The role of the ketyl radical in type II photoinitiators

Scientists have evaluated the influence of the photoinitiating system on the properties of photopolymerised methylmethacrylate.

A laboratory.
It was demonstrated that the glass transition temperature of the final crosslinked network could be controlled by the addition of a monoallylated monomer. Image source: kwanchaift - (symbol image).
Three photoinitiating systems (PIS) exhibiting different photoinitiation mechanisms were used for radical photopolymerisation 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 was found that the type II photoinitiating system leads to relatively low Mn and D̄ values, a fact partly attributed to the ketyl radical that acts as a terminating agent.

Highly efficient PIS 

When adding a triazine derivative to this system, the PIS turns to be highly efficient. High Mn values were found and the D̄ 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 Mn and low D̄. These differences affect the Tg values, a fact which is highlighted when comparing the behavior of the RAFT agent with that of the photocyclic initiating system. The decrease of D̄ leads to higher Tg by avoiding the self-plasticisation due to short chains.

The study has been published in Polymer Chemistry, Issue 9, 2021.

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