The Initiation Reaction
The initiations result from coordination of the cation catalysts with the oxygen of the monomers to form oxonium ions [48, 49]. This weakens the oxygen-carbon bonds and leads to ring openings after reactions with a second molecule of the monomer. New oxonium ions are generated in the process:
Some active oxonium salts are [48-50]: [(C2H2O),O]+ BF4 ̄, [(C2H5O)зO]+ SbCl [(C2H5O)2O]+ FeClTM, and [(C2H5O)3O] +AICl4 , Examples of carbon cations that can initiate polymerizations of tetrahydrofuran, as well as some other cyclic ethers are:
The initiation mechanisms, however, by many carbon cations as, for instance, by triphenylmethyl cations, are not straightforward. Initially, hydride ions are abstracted from the monomers to form triphenylmethanes [51-53]. Simultaneously, acids are released from the counterions. The acids become stabilized by complexing with monomers. After that, the complexes react slowly with additional monomers to form the propagating oxonium ions. This makes the acids the real initiators:
Other initiators for tetrahydrofuran polymerizations also include Lewis acids in combinations with "promoters." These are complexes of Lewis acids, like BF3, SnCL1, or C2H5AICl2 with epirane compounds like epichlorohydrin [42]. The small ring compounds are mote reactive toward many Lewis acids, or protonic acids, then tetrahydrofuran and act as promoters of the initiation reactions. The initiations in the presence of small quantities of oxirane compounds, for instance, can be illustrated as follows:
Strong Bronsted acids form when diaryliodonium salts. like BF4-, ASF6-, PF6- and SbF6- are reduced with compounds like ascorbic acid in the presence of copper salts, such acids also initiate the polymerizations of tetrahydrofuran, cyclohexene, and s-trioxane [54].
