Complex Macromolecular Architectures (Synthesis, Characterization, and Self-Assembly) || Complex Macromolecular Chimeras

15

Complex MacromolecularChimeras

Hermis Iatrou, Marinos Pitsikalis, Georgios Sakellariouand Nikos HadjichristidisDepartment of Chemistry, University of Athens, Panepistimiopolis, Zografou, 15771

Athens, Greece

15.1 Introduction

The synthetic polypeptides, although far from being strictly monodisperse or constructed from

a precise sequence and composition of a-amino acid residues, possess the ability, as their

natural relative – proteins, to form a-helix and b-sheet motifs. In order to approach the

perfection found in nature, these synthetic polypeptides must possess the lowest possible

molecular weight and compositional dispersities. Thus, their self-assembly into precisely

defined nanostructures, a requirement for appropriate functionality, will be favored. Such

nanostructures can then serve in the construction of nanomaterials and devices for biomedical

and pharmaceutical applications (Duncan, 2003).

For many years, synthetic homo/copolypeptides have been obtained by the ring-opening

polymerization (ROP) of a-amino acid N-carboxyanhydrides (NCAs) (Leuchs, 1906),

(Leuchs and Manasse, 1907), (Leuchs and Geiger, 1908), (Szwarc, 1965), (Sekiguchi, 1981)

generally using primary amines (e.g. n-hexylamine) as initiators, under an inert atmosphere.

However, attempts to synthesize well-defined living polypeptides (Scheme 15.1) with amino

initiators have been plagued, for more than 50 years, by unwanted polymerization mecha-

nisms and termination reactions (Kricheldorf, 2006). This problem has been overcome by

replacing the amino-initiators with organonickel (Deming, 1997) or ammonium chloride

(Dimitrov and Schlaad, 2003) and very recently by hexamethyldisilazane (Lu and Cheng,

2007) initiators.

Complex Macromolecular Architectures: Synthesis, Characterization, and Self-Assembly, First Edition.

Edited by Nikos Hadjichristidis, Akira Hirao, Yasuyuki Tezuka and Filip Du Prez.

© 2011 John Wiley & Sons (Asia) Pte Ltd. Published 2011 by John Wiley & Sons (Asia) Pte Ltd. ISBN: 978-0-470-82513-6

Alternatively, high-vacuum techniques (HVT) have been employed to create living con-

ditions for the amino-initiated ROP of NCAs. Apparently with this method, the unwanted

polymerization mechanism (activated monomer) is either avoided or is insignificant, thus

favoring the desired normal amine mechanism (Aliferis et al., 2004).

All these living systems lead to well-defined homo/copolypeptides with high molecular

weight and structural homogeneity (Aliferis et al., 2005; Papadopoulos et al., 2005; Hanski

et al., 2006; Iatrou et al., 2007). Furthermore, these systems can be employed for the synthesis

of polymer/polypeptide hybrid block copolymers, molecular chimeras (Schlaad and

Antonietti, 2003), a term borrowed from the Greek mythology. The Chimera (in Greek

Xı́maira) was an awesome fire-breathing she-monster (Homer, C8th BC) usually represented

as an hybrid of a lion, goat and serpent (Figure 15.1). Biosciences started using the term

chimera long ago to describe an organism composed of tissues that are genetically different.

Polymer–polypeptide hybrid block copolymers are able to self-assemble either in bulk

(microphase separation) or in selective solvents (micellization), like conventional block

copolymers, and in addition are organized within the microphases into a-helix or b-sheet

Scheme 15.1

Figure 15.1 The Chimera, a fire-breathing she-monster of the Greek mythology (Homer, Iliad) usually

represented as a hybrid of a lion, goat, and serpent.

462 Complex Macromolecular Architectures

motifs (Klok and Lecommandoux, 2006) thus leading to a rich variety of nanostructures

(Hadjichristidis et al., 2003).

The variety of the nanostructures could become even richer andmore complex inmultiblock

copolymers, multiblock multicomponent (e.g. terpolymers) or in nonlinear (e.g. miktoarm

stars) (Lodge, 2003) chimeras, approaching that found in nature. Therefore, the synthesis of

well defined complex chimeras adds another dimension to the suprastructural hierarchy of the

block copolymers with potential biomedical applications (Deming, 2007).

