Mechanism and kinetics of RAFT-mediated graft polymerization of styrene on a solid surface. 1. Experimental evidence of surface radical migration

The mechanism and kinetics of RAFT-mediated graft polymerization of styrene initiated on a silica particle were studied, where RAFT is the reversible addition-fragmentation chain transfer (RAFT) process. To prepare probe graft polymers with a dithiobenzoyl (X) group at the chain end, oligomeric polystyrene (PS) was grafted on a silica particle by the surface-initiated atom transfer radical polymerization (ATRP) technique, and then the terminal halogen atom of the graft polymer was converted to the X group by the reaction with 1-phenylethyl dithiobenzoate in the presence of CuBr/dHbipy complex in toluene. The graft polymerization of styrene on the PS-X-grafted silica particle was carried out in the presence of a free RAFT agent. After the polymerization, the graft chain was cleaved from the silica particle by treatment with HF and characterized. The GPC analysis revealed that the graft polymers, while their M_n increased linearly with increasing monomer conversion, gave a GPC curve with a prominent shoulder assignable to the recombined (dead) polymer. It was proved that the enhanced recombination is specific to the RAFT-mediated graft polymerization and is due to the effective migration of radical on the surface by sequential degenerative (exchange) chain transfer. This is a kind of reaction-diffusion process.