Crowded carbenium ions

Hydride abstraction occurs when the monomer is sterically crowded, as with a-methylstyrene [Eq. (40)]. This generates a carbenium ion, which then adds to additional monomer. 

Opening of the three-center bond affords the carbenium ion of greatest stability according to the well-known Markovnikov rule. It should be pointed out that except in extremely crowded cases such as adamantylideneadaman-tane, there is no direct evidence that the three-center n-bonded complex is an intermediate, and thus it is only a depiction of the transition state or high-lying intermediate of the addition reaction. 

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