Although, there are many reviews on linear diblock chimeras, only a very few have

addressed the synthesis of polypeptide-based materials with different macromolecular archi-

tectures (Hadjichristidis et al., 2009; Deming, 2006; Deming, 2000). In this chapter our

discussion on the synthesis of well-defined complex chimeras, including linear multiblock co/

terpolymer, as well as nonlinear copolymer chimeras (e.g. star, graft, block-graft, dendritic),

will hopefully enrich the current literature on macromolecular chimeras. This chapter will not

address the self-assembly of these complex chimeras since this issue is discussed in another

chapter of this volume.

15.2 Linear Multiblock Chimeras

Research on the synthesis of polypeptide hybrid diblock copolymers began in the mid-1970s

(Yamashita et al., 1975). Advances in polymer synthesis, culminating in the development of

controlled/living polymerization techniques and click chemistry (Sumerlin and Vogt, 2010), in

combination with the living ROP of NCAs, allowed for the preparation not only of simple

diblock but also of a wide variety of multiblock Chimeras. In this section we will describe the

materials synthesized according to the macroinitiators used for the ROP of the NCAs: amino-

and transition-metal complexes.

15.2.1 Primary Amine Macroinitiators

o-Functionalized polymers carrying amines on both ends can be used asmacroinitiators for the

ROP of NCAs. This remains the most widely applied method for the synthesis of a plethora of

linear multicomponent hybrid block co/terpolymers. Themajor advantage of thismethod is the

fact that several living/controlled polymerization techniques are able to prepare a variety of

well-defined polymers with a high degree of amine functionalization.When combinedwith the

recently developed ROP of NCAs under high vacuum, these methods lead to well-defined

poplypeptide hybrids.

Most of these hybrid copolymers involve the use of telechelic poly(ethylene oxide), PEO,

carrying amine end groups. This interest can be attributed to the fact that the amino-PEOs are

commercially available in a variety of molecular weights with narrow molecular-weight

distributions, are water soluble and biocompatible. Furthermore, these PEO polypeptide

hybrids can form amphiphilic, double-hydrophilic and rod–coil–rod block copolymers,

leading to materials with very interesting properties in solution and in bulk.

a,o-Diamino-PEOswere employed as difunctionalmacroinitiators for the synthesis ofABA

triblock copolymers, where A the polypeptide and B is PEO. Using this difunctional initiator,

PBLG-b-PEO-b-PBLG (BLG: g-benzyl-L-glutamate)(Cho, Kim and Komoto, 1990; Floudas

et al., 2003), PZLL-b-PEO-b-PZLL (ZLL: e-carbobenzyloxy L-lysine) (Cho et al., 1994;

Complex Macromolecular Chimeras 463

Yang et al., 2005) and poly[(DL-Val)-co-(DL-Leu)]-b-PEO-b-poly[(DL-Val)-co-(DL-Leu)]

(Cho et al., 2003) have been prepared. Earlier studies resulted in poorly defined products, since

diblocks and homopolymers were present in the final reaction products.

The better purification of the reagents (NCA, initiator, solvent) was found to offer a higher

level of structural control. Thus, using a PEO-NH2macroinitiator and employing high-vacuum

techniques, the synthesis of the PEO-b-PZLL-b-PBLG triblock terpolymers was achieved

(Karatzas et al., 2009) by sequential polymerization. Size-exclusion chromatography (SEC)

analysis revealed that the macroinitiator was quantitatively consumed and that the final

products displayed monomodal peaks of relatively narrow molecular-weight distribution. In

addition, the molecular weights were very close to the stoichiometric values, indicating that

when using the high-vacuum techniques the polymerization was free of termination reactions.

Hybrid block copolymers with aliphatic polyesters, such as poly(e-caprolactone), PCL and

polylactide, PLA, represent a very interesting class of polymeric materials. Due to the low

immunogenicity, biocompatibility, biodegradability and excellent mechanical properties of

aliphatic polyesters, these hybrids have found applications in tissue engineering and drug

delivery (Langer, 2000; Tomson et al., 1995;Gombotz andPetit, 1995).Moreover, the presence

of the polyesters modifies the stability of the hybrids, since it reduces the enzymatic

degradation of the polypeptides. Finally, the formation of crystalline polyester domains greatly

influences the self-assembly behavior of the hybrid structures.

