers, the ‘graft‐onto’ route

Sample Mn calc [g/mol]

Mn* [g/mol]

PDI

Acetonide-P(G1)-1 19 400 27 000 1.7 OH-P(G1) 17 400 20 800 1.72 Acetonide-P(G2) 33 000 54 900 1.5 OH-P(G2) 29 000 19 200 2.0 Acetonide-P(G3) 60 200 84 000 2.0 *Mn was obtained using universal calibration in THF.

In Figure 41 the SEC‐traces of the dendronized polymers by the ‘graft‐onto’ route are shown. As can be seen, the curves are monomodal and do not have any high or low molecular weight shoulders.

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Figure 41. SEC chromatogram () P(G1) (×) P(G2) (∆) P(G3) MALDI‐TOF analysis of these polymers was not possible due to difficulties in the ionization step. A G4 polymer was synthesized from P(G3). However, unwanted hydrolysis of the acetonide‐group was observed during the NMR analysis, since long acquisition times were required. Therefore no reliable data has been acquired for this polymer.

5. Conclusions

50

5 CONCLUSIONS Atom Transfer Radical Polymerization (ATRP) has successfully been used in order to obtain new, complex macromolecular architectures. A dendritic‐linear hybrid structure was obtained by reacting poly(3‐ethyl‐3‐(hydroxymethyl)oxetane) with 2‐bromoisobutyryl bromide and using the resulting macromolecule as a macroinitiator in ATRP of MA. The resulting polymers had low PDI and the length of the linear arms could be controlled. Conversions as high as 65 % were reached, while maintaining control over the multifunctional system. A graft copolymer between cellulose and surface confined PMA has been synthesized. A bromo‐ester modified cellulose filter paper, Whatman 1, was grafted with PMA, utilizing ATRP. FT‐IR, contact angle measurements, gravimetric measurements and AFM clearly showed that the cellulose fibers could be modified this way and that the amount of PMA grafted onto the cellulose could be controlled. A block‐copolymer of MA and HEMA was also grafted onto cellulose. This showed that the chain‐ends from the PMA‐grafting were intact and could be used for further grafting. The second block of PHEMA dramatically changed the hydrophobic behavior of the PMA‐grafted filter paper and the paper quickly absorbed water. Dendronized polymers of generation one, two and three were synthesized from acrylic macromonomers based on bis‐MPA. In the macromonomer route, macromonomers of each generation were polymerized by ATRP. The polymerizations resulted in polymers with low PDI values. The kinetics of the reactions were investigated, and the polymerizations followed first‐order kinetics when ethyl 2‐bromopropionate was used as initiator. In the ‘graft‐onto’ route, dendrons were divergently attached to a dendronized polymer of generation one, that had been obtained by ATRP.

6. Future work

51

6 FUTURE WORK Modification of cellulose with controlled/”living” radical polymerization techniques is an interesting field that might yield new and improved applications for cellulose. However, the reaction conditions and work‐up procedures need to be improved. In order to investigate the polymerization from the surface, and conclude that it is controlled, it is necessary to detach the polymer from the surface and analyze it separately. The versatility of the surface modification will be further explored by the use of various monomers such as styrene, acryl amide and various dendritic monomers. Ideally, the surface properties of the fibers can be tailored by the appropriate choice of monomers. It would also be interesting to investigate other cellulose substrates. The use of these fibers in composites should be investigated further. The grafted fibers should be analyzed with other methods. For example, it could be interesting to investigate the barrier properties. The importance of a spacer in the ATRP of dendronized macromonomers should be explored. This could be conducted by varying the spacer length. The dendronized polymers need to be characterized further. The conformation of the polymers in solution, as well as the rheological properties of the polymers, need to be investigated. Also, other controlled radical polymerization techniques should be investigated for dendronized macromolecules, especially reverse‐addition fragmentation chain‐transfer (RAFT).

7. Acknowledgements

52

7 ACKNOWLEDGEMENTS First and foremost I would like to thank the person without whom this thesis never would have been: my outstanding supervisor Ass. Prof. Eva Malmström. Eva, thank you for your constant support, your great ideas and your enthusiasm throughout the years. You are such a wonderful person and it has been an honour to be your Ph D student. I would also like to thank the other two bosses of “Ytbehandlingsgruppen”; Prof Anders Hult, for always being in the way, and Ass. Prof. Mats Johansson for being the one person who can speak nothing but French at parties. Head of the department Prof. Ann‐Christine Albertsson, Prof. Sigbritt Karlsson, Prof. Bengt Stenberg and Prof. Ulf Gedde are thanked for all their hard work over the years. All technical and administrative staff are thanked, especially the ones I have harassed the most; Maggan, Victoria, Ove and Inger. The Swedish Research Council is gratefully acknowledged for financial support. To my “Ytbehandlingsgrupp”, the one I started in and the one I now finish in: thank you for being a constant source of amazement and joy to me. I am grateful to have met each and every one of you! Thanks to Hasse for being the most natural man I have ever met, Peter for being the strangest man I have ever met, Johan and Daniel for all silly pranks, Linda S for making me drink too much tequila, Ci for being the groups mam, Geraldine for being a great house‐mate, Robert “the Bob” for making me like him so much, Andreas N for being such a cute and helpful “lab‐tomte”, Linda F for her laughter that came just in time for me, Josefina for being so like me at times, Emma for her sweet personality, Hanna for making Peter do the dishes in the lab, Daniel N for being the other member of the all‐girl badminton team and Magnus for the biggest lunch portions I have ever seen.

7. Acknowledgements

53

I would also like to thank all the diploma students I have been privileged to work with, especially Robert “lill‐Bob”, for brightening my days during these last, stressful six months. All other PhD students at the department are thanked for all their help throughout the years‐thank you!! I would like to thank all my friends, especially: Erica “Irri”, Katha “Spökis”, Katties, Fille‐Fjonk, Challe‐balle, Johan “Le jaune pulle”, Mickan, Lena, Fredrik P, Jocke and Nicholas. You mean the world to me! To one of my closest friends Fredrik “Fredan” von Kieseritzky, for creating the world as I know it. What would I ever do without you? To the Forsberg and Nyh families for letting me feel as part of their family. I would especially like to thank Nils “Lill‐Bobbo” Forsberg for bearing to be around me so much. To my American family, the Cranstons. For all their love and support and for taking care of me through some of the darkest times of my life. To my family: Favoritmorbror Björn, moster Karin, moster Agneta, kusin Ulrika, kusin‐man Per, kusin‐barn Joel, kusin Jonas, mormor, morfar, farmor and farfar. To my little brother Johan for his outstanding personality, charm and wonderful sense of humor. To my mother Barbro for her strength, positivism, kindness and love. To my father Karl for being my greatest inspiration (I will miss you for the rest of my life). I am so lucky to have been raised in a family such as ours! I love each and every one of you! And finally to Per, my “putabanana”, for being brave enough to marry me. Your love is what gets me through the day. Thank you for always making me so happy! Lavv jo!!!!!

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