可逆加成-断裂链转移聚合中加成与断裂的动力学：一项从头算研究。

Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia.

J Phys Chem A. 2005 Feb 17;109(6):1230-9. doi: 10.1021/jp046131u.

High-level ab initio calculations of the forward and reverse rate coefficients have been performed for a series of prototypical reversible addition fragmentation chain transfer (RAFT) reactions: R* + S=C(Z)SCH3 --> R-SC*(Z)SCH3, for R = CH3, with Z = CH3, Ph, and CH2Ph; and Z = CH3, with R = (CH3), CH2COOCH3, CH2Ph, and C(CH3)2CN. The addition reactions are fast (ca. 10(6)-10(8) L mol(-1) s(-1)), typically around three orders of magnitude faster than addition to the C=C bonds of alkenes. The fragmentation rate coefficients are much more sensitive to the nature of the substituents and vary from 10(-4) to 10(7) s(-1). In both directions, the qualitative effects of substituents on the rate coefficients largely follow those on the equilibrium constants of the reactions, with fragmentation being favored by bulky and radical-stabilizing R-groups and addition being favored by bulky and radical-stabilizing Z-groups. However, there is evidence for additional polar and hydrogen-bonding interactions in the transition structures of some of the reactions. Ab initio calculations were performed at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory, and rates were obtained via variational transition state theory in conjunction with a hindered-rotor treatment of the low-frequency torsional modes. Various simplifications to this methodology were investigated with a view to identifying reliable procedures for the study of larger polymer-related systems. It appears that reasonable results may be achievable using standard transition state theory, in conjunction with ab initio calculations at the RMP2/6-311+G(3df,2p) level, provided the results for delocalized systems are corrected to the G3(MP2)-RAD level using an ONIOM-based procedure. The harmonic oscillator (HO) model may be suitable for qualitative "order-of-magnitude" studies of the kinetics of the individual reactions, but the hindered-rotor (HR) model is advisable for quantitative studies.

针对一系列典型的可逆加成-断裂链转移（RAFT）反应：R* + S=C(Z)SCH3 --> R-SC*(Z)SCH3，已进行了高水平的从头算来计算正向和逆向速率系数。其中，R = CH3，Z = CH3、Ph和CH2Ph；以及Z = CH3，R = (CH3)、CH2COOCH3、CH2Ph和C(CH3)2CN。加成反应很快（约10(6)-10(8) L mol(-1) s(-1)），通常比加成到烯烃的C=C键快约三个数量级。断裂速率系数对取代基的性质更为敏感，范围从10(-4)到10(7) s(-1)。在两个方向上，取代基对速率系数的定性影响在很大程度上与对反应平衡常数的影响一致， bulky和自由基稳定的R基团有利于断裂，而bulky和自由基稳定的Z基团有利于加成。然而，有证据表明在某些反应的过渡结构中存在额外的极性和氢键相互作用。从头算在G3(MP2)-RAD//B3-LYP/6-31G(d)理论水平上进行，速率通过变分过渡态理论结合低频扭转模式的受阻转子处理获得。研究了对该方法的各种简化，以确定研究更大的聚合物相关体系的可靠程序。结果表明，使用标准过渡态理论，结合RMP2/6-311+G(3df,2p)水平的从头算，可能会得到合理的结果，前提是使用基于ONIOM的程序将离域体系的结果校正到G3(MP2)-RAD水平。谐振子（HO）模型可能适用于对单个反应动力学进行定性的“数量级”研究，但对于定量研究，建议使用受阻转子（HR）模型。