The general synthetic scheme for the polyester hybrids starts with the synthesis of the

diamino polyester (A) macroinitiators, followed by the ROP of the desired NCA to give the

corresponding BAB hybrids (B are the polypeptide chains).

The seven-membered cyclic initiator 2,2-dibutyl-2-stanna-1,3-dioxepane was used by

Kricheldorf et al. (1998) to produce macrocyclic PCL (Scheme 15.2). Subsequent reaction

of the two OHs with 4-nitrobenzoyl chloride yielded the corresponding a,o-dinitrobenzoylPCL, which was converted to the a,o-diamine PCL by catalytic hydrogenation. These

polymers were then employed as macroinitiators for the polymerization of the Gly-, Ala-,

Phe- andBLG-NCAs to afford the respective triblock chimeras (Kricheldorf andHauser, 2001).

The low nucleophilicity of the 4-aminobenzoyl end-groups prevented the aminolytic cleavage

of the PCL chains during synthesis and storage. The sampleswere characterized by viscometry,

IR and NMR spectroscopy. The yields of the NCA polymerization were high but not

quantitative (between 60 and 80%). The poor solubility of the polypeptides in organic solvents

rendered impossible the determination of the molecular-weight distribution of the copolymers

by SEC. However, there was evidence (NMR) that during the BLG-NCA polymerization, a

mixture of di- and triblocks was obtained, due to the low initiation rate compared to that of

propagation for this specific monomer.

Hybrid terpolymers have also been prepared by Deng et al. (2005) using poly(ethylene

oxide)-b-poly(L-lactic acid) (PEG-b-PLLA) macroinitiators (Scheme 15.3). PEG-b-PLLA-

OH was prepared by the ROP of LLA using monomethoxy PEG-OH in the presence

of Sn(Oct)2. The end-hydroxyl group was transformed to amino-group by reaction with

tert-butoxycarbonyl-L-phenylalanine in the presence of dicyclohexylcarbodiimide, DCC. The

deprotection of the amino group was performed under anhydrous conditions with trifluor-

oacetic acid at 0 �C, affording the PEG-b-PLLA-NH2 macroinitiator, which was further used

for the ROP of BLG-NCA leading to the synthesis of the triblock terpolymer PEG-b-PLA-b-

PBLG. Characterization of the products revealed that the macroinitiators were efficiently

prepared andwere quantitatively consumed during the polymerization reactions. SEC analysis


and NMR spectroscopy confirmed that rather well-defined products of relatively narrow

molecular-weight distribution were prepared.

The procedure developed by Chen et al. was adopted by Xia, Whittaker et al. for the

synthesis of PEG-b-PLLA-b-PLL triblock terpolymers (Peng et al., 2009), as shown in

Scheme 15.4. The amine end groups of the PEG-b-PLA-NH2 block copolymers served as

macroinitiators for the polymerization of ZLL-NCA leading to the preparation of the desired

terpolymers. The samples were characterized only by NMR and IR spectroscopy.

Scheme 15.2


Linear pentablock terpolymers PBLL-b-PBLG-b-PS-b-PBLG-b-PBLL (BLL: tert-butylox-

ycarbonyl-L-lysine, PS: polystyrene) were recently synthesized by Karatzas et al.

(2008) using a,o-diamino-PS. This polymer was prepared by anionic polymerization of

styrene with sodium/naphthalene (difunctional initiator) and termination with 1-(3-bromo-

propyl)-2,2,5,5-tetramethyl-aza-2,5-disilacyclopentane, and of deprotection by successive

Scheme 15.3


precipitations of the polymer in methanol. The a,o-diamino-PS was then used for the

sequential polymerization of BLG-NCA and BLL-NCA under high vacuum (Scheme 15.5).

The molecular weights were in very close agreement with the stoichiometric values and the

molecular-weight distributions very narrow, indicating that well-defined samples were pre-

pared with this methodology.

