Tapered (cone-shaped) bottlebrush polymers were synthesized for the first time by ring-opening metathesis polymerization (ROMP) using a sequential-addition of macromonomers (SAM) strategy. Polystyrene macromonomers with molecular weights that increased from 1 to 10 kg mol −1 were polymerized in sequence to high conversion, yielding tapered bottlebrush polymers that could be visualized by atomic force microscopy (AFM).

We report here on three constitutionally isomeric peptides, each of which contains two glutamic acid residues and two lysine residues functionalized with S-aroylthiooximes (SATOs), termed peptide–H2S donor conjugates (PHDCs). SATOs decompose in the presence of cysteine to generate hydrogen sulfide (H2S), a biological signaling gas with therapeutic potential. The PHDCs self-assemble in aqueous solution into different morphologies, two into nanoribbons of different dimensions and one into a rigid nanocoil. The rate of H2S release from the PHDCs depends on the morphology, with the nanocoil-forming PHDC exhibiting a complex release profile driven by morphological changes promoted by SATO decomposition. The nanocoil-forming PHDC mitigated the cardiotoxicity of doxorubicin more effectively than its nanoribbon-forming constitutional isomers as well as common H2S donors. This strategy opens up new avenues to develop H2S-releasing biomaterials and highlights the interplay between structure and function from the molecular level to the nanoscale.

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.

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.