Ring-opening Metathesis Polymerization Research Articles

A Facile Strategy for the Development of Recyclable Multifunctional Liquid Crystal Polymers via Post‐Polymerization Modification and Ring‐Opening Metathesis Polymerization

AbstractPost‐polymerization modification (PPM) offers a versatile approach for engineering multifunctional polymers, but this advantage has not been fully exploited to fabricate multifunctional liquid crystal polymers (LCPs). Here, we design a facile synthetic approach towards multifunctional LCP by combining the ring‐opening metathesis polymerization (ROMP) with PPM, in which ROMP helps to prepare a reactive LCP precursor with high molecular weight, and PPM provides a facilitation to introduce functional groups into the precursor. Consequently, a photo‐ and humidity‐responsive linear LCP (LLCP) is demonstrated to show the potential of this synthetic strategy to diversify functions of the LCPs. Under light irradiation and humidity changes, the deformation modes of the LLCP films are converted to complex shapes (bending, twisting, and curling). The obtained dual‐responsive LLCP with high molecular weight possesses excellent processability and recyclability, making it possible to construct 3D shape actuators with programmable deformation behaviors under light/humidity.

On-Demand Cross-Linkable Bottlebrush Polymers for Voltage-Driven Artificial Muscles.

Dielectric elastomer actuators (DEAs) generate motion resembling natural muscles in reliability, adaptability, elongation, and frequency of operation. They are highly attractive in implantable soft robots or artificial organs. However, many applications of such devices are hindered by the high driving voltage required for operation, which exceeds the safety threshold for the human body. Although the driving voltage can be reduced by decreasing the thickness and the elastic modulus, soft materials are prone to electromechanical instability (EMI), which causes dielectric breakdown. The elastomers made by cross-linking bottlebrush polymers are promising for achieving DEAs that suppress EMI. In previous work, they were chemically cross-linked using an in situ free-radical UV-induced polymerization, which is oxygen-sensitive and does not allow the formation of thin films. Therefore, the respective actuators were operated at voltages above 4000 V. Herein, macromonomers that can be polymerized by ring-opening metathesis polymerization and subsequently cross-linked via a UV-induced thiol-ene click reaction are developed. They allow us to fast cross-link defect-free thin films with a thickness below 100 μm. The dielectric films give up to 12% lateral actuation at 1000 V and survive more than 10,000 cycles at frequencies up to 10 Hz. The easy and efficient preparation approach of the defect-free thin films under air provides easy accessibility to bottlebrush polymeric materials for future research. Additionally, the desired properties, actuation under low voltage, and long lifetime revealed the potential of the developed materials in soft robotic implantable devices. Furthermore, the C-C double bonds in the polymer backbone allow for chemical modification with polar groups and increase the materials' dielectric permittivity to a value of 5.5, which is the highest value of dielectric permittivity for a cross-linked bottlebrush polymer.

Synthesis and Ring-Opening Metathesis Polymerization of a Strained trans-Silacycloheptene and Single-Molecule Mechanics of Its Polymer.

The cis- and trans-isomers of a silacycloheptene were selectively synthesized by the alkylation of a silyl dianion, a novel approach to strained cycloalkenes. The trans-silacycloheptene (trans-SiCH) was significantly more strained than the cis isomer, as predicted by quantum chemical calculations and confirmed by crystallographic signatures of a twisted alkene. Each isomer exhibited distinct reactivity toward ring-opening metathesis polymerization (ROMP), where only trans-SiCH afforded high-molar-mass polymer under enthalpy-driven ROMP. Hypothesizing that the introduction of silicon might result in increased molecular compliance at large extensions, we compared poly(trans-SiCH) to organic polymers by single-molecule force spectroscopy (SMFS). Force-extension curves from SMFS showed that poly(trans-SiCH) is more easily overstretched than two carbon-based analogues, polycyclooctene and polybutadiene, with stretching constants that agree well with the results of computational simulations.

