Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials

Pd-Catalyzed Ring-Opening Polymerization of Cyclobutanols through C(sp3)-C(sp3) Bond Cleavage

A new approach to ring-opening polymerization (ROP) based on C(sp3)-C(sp3) bond cleavage is reported. This process is based on the ability of Pd to promote both the β-carbon elimination of a bifunctional cyclobutanol precursor and the C-C coupling process with the resulting Pd-alkyl intermediate. Consequently, novel polyketone materials are obtained. Owing to the modular synthesis of the used cyclobutanol monomers, the present ROP reaction allows the introduction of substitution patterns in the polymeric chain that are not accessible by current polyketone synthesis methodologies. We have explored in detail the initiation, propagation, and termination steps of this new polymerization process.

Reactive Processing of Furan-Based Monomers via Frontal Ring-Opening Metathesis Polymerization for High Performance Materials

Frontal ring-opening metathesis polymerization (FROMP) presents an energy-efficient approach to produce high-performance polymers, typically utilizing norbornene derivatives from Diels-Alder reactions. This study broadens the monomer repertoire for FROMP, incorporating the cycloaddition product of biosourced furan compounds and benzyne, namely 1,4-dihydro-1,4-epoxynaphthalene (HEN) derivatives. A computational screening of Diels-Alder products is conducted, selecting products with resistance to retro-Diels-Alder but also sufficient ring strain to facilitate FROMP. The experiments reveal that varying substituents both modulate the FROMP kinetics and enable the creation of thermoplastic materials characterized by different thermomechanical properties. Moreover, HEN-based crosslinkers are designed to enhance the resulting thermomechanical properties at high temperatures (>200 °C). The versatility of such materials is demonstrated through direct ink writing (DIW) to rapidly produce 3D structures without the need for printed supports. This research significantly extends the range of monomers suitable for FROMP, furthering efficient production of high-performance polymeric materials.

AI-assisted discovery of high-temperature dielectrics for energy storage

Electrostatic capacitors play a crucial role as energy storage devices in modern electrical systems. Energy density, the figure of merit for electrostatic capacitors, is primarily determined by the choice of dielectric material. Most industry-grade polymer dielectrics are flexible polyolefins or rigid aromatics, possessing high energy density or high thermal stability, but not both. Here, we employ artificial intelligence (AI), established polymer chemistry, and molecular engineering to discover a suite of dielectrics in the polynorbornene and polyimide families. Many of the discovered dielectrics exhibit high thermal stability and high energy density over a broad temperature range. One such dielectric displays an energy density of 8.3 J cc-1 at 200 °C, a value 11 × that of any commercially available polymer dielectric at this temperature. We also evaluate pathways to further enhance the polynorbornene and polyimide families, enabling these capacitors to perform well in demanding applications (e.g., aerospace) while being environmentally sustainable. These findings expand the potential applications of electrostatic capacitors within the 85-200 °C temperature range, at which there is presently no good commercial solution. More broadly, this research demonstrates the impact of AI on chemical structure generation and property prediction, highlighting the potential for materials design advancement beyond electrostatic capacitors.

Supersoft Norbornene-Based Thermoplastic Elastomers with High Strength and Upper Service Temperature

With over 6 million tons produced annually, thermoplastic elastomers (TPEs) have become ubiquitous in modern society, due to their unique combination of elasticity, toughness, and reprocessability. Nevertheless, industrial TPEs display a tradeoff between softness and strength, along with low upper service temperatures, typically ≤100 °C. This limits their utility, such as in bio-interfacial applications where supersoft deformation is required in tandem with strength, in addition to applications that require thermal stability (e.g., encapsulation of electronics, seals/joints for aeronautics, protective clothing for firefighting, and biomedical devices that can be subjected to steam sterilization). Thus, combining softness, strength, and high thermal resistance into a single versatile TPE has remained an unmet opportunity. Through de novo design and synthesis of novel norbornene-based ABA triblock copolymers, this gap is filled. Ring-opening metathesis polymerization is employed to prepare TPEs with an unprecedented combination of properties, including skin-like moduli (<100 kPa), strength competitive with commercial TPEs (>5 MPa), and upper service temperatures akin to high-performance plastics (≈260 °C). Furthermore, the materials are elastic, tough, reprocessable, and shelf stable (≥2 months) without incorporation of plasticizer. Structure-property relationships identified herein inform development of next-generation TPEs that are both biologically soft yet thermomechanically durable.

