Polystyrene-block-poly(butyl acrylate) and polystyreneblock-poly[(butyl

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1 Macromol. Rapid Commun. 2000, 21, Communication: Polystyrene-block-poly(butyl acrylate) and polystyrene-block-poly[(butyl acrylate)-co-styrene] block copolymers were prepared in an aqueous dispersed system via controlled free-radical miniemulsion polymerization using degenerative iodine transfer. The first step is batch miniemulsion polymerization of styrene in the presence of C 6 F 13 I as transfer agent. The second step consists of the addition of butyl acrylate to this seed latex, either in one shot or continuously. The addition was started before the consumption of styrene was complete in order to perform a copolymerization reaction able to moderate the rate of propagation in the butyl acrylate polymerization step and, therefore, to favor the transfer reaction. Kinetics of polymerization and control of the molar masses were examined according to the experimental conditions and particularly to the rate of butyl acrylate addition. The formed block copolymers were analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR). Evolution of M n with conversion for the second block (straight line: theoretical M n) Polystyrene-block-poly(butyl acrylate) and polystyreneblock-poly[(butyl acrylate)-co-styrene] block copolymers prepared via controlled free-radical miniemulsion polymerization using degenerative iodine transfer C. Farcet, 1 M. Lansalot, 1 R. Pirri, 2 J. P. Vairon, 1 B. Charleux* 1 1 Laboratoire de Chimie Macromoléculaire, UMR 7610, Université Pierre et Marie Curie, Tour 44, 1er étage, 4, place Jussieu Paris Cedex 05, France charleux@ccr.jussieu.fr 2 ATOFINA, Groupement de Recherches de Lacq, B.P. n8 34, Lacq, France (Received: March 9, 2000; revised: June 1, 2000) Introduction One of the techniques to control the molar mass and molar mass distribution in radical polymerization is the so-called degenerative transfer based on the exchange of a terminal iodine atom between a functionalized dormant chain and an active one. [1 6] Initially, polymerizations were described in bulk or solution and led to the preparation of homopolymers and block copolymers with predetermined molar mass, controlled end-functionalization and relatively narrow molar mass distribution. Since radical polymerization is tolerant to water, emulsion techniques can also be applied providing that the transfer agent used is stable in the presence of water. We have demonstrated earlier that controlled radical polymerization of styrene could be performed in aqueous miniemulsion at 70 8C, using perfluorohexyl iodide as transfer agent (C 6 F 13 I), [7] The transfer agent efficiency was 100% and the target molar masses were always reached at complete monomer conversion. This result was not obtained in conventional emulsion polymerization for which the efficiency never exceeded 50%. Controlled polymerization in miniemulsion was achieved very simply by the addition of the transfer agent to the monomer phase before emulsification. The experimental conditions were not Macromol. Rapid Commun. 2000, 21, No. 13 i WILEY-VCH Verlag GmbH, D Weinheim /2000/ $ /0

2 922 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux otherwise changed with respect to a classical miniemulsion polymerization, [8] except that the usually used cosurfactant or hydrophobe was not needed anymore (C 6 F 13 I had a similar effect). In batch conditions, polystyrene with M n as high as g N mol 1 could be obtained with relatively narrow molar mass distribution (M w/m n = 1.5). Moreover, chain extension could be performed by the slow continuous addition of a second load of styrene. Under such conditions, a linear increase of M n with monomer conversion was observed. However, in