Influence of the photoinitiating system on the properties of photopolymerized methylmethacrylate: the role of the ketyl radical in type II photoinitiators

Abstract

Three photoinitiating systems (PIS) exhibiting different photoinitiation mechanisms were used for radical photopolymerization of methylmethacrylate (MMA). Namely, a photoactive RAFT agent (N,N benzyl dimethyldithiocarbamate), a type II photoinitiating system (isopropylthioxanthone/amine) and a photocyclic initiating system (isopropylthioxanthone/amine/triazine) were compared. It is found that the type II photoinitiating system leads to relatively low M_n and values, a fact partly attributed to the ketyl radical that acts as a terminating agent. When adding a triazine derivative to this system, the PIS turns to be highly efficient. High M_n values were found and the values dramatically increased. This behaviour is attributed to the disappearance of the ketyl radical by virtue of a photocycle. By contrast, the use of a photoactive RAFT agent leads to both high M_n and low . These differences affect the T_g values, a fact which is highlighted when comparing the behavior of the RAFT agent with that of the photocyclic initiating system. The decrease of leads to higher T_g by avoiding the self-plasticization due to short chains.