An infrared investigation of the catalyst formation in the system Ni(acac)2, C3H4, (iBu)3AL for propadiene polymerization

The catalyst formation in the system Ni(acac)2, C3H4, (iBu)3Al was investigated by means of infrared spectroscopy. It was found that the Al(acac)3 and (iBu)2Al(acac) formed are both by-products of catalyst formation without a catalytic activity. Ni(acac)2 loses its acac groups forming the unstable (iBu)2Ni compound; without C3H4 being present, this compound disproportionates to Ni metal and isobutane and isobutene. In the presence of C3H4 an allyl-nickel complex is formed, which reacts with (iBu)3Al to give the actual catalyst, possibly a bimetallic allyl—nickel-aluminium complex. Catalysts such as Ni(acac)2, C3H4, (iBu)3Al and (πC3H5)2Ni with or without (iBu)3Al all selectively give 1, 2, 1, 2-polypropadiene. A Lewis base like pyridine not only decreases the polymerization rate but also changes the selectivity towards the formation of 1, 2, 2, 1-polymer

Molecular weight determination for 1,2,1,2-polypropadiene

By using standard bromination conditions, the insoluble 1,2,1,2-polypropadiene (formed by Ni(acac)2 or Co(acac)2 or 3, C3H4, (iBu)3Al catalysts) is transformed into a soluble bromopolypropadiene. Using this technique, determination of molecular weight becomes possible. It was found that the molecular weight increases with polymerization time until a steady value is reached. As the polymer yield continues to increase when a constant molecular weight has been reached, chain transfer must occur. The molecular weight of polybromopropadiene was independent of the concentrations of the catalyst components. From experiments with crosslinked polymers and from theoretical considerations, it was deduced that the low solubility of the original 1,2,1,2-polypropadiene is due to its high crystallinity

The polymerization of propadiene by Ni(acac)2, C3H4, RnAlX3−n catalysts

Catalyst formation in the system Ni(acac)2, C3H4, RnAlX3−n was studied. Polymerization experiments showed that, by replacing ionic groups such as acac−, Br−, Cl− with alkyl or hydride groups, an active catalyst is obtained. Electrolysis of Ni(acac)2 in tetrahydrofuran also gives an active catalyst. Lewis acids like (iBu)3Al and Et3Al increase the polymerization rate, while Lewis bases like pyridine and triphenylphosphine not only decrease the rate but also change selectivity. The selectivity is not changed if different transition metals (e.g. Co, Pd, Ni) are used. Kinetic measurements show a first order dependence on Ni. The dependence on (iBu)3Al changes from first to zero order with increasing Al/Ni ratio. This can be explained by assuming that the very active catalyst is formed via an equilibrium between a nickel complex and (iBu)3Al. A first order deactivation of the nickel catalyst is observed; it is faster during polymerization than during ageing of the catalyst