Regioselective Palladium-Catalyzed Chain-Growth Allylic Amination Polymerization of Vinyl Aziridines

Organometallic-mediated chain growth polymerization of readily accessible chemical building blocks is responsible for important commercial and technological advances in polymer science, but the incorporation of heteroatoms into the polymer backbone through these mechanisms remains a challenge. Transition metal π-allyl complexes are well-developed organometallic intermediates for carbon-heteroatom bond formation in small-molecule catalysis yet remain underexplored in polymer science. Here, we developed a regioselective palladium-phosphoramidite-catalyzed chain-growth allylic amination polymerization of vinyl aziridines for the synthesis of novel nitrogen-rich polymers via ambiphilic π-allyl complexes. The polymerization accessed a linear microstructure with four carbons between each nitrogen, which is challenging to achieve through other chain-growth polymerization approaches. The highly regioselective allylic amination polymerization demonstrated the characteristics of a controlled polymerization and was able to achieve molar masses exceeding 20 kg mol-1 with low dispersities (D̵ < 1.3). The identification of the polymer structure and well-defined chain ends were supported by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and chain extension experiments demonstrate opportunities for building more complex materials from this method. A Hammett study was performed to understand the role of the catalyst and monomer structure on regioselectivity, and the data supported a mechanism wherein regioselectivity was primarily controlled by the ligand-metal complex. Postpolymerization desulfonylation provided access to a novel polyamine that demonstrated broad anticancer activity in vitro, which highlights the benefits of unlocking novel polyamine microstructures through regioselective chain-growth allylic amination polymerization.

Bottlebrush Networks: A Primer for Advanced Architectures

Bottlebrush networks (BBNs) are an exciting new class of materials with interesting physical properties derived from their unique architecture. While great strides have been made in our fundamental understanding of bottlebrush polymers and networks, an interdisciplinary approach is necessary for the field to accelerate advancements. This review aims to act as a primer to BBN chemistry and physics for both new and current members of the community. In addition to providing an overview of contemporary BBN synthetic methods, we developed a workflow and desktop application (LengthScale), enabling bottlebrush physics to be more approachable. We conclude by addressing several topical issues and asking a series of pointed questions to stimulate conversation within the community.

Tailoring the Structure-Property Relationship of Ring-Opened Metathesis Copolymers for CO2-Selective Membranes

In this work, we explore the use of ring-opening metathesis polymerization (ROMP) facilitated by a second-generation Grubbs catalyst (G2) for the development of advanced polymer membranes aimed at CO2 separation. By employing a novel copolymer blend incorporating 4,4'-oxidianiline (ODA), 1,6-hexanediamine (HDA), 1-adamantylamine (AA), and 3,6,9-trioxaundecylamine (TA), along with a CO2-selective poly(ethylene glycol)/poly(propylene glycol) copolymer (Jeffamine2003) and polydimethylsiloxane (PDMS) units, we have synthesized membranes under ambient conditions with exceptional CO2 separation capabilities. The strategic inclusion of PDMS, up to a 20% composition within the PEG/PPG matrix, has resulted in copolymer membranes that not only surpass the 2008 upper limit for CO2/N2 separation but also meet the commercial targets for CO2/H2 separation. Comprehensive analysis reveals that these membranes adhere to the mixing rule and exhibit percolation behavior across the entire range of compositions (0-100%), maintaining robust antiplasticization performance even under pressures up to 20 atm. Our findings underscore the potential of ROMP in creating precisely engineered membranes for efficient CO2 separation, paving the way for their application in large-scale environmental and industrial processes.

Approaching Tryptophan-Derived Polynorbornene Fluorescent Chemosensors: Synthesis, Characterization, and Sensing Ability for Biomedical Applications as Biomarkers for Detecting Fe2+ Ions

We present a novel group of tryptophan (Trp)-based fluorescent polymeric probes synthesized via ring-opening metathesis polymerization (ROMP) of Trp-derived norbornene monomers. These probes, in mono- and disubstituted forms, incorporate amide and ester anchoring groups. The quantity of Trp substituents did not affect fluorescence selectivity but influenced quenching percentage. Poly-diamide-Trp, Poly-monoamide-Trp, Poly-diester-Trp, and Poly-monoester-Trp probes displayed selective detection of Fe2+ and Fe3+ ions with fluorescence on-off characteristics. Poly-diamide-Trp and Poly-monoamide-Trp exhibited a limit of detection (LOD) for Fe2+ and Fe3+ ions of 0.86-11.32 μM, while Poly-diester-Trp and Poly-monoester-Trp showed higher LODs (21.8-108.7 μM). These probes exhibited high selectivity over Fe2+, a crucial metal ion in the body known for its redox properties causing oxidative stress and cell damage. Cell cytotoxicity tests in various cell types confirmed biocompatibility. Additionally, Poly-diamide-Trp displayed excellent cell permeability and iron ion detection in EA.hy926 cells, suggesting potential for bioimaging and clinical applications.

