RAFT Copolymerization of Vinyl Acetate and N-Vinylcaprolactam: Kinetics, Control, Copolymer Composition, and Thermoresponsive Self-Assembly

Well-defined thermoresponsive amphiphilic statistical copolymers based on N-vinylcaprolactam (VCL) and vinyl acetate (VAc) were successfully synthesized by RAFT/MADIX polymerization mediated by O-ethyl-S-(1-ethoxycarbonyl)ethyldithiocarbonate chain transfer agent at 65 degrees C. We achieved the synthesis of a series of copolymers with controlled molar masses and low dispersities over a wide range of monomer feed ratios (0 < f(VAc,0) < 1). Both conventional linearization methods and a nonlinear least-squares methods (NLLS) were applied to estimate reliable values of reactivity ratios for VAc and VCL radical polymerization in regards to the disparate values previously reported in the literature. The highest measurement precision was observed for the NLLS method based on the integrated form of the copolymerization equation developed by Skeist. The calculated values of the reactivity ratios (r(VAc) = 0.33 +/- 0.10 and r(VCL) = 0.29 +/- 0.15) perfectly fitted the drift in monomer feed ratio versus conversion. The VCL and VAc monomer units are distributed homogeneously along the copolymer chains. Both the cloud point temperatures and glass transition temperatures evolve linearly with the copolymer composition. Analyses of the aqueous solutions of the amphiphilic copolymers by means of dynamic and static light scattering show that P(VAc-co-VCL) copolymers undergo a reversible temperature-induced conformational change between large aggregates (T > T-cloud point) and either unimers for F-VAc inferior to 0.3 (N-agg = 1, D-h similar to 7 nm) or small aggregates for F-VAc = 0.53 (N-agg = 3, D-h similar to 14 nm).