Department of Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA.
J Phys Chem A. 2013 Mar 28;117(12):2605-18. doi: 10.1021/jp3100798. Epub 2013 Mar 13.
This article presents a computational study of chain transfer to monomer (CTM) reactions in self-initiated high-temperature homopolymerization of alkyl acrylates (methyl, ethyl, and n-butyl acrylate). Several mechanisms of CTM are studied. The effects of the length of live polymer chains and the type of monoradical that initiated the live polymer chains on the energy barriers and rate constants of the involved reaction steps are investigated theoretically. All calculations are carried out using density functional theory. Three types of hybrid functionals (B3LYP, X3LYP, and M06-2X) and four basis sets (6-31G(d), 6-31G(d,p), 6-311G(d), and 6-311G(d,p)) are applied to predict the molecular geometries of the reactants, products and transition sates, and energy barriers. Transition state theory is used to estimate rate constants. The results indicate that abstraction of a hydrogen atom (by live polymer chains) from the methyl group in methyl acrylate, the methylene group in ethyl acrylate, and methylene groups in n-butyl acrylate are the most likely mechanisms of CTM. Also, the rate constants of CTM reactions calculated using M06-2X are in good agreement with those estimated from polymer sample measurements using macroscopic mechanistic models. The rate constant values do not change significantly with the length of live polymer chains. Abstraction of a hydrogen atom by a tertiary radical has a higher energy barrier than abstraction by a secondary radical, which agrees with experimental findings. The calculated and experimental NMR spectra of dead polymer chains produced by CTM reactions are comparable. This theoretical/computational study reveals that CTM occurs most likely via hydrogen abstraction by live polymer chains from the methyl group of methyl acrylate and methylene group(s) of ethyl (n-butyl) acrylate.
本文对烷基丙烯酸酯（甲基丙烯酸甲酯、丙烯酸乙酯和丙烯酸正丁酯）自引发高温均聚合中的链转移至单体（CTM）反应进行了计算研究。研究了几种 CTM 机制。理论上研究了活聚合物链的长度和引发活聚合物链的单自由基的类型对所涉及反应步骤的能垒和速率常数的影响。所有计算均采用密度泛函理论进行。使用了三种混合泛函（B3LYP、X3LYP 和 M06-2X）和四种基组（6-31G(d)、6-31G(d,p)、6-311G(d) 和 6-311G(d,p)）来预测反应物、产物和过渡态的分子几何形状以及能垒。过渡态理论用于估计速率常数。结果表明，从甲基丙烯酸甲酯的甲基、丙烯酸乙酯的亚甲基和丙烯酸正丁酯的亚甲基中提取氢原子（由活聚合物链提取）是 CTM 最可能的机制。此外，使用 M06-2X 计算的 CTM 反应速率常数与使用宏观机理模型从聚合物样品测量得到的速率常数吻合良好。速率常数值随活聚合物链的长度变化不大。叔自由基的氢提取的能垒高于仲自由基的氢提取，这与实验结果一致。由 CTM 反应产生的死聚合物链的计算和实验 NMR 谱具有可比性。这项理论/计算研究表明，CTM 最有可能通过活聚合物链从甲基丙烯酸甲酯的甲基和丙烯酸乙酯（正丁基）丙烯酸酯的亚甲基中提取氢原子发生。