Reversible strain-promoted DNA polymerization

Molecular strain can be introduced to influence the outcome of chemical reactions. Once a thermodynamic product is formed, however, reversing the course of a strain-promoted reaction is challenging. Here, a reversible, strain-promoted polymerization in cyclic DNA is reported. The use of nonhybridizing, single-stranded spacers as short as a single nucleotide in length can promote DNA cyclization. Molecular strain is generated by duplexing the spacers, leading to ring opening and subsequent polymerization. Then, removal of the strain-generating duplexers triggers depolymerization and cyclic dimer recovery via enthalpy-driven cyclization and entropy-mediated ring contraction. This reversibility is retained even when a protein is conjugated to the DNA strands, and the architecture of the protein assemblies can be modulated between bivalent and polyvalent states. This work underscores the utility of using DNA not only as a programmable ligand for assembly but also as a route to access restorable bonds, thus providing a molecular basis for DNA-based materials with shape-memory, self-healing, and stimuli-responsive properties.

Unraveling Reactivity Differences: Room-Temperature Ring-Opening Metathesis Polymerization (ROMP) versus Frontal ROMP

In this study, we explore the distinct reactivity patterns between frontal ring-opening metathesis polymerization (FROMP) and room-temperature solventless ring-opening metathesis polymerization (ROMP). Despite their shared mechanism, we find that FROMP is less sensitive to inhibitor concentration than room-temperature ROMP. By increasing the initiator-to-monomer ratio for a fixed inhibitor/initiator quantity, we find reduction in the ROMP background reactivity at room temperature (i.e., increased resin pot life). At elevated temperatures where inhibitor dissociation prevails, accelerated frontal polymerization rates are observed because of the concentrated presence of the initiator. Surprisingly, the strategy of employing higher initiator loading enhances both pot life and front speeds, which leads to FROMP rates exceeding prior reported values by over 5 times. This counterintuitive behavior is attributed to an increase in the proximity of the inhibitor to the initiator within the bulk resin and to whether the temperature favors coordination or dissociation of the inhibitor. A rapid method was developed for assessing resin pot life, and a straightforward model for active initiator behavior was established. Modified resin systems enabled direct ink writing of robust thermoset structures at rates much faster than previously possible.

Access to Functionalized Materials by Metal-Free Ring-Opening Metathesis Polymerization of Active Esters and Divergent Postpolymerization Modification

Metal-free ring-opening metathesis polymerization (MF-ROMP) is an emerging polymerization strategy that provides access to ROMP materials by using organic initiators and photoredox catalysts. Unlike metal-mediated ROMP, MF-ROMP is not highly tolerant toward functionalized monomers. Herein, we report that pentafluorophenyl esters are polymerizable under MF-ROMP conditions to produce homopolymers, statistical copolymers, and block copolymers. Amine coupling agents were then used to install a range of functional groups via acyl substitution including alkynes, amino acid derivatives, fluorophores, and redox active moieties. Overall, these findings provide a framework to prepare functionalized ROMP polymers without the risk of metal contamination.

Saltwater-Induced Rapid Gelation of Photoredox-Responsive Mucomimetic Hydrogels

Shear-thinning hydrogels represent an important class of injectable soft materials that are often used in a wide range of biomedical applications. Creation of new shear-thinning materials often requires that factors such as viscosity, injection rate/force, and needle gauge be evaluated to achieve efficient delivery, while simultaneously protecting potentially sensitive cargo. Here, a new approach to establishing shear-thinning hydrogels is reported where a host-guest cross-linked network initially remains soluble in deionized water but is kinetically trapped as a viscous hydrogel once exposed to saltwater. The shear-thinning properties of the hydrogel is then "switched on" in response to heating or exposure to visible light. These hydrogels consist of polynorbornene-based bottlebrush copolymers with porphyrin- and oligoviologen-containing side chains that are cross-linked through the reversible formation of β-cyclodextrin-adamantane inclusion complexes. The resultant viscous hydrogels display broad adhesive properties across polar and nonpolar substrates, mimicking that of natural mucous and thus making it easier to distribute onto a wide range of surfaces. Additional control over the hydrogel's mechanical properties (storage/loss moduli) and performance (adhesion) is achieved post-injection using a low-energy (blue light) photoinduced electron-transfer process. This work envisions these injectable copolymers and multimodal hydrogels can serve as versatile next-generation biomaterials capable of light-based mechanical manipulation post-injection.

Flow Optimization of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is a convenient metal-free method to produce a variety of ROMP polymers. Transitioning MF-ROMP from a batch to a continuous flow process has yet to be demonstrated and could potentially benefit the production efficiency, safety, and modularity of reaction conditions. We designed and evaluated continuous flow and droplet flow setups and compared the results for MF-ROMP across a short series of common monomers. By using the droplet flow reactor setup, we achieved flow conversions comparable to that of batch and circumvented issues with diffusion-limited mixing and air exposure.

