And Polystyrene- Block-Poly[(Butyl Acrylate)-Co-Styrene]

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Macromol. Rapid Commun. 2000, 21, 921–926 921

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 C6F13I 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 per- form 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 Evolution of M with conversion for the second block (straight n — chromatography (SEC), differential scanning calorimetry line: theoretical Mn) (DSC) and nuclear magnetic resonance (NMR).

Polystyrene-block-poly(butyl acrylate) and polystyrene- block-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 Macromole´culaire, UMR 7610, Universite´ Pierre et Marie Curie, Tour 44, 1er e´tage, 4, place Jussieu 75252 Paris Cedex 05, France [email protected] 2 ATOFINA, Groupement de Recherches de Lacq, B.P. n8 34, 64170 Lacq, France (Received: March 9, 2000; revised: June 1, 2000)

Introduction used is stable in the presence of water. We have demon- One of the techniques to control the molar mass and strated earlier that controlled radical polymerization of molar mass distribution in radical polymerization is the styrene could be performed in aqueous miniemulsion at so-called degenerative transfer based on the exchange of 708C, using perfluorohexyl iodide as transfer agent [7] a terminal iodine atom between a functionalized dormant (C6F13I), The transfer agent efficiency was 100% and chain and an active one.[1–6] Initially, polymerizations the target molar masses were always reached at complete were described in bulk or solution and led to the prepara- monomer conversion. This result was not obtained in tion of homopolymers and block copolymers with prede- conventional emulsion polymerization for which the effi- termined molar mass, controlled end-functionalization ciency never exceeded 50%. Controlled polymerization and relatively narrow molar mass distribution. Since radi- in miniemulsion was achieved very simply by the addi- cal polymerization is tolerant to water, emulsion techni- tion of the transfer agent to the monomer phase before ques can also be applied providing that the transfer agent emulsification. The experimental conditions were not

Macromol. Rapid Commun. 2000, 21, No. 13 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1336/2000/1308–0921$17.50+.50/0 922 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux

otherwise changed with respect to a classical miniemul- acrylic ester)s via reversible addition-fragmentation sion polymerization,[8] except that the usually used cosur- transfer to a macromonomer.

factant or hydrophobe was not needed anymore (C6F13I had a similar effect). In batch conditions, polystyrene — –1 Experimental part with Mn as high as 47000 g N mol could be obtained — — with relatively narrow molar mass distribution (Mw /Mn = Materials 1.5). Moreover, chain extension could be performed by Styrene (St) and butyl acrylate (BA) were distilled under the slow continuous addition of a second load of styrene. — reduced pressure before use. Water-soluble radical initiator Under such conditions, a linear increase of Mn with 4,49-azobis(4-cyanopentanoic acid) (ACPA, 75%, remainder

monomer conversion was observed. However, in contrast water, Aldrich), transfer agent perfluorohexyl iodide (C6F13I, to batch polymerization, the molar mass distribution 99%, Aldrich), anionic surfactant sodium dodecyl sulfate broadened during the monomer addition process. This (SDS, 98%, Acros) and buffer sodium hydrogen carbonate result which was typically observed under starve-feed (NaHCO3 , Prolabo) were used as received. conditions was explained by the high internal viscosity of the particles, reducing the rate of bimolecular exchange Miniemulsion polymerization procedures between an active macromolecule and a dormant one, The batch miniemulsion polymerization procedure for the which is the key step to ensure good control. The success- synthesis of the polystyrene first block was the same as ful chain extension with styrene prompted us to apply the described previously.[7] After 50 min of polymerization at same technique with another monomer in order to synthe- 708C, the conversion of styrene was between 80 and 90%, size block copolymers in an aqueous dispersion. In this approximately. At this stage, except for experiment ME1, work, the second monomer chosen was butyl acrylate, second monomer BA was added either in one shot (ME5) or and the polymerization was performed by adding this continuously at a controlled flow rate (ME2, ME3, ME4). monomer to a polystyrene seed latex prepared by degen- For ME1, the addition was started after 2 h of styrene homo- erative iodine transfer polymerization in miniemulsion. polymerization, in order to guarantee complete conversion of The synthesis of polystyrene-block-poly(butyl acrylate) the first monomer. During BA addition and polymerization using this same technique in bulk has already been period, samples were withdrawn at regular time intervals in order to monitor the overall monomer conversion, using described.[3] Nevertheless, to our knowledge, it is the first gravimetry. All the conversion data were calculated as time it is applied in a dispersed system. Moreover, regard- weight fractions with respect to the overall amount of mono- less of the controlled polymerization technique used, only mers added at the end of the reaction and were systemati- one example of block copolymer prepared in an aqueous cally corrected with respect to the amounts of polymer and dispersed system has already been reported in the scienti- monomer removed for sampling. The experimental condi- fic literature.[9] It concerned the synthesis of poly(meth- tions are reported in Tab. 1.

