University of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy.
Dalton Trans. 2013 Feb 28;42(8):2792-802. doi: 10.1039/c2dt31264f.
A series of group 4 metal tetracarbamates M(O(2)CNR(2))(4) (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; (i)Pr, 1c; M = Hf, R = Et, 1d; R = (i)Pr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of (i)PrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94,000 g mol(-1). The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO(2) and NHR(2). The polymerization mediated by Ti(O(2)CNEt(2))(4) involves the release of a radical fragment [O(2)CNEt(2)]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).
一系列第 4 族金属四异氰酸酯 M(O(2)CNR(2))(4)（M = Ti，R = Et，1a；M = Zr，R = Et，1b；(i)Pr，1c；M = Hf，R = Et，1d；R = (i)Pr，1e）被用作 rac-丙交酯溶液聚合中的催化前体。添加 (i)PrOH 可增加钛配合物而不是锆和铪配合物的活性。异氰酸酯配体的结构显著影响聚合物的摩尔质量；具有异丙基异氰酸酯配体的配合物可得到摩尔质量高达 94,000 g mol(-1)的 PLA。通过光谱和计算分析概述了聚合反应初始阶段的主要机理方面。对于锆和铪配合物，在室温下，异氰酸酯配体和一个丙交酯分子的 CH 单元之间建立相互作用。这种相互作用随后是丙交酯向异氰酸酯 O-原子的高温质子转移，提供具有 CO(2)和 NHR(2)释放的催化活性烷氧基配合物。Ti(O(2)CNEt(2))(4)介导的聚合涉及自由基片段 [O(2)CNEt(2)]˙的释放，随后生成 Ti(III)中心。增长链是与 Ti(IV)中心配位的醇盐配体，并且包含主要位于链尾部的自由基（DFT，EPR）。
