In-depth analysis of radical polymerization kinetics and mechanism by highly time-resolved electron paramagnetic resonance spectroscopy in conjunction with pulsed-laser initiation

在精确的扩散、终止和转移各个步骤的速率系数的基础上详细了解自由基聚合动力学，对于基本理解这些基本反应和模拟技术聚合过程是非常重要的。在我们团队中，我们发展了SP-PLP-EPR方法，将单脉冲(SP)诱导的脉冲激光聚合(PLP)与电子顺磁共振(EPR)光谱相结合。激光脉冲后的自由基浓度直接以微秒到毫秒的时间分辨率进行监测。该方法提供了与自由基终止的链长相关的完美途径，如已证明的对甲基丙烯酸甲酯、苯乙烯和乙酸乙烯酯的终止。由于可以监测多种自由基的浓度，SP-PLP-EPR技术还可以研究水溶液中丙烯酸酯聚合和丙烯酰胺聚合的终止和转移动力学。对于这些单体，在典型的聚合条件下，二次链端自由基可以通过1，5-氢转移反应(反咬)进行内部转移，从而产生具有非常不同的动力学性质的中链自由基。对几种自由基物种的分析还可以对RAFT和ATRP等可逆失活聚合反应进行详细的动力学和力学分析。SP-PLP-EPR方法的有利特征是几乎瞬间产生强烈的小自由基爆发，并结合高灵敏的EPR检测来自初级光引发剂衍生物种的自由基的时间演化。该方法适用于整个自由基聚合领域，即广泛的单体和溶剂环境。提交的项目建议书集中在以下主题的SP-PLP-EPR研究：1.单体转化率高的本体自由基聚合终止速率系数的测量2。从单个SP-PLP-EPR实验中推导出完全和部分电离单体自由基聚合的终止、转移和传播速率系数3.设计双脉冲PLP-EPR实验来测试不同链长的自由基的交叉末端模型4.测量丙烯酸酯的回刺率作为烷基酯大小的函数本项目旨在展示SP-PLP-EPR技术在详细和准确测量整个自由基聚合领域的相关速率系数方面的巨大和无与伦比的潜力。

Detailed knowledge about radical polymerization kinetics on the basis of accurate rate coefficients for the individual steps of propagation, termination and transfer is of enormous relevance for the fundamental understanding of these elementary reactions and for the simulation of technical polymerization processes. Within our group, the SP-PLP-EPR method has been developed, which connects pulsed laser polymerization (PLP) induced by a single pulse (SP) with electron paramagnetic resonance (EPR) spectroscopy. Radical concentrations after laser pulsing are directly monitored with a time resolution of microseconds to milliseconds. The method provides perfect access to the chain-length-dependence of radical termination, as has been demonstrated for methyl methacrylate, styrene and vinyl acetate. As the concentration of more than one type of radicals may be monitored, the SP-PLP-EPR technique also allows for studies into the termination and transfer kinetics of acrylate polymerization and of acrylamide polymerization in aqueous solution. With these monomers, secondary chain-end radicals, at typical polymerization conditions, may undergo internal transfer by a 1,5-hydrogen shift reaction (backbiting) thus producing midchain radicals with very different kinetic properties. Analysis of several radical species also allows for detailed kinetic and mechanistic analyses of reversible deactivation polymerizations such as RAFT and ATRP.The favourable signature of the SP-PLP-EPR method is due to the almost instantaneous production of an intense burst of small radicals in conjunction with the highly sensitive EPR detection of the time evolution of radicals originating from the primary photoinitiator-derived species. The method is applicable within the entire field of radical polymerization, i.e., to a wide range of monomers and solvent environments.The submitted project proposal focuses on SP-PLP-EPR studies into the following topics:1. Measurement of termination rate coefficients for bulk radical polymerizations up to high degrees of monomer conversion2. Deducing termination, transfer and propagation rate coefficients for radical polymerizations of fully and partially ionized monomers from a single SP-PLP-EPR experiment 3. Design of double-pulse PLP-EPR experiments for testing cross-termination models of radicals differing in chain length 4. Measurement of the backbiting rate of acrylates as a function of alkyl ester sizeThe project aims at demonstrating the enormous and unrivalled potential of the SP-PLP-EPR technique toward the detailed and accurate measurement of relevant rate coefficients for the entire field of radical polymerization.