[ACS Symposium Series] Controlled/Living Radical Polymerization Volume 944 (From Synthesis to Materials) || Borane-Mediated Control Radical Polymerization: Synthesis of Chain End Functionalized

Chapter 27

Borane-Mediated Control Radical Polymerization: Synthesis of Chain End Functionalized

Fluoropolymers

T. C. Chung, H. Hong, Z. C. Zhang, and Z. M. Wang

Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802

This paper summarizes our experimental results in a new family of borane-based control radical initiators and their unique application in the synthesis of fluoropolymers containing one or more terminal functional groups. The chemistry is advantaged by its simplicity of borane initiator and mild reaction condition, and applicable to a broad range of fluoromonomers, including VDF, f-acrylic, etc. Two type borane initiators, including cycloborane and functional borane, will be discussed to illustrate the functionalization scheme. The control radical polymerization is characterized by predictable molecular weight, narrow molecular weight distribution, formation of diblock copolymer, and tolerance to many functional groups that usually cause chain transfer reactions in regular free radical polymerization. In turn, the chain end functionalized fluoropolymers exhibit very high surface activities in the polymer/inorganic composites. For example, the polymer can exfoliate clay interlayer structure in PVDF/clay nanocomposite, and maintaining the disorder state even after further mixing with neat (unfunctionalized) polymer.

© 2006 American Chemical Society 387

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In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

388

Fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), poly(vinylidine fluoride) (PVDF), poly(vinylidine-co-hexafluoropropene) (VDF/HFP elastomer), etc., exhibit an unique combination of properties, including thermal stability, chemical inertness (acid and oxidation resistance), low water and solvent absorptivities, self-extinguishing, excellent weatherability, very interesting surface properties, and becoming important electric and electronic properties. They are commonly used in many high-end applications, such as aerospace, automotive, textile finishing, and microelectronics (/). However, fluoropolymers also have some drawbacks, including limited processibility, poor adhesion to substrates, limited crosslinking chemistry, and inertness to chemical modification, which limit their applications when interactive and reactive properties are paramount. Functionalization of fluoropolymers, having specific functional groups, have been a constant research interest in the past decades. Most of research approaches have been focusing on copolymerization of fluorinated monomers with functional comonomers to form functional fluoro-copolymers containing pendent functional groups (2-<5). Few reports discussed the preparation of fluoropolymers containing terminal functional groups (7-70). Recently, Saint-Loup et al. (77) prepared telechelic VDF/HFP elastomers containing two opposing hydroxy terminal groups by using hydrogen peroxide as an initiator. However, many side reactions occur in this polymerization, and the final product contains not only hydroxy terminal groups but also carboxylic acid terminal groups, as well as some unsaturated terminal groups.

Living radical polymerization provides a very useful method to prepare a wide range of polymers with well-defined molecular structures; i.e. narrow molecular weight distribution, control molecular weight, and desirable polymer chain ends. Early attempts to realize living radical polymerization involved the concept of reversible termination of the growing polymer chains by iniferters (72,75), such as Ν,Ν-diethyldithiocarbamate derivatives, with some success. The first living radical polymerization was observed in reactions involving a stable nitroxyl radical, such as TEMPO (14, 75), which does not react with monomers but forms a reversible end-capped propagating chain end. Usually, the reactions have to be carried out at an elevated temperature (>100 °C) to obtain a sufficient concentration of propagating radicals for monomer insertion. Subsequently, several research groups have replaced the stable nitroxyl radical with transition metal species or reversible chain transfer agents as the capping agents to mediate living free radical systems. These polymerization reactions follow the mechanisms of atom transfer radical polymerization (ATRP) (16,17) or reversible addition-fragmentation chain transfer (RAFT) (75), respectively. Overall, these systems have a central theme-reversible termination via equilibrium between the active and dormant chain ends at an elevated temperature.

