Industrial Importance of Alkenes
Dimerization, Oligomerization and Polymerization of Alkenes
In the presence of acids, alkenes add a molecule of water (see hydration of isobutene). Carrying out the reaction in an organic solvent, such as isobutene, and increasing the concentration of the acid with concomitant large reduction of water, isobutene as the only Lewis base present adds to the isobutyl carbenium ion to form a dimer from two isobutene monomers.
The products of the dimerization are the isomeric trimethylpentenes (a) and (b). According to Markovnikov´s rule, the more stable intermediate carbenium ion is formed in this electrophilic addition. Subsequent deprotonation results in a mixture of the two products (a) and (b) with (b) being the predominant product because of higher stabilization of its double bond by the +I effect of the alkyl substituents and hyperconjugation. Without taking special measures, additional isobutene molecules are added to the dimers in the same way and in the end oligomers or polymers are formed.
Because of the cationic intermediate, this type of polymerization is called cationic polymerization. Termination of the reaction occurs by deprotonation or addition of a negatively charged Lewis base. Other types of polymerization are the radical polymerization (e.g. of ethylene), anionic polymerization (e.g. of butadiene), and metal-catalyzed polymerization (e.g. the Ziegler-Natta-type polymerizations).
These polymerizations are of great industrial importance.