Solvent Effects in Grafting-through Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) utilizing Grubbs' third-generation catalyst ((H2IMes)(Cl)2(pyr)2RuCHPh) shows the characteristics of living polymerizations, including molecular weights increasing with monomer conversion and the ability to make (multi)block copolymers. However, irreversible termination reactions still occur due to catalyst decomposition, leading to terminated chains, especially in the context of sterically demanding monomers such as macromonomers (MMs). In this work, we performed identical ROMP reactions on three different MMs in six solvents commonly used in ROMP with varying levels of purity. The solvents included ethyl acetate (EtOAc), dichloromethane (CH2Cl2), chloroform (CHCl3), toluene, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). All polymerizations were conducted under air targeting a bottlebrush polymer backbone degree of polymerization (Nbb) of 100. All three MMs included a norbornene on the α chain end and had a molecular weight (Mn) of ∼4 kg/mol. They included one polystyrene MM with a bromine on the ω chain end and two poly(n-butyl acrylate) MMs with either a bromine or a trithiocarbonate group on the ω chain end. Solvent choice, and in some cases the level of purity, led to significant differences in the propagation rate in these ROMP grafting-through reactions. Of the solvents tested, the propagation rates in EtOAc and CH2Cl2 were approximately 4-fold and 2-fold faster, respectively, than that in CHCl3, toluene, and THF for all MMs. Propagation was much slower in DMF for the polystyrene MM than all the other solvents and on par with the slower solvents for the two poly(n-butyl acrylate) MMs tested. The purity of the solvent in some cases had a profound effect on the propagation rate: In the case of EtOAc, purification led to a 2-fold decrease in propagation rate; in contrast, purification of THF was required to observe full conversion of a MM to a bottlebrush polymer. The functional group on the ω chain end did not influence the rate of ROMP. Utilizing UV–vis spectroscopy to measure catalyst decomposition, the main polymer termination route in ROMP, we uncovered dramatic solvent effects, where the catalyst decomposed over 10 times faster in THF and DMF than in toluene. Finally, studies targeting Nbb = 500 or 1000 revealed that toluene, EtOAc, and CH2Cl2 demonstrated the highest degree of "livingness" in ROMP. These results will enable the synthesis of complex polymer architectures using ROMP with a high degree of living character.