In our group, we have been studying a new free radical initiation system based on the oxidation adducts of organoborane and oxygen, which contains boroxyl radical - a mirror-image of the stable nitroxyl radicals - as shown in

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Scheme 1. Our early interest in the borane/oxygen radical initiator stemmed from the desire to develop a new effective route in the funetionalization of polyolefins (19-22) (i.e. ΡΕ, PP) and block/graft copolymers (23-26), which has been a longstanding scientific challenge area with great potential for industrial applications.

electron withdrawing electron donating

Scheme 1

The unexpected good control in the incorporation of borane groups to polyolefin by metallocene catalysis and the subsequent radical chain extension by the incorporated borane groups prompted us to examine this free radical polymerization mechanism in greater details. Several relatively stable borane-based radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion (27) and producing block copolymers by sequential monomer addition (28). This stable radical initiator system was recently extended to the polymerization of fluorinated monomers, which can effectively occur in bulk and solution conditions. Some interesting ferroelectric fluoro-terpolymers (29), showing large electromechanical response, have been prepared with high molecular weight and controlled polymer structure with narrow molecular weight and composition distributions. In this paper, we will focus on the application of this stable borane initiator technology to prepare fluoropolymers having one or two terminal functional groups, which includes the published results in fluorinated acrylate (30, 31) and new observation in VDF-based polymers.

Experimental

Synthesis of 8-boraindane Initiator

Under Ar atmosphere at 0° C, 21.6 g (0.2 mol) of 1,3,7-octatriene in 50 ml of THF solution was added dropwise with 200 ml (1.0 M) of borane THF complex in THF solution. After the addition was complete, stirring continued for 1 hour at 0° C. Then the mixture was refluxed for 1 hour before THF was removed completely under vacuum at room temperature. The attained white

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solid was heated to 210°C for 3 hours then 9.6 g of 9-bora-indane (yield: 41%) was distilled from the mixture at about 50 °C to 60 °C (0.3 mmHg). The spectra data were as follows: ^ - N M R (25° C in CDC13) δ .08-1.6 ppm (m); n B - N M R (25°C in CDC13) δ 91.14 ppm (s); 1 3 C-NMR (25°C in CDCI3) δ 21.9 ppm (b, CH 2-B), δ 25.6 ppm (s, CH 2), δ 26.3 ppm (s, CH 2), δ 27.4 ppm (b, CH 2-B), δ 28.4 ppm (s, CH 2), δ 31.6 ppm (s, CH 2), δ 34.4 ppm (s, CH 2), δ 42.4 ppm (b, CH-B).

Synthesis of [(C2H50)3SiCH2CH2]3B Functional Initiator

In a 500 ml flame-dried flask equipped with a magnetic stir bar, 250 ml of dry THF and 35 g (180 mmol) of vinyltriethoxylsilane was injected under argon. After cooling the solution to 0° C, 60 ml of B H 3 in THF (1.0 M) was added. The mixture was stirred at 0° C for 4 hours and then was warmed to ambient temperature for 1 hour to assure complete hydroboration reaction. After solvent-removal, the product was subjected to vacuum distillation at 170° C to obtain 23.4 g of tri-(triethoxylethylsilyl)borane product. *H NMR spectrum indicates the hydroboration reaction involving mainly anti-markovnikov addition (>90%). The spectra data were as follows: n B - N M R (25°C in CDC13) δ 81.52 ppm (s); *H-NMR (25° C in CDC13) δ 0.52-0.58 ppm (b, CH2-Si), δ 1.14 ppm (s, CH 3), δ 1.44 ppm (b, CH-B). δ 3.72 ppm (s, CH r O) .