contrast to batch polymerization, the molar mass distribution broadened during the monomer addition process. This result which was typically observed under starve-feed conditions was explained by the high internal viscosity of the particles, reducing the rate of bimolecular exchange between an active macromolecule and a dormant one, which is the key step to ensure good control. The successful chain extension with styrene prompted us to apply the same technique with another monomer in order to synthesize block copolymers in an aqueous dispersion. In this work, the second monomer chosen was butyl acrylate, and the polymerization was performed by adding this monomer to a polystyrene seed latex prepared by degenerative iodine transfer polymerization in miniemulsion. The synthesis of polystyrene-block-poly(butyl acrylate) using this same technique in bulk has already been described. [3] Nevertheless, to our knowledge, it is the first time it is applied in a dispersed system. Moreover, regardless of the controlled polymerization technique used, only one example of block copolymer prepared in an aqueous dispersed system has already been reported in the scientific literature. [9] It concerned the synthesis of poly(methacrylic ester)s via reversible addition-fragmentation transfer to a macromonomer. Experimental part Materials Styrene (St) and butyl acrylate (BA) were distilled under reduced pressure before use. Water-soluble radical initiator 4,49-azobis(4-cyanopentanoic acid) (ACPA, 75%, remainder water, Aldrich), transfer agent perfluorohexyl iodide (C 6 F 13 I, 99%, Aldrich), anionic surfactant sodium dodecyl sulfate (SDS, 98%, Acros) and buffer sodium hydrogen carbonate (NaHCO 3, Prolabo) were used as received. Miniemulsion polymerization procedures The batch miniemulsion polymerization procedure for the synthesis of the polystyrene first block was the same as described previously. [7] After 50 min of polymerization at 708C, the conversion of styrene was between 80 and 90%, approximately. At this stage, except for experiment ME1, second monomer BA was added either in one shot (ME5) or continuously at a controlled flow rate (ME2, ME3, ME4). For ME1, the addition was started after 2 h of styrene homopolymerization, in order to guarantee complete conversion of the first monomer. During BA addition and polymerization period, samples were withdrawn at regular time intervals in order to monitor the overall monomer conversion, using gravimetry. All the conversion data were calculated as weight fractions with respect to the overall amount of monomers added at the end of the reaction and were systematically corrected with respect to the amounts of polymer and monomer removed for sampling. The experimental conditions are reported in Tab. 1. Tab. 1. Sequential miniemulsion polymerization of styrene and butyl acrylate in the presence of C 6 F 13 I at 708C. Experimental conditions: water: 167 g; NaHCO 3 : 0.15 g; SDS: 0.47 g; ACPA: 0.20 g; NaOH: 0.05 g; high molar mass polystyrene (M w = g/ mol): 0.18 g; C 6 F 13 I: 1.74 g (0.1 mol/l with respect to monomers); St: 17.5 g; BA: 22.3 g (continuous addition) Expt. Polystyrene first block Block copolymerization St conv. a) Theor. M n after 50 min in g/mol Exp. M n in g/mol (M w/m n) D in nm/ N p in ml 1 b) R add in g/h R p in g/h Overall conv. Theor. M n in g/mol Exp. M n in g/mol (M w/m n) Final D in nm/n p in ml 1 T g in8c ME1 1.0 (0.49) / (2.01) (after 120 min) (1.47) (2.82) 133/ ME (0.44) / (1.80) (1.51) (2.05) 109/ ME (0.36) / c) (1.67) (1.49) 15 d) (2.17) 140/ ME (0.41) / (1.52) (1.39) (A3) 105/ ME (0.45) / Shot (A3) 126/ (1.66) a) Conversion with respect to styrene (in parenthesis: overall conversion, including styrene and butyl acrylate). b) Rate of BA addition in g/h. c) Before 120 min. d) After 120 min.

