Neutral Salt Effects on Cationic Polymerization of N-Vinylcarbazole Initiated by Protonic Acids

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Cationic polymerization of N-vinylcarbazole(VCZ) initiated by perchloric acid or trichloroacetic acid(TCA) in three different solvents(nitrobenzene(NB), ethylene dichloride(EDC), and benzene(Bz)) was studied at 30°C in the presence and absence of neutral electrolytes(tetra-n-butylammonium perchlorate(TBP) and tetraethylammonium perchlorate(TEP)). The kinetic quantity determined was the chain-transfer constant to methanol. Since chain-transfer constant is affected by the nature of kinetic chain carrier, the study of neutral salt effects on chain-transfer constants provides useful information on whether the cationic chain carrier is free or ion-paired. Measurements for six combinations of three solvents and two initiators either in the presence of TBP or in its absence afforded the following pieces of information.(1) The presence of TBP enhanced the chain-transfer constant in polar media. However, no neutral salt effects were observed in benzene, indicating that the growing carbonium ion in benzene is tightly ion-paired and the addition of TBP cannot affect the ionic association.(2) In EDC, the chain-transfer constants of methanol are very much enhanced by the addition of neutral electrolyte. The results were interpreted assuming that the rate of propagation decreases in the presence of TBP whereas the rate of chain-transfer reaction is relatively unaffected. A study of the deuterium isotope effect using methanol-d supports this view.(3) In NB, the propagating species in the absence of TBP seem to be free carbonium ions since the kind of initiator does not influence the chain-transfer constant. In this system, the growing cation would be strongly solvated and stabilized.(4) Dependence of the rate of polymerization on neutral salts was also investigated. The rate of polymerization is, however, a complex function of various rate constants and conclussion is difficult.Chain-transfer constants are very sensitive to the nature of propagating species and their determinations as functions of neutral electrolyte concentration would provide a useful procedure to study the ionic equilibria of the carbonium ion and ion pairs.