Synthesis of Telechelic Poly(trifluoroethyl acrylate) with Two Terminal OH Groups Using 8-Bora-indane/02 initiator

In a 150 ml flask, 40 ml of THF, 6 ml (50 mmol) of 2,,2',2'-trifluoroethyl acrylate (TFEA) monomer, and 70 mg (0.6 mmol) of 8-bora-indane were introduced under argon. After injecting 5 ml of 0 2 , the solution was mixed for about 5 minutes. The solution was then kept at room temperature for various times before exposing the solution to air that stops the reaction. The solution was then poured into 200 ml of well stirred methanol to quench the polymerization and precipitate PTFEA polymer. To assure complete oxidation of all borane moieties, the isolated polymer was then re-dissolved in 20 ml THF before adding 0.2 ml (6N) NaOH solution, followed by dropwise 0.4 ml, 33% H 2 0 2 at 0° C. The resulting mixture was stirred for 1 hour to complete the oxidation. After cooling to room temperature, the solution was purred into 200 ml of well stirred methanol. The precipitated telechelic poly(trifluoroethyl acrylate) was collected, washed, and dried in vacuum at 60° for 2 days, then was characterized by Gel Permeation Chromatography (GPC) and lH and 1 3 C NMR-DEPT measurements.

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Synthesis of poly(methyl methacrylate-b-trifluoroethyl acrylate) diblock copolymer Using 8-Bora-indane/02 initiator

In a flame-dried 50 ml flask, 5.0 ml (50 mmol) of M M A and 70 mg (0.6 mmol) of 8-bora-indane were mixed under argon. To this mixture 5.0 ml of 0 2

(0.2 mmol) was injected, following by vigorous shaking to assure complete mixing. The system was then kept at room temperature for 20 min, followed by removal of all the volatiles by vacuum distillation. About 5.0 ml of 2',2',2'-trifluoroethyl acrylate (TFEA) was subsequently injected into the system. The mixture was shaken vigorously to dissolve the solid as soon as possible. After complete dissolution, the solution was kept at room temperature for 1 hour before adding 10 ml of acetone to reduce the viscosity and then opening the system to air to oxidize all the borane moieties. The solution was then poured into 200 ml of well stirred methanol. The precipitated telechelic diblock polymer was collected, washed, and dried in vacuum at 60°C for 2 days. The resulting telechelic poly(methyl methacrylate-b-trifluoroethyl acrylate) diblock copolymer was characterized by GPC and *H and 1 3 C NMR-DEPT measurements.

Synthesis of PVDF Polymers with A Terminal (C 2H sO) 3Si Group

In a typical reaction, 2.9 g of [(C 2H 50) 3SiCH 2CH 2] 3B (5 mmol) was dissolved in 100 ml of CH2C12 in a dry box, the reactor was then connected to a vacuum line, and 25.6 g of VDF (400 mmol) was condensed into a autoclave reactor under vacuum by liquid nitrogen. VDF has a vapor pressure of ~ 40 atm at 25 °C. About 2.5 mmol 0 2 was charged into the reactor to oxidize borane moiety and initiate the polymerization that was carried out at ambient temperature for 4 hours. After releasing the pressure, the mixture was transferred into a flask containing 100 ml of hexane. After stirring for 30 min, the polymer powder was filtered, washed, and then dried under vacuum at 60°C for 6 hours. About 21 g of polymer was obtained with yield of 82 %. The resulting polymer was characterized by intrinsic viscosity (Mv) and *H NMR measurements.

Preparation of PVDF/CIay Nanocomposite.

In a typical example, a PVDF-t-Si polymer (Tm = 170° C, M v = 30,000 g/mol) was mixed with Na+-mmt clay. Static melt intercalation was employed by heating the mixture at 190° C for 3 hr under nitrogen condition. The resulting PVDF-t-Si/Na+-mmt nanocomposite shows a featureless XRD pattern, indicating the formation of an exfoliated clay structure. The resulting binary PVDF-t-Si/Na+-mmt exfoliated nanocomposite was further melting mixing (50/50 weight

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ratio) with commercial neat PVDF (Mn= 70,000 and Mw = 180,000 g/mol). The resulting ternary PVDF/ PVDF-t-Si/Na+-mmt nanocomposite also shows a featureless XRD pattern.

