Bottlebrush Polymer Synthesis by Ring-Opening Metathesis Polymerization: The Significance of the Anchor Group

Design, Synthesis, and Self-Assembly of Janus Bottlebrush Polymers

Janus bottlebrush polymers are a class of special molecular brushes, which have two immiscible side chains on the repeating unit of the backbone. The characteristic architectures of Janus bottlebrush polymers enable unique self-assembly properties and broad applications. Recently, remarkable advances of Janus bottlebrush polymers have been achieved for polymer chemistry and material science. This review summarizes the synthetic strategies of Janus bottlebrush polymers, and highlights the self-assembly applications. Finally, the challenges and opportunities are proposed for the further development.

Alternating Cascade Metathesis Polymerization of Enynes and Cyclic Enol Ethers with Active Ruthenium Fischer Carbenes

Ruthenium alkoxymethylidene complexes have rarely been demonstrated as active species in metathesis reactions and are frequently regarded as inert. Herein, we highlight the ability of these Fischer-type carbenes to participate in cascade alternating ring-opening metathesis polymerization through their efficient alkyne addition reactions. When enyne monomers are combined with low-strain cyclic vinyl ethers, a controlled chain-growth copolymerization occurs that exhibits high degrees of alternation (>90% alternating diads) and produces degradable poly(vinyl ether) materials with low dispersities and targetable molecular weights. This new method is amenable to the synthesis of alternating diblock polymers that can be degraded to small-molecule fragments under aqueous acidic conditions. This work furthers the potential of Fischer-type ruthenium alkylidenes in polymerization strategies and presents new avenues for the generation of functional metathesis materials.

Impact of Multivalence and Self-Assembly in the Design of Polymeric Antimicrobial Peptide Mimics

Antimicrobial resistance is an increasingly serious challenge for public health and could result in dramatic negative consequences for the health care sector during the next decades. To solve this problem, antibacterial materials that are unsusceptible toward the development of bacterial resistance are a promising branch of research. In this work, a new type of polymeric antimicrobial peptide mimic featuring a bottlebrush architecture is developed, using a combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and ring-opening metathesis polymerization (ROMP). This approach enables multivalent presentation of antimicrobial subunits resulting in improved bioactivity and an increased hemocompatibility, boosting the selectivity of these materials for bacterial cells. Direct probing of membrane integrity of treated bacteria revealed highly potent membrane disruption caused by bottlebrush copolymers. Multivalent bottlebrush copolymers clearly outperformed their linear equivalents regarding bioactivity and selectivity. The effect of segmentation of cationic and hydrophobic subunits within bottle brushes was probed using heterograft copolymers. These materials were found to self-assemble under physiological conditions, which reduced their antibacterial activity, highlighting the importance of precise structural control for such applications. To the best of our knowledge, this is the first example to demonstrate the positive impact of multivalence, generated by a bottlebrush topology in polymeric antimicrobial peptide mimics, making these polymers a highly promising material platform for the design of new bactericidal systems.

Color, structure, and rheology of a diblock bottlebrush copolymer solution

A structure-property-process relation is established for a diblock bottlebrush copolymer solution, through a combination of rheo-neutron scattering, imaging, and rheological measurements. Polylactic acid-b-polystyrene diblock bottlebrush copolymers were dispersed in toluene with a concentration of 175 mg ml-1, where they self-assembled into a lamellar phase. All measurements were carried out at 5 °C. The solution color, as observed in reflection, is shown to be a function of the shear rate. Under equilibrium and near-equilibrium conditions, the solution has a green color. At low shear rates the solution remains green, while at intermediate rates the solution is cyan. At the highest rates applied the solution is indigo. The lamellar spacing is shown to be a decreasing function of shear rate, partially accounting for the color change. The lamellae are oriented 'face-on' with the wall under quiescence and low shear rates, while a switch to 'edge-on' is observed at the highest shear rates, where the reflected color disappears. The intramolecular distance between bottlebrush polymers does not change with shear rate, although at high shear rates, the bottlebrush polymers are preferentially aligned in the vorticity direction within the lamellae. We therefore form a consistent relation between structure and function, spanning a wide range of length scales and shear rates.

