Pi-Complex Behavior in Catalyst-Transfer Condensation Polymerizations

[摘要] Conjugated polymers combine mechanical flexibility with light absorption/emission and charge conductivity, stimulating their use in transistors, light emitting diodes, and solar cells.Living, chain-growth methods enable polymeric properties (e.g., miscibility, morphology, charge transport) to be tailored by controlling molecular weight, end-groups, and copolymer sequence.However, synthetic challenges have limited the scope of conjugated copolymers produced in this fashion.The distinguishing feature of chain-growth polymerizations of conjugated monomers is that the catalyst remains associated with a single polymer chain during the polymerization.This association is due to a catalyst-polymer π-complex, which may be able to migrate along the polymer to react at both ends—preventing sequence and end-group control. We have recently demonstrated that IPrPd(3-chloropyridine)Cl2 is a promising catalyst for thiophene and phenylene copolymers, which were previously inaccessible. However, control over polymer sequence can only be exerted if the catalyst reacts at a single polymer end. This thesis details how the π-complex directs intramolecular reactivity using four popular precatalysts in catalyst-transfer condensation polymerizations, how π-complex reactivity is heavily influenced by the polymer, and catalyst design strategies to overcome reactions at both polymer ends.Chapter 1 discusses the advantages of chain-growth polymerization over conventional, step-growth polymerizations to control polymer Mn, Đ, and copolymer sequence. Despite the limited monomer scope, several examples are discussed that illustrate how copolymer sequence can improve solar cell stability when employed as the main component or an additive. We introduce how preferential π-binding and catalyst migration present major obstacles to copolymerizations, but that the relationship between catalyst/polymer properties and their π-complex behavior is limited.Chapter 2 details an end-capping model system that investigates the strength of catalyst-polymer π-complexes in catalyst-transfer condensation polymerization. The π-complex is challenged by forcing end-to-end migration across a poly(3-alkylthiophene) backbone with an excess of highly reactive competitive agent in the reaction mixture. Near quantitative migration products suggest that all catalysts bind to the polymer tight enough to resist chain-transfer, but the catalyst is highly mobile and can migrate across many repeat units to react at both ends.Chapter 3 expands our investigation into end-to-end catalyst migration by changing the polymer. Here we observe large differences in the interactions between the different catalysts and poly(phenylene) that are influenced by the ligand identity and transition metal. The random-walking statistical model, commonly used to describe migration, provides an incomplete fit with our experimental results.Chapter 4 investigates precatalysts that transfer a reactive ligand to the polymer during initiation to prevent reaction on both polymer ends (enabled by end-to-end migration). We identify and optimize a promising lead, but reactive ligand transfer during initiation was incomplete. Polymerization additives improved precatalyst control over molecular weight and dispersity, but had little effect on reactive ligand transfer during initiation.Chapter 5 describes how our investigations into π-complex stability/catalyst mobility could relate to polymerization and/or copolymerization behavior. We note that catalyst migration can be a highly desirable trait for expanding the monomer and comonomer scope, which are major limitations in catalyst-transfer condensation polymerizations. Continued efforts are required to elevate these living, chain-growth methods into widespread use for conjugated polymer synthesis.

[发布机构] University of Michigan

[效力级别] polymer

[关键词] catalyst;polymer;conjugated polymer;mechanism;CTP;CTCP;Chemistry;Science;Chemistry