Stereochemical Aspects of the Polymerization of Cis and Trans Thiiranes Using Chiral Initiators
Stereochemical rules and kinetic behavior of the resolution reaction were established in the caso of ring-opening polymerization of monosubstituted cyclic monomers using chiral initiators.
The use of the same initiators is now applied to disubstituted monomers.
Trans dimethylthiirane, a racemic monomer, mixture of two stereoisomers bearing two asymmetric carbons of identical configuration, give by polymerization optically inactive composed of -RS- units. This confirms the complete inversion of the attached assymetric carbon during ring-opening as established previously by Vandenberg. Polymers obtained are crystalline and of high molecular weight. Unusual kinetics were observed for the resolution reaction in which the sign of the unreacted monomer changed at a definite conversion.
The polymerization of cis dimethylthiirane, and achiral monomer of meso configuration, produces optically active polymers. This result shows that the initiator is able to recognize the donfiguration of an asymmetric carbon in a molecule and to direct an attack on a preferential side in agreement with a homochirality rule. The enantiomeric distribution in the polymer chain, i. e. the proportion of -RR- and -SS- type configurational units, was established by NMR and from cationic degradation studies. The formation of disulfide linkages in substantial amounts was observed with some of the initiator systems used. However, the polymers are generally crystalline and their melting points are different from those of optically inactive polymers prepared with a chiral initiators.
KeywordsCotton Effect Initiator System Disulfide Linkage Rotatory Power Configurational Unit
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