Biguanide Mono- and Bimetallic Zinc Complexes for the Highly-Efficient 1 Ring-Opening (Co)polymerization of Lactides

Abstract

One of the most urgent social demands on polymer chemistry is the design of an inexpensive, efficient, robust and non-toxic catalyst for the preparation of biodegradable polymers with good control of its properties. The dissymmetric ditopic doubly-deprotonable biguanide proligand (substituted 4,6-dimethoxypyrimidin-2-yl-guanidine – 1) with dynamic behavior forms monometallic as well as bimetallic complexes when reacted with one or two equivalents of Et2Zn or Zn[N(SiMe3)2]2. Zinc atoms primarily occupy a position within the six-membered ring. In complex 5, their positions can be interchanged, which could explain the ability of this “dissymmetric” bimetallic complex to promote the ring-opening polymerization (ROP) of lactide (LA) at both Zn sites to form polylactide (PLA) chains with a unimodal molecular-weight distribution. In 3 and 5, the adjacent ethylzinc- and zinc-amide moieties are coordinated in a bidentate fashion by the guanidinate-like part of the ligand. The ethylzinc complexes 2 and 3 are inactive in the ROP of rac-lactide, whereas the performance of the zinc amides 4 and 5 activated by iPrOH is among the highest observed. One-pot preparation of various di- or oligoblock copolymers is possible by the consecutive living copolymerization of β-butyrolactone and L-, D- or rac-lactides, leading to the precise control of its microstructure.