Catalyst design for ethylene polymerisation : a study on long-chain branching

• Main Author: Gragert, Maria Magdalena
• Other Authors: Britovsek, George J. P.
• Published: Imperial College London 2014
• Subjects: 668.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.712817

Polyethylene (PE) is the most produced polyolefin worldwide. Since recently discovered long-chain branches (LCB) have been shown to improve the processability, this thesis concentrates on the synthesis of new catalysts capable of producing long-chain branched polyethylene (LCB PE), the investigation of the mechanism which leads to LCB, and the control of the branching degree. The synthesis of new Group IV metal complexes with aromatic carbon-donor ligands which are η1-bound to the metal centre is described. These ligands include biphenyls, diphenylpyridines and terphenyls. Furthermore, an amido complex has been prepared and its synthesis and characterisation is presented herein. All complexes with carbon-based ligands are highly sensitive to air and moisture. Only the use of metal precursors with electron donating ligands enabled the formation of such complexes and it is concluded that electron donating ligands stabilise the electron deficient metal centre and metal-carbon bond. Polymerisation studies showed that the new complexes are active ethylene polymerisation catalysts. Their activity usually increases with temperature. The effect of hydrogen on the polymerisation activity cannot be predicted. Sensitive rheology measurements of the polymer melts revealed the presence of long-chain branches and it has been shown that the Group IV metal complexes with carbon-based ligands described in this thesis are producing LCB PE. Initial experiments towards the investigation of the long-chain branch forming mechanism are described within this thesis. Although not finished, a suitable route to complexes bearing a long alkyl chain has been established, which can be pursued in further studies. A series of tertiary-alkyl amines has been prepared and reacted with a silane to give the targeted aminosilane ligand. The effect of alkyl chain substituents on a cyclopentadienyl ligand in metallocenes has additionally been investigated with respect to the formation of LCB. In order to control the number of LCB in a PE resin, a bicatalytic system involving a bis(imino)pyridineiron catalyst that produces linear PE, an α-diiminenickel catalyst that produces branched PE and a chain transfer agent (CTA) was investigated. The combination of chain shuttling and chain walking as common mechanisms involved in ethylene polymerisation with late transition metal complexes showed to be a promising approach for the control of branching in PE while using ethylene as only monomer feed. Not only can branching be induced by carefully selecting the catalyst ratio and amount of CTA, but it has been shown that chain straightening occurs.