Is the cationic polymerization of iso-olefins in a moderately concentrated sulfuric acid a real carbenium-ion reaction?

Abstract

The different reaction products resulting from 2-methyl-2-butene transformations in 95 and 65% sulfuric acid at 0°C clearly indicate the different mechanisms of the acid catalyzed ‘conjunct olygomerization’ and cationic polymerization of olefins in the concentrated and more dilute acid. The formation of paraffinic oligomers with even and odd numbers of carbon atoms in the 95% acid indicates the predominant role of hydride transfer and cracking. This should be considered as an evidence of the real carbenium-ion mechanism of ‘conjunct olygomerization’, although the steady state concentration of alkyl carbenium ions is below the limit of 13 C NMR detection. The absence of these reactions in a moderately dilute acid indicates a different mechanism of cationic polymerization. The latter represents an oxonium ion reaction where similar to transformations of olefins on zeolites the carbenium ions are formed only as transition states.