Structure-reactivity relationship of catalysts for ring-opening polymerization of some oxiranes

Abstract

This paper will review structure--reactivity relationship of organozinc and other organometallic catalysts in the ring—opening polymerizations of some oxiranes. The catalytic behavior of diethylzinc-water system was changeable in response to the ratio of diethylzinc to water. GPC study of a poly(methyloxirane) sample prepared by Et2Zn/H20(l/O.l) showed the cationic nature of this catalyst system in contrast with the anionic nature of Et2Zn/H20(l/l) system. Any of catalyst systems consisting of Et2Zn and alcohol did not exhibit cationic nature irrespective of the mole ratio of the two components. Active species operative in the Et2Zn/CH3OH system was shown to be zinc dimethoxide. The reaction mechanism for the stereospecific polymerization of methyloxirane catalysed by zinc dimethoxide was elucidated in terms of the enantiomorphic catalyst site control model. Results of stereoselective and asymmetric selective (or stereoelective) polymerizations of some thiirane and oxirane with optically active organo—zinc or —cadmium catalyst were discussed in the light of recent studies with our model catalyst having a welldefined structure. Poly(methyloxirane) prepared with Nmethyl-a, ,y, c5-tetraphenylporphin/ZnEt2 as catalyst was found to have a narrow molecular weight distribution and an unusual percentage of the triad tacticities. Since the porphin catalyst system was able to form a living system, it was possible to prepare block copolymers with 100% efficiency. A novel type of asymmetric selective (or stereoelective) polymerization of epichlorohydrin was found to take place with an optically active salen-type complex as catalyst. The nature of some catalysts for copolymerization of oxirane with carbon dioxide was also discussed in comparison with that for oxirane homopolymerization. 'We have long been interested in the reactivity and stereospecificity of organozinc compounds as catalyst for oxirane polymerization (Refs. 1-5). In this paper, the writer will review structure--reactivity relationship of organozinc and other organometallic catalysts in the ring-opening polymerizations of some oxiranes. 1. DIETHYLZINC-WATER AND DIETHYLZINC-ALCOHOL SYSTEM AS CATALYST FOR METHYLOXIRANE POLYMERIZATION It is widely recognized that Et2Zn-H20 system is the most active catalyst for the stereospecific oxirane polymerization (Ref. 6). A variety of chemical species operative in the polymerization reaction were previously shown to be formed according to the following equations: ZnEt2 + H20 — EtZnOH + EtH