Temperature\nEffects in Conventional and RAFT Photopolymerization

Photochemical processes are often thought to be temperature-independent.\nHowever, photochemical polymerization involves photochemical processes\nsuch as light-driven radical generation coupled with thermal-driven\nreactions such as monomer propagation. The apparent activation energy\nof propagation, <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>), of a series of three monomers, methyl acrylate (MA), methyl\nmethacrylate (MMA), and styrene (STY), are deduced from Arrhenius\nanalysis of conventional and RAFT photopolymerization of these monomers\nacross a range of corresponding temperatures. The deduced <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>) was compared\nwith the benchmarked <i>E</i><sub>A</sub>(<i>k</i><sub>p</sub>) derived from pulse laser polymerizations coupled with\nsize exclusion chromatography (PLP-SEC). For conventional photopolymerization\nof MA, MMA and STY, the relatively small discrepancy between the photopolymerization-derived <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>) and the <i>E</i><sub>A</sub>(<i>k</i><sub>p</sub>) from PLP-SEC\nwas rationalized due to temperature-induced changes in termination.\nThe deviation between the <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>) measured in RAFT photopolymerization and <i>E</i><sub>A</sub>(<i>k</i><sub>p</sub>) from PLP-SEC depends\non the retardation strength in RAFT polymerizations. MMA and STY monomers\nare characterized with minimal retardation and recorded excellent\nagreement in PLP-SEC and RAFT-derived <i>E</i><sub>p</sub> values. However, the RAFT photopolymerization of MA, which is subject\nto strong retardation, had a much larger <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>) than the <i>E</i><sub>A</sub>(<i>k</i><sub>p</sub>) from PLP-SEC. The high apparent <i>E</i><sub>A</sub>(<i>R</i><sub>p</sub>) in RAFT polymerization\nof MA is likely due to the added influence of temperature-induced\nchanges in the RAFT equilibrium. Overall, these results rationalize\ntemperature-dependent effects in photochemical reactions.