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 polymeri- zation using degenerative iodine transfer. The first step is batch miniemulsion polymerization of styrene in the pre- sence 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.