Initiator-coinitiator complex mechanism

Many polymerizations exhibit a maximum polymerization rate at some ratio of initiator to coinitiator [Biswas and Kabir, 1978, 1978 Colclough and Dainton, 1958 Taninaka and Minoura, 1976]. The polymerization rate increases with increasing [initiator]/[coinitiator], reaches a maximum, and then either decreases or levels off. Figure 5-1 shows this behavior for the polymerization of styrene initiated by tin(IV) chloride-water in carbon tetrachloride. The decrease in rate at higher initiator concentration is usually ascribed to inactivation of the coinitiator by initiator. The inactivation process in a system such as SnCl4-H20 may involve hydrolysis of Sn—Cl bonds to Sn—OH. There is experimental evidence for such reactions when comparable concentrations of coinitiator and initiator are present. However, the rate maxima as in Fig. 5-1 are observed at quite low [initiator]/[coinitiator] ratios where corresponding experimental evidence is lacking. An alternate mechanism for the behavior in Fig. 5-1 is that initiator, above a particular concentration, competes successfully with monomer for the initiator-coinitiator complex (V) to yield the oxonium salt (VI), which... 

Lewis acids initiate polymerization through the formation of carbonium ions, and Plesch (17) has proposed that a suitable coinitiator is necessary to produce these ions. The mechanism for the initiation of the homopolymerization of epoxy resins by BF3 complexes has been proposed by Arnold (11) to proceed as follows ... 

Lewis acids have been shown to initiate polymerization through carbonium ion formation and therefore the presence of a suitable coinitiator has been found necessary in order to generate these ions. The polymerization of epoxy resins by BF3 complexes has been proposed to follow the mechanism shown in Scheme 29. 

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