Trioxane polymerization, hydride transfer

An additional termination in the trioxane polymerization is chain transfer to monomer hy hydride ion transfer, which results in terminating the propagating chain with a methoxyl group while carbocation XXII reinitiates polymerization [Kern et al., 1966 Weissermel et al., 1967]. 

In the homopolymerization of dioxolane below 30°C. tertiary oxonium ions exist exclusively (2, 5). Otherwise hydride transfer would occur (carbonium ions abstract hydride from monomeric cyclic formats) (II, 16). In trioxane polymerization, however, at least some of the active chain ends are carbonium ions they cause hydride transfer and elimination of formaldehyde (9, II, 13). Thus, in copolymerization we must expect two different kinds of structures for cationic chains with terminal trioxane unit. Oxonium ions (I) and carbonium ions (II) may have different reactivity ratios in the copolymerization, but hopefully this does not cause severe disturbance since I and II seem to be in a fast kinetic equilibrium with each other (3). Hence, we expect [I]/[II] to be constant under similar reaction conditions. 

Although the corresponding quantitative measurements of the carbenium-oxoniumion equilibrium in the polymerization of 1,3,5-ttioxane have not yet been carried out, one can assume that the proportion of oxonium ions relative to this monomer will be lower than with 1,3-dioxolane which is much more nucleophilic. Moreover, the importance of hydride anion transfer may be higher for 1,3,5-trioxane polymerization. Thus, in spite of 1,3,5-trioxane itself beitig a much weaker hydride donating agent than 1,3-dioxolane, the hydride anion transfer processes play an important role in the polymerization of the former. 

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