Results and Discussion

In this paper, we discuss two borane initiators, including cycloborane (I) and silane containing borane (II) shown in Scheme 2, which can introduce one or more functional groups in the beginning of perfluorinated polymers, such as poly(vinylidene fluoride) (PVDF) and fluorinated acrylic polymers.

X Β - / x ̂

B-CH 2-CH 2-Si-(OR) 3

d) (Π) Scheme 2

Chain end functionalized acrylic fluoropolymers (30, 31)

Equation 1 illustrates the preparation of chain end functionalized acrylic fluoropolymer by using the 8-boraindane initiator (I). The cyclic B-C bond in 5-member ring of 8-boraindane (I) may be preferly oxidized under a controlled oxidation condition to form a peroxide compound (C-0-0-BR 2) (A) that initiates control radical polymerization of 2,2,2-trifluoroethylacrylate (TFEA) monomers monomers at ambient temperature. The C-0-0-BR 2 species is decomposed to an alkoxyl radical (C-O*) and a borinate radical (*0-BR2) in the presence of fluoro-monomers. The alkoxyl radical is active in initiating the polymerization of TFEA. On the other hand, the borinate radical is too stable to initiate polymerization due to the back-donating of electrons to the empty p-orbital of boron. However, this "stable" borinate radical may form a reversible bond with the radical at the growing chain end to form dormant species and prolong the lifetime of the propagating radical. In the whole polymerization process, the mono-oxidized bicycloborane residue remains bonded to the beginning of the polymer chain (B), despite the continuous growth of the polymer chain. After terminating the control radical polymerization, the two unreacted cyclic B-C bonds in the borane residue can be completely interconverted to functional groups, such as two OH groups by NaOH/H 20 2 reagent. The resulting poly(2,2,2-trifluoroethylacrylate) (C) contains two OH groups located at the beginning of polymer chain.

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Table 1 summarizes several comparative polymerization runs in the preparation of telechelic PTFEA polymers. The polymer molecular weight is linearly increased with the monomer conversion, and polymers maintain relatively narrow molecular weight distribution throughtout the polymerization process, implying a "stable" propagation without significant termination and chain transfer reactions. This chemistry is applicable to many acrylate and methacrylate monomers, including fluorinated and unfluorinated ones and their mixtures (30, 31). Apparently, a constant number of active sites are formed after the oxidation reaction, which maintained reactivity throughout the polymerization process (30). This controlled radical polymerization was also evidenced by end group analysis and diblock copolymers, such as poly(methyl methacrylate-b-trifluoroethyl acrylate), by sequential monomer addition (discussed later).

Table 1. A summary of TFEA polymerization8 by 8-bora-indane/02 in THF

Run Time Conversion Mn" Mw" PD1 (hr) (%) (g/mole) (g/mole) (Mw/Mn)

1 2.0 12 7,000 14,000 2.0 2 4.0 19 12,000 23,500 2.0 3 6.0 40 25,000 49,000 1.9 4 8.0 52 30,500 52,500 1.7 5 10 60 33,000 56,000 1.6

a. Reaction temperature: 25° C. b. Molecular weight determined by GPC.

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ΊΓ ΊΓ

IT

OP* 200

Figure 1. (top) 1H and (bottom) 13C NMR-DEPT135 spectra of PTFEA prepared by 8-bora-indane/02 in benzene at 0 °C. (Reproduced with permission

from Macromolecules 2004, 37, 6260-6263. Copyright 2004 American Chemical Society.)

