Gradient copolymers of styrene–methyl acrylate and styrene–acrylic acid by organostibine-mediated controlled/living radical polymerization and their glass transition behaviors

Gradient copolymers of styrene–methyl acrylate (poly(St-grad-MA)) were synthesized by semi-batch organostibine-mediated controlled/living radical polymerization (SBRP) in bulk at 60 °C. The number-average molecular weights of the copolymers increase linearly with the total conversion, and the polydispersity indices of all the final copolymers are less than 1.20. The relationships of the cumulative composition (F_cum) and instantaneous composition (F_inst) with the degree of polymerization demonstrate the formation of different gradient chain structure and composition of copolymers. Furthermore, amphiphilic gradient copolymers of styrene–acrylic acid (poly(St-grad-AA)) with a small amount of residual MA units were achieved through the hydrolysis of poly(St-grad-MA) under basic conditions. The glass transition temperature (T_g) behaviours of poly(St-grad-MA) and poly(St-grad-AA) were compared in detail. T_g values of poly(St-grad-MA) increase with increasing F_cum,St, while poly(St-grad-AA) has the maximum T_g value and both the two kinds of copolymers have the maximum T_g breadth values with F_cum,St range of 0.4–0.5. The T_g breadth values of poly(St-grad-AA) copolymers are larger than that of the corresponding poly(St-grad-MA) due to the more strongly segregating components of St–AA than St–MA. The typical wide T_g breadths of all the copolymers further prove their gradient structures.