Tunable structural color of bottlebrush block copolymers through direct-write 3D printing from solution

Additive manufacturing of functional materials is limited by control of microstructure and assembly at the nanoscale. In this work, we integrate nonequilibrium self-assembly with direct-write three-dimensional (3D) printing to prepare bottlebrush block copolymer (BBCP) photonic crystals (PCs) with tunable structure color. After varying deposition conditions during printing of a single ink solution, peak reflected wavelength for BBCP PCs span a range of 403 to 626 nm (blue to red), corresponding to an estimated change in d-spacing of >70 nm (Bragg- Snell equation). Physical characterization confirms that these vivid optical effects are underpinned by tuning of lamellar domain spacing, which we attribute to modulation of polymer conformation. Using in situ optical microscopy and solvent-vapor annealing, we identify kinetic trapping of metastable microstructures during printing as the mechanism for domain size control. More generally, we present a robust processing scheme with potential for on-the-fly property tuning of a variety of functional materials.

Forcing single-chain nanoparticle collapse through hydrophobic solvent interactions in comb copolymers

We demonstrate that we can tune the chain collapse of comb copolymers into single-chain nanoparticles upon UV irradiation through solvency control.

Tailored silyl ether monomers enable backbone-degradable polynorbornene-based linear, bottlebrush and star copolymers through ROMP

Ring-opening metathesis polymerization of norbornene-based (macro)monomers is a powerful approach for the synthesis of macromolecules with diverse compositions and complex architectures. Nevertheless, a fundamental limitation of polymers prepared by this strategy is their lack of facile degradability, limiting their utility in a range of applications. Here we describe a class of readily available bifunctional silyl ether-based cyclic olefins that copolymerize efficiently with norbornene-based (macro)monomers to provide copolymers with backbone degradability under mildly acidic aqueous conditions and degradation rates that can be tuned over several orders of magnitude, depending on the silyl ether substituents. These monomers can be used to manipulate the in vivo biodistribution and clearance rate of polyethylene glycol-based bottlebrush polymers, as well as to synthesize linear, bottlebrush and brush-arm star copolymers with degradable segments. We expect that this work will enable preparation of degradable polymers by ROMP for biomedical applications, responsive self-assembly and improved sustainability.

Amphiphilic multicomponent molecular brushes

Multicomponent molecular brushes containing amphiphilic polymer moieties are promising objects of research of macromolecular chemistry. The development of stimulus-responsive systems sensitive to changes in environmental parameters, based on the molecular brushes, opens up new possibilities for their applications in medicine, biochemistry and microelectronics. The review presents the current understanding of the structures of main types of amphiphilic multicomponent brushes, depending on the chemical nature and type of coupling of the backbone and side chains. The approaches to the controlled synthesis of multicomponent molecular brushes of different architecture are analyzed. Self-assembly processes of multicomponent molecular brushes in selective solvents are considered.

The bibliography includes 259 references.

Mechanistic and Kinetic Studies of the Ring Opening Metathesis Polymerization of Norbornenyl Monomers by a Grubbs Third Generation Catalyst

The mechanism of ring-opening metathesis polymerization (ROMP) for a set of functionalized norbornenyl monomers initiated by a Grubbs third generation precatalyst [(H₂IMes)(pyr)₂(Cl)₂Ru═CHPh] was investigated. Through a series of ¹²C/¹³C and ¹H/²H kinetic isotope effect studies, the rate-determining step for the polymerization was determined to be the formation of the metallacyclobutane ring. This experimental result was further validated through DFT calculations showing that the highest energy transition state is metallacyclobutane formation. The effect of monomer stereochemistry (exo vs endo) of two types of ester substituted monomers was also investigated. Kinetic and spectroscopic evidence supporting the formation of a six-membered chelate through coordination of the proximal polymer ester to the Ru center is presented. This chelation and its impact on the rate of polymerization are shown to vary based on the monomer employed and its stereochemistry. The combination of this knowledge led to the derivation of a generic rate law describing the rate of polymerization of norbornene monomers initiated by a Grubbs third generation catalyst.

