A systematic series of 16 amphiphilic bottlebrush block copolymers (BCPs) containing polystyrene and poly(N-acryloylmorpholine) (PACMO) side chains were prepared by a combination of atom-transfer radical polymerization (ATRP), photoiniferter polymerization, and ring-opening metathesis polymerization (ROMP). The grafting-through method used to prepare the polymers enabled a high degree of control over backbone and side-chain molar masses for each block. Surface tension measurements on the self-assembled amphiphilic bottlebrush BCPs in water revealed an ultralow critical micelle concentration (cmc), 1−2 orders of magnitude lower than linear BCP analogues on a molar basis, even for micelles with >90% PACMO content. Combined with coarse-grained molecular dynamics simulations, fitting of small-angle neutron scattering traces (SANS) allowed us to evaluate solution conformations for individual bottlebrush BCPs and micellar nanostructures for self-assembled macromolecules. Bottlebrush BCPs showed an increase in anisotropy with increasing PACMO content in toluene-d 8 , which is a good solvent for both blocks, reflecting an extended conformation for the PACMO block. SANS traces of bottlebrush BCPs assembled into micelles in D 2 O, a selective solvent for PACMO, were fitted to a core−shell−shell model, suggesting the presence of a partially hydrated inner shell. Results showed an average micelle diameter of 40 nm with combined shell diameters ranging from 16 to 18 nm. A general trend of increased stability of micelles (i.e., resistance to precipitation) was observed with increases in PACMO content. These results demonstrate the stability of bottlebrush polymer micelles, which self-assemble to form spherical micelles with ultralow (<70 nmol/L) cmc's across a broad range of compositions.

Nature shows us that complex molecular architectures lead to unique material properties, and these observations have driven polymer scientists to synthesize complex architectures in an effort to discover how topology influences properties in synthetic polymers. In this Perspective, we discuss a variety of complex architectures synthesized using ringopening metathesis polymerization (ROMP), including multiblock linear polymers, bottlebrush homopolymers and (multi)block copolymers, dendronized polymers, star polymers, and polymer−biomolecule conjugates. Traditional and recently developed synthetic methods, including polymerization-induced self-assembly, copolymerization to create gradient structures, and engineering approaches to making complex topologies using ROMP, are also reviewed. In this context, we highlight emerging applications stemming from these materials, including drug delivery vehicles, nanoscale constructs, and components in light refraction or energy storage, among others. Finally, we conclude with an indepth discussion on practical considerations in ROMP that enable the highest level of control when synthesizing complex polymer topologies from sterically demanding or otherwise challenging (macro)monomers. Our hope is that this Perspective will guide scientists synthesizing complex polymer architectures toward new and innovative materials with the potential for unique properties and applications.

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.

We report a synthetic route toward a family of functional COS/H 2 S-releasing N-substituted Nthiocarboxyanhydrides (NTAs) with functionalities to accommodate popular conjugation reactions, including olefin cross metathesis, thiol-ene, and copper-catalyzed azide-alkyne cycloaddition. The N-substituted NTAs were attached to small molecules, polymers, and a protein to synthesize novel H 2 S donors convergently. All conjugates showed sustained H 2 S release kinetics.

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs’ third-generation catalyst [G3, (H2IMes)(Cl)2(pyr)2RuCHPh] is widely used to make bottlebrush polymers by polymerization of a macromonomer (MM), typically a low molecular weight polymer functionalized with a norbornene. Termed the grafting-through method, this strategy requires a high degree of living character (“livingness”) to form well-defined bottlebrush polymers. Here, we studied how various anchor groups, the series of atoms connecting the polymerizable norbornene unit to the polymer side-chain, affect livingness in ROMP in a series of exo-norbornene polystyrene MMs. First, we calculated the HOMO and HOMO/LUMO gap energies of MM structures containing five different anchor groups using density functional theory methods, finding that these energies spanned a range of 10 kcal/mol. We then performed kinetics experiments on each MM with target backbone degrees of polymerization (N bb) of 100 to measure the propagation rate constant (k p,obs) under identical conditions. A positive correlation between the HOMO energy and measured k p,obs values emerged, revealing a 7-fold variation in k p,obs values across the five MMs, suggesting different degrees of livingness among the anchor groups. A series of studies targeting N bb values ranging from 100 to 2000 further highlighted these differences: The MMs with high k p,obs values reached higher conversions at high target N bb values with lower dispersities (D̵) than the MMs with lower k p,obs values. Finally, we evaluated the synthesis of bottlebrush pentablock copolymers using the MMs at the two extremes by injecting an MM aliquot into a catalyst solution five consecutive times, allowing for polymerization of each block before the next injection. MM conversion at each step was higher, and the D̵ values for each block were lower for the MM with the highest k p anchor group compared to the lowest k p anchor group. Taken together, these studies highlight how the anchor group dramatically affects both k p and livingness in ROMP, which is crucial for the synthesis of precise bottlebrush (co)polymers.