Discussion
Which one of the following polymerization methods is used for the synthesis of butyl rubber?
Answer: C
Butyl rubber is made by cationic polymerization.
Butyl rubber is a copolymer of isobutylene with a few per cent of isoprene. Isobutylene carries two electron-donating methyl groups on one carbon, which stabilise a carbocation, so it polymerises readily by a cationic mechanism, using a Lewis acid such as AlCl3 with a trace of water as co-initiator, in methyl chloride at about -100 °C.
The same electron-rich double bond makes anionic polymerization impractical. Ring opening needs a cyclic monomer, and condensation polymerization needs monomers with two reactive functional groups that release a small molecule, neither of which applies to isobutylene.
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