Step-growth polymerization is another technique leading to telechelic amine macro-

initiators. The Yamamoto coupling polymerization of 2,7-dibromo-9,9-dihexylfluorene

was performed by Kong et al. (2004) followed by the end-capping reaction with N-(p-

bromobenzyl)phthalimide. Deprotection with hydrazine afforded the telechelic amine

macroinitiators, which were subsequently used for the polymerization of BLG-NCA.

Due to the nature of the step-growth polymerization the molecular-weight distribution,

of the triblock copolymers was relatively broad (Mw/Mn values higher than 2.0)

(Scheme 15.6).

An amine-functionalized TEMPO radical was employed by Karatzas et al. (2009) for

the synthesis of poly(N-vinyl pyrrolidone), PNVP, copolymers with PBLG and PZLL

blocks, PNVP-b-PBLG-b-PZLL (Scheme 15.7). The polypeptide blocks were initially

prepared using the amino group as initiator of the ROP of NCAs. The TEMPO

moiety did not interfere with the ROP and polypeptides with very narrow molecular-weight

Scheme 15.4


distributions were obtained. The final step involved the nitroxide-mediated polymerization

of NVP leading to chimeras with relatively low polydispersity (1.16–1.22) and controlled

molecular weights (22.6–96.5� 103 gmol�1).

15.2.2 Transition-Metal Complex Macroinitiators

Zero-valent metal complexes cannot be used directly for the synthesis of polypeptide hybrid

block copolymers. However, Na-allyloxycarbonyl-amino acid allyl amides can be used as

universal precursors for the amido-amidate nickelacycle initiators (Scheme 15.8). As shown in

Scheme 15.8, the Na-allyloxycarbonyl-amino acid derivatives may undergo tandem oxidative

additions to nickel(0) giving the nickelacycle initiators. Thismethodwas initially employed for

the synthesis of block copolypeptides and was then expanded by Deming and coworkers to a

variety of hybrid structures.

This methodology was applied for the synthesis of PBLG-b-polyoctenamer-b-PBLG and

PBLG-b-polyethylene-b-PBLG triblock chimeras (Brzezinska and Deming, 2001)

(Scheme 15.9). At the beginning, acyclic diene metathesis, ADMET, polymerization was

employed for the synthesis of a,o-diamino-functionalized polyoctenamer using Grubbs’

catalyst RuCl2-(¼CHPh)(PCy3)2. This product was converted to the bisallyloxycarbonyl-L-

leucine -terminated polyoctenamer, followed by reaction with [1,2-bis(diethylphosphino)

ethane]Ni(COD), depeNi(COD), to produce the active sites for the ROP of BLG-NCA.

After the NCA polymerization, the Wilkinson catalyst was employed to hydrogenate the

polyoctenamer block, affording the corresponding polyethylene block. This methodology

was also applied for the synthesis of triblock copolymers where the middle block was

either poly(ethylene glycol), PEG, or poly(dimethyl siloxane), PDMS (Brzezinska

et al., 2002).

Scheme 15.5


It is well known that amido-metallacycle end groups are the active species for the synthesis

of polypeptides by transition-metal complexes. Therefore, electrophilic reagents, such as

isocyanates, can react with these end groups through formation of stable urea linkages. Based

on this concept, excess isocyanate end-capped PEG was reacted with living PBLG to produce

PBLG-b-PEG (Brzezinska et al., 2002) (Scheme 15.10). SEC and NMR measurements

revealed that the coupling reaction was near quantitative. This work was further expanded

to the synthesis of CABAC pentablock terpolymers (C: PEG, A: PBLG and B: polyoctenamer,

PEG or PDMS). The synthesis was achieved through the coupling reaction of the living ABA

triblock copolymers with isocyanate end-capped PEG. Repeated precipitation from THF

solutions into methanol was employed for the purification of the pentablock chimeras. SEC

Scheme 15.6


analysis confirmed that monomodal traces of relatively narrow molecular-weight distribution

were obtained.

15.3 Nonlinear Chimeras

Controlled/living polymerization techniques in combination with living ROP of NCAs can

lead not only to linear but also to nonlinear chimeras. In this section we will describe the

materials synthesized according to their structure: star, comb, brush-block, and dendritic-like

chimeras.

15.3.1 Star Chimeras

Star polymers are branched polymers consisting of several linear chains emanating from a

central core. Among the general synthetic routes (Mishra andKobayashi, 1999; Hadjichristidis

etal., 2001;2006;2007) for starpolymers, theuseofmultifunctional initiatorshaspreferentially

been used for the synthesis of polypeptide-based stars.