3 Polystyrene-block-poly(butyl acrylate) and polystyrene-block-poly[(butyl acrylate)-co-styrene] Latex characterization The final latexes had a solid content of 20%, they were stable without formation of coagulum. The particle diameter was measured by dynamic light scattering using the Zetasizer4 from Malvern. For two experiments (ME2, ME3), the particle size distribution was determined by capillary hydrodynamic fractionation (CHDF, from MATEC). Polymer characterization Polymers were recovered from the latexes by water evaporation. Molar masses were measured by size exclusion chromatography (SEC) with tetrahydrofuran as eluant at a flow rate of 1 ml N min 1. The SEC system was equipped with three columns from Shodex (KF 802.5; KF 804L; KF 805L) thermostatted at 30 8C; a differential refractive index detector was used, and molar masses were derived from a calibration curve based on polystyrene standards. Block copolymers were also analyzed with a second SEC system equipped with a double detection: differential refractive index and UV (254 nm); separation was performed with four PL gel 10 l columns (100, 500, 10 3 and 10 4 Å). The glass transition temperature (T g ) was measured by differential scanning calorimetry (DSC7 from Perkin-Elmer) in a temperature range from 1008C to 1508C, at a scanning rate of 208C N min 1. 1 H and 13 C NMR spectra of the polymers were run in CDCl 3 solution at room temperature using a Bruker AC200 apparatus, operating at a frequency of 200 MHz for 1 H and 50.3 MHz for 13 C. The chemical shift scale was calibrated on the basis of the solvent peak (7.24 ppm for 1 H and 77.0 ppm for 13 C). The composition of the copolymers was calculated from the proton NMR analysis by integrating the aromatic protons of the styrene units ( ppm, 5 H) and CH 2 ester protons of the butyl acrylate units ( ppm, 2 H). Results and discussion When a degenerative transfer process is applied to control the polymerization under batch conditions, it is essential that the rate constant of transfer to the chain transfer agent is higher than the rate constant of propagation (C tr = k tr /k p A 1). [7] Otherwise, the monomer is totally consumed before the transfer agent; consequently, the theoretical molar mass is not reached (the experimental value is larger than the theoretical one), and the remaining transfer agent molecules can disturb the following polymerization steps. With butyl acrylate, this goal is difficult to achieve since this monomer has a very high propagation rate constant. [10] For instance, the transfer constant to C 6 F 13 I was found to be below 0.1 while it was 1.4 for the polymerization of styrene in bulk at 708C. [7] Therefore, in order to achieve a controlled polymerization of BA by a degenerative transfer process under slow exchange conditions, it is necessary to reduce the rate of propagation with respect to that of transfer. A first possibility is to work under monomer starve-feed conditions. [11] A second one is to copolymerize it with a few percents of another monomer able to both moderate the overall rate of propagation and enhance the transfer reaction. This is expected to be the case with styrene. For instance, as in bulk, the overall rate of St and BA emulsion copolymerization is much lower than the rate of BA homopolymerization. [12] Moreover, the reactivity ratios below 1 (r BA = 0.18; r St = 0.66 in emulsion [13] ) ensure the incorporation of styrene as isolated units when a small proportion of this monomer is used, which should not drastically modify the properties of the poly(ba) block. This latter is expected to exhibit a tapered structure with an increasing proportion of BA towards the x-end of the chain. For the synthesis of polystyrene-block-poly(butyl acrylate) block copolymers in miniemulsion, we chose to apply both techniques simultaneously. The second monomer (BA) was added to the polystyrene seed latex either in one shot or continuously with various flow rates. Usually, the addition was started before complete consumption of styrene (below 90%) except in one case, in which the addition was started after complete conversion. We have examined the effect of such experimental conditions on the kinetics and on the control over the molar masses. Kinetics of BA polymerization When the addition of BA was started before complete consumption of styrene, various rates of addition were tested. As shown in Tab. 1 and Fig. 1, faster polymerization was achieved when a shot addition process was applied (ME5). Performing a continuous addition, the polymerization was slower when the rate of addition (R add ) was smaller. Nevertheless, except for experiment ME2, for which the rate of polymerization (R p ) was close to the rate of addition, for the other reactions, R p was far below R add. This result indicates that no addition was actually performed under monomer-starved conditions. The polymerization rate was not only controlled by the addition rate but also by the instantaneous St/BA molar Fig. 1. Overall conversion versus time for miniemulsion polymerizations with addition of BA after 50 min of styrene homopolymerization

