Chapter 25

Controlled Radical Polymerization Methods for the Synthesis of Nonionic Surfactants for CO2

D E . Betts, T. Johnson, D. LeRoux, and J . M . DeSimone1

Department of Chemistry, C B 3290, Venable and Kenan Laboratories, University of North Carolina at Chapel Hill, Chapel Hill, N C 27599-3290

Several examples of fluorocarbon containing amphiphilic block copolymers have been synthesized using controlled free radical polymerization methods. These block copolymers contain a C O 2 soluble fluorocarbon acrylate or methacryate block and a CO2 insoluble block that is either lipophilic or hydrophilic. The synthesis of these block copolymers is wel l suited to the controlled free radical polymerizat ion methods of iniferter and atom transfer radical polymerization ( A T R P ) . Addi t iona l ly , the synthesis of block copolymers containing a poly(dimethylsiloxane) (PDMS) block using a combined l iving anionic and iniferter synthetic technique is described. These block copolymers have been characterized by G P C , 1H-NMR, and S A N S , and their solubility in CO2 has been examined. Potential uses for these block copolymers include surfactants for dispersion polymerizat ions in C O 2 and cleaning and surface treatment technologies.

Only a few classes of polymers show appreciable solubility in supercritical carbon dioxide under relatively "mild" conditions (T < 100 °C, Ρ < 5000 psi): amorphous or low melting fluoropolymers and silicones (7-5). These CO2 soluble materials have been termed "C02 -ph i l i c " because they have high solubility at relatively low temperatures and pressures (6). Conventional hydrocarbon polymers, either hydrophilic or lipophilic, are relatively insoluble in CO2 (7) and are therefore termed "C02 -phob ic . " However, some block copolymers containing a C 0 2 - p h o b i c hydrocarbon segment and a C02-phil ic fluorocarbon segment are soluble in CO2 (2,3). These amphiphilic block copolymer materials are currently being studied for their use as surfactants in the dispersion polymerization of several different classes of monomers in CO2 (8,9). In addition, they are also being used in cleaning and surface treatment technologies that take advantage of the enhanced emulsification capabilities afforded by surfactant modified C O 2 (10). Micel les may form when block copolymers are dissolved in a solvent that is selectively good for one of the blocks but is a nonsolvent for the other block (11-13). For A B diblock copolymers dissolved in CO2, where the A block is a C02 -phi l ic block and the Β block is C02-phobic, we have shown that the

'Corresponding author

418 © 1998 American Chemical Society

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Β blocks aggregate to form the core of a micelle which is surrounded by a corona of solvated A blocks. The C02-phobic block may be either lipophilic or hydrophilic. With in the C02 -phobic core of the micelle, the solubilization of otherwise CO2 insoluble materials is achieved (70).

Controlled free radical polymerization techniques provide the best route for the preparation of these amphiphilic block copolymers due to the relative ease of synthesis and the limited mechanisms by which the fluorocarbon monomers can be polymerized. In addition, since most fluoropolymers are insoluble in common organic solvents, the compatibility of controlled free radical polymerization techniques with a wide range of solvents makes these techniques the most versatile methods available (14,15). Specifically, iniferter (16) and atom transfer radical polymerization (ATRP) (17,18) are attractive routes to the synthesis of these block copolymers. We report the synthesis of both lipophilic and hydrophilic hydrocarbon-fluorocarbon block copolymers using both iniferter and A T R P polymerization techniques. We also report the synthesis of silicone block copolymers by crossover from a living anionic polymerization to iniferter (19).

The concept of iniferter was first demonstrated by T. Otsu in 1982 for the synthesis of polymers by a radical mechanism with a controlled structure and functionality (16,20). The technique was subsequently used for the synthesis of block copolymers (21,22). The term "iniferter" comes from the three-fold role of the compound: initiator, chain transfer agent, and reversible terminating agent. The reversible termination of the polymer chain allows for the synthesis of block copolymers. The iniferter technique makes use of a photoactive compound that upon photolysis yields an initiating fragment (R») and a stable free radical species (S»), typically a thiuram, which serves as a reversible capping agent (Figure 1). The end capping agent does not initiate polymerization. Iniferter polymerizations typically give molecular weight polydispersity indices (PDIs) of 1.4 - 1.8. In contrast to iniferter, A T R P employs a haloalkane that serves as the initiator along with a copper(I) catalyst and a dipyridyl ligand that serves to help solubilize the copper catalyst. A T R P polymerizations are typically run at 80 - 130 °C. In A T R P , initiation occurs by the abstraction of the halogen atom from the haloalkane by the copper(I) catalyst giving a carbon-centered radical that initiates polymerization, and a copper(II) species. The radical polymerization is controlled by the reversible transfer of the halogen atom between the growing polymer chain and the copper species (Figure 1). Polymers synthesized by A T R P typically have PDIs less than 1.3, but values less than 1.1 have been reported (17,18,23). Both iniferter and A T R P give well defined polymers with controlled structure, functionality and polymer molecular weight.

