Synthesis and X-ray Crystal Structure of Dichloro[N-{6-methyl-2-pyridyl)methyl}-(S)-1-phenylethylamine]zinc(II) and Its Catalytic Application to rac-Lactide Polymerization

Abstract

Poly(lactic acid) or polylactide (PLA) is a biodegradable synthetic polymer. A typical degradation time is months to years for L-PLA and weeks to months for rac-PLA. This time scale is several orders of magnitude shorter than that of petroleum-based polymers, which makes it possible to compost PLA waste just into soil amendments. In addition to this green benefit, PLA’s raw material is from annually renewable resources such as corn and sugar beats or even from the food wastes. Moreover, basic physical properties of PLA that include narrow MWD, low haze, high mechanical strength, high gas permeability and easy processibility are acceptable enough to utilize this material in many applications such as fiber, film, and blow molding. Due to these merits not to mention skyscraping natural oil price, PLA have emerged as an environmentally friendly plastic materials that can replace many petroleum-based ones. It has been known that the catalyst-initiated ring-opening polymerization reaction of purified lactide is advantageous over the condensation reaction of lactic acid in terms of a higher polymerization rate, a higher MW, and a better conversion. Among many different kinds of metal species for the polymerization catalysts, relatively non hazardous magnesium and zinc, or aluminum have attracted more attention than highly toxic tin, lead and bismuth, or lanthanides because the major application fields of PLA are biomedical and food packaging and even ppm level catalyst residues are potentially harmful for these application. Along this line, we attempted to prepare PLA with the newly synthesized zinc complex ligated by the chiral N-{6-methyl-2-pyridinyl)methyl}-(S)-1-phenylethylamine (MPMA) ligand. Instead of a direct complexation reaction between MPMA and dialkylzinc to prepare an active catalyst species as in our previous work, we first synthesized air and moisture stable (MPMA)ZnCl2 complex and generated the active catalyst species by treating it with benzylmagnesium chloride (BnMgCl) in situ. Herein, we report the synthesis of MPMA ligand as well as its ZnCl2 complex, the X-ray crystal structural characterization of the complex, and the polymerization results of rac-lactide with in situ generated (MPMA)ZnBn2.