RAFT polymerization of methyl acrylate in carbon dioxide

Reversible additon fragmentation chain transfer (RAFT) polymerizations of methyl acrylate (MA) in solution containing either 22 vol.-% CO₂ or toluene were performed at 80°C and 300 bar using cumyl dithiobenzoate (CDB) at concentrations between 1.8 × 10^-3 to 2.5 × 10 ^-2 mol · L^-1 as the RAFT agent. Product molecular weight distributions and average molecular weight indicated the successful control of MA polymerization in CO₂, even at low CDB concentrations. RAFT polymerization rates were strongly retarted by CDB and were lower in CO₂ than in toluene solution. The enhanced fluidity associated with the addition of CO₂ to the polymerizing system provided access to mechanistic details of RAFT polymerization. The data of the present study into MA, together with our recent results on RAFT polymerization of styrene in solution of CO₂ and of toluene, suggest that self-termination of intermediate RAFT radicals is responsible for retardation in case of high concentrations of this intermediate and in case of enhanced fluidity, which may be achieved by polymerization in solution of CO₂.