Tungsten Oxo and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes for the Visible-Light-Induced Ring-Opening Metathesis Polymerization of Dicyclopentadiene

A system for the visible-light-controlled ring-opening metathesis polymerization (ROMP) of endo,endo-dicyclopentadiene (DCPD) was developed by combining thermally latent group 6 olefin metathesis initiators, whose vacant binding site is occupied by an N-heterocyclic carbene (NHC), with a photoredox catalyst. Similar ruthenium-based systems have previously been utilized in numerous photolithographic applications. Here, we were able to adapt the approach to group 6 metathesis catalysts for the first time. Extensive screening of 31 different olefin metathesis initiators and five different photoredox catalysts revealed that both tungsten oxo and tungsten imido alkylidene NHC complexes together with 2,4,6-triphenylpyrylium tetrafluoroborate (TPPT) as a photocatalyst are most suitable for the desired application. We propose that, upon irradiation of a mixture of the corresponding olefin metathesis catalyst and TPPT with visible light (440 nm), the excited pyrylium compound oxidizes the NHC ligand and a metathesis-active Schrock-type catalyst is formed. This system can be applied to two-dimensional (2D) photolithography in the ROMP of DCPD. Photomasks were employed to produce cross-linked polymers with great spatial resolution within minutes by applying visible light as the sole stimulus giving access to polymers those compared to many other systems do not require thermal or UV post-curing. Notably, mixtures of the two catalysts and DCPD were latent for several days, and catalyst loadings as low as 100 ppm were sufficient for the desired applications proving the robustness and high activity of the developed system.

Synthesis of dendronized PAMAM grafted ROMP polymers

Oxanorbornene cored 0.5, 1.5 and 2.5 generation ester terminated PAMAM dendronized monomers with 2 and 6 carbon-containing alkyl spacer in the middle of oxanorbornene core and dendronized branching were synthesized. Monomers were polymerized through the ring-opening metathesis polymerization (ROMP) technique by using 2nd and 3rd generation Grubbs catalysts to get dendronized ROMP polymers. It was found that Grubbs 3rd generation catalyst works well for 0.5 generation dendronized ROMP polymer whereas Grubbs 2nd generation catalyst works for the higher generation. We also found the linker effect during polymerization as six-carbon linker monomers polymerize with high yield along with higher generation. Finally, dendronized ROMP-based PAMAM encapsulated zero valent Cu nanoparticle was synthesized by using a reducing agent, NaBH4. The catalytic activity of this nanoparticle was further investigated from the reduction of 4-nitrophenol to 4-aminophenol.

Ferrocene-anchored poly(norbornene) for ATP sensing using indicator displacement assay

A new amphiphilic polynorbornene-based copolymer P1 was designed and synthesized successfully by ring-opening metathesis polymerization (ROMP) of polyethylene glycol (PEG) containing monomer NB-PEG and ferreocene-anchored monomer NB-Fc. The copolymer P1 can bind with anionic uranine dye (UD) through multiple interactions to form no fluorescent P1/UD nano-structure ensemble in aqueous solution. The P1/UD ensemble can be used as indicator displacement assay (IDA) for ATP detection with a high sensitivity and selectivity. Upon the addition of ATP, the fluorescence is significant enhanced for displacement of UD from P1/UD ensemble. The LOD value of P1/UD ensemble toward ATP is as low as 35.9 nM. The P1/UD ensemble exhibits an excellent photo-stability and biocompatibility, and has been successfully used for imaging ATP in living cells.

