Stasiw Daniel E, Mandal Mukunda, Neisen Benjamin D, Mitchell Lauren A, Cramer Christopher J, Tolman William B
Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Minnesota Supercomputing Institute (MSI), University of Minnesota , 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States.
Inorg Chem. 2017 Jan 17;56(2):725-728. doi: 10.1021/acs.inorgchem.6b02849. Epub 2016 Dec 22.
Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d by H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local minimum. Nor were local minima for cis-bound precomplexes found, suggesting a concerted coordination-insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the "resting" ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymerization catalysts.
在110°C下，于氘代甲苯中通过核磁共振氢谱（¹H NMR）监测使用旨在防止无效反式配位的烷氧基铝催化剂（1）引发的ε-己内酯（CL）聚合反应，并将浓度与时间数据拟合为一级速率表达式。将1的半衰期与许多其他烷基铝和烷氧基铝配合物的值进行比较，结果表明1对CL聚合反应的活性要低得多。采用密度泛函理论计算来理解动力学缓慢的原因。结果发现，1的配体骨架的优化几何结构确实使CL难以发生反式配位：未发现反式配位中间体是局部极小值。也未发现顺式配位预络合物的局部极小值，这表明聚合物引发和增长过程存在协同配位插入。1的反应迟缓归因于将“静止”配体几何结构变形为在相应过渡态结构几何中提供最佳催化作用的几何结构所需的高骨架畸变能，因此表明在高效聚合催化剂设计中需要考虑配体的灵活性。
