Controlled carbocationic polymerizations of bicyclo[2,2,1]hepta-2,5-diene with AlCl3 in the presence of electron pair donor components

The cationic isomerization polymerization of bicyclo[2.2.1]hepta-2,5-diene (norbornadiene) was investigated. The synthesis of gel-free poly(3,5-tricyclo[2.2.1.0^2,6]heptylene) was achieved at a reaction temperature of 0°C. These polymerizations were initiated by AlCl₃ in the presence of the weakly donating additives nitrobenzene or nitromethane. NMR spectra show that the polymer contains a small fraction (between 2 and 4 mol-%) of branched structures in addition to a small amount of 5,7-linked bicyclic olefin units (approximately 2%). A reaction mechanism involving non-classical carbocation intermediates is proposed in order to account for the 5,7-enchainment of the bicycloheptenylene units, which contrasts with earlier studies on cationic norbornadiene polymerizations. Further, pentamethylbenzene was utilized as a chain transfer reagent in order to adjust the number average molecular weight M _n to values below 2500.