Polymerisation of Tiiranes

Although thiacyclic monomers do not play such an important role in coordination polymerisation as oxacyclic monomers, some polymerisation of tiiranes with coordination catalysts deserves attention, especially considering the polymerisation stereochemistry  

Note that some general considerations of ring-opening polymerisation originate from studies of the coordination polymerisation of tiiranes. Other thiacyclic monomers that contain an endocyclic sulphur atom and an exocyclic oxygen atom, such as monothiocarbonates, are instead analogous to five-membered cyclic carbonates. 

The ruthenium(II) - ethylene episulphide complex [152] may serve as a model for the coordination of tiiranes at the catalyst active site. 

Polymerisations of tiiranes have been carried out in the presence of various catalysts containing multinuclear species, such as those formed in diethylzinc or dimethylcadmium-water, alcohol, diol, thiol or carboxylic acid systems, as well as catalysts containing mononuclear species, such as porphinatozinc thiolate (Tables 9.1 and 9.5). 

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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. 

The enantiomorphic catalyst sites control mechanism was found to operate in the stereospecific polymerisation of tiiranes. Sigwalt et al. [79,153] found that... 

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] ... 

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