CO2-Responsive polymers and their use in catalysis

Abstract

CO2-Responsive polymers, when attached to homogeneous or heterogeneous catalysts, can be used to facilitate catalyst recovery post-reaction, or control catalyst product selectivity. Two examples of CO2-responsive polymers in catalysis will be given: A.) A self-functionalizing atom transfer radical polymerization (ATRP) catalyst and initiator “Ru-InitPhos” was developed to tether CO2-responsive polymers to a tridentate phosphine-ruthenium catalyst complex. These polymers provide on-demand phase transfer and catalyst recover with the addition and removal of CO2 from the system. B.) Insoluble CO2-responsive polymers were demonstrated to reversibly control the product selectivity of supported heterogenous metal nanoparticles used in hydrogenations with addition and removal of CO2 into the reaction vessel. Published homogeneous CO2-responsive catalysts often focus on recovery of the ligand and not the metal center. This work aims to recover both the ligand and metal atom by separating the responsive and ligand functionalities. Using the newly developed self-functionalizing catalyst “Ru-Initphos”, CO2-responsive polymers were grown directly from the ligand backbone. These polymer-tagged catalysts were used in hydrogenation reactions and then recovered post-reaction using their CO2-switchable solubility. Beyond CO2-responsive polymers, Ru-InitPhos was used to catalyze its self-functionalization with thermo-responsive and hydrophobic polymers for recovery after use in hydrogenation. Further work in this thesis demonstrates the use of insoluble CO2-responsive polymers as the support for metal nanoparticles for use as switchable selective hydrogenation catalysts. Catalysts were prepared using three different metals including ruthenium, rhodium, and palladium. Ruthenium nanoparticles supported on CO2-responsive polymeric materials showed switchable selectivity in the hydrogenation of carbonyl groups and bicyclic heterocycles. Catalysts prepared with rhodium particles supported on CO2-responsive polymer gels demonstrated switchable selectivity in aryl ring hydrogenations. CO2-responsive palladium catalysts showed the least switchable selectivity in hydrogenation with a change in product distribution based on the addition or removal of CO2 observed with only one substrate, benzonitrile. To better understand how CO2-switchable materials behave in more complex systems, a mathematical model describing the protonation behaviour of two-base solutions was developed. This model can be used to help guide the selection of CO2-responsive materials for use in two-component switchable systems.