Joint Laboratory of Polymer Science and Materials, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
J Phys Chem A. 2010 Feb 25;114(7):2697-700. doi: 10.1021/jp910712v.
The mechanism of ethylene insertions into eight tertiary diamine/n-butyllithium complexes has been studied at the BLYP/DNP level. In contrast to the cationic coordination polymerization in which a strong coordination complex between ethylene and the metal center is formed prior to ethylene insertion, there is only a weak van der Waals complex between ethylene and tertiary diamine/n-butyllithium complex. After crossing a four-membered-ring transition state, ethylene inserts into the Li-C bond. The insertion barriers for the eight reactions are in the range of 6.9-11.0 kcal/mol, comparable to those of ethylene cationic coordination polymerizations. However, the polymerization activities of ethylene anionic polymerizations are much lower than those of cationic coordination polymerizations. Comparing the energy profiles of these ethylene anionic polymerizations with those of cationic coordination polymerizations, it can be found that the transition states in the ethylene anionic polymerizations are higher in energy than the reactants, while the transition states in ethylene cationic coordination polymerizations are lower than the reactants. Therefore, ethylene anionic polymerizations need additional energy to climb the energy barriers, while the energies for overcoming the transition states in the cationic coordination polymerizations can be obtained from reactants that are higher in energy than the reactants. We reason the differences in their energy profiles could be one of the reasons for the lower activity of ethylene anionic polymerization than ethylene cationic coordination polymerization despite their comparable insertion barriers.
在 BLYP/DNP 水平上研究了乙烯插入八个叔二胺/正丁基锂配合物的机理。与阳离子配位聚合不同，阳离子配位聚合中乙烯与金属中心在乙烯插入之前形成强配位络合物，而在叔二胺/正丁基锂配合物中，乙烯与配合物之间只有较弱的范德华络合物。经过一个四元环过渡态后，乙烯插入到 Li-C 键中。这八个反应的插入势垒在 6.9-11.0 kcal/mol 范围内，与乙烯阳离子配位聚合相当。然而，乙烯阴离子聚合的聚合活性远低于阳离子配位聚合。将这些乙烯阴离子聚合的能量曲线与阳离子配位聚合的能量曲线进行比较，可以发现乙烯阴离子聚合的过渡态的能量高于反应物，而乙烯阳离子配位聚合的过渡态的能量低于反应物。因此，乙烯阴离子聚合需要额外的能量来克服能垒，而阳离子配位聚合中克服过渡态所需的能量可以从能量高于反应物的反应物中获得。我们认为，它们的能量曲线的差异可能是乙烯阴离子聚合活性低于乙烯阳离子配位聚合的原因之一，尽管它们的插入势垒相当。
