Antisteric process

I = 0.44).When the same reagents are reacted 1n (1 0,5) amounts tne choice is opposite (1=4 antisteric process). 

The stereoelective choice of monomers is however in agreement with configuration rules as seen from the sign of optical activity of unreacted monomer. Homosteric and antisteric processes are observed with considerable amount of a-scission (39). According to the chemical composition of these initiators a cationic character of the latter seems to be excluded and therefore this particular behaviour could be due to some steric reasons which are not yet completely understood. New studies are now in progress. 

It was foimd however, that in some other cases the choice corresponds to opposite configuration, that is to an antisteric type process. 

Fig. 6. Polymerization of methyl thiirane using various chiral initiators. Variation of the optical activ-ity ajj (neat, dm) of residual monomer with conversions. homosteric type process using ZnEtj — (-) -tBu—CHOH—CHjOH (1 1) as initiator system antisteric type process using CdMej —(-) -tBu—CHOH—CH OH (1 1) as initiator system...

When the monomer chosen corresponds in configuration to the chiral agent used, the process is called homosteric when it is of opposite configuration the process is called antisteric . 

By extension one may call a homosteric type initiator an initiator which gives an homosteric stereoelection with a given monomer. Antisteric type process were observed in the case of methyl thiirane [19, 23, 24] and more recently in the case of methoxy methyl thiirane [25] and ethyl thiirane [26]. However, f-butyl thiirane did not give inversion with conventional antisteric initiators. This could be due to its high sterical hindrance. 

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