Scheme 15.7

Scheme 15.8


This method is referred to as the “core-first” or “arm-out” or divergent approach. According

to this procedure, multifunctional compounds capable of simultaneously initiating the

polymerization of several arms are used. There are several requirements a multifunctional

initiator has to fulfill in order to produce star polymers with controllable molecular weights,

uniform arm lengths, and lowmolecular-weight distribution.All initiating sitesmust be equally

Scheme 15.9


reactive and the initiation rate must be higher than the propagation rate. The characterization of

the star polymers produced by this method is difficult, since the molecular weight of the arms

cannot be measured directly. The number of arms can be defined indirectly by several methods,

such as end-group analysis, determination of the branching parameters, which are the ratios of

either the mean square radius of gyration, or intrinsic viscosity or hydrodynamic radius of the

star to the corresponding linearwith the samemolecularweight. Finally, the determinationof the

functionality can be achieved by isolation of the arms after cleavage (i.e. hydrolysis), if possible,

and subsequent analysis. This approach is the most widely used for the synthesis of polypeptide

stars. Using this methodology and a combination of different polymerization techniques several

polypeptide-based star hybrid polymers have been prepared.

o-Amino 4-arm PEO stars were efficiently used by Karatzas et al. (2009) for the ROP of

BLG-NCA leading to (PEO-b-PBLG)4 star-block copolymers (Scheme 15.11). The multi-

functional macroinitiator was quantitatively consumed and thus giving a product of

relatively narrow molecular-weight distribution and molecular weight very close to the

stoichiometric value.

A combination of ATRP and ROP was reported by Abraham et al. (2006) for the synthesis of

(PS-b-PBLG)3 3-arm star-block chimeras. ATRP techniques were initially adopted for the

synthesis of 3-arm PS star using a suitable cynurate-based initiator. The end-bromine groups

were transformed to amines by a three-step azidation route followed by conversion to nickel-

amine complex macroinitiators suitable for the polymerization of BLG-NCA (Scheme 15.12).

The functionality of the precursor PS star was confirmed by the measurement of the molecular

weights of the stars and the corresponding arms. The armswere isolated by the hydrolysis of the

ester groups at the core. The final products were obtained in moderate yields and possessed

relatively narrow molecular-weight distributions (1.1<Mw/Mn< 1.4).

The polyamidoamine with four terminal amine groups (G0-PAMAM) was employed by

Dong et al. as tetrafunctional initiator for the polymerization of BLG-NCA leading to the four-

arm (PBLG)4 stars (Qiu et al., 2009). The terminal amine groupswere further functionalized by

reaction with 2-bromo-2-methylpropionyl bromide to introduce initiation sites for ATRP.

Scheme 15.11

Scheme 15.10


These sites were then employed for the polymerization of D-gluconamidoethyl methacrylate

providing the corresponding star-block copolymers (Scheme 15.13). The reaction sequence

was monitored by NMR and IR spectroscopy and SEC revealing that rather well-defined

structures having relatively low polydispersities (1.18<Mw/Mn< 1.45) were obtained.

Miktoarm star polymers, consisting of chemically different arms, including polypeptide

chains have also been synthesized. A combination of ATRP and ROP were employed by

Babin et al. (2005; 2008) for the synthesis of PS(PBLG)2 miktoarm star chimeras

(Scheme 15.14). Styrene was first polymerized by ATRP, followed by reaction with a

Scheme 15.12


large excess of 1-aminotriethylenetriamine for the nucleophilic substitution of the end-

bromine group, leading to PS chains bearing two amine end groups. Due to the nonquantitative

nature of this reaction, the unfunctionalized PS chains were removed by selective precipitation

in hexane after treatment of the PS(NH2)2 with aqueous HCl. The o-amine groups were

subsequently used for the polymerization of BLG-NCA, to afford the desired structures.

Hydrolysis with an excess of hydrogen bromide in trifluoroacetic acid led to the cleavage of the

benzyloxycarbonyl protective groups to afford the corresponding amphiphilic miktoarm star

chimeras. SEC analysis revealed that the PS(NH2)2 macroinitiators were quantitatively

consumed and that the molecular-weight distributions of the final products were fairly narrow

(1.09<Mw/Mn< 1.22).

