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, E_A(R_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 E_A(R_p) was compared with the benchmarked E_A(k_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 E_A(R_p) and the E_A(k_p) from PLP-SEC was rationalized due to temperature-induced changes in termination. The deviation between the E_A(R_p) measured in RAFT photopolymerization and E_A(k_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 E_p values. However, the RAFT photopolymerization of MA, which is subject to strong retardation, had a much larger E_A(R_p) than the E_A(k_p) from PLP-SEC. The high apparent E_A(R_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.

中文翻译：

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常规和 RAFT 光聚合中的温度效应


光化学过程通常被认为与温度无关。然而，光化学聚合涉及光化学过程，例如光驱动的自由基生成以及热驱动的反应，例如单体传播。丙烯酸甲酯 （MA）、甲基丙烯酸甲酯 （MMA） 和苯乙烯 （STY） 等一系列三种单体的传播表观活化能 E_A（R_p） 是从这些单体在相应温度下的常规和 RAFT 光聚合的 Arrhenius 分析中推导出来的。将推导出的 E_A（R_p） 与脉冲激光聚合-尺寸排阻色谱 （PLP-SEC） 得出的基准 E_A（k_p） 进行比较。对于 MA、MMA 和 STY 的常规光聚合，由于温度诱导的终止变化，光聚合衍生的 E_A（R_p） 和 PLP-SEC 的 E_A（k_p） 之间的相对较小的差异被合理化。在 RAFT 光聚合中测得的 E_A（R_p） 与 PLP-SEC 的 E_A（k_p） 之间的偏差取决于 RAFT 聚合中的延迟强度。MMA 和 STY 单体的特征是具有最小的延迟，并且在 PLP-SEC 和 RAFT 衍生的 E_p 值中记录了极好的一致性。 然而，MA 的 RAFT 光聚合反应受到强延迟，其 E_A（R_p） 比 PLP-SEC 的 E_A（k_p） 大得多。MA 的 RAFT 聚合中的高表观 E_A（R_p） 可能是由于温度诱导的 RAFT 平衡变化的额外影响。总体而言，这些结果合理化了光化学反应中的温度依赖性效应。