Studies on polymerizations initiated by syncatalytic systems based on aluminium organic compounds, 7. Reaction mechanisms

The polymerizations of isobutene and its copolymerizations with isoprene by syncatalytic systems consisting of an aluminium compound R2AlX (with R = alkyl and X = alkyl or halogen) and of a halogen or an interhalogen compound are initiated by the addition of a positive halogen ion to the monomer. Chlorine is the best co‐initiator. Whereas in this syncatalyzed polymerization of isobutene the initiation is relatively slow and monomer transfer and termination are unimportant, in the polymerization of isobutene by AlCl3 or EtAlCl2 the initiation is very fast and there is at least one efficient termination. Our interpretation of these results includes a discussion of the different initiations and chain‐breaking reactions. In the syncatalyzed polymerizations the initiation rate is governed by the relatively slow interaction of initiator and co‐initiator. Since the termination reactions are relatively unimportant, second and subsequent portions of monomer polymerize completely and faster than the first. The polymerizations initiated by AlCl3 or EtAlCl2 stop at incomplete conversion because of a termination which is chlorination of the carbenium ions by the anions; the AlCl3 thus formed is complexed by the unreacted monomer.