Enantioasymmetric polymerisation

The first enantioasymmetric polymerisation of (R, propylene oxide, reported in 1962, was carried out in the presence of the diethylzinc (+)-borneol catalyst [23,24]. Other optically active catalysts for propylene oxide stereoelective polymerisation, e.g. diethylzinc (R)-( )-3,3-dimethyl-1,2-butanediol were described later on [25]. 

As regards the dimethylcadmium-(7 )-(—)-3,3-diethyl-1,2-butanediol (1 1) catalyst, it was characterised by a lower stereoelectivity than the diethylzinc-(R)-(—)-3,3-dimethyl- 1,2-butanediol (1 1) catalyst in enantioasymmetric polymerisations of propylene sulphide, cA-2-butene sulphide and cyclohexene sulphide, but the elected chirality was opposite to that found for polymers obtained with the zinc-based catalyst [159]. 

The above stereospecific tiirane polymerisations have generally been run in heterogeneous systems. Such conditions essentially make it impossible to determine the detailed structure of active species involved in these polymerisations. Thus, enantiosymmetric and enantioasymmetric polymerisations of propylene sulphide have also been studied in a homogeneous phase by using chiral cadmium thiolates of cysteine esters and chiral cadmium carboxylates of cysteine and methionine [157,160-164]. The most studied is living polymerisation using the cadmium derivative of the isopropyl ester of (.S)-cysteine [160] ... 

Characterise the stereoselective (enantiosymmetric) polymerisation and stereoselective (enantioasymmetric) polymerisation of heterocyclic monomers. Give examples. 

Polymerisations of tiiranes in the presence of coordination catalysts containing multinuclear species have been extensively studied in terms of their stereoselective and stereoelective behaviour. For monosubstituted tiiranes, the polymerisation can proceed enantiosymmetrically and lead to a mixture of isotactic chains of opposite configurations. By using optically active catalysts, the polymerisation may occur enantioasymmetrically, with the enchainment of only one of the two enantiomers. 

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