Catalytic Living ROMP: Synthesis of Degradable Star Polymers

Star polymers have attracted considerable attention over the past few years due to their distinctive physical and chemical attributes that are different from conventional linear polymers. Here, we present a one-pot synthesis of narrowly dispersed and degradable homoarm and miktoarm star polymers exploiting the catalytic living ring-opening metathesis polymerization (ROMP) mechanism. Several complex polymeric architectures (such as A3-, A4-, A6-, A2B-, A3B-, and AB2-type star polymers) were synthesized quite straightforwardly by using appropriate vinyl ether chain transfer agents. SEC, 1H NMR, and DOSY NMR spectroscopy were employed to analyze and characterize all of the synthesized polymers. We believe that this sustainable and environmentally friendly synthesis of star polymers could become an important synthetic tool for polymer engineers working on supramolecular, industrial or biomedical applications.

ENGINEERED INTRAVENOUS THERAPIES FOR TRAUMA

Trauma leading to severe hemorrhage and shock on average kills patients within 3 to 6 hours after injury. With average prehospital transport times reaching 1-6 hours in low- to middle-income countries, stopping the bleeding and reversing hemorrhagic shock is vital. First-generation intravenous hemostats rely on traditional drug delivery platforms, such as self-assembling systems, fabricated nanoparticles, and soluble polymers due to their active targeting, biodistribution, and safety. We discuss some challenges translating these therapies to patients, as very few have successfully made it through preclinical evaluation in large-animals, and none have translated to the clinic. Finally, we discuss the physiology of hemorrhagic shock, highlight a new low volume resuscitant (LVR) PEG-20k, and end with considerations for the rational design of LVRs.

Controlled Ring Opening Metathesis Polymerization of a New Monomer: On Switching the Solvent-Water-Soluble Homopolymers to Degradable Copolymers

A new heterocyclic monomer is developed via simple Diels-Alder reaction which is reluctant to polymerize in dichloromethane (DCM) whereas undergoes facile polymerization in tetrahydrofuran with excellent control over molecular weight (Mn ) and dispersities (Đ) using Grubbs' third generation catalyst (G3). The deprotection of the tert-butoxycarbonyl group from the polymeric backbone yielded a water-soluble ring opening metathesis polymerization (ROMP) polymer easily. Moreover, in DCM this new monomer copolymerizes with 2,3-dihydrofuran under catalytic living ROMP conditions to give backbone degradable polymers. All the synthesized polymers are characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. It is believed that this new route to water soluble ROMP homopolymers as well as the cost-effective and environmentally friendly route to degradable copolymers and block-copolymers could find applications in biomedicine in the near future.

Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene

We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from bicyclo[4.2.0]oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo[4.2.0]oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening and the rate of reaction of the resulting carbene with cyclohexene (B monomer). The resulting Ru-cyclohexylidenes underwent reversible ring opening metathesis with cyclohexene. However, the thermodynamic equilibrium disfavored cyclohexene ring opening. Utilization of triphenylphosphine forms a more stable PPh3 ligated complex, which suppresses the reverse ring closing reaction and allowed direct measurements of the forward rate constants for formation of various A-B and A-B-A' complexes through carbene-catalyzed ring-opening metathesis and thus gradient polymer structure-determining steps. The relative rate of the propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. In addition, the rate of cyclohexene ring-opening catalyzed by the propyl bicyclooctene is 1.4 times faster than when catalyzed by the ethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening by propyl-based Ru-hexylidene are 1.6-fold faster than ethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constants into reactivity ratios of bicyclo[4.2.0]amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.

Sequestration of Ruthenium Residues via Efficient Fluorous-enyne Termination

The creation of polymers without metal contamination remains a significant challenge for metathesis-based polymerization techniques and has complicated applications in biomedical and electronic applications. This communication reports a new approach for the removal of ruthenium byproducts through the design of an enyne terminator for metathesis polymerization that contains a fluorous tag. Upon reaction of a living polymer chain with the enyne, the ruthenium center is captured as a stable sulfur-chelated complex that can be efficiently removed after a single filtration through a fluorous cartridge. Levels of ruthenium residues as determined by ICP-MS were found to depend on the monomer structure, eluting solvent, and the degree of polymerization targeted. Ruthenium residues were minimized to low ppm levels (4-75 ppm) for most samples examined and also led to the improved thermal stability of the final materials. This represents the most efficient single method for removal of ruthenium residues from metathesis polymerization products.

Inflammation-Responsive Micellar Nanoparticles from Degradable Polyphosphoramidates for Targeted Delivery to Myocardial Infarction

Nanoparticles that undergo a localized morphology change to target areas of inflammation have been previously developed but are limited by their lack of biodegradability. In this paper, we describe a low-ring-strain cyclic olefin monomer, 1,3-dimethyl-2-phenoxy-1,3,4,7-tetrahydro-1,3,2-diazaphosphepine 2-oxide (MePTDO), that rapidly polymerizes via ring-opening metathesis polymerization at room temperature to generate well-defined degradable polyphosphoramidates with high monomer conversion (>84%). Efficient MePTDO copolymerizations with norbornene-based monomers are demonstrated, including a norbornenyl monomer functionalized with a peptide substrate for inflammation-associated matrix metalloproteinases (MMPs). The resulting amphiphilic peptide brush copolymers self-assembled in aqueous solution to generate micellar nanoparticles (30 nm in diameter) which exhibit excellent cyto- and hemocompatibility and undergo MMP-induced assembly into micron-scale aggregates. As MMPs are upregulated in the heart postmyocardial infarction (MI), the MMP-responsive micelles were applied to target and accumulate in the infarcted heart following intravenous administration in a rat model of MI. These particles displayed a distinct biodistribution and clearance pattern in comparison to nondegradable analogues. Specifically, accumulation at the site of MI competed with elimination predominantly through the kidney rather than the liver. Together, these results suggest this as a promising new biodegradable platform for inflammation targeted delivery.