(PLLA)2PBLG 3-miktoarm stars were prepared by Sun et al. (2009) using the trifunctional

initiator 2-benzyloxycarbonylamino-1,3-propanediol (Scheme 15.15). The two available hy-

droxyl groups were employed for the polymerization of L-lactide in the presence of Sn(Oct)2.

Subsequent cleavage of the benzyloxycarbonyl protective group, with hydrogen bromide in

acetic acid, afforded PLLA chains with a central amine group, (PLLA)2NH2, which served as

macroinitiator for the polymerization ofBLG-NCA togive the desiredmiktoarm.NMRanalysis

revealed that the (PLLA)2NH2 macroinitiator was efficiently prepared. SEC experiments

confirmed the complete consumption of themacroinitiator and the absence of any homopolymer

trace. However, the molecular-weight distributions of the final products were not measured due

to the self-assembly of the miktoarm star copolymers in the solvent used for SEC.

A commercially available PEO bearing two central- and two end-amine groups was

employed by Cho et al. (2000) as a macroinitiator for the polymerization of BLG-NCA,

leading to the synthesis (PEO-b-PBLG)2(PBLG)2 4-miktoarm star copolymers,

(Scheme 15.16). The molecular weights were determined by NMR analysis. However,

details concerning the efficiency of this synthetic approach and the characterization of the

products were not provided.

Taking advantage of the unique features of anionic polymerization to produce end- or in-

chain amino-functionalized polymers, as well as of the living nature of the ROP of NCAs under

Scheme 15.13


high-vacuum conditions, a variety of well-defined miktoarm star hybrids was synthesized

and fully characterized by Karatzas et al. (2008). These structures include: (PS)2(PBLG),

(PS)2(PBLL), (PS)(PI)(PBLG), (PS)(PI)(PBLL), (PS)2[P(a-MeS)](PBLG), (PS)2[P(a-MeS)]

(PBLL), (PS)2(PBLG)2 and (PS)2(PBLL)2 (Scheme 15.17). The synthetic strategy involved the

preparation of diphenyl ethylene, DPE, functionalized polymers [DPE-chain-end-functiona-

lized PI (PI-D), DPE-in-chain-functionalized polystyrene (PS-D-PS) and DPE-in-chain-

difunctionalized polystyrene], shown in Scheme 15.18. These structures were subsequently

activated by reaction with a living polymer chain or s-BuLi followed by reaction with 1-(3-

bromopropyl)-2,2,5,5-tetramethyl-aza-2,5-disilacyclopentane. The silyl-protected group was

cleaved by treatment with HCL or p-toluenesulfonic acid, leading to the synthesis of the

corresponding amine-functionalized polymers. These products were employed as macroini-

tiators for the polymerization of BLG- and BLL-NCAs to afford the desired miktoarm stars.

Combined characterization by NMR, SEC and light-scattering measurements confirmed the

Scheme 15.14


efficiency of this synthetic scheme and the homogeneity of prepared products. The only

restriction of this procedure was the steric hindrance of the in-chain functionalized macro-

initiators. This became obvious from the fact that (PS)2(NH2)2 macroinitiators led tomiktoarm

stars with broader molecular-weight distributions, whereas the more sterically hindered

(PS)4(NH2)2 macroinitiators afforded multimodal distributions.

15.3.2 Comb, Brush-Block, Dendritic-Like Chimeras

The limitations in achieving true living polymerization conditions of NCAs have, in the past,

rendered difficult the synthesis ofmore complex structures containing polypeptide chains, such

Scheme 15.15

Scheme 15.16


as comb-shaped chimeras. Three general methods have been developed for the synthesis

of randomly branched graft copolymers: (a) the “grafting-onto”, (b) the “grafting-from”

and (c) the macromonomer method (or “grafting-through” method) (Pitsikalis et al., 1998;

Hadjichristidis et al., 2001; 2003b; 2006).

In the “grafting-from” method active sites are generated randomly along the backbone.

