Ethylene oligomerization and polymerization with novel transition metal catalysts and trithiocarbonate chain transfer agent

Abstract

This thesis deals with ethylene oligomerization and polymerization with novel transition metal catalysts and trithiocarbonate chain transfer agent. Chapter 1 introduces polyolefin, linear α-olefin and functional polyolefin. In Chapter 2, a series of $\alpha$ -diimine ligands and corresponding Ni complexes having pentacyclic structure were prepared by TiCl4/Dabco catalytic system. Most of the synthesized catalysts exhibited good activities for ethylene polymerizations in the presence of MAO and produced polyethylenes with branching densities similar to a typical $\alpha$ -diimine nickel catalyst. Increased steric bulkiness of the complexes leads to high activity, and the molecular weights of polyethylenes were varied as the structure of ligands and polymerization conditions were changed. An interesting feature of these complexes is observed in the polymer branching. It has been known that the branching density increases as steric bulkiness of the ligand increases with conventional $\alpha$ -diimine catalysts. But the polyethylenes formed by these catalysts at the same temperature show similar branching density while those branch distribution is markedly changed as the steric bulkiness varies, and thus the thermal property of the polyethylens is significantly affected. Change of the branch distribution as steric tuning of the ligands is attributed to the substantially different environment of active species created by the asymmetric geometry of the ligand. In Chapter 3, a series of bis(diphenylphosphino)amine ligands with amine donors tethered to nitrogen of the ligand backbones have been synthesized and transformed to ethylene oligomerization catalysts with chromium. Ethylene oligomerzation properties of the catalysts with MAO cocatalysts were investigated. It was found that catalytic properties were highly dependent on the structural and electronic nature of them. Interestingly, the catalyst having a similar ligand to the conventional diphosphinoamine catalysts produced statistical distribution of α-olefins via nonselective mechanism, but many of the catalysts made from ligands with amine donors showed ethylene tetramerization behaviors. Increased temperature from $30^\circ C$ to $60^\circ C$ leaded to the enhancement of 1-hexene and 1-octene selectivity while preserving good activity, which was significantly different from conventional diphosphinoamine catalysts. Thermal stability of the catalysts was also confirmed through tetramerization study at high temperature. In Chapter 4, Radical polymerization of ethylene with diethyl fumarate (DEF) leads to the formation of the alternating copolymer with minimum ethylene incorporation of 43.5 mol%. Systematic study was carried out on the effects of reaction conditions such as temperature, ethylene pressure and the presence of Lewis acid on the polymerization behavior. As the factors affecting the polymerization changed, the alternating copolymers with controlled molecular weights from 6,000 and 16,000 were obtained. At last, we demonstrated that controlled polymerization of ethylene and DEF via RAFT method produced the polyethylene copolymers having a highly alternating character.