EFFECT OF ALUMINUM ALKYLS ON THE SYNTHESIS OF SYNDIOTACTIC POLYSTYRENE WITH TITANIUM COMPLEXES METHYLALUMINOXANE CATALYTIC-SYSTEMS

Syndiotactic polystyrene (sPS) was synthesised using homogeneous catal ytic systems based on titanium derivatives (tetraethyl orthotitanate, (cyclopentadienyl)titanium trichloride), methylaluminoxane (MAO) and a luminium alkyls (trimethylaluminium, triisobutylaluminium). The polyme rs were characterised by gel permeation chromatography (GPC) and nucle ar magnetic resonance (n.m.r.) spectroscopy. Trimethylaluminium was fo und to decrease both the catalytic activity and the polymer molecular weight. Triisobutylaluminium causes a reduction in activity only for l ow ( < 600) methylaluminoxane/titanium molar ratios. While the molecul ar weight decrease is almost linear with the increase of the amount of aluminium in the catalytic system in the case of trimethylaluminium, a very sharp decrease of molecular weight is observed by addition of t riisobutylaluminium even in moderate amounts. The results were interpr eted on the basis of the different ability of the two aluminium alkyls to behave as chain transfer agents. Moreover, the reactions of termin ation and re-initiation of the polymer chains were examined taking int o account the end-group structures,and a polymerisation mechanism to e xplain the experimental results was put forward. (C) 1997 Elsevier Sci ence Ltd.