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

Abstract

We describe the synthesis of the new Zn–N‐heterocyclic carbene (NHC) alkoxide complexes [(S,CNHC)ZnCl(OBn)]2 (5) and [(O,CNHC)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,CNHC)ZnCl(Et)] (3) and [(O,CNHC)ZnCl(Et)] (4), and species 3 and 4 were obtained from the reaction of ZnEt2 with the N‐methyl‐N'‐ethylphenylsulfide (1⋅HCl) and N‐methyl‐N'‐ethylmethylether (2⋅HCl) 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 ZnII 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.

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

“…When rac-lactide is used, no clear preference for (R,R)-lactide vs. (S,S)-lactide insertion was found, therefore atactic polymer formation is predicted. A thorough DFT study of latide ROP initiated by an NHC zinc complex was also reported and showed the assistance of the second metal center [45]. These calculations provided valuable information on the design and development of homogeneous transition and main-group metal polymerization catalysts.…”

Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis, Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

“…Multinuclear metal catalysts, such as yttrium, rare earths, group IV, iron, magnesium, copper, zinc, and aluminum have received enormous attention for ROP of cyclic esters. Tolman and co‐workers reported that the polymerization rate for LA with dinuclear zinc catalysts compares favourably to the fastest catalysts .…”

The Role of Transesterification in SnOct2‐Catalyzed Polymerizations of Lactides

“…The present work had the purpose to elucidate how largely different temperatures and reaction times influence nature and extent of transesterification reactions and, thus, the molecular weight distribution (MWD) of the resulting polylactides. For the sake of completion two recent publications dealing with reaction mechanisms of lactide in the presence of zinc catalysts should be mentioned, but these references are not relevant for the discussion of the present work …”