Frontal Polymerizations: From Chemical Perspectives to Macroscopic Properties and Applications

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

The Fluorocarbene Exploit: Enforcing Alternation in Ring-Opening Metathesis Polymerization

Fluoroalkenes are known to be notoriously reluctant substrates for olefin metathesis due to the generation of thermodynamically stable Fischer-type fluorocarbene intermediates, which invariably fail to undergo further reaction. In the present disclosure, we find that fluorine substitution on the sp² carbon also strictly suppresses homopolymerization of norbornene derivatives (NBEs), and this can be harnessed to achieve alternating ring-opening metathesis polymerization (ROMP) with an appropriately electron-rich comonomer. Dihydrofuran (DHF) is thereby shown to undergo alternating ROMP with fluorinated norbornenes, the perfectly alternating structure of the resulting copolymer having been unambiguously elucidated by ¹H, ¹⁹F, and ¹³C NMR analyses. Furthermore, we find that the degradability of the resultant copolymers in acidic media via hydrolysis of enol ether moieties in the backbone can be predictably modulated by the number of fluorine atoms present in the NBE comonomer, affording an opportunity to engage with the desirable physical properties of fluorinated polymers while limiting their attendant environmental degradability issues.

Covalently bridged pillararene-based polymers: structures, synthesis, and applications

Covalently bridged pillararene-based polymers (CBPPs) are a special class of macrocycle-based polymers in which multiple pillararene monomers are attached to the polymer structures by covalent bonds. Owing to the unique molecular structures including the connection components or the spatial structures, CBPPs have become increasingly popular in applications ranging from environmental science to biomedical science. In this review, CBPPs are divided into three types (linear polymers, grafted polymers, and cross-linked polymers) according to their structural characteristics and described from the perspective of synthesis methods comprehensively. In addition, the applications of CBPPs are presented, including selective adsorption and separation, fluorescence sensing and detection, construction of supramolecular gels, anticancer drug delivery, artificial light-harvesting, catalysis, and others. Finally, the current challenging issues and comprehensive prospects of CBPPs are discussed.

Synthesis and Ring-Opening Metathesis Polymerization of o-Dialkoxy Paracyclophanedienes

The highly strained ortho-diethylhexyloxy [2.2]paracyclophane-1,9-diene (M1) can be synthesized by ring contraction of a dithia[3.3]paracyclophane using a benzyne-induced Stevens rearrangement. This paracyclophanediene undergoes ring-opening metathesis polymerization to give well-defined 2,3-dialkoxyphenylenevinylene polymers with an alternating cis/trans alkene stereochemistry and controllable molecular weight. Fully conjugated block copolymers with electron-rich and electron-deficient phenylene vinylene polymer segments can be prepared by sequential monomer additions. These polymers can be readily isomerized to the all-trans stereochemistry polymer. The optical and electrochemical properties of these polymers were investigated by theory and experiment.

Practical Route for Catalytic Ring-Opening Metathesis Polymerization

Norbornene derivatives are typical monomers for ring-opening metathesis polymerization (ROMP) for synthesizing highly functional polymers. However, the lack of catalytic methods, that is, the lack of readily available chain transfer agents (CTAs) for these monomers has been a significant cost limitation when large-scale syntheses are required. Here, we report commercially available styrene and its derivatives as efficient regioselective CTAs for the catalytic synthesis of metathesis polymers requiring up to 1000 times less ruthenium than in classical ROMP experiments. The molecular weight of the synthesized polymers was controlled by the monomer-to-CTA ratio. Low molecular weight ROMP polymers known for their antimicrobial properties were also synthesized on a gram scale in this report. Polymers were characterized by SEC, ¹H NMR spectroscopy, and isotopically resolved MALDI-TOF MS. This approach describes a greener, more cost-effective, and eco-friendly methodology for the preparation of metathesis-based materials on the multigram scale.

