Date of Award
Doctor of Philosophy
Brian K. Long
Craig E. Barnes, Alexei P. Sokolov, Gila E. Stein
This dissertation describes advances made within the Long Research Group to design single-site catalysts for olefin polymerizations, and for the synthesis of thermally cross-linkable polyolefins. We have 1) designed thermally robust Ni-based catalysts for ethylene polymerization, 2) expanded this thermal stability enhancement strategy to Co-based catalysts, and 3) developed thermally cross-linkable polyethylene that is facilitated by the rearrangement of a co-monomer.Catalysts employing late transition metals have been heavily studied for olefin polymerizations but their implementation in industry remains limited due to a variety of drawbacks. One specific limitation is the general thermal instability of these catalysts at temperatures commonly used for industrial polymerizations. We will herein demonstrate that the precatalyst bis[(2,6-dibenzhydryl-4-methylimino)acenaphthene] nickel(II) dibromide can be used to dramatically enhance the thermal stability of this family of Ni-based catalysts. This precatalyst proved to be thermally robust for ethylene polymerizations at temperatures as high as 90 °C and showing living polymerization behavior at temperatures as high as 75 °C.This bulky ligand was further expanded to sterically demanding Fe- and Co-based olefin polymerization catalysts bearing 2,6-bis(biphenylmethyl)-4-methylaniline substituted bis(imino)pyridine ligands were synthesized and evaluated for ethylene polymerization. Tthe extreme ligand bulk mitigated detectable chain-transfer to aluminum and associative chain-transfer events. These bulky Co catalysts display great thermal stability up to 80 °C and show enhanced thermal stability at 90 °C. These observations are attributed to the extreme steric demand by which the ligand mitigates catalyst transfer, deactivation, and decomposition.Lastly, materials that are accessible using catalysts currently employed in Industry. Industrial methods to cross-linked polyethylene are polymer irradiation and the incorporation of peroxides. These methods suffer from lack of control over cross-link bond formation and can result in a tacky polymer. We developed a thermally cross-linkable polyethylene that utilizes benzocyclobutene as a co-monomer. After polymerization, non-cross-linked films were formed and cross-linked by increasing the temperature to promote the thermal rearrangement of benzocyclobutene. These co-monomers can undergo cycloaddition with other activated benzocyclobutene co-monomers leading to covalently linked polyethylene chains. It will be demonstrated that cross-linking at temperatures above 200 °C yield cross-linked PEX films that show up to 82 % gel percent content.
Mitchell, Nolan Evan, "Development of Late Transition Metal Catalysts and Advanced Polyolefins. " PhD diss., University of Tennessee, 2018.
Available for download on Thursday, August 15, 2019