Tab. 1. Sequential miniemulsion polymerization of styrene and butyl acrylate in the presence of C F I at 708C. Experimental con- 6 13 — ditions: water: 167 g; NaHCO3 : 0.15 g; SDS: 0.47 g; ACPA: 0.20 g; NaOH: 0.05 g; high molar mass polystyrene (Mw = 330000 g/ mol): 0.18 g; C6F13I: 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

a) — — b) — — St conv. Theor. Mn Exp. Mn D in nm/ Radd Rp Overall Theor. Mn Exp. Mn Final D Tg –1 after 50 min in g/mol in g/mol Np in mL in g/h in g/h conv. in g/mol in g/mol in nm/Np in8C — — — — –1 (Mw /Mn) (Mw /Mn) in mL

ME1 1.0 (0.49) 5000 4500 111/1.461014 15.1 13 0.75 7700 6500 (2.01) (after (1.47) 0.99 10200 8800 (2.82) 133/1.661014 120 min) ME2 0.90 (0.44) 4500 4400 87/2.661014 6.7 6 0.75 7700 7500 (1.80) (1.51) 0.98 10000 9500 (2.05) 109/2.861014 –15 ME3 0.75 (0.36) 3700 4200 107/1.261014 15.1 9c) 0.70 7140 7800 (1.67) (1.49) 15d) 0.98 10000 9200 (2.17) 140/1.361014 –11.5 ME4 0.83 (0.41) 4200 4800 86/2.561014 44.4 13 0.56 5800 7100 (1.52) (1.39) 0.99 10200 10000 (A3) 105/2.961014 –7.5 ME5 0.82 (0.45) 4600 3600 98/1.761014 Shot 17 0.99 10100 18000 (A3) 126/1.861014 (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. Polystyrene-block-poly(butyl acrylate) and polystyrene-block-poly[(butyl acrylate)-co-styrene] ... 923

Latex characterization monomer able to both moderate the overall rate of propa- The final latexes had a solid content of 20%, they were stable gation and enhance the transfer reaction. This is expected without formation of coagulum. The particle diameter was to be the case with styrene. For instance, as in bulk, the measured by dynamic light scattering using the Zetasizer4 overall rate of St and BA emulsion copolymerization is from Malvern. For two experiments (ME2, ME3), the parti- much lower than the rate of BA homopolymerization.[12] cle size distribution was determined by capillary hydrody- Moreover, the reactivity ratios below 1 (rBA = 0.18; rSt = namic fractionation (CHDF, from MATEC). 0.66 in emulsion[13]) ensure the incorporation of styrene as isolated units when a small proportion of this monomer Polymer characterization is used, which should not drastically modify the proper- ties of the poly(BA) block. This latter is expected to exhi- Polymers were recovered from the latexes by water evapora- bit a tapered structure with an increasing proportion of tion. Molar masses were measured by size exclusion chroma- BA towards the x-end of the chain. For the synthesis of tography (SEC) with tetrahydrofuran as eluant at a flow rate polystyrene-block-poly(butyl acrylate) block copolymers of 1 mL N min–1. The SEC system was equipped with three columns from Shodex (KF 802.5; KF 804L; KF 805L) ther- in miniemulsion, we chose to apply both techniques mostatted at 308C; a differential refractive index detector simultaneously. The second monomer (BA) was added to was used, and molar masses were derived from a calibration the polystyrene seed latex either in one shot or continu- curve based on polystyrene standards. Block copolymers ously with various flow rates. Usually, the addition was were also analyzed with a second SEC system equipped with started before complete consumption of styrene (below a double detection: differential refractive index and UV 90%) except in one case, in which the addition was (254 nm); separation was performed with four PL gel 10 l started after complete conversion. We have examined the columns (100, 500, 103 and 104 A˚ ). The glass transition tem- effect of such experimental conditions on the kinetics and perature (Tg) was measured by differential scanning calori- on the control over the molar masses. metry (DSC7 from Perkin-Elmer) in a temperature range from –1008C to 1508C, at a scanning rate of 208C N min–1. 1 13 H and C NMR spectra of the polymers were run in CDCl3 Kinetics of BA polymerization solution at room temperature using a Bruker AC200 appara- tus, operating at a frequency of 200 MHz for 1H and 50.3 When the addition of BA was started before complete MHz for 13C. The chemical shift scale was calibrated on the consumption of styrene, various rates of addition were basis of the solvent peak (7.24 ppm for 1H and 77.0 ppm for tested. As shown in Tab. 1 and Fig. 1, faster polymeriza- 13C). The composition of the copolymers was calculated tion was achieved when a shot addition process was from the proton NMR analysis by integrating the aromatic applied (ME5). Performing a continuous addition, the protons of the styrene units (6.3–7.3 ppm, 5 H) and CH2 polymerization was slower when the rate of addition ester protons of the butyl acrylate units (3.4–4.2 ppm, 2 H). (Radd) was smaller. Nevertheless, except for experiment ME2, for which the rate of polymerization (Rp) was close to the rate of addition, for the other reactions, R was far Results and discussion p below Radd . This result indicates that no addition was When a degenerative transfer process is applied to control actually performed under monomer-starved conditions. the polymerization under batch conditions, it is essential The polymerization rate was not only controlled by the that the rate constant of transfer to the chain transfer addition rate but also by the instantaneous St/BA molar agent is higher than the rate constant of propagation (Ctr [7] = ktr /kp A 1). 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