Self-Healable, Stretchable, and Super-Soft Bottlebrush Polyester Elastomers for Highly Sensitive Flexible Sensors

While much effort has been devoted to the creation of super-soft elastomers, there is a dearth of research aimed at self-healable and super-soft renewable elastomers. Herein, self-healing and super-soft renewable elastomers are developed based on bottlebrush architecture design and dynamic associative transesterification. The bottlebrush polymer precursors are generated by ring-opening metathesis polymerization (ROMP) of the norbornene-terminated macromonomer poly(ε-caprolactone-co-l-lactide) (PCLLA) (NB-PCLLA). After introducing alkyne groups onto the end of PCLLA side chains, the bottlebrush polymers are cross-linked to achieve elastomer networks via a UV-induced “click” thiol-yne reaction between the terminal alkyne groups and the dithiothreitol (DTT) additive. The resulting materials exhibit a shear modulus as low as 17 kPa, high stretchability with breaking strains as large as 315%, and a self-healing efficiency of 95.6% at an elevated temperature of 80 °C. Taking the super-softness advantage, flexible sensor devices are assembled based on the prepared elastomer, which are highly sensitive to deformation with a gauge factor of 0.347. This work demonstrates that high-performance biodegradable materials with unique properties that conventional linear polymers impossibly possess can be easily constructed by molecular architecture design using bottlebrush polymers as building blocks.

3D Printed Bioactive PLGA Dermal Scaffold for Burn Wound Treatment

Burn injury represents a major global public healthcare problem and has a significant health-economics impact. In this study, we report on a 3D printed poly(lactic-co-glycolic acid) (PLGA) dermal scaffold containing bioactive PLGA for burn wound healing. Bioactive brush copolymers containing pendant side chains of PLGA and PEGylated Arg-Gly-Asp tripeptide (RGD) or hyaluronic acid (HA) were synthesized by ring-opening metathesis polymerization (ROMP). These copolymers exhibited good thermal stability for material processing using melt-extrusion-based methods. The copolymers were blended with commercial PLGA, extruded into filaments and 3D printed using fused filament fabrication (FFF) methods with incorporated porosities. The 3D printed scaffolds demonstrated good biocompatibility in in vitro cell assays and in vivo murine models. Porcine study based on partial thickness burn wound model showed that these PLGA scaffolds facilitated re-epithelization with reduced inflammation as compared to the clinical gold standard for second-degree burn wound treatment, Biobrane. The bioactive PLGA scaffolds presented herein are beneficial in wound healing and have therapeutic potential in burn wounds treatment.

Star-to-Bottlebrush Transition in Extensional and Shear Deformation of Unentangled Polymer Melts

A series of model poly((±)-lactide) (PLA) graft copolymers was synthesized by ring-opening metathesis polymerization and used to probe the star-to-bottlebrush transition in shear and extensional flows. Ten samples with backbone degrees of polymerization 11 ≤ Nbb ≤ 420 were investigated using small-amplitude oscillatory shear (SAOS) and extensional rheometry measurements. Each contained one PLA side chain of length Nsc = 72 per two backbone repeating units on average (graft density of z = 0.5). The star-like to bottlebrush transition was identified at Nbb = 50–69 using SAOS. In extension, melt strain hardening is absent in the star-like melts (Nbb ≤ 50) but is prominent in the bottlebrush limit (Nbb > 69). The onset of melt strain hardening occurs at a time scale equivalent to the Rouse time of the backbone. A molecular interpretation of these results builds upon recent conjectures related to strain-induced increases in interchain friction in bottlebrush polymers. These findings will be useful in designing bottlebrush melts that strain harden, which is critical in various types of processing methods involving extensional flows, including foaming, 3D printing, and film-blowing.

