Investigating the fundamentals of ring-opening metathesis polymerization to synthesize large, well-defined, bottlebrush polymers

Ring-opening metathesis polymerization (ROMP) is a robust synthetic technique for synthesizing complex polymer architectures (topologies). To achieve complex architectures, specifically bottlebrush polymers, using ROMP, attaining the highest degree of living character is essential. As the molecular weight of the side chain or backbone increases, the "livingness" of the polymerization suffers due to premature catalyst degradation. Attaining large, well-defined, bottlebrush polymers requires precision so it was our goal to determine how seemingly simple reaction variables could affect the rates of propagation and achievable conversion, as well as why these variables have these effects. We tested several reaction parameters to understand how they affect the rate of polymerization, the rate of catalyst degradation, and the conversion that can be reached. We performed a systematic study using six organic solvents to determine the propagation rate of three macromonomers (MMs), one polystyrene and two poly(n-butyl acrylate) MMs, in ROMP with varying side chain chemistries and end groups, as well as rate of catalyst degradation in each of the solvents. We determined that solvent affected that rate of propagation primarily by interacting with the catalyst, while there was some evidence of polymer sidechain chemistry affecting the rate. We found that ethyl acetate (EtOAc) and CH2Cl2 had the highest rates of propagation compared to the other solvents, while DMF and THF were the slowest. UV-Vis testing on the catalyst in each solvent revealed that DMF and THF had fast rates of catalyst decomposition, while toluene was much slower to decompose. From these experiments we learned that toluene, despite its slower propagation rate, has the most living character, due to its slower rate of decomposition. We also learned that purification greatly affects the propagation rate, with THF requiring purification to have any conversion to bottlebrush polymer, while purification of EtOAc slows the rate of propagation almost 2-fold. From the decrease in rate after purification, and the conclusion that it was due to an acetic acid impurity in the impure EtOAc, we decided to systematically test small molecule additives and found that acids can increase the propagation rate and the conversion of the polynorbornene backbone achievable in ROMP reactions. Notably, in reactions performed in DMF with added CF3COOH we were able to polymerize a norbornene-functionalized unprotected peptide, which was insoluble in most organic solvents, to a higher conversion than in DMF without the added acid. We learned from our research that changing reaction variables can lead to substantial changes in the rate of propagation as well as the achievable conversion in bottlebrush polymer synthesis. By understanding this we can further test other reaction variables and do systematic studies on atmosphere and temperature. We hope this research and future fundamental research can guide scientists toward synthesizing large, well-defined, complex polymer architectures using ROMP.