The initiation of polymerisations by aluminium halides

Abstract

Abstract The polymerisation of isobutylene by aluminium chloride and bromide in corresponding alkyl halide solution was studied by conductivity measurements and radio‐tracer techniques in high‐vacuum apparatus. Some of the reaction mixtures were killed with tritiated water. The conductivity changes during and after polymerisation and the unexchangeable tritium in the polymers support the theory that initiation is by addition of AlX to the double bond of the monomer with consequent formation of a carbenium ion which then propagates. Subsidiary experiments with styrene and 2,5‐norbornadiene and with a saturated hydrocarbon confirmed our conclusions. Contrary to accepted views, we found tertiary carbenium tetrahaloaluminates to be stable electrolytes in our very pure alkyl halide solvents. The very slow addition of isobutylene to solutions of aluminium halide in alkyl halide reduced their conductivity markedly and produced non‐reacting mixtures from which the monomer but no polymer could be recovered. This is shown to prove the long suspected complexing of isobutylene with aluminium halide and rules out all theories of initiation based on interaction of initiator molecules with monomer. It is shown that our theories can explain most features of the polymerisations; our results are compared with those of others.