The iniferter controlled free radical polymerization technique is effective for the synthesis of a wide range of hydrophilic, l ipophil ic and fluorocarbon polymers inc luding poly(2-dimethylaminoethyl methacrylate) ( P D M A E M A ) , poly(2-hydroxyethyl methacrylate) ( P H E M A ) , polystyrene (PS), polyvinyl acetate) (PVAc) , poly(l , l ' -dihydroperfluorooctyl acrylate) ( P F O A ) , and poly(l , l ' -dihydroperfluoro-octyl methacrylate) ( P F O M A ) . While the types of monomers that can be used with A T R P are more limited than for iniferter (e.g. F O A and acetates do not work), A T R P gives much more controlled and well-defined polymers. A T R P has proven to be effective for the polymerization of a broad range of lipophilic hydrocarbon monomers including acrylates, methacrylates and styrenics as well as fluorocarbon monomers such as F O M A , 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSEA) , 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate ( F O S E M A ) , l,r,2,2"-tetra-hydroperfluorooctyl methacrylate (TM), and l,r,2,2'-tetrahydroperfluorooctyl acrylate (TA-N) (24). Although A T R P is quite useful for making lipophilic-fluorocarbon block copolymers, the synthesis of hydrophilic-fluorocarbon block copolymers must overcome the complexation of the copper catalyst species to the polar groups frequently encountered in hydrophilic monomers. One solution is the use of protected hydrophilic monomers such as 2-(trimethylsilyloxy)ethyl methacrylate ( H E M A - T M S ) and tert-

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a.) R-S R. + S * b.) R - X + C u ( I ) — • R* + C u ( I I ) X

R* + M • R-Μ· R* + Ρ • R-Ρ ·

R - M . + S- R - M - S R ~ P - + C u ( I I ) x P " X + Cu(I> *deact. * d e a c t -

Figure 1. Mechanism of Polymerization a.) Iniferter b.) A T R P

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butylacrylate (f-BuA) to yield P H E M A and poly(acrylic acid) respectively upon deprotective hydrolysis after polymerization.

C H 2 = C

R /

\ c=o

R /

C H 2 — C

R

C H 9 = C

Ο \

R = H F O A R = C H 3 F O M A * = 25% branching

ο \

C H 2 c 7 F i 5 C H 2 C H 2 ( C F 2 ) 6 - i o C F 3

R = Η T A - N R = C H 3 T M

\ / C = o

° \ f CH2CH2NS0 2 (CF 2 )6 - ioCF3

R = H F O S E A R = C H 3 F O S E M A

Both the iniferter and A T R P controlled free radical polymerization techniques offer the same advantages as conventional free radical polymerizations: relative ease of synthesis and monomer compatibility. Additionally, they offer the ability to control the polymer structure and functionality thus affording polymers with narrow poly-dispersities, targeted molecular weights and complex architectures.