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Figure 1 shows ! H and 1 3 C NMR-DEPT 135 spectra of the resulting telechelic poly(trifluoroethyl acrylate) having two terminal OH groups (run 1 in Table 1). In *H NMR spectrum, there are two weak peaks at 3.7-4.0 ppm, indicating the existence of two type OH groups, and some expected chemical shifts between 1.7-2.8 ppm for C H 2 and CH groups and a strong peak at 4.6 ppm for 0-CH 2 group in the PTFEA chain. To provide direct evidence for the existence of both primary and secondary OH groups, the telechelic polymer was also examined by 1 3 C NMR (DEPT-135). In addition to three expected chemical shifts at 25.4-35.6 (negative), 41.1 (positive), and 60.1 (negative) ppm, corresponding to the CH 2 , CH, and O-C H 2 groups, respectively, in the PTFEA backbone, there are two distinctive chemical shifts - one negative peak at 68.2 ppm, corresponding to the primary C H 2 -OH group, and one positive peak at 77.6 ppm, corresponding to the secondary CH-OH group. For quantitative end group analysis, both OH groups in the telechelic PTFEA polymer were completely converted to the corresponding silane derivative by reacting with Cl-Si(CH3)3 reagent. A new chemical shift at 0.15 ppm corresponding to ~0-Si(CH 3) 3 is clearly observed with a reasonable intensity for qualitative analysis. The peak intensity ratio between two peaks (4.6 and 0.15 ppm) and the representing protons indicate about two OH groups per PTFEA chain.

τ ι 1 r 20 30 40 50

Elution Time (minutes)

Figure 2. GPC curve comparison between (a) PMMA and (b) PMMA-b-PTFEA diblock Copolymer Prepared byUsing 8~Bora-indane/02 initiator. (Reproduced with permission from Macromolecules 2004, 37, 6260-6263. Copyright 2004

American Chemical Society.)

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It is also possible to extend the functional borane-mediated control radical polymerization to block copolymers by means of sequential monomer addition. After completing the polymerization of a first monomer to form a first polymer "block", a second monomer is introduced into the reaction mass to polymerize the second monomer to form a second polymer "block". After terminating the living polymerization, the partially oxidized borane residue located at the beginning of polymer chain can be completely interconverted to two reactive functional groups. Figure 2 compares GPC curves of a telechelic poly(methyl methacrylate-b-trifluoroethylacrylate) diblock copolymer (graph b) and the corresponding poly(methyl methacrylate) homopolymer (graph a). The molecular weight almost doubles from the homopolymer (Mn= 12,400 and Mw= 24,000 g/mol) to the diblock copolymer (Mn= 32,800 and Mw= 58,000 g/mol) without a broadening in the molecular weight distribution.

(b)

(a)

" ~ Γ " ~ Ι r — — ι ι I ι I · ι < ι p p m 10 8 6 4 2 0

Figure 3. !HNMR spectra between (a) PMMA and (b) PMMA-b-PTFEA diblock Copolymer Prepared by 8-Bora-indane/Oi initiator. (Reproduced with

permission from Macromolecules 2004, 31, 6260-6263. Copyright 2004 American Chemical Society.)

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Figure 3 compares the same two polymers, poly(methyl methacrylate-b-trifluoroethylacrylate) diblock copolymer (top) and the corresponding poly(methyl methacrylate) homopolymer (bottom). The incorporation of second polymer block is clearly evidenced with strong new peaks for PTFEA. Basically, the copolymer composition is controlled by the monomer feed ratio and reaction time. The combined experimental results strongly indicate 8-boraindane as a clear and effective control radical initiator for both fluoro- and non-fluoro acrylic monomers. This bicyclic initiator provides the route to prepare not only well-defined homo- and block copolymers but also telechelic polymers having two OH groups at the same polymer chain end.

Chain end functionalized PVDF

The borane based radical initiators are also active in polymerization of olefinic fluoro monomers (52), such as VDF (vinylidene fluoride), TrFE (trifluoroethylene), HFP (hexafluoropropene), and their mixtures, at ambient temperature. During the propagating reaction, a coordination intermediate (III) may be formed due to the B-F acid-base complex between the active site and the incoming monomer, as illustrated in Scheme 3. Such an interaction may enhance the reactivity of fluoromonomer and minimize side reactions, and therefore produce functional fluoro-copolymers with the controlled molecular structures. As the alkoxyl radical intiatae the polymerization, the "dormant" borinate radical may form a reversible bond with the radical at the growing chain end to prolong the lifetime of the propagating radical and reduce chain transfer reactions.

