Overcoming Challenges in Reversible Addition–Fragmentation Chain Transfer Polymerization using Photoinduced Electron/Energy Transfer Catalysis

Abstract

The development of polymerization methodologies is discussed, with an emphasis on addressing two limitations in reversible addition–fragmentation chain transfer (RAFT) polymerization. Both methods employ photoinduced electron/energy transfer (PET) catalysis to generate radicals within the polymerization system. PET catalysis was selected as the initiation pathway because the rate of radical introduction is tunable across a wide range of conditions, including, but not limited to, photocatalyst identity, photocatalyst concentration, wavelength of light, light intensity, and temperature. The first limitation that is discussed is the incorporation of a single monomer unit at a defined position within the backbone of a polymer chain. Previously, a single monomer unit could be incorporated only at the beginning or end of a polymer chain in reversible-deactivation radical polymerization (RDRP), or otherwise, single units could undergo multiple incorporations. Using expansive condition screening with PET-RAFT polymerization, a set of conditions was identified that resulted in a single-unit monomer insertion (SUMI) within the polymer backbone. The reaction depended on polymer concentration, monomer concentration, and temperature, and resulted in no detectable double- or higher-order insertions. The second limitation addressed was the blocking order requirements associated with both RAFT polymerization and RDRP. This limitation had previously been studied in the field, but it either exhibited termination reactions or applied only to specific systems. Using in-depth kinetic experiments and computational studies, we identified unique conditions that enabled us to overcome the blocking-order requirements associated with RDRP. Polymer and photocatalyst concentrations were crucial to the success of the reaction. Using this method, the impact of blocking order on material properties was evaluated and found to affect material behavior significantly. The method yielded a novel high-molecular-weight thermoplastic elastomer that retained its shape. Expanding on the second method, we sought to maximize chain end fidelity and further elucidate the underlying rates of the technique. By coupling the development of new characterization methods to quantify rates within the system and extensive kinetic experiments, the ratio of the rate of trithiocarbonate activation to the rate of trithiocarbonate termination could be measured. By optimizing conditions to achieve a high ratio of trithiocarbonate activation to termination, a significant increase in chain end fidelity relative to the previously identified conditions was achieved and led to a deeper understanding of which conditions are vital to the method. Lastly, the impact of PET-RAFT polymerization on the uniformity and properties of polymer networks is discussed. In this study, PET-RAFT polymerization yielded controlled networks initially, but could not yield controlled chain extended networks, resulting from decreased chain mobility in the networks. However, PET-RAFT polymerization enabled access to tunable properties, resulting in changes to the hydrophobicity and swelling ratios of the networks.