Primary structure and chiroptical properties of polydiasteriomers obtained by steroelective polymerization of N-[(S)-sec-butyl] and N-[(R)-sec-butyl-N-methyl-N-(R,S)-thiirane-2-ylmethyl] amine isomers

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Stereoisomers of N-( sec-butyl)- N-methyl- N-((R,S)-thiirane 2-ylmethyl) amine ((S)- sec-butyl, optical purity O.P. ∼94% and (R)- sec-butyl, O.P. ∼26%) were polymerized using a chiral initiator system (1:1) ZnEt 2 R(-)3,3- dimethyl-1,2- butanediol (R(-)BMBD) (stereoelective polymerization). In agreement with the homosteric character of this initiator, the configuration of the asymmetric centre located in the heterocycle of the elected diastereoisomer is shown to be R from c.d. spectra of residual monomers, whatever the configuration of the asymmetric centre located in the pendant group. O.r.d. and c.d. spectra of stereoelected polydiastereoisomers are described and compared to those of corresponding polydiastereoisomers bearing racemic main-chain chiral centres. 13C n.m.r. spectra of different polydiastereoisomers are reported. The stereosensitivity to tacticity of the main-chain carbons is poor but some of the side-chain carbon atoms are very sensitive to the presence of two asymmetric centres per repeat unit. The optical purity of main-chain chiral centres is deduced from the split resonance of one of these carbons atoms. It is shown that chiroptical properties of polydiastereoisomers primarily depend on configurations of the two chiral sites without obvious contributions from macromolecular conformations. The 242 nm c.d. band assigned to the n → σ∗ electronic transition of sulphur chromophores is sensitive only to the optical purity of main-chain chiral centres and can be used to evaluate the configurational enrichment due to the stereoelective polymerization.