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Abstract

The alkoxy zinc complexes (±)-[(NNHOt-Bu)Zn(OCH₂Ph)]₂ 4, (±)-[(NNOt-Bu)Zn(OCH₂Ph)]₂ 5, and 6 (±)-[(NNOCm)Zn(OCH₂Ph)]₂ were prepared from the reactions of the corresponding alkyl zinc complexes (±)-(NNHOt-Bu)Zn(CH₂CH₃) 1, (±)-(NNOt-Bu)Zn(CH₂CH₃) 2, and (±)-(NNOCm)Zn(CH₂CH₃) 3, respectively, with benzyl alcohol (PhCH₂OH). All zinc compounds 1-6 were characterized by ¹H and ¹³C{¹H} NMR spectroscopy. The molecular structures of 2-6 were characterized by single-crystal X-ray crystallography. The alkyl zinc complexes are mononuclear with a tridentate coordination mode and a distorted tetrahedral geometry around the zinc metal centers. The alkoxy zinc catalysts show a dimeric feature containing two zinc metal centers bridging through the benzyl alkoxy oxygen atoms with different coordination modes for all three catalysts. The solution denticities of 4-6 were further investigated through 1D and 2D NMR spectroscopy experiments. Exchange studies with pyridine and complexes 4-6 were also conducted. The catalytic activities of alkyl and alkoxy zinc complexes 1-6 toward ring opening polymerization (ROP) of racemic lactide (rac-LA) have been studied by ¹H NMR spectroscopy, Gel permeation chromatography, and MALDI-TOF mass spectrometry. Experimental results indicate that the alkyl zinc compounds 1-3 show relatively less activity toward ROP of cyclic esters than the alkoxy zinc compounds 4-6. Catalysts 5 and 6 demonstrate a better control over experimental molecular weights and dispersity values of PLA, and more stereoselective than 4. Catalyst 4 undergoes the depolymerization reactions. Different transesterification side reactions operate at low equivalency polymerization studies to a different extent for all alkoxy zinc complexes. Immortal ROP of alkyl zinc compounds 1-3 was investigated in the presence of a chain transfer agent. The obtained results reveal the controlled nature of iROP.