Polymer Microstructure Regulation in Olefin Polymerization by Pd-diimine Catalysts
Basbug Alhan, Hatice Elmas
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While the origins are a pervasive approach to olefin polymerization have encompassed early transition metals, late transition metals act as the most ubiquitous platforms to obtain functional polyolefins. Following the development of α-diimine palladium and nickel catalysts, which proved to be more tolerant than their early transition metal counterparts, research has investigated their mechanism, catalyst design, and monomer scope. While many approaches tackle catalyst design to tune reactivity, the application of a controlled stimulus has seen recent growth. One such approach encompasses metal-organic insertion light initiated radical (MILRad) polymerization; a method established in the Harth group. The investigation of the Pd-diimine methacrylate chelate opening and activation led to the discovery that Lewis acid cocatalysts can enable an exchange of the ancillary ligand which can have profound effects on the polyolefin microstructure. Chapter 1 of this thesis explores this new mechanism on how Lewis acids achieve a change in branching and architecture and is studied with a series of Lewis acids and is demonstrated in various olefins. The main breakthrough of this work was the realization that with the addition of a cocatalyst during polymerization, the structure of the polymer is changed, and polyolefin branched block copolymers can be generated without using a secondary catalyst. This achievement led to a further investigation on the influence of the ancillary ligand quantity on isomerization, polymerization, and branching which is discussed in Chapter 2. We discovered that a tuning between these pathways is possible and switching between these routes can be maintained which had not been described before. In Chapter 3, novel polymer architectures were designed based on the discovered branching modulation. Here, we targeted gradient polyolefin architectures which have not been described so far and are anticipated to play a major role in tuning polyolefin properties in terms of their elasticity and other mechanical properties. This thesis laid the foundation and opened up a new venue to access polyolefins block copolymers and gradient copolymers by the discovery of the importance and mechanistic features of added cocatalysts such as Lewis acids, and ancillary ligands interacting with Pd-diimine Brookhart type catalysts.