C6F13I 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 Fig. 1. Overall conversion versus time for miniemulsion poly- [11] work under monomer starve-feed conditions. A second merizations with addition of BA after 50 min of styrene homo- one is to copolymerize it with a few percents of another polymerization 924 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux

— Fig. 3. Evolution of M with overall conversion for the second n — Fig. 2. Comparison of experiments ME3 and ME1 performed block (straight line: theoretical Mn) 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 Fig. 4. Size exclusion chromatograms of ME2 (differential incomplete St conversion) with the same flow rate. It can refractive index versus elution volume) be seen in Fig. 2 that in the latter case, the polymerization was initially much slower compared to the case where second block and indicate that PSt-I chains played the styrene was absent from the reaction medium. Neverthe- expected role as transfer agent in the second stage of the less, the polymerization became faster with progressing polymerization. However, as it was previously observed [7] overall conversion and, eventually, Rp reached a final for the chain extension with styrene , the polydispersity value close to Radd . 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 ter- Evolution of the molar masses with monomer mination reactions. An explanation which was proposed conversion at that time[7] was that the increasing viscosity of the par- The molar masses as measured by SEC are plotted versus ticles would more drastically affect exchange reactions the overall monomer conversion in Fig. 3. The evolution between macromolecules than the propagation. Another is linear and follows the theoretical line indicating that interpretation might also be suggested. In a seeded emul- the concentration of chains remained constant throughout sion polymerization with water-soluble initiator and con- the reaction. It equals the initial concentration of the tinuous addition of monomer, it can be supposed that the transfer agent and, consequently, the concentration of the polymerization takes place predominantly near the parti- end-functionalized polystyrene chains, PSt-I, after the cle/water interface, which should also strongly disadvan- first step. The SEC chromatograms were carefully exam- tage exchange reactions. In addition, as reported in ined. A complete shift of the polystyrene peak towards Tab. 1, the final molar mass distributions were found to higher molar masses could be observed (Fig. 4). More- be broader when the monomer addition was faster and, as over, UV and refractometric traces of the block copoly- a consequence, when the polymerization was faster. mers perfectly superimpose indicating the presence of Therefore, there is another source of molar mass broaden- polystyrene in all the detected chains. No peak corre- ing in the case of BA. In addition to the decrease in the sponding to high molar mass homopolymeric poly(butyl rate of the degenerative transfer reaction, there is a con- acrylate) could be evidenced. Those results support the comitant increase in the rate of propagation when either formation of block copolymers with a pure polystyrene the addition rate is higher or the overall conversion pro- first block and an either poly(BA) or poly[(BA)-co-(St)] gresses and styrene is progressively consumed. Polystyrene-block-poly(butyl acrylate) and polystyrene-block-poly[(butyl acrylate)-co-styrene] ... 925