By using functional borane initiators, containing some specific functional groups that are stable during the borane oxidation and polymerization process, it's possible to prepare olefinic fluoropolymers containing terminal functional group(s). Equation 2 illustrates an example, in which the preparation of silane terminated poly(vinylidene fluoride) [PVDF-t-Si(OR)3] by using a silane containing borane initiator.

( I l l ) "Coordination

Radical Addition Process"

Scheme 3

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CH2=CF2

χ /

CH2=CH-Si-(OR)3

H-BX2/THF

02 Xs. B-0-0-CH2-CH2-Si-(OR)3 -* ^B-CH 2.CH 2-Si-(OR) 3

(Α') (Π)

-PVDF Si-(0R)3 (B1)

Equation 2

The functional borane initiator (II), containing a silane group, was quantitatively prepare by simple hydroboration reaction of vinylsilane (commercial available) with a borane compound containing at least one B-H group at ambient temperature. The subsequent mono-oxidation reaction of functional borane initiator (II) with a control quantity of oxygen is spontaneously occurred at room temperature to form the corresponding peroxylborane (Α') for initiating polymerization. This oxidation reaction can be carried out in situ during the control radical polymerization with the presence of monomers. The resulting fluoropolymers (Β') has a terminal silane group at the beginning of polymer chain. Table 2 summarizes the experimental conditions and results of VDF polymerization reactions initiated by [(C 2H 50) 3SiCH 2CH 2] 3B/0 2 system, which results in PVDF-t-Si(OC2H5)3 (silane-terminated PVDF) polymers. The monomer conversion under a constant reaction time (4 hours) is basically controlled by the active sites governed by borane and oxygen concentrations.

Table 2. Summary of PVDF Polymers Containing A Terminal SilaneGroup Prepared by [(CîHsOkSiCHîCHîbB/Oi Initiator

Run Β o2 VDF Yield

(mmol) (mmol) (mmol) (%) 4 5.0 1.0 400 32 5 5.0 2.5 400 82 6 5.0 5.0 400 73 7 10.0 5.0 400 86

Figure 4 shows *H NMR spectrum of a PVDF-t-Si(OC2H5)3 polymer (Mv= 10,600 g/mole), with two insets of the expanded regions exhibiting chain end and side chain structures. Two major chemical shifts at δ= 2.3 ppm (weak) and δ= 2.9 ppm (strong) corresponds to C H 2 units with head-to-head and head-to-tail monomer sequence in the PVDF backbone, respectively. In the expanded

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regions, there are two dominate chemical shifts at 1.15 ppm (CH3) and 3.75 ppm (OCH2), almost identical with those in [(C2H50)3SiCH2CH2]3B initiator, which represent the existance of terminal (C 2H 50) 3Si silane group in PVDF polymer chain end. In addition, the peak intensity ratio between chain end and main chain indicates about one silane per polymer chain, using polymer molecular weight estimated from intrinsic viscosity and Mark-Houwink equation. Recently, some solubility studies by immobilizing PVDF-t-Si(OC2H5)3 polymer chains onto a silicate substrate also indicate that most of polymer chains contain silane group. It's interesting to note that the weak chemical shift at 6.3 ppm (CF2H) is associated with the conventional intra-chain transfer process (33).

Jlu-

ppm 8 6 4 2 0

Figure 4. lHNMR spectrum ofPVDF-t-Si(OC2H5)3polymer (Mv= 10,600 g/mole), with two insets of the expanded regions.

