New views on cationic polymerizations induced by ionizing radiations

This paper is about a reinterpretation of the cationic polymerizations of hydrocarbons (HC) and of alkyl vinyl ethers (VE) by ionizing radiations in bulk and in solution. It is shown first that for both classes of monomer, M, in bulk ([M] = mB) the propagation is unimolecular and not bimolecular as was believed previously. This view is in accord with the fact that for many systems the conversion, Y, depends rectilinearly on the reaction time up to high Y. The growth reaction is an isomerization of a rc-complex, P+nM, between the growing cation P+n and the double bond of M. Therefore the polymerizations are of zero order with respect to m, with first-order rate constant k+p1. The previously reported second-order rate constants K+p are related to these by the equation K+p ≡ K+pmB If the monomer contains a group, other than the double bond, that can complex with the carbenium ion, such as an aromatic ring or a heteroatom, the resulting complexes, P+nG, are not propagators; in such systems the concentration of the propagators, [P+nM], is less than the total concentration, c, of cations. This newly recognized effect makes the k+p1 of most monomers calculated from c too small; it may be at least partly responsible for the rate of polymerization of styrene being significantly less than that of isobutene, and for the rate of polymerization of the VE being much less than that of the HC.