Experimental

Materials. Tetraethylthiuram disulfide (TD, Aldrich) was recrystallized twice from methanol and the purity was checked by i H - N M R . sec-Butyllithium (1.3 M in cyclo­hexane, Aldr ich) , c o p p e r © bromide (99.999%, Aldr ich) , 2,2 '-dipyridyl (99+%, Aldrich), ethyl 2-bromoisobutyrate (98%, Aldrich) and diethyldithiocarbamate sodium salt (Aldrich) were used as received. Benzyl Ν,Ν-diethyldithiocarbamate (BDC) and xylylene bis(N,N-diethyl dithiocarbamate) ( X D C ) were synthesized as described in the literature (15,21,22). Styrene (Aldrich), vinyl acetate (Aldrich), 1,1-dihydroperfluoro-octyl acrylate ( F O A , 3M) , and 1,1-dihydroperfluorooctyl methacrylate ( F O M A , 3M) were passed through an alumina column to remove the inhibitor and deoxygenated by purging with argon prior to use. Hexamethylcyclotrisiloxane ( D 3 , Acros) was kindly provided by Bayer and was purified by vacuum sublimation, l-(dimethylchloro-silyl)-2-(p,m chloromethyl phenyl) ethane was synthesized as described in the literature (25). Cyclohexane (Fisher) was stirred over concentrated sulfuric acid for ca. 2 weeks, decanted, and distil led over sodium under an argon atmosphere prior to use. Tetrahydrofuran (THF, Mallinckrodt) was distilled from a sodium/benzophenone solution under argon atmosphere prior to use. Methanol (Mallinckrodt), heptane

(Mallinckrodt), acetone (Mallinckrodt), and α ,α ,α- t r i f luoroto luene (TFT, Aldr ich , 99+%) were used as received. l,l,2-trichloro-2,2,l-trifluoroethane (F-113, DuPont) was used as received. Carbon dioxide (SFC/SFE Grade) was kindly provided by A i r Products and was used as received.

Iniferter. Synthesis of Polystyrene-Iniferter (TD-PS). Polystyrene was

synthesized by the iniferter controlled free radical method using T D as the iniferter as reported earlier (75). The polymerizations were conducted in bulk and stopped at about 50% conversion. In a typical polymerization, 1.90 g of T D was dissolved in 15.0 g of deinhibited styrene monomer. The solution was then purged with argon and placed in a constant temperature bath at 70 °C for 15 hours. A t the end of the reaction, the viscous

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polymer solution was diluted with T H F and then the polymer was recovered by precipitation into a ten fold excess of methanol and dried under vacuum. (Gravimetric yield = 7.9 g, 53.9%, M n = 3.2 kg/mol, PDI = 1.9). The polymer was purified twice by dissolution in T H F and precipitation into methanol (reprecipitation yield = 89%). The M n and PDI of the purified polymer were determined to be 3.7 kg/mol and 1.7, respectively.

Synthesis of Polyvinyl Acetate)-Iniferter (BDC-PVAc) . Polyvinyl acetate) was synthesized using B D C as the iniferter. The polymerizations were conducted in bulk and stopped at about 20% conversion. In a typical polymerization, 40 g of deinhibited vinyl acetate monomer was added to a quartz flask equipped with a stir bar and then 1.0 g of B D C was added along with 2.2 mg of T D . (The T D added does not initiate polymerization of the vinyl acetate but serves to better control the polymerization of the reactive monomer by increasing the concentration of the end capping agent in solution through chain transfer (26).) The flask was then sealed with a septum and purged with argon. The solution was photolyzed at room temperature with stirring for 30 hours in a sixteen bulb Rayonet equipped with 350 nm wavelength bulbs. The polymer was then collected by precipitation into heptane and dried under vacuum giving a crude yield of 8.7 g, or 21.7% ( M n = 4.7 kg/mol, P D I = 1.5). The polymer was purified twice by dissolution in acetone and precipitation into heptane (reprecipitated yield = 8.3 g, 95.4%). The M n and PDI of the purified polymer were 4.4 kg/mol and 1.6, respectively.

Block Copolymer Synthesis. The diblock copolymers were synthesized by the subsequent polymerization of a fluorocarbon monomer using either T D - P S , B D C - P V A c or X D C - P D M A E M A as the macroiniferter. The synthesis of X D C -P D M A E M A was conducted in a manner similar to the synthesis of B D C - P V A c but using X D C . Polymerizations were conducted in T F T as the solvent. Whi le the polymerizations with B D C - P V A c were homogeneous throughout the reaction, the polymerizations with TD-PS were initially cloudy but became clear after a few hours of irradiation. A s an illustration, the synthesis of PVAc-2?-PFOA is described herein. In a typical experiment, 2.0 g B D C - P V A c ( M n = 4.4 kg/mol) was dissolved in 25 m L of T F T in a quartz flask equipped with a stir bar. F O A monomer (18.0 g) was then added and the flask was purged with argon. After purging, the solution was photolyzed at room temperature with stirring for 30 hours in a sixteen bulb Rayonet equipped with 350 nm bulbs. The polymer was then collected by precipitation into methanol and dried under vacuum (yield = 12.7 g, 61.9%). The block copolymer was purified by Soxhlet extraction using methanol for ca. 2 days to remove any unreacted P V A c homopolymer. The composition and relative molecular weights of the blocks in the block copolymer were determined by * H - N M R ( M n P V A c = 4.4 kg/mol, 34.9 mol %; P F O A M n = 43.1 kg/mol, 65.1 mol %).

