A density functional theory study of the mechanisms of scandium-alkoxide initiated coordination–insertion ring-opening polymerization of cyclic esters

Ring-opening polymerization of lactides and ε-caprolactone catalyzed by Zn(II) aryl carboxylate complexes supported by 4-pyridinyl schiff base ligands

Synthesis and catalytic studies of aryl carboxylate Zn (II) complexes is reported. Reaction of substituted (E)-N-phenyl-1-(pyridin-4-yl)methanimine with a methanolic solution of Zn(CH₃COO)₂ and substituted aryl carboxylate co-ligands gave heteroleptic Zn(II) complexes; [Zn(C₆H₅COO)₂(L1)]₂ (1), [Zn(C₇H₇COO)₂(L1)]₂ (2), [Zn (4-F-C₆H₄COO)₂(L1)]₂ (3), [Zn(C₆H₅COO)₂(L2)]₂ (4), [Zn(C₇H₇COO)₂(L2)]₂ (5), [Zn (4-F-C₆H₄COO)₂(L2)]₂ (6), [Zn(C₆H₅COO)₂(L3)]₂ (7), [Zn(C₇H₇COO)₂(L3)]₂ (8), [Zn (4-F-C₆H₄COO)₂(L3)]₂ (9). The molecular structures of complexes 1 and 4 are dinuclear with the zinc atom in complex 1 adopting a distorted trigonal bipyramidal geometry in a bi-metallacycle while complex 4 is square pyramidal where all four benzoate ligands bridge the zinc metals in a paddle wheel arrangement. All complexes successfully initiated mass/bulk ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) and lactides (LAs) monomers with or without alcohol co-initiators at elevated temperatures. Complexes 1, 4 and 6 containing the unsubstituted benzoate co-ligands were the most active in their triad; with complex 4 being the most active (k _app) of 0.3450 h^-1. The physicochemical properties of the polymerization products of _l-lactide and rac-lactide in toluene revealed melting temperatures (T_m) between 116.58 °C and 188.03 °C, and decomposition temperatures between 278.78 °C and 331.32 °C suggestive of an isotactic PLA with a metal capped end.

Comparative bindings of lactones, lactide, and cyclic carbonates: experimental insights into the coordination step of polymerization

Comparative bindings of several renowned monomers were investigated experimentally using B(C₆F₅)₃ as a Lewis acid model for the coordination step in ring-opening polymerization. A complete series of the X-ray crystal structures of the B(C₆F₅)₃ adducts with the monomers was reported. The X-ray structural studies and spectroscopic data revealed a coordination strength in the order lactones > tetrahydrofuran > cyclic carbonates > lactide.

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

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

The Role of Alkoxide Initiator, Spin State, and Oxidation State in Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Iron Bis(imino)pyridine Complexes

Density functional theory (DFT) is employed to characterize in detail the mechanism for the ring-opening polymerization (ROP) of ε-caprolactone catalyzed by iron alkoxide complexes bearing redox-active bis(imino)pyridine ligands. The combination of iron with the non-innocent bis(imino)pyridine ligand permits comparison of catalytic activity as a function of oxidation state (and overall spin state). The reactivities of aryl oxide versus alkoxide initiators for the ROP of ε-caprolactone are also examined. An experimental test of a computational prediction reveals an Fe(III) bis(imino)pyridine bis-neopentoxide complex to be competent for ROP of ε-caprolactone.