Steric Control in Polymerizations of Oxiranes

Explain the mechanism postulated by Tsunita of steric control in polymerizations of oxiranes with the aid of oiganozinc compounds, giving the structure of the catalyst and the mode of monomer insertion and the mode or ring opening. 

Discuss the general characteristics of steric control in the polymerizations of oxiranes. 

Tsuruta, T. Molecular level approach to the origin of the steric control in organozinc catalyst for oxirane polymerization. Makromol. Chem., Macromol. Symp. 1986,6, 23-31. 

Stereoelective polymerization of a racemic monomer is a sterical control polymerization process in which one of the monomeric antipodes is preferentially polymerized. If the conversion is not complete, both polymer and recovered monomer are optically active. It is synonymous with asymmetric-selection polymerization the yield can be simply evaluated from the optical activity of unreacted monomer. This theoretically important method of synthesis concerns, for the moment, mainly the polymerization of racemic three-membered heterocycles (oxiranes, thiiranes and aziridines) and of a-amino-7V-carboxy acid-anhydrides (or Leuch s salts). 

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