Silicone block copolymer synthesis. Synthesis of PDMS-iniferter. The PDMS-inifer ter was synthesized by

the l iv ing anionic polymerization of D3 according to procedures described in the literature. A 35% D3/cyclohexane solution (17.5 g, 50 mL) was added via syringe to a flame dried round bottom flask equipped with a stir bar and under argon atmosphere. &?c-butyllithium (0.45 m L , 5.85 χ 10 - 4 mol) was then added via syringe to the flask to give a targeted molecular weight of 30 kg/mol. The solution was allowed to stir for about 2 h and then 20 v/v % (10 mL) of dry T H F was added to start the chain propagation. The reaction was allowed to proceed for two days and then was terminated with a six fold molar excess (0.87 g, 3.51 χ 10 - 3 mol) of l-(dimethylchloro-silyl)-2-(/?,ra chloromethyl phenyl) ethane. Upon adding the terminating agent, the precipitation of l i thium chloride was observed and the solution was left to stir for

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several hours. The P D M S was then precipitated into methanol and isolated using a separatory funnel. The polymer was dried under vacuum and its weight and yield determined (yield = 14.8 g, 84.5%).

The P D M S was then functionalized to afford the PDMS-iniferter. The polymer was dissolved in ca. 30 m L T H F and a three fold molar excess of diethyldithio-carbamate sodium salt (0.40 g, 1.76 χ 10 - 3 mol) was added. The yellow solution was stirred overnight at room temperature. The PDMS-iniferter was precipitated into ethanol and dried under vacuum. The PDMS-iniferter was then purified by dissolving in hexanes and washing with a 3:1 methanol:water mixture. After washing, it was collected and dried under vacuum (yield = 13.1 g, 95%). The M n and P D I of the polymer were 26.8 kg/mol and 1.10, respectively.

PDMS Block Copolymer Synthesis. B lock copolymers of P D M S with vinyl monomers such as F O A and V A c were synthesized. The synthesis of PDMS-&-P V A c is described as an example. Deinhibited vinyl acetate monomer (1.0 g), 5.0 g of the PDMS-iniferter ( M n = 26.8 kg/mol), and 15 m L of T H F were added to a quartz flask equipped with a stir bar. The flask was then purged with argon and photolyzed for 30 h in a 16 bulb Rayonet equipped with 350 nm bulbs. A t the end of the reaction, the polymer was precipitated into methanol, collected, and dried under vacuum (yield = 4.3 g, 70.5%). Molecular weight determination of the second block was determined by subtracting the molecular weight of the P D M S block synthesized from the total block copolymer molecular weight as determined by G P C ( M n block copolymer = 39.8 kg/mol, PDI = 1.4; M n P V A c block = 13.0 kg/mol, 29.5 mol %).

A T R P . Synthesis of Poly(methyl methacrylate) ( P M M A ) . Into a round

bottom flask equipped with a stir bar was added 20.0 g deinhibited methyl methacrylate monomer, 0.78 g (4 χ Ι Ο - 3 mol) ethyl 2-bromoisobutyrate, 0.58 g (4 χ 10" 3 mol) copper(I) bromide, 1.87 g (1.2 χ 10" 2 mol) dipyridyl, and 20 m L ethyl acetate as solvent. The flask was then sealed with a septum and purged with argon. After purging, the flask was placed into a 100 °C oi l bath for 5.5 hours. The reaction was heterogeneous due to the insolubility of the copper catalyst. A t the end of the reaction, the reaction solution was diluted with ethyl acetate and passed through a short alumina column to remove the copper catalyst. The polymer was precipitated into methanol, collected, and dried under vacuum (yield = 14.9 g, 74.6%, M n = 8.1 kg/mol, PDI = 1.3).

