Stable carbonium ion salts as initiators

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

In the case of AT-vinylcarbazole at 0 and —25°C, initiation has been shown to be the direct addition of the carbonium ion (C7H7) to monomer, a process which appears to take place virtually instantaneously. Under these circumstances the concentration of active centres is assumed equal to the initial salt concentration, [I]q. Significant termination appears to be absent during kinetic lifetimes, (up to 100% conversion) and polymer molecular weights are limited only by transfer reactions. The experimental rate law can be expressed as 

Polymerization of Al-vinylcarbazole by tropylium salts in methylene chloride solution 

Counter-ion Temperature (°C) io-=fep (1 mole sec ) Activation Energy (kcal mole ) 

In the case of alkyl vinyl ethers [27, 30] reaction rates were again high and the same experimental technique was used. However, the initiation reaction did not appear to be as fast as that in the polymerization of Al-vinylcarbazole, and the conversion/time data showed evidence of an initial acceleration to a maximum rate of polymerization, particularly in runs carried out at —25°C. The mechanism of initiation was assumed to involve direct addition of the initiating carbonium ion to the double bond of the monomer, in the light of related evidence from similar reaction in the presence of strong nucleophiles [80, 81]. At 0°C there was also an indication of a contribution from a termination reaction. Polymer yields were always in excess of 75%, however, and the termination process was neglected in the kinetic analysis. The simple scheme envisaged is ki 

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