Borane-Mediated Control Radical Polymerization - American

Chapter 27

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

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch027

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

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

387


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 fluorocopolymers containing pendent functional groups (2-100 °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


389 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 long standing 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 boranebased 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 VDFbased 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


390

solid was heated to 210°C for 3 hours then 9.6 g of 9-bora-indane (yield: 41%) was distilledfromthe mixture at about 50 °C to 60 °C (0.3 mmHg). The spectra data were as follows: ^ - N M R (25° C in CDC1 ) δ .08-1.6 ppm (m); B - N M R (25°C in CDC1 ) δ 91.14 ppm (s); C-NMR (25°C in C D C I 3 ) δ 21.9 ppm (b, CH -B), δ 25.6 ppm (s, CH ), δ 26.3 ppm (s, CH ), δ 27.4 ppm (b, CH -B), δ 28.4 ppm (s, CH ), δ 31.6 ppm (s, CH ), δ 34.4 ppm (s, CH ), δ 42.4 ppm (b, CH-B). n

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Synthesis of [(C H 0) SiCH CH ] B Functional Initiator Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch027

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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 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 solventremoval, 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: B - N M R (25°C in CDC1 ) δ 81.52 ppm (s); *H-NMR (25° C in CDC1 ) δ 0.52-0.58 ppm (b, CH -Si), δ 1.14 ppm (s, CH ), δ 1.44 ppm (b, CH-B). δ 3.72 ppm (s, C H O ) . 3

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Synthesis of Telechelic Poly(trifluoroethyl acrylate) with Two Terminal OH Groups Using 8-Bora-indane/0 initiator 2

,

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 , 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 0 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 H and C NMRDEPT measurements. 2

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391 Synthesis of poly(methyl methacrylate-b-trifluoroethyl acrylate) diblock copolymer Using 8-Bora-indane/0 initiator 2

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 (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 C NMR-DEPT measurements.


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

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In a typical reaction, 2.9 g of [(C H 0) SiCH CH ] B (5 mmol) was dissolved in 100 ml of CH C1 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 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 (M ) and *H NMR measurements. 2

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Preparation of PVDF/CIay Nanocomposite. In a typical example, a PVDF-t-Si polymer (T = 170° C, M = 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-tSi/Na -mmt exfoliated nanocomposite was further melting mixing (50/50 weight m

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392 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 - /

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

2

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d)

(Π) Scheme 2

Chain end functionalized acrylicfluoropolymers(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 5member ring of 8-boraindane (I) may be preferly oxidized under a controlled oxidation condition to form a peroxide compound (C-0-0-BR ) (A) that initiates control radical polymerization of 2,2,2-trifluoroethylacrylate (TFEA) monomers monomers at ambient temperature. The C-0-0-BR species is decomposed to an alkoxyl radical (C-O*) and a borinate radical (*0-BR ) in the presence of fluoromonomers. 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 0 reagent. The resulting poly(2,2,2-trifluoroethylacrylate) (C) contains two OH groups located at the beginning of polymer chain. 2

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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, includingfluorinatedand 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).

8

Table 1. A summary of TFEA polymerization by 8-bora-indane/0 in THF 2

Run 1 2 3 4 5

Time (hr) 2.0 4.0 6.0 8.0 10

Conversion Mn" (g/mole) (%) 12 19 40 52 60

7,000 12,000 25,000 30,500 33,000

Mw" PD1 (g/mole) (Mw/Mn) 14,000 23,500 49,000 52,500 56,000

2.0 2.0 1.9 1.7 1.6

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



394

IT OP* 200

ΊΓ

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Figure 1. (top) 1H and (bottom) C NMR-DEPT135 spectra ofPTFEA prepared by 8-bora-indane/0 in benzene at 0 °C. (Reproduced with permissi from Macromolecules 2004, 37, 6260-6263. Copyright 2004 American Chemical Society.) 2


395 !

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Figure 1 shows H and C NMR-DEPT 135 spectra of the resulting telechelic poly(trifluoroethyl acrylate) having two terminal OH groups (run 1 in Table 1). In *H N M R 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 1.72.8 ppm for C H and C H groups and a strong peak at 4.6 ppm for 0 - C H 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 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 C H , CH, and OC H 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 OH group, and one positive peak at 77.6 ppm, corresponding to the secondary CHOH 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(CH ) reagent. A new chemical shift at 0.15 ppm corresponding to ~0-Si(CH ) 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. 2

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30 40 Elution Time (minutes)

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Figure 2. GPC curve comparison between (a) PMMA and (b) PMMA-b-PTFEA diblock Copolymer Prepared byUsing 8~Bora-indane/0 initiator. (Reproduced with permissionfromMacromolecules 2004, 37, 6260-6263. Copyright 2004 American Chemical Society.) 2



396 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-btrifluoroethylacrylate) 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.

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Borane-Mediated Control Radical Polymerization - American

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