Mechanism of Cationic Polymerization

Cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers a charge to a vinyl monomer which then becomes reactive. This reactive monomer goes on to react with other monomers to form a polymer. Let A be the catalyst, RH the co-catalyst and M the monomer, then the individual steps can be represented as follows:

A + RH ⇔ H⁺AR^-

H⁺AR^- + M → HM⁺AR^-

HM_n⁺AR^- + M → HM_n+1⁺AR^-

HM_n⁺AR^- → M_n + H⁺AR^-

HM_n⁺AR^- + M → M_n + HM⁺AR^-

Both the initiation and termination reaction involves the transfer of a proton. Other cations may of course be transferred in some other cases.

In a homogenous system, the rate of initiation is usually proportional to the concentration of the catalyst complex and also to the concentration of the monomer. Its concentration is usually below one percent of that of the monomer. Often the chemical nature of the initiating catalyst and its concentration is unknown. For this reason, the rate of initiation is usually expressed in terms of the total concentration of either the catalyst or the co-catalyst, whichever is present in stoichioemetric deficiency. Calling this concentration [C], the rate of initiation can be written as k_i [C] or k_i [C] [M] depending on whether the initiation complex H⁺AR^- is mostly converted to a reactive chain-growth monomer HM⁺AR^- or not. The rate of initiation R_i is then given by

R_i = k_i [H⁺AR^-] [M] = k_i K [A] [RH] [M]

Herein is K the rate constant of the catalyst-cocatalyst complex formation and k_i is the rate constant of the initiation step.

The termination rate should be of second order, that is, it should be proportional to the anion concentration [AR^-] and the concentration of growing chains [M+]. However, it is of first order since the anion remains always in the vicinity of the growing chain center:

R_t = k_t [M⁺]

Assuming steady state conditions, R_i = R_t, the rate of polymerization can be written as

R_p = k_p [M⁺] [M] = (K k_i / k_t) [A] [RH] [M]²

where [M⁺] is the concentration of all chain carriers ∑_nHM_n⁺AR^- .