Characterization of the block copolymers Although SEC analyses confirmed the controlled charac- ter 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 Tg’s of the two homopolymers but a single intermediate Tg (Tab. 1). Such a result, which is characteristic of a homogeneous system, is in favor of the absence of a large proportion of the incompa- tible homopolymers and reinforces the previous conclu- sions. 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. Fig. 5. 13C NMR analysis of the copolymers obtained at var- The structure of those copolymers was verified by NMR ious conversions (experiment ME3) spectroscopy. On the one hand, it was shown that the copolymers had the expected composition, as measured by 1H NMR spectroscopy. On the other hand, the compo- can be observed at the beginning (SAS, SAA and AAA), sition microstructure of the second block was determined while the intensity of the signal characteristic of long by 13C NMR with the help of previously published data poly(BA) sequences (AAA, 174.0–174.7 ppm) continu- on peak assignments for the various triads.[13] The conclu- ously increases with conversion. sions 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 copo- Conclusion lymerized with styrene. Indeed, the broad peak of the It has been demonstrated that the synthesis of block copo- polystyrene quaternary carbon shows the formation of lymers based on polystyrene and poly(butyl acrylate) ASA (BA/St/BA) and ASS triads (from 142.0 to could be performed very easily in an aqueous dispersed 144.5 ppm) at low BA conversion, whereas the initial system via controlled free-radical miniemulsion polymer- homopolystyrene peak was situated between 144.5 and ization using degenerative iodine transfer. Concerning the 146.5 ppm only. Correspondingly for the signal of the macromolecular characteristics of the block copolymers, carbonyl group, the presence of all three possible triads the results were similar to those previously obtained in 926 C. Farcet, M. Lansalot, R. Pirri, J. P. Vairon, B. Charleux

bulk:[3] the polydispersity index of the first polystyrene [3] K. Matyjaszewski, S. G. Gaynor, J.-S. Wang, Macromole- block was close to 1.5 and the formation of the poly(BA) cules 1995, 28, 2093. — — second block led to an increase of M /M to approxi- [4] S. G. Gaynor, J.-S. Wang, K. Matyjaszewski, Macromole- w n cules 1995, 28, 8051. mately 2. It is expected that a faster rate of exchange [5] A. Goto, K. Ohno, T. Fukuda, Macromolecules 1998, 31, would result in the formation of better-defined block 2809. copolymers with narrower molar mass distribution. [6] R. D. Puts, P. P. Nicholas, J. Milan, D. Miller, E. Elce, J. Nevertheless, this work shows that a reversible transfer Lee, N. Pourahmady, Polym. Prepr. (Am. Chem. Soc., Div. technique can be easily applied to heterogeneous condi- Polym. Chem.) 1999, 40, 415. [7] M. Lansalot, C. Farcet, B. Charleux, J.-P. Vairon, R. Pirri, tions because it does not require drastic modification of Macromolecules 1999, 32, 7354. the usual experimental conditions. [8] K. Landfester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 1999, 32, 5222. [9] a) J. Krstina, G. Moad, E. Rizzardo, C. L. Winzor, Macro- molecules 1995, 28, 5381; b) J. Krstina, C. L. Moad, G. Acknowledgement: The authors wish to thank Monique Con- Moad, E. Rizzardo, Macromol. Symp. 1996, 111, 13. tassot (UPMC, Paris) for DSC measurements. ATOFINA is [10] R. A. Lyons, J. Hutovic, M. C. Piton, D. I. Christie, P. A. acknowledged for financial contribution. Clay, B. G. Manders, S. H. Kable, R. G. Gilbert, Macromo- lecules 1996, 29, 1918. [11] A. H. E. Mu¨ller, D. Yan, G. Litvinenko, R. Zhuang, H. Dong, Macromolecules 1995, 28, 7335. [1] US 5439980 (1995), Daikin Industries, invs: Y. Yutani, M. [12] J. L. Guillaume, C. Pichot, A. Revillon, Makromol. Chem., Tatemoto; Chem. Abstr. 123, 287366r. Suppl. 1985, 10/11, 69. [2] J.-S. Wang, S. G. Gaynor, K. Matyjaszewski, Polym. Prepr. [13] M. F. Llauro-Darricades, C. Pichot, J. Guillot, Polymer (Am. Chem. Soc., Div. Polym. Chem.) 1995, 36, 465. 1986, 27, 889.