The Possible Role of Halonium Ions

Dialkylhalonium ions are known to be effective alkylating agents for arranatic compounds and their reactions with olefins have also been studied Dimethylbromonium hexa-fluoroantimonate has been shown to initiate the cationic polymerisation of butene-1 and isobutene in liquid SOj  

While initiation by halonium ions is certainly an interesting and little studied field in cationic polymerisation, it can be safely concluded that the mechanism of direct initiation in the cationic polymerisations discussed in this chapter cannot be ascribed to the formation of such species. 

Concerning the possible causes of the premature cessation of polymerisation in most systems characterised by direct initiation, we believe that the accumulation of anions derived from the Lewis acid is the most important sin e factor responsible. We have already discussed the possible reactions giving rise to an increasing concentration of these anions in the case of direct initiation by the neutral Lewis acid. To these must be added the formation of carbenium ions which cannot propa te either because they are sterical-ly hindered, as in the case of isobutene unsaturated dimers and trimers , or because they are too stable to attack the monomer, as in the case of termination by formation of allylic cations. The accumulation of these species is inevitably accompanied an equivalent rise in anion concentration. Whatever the origin(s) of this rise, a higher anion concentration can determine various detrimental effects on the survivd of active species  

There are no compelling reasons against a third mechanism of direct initiation, which we wish to suggest for the sake of completeness. It is in fact conceivable that a concerted, AdgS-type attack could take place between the Lewis acid and the olefin. This mechanism would resemble closely the one we propose in the next section to explain the way cocatalysis operates. Thus  

If the rate-determining step of this process is the formation of the carbenium ion, a second or third order dependence on Lewis acid concentration would be expected depending on whether the species MtX iMtXji/+j is in the form of ion pairs or free ions. This third mechanism is not inconsistent with the general phenomenology of direct initiation, but no specific evidence in its favour is available. 

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