Block Copolymer Synthesis. The previously synthesized P M M A block (3.24 g, M n = 8.1 kg/mol) was placed into a round bottom flask equipped with a stir bar. T F T (40 mL) was added as the solvent for the reaction and 5.7 χ 10"2 g (4 χ ΙΟ" 4

mol) copper(I) bromide and 0.19 g (1.2 χ Ι Ο - 3 mol) dipyridyl were added. Deinhibited F O M A monomer (20 g) was then added and the flask was sealed with a septum and purged with argon. After purging, the flask was placed in a 110 °C oil bath for 5.5 h. A t the end of the polymerization, the flask was removed from the oi l bath and the reaction solution was diluted and passed through a short alumina column to remove the insoluble copper catalyst. The polymer was then precipitated into heptane, collected, and dried under vacuum (yield = 5.9 g, 25.4%). The block copolymer was purified by Soxhlet extraction using T H F and the relative block lengths were determined by lU-N M R ( P M M A : M n = 8.1 kg/mol, 26.7 mol %; P F O M A : M n = 104.0 kg/mol, 73.3 mol%).

Characterization. The molecular weight data for the polymer samples were determined using a Waters 150-CV gel permeation chromatograph (GPC) with

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Ultrastyragel columns of 100, 500, 10 3 , and 10 4 Â porosities using T H F as the eluent and PS standards (Showa Denko). The molecular weight data for the P D M S homopolymers and block copolymers were determined using a modular Waters 610 G P C instrument with two Ultrastyragel columns of 10 3 and 10 4 Â porosities and a μStyragel H T linear column, using dichloromethane as the eluent and PS standards (Showa Denko). i H - N M R spectra were obtained from a Bruker W M 2 5 0 spectrometer. ϊΗ-NMR spectra of the hydrocarbon/fluorocarbon block coploymers were conducted in a mixed solvent of F - l 13 and either CDCI3 or d6-acetone.

Results and Discussion

For the synthesis of diblock copolymers using the iniferter polymerization technique as developed by Otsu, the C02-phobic block was synthesized first using either T D or B D C as the iniferter in conjunction with either heat or U V light, respectively. Addi t ional ly , for the formation of B A B triblock copolymers, the difunctional photoiniferter X D C was used. The polymer formed with terminal dithiocarbamate groups was then isolated, purified and characterized. The macroiniferter first block was then used, in the presence of U V light, to initiate the polymerization of the second monomer (CC>2-philic fluorocarbon) to form the block copolymer. U V irradiation of the macroiniferter causes the reactive end groups to dissociate generating polymeric radicals which, in the presence of additional monomer, initiate the polymerization of the second monomer and lead to the formation of the second block of the block copolymer (Figure 2). Through the use of this technique, block copolymers such as PS-&-PFOA, P S - 6 - P F O M A , P V A c - f c - P F O A , P V A c - f c - P F O M A , P F O A - Z ? - P V A c - Z ? - P F O A , P D M A E M A - f c - P F O M A and P T A N - 6 - P D M A E M A - f c - P T A N have been successfully synthesized.

Listed in Tables I and II are several examples of P V A c - ^ - P F O A and PS-b-P F O A diblock copolymers synthesized using the iniferter technique and the description of their solubility in C O 2 at 65 °C and 5000 psi. These conditions correspond to a C O 2 density of 0.846 g/cc. A s can be seen in comparison between the two materials, the P V A c block copolymers have a much higher solubility in C O 2 than the PS block copolymers. The P V A c block copolymers are soluble and form transparent solutions with a P V A c mole % of greater than 55%, compared to the PS block copolymers that are only soluble up to a PS mole % of around 30%. This is due to the much higher solubility of P V A c in C O 2 than PS: McHugh has shown that PS of molecular weight greater than 1000 g/mol is insoluble in C O 2 even at temperatures of 225 °C and 30,000 psi, while P V A c of a molecular weight of 125,000 g/mol is soluble in C O 2 at about 9000 psi and room temperature (7) .

Table I. PVAc-fr-PFOA Diblock Copolymers

M n (kg/mol) P V A c a

PDI Mole % P F O A b

M n (kg/mol) C 0 2 Solubility c

65 °C, 5k psi

4.4 1.6 34.9 43.1 clear 4.4 1.6 24.8 70.1 clear 10.3 1.5 55.8 43.1 clear 30.9 1.5 75.3 53.5 turbid

Characterized by G P C . bCharacterized by * H - N M R . c 4 % wt/vol; determined visually with a high pressure view cell.