Grafting Robust Thick Zwitterionic Polymer Brushes via Subsurface-Initiated Ring-Opening Metathesis Polymerization for Antimicrobial and Anti-Biofouling

In the present work, high-thickness zwitterionic polymer brushes based on imidazolium salts were successfully grafted via a novel subsurface-initiated ring-opening metathesis polymerization (subsurface-initiated ROMP) from polydimethylsiloxane (PDMS), and their antifouling performance was evaluated in detail. First, an initiator-embedded PDMS was prepared via copolymerization of PDMS prepolymer and ROMP initiator, and then zwitterionic polymer brushes were grafted via subsurface-initiated ROMP from surface to subsurface of the PDMS due to the implanted ROMP initiator. Results from a series of characterization methods such as infrared spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy proved the zwitterionic polymer brushes' successful grafting. The grafting thickness of zwitterionic polymer brushes via subsurface-initiated ROMP can reach the micron scale, and the as-prepared zwitterionic polymer based surfaces showed good lubricating properties compared to traditional surface-initiated ROMP, which hints that polymer brushes can be grafted not only on the surface but also on the subsurface of PDMS. The protein adhesion test and biofouling assay of zwitterionic polymer brushes were tested in the laboratory, and the results indicated that the zwitterionic polymer-functionalized PDMS can effectively resist the adhesion of bovine serum albumin and algae (Porphyridium and Dunaliella) and has good anti-bacterial activity against both Escherichia coli and Staphylococcus aureus.

Predicting Monomers for Use in Aqueous Ring-Opening Metathesis Polymerization-Induced Self-Assembly

Aqueous polymerization-induced self-assembly (PISA) is a well-established methodology enabling in situ synthesis of polymeric nanoparticles of controllable morphology. Notably, PISA via ring-opening metathesis polymerization (ROMPISA) is an emerging technology for block copolymer self-assembly, mainly due to its high versatility and robustness. However, a limited number of monomers suitable for core-forming blocks in aqueous ROMPISA have been reported to date. In this work, we identified seven monomers for use as either corona- or core-forming blocks during aqueous ROMPISA by in silico calculation of relative hydrophobicity for corresponding oligomeric models. The predicted monomers were validated experimentally by conducting ROMPISA using our previously reported two-step approach. In addition to predictive data, our computational model was exploited to identify trends between polymer hydrophobicity and the morphology of the self-assembled nano-objects they formed. We expect that this methodology will greatly expand the scope of aqueous ROMPISA, as monomers can be easily identified based on the structure-property relationships observed herein.

Dilute solution structure of bottlebrush polymers

Bottlebrush polymers are a class of macromolecules that have recently found use in a wide variety of materials, ranging from lubricating brushes and nanostructured coatings to elastomeric gels that exhibit structural colors. These polymers are characterized by dense branches extending from a central backbone and thus have properties distinct from linear polymers. It remains a challenge to specifically understand conformational properties of these molecules, due to the wide range of architectural parameters that can be present in a system, and thus there is a need to accurately characterize and model these molecules. In this paper, we use a combination of viscometry, light scattering, and computer simulations to gain insight into the conformational properties of dilute solution bottlebrush polymers. We focus on a series of model bottlebrushes consisting of a poly(norbornene) (PNB) backbone with poly(lactic acid) (PLA) side chains. We demonstrate that intrinsic viscosity and hydrodynamic radius are experimental observations sensitive to molecular architecture, exhibiting distinct differences with different choices of branches and backbone lengths. Informed by the atomistic structure of this PNB-PLA system, we rationalize a coarse-grained simulation model that we evaluate using a combination of Brownian dynamics and Monte Carlo simulations. We show that this exhibits quantitative matching to experimental results, enabling us to characterize the overall shape of the bottlebrush via a number of metrics that can be extended to more general bottlebrush architectures.

