Dinuclear Zinc-N-Heterocyclic Carbene Complexes for Either the Controlled Ring-Opening Polymerization of Lactide or the Controlled Degradation of Polylactide Under Mild Conditions
Dinuclear Zinc-N-Heterocyclic Carbene Complexes for Either the Controlled Ring-Opening Polymerization of Lactide or the Controlled Degradation of Polylactide Under Mild Conditions
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DOI：
10.1002/cctc.201301015
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发表时间：
2014-05-01
期刊：
影响因子：
4.5
通讯作者：
Aviles, Teresa
中科院分区：
文献类型：
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作者：
Fliedel, Christophe;Vila-Vicosa, Diogo;Aviles, Teresa
合成了用于丙交酯聚合的新型锌氮杂环卡宾(NHC)醇盐配合物[(S，C-NHC)锌氯(OBN)](2)(5)和[(O，C-NHC)锌氯(OBN)](2)(6)作为开环聚合引发剂。配合物5和6分别与N-甲基-N‘-乙基苯硫醚(1HCl)和N-甲基-N’-乙基甲醚(2HCl)咪唑盐类化合物发生醇解反应而得到。在所研究的条件下(反应条件：CH2Cl2或THF，室温)，苯甲酸锌物种5和6的溶液和固体结构数据均符合二聚体结构。对物种3和4的计算分析支持溶液中的二聚体结构。研究发现，锌-Ⅱ醇盐物种5和6可以有效地和可控地调节丙交酯的ROP以生成链长可控的聚乳酸，或者在甲醇等乙醇存在的情况下，通过广泛的酯交换反应控制聚乳酸的降解，得到主要产物乳酸甲酯。对络合物5引发的丙交酯的ROP进行了详细的密度泛函计算分析，结果表明，第二个锌中心辅助了丙交酯的工作配位插入机制，导致了较低的ROP过程，这一结果可能对未来设计定义明确和高性能的金属基催化剂具有重要意义。
We describe the synthesis of the new Zn-N-heterocyclic carbene (NHC) alkoxide complexes [(S,C-NHC)ZnCl(OBn)](2) (5) and [(O,C-NHC)ZnCl(OBn)](2) (6) for use as ring-opening polymerization (ROP) initiators for lactide polymerization. Complexes 5 and 6 are readily available through an alcoholysis reaction between BnOH and the corresponding Zn-NHC ethyl species [(S,C-NHC)ZnCl(Et)] (3) and [(O,C-NHC)ZnCl(Et)] (4), and species 3 and 4 were obtained from the reaction of ZnEt2 with the N-methyl-N'-ethylphenylsulfide (1HCl) and N-methyl-N'-ethylmethylether (2HCl) imidazolium salts, respectively. Both solution and solid-state structural data for Zn benzyloxide species 5 and 6 agree with dimeric structures under the studied conditions (reaction conditions: CH2Cl2 or THF, room temperature). A computational analysis of species 3 and 4 supports a dimeric structure in solution. The Zn-II alkoxide species 5 and 6 were found to mediate either the ROP of lactide (in an effective and controlled manner) to produce chain length-controlled polylactide (PLA) or, in the presence of an alcohol source such as MeOH, the controlled degradation of PLA through extensive transesterification reactions to afford methyl lactate as the major product. A thorough DFT computational analysis of the ROP of lactide initiated by complex 5 was performed, which revealed that the operating coordination-insertion mechanism was assisted by the second Zn center, leading to a lower-energy ROP process; this result may be of interest for the future design of well-defined and high-performance metal-based catalysts.