Macrothiols through free radical chain transfer: Synthesis, kinetics, and application in block copolymer synthesis

Macro chain transfer agents based on mercaptan terminated-polymethyl methacrylate (macrothiol, PMMA-SH) were obtained by low-conversion free radical polymerization of methyl methacrylate in presence of 1,6-hexane dithiol as chain transfer agent. The chain transfer constant of 0.82 ensured good functionality and relatively narrow dispersity for the formed macrothiol. On increasing the conversion, the molecular weight of the macrothiol increased and its functionality decreased. A kinetic model was proposed which could satisfactorily explain these variations. Free radical polymerization of styrene in presence of PMMA-SH as chain transfer agent furnished the PS-PMMA diblock copolymer. Styrene exhibited a transfer constant (Ctr) of 3.37 for the macrothiol which was practically the same as that for 1,6-hexane dithiol for the same monomer. The length of polystyrene block in the copolymer decreased with increase in concentration of the macrothiol in the reaction medium. However, the block length was more than that theoretically expected which was attributable to the effect of diffusion controlled kinetics. The block copolymers were characterized by elemental analysis and by gel permeation chromatography using multiple detectors. Attempts to synthesize macrothiol of butyl acrylate and styrene and corresponding block copolymers were not successful due to secondary reaction during chain transfer in these cases.