Synthesis, thermal and gas permeation properties of new silicon containing ROMP polytricyclodecadienes

New bis-trimethylsilyl-substituted poly(tricyclo[4.2.2.02,5]deca-3,9-diene) (PTDDSi2) and poly(tricyclo[4.2.2.02,5]deca-3,7,9-triene) (PTDTSi2) were synthesized by ring-opening metathesis polymerization of the corresponding new monomers in the presence of 1st ‒ 3d generation Grubbs’ catalysts. For monomer synthesis, the reaction of chlorosilanes with 1,3,5,7-cyclooctatetraene was studied for the first time. Polymerization conditions (catalyst type, temperature, monomer concentration) were selected to provide 77–96% yield of polymers. The monomer and polymer spatial structure had a significant effect both on their synthesis and on properties. The high Тg of PTDDSi2 (above the Td), as well as the easily proceeding retrodiene decomposition of the PTDTSi2 with the formation of bis(trimethylsilyl)benzene and polyacetylene (at 98 °C) indicated high rigidity of the polymer chain. A gas permeability investigations of mono-(PTDDSi), bis-Me3Si-substituted PTDDSi2 and hydrogenated product H-PTDDSi2 showed that PTDDs belong to the group of polymers with moderate gas permeability (PO2 up to 100 Barrer). Nonetheless, PTDDSi2 exhibited “solubility controlled permeability”, which is unusual for moderately permeable polymers.

Towards a synthetic macromolecular platform for biocompatible organometallic catalysis: From controlled polymer synthesis to catalysis in complex biological media

Incorporation of organometallic catalysts within macromolecular scaffolds is a promising approach to constructing biocompatible catalytic systems. Here, we report our recent progress in establishing a modular synthetic platform to prepare di-block brush-arm star copolymers (DBSPs), which were further used as scaffolds to construct catalytic systems that can function in complex biological media. First, a novel convergent graft-to approach based on ring-opening metathesis polymerization (ROMP) and enyne metathesis mediated conjugation was developed to synthesize DBSPs. By introducing active esters, functional DBSPs can be rapidly accessed through post-polymerization modification. DBSPs with fluorescent or radial probes were prepared, enabling structure-property study of the DBSPs. Further incorporation of organometallic catalysts into the DBSPs via either covalent conjugation or physical encapsulation afforded catalytic systems that are active in cellular environments.

(Arylimido)niobium(V)–Alkylidenes as the Catalysts for Ring-Opening Metathesis Polymerization (ROMP) of Cyclic Olefins: Z-Specific ROMP of Cyclooctene by Nb(CHSiMe3)(NC6H5)[OC(CF3)3](PMe3)2

(Arylimido)niobium(V)–alkylidene complexes, Nb(CHSiMe3)(NAr)[OC(CF3)3](PMe3)2 (Ar = C6H5 (4), 2,6-F2C6H3, C6F5 (6)), Nb(CHSiMe3)(N-2,6-Cl2C6H3)(OC6F5)(PMe3)2 (7), and Nb(CHSiMe3)(N-2,6-F2C6H3)(OC6Cl5)(PMe3)2, have been prepared and identified. The pentafluorophenylimido analogue 6 and 7 showed superior catalytic activities for the ring-opening metathesis polymerization (ROMP) of norbornene (TOF = 56.1, 53.7 s–1, respectively, at 25 °C) and the activities increased at 50 °C (TOF ≥ 352 s–1). The prepared alkylidenes catalyzed the ROMP of cis-cyclooctene (COE), and the activity increased at high temperature (up to 80 °C). The Z-specific ROMP of COE has been demonstrated especially by the phenylimido analogue 4 at 80 °C, affording polymers possessing olefinic double bonds with exclusive cis selectivity (99%).

Synthesis of poly(maleimide)s with promising performance via Diels–Alder reaction and ring-opening metathesis polymerization

Synthesis of poly(maleimide)s with promising performance via Diels–Alder reaction and ring-opening metathesis polymerization

Regiospecific Poly(ethylene-co-vinyl alcohol) by ROMP of 3-Acetoxycyclooctene and Postpolymerization Modification for Barrier Material Applications

