Novel catalysts for ethylene polymerisation and oligomerisation

Polyolefins, the generic name for the synthetic polymers based on ethylene, propylene, other non-functionalised a-olefins and reactive internal olefins, are the most common commodity plastics. The development of new single-site catalysts for olefin polymerisation is an area of ongoing research interest due to the versatility of the ligand fine-tuning, which allows the production of polymers with tailored microstructures and properties. Likewise, selective ethylene oligomerisation to 1-hexene or 1-octene can only be achieved with homogeneous single-site oligomerisation catalysts. The design of the ancillary ligand, the metal and the cocatalyst are key factors determining the selectivity of these catalysts. In the first part of the thesis (Chapter 2), we have studied the chemistry of group IV metal-based aluminium-pyrrolyl catalysts to probe their ethylene polymerisation activity and mechanistic behaviour. Several novel heterobimetallic aluminium-pyrrolyl complexes of titanium, zirconium and hafnium were synthesised and were fully characterised by means of single crystal X-ray diffraction, elemental analysis and NMR spectroscopy. However, in comparison to vanadium- and chromium-based aluminium-pyrrolyl catalysts, the catalytic performances of the group (IV)-based aluminium-pyrrolyl catalysts were not promising because none of these catalysts showed any self-activating catalytic behaviour or high thermal stability. The mono(aluminium-pyrrolyl) complexes afforded moderately active ethylene polymerisation catalysts (producing UHMWPE) upon activation with either methylaluminoxane (MAO) or triisobutylaluminium (TIBA)/[Ph3C]+[B(C6F5)4]-, albeit only at higher ethylene pressure. On the other hand, the bis(aluminium-pyrrolyl) group IV metal species did not yield active ethylene polymerisation catalysts, irrespective of the nature of the group IV metal applied. Intramolecular coordination of a chlorine to the active site as a possible reason for lack of, or only poor, catalytic behaviour of these catalysts was supported by density functional theory (DFT) calculations. The second objective of the research described in this thesis was to develop novel catalysts for ethylene oligomerisation. The fact that the fractionation of the oligomerisation mixture to extract the most requested a-olefins (1-hexene and 1-octene) is an energy-intensive process motivates the search for selective catalytic systems. However, designing catalysts which can produce 1-hexene and 1-octene selectively remains a great challenge. We investigated the ethylene oligomerisation capabilities of three different classes of chromium-based catalysts, namely: pyrrole-based chromium catalysts, pyrazole-based chromium catalysts and modified bis(diphenylphosphino)methane chromium catalysts. In chapter 3, we evaluated the ethylene oligomerisation capabilities of various dipyrrole-based ancillary ligands and the results were compared with the Chevron-Phillips trimerisation system. Activation of dipyrrole-chromium catalysts using triethylaluminium (TEA) as cocatalyst showed that these systems are less active and selective compared to the commercial Phillips trimerisation system. However, by using an alternative activation method (via combination of depleted-MAO (DMAO) and TIBA), an efficient way to increase both the activity and selectivity was discovered. Applying this novel activation method to the 2,5-dimethypyrrole-based system resulted in a nearly two-fold increase in the activity compared to the activity of the original Chevron-Phillips system. The fact that the new activation method resulted in a polymer free-trimerisation system also gives significant added value to this finding. Moreover, the higher activity and selectivity was obtained at relatively lower ethylene pressure and temperature (30 bar, 60 °C), whereas in the case of the commercial Phillips ethylene trimerisation system high pressure is required (50-100 bar) to get high activity. Furthermore, well-defined chromium complexes of dipyrrole ligands were synthesised, fully characterised and assessed for their ethylene oligomerisation activities. In the course of Chapter 4, the synthesis and the ethylene trimerisation capability of chromium(II) and chromium(III) heteroscorpionate complexes has been discussed. N2N¿ and N2O donor heteroscorpionate ligands with secondary and tertiary amine donors, as well as phenol and phenyl ether donors, were successfully coordinated to Cr(II) and Cr(III). The ethylene oligomerisation results show that very small change in the nature of the pendant donor atom (N, O) and/or the presence of ionisable hydrogen (N-H, O-H) can have a large effect on activity and/or selectivity. In almost all cases, the new complexes gave an increase in productivity compared to Cr{CH(Me2pz)3}Cl3, but disappointingly the selectivity towards 1-hexene was significantly lower than the selectivity obtained with Cr{CH(Me2pz)3}Cl3. In chapter 5, novel bis(diphenylphoshino)methane (DPPM)-based mono anionic ligands were prepared by the reaction of the in situ-generated DPPM- anion with different cumulenes. The reaction of DPPM- with CO2 and aliphatic isocyanates (CyNCO, t-BuNCO) occurred via the attack of the nucleophilic DPPM-anion to the electrophilic carbon atom of the cumulenes, in the process forming a carbon-carbon bond (classical addition). However, the use of aromatic isocyanate resulted in a "non-classical" type addition, forming a mixed valent P(III)/P(V)-containing ligand. We observed that this arrangement disappears upon complexation with chromium through an internal redox transformation. The connectivity may switch back and forward between "classical" and "non-classical" forms, depending on the steric requirement of the final complexes. While we found no evidence that this may be a fluxional behaviour (at least in the case of the diamagnetic lithium salt), this strange dynamic behaviour for a ligand system in principle holds some promises for catalysis. To this end, the catalytic testing of all the species presented in this chapter showed mainly non-selective behaviour with high catalytic activity. However, fine tuning the oligomerisation condition (DMAO as cocatalyst, Cr(acac)3 as metal precursor, methylcyclohexane as solvent) resulted in the selective formation of 1-hexene and 1-octene as the only liquid product. Nevertheless, the considerable amount of waxy polyethylene produced along with the liquid products makes the system less attractive from an industrial point of view. The research described in this thesis has given more insights into the structure-property relationship of several new ethylene oligo/polymerization catalysts