4 924 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux Fig. 2. Comparison of experiments ME3 and ME1 performed with the same rate of addition of BA (15.1 g N h 1 ) after 50 min and 120 min of styrene homopolymerization, respectively (straight lines: BA addition) ratio which was larger when the BA addition was slower leading to slower propagation owing to the copolymerization effect. This is also illustrated by experiment ME1 for which addition was performed after complete styrene conversion. In this case, the rate of polymerization was very close to the rate of addition (Fig. 2), indicating starve-feed conditions. This experiment can be compared with ME3 for which BA was added after 50 min (at incomplete St conversion) with the same flow rate. It can be seen in Fig. 2 that in the latter case, the polymerization was initially much slower compared to the case where styrene was absent from the reaction medium. Nevertheless, the polymerization became faster with progressing overall conversion and, eventually, R p reached a final value close to R add. Evolution of the molar masses with monomer conversion The molar masses as measured by SEC are plotted versus the overall monomer conversion in Fig. 3. The evolution is linear and follows the theoretical line indicating that the concentration of chains remained constant throughout the reaction. It equals the initial concentration of the transfer agent and, consequently, the concentration of the end-functionalized polystyrene chains, PSt-I, after the first step. The SEC chromatograms were carefully examined. A complete shift of the polystyrene peak towards higher molar masses could be observed (Fig. 4). Moreover, UV and refractometric traces of the block copolymers perfectly superimpose indicating the presence of polystyrene in all the detected chains. No peak corresponding to high molar mass homopolymeric poly(butyl acrylate) could be evidenced. Those results support the formation of block copolymers with a pure polystyrene first block and an either poly(ba) or poly[(ba)-co-(st)] Fig. 3. Evolution of M n with overall conversion for the second block (straight line: theoretical M n) Fig. 4. Size exclusion chromatograms of ME2 (differential refractive index versus elution volume) second block and indicate that PSt-I chains played the expected role as transfer agent in the second stage of the polymerization. However, as it was previously observed for the chain extension with styrene [7], the polydispersity index values increased with conversion (see Tab. 1). This can be ascribed to a decrease in the rate of exchange with respect to that of propagation rather than to extensive termination reactions. An explanation which was proposed at that time [7] was that the increasing viscosity of the particles would more drastically affect exchange reactions between macromolecules than the propagation. Another interpretation might also be suggested. In a seeded emulsion polymerization with water-soluble initiator and continuous addition of monomer, it can be supposed that the polymerization takes place predominantly near the particle/water interface, which should also strongly disadvantage exchange reactions. In addition, as reported in Tab. 1, the final molar mass distributions were found to be broader when the monomer addition was faster and, as a consequence, when the polymerization was faster. Therefore, there is another source of molar mass broadening in the case of BA. In addition to the decrease in the rate of the degenerative transfer reaction, there is a concomitant increase in the rate of propagation when either the addition rate is higher or the overall conversion progresses and styrene is progressively consumed.

5 Polystyrene-block-poly(butyl acrylate) and polystyrene-block-poly[(butyl acrylate)-co-styrene] Characterization of the block copolymers Although SEC analyses confirmed the controlled character of the polymerization and the formation of block copolymers, a more complete characterization, however, was performed in order to check both the structure of the block copolymers and a possible presence of homopolymers. It is in the very nature of the degenerative transfer process to lead to the formation of small portions of both homopolymers when a block copolymer is prepared. The polymerization kinetics follows a steady-state regime and, therefore, initiation and termination reactions exist similarly as in a conventional process throughout the polymerization. Moreover, when the two polymerization steps are performed