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C H o =

C H 3

I Î c=o I

O C H 2 C H 2 N ( C H 3 )

D M A E M A U V h v

C H 2 - S — C — N ( E t ) 2

B D C

Ç H 3 S

C H 2 - ^ C H 2 - C " ) - S — C — N ( E t ) 2

c=o I

O C H 2 C H 2 N ( C H 3 )

C H o

C H 3

I I I U V h v

C

I O C 7 F 1 5

F O M A P D M A E M A - f e - P F O M A

Figure 2. Synthesis of P D M A E M A - é - P F O M A Using the Iniferter Technique.

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Table II. PS-fr-PFOA Diblock Copolymers

M n (kg/mol) P S a

PDI Mole % P F O A b

M n (kg/mol) C O 2 Solubility 0

65 °C, 5k psi

3.7 1.7 37.0 27.5 cloudy 3.7 1.7 28.9 39.8 clear 3.7 1.7 20.9 61.2 clear 4.5 1.8 44.5 24.5 turbid 6.6 1.6 45.1 34.9 translucent 6.6 d 1.6 41.2 42.3 clear

Characterized by G P C . bCharacterized by ÏH-NMR. c 4 % wt/vol; determined visually with a high pressure view cell. d PS -&-PFOMA.

A n adaptation of the synthesis scheme developed by Kumar for the crossover from a l iving anionic synthesis to controlled free radical iniferter synthesis (27) was used for the synthesis of several P D M S - & - P F O A and P D M S - & - P V A c block copolymers. In this synthesis technique, the siloxane block is synthesized first using established living anionic polymerization techniques. A t the end of the polymerization the chain ends are terminated with l-(dimethylchlorosilyl)-2-(/7,m-chloromethyl phenyl)ethane to afford the benzyl chloride terminated poly (dimethyl siloxane). The terminal chloro group is then substituted with diethyldithiocarbamic acid to afford the diethyldithiocarbamate terminated P D M S . This macroiniferter-PDMS can then be used in conjunction with U V light to initiate the polymerization of a second monomer to yield a diblock copolymer (Figure 3). The results of the synthesis of a P D M S - Z ? - P V A c diblock copolymer is given in Table III. The anionic polymerization of the first block ( P D M S ) gave a very narrow molecular weight distribution with a P D I of 1.1. The polymerization of the second block (PVAc) using the iniferter technique broadened the polydispersity of the block copolymer due to the less "living" nature of the iniferter polymerization. The P D M S - & - P V A c block copolymer synthesized by this technique was also employed as a stabilizer for the dispersion polymerization of vinyl acetate in C 0 2 (25).

Table III. P D M S - ^ - P V A c

P D M S block Viny l Acetate block M n

PDI Mole % M n P D F Mole %

(kg/mol) (kg/mol)

26.8 1.1 70.5 13.0 1.4 29.5

a Polydispersity of the block copolymer.

Several hydrocarbon monomers, including acrylates, methacrylates, styrenics, and acetates, along with several fluorocarbon monomers have been successfully polymerized using the controlled free radical polymerization techniques (iniferter or A T R P ) . Of the block copolymers synthesized, the PS-fc-PFOA, PS- fc -PFOMA and

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P V A c - b - P F O A block copolymers have been studied by small angle neutron scattering (SANS) in C O 2 and found to self assemble into micelles with a hydrocarbon core and a fluorocarbon corona (10). Furthermore, these block copolymers have been found to be effective stabilizers for dispersion polymerizations in C O 2 (9).

S A N S studies of several examples of the P S - & - P F O A block copolymers synthesized using the iniferter technique have shown that these amphiphiles self associate in C O 2 to form spherical micelles with a PS hydrocarbon core stabilized by a P F O A fluorocarbon corona. It was also noted that the size and aggregation number of the micelles changed slightly based on the density of the C O 2 medium. These block copolymers were also shown to solubilize C O 2 insoluble 500 g/mol polystyrene oligomer into the C O 2 continuous phase (10). The S A N S studies were complemented with high pressure, high-resolution N M R spectroscopy of these block copolymers in supercritical C O 2 (29). ! H - N M R studies revealed that at low C O 2 densities only the resonances from the P F O A blocks are visible, indicative of the PS blocks being immoblized in micelle cores. But with increasing C O 2 density, the P F O A resonances shift upfield, and N M R signals from the PS blocks grow in and also shift upfield. The appearance of the PS resonances arises from the increased mobilization of the PS blocks in the micelle core as the core is plasticized and the P F O A solubility is increased by the denser C O 2 .

