Studies on cobalt ethylene polymerisation catalysts bearing bis(imino)pyridine ligands strongly indicate that the activated species is not the anticipated cobalt(II) alkyl cation.

The synthesis and characterisation of a series of cationic bis(imino)pyridine iron and cobalt complexes of the type [LMCl(D)]SbF 6 (D = CH 3 CN or thf ) and [LM(R 2 acac)]SbF 6 (R = CH 3 , CF 3 , Ph) are described {L = 2,6-bis [1-(2,6diisopropylphenylimino)ethyl]pyridine, M = Fe or Co}. The solid state structures of these five-coordinate complexes vary between square-based pyramidal and trigonal bipyramidal geometries, depending on the ligands used. Attempts to synthesise cationic metal alkyl complexes of the type [LMR] ϩ were unsuccessful. However, these complexes serve as highly active ethylene polymerisation catalysts when activated with MAO. Polymerisation activities are comparable to the activities obtained with neutral dichloride precursors [LMCl 2 ] and the resulting polymer properties are nearly identical, suggesting that in all cases the same active species is being generated. The polymerisation activity is not inhibited by the presence of donors such as thf or CH 3 CN and these cationic precursors can be activated with less co-catalyst than is normally used for neutral dichloride pre-catalysts. As little as 10 equiv. TMA, in combination with 1 equiv. B(C 6 F 5 ) 3 , afford a highly active polymerisation system. Co-polymerisation studies of ethylene with polar monomers such as methyl methacrylate (MMA) or styrene resulted in polymer production with high activities. However, in both cases no co-polymer is obtained. The activity of the catalyst is significantly reduced in the presence of methyl acrylate (MA) or 2-vinyl-1,3-dioxolane (VDO) and again no co-polymer is produced. Polar monomers such as vinyl acetate, acrolein and acrylonitrile deactivate the catalyst. DALTON

A series of co-oligomerization and co-polymerization reactions of C 2 H 4 /C 2 D 4 (1:1) mixtures have been carried out using various transition metal catalysts based on Cr, Co and Fe in combination with MAO. The oligomeric α-olefin products have been analysed by GC and GC/MS and the experimental results have been compared with the theoretical mass spectra derived from mathematical models. Solid polymer samples have been analysed by 13 C{ 1 H} and 13 C DEPT-135 NMR spectroscopy. C 2 H 4 /C 2 D 4 co-oligomerization can be used as a method to differentiate between a metallacyclic or a Cossee-type chain growth mechanism in oligomerization systems. In the case of a metallacyclic mechanism, no H/D scrambling is observed whereas for a Cossee-type mechanism, similar rates of chain propagation and chain termination (β-H elimination) result in rapid H/D scrambling of the C 2 H 4 /C 2 D 4 feed. This method is therefore limited to oligomerization systems and cannot be applied in polymerization systems where the rate of chain propagation is much faster than the rate of chain termination.

The activation of bis(imino)pyridine cobalt(II) precatalysts by MAO leads initially to a bis(imino)pyridine cobalt(I) cationic species with no cobalt-C(alkyl) bond into which insertion can occur. Mechanistic studies have shown that the initiation of polymerization from this species involves incorporation of alkyl groups from the cocatalyst, most likely involving attack of methide anion (from the counteranion) on a cobalt-ethylene species.

A series of bis(imino)pyridine ligands containing aryl substituents with o-trifluoromethyl units, along with their cobalt and iron complexes, has been prepared and characterized. The crystal structures of several complexes are reported. Both the cobalt and iron complexes, when activated by methylaluminoxane cocatalysts, afford much higher olefin oligomerization/polymerization activities than their nonfluorinated relatives. Enhanced performance is seen not only in higher peak activities but also in longer catalyst lifetimes, suggesting that the trifluoromethyl group significantly improves catalyst stability. The most impressive activity increase is observed for a cobalt catalyst combining an o-CF3 and an o-F unit. This system is more active than its iron counterpart in ethylene polymerization, reaching >100 000 g mmol-1 h-1 bar-1. Propene, 1-butene, and 1-hexene are oligomerized by this catalyst at rates much higher than for nonfluorinated relatives. Highly linear dimers predominate in each case, the remainder being trimeric and tetrameric products. The principal product of propene dimerization is 1-hexene (60−73% of total), whereas for 1-butene and 1-hexene internal olefins are obtained (E isomers predominate). No isomerization of 1-hexene is seen under the reaction conditions. The catalysts operate by a mechanism involving (1,2)- followed by (2,1)-insertion steps. Combinations of chain growth and step growth account for the formation of linear trimers and tetramers of propene.