Abstract

Photopolymerizations in Continuous Flow: From Highly Efficient to Overefficient Reactions

Thomas Junkers, Professor, Hasselt University

The development of continuous tubular photoflow reactors has pushed the field of chemical synthesis via photoinduced reactions significantly. Next to the classical advantages of continuous flow techniques (simple scalability, close to ideal heat transfer and high operation stability), photoflow reactors give access to relatively large volumes while keeping optical pathlengths at any instance small and hence avoiding occurrence of light intensity gradients within the reactor. Focusing on photoinduced controlled radical polymerizations (reversible addition fragmentation transfer, RAFT, atom transfer radical polymerization, ATRP and cobalt-mediated radical polymerization, CMRP), we demonstrate how flow reactors can be used to produce polymers with very high structural integrity under mild reaction conditions in a robust way by employing simple UV light sources. photoATRP and photoCMRP rely on photoelectron transfer reactions, while photoRAFT is induced by conventional radical photoinitiators. Further, also photoclick reactions are investigated, namely cycloadditions and radical thiol-ene reactions. Limitations in the thiol-ene reaction efficiency are identified and routed to over-efficiency of light absorption. In absence of any dark volume reservoir in the reactor, decay of the initiator is too fast, and reactions come to stop before full conversion of the ene components can be reached.