Ethylene vinyl alcohol (EVOH) is an oxygen barrier polymer used to prevent premature degradation of food, pharmaceuticals, and other products due to its semicrystallinity, strong intermolecular interactions, and consequently low free volume. EVOH is made using traditional free-radical copolymerization, which leads to little structural regularity. We utilized ring-opening metathesis polymerization (ROMP) to determine how regioregularity impacts the barrier properties of EVOH-related materials. A regioregular (head-to-tail) polymer was synthesized from ROMP of 3-acetoxycyclooctene, followed by hydrogenation and deacylation to give a linear, highly regioregular EVOH (PH3OHCOE) containing the equivalent of 75 mol % ethylene units. The same process was carried out with 5-acetoxycyclooctene, but the resulting polymer (PH5OHCOE) is regiorandom. Both polymers were compared to an industry benchmark, EVOH-44, containing 44 mol % ethylene units. After processing, differential scanning calorimetry showed that the semicrystalline PH3OHCOE had a higher melting temperature and enthalpy of melting compared to semicrystalline PH5OHCOE, indicating that PH3OHCOE is more crystalline. This was confirmed by wide-angle X-ray scattering (WAXS). WAXS, rheological studies, and polarized optical microscopy showed that PH3OHCOE has a more well-defined crystal structure, a higher degree of hydrogen-bonding between −OH groups, and a higher glass transition temperature compared to PH5OHCOE. These differences were also highlighted in their tensile behavior, where PH3OHCOE and EVOH-44 exhibited brittle failure compared to the ductile behavior observed for PH5OHCOE. Oxygen barrier testing demonstrated that regioregular PH3OHCOE had an oxygen permeability more than a factor of 3 lower than regiorandom PH5OHCOE but still higher than EVOH-44, while water barrier testing showed that PH3OHCOE had the lowest water permeability, more than 6 times lower than EVOH-44. These results highlight the importance of regioregularity on the barrier properties of EVOH-like materials and show that structural regularity can lower oxygen permeability while maintaining low water permeability at the low vinyl alcohol content.

Photodimerization of anthracene- and carborane-bearing polymers obtained by ring opening metathesis polymerization

Random copolymers with pendant anthracene and carborane units were obtained by ring-opening metathesis polymerization (ROMP). The copolymers exhibit a variety of emission behaviors in solution and mechanochromic behavior as solids. Photodimerization and photocleavage of the anthracene groups in the copolymers at 350 and 254 nm, respectively, was studied by UV–vis spectroscopy. Thermal properties of the copolymers were evaluated under nitrogen and in air in the range of 25–600 °C.

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.

Crosslinked Polynorbornene-Based Anion Exchange Membranes with Perfluorinated Branch Chains

To investigate the effect of perfluorinated substituent on the properties of anion exchange membranes (AEMs), cross-linked polynorbornene-based AEMs with perfluorinated branch chains were prepared via ring opening metathesis polymerization, subsequent crosslinking reaction, and quaternization. The crosslinking structure enables the resultant AEMs (CFnB) to exhibit a low swelling ratio, high toughness, and high water uptake, simultaneously. In addition, benefiting from the ion gathering and side chain microphase separation caused by their flexible backbone and perfluorinated branch chain, these AEMs had high hydroxide conductivity up to 106.9 mS cm-1 at 80 °C even at low ion content (IEC < 1.6 meq g-1). This work provides a new approach to achieve improved ion conductivity at low ion content by introducing the perfluorinated branch chains and puts forward a referable way to prepare AEMs with high performance.

Grafting-To and From for Multiplexed Chemical-Warfare-Agent Responsive Polymer Brushes

