Heterometallic Catalysts for Olefin Polymerization



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Coordinative insertion polymerization is a useful method for polyethylene synthesis, due to its ability to give narrowly dispersed polyolefins under mild reaction conditions. Polymerizations using early transition metal catalysts, however, are not compatible with polar monomers due to irreversible coordination inhibition by functional groups. The discovery of late transition metal (e.g.,Ni, Pd) catalysts for olefin polymerization was an exciting breakthrough because they allowed the preparation of polar-group functionalized polyethylenes. However polymerizations using polar monomers is slow because they tend to form metal-chelated intermediates. To solve the coordination inhibition problem, we designed heterobimetallic complexes that could cooperatively react. Ideally, the primary metal is active for polymerization and the secondary metal assists in monomer binding and insertion. In Chapter 2, we prepared a new class of nickel complexes bearing the 1,2,3-triazole-4-carboxamidate group. Upon activation by treatment with Ni(COD)2, the first generation nickel complexes are active as catalysts for ethylene polymerization. However, upon the addition of zinc triflate, we could not obtain discrete bimetallic complexes. These results suggest that the use of dinucleating ligands is needed to provide well-defined nickel-zinc catalysts. To improve zinc chelation by the nickel triazolecarboxamide catalysts, we introduced picolyl donors to the γ-positions of their triazole rings. We prepared a new series of organometallic nickel complexes ligated by 1-picolyl-4-carboxamidate-1,2,3-triazole donors. Solution metal binding studies by NMR spectroscopy indicated that their reactions with zinc chloride led to the formation of nickel-zinc heterometallics with undefined compositions. These nickel-zinc complexes showed increasing ethylene polymerization activity over the course of 3 h and provided polymers with multimodal distributions. In the final part of this dissertation, we synthesized a new type of phosphine-phosphonate ester ligand containing diethylene glycol chains for the preparation of heterobimetallic complexes. Our heterobimetallic complexes exhibit significant enhancement in catalytic activity and thermal stability compared to their parent monometallic catalysts. Most excitingly, by performing reactions in THF instead of toluene, we were able to expand the use of secondary metal salts beyond those of the alkali series. The most active heterobimetallic catalysts for ethylene and co-polymerization were generated using either Zn2+ or Co2+ with nickel.



Nickel catalysts, Olefin polymerization


Portions of this document appear in: Cai, Zhongzheng, Dawei Xiao, and Loi H. Do. "Heterobimetallic Catalysts in Coordination Insertion Polymerization." (2019). And in: Xiao, Dawei, and Loi H. Do. "Triazolecarboxamidate Donors as Supporting Ligands for Nickel Olefin Polymerization Catalysts." Organometallics 37, no. 2 (2018): 254-260. And in: Xiao, Dawei, and Loi H. Do. "In Situ Generated Heterometallic Nickel–Zinc Catalysts for Ethylene Polymerization." Organometallics 37, no. 18 (2018): 3079-3085.