These PS-&-PFOA block copolymers were also studied in the solid phase by X -ray photoelectron spectroscopy (XPS) (30). P F O A is a very low surface energy material (ca. 11 dyn/cm) while PS has a fairly high surface energy (40 dyn/cm); these materials therefore are expected to strongly phase separate. X P S studies confirmed that P F O A preferentially exists at the air/surface interface. The enhanced mole % of P F O A at the surface was also shown to increase upon annealing as this increased the polymer chain mobility, allowing the P F O A chains to move to the surface. For an unannealed PS-&-PFOA sample comprised of 50 mole % PS, less than 20 mole % PS was found at the surface. The mole % PS was observed to further decrease to less than 10% upon annealing.

Additionally, these block copolymers are effective stabilizers in the dispersion polymerization of styrene in C O 2 . In using these block copolymers as stabilizers it was observed that the length of the anchoring block (PS) relative to the length of the solubilizing block (PFOA) is very important to the stability of the latex formed and the particle size obtained. The best results were obtained using the block copolymers with the longer anchoring blocks (Table IV) (9).

Table IV. Results of Styrene Polymerizations in C 0 2 a

Stabilizer b Stabilizer yield M n Particle size

(kg/mol) cone, (w/v) (%) (kg/mol) (urn)

none 0 22.1 3.8 _

P S ^ - f c - F F O A 1 ™ 4 72.1 19.2 0.40

P S 4 - 5 - & - P F O A 2 4 - 5 4 97.7 22.5 0.24 PS 6 - 6 -Z?-PFOA 3 4 -9 4 93.6 23.4 0.24

a Polymerizations in C O 2 at 3000 psi and 65 °C in a 10 m L high pressure view cell with 2.0 g styrene and 2.4 χ ΙΟ" 2 Μ ΑΓΒΝ. b F o r simplicity, the block copolymer molecular weights are listed as P S < M n > - f c - P F O A < M n > .

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Based upon the results obtained from the S A N S studies of the P S - & - P F O A block copolymers which showed that the size of the micelle changed slightly with pressure, the concept of a critical micelle density ( C M D ) was developed. Below the critical density, the surfactants are predominantly located in micelles. But, at higher densities, the block which was previously insoluble and in the core of the micelle is solubilized in the denser C O 2 medium and the surfactant exists as unimers in solution (Figure 4) (10,31). Due to the higher C O 2 solubility of P V A c compared to PS (7) , P V A c - & - P F O A block copolymers were synthesized by the iniferter technique with the idea that these block copolymers would exhibit greater changes in solution aggregation behavior than the PS-&-PFOA block copolymers and thus clearly demonstrating the existence of a C M D . Indeed, the existence of a C M D was clearly seen by S A N S for a P V A c - b - P F O A block copolymer. A t lower densities the P V A c block is relatively insoluble leading to the formation of micelles in solution, but at higher densities the P V A c block is solubilized leading to a decrease in the aggregation number and the breakup of the micelles into unimers.

In P V A c dispersion polymerization studies in C O 2 with these P V A c - & - P F O A block copolymers these block copolymers proved to be effective stabilizers, affording stable latexes, high yield, and high molecular weight P V A c (28).

When using A T R P to make block copolymers, a haloalkane (e.g. (1-bromoethyl)benzene, ethyl 2-bromoisobutyrate) was used as the initiator in conjunction with either copper(I) chloride or bromide and 2,2'-dipyridyl as the catalyst. The initiatoncopper haliderdipyridyl mole ratio typically used was 1:1:3. In the presence of sufficient heat, the halogen atom is believed to dissociate from the haloalkane initiator giving an initiating alkyl-radical which initiates polymerization of the first monomer to form the first block. After polymerization of the first block is complete, the halogen endcapped polymer is isolated, purified, and characterized. The formation of the second block is accomplished by sufficiently heating a solution of the redissolved first block, the second monomer, additional copper halide, and dipyridyl catalyst (Figure 5). Using A T R P , various block copolymers have been synthesized including poly(methyl methacrylate)-fc-PFOMA, P H E M A - f c - P F O M A , PS-fc-PFOMA, poly(i-butylacrylate)-Z?-P F O M A , and poly(ethylhexyl acrylate)-fc-PFOMA (Table V ) .

