Influence of basicity and ring strain

The factors most commonly mentioned as important in studies of copolymerization of these cyclic compounds are relative basicity and ring strain. It seems fairly well established that relative basicity plays an important role in controlling the relative reactivity of cyclic ethers. This lends further support to the belief that propagation proceeds by nucleophilic attack of monomer oxygen in the cationic copolymerizations. Basicity seems to be less important than other factors such as ring strain in determining relative reactivity of cyclic sulphides. 

Relative basicities of cyclic ethers have been determined by several methods [146,152,153]. By one method Wirth and Slick [152] obtained relative basicities by infrared from a study of the equilibrium constant at 27°C for the distribution of BF3 between two ethers in benzene solution. In another case, Yamashita et al. [146], following a method similar to that of Gordy and Stanford [154], measured the infrared absorption spectra of cyclic ethers in the presence of 0.1 mole r methanol-d. They then calculated the basicity of the monomers from the shift value of O—D stretching band. Nuclear magnetic resonance measurements of the equilibrium constant between BF3. Et O and the cyclic ether were also used [146]. 

Similarly, base strengths of cyclic sulphides have been determined by several methods. Stille and Empen [66] used the position of the wavelength of maximum absorption in the ultraviolet as one measure of electron density of the sulphur atom. In this study it was assumed that the sulphur atom with the highest electron density would have the longest 

Yamashita et al. [146] point out some features of the basicity of cyclic ethers. The basicity is affected by chemical structure, ring size, and substituents. The ring size affects basicity in the order 4 5 6 3. In 5-membered rings the basicity order is ether lactone formal. Methyl substitution increases the basicity and chloromethyl substitution decreases basicity. The relative basicities of the monomers are broadly in agreement with their reactivity in copolymerization experiments [58, 122]. 

The order of basicity found for the cyclic sulphides on the basis of ring size is 5 6 4 3. This order is different from that in cyclic ethers. Stille and Empen [66] state that this difference has been ascribed to the differences in heteroatom size, differences in ring size (ring strain), and also to differences in polarizability between oxygen and sulphur atoms. Basicity did not correlate well with reactivity in the sulphide series. Ring strain seemed to be more important. However, it should be noted that the reactivity measured in the sulphide case was in homopolymerizations. Very few copolymerization studies have been carried out so far. 

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