Solvent Effects in Grafting-through Ring-Opening Metathesis Polymerization

Abstract

Ring-opening metathesis polymerization (ROMP) utilizing Grubbs' third-generation catalyst ((H2IMes)(Cl)2(pyr)2RuCHPh) shows 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 (MM). 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 molecular weights (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 level of purity, led to significant differences in the propagation rate in these ROMP grafting-through reactions. Of the solvents tested, propagation rates in EtOAc and CH2Cl2 were approximately 4-fold and 2-fold faster, respectively, than 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 MM to bottlebrush polymer. The functional group on the ω chain end did not influence the rate of ROMP. Utilizing UV-Vis spectroscopy to measure catalyst 3 decomposition, the main polymer termination route in ROMP, we uncovered dramatic solvent effects, where the catalyst decomposed over ten 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.

Complex Polymer Architectures Using Ring-Opening Metathesis Polymerization: Synthesis, Applications, and Practical Considerations

“…The as-received and distilled THF showed <3% conversion to BB polymer, so k p,obs could not be determined in these two cases. Reproduced with permission from reference . Copyright 2022 American Chemical Society.…”

“…206 Focusing on G3, a recent study from our lab compared solvent type and purity for the synthesis of bottlebrush polymers using grafting-through ROMP studying six commonly used solvents (Figure 26). 207 We found several key factors in selecting a solvent for a G3catalyzed ROMP under air: First, purification of the solvent is unnecessary in most cases, although it was required for THF. Second, solvent influences the rate of propagation and the rate of catalyst decomposition, with EtOAc leading to a propagation rate ∼2−4 times faster than the other tested solvents.…”

Radical–radical coupling effects in the direct-growth grafting-through synthesis of bottlebrush polymers using RAFT and ROMP

“…Bisnorbornenyl species can also form even when radical polymerizations are not employed in the MM synthesis; for example, the diol impurity present in commercial monofunctionalized polyethylene glycol (PEG) can affect the synthesis of bottlebrush polymers. 13 Thus, synthetic methods to reduce the presence of bisnorbornenyl species in MMs are needed to achieve well-defined bottlebrush polymers, which are under investigation for the synthesis of complex polymer topologies [14][15][16][17][18] and applications including biomedicine, [19][20][21][22][23][24][25] electronic and transport materials, 26,27 elastomers, 6,[28][29][30][31][32] photonic crystals, [33][34][35][36][37] and nanoporous materials. [38][39][40][41] Two approaches are typically used to synthesize MMs monofunctionalized with a norbornene.…”

Tough, Self-Healing, Recyclable Bottlebrush Polyurethane Elastomer with a Skin-like Strain-Adaptive-Strengthening Property

“…Bottlebrush polymers exhibit very interesting physical features, which are manifested in a unique self-assembly under specific conditions. − Generally, the synthesis of bottlebrush polymers is mainly achieved via three approaches: grating-onto, , grafting-through, − and grafting-from. , In the approach of grafting-onto, a macroinitiator bearing initiating groups is first obtained, followed by transformation of prepolymerized side chains. Grafting-through demands a homopolymerization of macromonomers in the presence of qualified catalysts. − The method of grafting-from involves a macroinitiator and a subsequent polymerization of side chains. To obtain well-defined polymeric structure, size exclusion chromatography (SEC) provides a relative accurate evaluation of molecular weight and polydispersity.…”