Macrochain Transfer Agents for Catalytic Ring-Opening Metathesis Polymerization

A monosubstituted 1,3-diene derivative attached to a polymer is demonstrated to act as a macrochain transfer agent in catalytic ring-opening metathesis polymerization. PEG- and PLA-based macrochain transfer agents were synthesized in a few steps and were characterized using NMR spectroscopy, size exclusion chromatography (SEC) and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-ToF) mass spectrometry. Poly(l-lactide) based diblock copolymer, poly(ethylene glycol)-based diblock, and triblock (ABA type) copolymers of varied chain lengths were prepared catalytically in a one-pot approach via metathesis polymerization. Block copolymers were characterized by SEC and showed monomodal molecular weight distributions. Moreover, DOSY NMR spectroscopy further proved the block microstructures of the synthesized polymers.

Chain transfer agents for the catalytic ring opening metathesis polymerization of norbornenes

Here, we present a detailed study of the metathesis activity of conjugated 1,3 diene derivatives in ring opening metathesis polymerization (ROMP) using Grubbs' 3rd generation catalyst (G3). A comprehensive screening of those derivatives revealed that monosubstituted 1,3 dienes show similar reactivities towards G3-alkylidenes as norbornene derivatives. Therefore, they represent perfect candidates for chain transfer agents in a kinetically controlled catalytic ROMP. This unprecedented reactivity allowed us to catalytically synthesize mono-end-functional poly(norborneneimide)s on the gram scale. Much more complex architectures such as star-shaped polymers could also be synthesized catalytically for the very first time via ROMP. This inexpensive and greener route to produce telechelic ROMP polymers was further utilized to synthesize ROMP block copolymers using bifunctional ROMP and ATRP/NCL initiators. Finally, the regioselective reaction of G3 with 1,3 diene derivatives was also exploited in the synthesis of a ROMP-PEG diblock copolymer initiated from a PEG macroinitiator.

Expedient Synthesis and Ring-Opening Metathesis Polymerization of Pyridinonorbornenes

Pyridine-containing polymers are promising materials for a variety of applications from the capture of contaminants to the self-assembly of block copolymers. However, the innate Lewis basicity of the pyridine motif often hampers living polymerization catalyzed by transition-metal complexes. Herein, we report the expedient synthesis of pyridinonorbornene monomers via a [4+2] cycloaddition between 2,3-pyridynes and cyclopentadiene. Well-controlled ring-opening metathesis polymerization was enabled by careful structural design of the monomer. Polypyridinonorbornenes exhibited high Tg and Td, a promising feature for high-temperature applications. Investigation of the polymerization kinetics and of the reactivity of the chain ends shed light on the influence of nitrogen coordination on the chain-growth mechanism.

Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition

The [Ru(PPh3)2Cl-piperidine(4-aminomethyl)] complex (mono-Ru) was synthesized from [Ru(PPh3)3Cl2] and 4-(aminomethyl)piperidine, whereas the [(PPh3)PdCl(Shiff-pip)] complex (mono-Pd) was obtained by reacting [Pd(PPh3)2Cl2] with its respective Schiff base ligand, both at a 1:1 molar ratio. The heterobimetallic [RuCl2(PPh3)2](μ-Schiff)Pd(PPh3)Cl] complex (Ru/Pd) was synthesized via a one-pot, three-component reaction of mono-Ru, [(Pd(PPh3)2Cl2] and salicylaldehyde. All complexes were fully characterized by FTIR, UV-Vis, and NMR spectroscopy, as well as elemental analysis, MALDI-TOF mass spectrometry, cyclic voltammetry, and computational studies. Ru/Pd was able to polymerize norbornene (NBE) by two different mechanisms: ROMP and vinyl polymerization. The Ru fragment was active for ROMP of NBE, reaching yields of 68 and 31% for mono-Ru and Ru/Pd, respectively, when the [NBE]/[Ru] = 3000 molar ratio and 5 μL EDA addition were employed at 50 °C. The poly(norbornene) (polyNBE) obtained presented an order of magnitude of 104 g mol−1 and Ð values between 1.48 and 1.79. For the vinyl polymerization of NBE, the Pd fragment was active using MAO as a cocatalyst, reaching a yield of 47.0% for Ru/Pd and quantitative yields for mono-Pd when [Al]/[Pd] = 2500 and [NBE]/[Pd] = 20,000 molar ratios were employed, both at 60 °C.

Tuning Single-Polymer Growth via Hydrogen Bonding in Conformational Entanglements

Synthetic polymers have widespread applications in daily life and advanced materials applications. Making polymers efficiently and controllably is highly desired, for which modulating intramolecular and intermolecular interactions have been an effective approach. Recent real-time single-polymer growth studies uncovered nonequilibrium conformational entanglements that form stochastically under living polymerization conditions and which appear to plausibly play key roles in controlling the polymerization kinetics and dispersion. Here, using magnetic tweezers measurements, we study the real-time polymerization dynamics of single polynorbornene-based polymers in which we systematically tune the hydrogen-bonding interactions by titrating the OH content in the monomers and the formed polymers during ring opening metathesis polymerization. Using norbornenes with and without a hydroxyl group and a nonreactive monomer analogue, we show that intrachain and intermolecular hydrogen bonding compete, and both alter the microscopic properties of the nonequilibrium entanglements, leading to surprising multiphasic dependences of polymerization dynamics on the polymer's OH content. We further formulate a simple model to rationalize quantitatively the observed multiphasic behaviors by considering the different scaling relations of intrachain and intermolecular hydrogen bonding on the OH content. These results provide insights into the interconnected roles of intra-/intermolecular interactions, polymer chain conformations, and free monomers in solution in affecting polymerization kinetics and dispersion, and point to new opportunities in manipulating polymerization reactions.