Surface-tethered polymers have important applications in functional polymer coatings, particularly for the development of chemically responsive surfaces. Here, we combined the traditional grafting-to and grafting-from methods to create a new surface grafting strategy, termed grafting-to and from, using surface-initiated ring-opening metathesis polymerization (SI-ROMP). In this method, poly(pentafluorophenyl methacrylate) is grafted to an amine terminated surface. Surplus reactive esters after reaction with surface amines render this polymer a connecting or tie layer that can be further reacted to provide dense ROMP initiation sites. This amplification of grafting sites results in thick and environmentally stable polymer brushes upon SI-ROMP. With the goal of developing polymer-grafted breathable membranes that autonomously react to multiple chemical warfare agents (CWAs), we demonstrate the benefit of this method by employing amine reactive monomers in the grafting-from step. This enables diverse postsynthetic functionalization for the facile screening of chemical motifs to enhance response capabilities to mustard blister agents. Surface-tethered triarylmethanol-containing polymers with four distinct functional groups are prepared and challenged with the vapor of 2-chloroethyl ethyl sulfide (CEES), a simulant of mustard agent, in humid air. Importantly, hydroxyl groups effectively improve CWA response and the resulting polymer brushes show chain collapse after both CEES and diethylchlorophosphate (DCP) treatment. Our results illustrate that the grafting-to and from method can be used to grow functional and robust polymer coatings for various applications.

Rapid Controlled Synthesis of Large Polymers by Frontal Ring-Opening Metathesis Polymerization

Tunable molecular weight and well-defined polydispersity are the hallmarks of a controlled polymerization. This process relies on vanishingly small termination rates, minimal chain transfer, and initiation rates faster than propagation rates. Ring-opening metathesis polymerization (ROMP) is a well-known controlled polymerization based on the opening of strained cyclic olefins. The exothermic nature of ROMP allows rapid conversion of neat monomers to polymers through frontal ROMP (FROMP). Unlike traditional ROMP, FROMP uses the exothermic heat from the opening of strained cyclic olefins to thermally activate the initiator that sustains the propagation of a cascading reaction front. Although the reaction mechanisms for ROMP and FROMP are the same, the reaction conditions differ greatly, especially in the temperature and monomer concentration. The ability to control the polymerization under FROMP conditions has yet to be investigated, as well as its potential in the synthesis of well-defined polymers without the use of solvents and with minimal energy input. Here, we show that FROMP rapidly transforms monomers into polymers of high-molecular weight (Mn) with good fidelity and low dispersity (Đ). Specifically, the synthesis of polymers with Mn up to 700 kg/mol and Đ of 1.5 was achieved with a rapid, solvent-free, and oxygen-tolerant frontal polymerization technique. Further control of the polymerization was possible with the addition of a phosphite ligand that lowered the Đ to 1.2. We anticipate that controlled FROMP will become a valuable macromolecular synthetic tool due to its reliability, speed, scalability, and simplicity.

Microporous polynorbornenes bearing carbocyclic substituents: Structure-property study

Four sets of polymers from norbornene-type monomers with various carbocyclic groups were prepared to perform the structure-property study and to evaluate the effect of main- and side-chains on the porosity of resultant polymers. As monomers, norbornenes with phenyl-, cyclohexyl-, norbornyl-, and framed carbocyclic-groups were designed and synthesized using the Diels-Alder reaction in good yields. All these monomers were involved in ring-opening metathesis and vinyl-addition polymerization to form polymers with various structure of main-chains. The porosity was investigated with help of low temperature nitrogen adsorption/desorption method, and the sorption of H2, CO2, CH4 were also assessed for the most promising synthesized polymers. The obtained polymers exhibited large BET surface areas (up to 740 m2/g), while CO2 and H2 uptake reached 3.0 and 0.18 mmol/g, correspondingly. The porosity of polymers was shown to be determined by the nature of both backbones and side-chains, and it seems that the influence of side-chains is the main one. The largest values of BET surface areas were achieved for polynorbornenes with framed carbocyclic groups. At the same time, vinyl-addition polynorbornenes, which have more rigid main-chains, also provided larger BET surface areas compared to isomeric metathesis ones. Therefore, new valuable relationships between polymer structure and its porosity were established, and an efficient strategy to readily available soluble microporous polymers from norbornene derivatives was suggested. The combination of comparatively simple synthesis, high porosity and solubility in organic solvents can be considered as a key advantage of the synthesized polymers.