These sites are capable of initiating the polymerization of a second monomer, leading to the

synthesis of graft copolymers. This method is widely used for the synthesis of graft hybrids,

especially with polypeptide side chains. Amine groups were generated along the backbone and

then served as initiation sites for theROPofNCAs.An early example includes the conventional

radical copolymerization of N-benzylacrylamide with N-(3-aminopropyl)methacrylamide

hydrochloride. The copolymer obtained was then dehydrochlorinated to afford free primary

amine groups, subsequently functioning as initiation sites for the polymerization of Ala-NCA

(Boulahia et al., 1989). (Scheme 15.19) In another study, N-methyl-N-(4-vinylphenethyl)

ethylenediamine was copolymerized with 2-hydroxyethyl methacrylate. The side amine

groups were subsequently used to copolymerize the NCAs of b-benzyl L-aspartate and

b-(4-phenylazobenzyl) L-aspartate to give the graft copolymer chimera (Aoyama

et al., 1990a; 1990b (Scheme 15.20) These structures were not characterized thoroughly in

terms of the molecular and structural characteristics.

The “grafting-from” approach was also employed in more recent studies. N-tert-butox-

ycarbonyl-N0-(2-methacryloyl)-1,3-diaminopropane was prepared by Zhang et al. (2005)

and polymerized by free-radical polymerization. The quantitative deprotection of the

amine group was performed in a mixture of dichloromethane and trifluoroacetic acid,

Scheme 15.17


followed by oligomerization of the BLG- or ZLL-NCAs affording the corresponding molecu-

lar brushes (Scheme 15.21). The samples were characterized by NMR and AFM techniques.

A similar methodology was applied by Xiang et al. (2009) for the synthesis of chitosan

grafted with poly(L-tryptophan) chains (Scheme 15.22). Deacetylated chitosan (deacetylation

degree of amino groups 75%) was employed as a multifunctional macroinitiator for the

polymerization of the L-tryptophan-NCA in ethyl acetate solutions leading to the synthesis of

chitosan-g-poly(L-tryptophan) graft copolymers. The products were analyzed by NMR and

IR spectroscopy.

Using the grafting-from methodology amphiphilic block-brush copolymers were prepared

by Cai et al. (2009). Amine-terminated PEO was employed as macroinitiator for the

polymerization of BLG-NCA leading to the synthesis of PEO-b-PBLG block copolymers.

Subsequent aminolysis of the benzyl ester groups of the PBLG block using 1,2-diaminoethane

led to the synthesis of the corresponding PEO-b-poly[(2-aminoethyl)-L-glutamate)], PEO-b-P

(ELG-NH2), copolymers. The amine moieties of the P(ELG-NH2) blocks served as initiators

for the polymerization of BLG-NCA leading to the synthesis of the desired block-

brush copolymer, PEO-b-[PELG-g-PBLG] (Scheme 15.23). The reaction sequence was

monitored by SEC. It was found that the molecular-weight distribution of the initial PEO-

b-PBLG block copolymer (Mw/Mn¼ 1.2) was substantially broadened after the grafting from

Scheme 15.18


procedure (1.6<Mw/Mn< 2.0). The molecular weights of the samples were obtained by

NMR spectroscopy.

The thirdmethod for the synthesis of graft copolymers is based onmacromonomers,which is

an oligomeric or polymeric chain bearing a polymerizable end group.Macromonomers having

two polymerizable end groups have also been reported. Copolymerization of preformed

macromonomers with another monomer yields graft copolymers. In an early report,

m,p-vinylbenzylamine was used as initiator for the oligomerization of DL-phenylalanine-

NCA, leading to narrow molecular-weight distribution macromonomers of low molecular

weight (Scheme 15.24). Thesemacromonomers were then copolymerizedwithMMAand S, to

give PMMA-g-PPhe and PS-g-PPhe graft copolymers, respectively (Takaki et al., 1987).

Polystyrenes dendronized with L-Lysine dendrons of different generations were prepared by

the procedure given in Scheme 15.25 (Lubbert et al., 2005). 4-Vinylbenzylamine was linked

with the appropriate protected L-Lysine dendrons under standard peptide coupling conditions,

affording the corresponding dendronized macromonomers. The macromonomers were then

subjected to conventional radical polymerization to give productswith broadmolecular-weight

distributions.Monomer conversion and polymermolecular weight were found to increasewith

increasing dendron size.

