New high-activity zinc(II)-based catalysts for ring-opening polymerization: Synthesis of stereoregular biodegradable polymers.

摘要

Biodegradable polymers, such as polycarbonates and polyesters, have drawn considerable recent interest as materials in biomedical, pharmaceutical, and agricultural applications, as well as environmentally benign packaging. Our goal is to design well-defined, single-site catalysts for cyclohexene oxide (CHO)/CO₂ copolymerization to produce poly(cyclohexene carbonate) and for rac-lactide polymerization to produce poly(lactic acid).; Several methodologies have been developed to synthesize β-diiminate zinc acetate, methoxide and isopropoxide complexes as a new class of well-defined, highly active catalysts for the synthesis of highly alternating, high molecular weight, and monodisperse CHO/CO₂ copolymers. The crystal structures reveal that all complexes exist as dimers in the solid state. The structures of the complexes both in solid state and solution will be discussed. The polymerization reactions were operated under exceptionally mild conditions: low CO₂ pressure and mild temperature. In terms of ligand modifications, it was determined that bulky substituents on the β-diimine ligands are essential for high activity. By tuning the electronics of the complexes, the turnover frequency (TOF) has been increased dramatically. The highest TOF has been achieved up to 864 mole of monomer unit per mole of zinc per hour. Mechanistic studies of CO₂ insertion and epoxide ring-opening were carried out. Atactic polycarbonate was formed due to the achiral nature of β-diimine ligand. The cyclohexyl ring is trans in configuration, consistent with inversion of stereochemistry through epoxide ring-opening.; A novel class of C₁ symmetric imine-oxazoline (IOX) ligands and complexes has been synthesized. To optimize this class of catalysts for asymmetric copolymerization of cyclohexene oxide and CO₂, systematic ligand modifications were carried out. The chiral IOX(iPr, tBu)ZnN(TMS)₂ (4.17) complex gave completely alternating copolymer in which the percent ee of the chiral diol unit was controlled to be 71%.; β-Diiminate zinc isopropoxide and methyl lactate complexes have been developed as single-site, living initiators for the polymerization of rac-lactide to form heterotactic PLA. The narrow polydispersity and the experimentally determined linear correlation between M_n and percent conversion are indicative of a living polymerization, as well as a single type of reaction. Mass spectrometry experiments reveal that the isopropoxide and methyl lactate groups initiate polymerization, while the β-diiminate ligand remains chelated to the zinc center.