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

The formation of complexes P+nM 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+nM reduces the population of ion‐pairs, P+nA−, 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+nM increases the degree of dissociation α of P+nA−, 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+nM is in greater excess over the slow P+nA− for more strongly complexing n‐donors than for the π‐donors. ‐ (iv) Since the nature of the P+nM is different for every monomer, a comparison of their propagation rate‐constants k+pM 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.