Tantalum- and titanium-based catalytic systems for the synthesis of hyperbranched polyethene

Hyperbranched polyethene was synthesized using a novel TaCl5/alkylaluminum halide catalyst system. The polymers were viscous oils with a ratio of meth yl hydrogens/total alkyl hydrogens of 0.25-0.40 and weight-average molecula r weights of 600-4000 daltons. Both branching and molecular weight can be v aried by changing reaction conditions and catalyst composition. The system can also be modified to synthesize 1-alkenes (principally, 1-hexene). A sec ond system, involving TiCl4/alkylaluminum halide, also gave hyperbranched p olyethene, with a ratio of methyl hydrogens/total alkyl hydrogens of 0.10-0 .25. Weight-average molecular weights varied from 900 to 1800 daltons. High er selectivity to liquid product was achieved through the addition of dihyd rogen. The mechanism of the formation of branched polyethene appears to inv olve, for the most part, oligomerization of ethene to 1-alkenes (principall y 1-hexene) by a transition metal alkyl species followed by cationic oligom erization of the 1-alkenes by Lewis acidic species. The addition of tetraal kylammonium chloride to the TaCl5/Et3Al system resulted in a change in prod uct selectivity from all polyethene to >65% 1-hexene. The highly selective trimerization of ethene by the tantalum system is unusual.