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Growth Kinetics of Single Polymer Particles in Solution via Active-Feedback 3D Tracking

The ability to directly observe chemical reactions at the single-molecule and single-particle level has enabled the discovery of behaviors otherwise obscured by ensemble averaging in bulk measurements. However powerful, a common restriction of these studies to date has been the absolute requirement to surface tether or otherwise immobilize the chemical reagent/reaction of interest. This constraint arose from a fundamental limitation of conventional microscopy techniques, which could not track molecules or particles rapidly diffusing in three dimensions, as occurs in solution. However, many chemical processes occur entirely in the solution phase, leaving single-particle/-molecule analysis of this critical area of science beyond the scope of available technology. Here, we report the first kinetics studies of freely diffusing and actively growing single polymer-particles at the single-particle level freely diffusing in solution. Active-feedback single-particle tracking was used to capture three-dimensional (3D) trajectories and real-time volumetric images of freely diffusing polymer particles (D ≈ 10^-12 m²/s) and extract the growth rates of individual particles in the solution phase. The observed growth rates show that the average growth rate is a poor representation of the true underlying variability in polymer-particle growth behavior. These data revealed statistically significant populations of faster- and slower-growing particles at different depths in the sample, showing emergent heterogeneity while particles are still freely diffusing in solution. These results go against the prevailing premise that chemical processes in freely diffusing solution will exhibit uniform kinetics. We anticipate that these studies will launch new directions of solution-phase, nonensemble-averaged measurements of chemical processes.

Synthesis and Characterization of High Viscosity Cationic Poly(Proline-Epichlorohydrin) Composite Polymer with Antibacterial Functionalities

We report microbial resistance and catalytic activity of high viscosity cationic poly(proline-epichlorohydrin) composite (PRO-EPI) in the aqueous system. The PRO-EPI was prepared by a simple polycondensation, followed by FTIR, ¹H NMR, SEM, DLS, viscosity, and DSC/TGA characterization. Several concentrations of the PRO-EPI were tested against Gram-negative (E. coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) microorganisms. The antimicrobial screening revealed that PRO-EPI was a potent antimicrobial agent with the least inhibitory concentrations (MICs) of 128 µg/mL against Gram-negative microorganisms. The PRO-EPI indicated no inhibitory effect against Gram-positive microorganisms. It was determined that PRO-EPI contains polymeric-quaternary ammonium compounds that inactivate the Gram-negative microorganisms by a dual mode of action and carries domains for electrostatic interaction with the microbial membrane and an intracellular target. To study the removal of toxic industrial wastewater, congo red (CR) was tested using sodium borohydride as a reducing agent. Adsorption was achieved within 20 min at a rate constant of 0.92 ks⁻¹. UV–vis spectra showed that the removal of CR in the reaction solution was due to the breakup of the azo (–N=N–) bonds and adsorption of aromatic fragments. PRO is biodegradable and non-toxic, and PRO-EPI was found to be both antimicrobial and also acts as a catalyst for the removal of congo red dye.

Controlled Alternating Metathesis Copolymerization of Terminal Alkynes

Terminal alkynes display high reactivity toward Ru-carbene metathesis catalysts. However, the formation of a less reactive bulky carbene hinders their homopolymerization. Simultaneously, the higher reactivity of alkynes does not allow efficient cross propagation with sterically less-hindered cycloalkene monomers, resulting in inefficient copolymerization. Nonetheless, terminal alkynes undergo rapid cross-metathesis with vinyl ethers. Therefore, an efficient cross propagation can be achieved with terminal alkynes and cyclic enol ether monomers. Here, we show that terminal alkyne derivatives can be copolymerized in an alternating fashion with 2,3-dihydrofuran using Grubbs' third generation catalyst (G3). A linear relationship of the number-average molecular weight versus monomer to initiator ratio and block copolymer synthesis confirmed a controlled copolymerization. The SEC and NMR analyses of the synthesized copolymers confirmed the excellent control over molecular weight and exclusive alternating nature of the copolymer. The regioselective chain transfer of G3 to vinyl ether and the high reactivity of the Fischer-type Ru carbene toward terminal alkynes was also exploited for polymer conjugation. Finally, the presence of an acid labile backbone functionality in the synthesized alternating copolymers allowed complete degradation of the copolymer within a short time interval which was confirmed by SEC analyses.

