Charge Transfer Complexes in Ionic Initiations
Reactions between donor and acceptor molecules, however, can also result in some charge transfers that yield ion-radicals and subsequent polymerization by ionic mechanism:
The nonbonding form predominates in the ground state while the charge transfer predominates in the excited state. The energy separation between the ground state and the first excited state is small. If the ionization potential of the donor molecule is low and the acceptor molecule has a strong electron affinity, transfer of an electron can occur to a significant extent even in the ground state. Mutual oxidation–reduction takes place:
Some vinyl compounds, i.e., vinyl ethers, can function as donor molecules because they possess a low ionization potential. The acceptors can be neutral molecules like, quinones, anhydrides, nitrile compounds, etc. They can also be ionic intermediates, such as metal ions, ionized acids, and carbon cations. An interaction of an acceptor with a donor is followed by a subsequent collapse of the charge transfer complex. This can results in formation of cation-radicals that are capable of initiating cationic polymerizations [57]. The exact mechanism of the reaction of cation-radicals with olefins is still not completely determined. One example is a combination of an alkyl vinyl ether (donor) with vinylidene cyanide (acceptor) that results in ionic polymerizations [58]. The reaction actually contains the ingredients of both cationic and anionic type polymerizations [59]:
where [CT] represents a charge transfer complex. Solvation of the charged species accelerates the transfer of electrons and the ionizations are enhanced by polar solvents [57]. Charge transfer reactions studies with tetracyanoethylene, an acceptor, and N-vinylcarbazole, a donor, in benzene solution demonstrated that both cation-radicals and anion-radicals form. This can be used in a subsequent cationic polymerization [60]:
A similar reaction takes place between chloranil and N-vinyl carbazole [61]:
Alkyl vinyl ethers also polymerize in the presence of strong acceptors like tetracyanoben zoquinone, 2,3-dichloro-5,6-dicyano-p-benzoquinone, and tetracyanoethylene [60]. A similar reaction mechanism was proposed [60]. Solutions of maleic anhydride in ether will initiate cationic polymerizations of isobutyl vinyl ether or N-vinyl carbazole, if subjected to attacks be free radicals. The same is true if the solutions are irradiated with ultraviolet light or gamma rays [62, 63]. Also, active species are generated from reactions of aldehydes or ketones with maleic anhydride when attacked by free radicals or irradiated by UV-light, or gamma rays from 60Co [63]. These active species are presumed to be formed through charge-transfer reactions that occur between the electron acceptors, p-acids, or electron donors, p bases, which form cations [64]. Some Lewis acids can form charge transfer complexes with monomers that yield cation radicals when irradiated with ultraviolet light [65, 66]. This was shown with such Lewis acids as VC14, TiCl4, and TiBr4 in polymerizations of isobutylene. The charge transfer complexes collapse after irradiation [66]:
