Temperature Effects in Conventional and RAFT Photopolymerization.

Photochemical processes are often thought to be temperature-independent. However, photochemical polymerization involves photochemical processes such as light-driven radical generation coupled with thermal-driven reactions such as monomer propagation. The apparent activation energy of propagation, ( ), of a series of three monomers, methyl acrylate (MA), methyl methacrylate (MMA), and styrene (STY), are deduced from Arrhenius analysis of conventional and RAFT photopolymerization of these monomers across a range of corresponding temperatures. The deduced ( ) was compared with the benchmarked ( ) derived from pulse laser polymerizations coupled with size exclusion chromatography (PLP-SEC). For conventional photopolymerization of MA, MMA and STY, the relatively small discrepancy between the photopolymerization-derived ( ) and the ( ) from PLP-SEC was rationalized due to temperature-induced changes in termination. The deviation between the ( ) measured in RAFT photopolymerization and ( ) from PLP-SEC depends on the retardation strength in RAFT polymerizations. MMA and STY monomers are characterized with minimal retardation and recorded excellent agreement in PLP-SEC and RAFT-derived values. However, the RAFT photopolymerization of MA, which is subject to strong retardation, had a much larger ( ) than the ( ) from PLP-SEC. The high apparent ( ) in RAFT polymerization of MA is likely due to the added influence of temperature-induced changes in the RAFT equilibrium. Overall, these results rationalize temperature-dependent effects in photochemical reactions.