Tiirane Coordination Polymerisation

1 Tiirane Coordination Polymerisation with Catalysts Containing Multinuclear Species 

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 

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

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]. 

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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. 

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