Developments in the theory of cationic polymerisation. Part IX some effects of the complex formation between cations and monomers

The formation of complexes P + n M between a propagating carbenium ion P + n and the monomer, M, is considered from several points of view ‐ (i) Under the most usual conditions the formation of the P + n M reduces the population of ion‐pairs, P + n A − , drastically. This can account for the frequently found nil‐effect of common ion salts on the rate of attack of carbenium ions on alkenes in initiation, model reactions, and polymerisations. ‐ (ii) The formation of P + n M increases the degree of dissociation α of P + n A − , so that all the estimates of α on which kinetic schemes have been based, are too low. ‐ (iii) The very high polymerisation rates found for n‐donor monomers, say 4‐MeO‐styrene or N‐vinylcarbazole compared to styrene, may be attributable not, primarily, to a difference in rate‐constants, but to a difference in ionic population. This is because the fast P + n M is in greater excess over the slow P + n A − for more strongly complexing n‐donors than for the π‐donors. ‐ (iv) Since the nature of the P + n M is different for every monomer, a comparison of their propagation rate‐constants k + p M is hardly meaningful and valid comparisons can only be made between the k + p of the P + n . The measurement of these requires extreme conditions, either extrapolations to [P + n ] = 0 and [M] = 0, or the use of highly polar solvents in which neither M nor the anion can compete with the solvent for the solvation of the P + n .