The Rôle of Carbocations in Cationic Polymerization (Polycondensation) of π-, σ-, and n-Donor Monomers

The rôle of carbocations in cationic polymerizations is discussed, differentiating π- (i.e. alkene and arene), σ- (i.e. alkane) and n-donor (i.e. ether, sulfide, nitrogen compound) monomers as the reactive nucleophiles. The mechanism of the initiation step in alkene polymerization is evaluated considering both protic (i.e. coinitiator containing) and aprotic systems. The aluminum bromide catalyzed polymerization of isobutylene, as an example, can involve besides the conjugate protic acid, bromoalanation intermediates in aprotic media, which, however, themselves rapidly can undergo elimination, forming HBr in the originally acid free system. The involvement of dialkylhalonium ions in alkyl halide solvent systems is also of substantial interest and may explain increasing deactivation in the order RI > RBr > RCl. Oxidative polycondensation of π-donor arenes (such as benzene) and σ-donor alkanes (such as methane) is discussed in the context of the general carbocation concept, involving trivalent carbenium ion intermediates in the former and pentacoordinated carbonium ions in the latter case, as the reactive alkylating agents responsible for the ionic propagation reactions. n-Donor monomers, such as ethers, sulfides or miscellaneous nitrogen compounds, undergo polymerization via nucleophilic displacement reactions on the polarized carbocationic sites of intermediate alkylated or protonated onium ions.