Continuous Microflow PhotoRAFT Polymerization

Abstract

PhotoRAFT (reversible addition-fragmentation radical transfer) polymerizations are investigated for reactions induced by conventional radical photoinitiators. As demonstrated, this rather simple photoRAFT reaction shows similar outcomes compared to other recently introduced photopolymerizations, such as photoelectron transfer RAFT or copper-mediated photopolymerization. Despite the general notion that classically initiated photoRAFT yields unsatisfactory results, it is shown that good results can be achieved when conditions are selected accordingly. Not only the type of initiator is of importance, also light intensity, RAFT agent to initiator concentration, and reaction temperature are of importance. For the initiator benzoin, optimal polymerizations are obtained when the initiator is used in a ratio of 0.25 to the initial RAFT agent at 60 degrees C reaction temperature and 30 mW cm(-2) light intensity (365 nm). Chain lengths of the polymer can be tuned efficiently and block copolymers are accessible from the process despite some slight loss in chain-end fidelity during polymerizations. Additionally, the choice of initiator is shown to have a large effect on the polymerization, which can be routed to different decomposition rate coefficient under the same illumination conditions. Decomposition rates of the photoinitiators are under flow conditions very high, and polymerizations proceed to completion after all initiator is used up via a photoiniferter mechanism.