A polyamidoamine dendron (PAMAM) carrying a Boc-protected amine group was

also synthesized by Harada et al. (2006; 2009). The protective group was then removed

Scheme 15.19


and the primary amine group was employed as initiator for the polymerization of ZLL-NCA

leading to linear-dendritic block copolymer (Scheme 15.26).

Linear/dendron-like polypeptide hybrid structures were prepared by Dong et al. by

combination of ROP and click chemistry (Hua et al., 2009). The synthesis of linear PCL/

dendron-like PBLG, PCL-b-(Dm-PBLG), hybrid copolymers having 2m (m¼ 0,1,2,3) PBLG

branches is described in Scheme 15.27. Benzyl alcoholwas used as initiator and Sn(Oct)2 as the

catalyst for the ROP of e-CL. The end-hydroxyl group was then quantitatively tosylated and

was further transformed into azide-terminated PCL, PCL-N3. Propargyl focal point PAMAM-

typed dendrons with 2m (m¼ 0,1,2,3) primary amine groups, Dm, were synthesized and then

click conjugated with the PCL-N3 to produce amine-terminated PCL dendrons. The available

amine groups were employed as initiation sites for the polymerization of BLG-NCA leading to

the synthesis of the desired structure. SEC analysis revealed that the products had moderate

polydispersities (1.13<Mw/Mn< 1.36). Detailed characterization by SEC, MALDI, NMR

spectroscopy, DSC and WAXD verified the synthesis of the target structures.

Using similarmethodology the synthesis of asymmetric PEO-b-(Dm-PBLG) and symmetric

(Dm-PBLG)-b-PEO-b-(Dm-PBLG) hybrid copolymers having 2m (m¼ 0,1,2,3) PBLG

Scheme 15.20


Scheme 15.21

Scheme 15.22


Scheme 15.23

Scheme 15.24


branches was reported by Peng et al. (2009) (Scheme 15.28). Both the “arm-first” and

“core-first” strategies were employed for the synthesis of the desired products. According

to the “arm-first” approach, the propargyl focal point dendrons, Dm having 2m (m¼ 0,1,2,3)

primary amine groups were first used for the ROP of BLG-NCA, leading to the formation of

“clickable” dendron-like PBLGpolymers having 2m branches. These structureswere then click

conjugated with azide-terminated PEO at the one or both chain ends (PEO-N3, N3-PEO-N3)

producing the asymmetric and the symmetric block hybrids, respectively. Alternatively,

according to the “core-first” approach the propargyl focal point dendrons were first click

conjugated with PEO-N3 or N3-PEO-N3 generating primary amine-terminated PEO dendrons,

which were subsequently used for the ROP of BLG-NCA leading to the synthesis of the target

structures. SEC analysis, NMR and FT-IR spectroscopy revealed that the products were rather

well defined and had relatively narrow molecular-weight distributions.

Hyperbranched amphiphilic multiarm polyethyleneimine-PBLG copolymers were

prepared by Tian et al. (2006) The synthetic approach involved the ROP of BLG-NCA,

Scheme 15.25


Scheme 15.26


Scheme 15.27

Scheme 15.28


initiated by a hyperbranched polyethyleneimine (PEI) macroinitiator. No data concerning

the polydispersity indices of the PEI precursors or the final copolymers were provided. In

this work, the micellar properties in aqueous solutions of the multiarm copolymers

synthesized were examined.

15.4 Concluding Remarks

It is clear that the new developments in living/controlled polymerizations, as well as in the

living ROP of NCAs, hold tremendous promise for the synthesis of well-defined complex

chimerasmaterials with controllablemolecular weight, sequence, composition andmolecular-

weight distribution. Although much development is still required, these complex polypeptide-

based materials begin to rival their natural counterparts in terms of complexity and accuracy.

Such well-defined materials can self-assemble into precisely defined nanostructures, a

requirement for the development of new biomedical and pharmaceutical tools with a wide

range of tunable properties. Furthermore, studies on these novel chimeraswill shed light on the

natural phenomena associated with protein function and functionality.

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Complex Macromolecular Architectures (Synthesis, Characterization, and Self-Assembly) || Complex Macromolecular Chimeras