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, <i>E</i> _A(<i>R</i> _p), 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 <i>E</i> _A(<i>R</i> _p) was compared with the benchmarked <i>E</i> _A(<i>k</i> _p) 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 <i>E</i> _A(<i>R</i> _p) and the <i>E</i> _A(<i>k</i> _p) from PLP-SEC was rationalized due to temperature-induced changes in termination. The deviation between the <i>E</i> _A(<i>R</i> _p) measured in RAFT photopolymerization and <i>E</i> _A(<i>k</i> _p) 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 <i>E</i> _p values. However, the RAFT photopolymerization of MA, which is subject to strong retardation, had a much larger <i>E</i> _A(<i>R</i> _p) than the <i>E</i> _A(<i>k</i> _p) from PLP-SEC. The high apparent <i>E</i> _A(<i>R</i> _p) 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.