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

Abstract

In 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.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

“…Finally, very interesting results were obtained at the intersection of coordination catalysis and organocatalysis when LA ROP was initiated by ZnEt 2 and N , N -dimethyleminopyridine (DMAP) complex 61 [139]. DFT calculations of the stationary points and transition states were performed at the B3LYP [53] level with the 6-31G [144,145,146] and LANL2DZ [78] basis sets for three possible reaction pathways.…”

“…Mononuclear metal complexes studied in DFT modeling of lactide ROP [50,75,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140]. …”

“…The optimized structure of TS1′ and TS2′ were not obtained. Reprinted with permission from [139]. Copyright (2018) Wiley-VCH Verlag GmbH & Co.…”

Simple In-Based Dual Catalyst Enables Significant Progress in ε-Decalactone Ring-Opening (Co)polymerization

“…We recently used the association of Zn derivatives with tertiary amines (DMAP, TMP) to polymerize lactide (and caprolactone) and access macrocyclic polymers. 51,52 In addition, M Hillmyer reported in 2010 53 a detailed study on the ability of the InCl3/NEt3 association to act as catalyst for the controlled ROP of lactide. In the same article, preliminary results were disclosed on the ROP of several lactones including one example with ε-Me-ε-CL, suggesting that this dual catalytic system is able to propagate ROP via secondary In-alkoxide moieties.…”

Application of Diethylzinc/Propyl Gallate Catalytic System for Ring-Opening Copolymerization of rac-Lactide and ε-Caprolactone

“…Zinc salts, such as lactate [13], aceturate, or L-prolinate [27] were found to be active catalysts for ROP of LLA. Organic zinc complexes were also successfully used as initiators/catalysts for the homopolymerization of LA [28,29,30,31], CL [32,33] or copolymerization of CL, and LA [34,35,36]. Zinc-based catalysts may be a viable alternative to SnOct 2 due to low toxicity and programmable polymer chain microstructure.…”