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 (Fcum) and instantaneous composition (Finst) 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 (Tg) behaviours of poly(St-grad-MA) and poly(St-grad-AA) were compared in detail. Tg values of poly(St-grad-MA) increase with increasing Fcum,St, while poly(St-grad-AA) has the maximum Tg value and both the two kinds of copolymers have the maximum Tg breadth values with Fcum,St range of 0.4–0.5. The Tg 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 Tg breadths of all the copolymers further prove their gradient structures.

The first example of organostibine mediated controlled/living random copolymerization of styrene (St) and methyl methacrylate (MMA) was achieved by heating a solution of St/MMA/organostibine mediator at 100 °C or St/MMA/organostibine mediator/AIBN with various monomer feed ratios at 60 °C. The addition of AIBN significantly decreased the reaction temperature and enhanced the rate of copolymerization. The structure of poly(St-co-MMA) was verified by 1H NMR. The reactivity ratios at 60 °C were determined by the extended Kelen-Tüdős method to be γSt = 0.40 and γMMA = 0.44. The ln([M]0/[M]) increased linearly with increasing reaction time. The number-average molecular weights of poly(St-co-MMA) increased linearly with conversion. Poly(St-co-MMA) with expected number-average molecular weight and low polydispersity index was formed. The living characteristic was further confirmed by chain-extension of poly(St-co-MMA) to form poly(St-co-MMA)-b-PMMA.