Improved Nickel-Catalyzed Catalyst-Transfer Polycondensation via Ligand Design.

Chapter 1 provides an introduction to π-conjugated polymers and their applications. Understanding the structure-property relationships is crucial in expanding the application scope for π-conjugated polymers. Currently, the catalyst transfer polycondensation is used to synthesize well-defined polymers. This thesis details our efforts to overcome limitations, chain transfer and slow initiation, to promote controlled chain-growth polymerization. Chapter 2 demonstrates our efforts to minimize chain transfer in catalyst transfer polycondensation by modifying the electronics of ancillary ligands of Ni precatalysts. We showed that electron-rich ligands promote chain-growth behavior by stabilizing Ni-polymer π-complex and increasing the intramolecular oxidative addition rate. During this investigation, we found that the initiation is substantially slower than the propagation rate and that modification of ancillary ligands can not increase initiation rate selectively. Chapter 3 describes the importance of fast initiation in controlled catalyst transfer polycondensation and our efforts to selectively increase the initiation rate without increasing the propagation rate. We found that modifying the reactive ligands leads to change in the initiation rate. Computational studies demonstrated that the delocalization of charges on Ni catalyst by reactive ligands during the initiation is the key to increasing the initiation rate. Chapter 4 demonstrates our efforts to increase the initiation rate beyond the propagation rate by modifying Ni precatalysts. We selected a number of heteroaromatic and aromatic reactive ligands and calculated the activation barriers using the computational method developed in Chapter 3. The computational study allowed us to narrow down the suitable targets, which we synthesized and tested experimentally. We found that a biphenyl-based reactive ligand leads to a faster initiation rate than propagation rate and promotes controlled chain-growth behavior. Chapter 5 describes our findings, specifically the impact of ligand electronics on controlled catalyst trasfer polycondensation. Additionally, the future directions are presented, specifically implementing the computational method for other polymer syntheses, evaluating more electron-rich ancillary ligand for chain-growth, and exploring non-symmetric ancillary ligands to expand the ligand scope for current CTP method. The importance of thorough mechanistic studies and understanding of Ni precatalysts for expanding monomer scope and accessing copolymers are emphasized.