Getting into Shape: Reflections on a New Generation of Cylindrical Nanostructures' Self-Assembly Using Polymer Building Blocks

Cylinders are fascinating structures with uniquely high surface area, internal volume, and rigidity. On the nanoscale, a broad range of applications have demonstrated advantageous behavior of cylindrical micelles or bottlebrush polymers over traditional spherical nano-objects. In the past, obtaining pure samples of cylindrical nanostructures using polymer building blocks via conventional self-assembly strategies was challenging. However, in recent years, the development of advanced methods including polymerization-induced self-assembly, crystallization-driven self-assembly, and bottlebrush polymer synthesis has facilitated the easy synthesis of cylindrical nano-objects at industrially relevant scales. In this Perspective, we discuss these techniques in detail, highlighting the advantages and disadvantages of each strategy and considering how the cylindrical nanostructures that are obtained differ in their chemical structure, physical properties, colloidal stability, and reactivity. In addition, we propose future challenges to address in this rapidly expanding field.

Grubbs' and Schrock's Catalysts, Ring Opening Metathesis Polymerization and Molecular Brushes-Synthesis, Characterization, Properties and Applications

In this review, molecular brushes and other macromolecular architectures bearing a bottlebrush segment where the main chain is synthesized by ring opening metathesis polymerization (ROMP) mediated by Mo or Ru metal complexes are considered. A brief review of metathesis and ROMP is presented in order to understand the problems and the solutions provided through the years. The synthetic strategies towards bottlebrush copolymers are demonstrated and each one discussed separately. The initiators/catalysts for the synthesis of the backbone with ROMP are discussed. Syntheses of molecular brushes are presented. The most interesting properties of the bottlebrushes are detailed. Finally, the applications studied by different groups are presented.

A versatile synthetic strategy for macromolecular cages: intramolecular consecutive cyclization of star-shaped polymers

Cage-shaped polymers, or "macromolecular cages", are of great interest as the macromolecular analogues of molecular cages because of their various potential applications in supramolecular chemistry and materials science. However, the systematic synthesis of macromolecular cages remains a great challenge. Herein, we describe a robust and versatile synthetic strategy for macromolecular cages with defined arm numbers and sizes based on the intramolecular consecutive cyclization of highly reactive norbornene groups attached to each end of the arms of a star-shaped polymer precursor. The cyclizations of three-, four-, six-, and eight-armed star-shaped poly(ε-caprolactone)s (PCLs) bearing a norbornenyl group at each arm terminus were effected with Grubbs' third generation catalyst at high dilution. 1H NMR, SEC, and MALDI-TOF MS analyses revealed that the reaction proceeded to produce the desired macromolecular cages with sufficient purity. The molecular sizes of the macromolecular cages were controlled by simply changing the molecular weight of the star-shaped polymer precursors. Systematic investigation of the structure-property relationships confirmed that the macromolecular cages adopt a much more compact conformation, in both the solution and bulk states, as compared to their linear and star-shaped counterparts. This synthetic approach marks a significant advance in the synthesis of complex macromolecular architectures and provides a platform for novel applications using cage-shaped molecules with polymer frameworks.

Design, synthesis, and evaluation of heparan sulfate mimicking glycopolymers for inhibiting heparanase activity

Heparanase is an enzyme which cleaves heparan sulfate (HS) polysaccharides of the extracellular matrix. It is a regulator of tumor behavior, plays a key role in kidney related diseases and autoimmune diabetes. We report herein the use of computational studies to extract the natural HS-heparanase interactions as a template for the design of HS mimicking glycopolymers. Upon evaluation, a glycopolymer with 12 repeating units was determined to be the most potent inhibitor and to have tight-binding characteristics. This glycopolymer also lacks anticoagulant activity.

Well-Defined and Precision-Grafted Bottlebrush Polypentenamers from Variable Temperature ROMP and ATRP

Polypentenamer macroinitiators are synthesized through variable temperature ring opening metathesis polymerization of 3-cyclopentenyl α-bromoisobutyrate, which has sufficient ring strain (ΔH_p = -22.6 kJ mol^-1) to produce targeted molar mass (<5% from theoretical), low dispersity (1.17 ≤ Đ ≤ 1.23), and high conversion (∼72%). An initiation site for atom-transfer radical polymerization at every fifth backbone carbon allows "grafting-from" of styrene with quantitative initiation and linear molar mass increase with time. These bottlebrushes retain a low dispersity (Đ ≤ 1.34) at varying graft degrees of polymerization (5 ≤ N_sc ≤ 49) and have a glass transition temperature highly sensitized to graft length. Extension of the grafts with methyl methacrylate produces a core-shell brush polymer with high molar mass (>1000 kg mol^-1) and Đ = 1.33. This system exhibits high synthetic versatility and control with a unique flexible backbone to expand the suite of densely grafted polymers.

