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

One of the most urgent social demands on polymer chemistry is the design of an inexpensive, efficient, robust, and nontoxic catalyst for the preparation of biodegradable polymers with good control of its properties. The nonsymmetric 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. In monometallic complexes 2 and 4, zinc atoms primarily occupy a position within the six-membered ring. In the bimetallic complexes 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 ring-opening polymerization (ROP) of rac-lactide, whereas the performance of the zinc amides 4 and 5 activated by ⁱPrOH is among the highest observed. In complex 5, the positions of the two zinc ions can be interchanged, which could explain the ability of this “nonsymmetrical” bimetallic complex to promote the ROP of lactide (LA) at both Zn sites to form polylactide (PLA) chains with a unimodal molecular-weight distribution. One-pot preparation of various di- or oligoblock copolymers is possible by the sequential living copolymerization of β-butyrolactone and L-, D- or rac-lactides, leading to the precise control of its microstructure.