Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency
Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency
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DOI：
10.1016/j.eurpolymj.2020.109917
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发表时间：
2020-08
影响因子：
6
通讯作者：
Annabel Rae;Anand J. Gaston;Zoe Greindl;Jennifer A. Garden
中科院分区：
文献类型：
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作者：
Annabel Rae;Anand J. Gaston;Zoe Greindl;Jennifer A. Garden
众所周知，铝-烷基配合物是丙交酯开环聚合的引发剂，然而，尽管铝-氯化配合物具有易于合成和改善空气稳定性等优点，但仍未得到充分开发。虽然氯化铝配合物通常是较差的引发剂，但最近的研究表明，富含电子的氨基取代（salen）AlCl配合物可以在环氧化物存在的情况下有效地引发丙交酯聚合。本文中，我们报道了八种醚取代配合物作为丙交酯聚合的有效引发剂，其中将强给电子氨基交换为弱给电子甲氧基取代基保持了有效的引发，并将繁殖速率提高了四倍。对邻甲氧基取代、间甲氧基取代、对甲氧基取代和间甲氧基取代区域异构体的研究表明，邻甲氧基取代配合物的活性是其他区域异构体的两倍。动力学和光谱研究表明，引发效率受电子（正位和对位>元和元‘）的影响，更接近Al的取代基可以提高引发效率（邻位>元和元’ >元）。在内酯聚合过程中，给电子的邻位取代基往往会降低催化剂活性，而醚基可以作为σ-吸电子基团和π-给电子基团，从而提高增殖速率、引发率和活性控制。
Aluminium-alkyl complexes are well known as initiators for lactide ring-opening polymerisation, yet aluminium-chloride complexes remain underexplored despite benefits such as ease of synthesis and improved air-stability. While aluminium-chloride complexes are typically poor initiators, recent studies have shown that electron rich amino-substituted (salen)AlCl complexes can efficiently initiate lactide polymerisation in the presence of an epoxide. Herein, we report eight ether-substituted complexes as efficient initiators for lactide polymerisation, where exchanging strongly electron-donating amino groups for weaker electron-donating methoxy substituents maintains efficient initiation and also improves the propagation rate by a factor of four. Investigation ofortho-,meta-,para-andmeta’-methoxy-substituted regioisomers established that theortho-substituted complex was twice as active as the other regioisomers. Kinetic and spectroscopic studies suggest that the initiation efficiency is influenced by the electronics (orthoandpara>metaandmeta’), with substituents closer to Al giving improved initiation (ortho>paraandmeta’>meta). While electron-donatingortho-substituents often decrease catalyst activity in lactone polymerisation, here we show that ether groups can act as σ-electron-withdrawing groups and π-electron donors, to deliver improved propagation rates, initiation and tacticity control.