JEE Main · 2012 · 29 Apr 2012 · Offline · Polymers

Cationic polymerisation is started by a species able to create a carbocation on the monomer, which means a strong Lewis acid or strong protic acid. Of the choices, $\mathrm{AlC}{\mathrm{l}}_3$ is a powerful Lewis acid and readily supplies the electrophilic push needed to open the double bond and form the chain-carrying cation. The hydride $\mathrm{LiAl}{\mathrm{H}}_4$ is a reducing agent, $\mathrm{HN}{\mathrm{O}}_3$ is an oxidising acid, and butyllithium is a nucleophile that starts anionic rather than cationic growth. So $\mathrm{AlC}{\mathrm{l}}_3$ is the best initiator.