From Zn(C6F5)2 to ZnEt2‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

Abstract

AbstractIn this contribution, the structure‐reactivity relationships of ZnR2/DMAP (R=C6F5, C6H5 and C2H5) Lewis pairs in ring‐opening polymerization (ROP) of lactones were investigated by crystallographic analysis, density functional theory (DFT) calculations and kinetic studies. With decrease of ZnR2 Lewis acidity, the interaction between ZnR2 and DMAP weakens and the dissociation of ZnR2⋅2DMAP Lewis adducts become easier, which facilitates the activation of cyclic ester monomers. Thus, the ZnEt2/DMAP Lewis pair, bearing weakest interaction between Lewis acid and Lewis base, exhibits high catalytic activity and broad monomer adaptability. ZnEt2⋅2DMAP can convert lactide (LA) rapidly into polylactide (PLA) even at room temperature. Furthermore, a wide range of cyclic esters can also be polymerized using this dual catalyst, from small lactones such as β‐butyrolactone (β‐BL), δ‐valerolactone (δ‐VL) and ϵ‐caprolactone (ϵ‐CL) to the strainless macrolactone. The optimal reaction pathway, key species and active species in the ZnEt2⋅2DMAP catalytic ROP of LA were figured out by DFT calculations. The results clearly indicated that a DMAP‐LA exchange was necessary for activation of monomer, and the single molecular initiation is preferred. Meanwhile, the cyclic active species is more stable than the linear analogues, in which DMAP and ZnEt2 bond with each polymer chain end respectively and ZnEt2 interact with DMAP. The DFT calculation gives an account for the formation of the cyclic polyester in the ROP of LA by ZnR2‐based Lewis pairs.