Table V . Hydrocarbon-fluorocarbon block copolymers synthesized by A T R P

Hydrocarbon block M n PDI Fluorocarbon block M n

(kg/mol) (kg/mol)

P M M A 8.1 1.3 P F O M A 104.0 P(i-BuAc) 5.0 1.6 P F O M A 52.6 P H E M A 4.0 1.5 P F O M A 40.0

In the synthesis of these hydrocarbon-fluorocarbon block copolymers, the first block synthesized is typically the hydrocarbon block due to the relative ease of characterization (high solubility in common organic solvents) compared to the fluorocarbon block. The hydrocarbon block molecular weight was determined by G P C , and * H - N M R was used to determine conversion and polymer purity. Once the hydrocarbon-fluorocarbon block copolymer was made, it was purified by Soxhlet extraction with a good solvent for the hydrocarbon homopolymer block, and characterized by ϊ Η - N M R to determine the relative molecular weight of the second block in comparison to the first block.

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sec-BuLi + D 3

1. cyclohexane, T H F

2.

CH 3

I 3

C I - C H 2 - ^ y— CH2CH2— Si—CI

ÇH3 ÇH3

C 4 H 9 - ( S i - O ^ S Î - C H 2 C H 2 - ^ y - C H 2 C I

CH 3 CH 3

î +NaS—C-N(Et)2

CHo CHo 1 , 1

C 4 H 9 -(Si-0)^Si-CH 2 CH2-^ y — CH 2S-C-N(Et) 2

C H 3 C H 3

hv V A c

PDMS-£-P V A c

CHo

Figure 3. Synthesis of PDMS-fc-PVAc Via the Combination of Anionic and Iniferter Polymerization Techniques

Increasing CO 2 density

micelles CMD

unimers

Figure 4. Illustration of Critical Micelle Density (CMD)

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C H 3

I C H 2 = C

C = 0

O C H 3

P M M A

Br I

C H 3 — C H

I c=o I

O C H , C u B r + bpy

110 °C

C H 3

CH 3—CH-(CH2-C^-Br

C = 0 C = 0

I OCH3 O C H 3

110 °C

C u B r + bpy +

C H s

C H 2 = C I c=o I O C 7 F i 5

F O M A

P M M A - & - P F O M A

F igu re 5. Synthesis of P M M A - f t - P F O M A U s i n g A T R P

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Conclusion.

A T R P and iniferter controlled free radical polymerization techniques have been found to be effective for the synthesis of various hydrocarbon-fluorocarbon block copolymers. Several of the hydrocarbon-fluorocarbon block copolymers synthesized have been shown to be effective stabilizers for dispersion polymerizations in C O 2 . Additionally, these block copolymers have been studied using various analytical techniques including S A N S , ïH-NMR, and X P S . Many of the block copolymers synthesized are soluble in C O 2 under suitable conditions and have been observed to self assemble into micelles that display interesting solution behavior. These micelles have been observed to change in size based upon the density of the C O 2 phase, leading to the concept of a critical micelle density. X P S analysis showed these block copolymers to be strongly phase separated with the fluorocarbon block being dominant at the surface. Research is continuing to design and synthesize more effective and wel l defined stabilizers for dispersion polymerizations in C O 2 and the emulsification of C02 - insoluble materials. The latter has implications for the use of C O 2 in surface treatment and cleaning technologies.

Acknowledgments.

The authors have greatly appreciated Scott Gaynor and Kris Matyjaszewski for their assistance with the initial A T R P polymerizations. We gratefully acknowledge financial support from the N S F through a Presidential Faculty Fellowship ( J M D : 1993-1997), the Environmentally Benign Chemical Synthesis and Processing Program sponsored by the N S F and the E P A , and the Consortium for Polymeric Materials Synthesis and Processing in Carbon Dioxide sponsored by DuPont, A i r Products and Chemicals, Hoechst-Celanese, Eastman Chemical , B . F . Goodrich, Xerox , and Bayer. In addition, we thank Isco for the donation of a syringe pump.

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