Dinuclear Zinc-N-Heterocyclic Carbene Complexes for Either the Controlled Ring-Opening Polymerization of Lactide or the Controlled Degradation of Polylactide Under Mild Conditions

摘要

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.