Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators
Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators
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DOI：
10.1073/pnas.0602765103
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发表时间：
2006-10-17
影响因子：
11.1
通讯作者：
White, Andrew J. P.
中科院分区：
文献类型：
--
作者：
Hormnirun, Pirnpa;Marshall, Edward L.;White, Andrew J. P.
本文合成了一系列具有不同空间位阻和电子性质的Salen型铝配合物[Salen为N，N '-双（水杨醛亚胺）-1，2-乙二胺]，并研究了其对丙交酯的聚合作用。关键预催化剂的X-射线晶体结构揭示了三角双锥和正方锥之间的金属配位几何结构。苯氧基取代基和主链连接基对聚合都有显著影响。吸电子基团连接到苯氧基供体一般得到增加的聚合速率，而大的邻位取代基一般减慢聚合。绝大多数的引发剂提供的聚乳酸与全同立构的偏见，只有一个表现出偏向异源选择性。等选择性通常随着骨架连接体的柔性增加而增加，这被认为能够更好地适应增长聚合物链、插入单体单元和苯氧基供体上的取代基之间的任何潜在的空间冲突。
A series of aluminum salen-type complexes [where salen is N,N'-bis(salicylaidimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.