Enhanced Intrafibrillar Mineralization of Collagen Fibrils Induced by Brushlike Polymers

Biomimetic mineralization of collagen fibrils is an essential process because the mineralized collagen fibers constitute the basic building block of natural bone. To overcome the limited availability and high cost of the noncollagenous proteins (NCPs) that regulate the mineralization process of collagen, commercially available analogues were developed to replicate sequestration and templating functions of NCPs. The use of branched polymers in intrafibrillar mineralization applications has never been explored. In this work, two novel carboxyl-rich brushlike polymers, a carboxylated polyethylene glycol terpolymer (PEG-COOH) and a polyethylene glycol/poly(acrylic acid) copolymer (PEG-PAA), were synthesized and modified to mimic the sequestration function of NCPs to induce intrafibrillar mineralization of collagen fibrils. It was found that these synthetic brushlike polymers are able to induce intrafibrillar mineralization by stabilizing the amorphous calcium phosphate (ACP) nanoprecursors and subsequently facilitating the infiltration of ACP into the gap zone of collagen microfibrils. Moreover, the weight ratios of mineral to collagen in the mineralized collagen fibrils in the presence of these brushlike polymers were 2.17 ± 0.07 for PEG-COOH and 2.23 ± 0.03 for PEG-PAA, while it is only 1.81 ± 0.21 for linear PAA. Plausible mineralization mechanisms using brushlike polymers are proposed that offer significant insight into the understanding of collagen mineralization induced by synthetic NCP analogues.

Reversibly Cross-linkable Bottlebrush Polymers as Pressure-Sensitive Adhesives

Dynamically cross-linkable bottlebrush polymer adhesives were synthesized by the grafting-from strategy through a combination of ring-opening metathesis polymerization (ROMP) and photoiniferter polymerization. A norbornene-containing trithiocarbonate was first polymerized by ROMP to form the bottlebrush polymer backbone; this was followed by blue-light-mediated photoiniferter polymerization of butyl acrylate initiated by the poly(trithiocarbonate) to form the bottlebrush polymer. This strategy afforded well-defined bottlebrush polymers with molar masses in excess of 11 000 kg/mol. For un-cross-linked bottlebrush polymers, 180° peel tests revealed a cohesive failure mode and showed similar peel strengths (∼30 g/mm) regardless of the backbone polymer degree of polymerization (DP). The bottlebrush polymers were then treated with butylamine to remove the trithiocarbonate, liberating thiols on each side-chain terminus. In the presence of oxygen, these thiols readily cross-linked via disulfide bond formation. The cross-linked bottlebrush polymers with a backbone DP of 400 showed a greater than sixfold improvement in peel strength, whereas those with a backbone DP of 100 exhibited a twofold enhancement compared with un-cross-linked samples along with a change to adhesive failure. Triphenylphosphine readily reduced the disulfide bonds, effectively removing all cross-links in the bottlebrush network and allowing for recasting of the adhesive, which showed similar adhesive and rheological properties to the original un-cross-linked samples.

One-Shot Synthesis and Melt Self-Assembly of Bottlebrush Copolymers with a Gradient Compositional Profile

Morphological control plays a central role in soft materials design. Herein, we report the synthesis of a gradient bottlebrush architecture and its role in directing molecular packing in the solid state. Bottlebrush copolymers with gradient interfaces were prepared via one-shot ring-opening metathesis polymerization of exo- and endo-norbornene-capped macromonomers. Kinetic studies revealed a gradient compositional profile separating the two blocks along the backbone. Side-chain symmetric gradient bottlebrush copolymers exhibited a strong tendency to assemble into cylindrical microstructures, in contrast to their block copolymer analogs with sharp interfaces. Such exquisite architectural control of the interfacial composition affords a delicate handle to direct macromolecular assembly.