A Single Functionalization Agent for Heterotelechelic ROMP Polymers

Heterotelechelic polymers are an important class of materials finding applications in bioconjugation, imaging, sensing, and synthesis of organic/inorganic hybrid systems with interesting features. However, the synthesis of such polymers is challenging. Here, we report a mechanistically unique and most efficient method based on a single functionalization agent to prepare heterotelechelic polymers by a ring-opening metathesis polymerization. Different functionalization agents can be synthesized in one simple step from inexpensive commercial starting materials. The functionalization agents initially generate a functional initiator from commercial Grubbs' first-generation ruthenium benzylidene catalyst. During this process, a functional dihydrofuran derivative is produced. After functional initiation and propagation of a suitable monomer, the dihydrofuran derivative functionally terminates the polymerization yielding a primary alcohol-terminated heterotelechelic polymer. Molecular weight control is achieved by varying the ratio between monomer and Grubbs' first-generation catalyst. This method may emerge as a popular choice to prepare heterotelechelic polymers due to its simplicity and efficiency.

Metal-Free Ring-Opening Metathesis Polymerization with Hydrazonium Initiators

The ring-opening metathesis polymerization (ROMP) of cyclopropenes using hydrazonium initiators is described. The initiators, which are formed by the condensation of 2,3-diazabicyclo[2.2.2]octane and an aldehyde, polymerize cyclopropene monomers by a sequence of [3+2] cycloaddition and cycloreversion reactions. This process generates short chain polyolefins (M_n ≤9.4 kg mol^-1 ) with relatively low dispersities (Đ≤1.4). The optimized conditions showed efficiency comparable to that achieved with Grubbs' 2^nd generation catalyst for the polymerization of 3-methyl-3-phenylcyclopropene. A positive correlation between monomer to initiator ratio and degree of polymerization was revealed through NMR spectroscopy.

Bottlebrush Amphiphilic Polymer Co-Networks

We report the synthesis of novel poly(ethylene glycol) and poly(dimethyl siloxane) (PEG and PDMS, respectively) bottlebrush amphiphilic polymer co-networks (B-APCNs) with high gel fractions by a grafting-through ring-opening metathesis polymerization. By varying the volume fraction of PEG (ϕPEG), we alter the crystallinity of the networks, achieving complete suppression of PEG crystallinity at ϕPEG=0.35. Furthermore, we show that the crystallinity of these networks can be tuned to alter their moduli. Through dynamic mechanical analysis, we show that the storage and loss moduli of networks with completely suppressed crystallinity (ϕPEG=0.35) behave similarly to a PDMS homopolymer bottlebrush network. These bottlebrush networks represent an unexplored architecture for the field of amphiphilic polymer co-networks.

The Limited Palette for Photonic Block-Copolymer Materials: A Historical Problem or a Practical Limitation?

Block-copolymer self-assembly has proven to be an effective route for the fabrication of photonic films and, more recently, photonic pigments. However, despite extensive research on this topic over the past two decades, the palette of monomers and polymers employed to produce such structurally colored materials has remained surprisingly limited. In this Scientific Perspective, the commonly used block-copolymer systems reported in the literature are summarized (considering both linear and brush architectures) and their use is rationalized from the point of view of both their historical development and physicochemical constraints. Finally, the current challenges facing the field are discussed and promising new areas of research are highlighted to inspire the community to pursue new directions.

The Limited Palette for Photonic Block-Copolymer Materials: A Historical Problem or a Practical Limitation?

Block-copolymer self-assembly has proven to be an effective route for the fabrication of photonic films and, more recently, photonic pigments. However, despite extensive research on this topic over the past two decades, the palette of monomers and polymers employed to produce such structurally colored materials has remained surprisingly limited. In this Scientific Perspective, the commonly used block-copolymer systems reported in the literature are summarized (considering both linear and brush architectures) and their use is rationalized from the point of view of both their historical development and physicochemical constraints. Finally, the current challenges facing the field are discussed and promising new areas of research are highlighted to inspire the community to pursue new directions.

Cascade Alternating Metathesis Cyclopolymerization of Diynes and Dihydrofuran

Ruthenium alkoxymethylidene complexes have recently come into view as competent species for metathesis copolymerization reactions when coupled with appropriate comonomer targets. Here, we explore the ability of Fischer-type carbenes to participate in cascade alternating metathesis cyclopolymerization (CAMC) through facile terminal alkyne addition. The combination of diyne monomers and an equal feed ratio of low-strain dihydrofuran leads to a controlled chain-growth copolymerization with high degrees of alternation (>97% alternating diads) and produces degradable polymer materials with low dispersities and targetable molecular weights. When combined with enyne monomers, this method is amenable to the synthesis of alternating diblock copolymers that can be fully degraded to short oligomer fragments under aqueous acidic conditions. This work furthers the potential for the generation of functional metathesis materials via Fischer-type ruthenium alkylidenes.