Carbocation rearrangement reactions 1.2- methide shift

Since the first step in the reaction is the formation of a carbocation, migration of a methyl group in a 1,2-methide shift gives a rearranged tertiary carbocation. This rearranged carbocation accounts for most of the product. 

In some cases, the propagation reaction is accompanied by intramolecular rearrangements due to hydride ion (H ) or methide ion (CH3 ) shifts. Such polymerizations are referred to as isomerization polymerizations. Consider, for example, the polymerization of 3-methyl-1,2-butene in which the carbocation (XV), formed initially, isomeiizes by a 1,2-hydride shift. The resulting ion (XVI), being a tertiary carbocation, is more stable than (XV) which is a secondary carbocation. 

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