Oxiranes chiral initiator polymerization

In Figure 3 experimental data are plotted obtained for methyl oxirane which was polymerized with a chiral initiator resulting from the reaction of diethylzinc with ( )-3,3-dimethyl-1, 2-butanediol in (1 1) ratio [13]. 

The bulkiness of the alkyl substituent also exerts an enormous influence upon the nature of stereoselective polymerization of alkyloxiranes. When the i ,5-copolymerization of t-butyl-oxirane was carried out starting with a monomer mixture consisting of 5/5=76/24 using t-BuOK as initiator, 5-monomer was found to be incorporated into polymer chain preferentially over 5-monomer This is explained in terms of the growing chain control mechanism, in which the chiral structure of the growing polymer chain is responsible for the stereoselection. 

Influence of the nature of the initiator Initiators resulting from the reaction between an organometallic derivative and a chiral compound containing an acidic hydrogen were the most usually employed in the stereoelective polymerization of oxiranes and thiiranes. 

For example, in the case of oxiranes and thiiranes one must prepare first the corresponding chiral 1,2 diols, and the overall yields from starting materials to final cyclic monomers are sometimes less than 5%. Price and Osgan [1] were the first to polymerize with different initiators the dextrorotatory propylene oxide. Several other optically active oxiranes and thiiranes were successfully prepared and polymerized [2, 3]. 

Fig. 18. Mechanistic scheme of stereoelectrve polymerization of oxiranes and thiiranes using ZnEtj-chiral-l,2-diol initiator system.

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