Thietans and Thiiranes Cyclic Sulphides

A number of 1,3-episulphides (thietans) have been shown to polymerise readily by cationic means using stable salts as initiators (53). The most studied monomer is 3,3 -dimethylthietan (33,52,53) which gives reasonable rates of 

Polymerisations are characterised by a fast initiation process generating strained cyclic sulphonium ion active centres, which then propagate, before terminating by formation of stable linear tertiary sulphonium ions by reaction with sulphur residues on the polymer backbone, e.g. 

The bimolecular termination reaction can be neglected at low conversions since no linear sulphide residues are present initially, enabling a simpler interpretation of initial conversion/time curves to be made. In the earlier work the concentration of active centres was equated with the initial catalyst salt concentration, but later an XH NMR method of analysis was employed (137). As in the polymerisation of tetrahydrofuran it was anticipated that both free ions and ion pairs were likely to contribute to the propagation reaction and the calculated rate constant kP.pp e t, was described by 

Values of a were computed assuming the dissociation constant Kd, for the polymeric ion pair to be the same as that for some model sulphonium ion salts (see Table 1, Section III.B), and hence a plot of kpappBrent versus a allowed estimation of kp and kp separately. The results obtained are shown in Table 10. 

These data were confirmed in experiments involving addition of excess common counterion salt. In methylene chloride solvent the SbClg ion pair seems to be 

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