in a dispersed system, there is an additional possibility to form a mixture of homopolymers instead of a block copolymer. Indeed the formation of a new population of particles in the second step would lead, in the absence of PSt-I within the polymerization site, to the extensive formation of uncontrolled high molar mass homopolymeric poly(ba). As mentioned above, this latter was not observed in the SEC chromatograms. Furthermore, no secondary nucleation took place as evidenced by the expected increase in particle size after the second polymerization, and the constant number of particles throughout the reaction. Another argument is based on CHDF analyses of some latexes showing a single population of particles. The DSC thermograms of the block copolymers did not exhibit T g s of the two homopolymers but a single intermediate T g (Tab. 1). Such a result, which is characteristic of a homogeneous system, is in favor of the absence of a large proportion of the incompatible homopolymers and reinforces the previous conclusions. All the collected information support the existence of block copolymers with a pure polystyrene first block and a poly[styrene-co-(butyl acrylate)] second block when BA addition was started before complete St consumption. The structure of those copolymers was verified by NMR spectroscopy. On the one hand, it was shown that the copolymers had the expected composition, as measured by 1 H NMR spectroscopy. On the other hand, the composition microstructure of the second block was determined by 13 C NMR with the help of previously published data on peak assignments for the various triads. [13] The conclusions are illustrated in Fig. 5 and confirm the tapered structure of the poly[styrene-co-(butyl acrylate)] second block. It appears very clearly that BA was initially copolymerized with styrene. Indeed, the broad peak of the polystyrene quaternary carbon shows the formation of ASA (BA/St/BA) and ASS triads (from to ppm) at low BA conversion, whereas the initial homopolystyrene peak was situated between and ppm only. Correspondingly for the signal of the carbonyl group, the presence of all three possible triads 13 Fig. 5. C NMR analysis of the copolymers obtained at various conversions (experiment ME3) can be observed at the beginning (SAS, SAA and AAA), while the intensity of the signal characteristic of long poly(ba) sequences (AAA, ppm) continuously increases with conversion. Conclusion It has been demonstrated that the synthesis of block copolymers based on polystyrene and poly(butyl acrylate) could be performed very easily in an aqueous dispersed system via controlled free-radical miniemulsion polymerization using degenerative iodine transfer. Concerning the macromolecular characteristics of the block copolymers, the results were similar to those previously obtained in

6 926 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux bulk: [3] the polydispersity index of the first polystyrene block was close to 1.5 and the formation of the poly(ba) second block led to an increase of M w/m n to approximately 2. It is expected that a faster rate of exchange would result in the formation of better-defined block copolymers with narrower molar mass distribution. Nevertheless, this work shows that a reversible transfer technique can be easily applied to heterogeneous conditions because it does not require drastic modification of the usual experimental conditions. Acknowledgement: The authors wish to thank Monique Contassot (UPMC, Paris) for DSC measurements. ATOFINA is acknowledged for financial contribution. [1] US (1995), Daikin Industries, invs: Y. Yutani, M. Tatemoto; Chem. Abstr. 123, r. [2] J.-S. Wang, S. G. Gaynor, K. Matyjaszewski, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1995, 36, 465. [3] K. Matyjaszewski, S. G. Gaynor, J.-S. Wang, Macromolecules 1995, 28, [4] S. G. Gaynor, J.-S. Wang, K. Matyjaszewski, Macromolecules 1995, 28, [5] A. Goto, K. Ohno, T. Fukuda, Macromolecules 1998, 31, [6] R. D. Puts, P. P. Nicholas, J. Milan, D. Miller, E. Elce, J. Lee, N. Pourahmady, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40, 415. [7] M. Lansalot, C. Farcet, B. Charleux, J.-P. Vairon, R. Pirri, Macromolecules 1999, 32, [8] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 1999, 32, [9] a) J. Krstina, G. Moad, E. Rizzardo, C. L. Winzor, Macromolecules 1995, 28, 5381; b) J. Krstina, C. L. Moad, G. Moad, E. Rizzardo, Macromol. Symp. 1996, 111, 13. [10] R. A. Lyons, J. Hutovic, M. C. Piton, D. I. Christie, P. A. Clay, B. G. Manders, S. H. Kable, R. G. Gilbert, Macromolecules 1996, 29, [11] A. H. E. Müller, D. Yan, G. Litvinenko, R. Zhuang, H. Dong, Macromolecules 1995, 28, [12] J. L. Guillaume, C. Pichot, A. Revillon, Makromol. Chem., Suppl. 1985, 10/11, 69. [13] M. F. Llauro-Darricades, C. Pichot, J. Guillot, Polymer 1986, 27, 889.