One major advantage of the fluoropolymer having terminal functional group(s) in one end of polymer chain is that they exhibit very high surface activity on the silicate clay surfaces to exfoliate clay interlayer structure, even using the pristine clay material (without treatment with organic surfactants or acids). Figure 5 illustrates the interaction pattern between chain end fluoropolymer and the clay interlayer surfaces (34). The terminal functional group can anchor fluoropolymer chain to the clay surfaces between interlayers by chemical bond (such as Si-O-Si or Si-O-C bond). On the other hand, the rest unperturbed high molecular weight hydrophoblic and oleophobic fluoropolymer

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chain, disliking the hydrophilic clay surfaces, exfoliates the clay layer structure and maintains this disorder clay structure even after further mixing with neat (unfunctionalized) polymer that is compatible with the backbone of the chain end functionalized fluoropolymer.

• Residue after the reaction between clay surface and fluoropolymer terminal functional group, such as Si(R)n(OH)3.n

Si(R)n(OR)3.n, OH,NH2, COOH, anhydride, ammonium, immidazolium, sulfonium, phosphoniu.

Figure 5. Illustration of an exfoliated fluoropolymer/clay nanocomposite using chain end functionalizedfluoropolymer as the interfacial agent

Figure 6 shows X-ray diffraction study of a typical example. A PVDF-t-Si polymer containing a terminal C 2H 5OSi(CH 3) 2 group (Tm = 170 °C, M v = 30,000 g/mol) was mixed with Na+-mmt clay, which has an ion-exchange capacity of ca. 90 mequiv/100g (WM). Static melt intercalation was employed by firstly mixing and grounding PVDF-t-Si dried powder and Na+-mmt with 90/10 weight ratio in a mortar and pestle at ambient temperature. The XRD pattern (shown in Figure 6,a) of this simple mixture shows a (001) peak at 20 ~ 7, corresponding to Na+-mmt interlayer structure with a d-spacing of 1.45 nm. The mixed powder was then heated at 190° C for 3 hr under nitrogen condition. The resulting PVDF-t-Si/Na+-mmt nanocomposite shows a featureless XRD pattern (shown in Figure 6,b), indicating the formation of an exfoliated clay structure. The resulting binary PVDF-t-Si/Na+-mmt exfoliated nanocomposite was further melting mixing (50/50 weight ratio) with commercial neat PVDF (Mn= 70,000 and Mw = 180,000 g/mol). Firstly the PVDF-t-Si / Na+-mmt exfoliated nanocomposite and neat PVDF with 50/50 weight ratio were ground

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together in a mortar and pestle at ambient temperature. The mixed powder was then heated at 200° C for 3 hr under nitrogen condition. The resulting ternary PVDF/ PVDF-t-Si/Na+-mmt nanocomposite also shows a featureless XRD pattern (shown in Figure 6,c), indicating that the stable exfoliated structure in the binary PVDF-t-Si/Na+-mmt exfoliated nanocomposite is clearly maintained after further mixing with PVDF that is compatible with the backbone of PVDF-t-Si.

2Θ

Figure 6. X-ray diffraction patterns of (a) physical mixture of PVDF-t-Si and Na+-mmt (90/10 wt%), (b) the same mixture after static melt-intercalation, and

(c) the 50/50 mixture by weight of exfoliated PVDF-t-Si/Ν a*-mmt structure (from b) and neat PVDF.

Conclusion

We have developed two type borane-based control radical initiators, including cycloborane and functional borane, for preparing fluoropolymers containing one or more terminal functional groups. The chemistry is advantaged by its simplicity of borane initiator and mild reaction condition, and applicable to both olefinic and acrylic fluoromonomers. The control radical polymerization is characterized by predictable molecular weight, narrow molecular weight

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Acknowledgements

The authors would like to thank the Office of Naval Research and Daikin Corporation for the financial support.

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[ACS Symposium Series] Controlled/Living Radical Polymerization Volume 944 (From Synthesis to Materials) || Borane-Mediated Control Radical Polymerization: Synthesis of Chain End Functionalized