Ring-opening Metathesis Polymerization Research Articles

Amine-Containing Monomers for Ring-Opening Metathesis Polymerization: Understanding Chelate Effects in Aryl- and Alkylamine-Functionalized Polyolefins

Ring-opening metathesis polymerization (ROMP) of two different types of amine-functionalized monomers, aminonorbornenes (ANs) and aminocyclooctenes (ACs), has been studied using [(H2IMes)(PCy3)(Cl)2Ru = CHPh] Grubbs second generation catalyst, G2, and [(H2IMes)(pyr)2(Cl)2Ru = CHPh] Grubbs third generation catalyst, G3. Despite the known detrimental effects of unprotected amine functionalities on Ru-based ROMP catalysts, aminopolyolefins can be readily prepared using G2 and G3. The influence of the amine substituent of the monomer on the polymerization process, as probed by monitoring reaction kinetics, confirmed that the basicity/nucleophilicity of the amine group has a detrimental influence on the ROMP process. Reaction kinetics of homopolymerization of both these classes of monomers revealed faster polymerization of ACs than the more strained AN counterpart, which has been attributed to the favored chelated catalyst resting states in the case of ANs. Hammett studies show increased polymerization rates in the presence of electron-withdrawing aryl amine-containing monomers. These observed kinetic effects were used to advantage in the copolymerization of AN and AC monomers to access a gradient polymer. Polymerization kinetics of combined monomers displayed different reactivity profiles than observed during homopolymerization. Polyolefins with unprotected secondary alkyl amines could be synthesized by leveraging the unfavorable chelate formation in the case of AC monomers. The strategic selection of cyclic alkene and relative positioning of amine substituent allows for the diverse incorporation of secondary amines into polymers by ROMP.

Fast Ring-Opening Metathesis Polymerization of Tricyclic Oxanorbornene Derivatives

A new type of tricyclic oxanorbornene monomer was developed for living ring-opening metathesis polymerization (ROMP). A methyl group located at the endoposition of the oxanorbornene bicycle resulted in polymerization kinetics orders of magnitude slower than the corresponding hydrogen-substituted monomers because of steric effects. Both monomers showed well-controlled molecular weights and excellent dispersities with Grubbs’ third-generation catalyst. The slower endomethyl substituted monomers could even be polymerized in a well-controlled manner using Grubbs’ second-generation catalyst. Investigations of random copolymers demonstrated that the steric hindrance only affects the rate of propagation rather than the rate of initiation. Block copolymers between the fast and slow monomers could be prepared with good molecular weight and dispersity control.

Solvent Effects in Grafting-through Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) utilizing Grubbs’ third-generation catalyst ((H2IMes)(Cl)2(pyr)2RuCHPh) shows the characteristics of living polymerizations, including molecular weights increasing with monomer conversion and the ability to make (multi)block copolymers. However, irreversible termination reactions still occur due to catalyst decomposition, leading to terminated chains, especially in the context of sterically demanding monomers such as macromonomers (MMs). In this work, we performed identical ROMP reactions on three different MMs in six solvents commonly used in ROMP with varying levels of purity. The solvents included ethyl acetate (EtOAc), dichloromethane (CH2Cl2), chloroform (CHCl3), toluene, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). All polymerizations were conducted under air targeting a bottlebrush polymer backbone degree of polymerization (Nbb) of 100. All three MMs included a norbornene on the α chain end and had a molecular weight (Mn) of ∼4 kg/mol. They included one polystyrene MM with a bromine on the ω chain end and two poly(n-butyl acrylate) MMs with either a bromine or a trithiocarbonate group on the ω chain end. Solvent choice, and in some cases the level of purity, led to significant differences in the propagation rate in these ROMP grafting-through reactions. Of the solvents tested, the propagation rates in EtOAc and CH2Cl2 were approximately 4-fold and 2-fold faster, respectively, than that in CHCl3, toluene, and THF for all MMs. Propagation was much slower in DMF for the polystyrene MM than all the other solvents and on par with the slower solvents for the two poly(n-butyl acrylate) MMs tested. The purity of the solvent in some cases had a profound effect on the propagation rate: In the case of EtOAc, purification led to a 2-fold decrease in propagation rate; in contrast, purification of THF was required to observe full conversion of a MM to a bottlebrush polymer. The functional group on the ω chain end did not influence the rate of ROMP. Utilizing UV–vis spectroscopy to measure catalyst decomposition, the main polymer termination route in ROMP, we uncovered dramatic solvent effects, where the catalyst decomposed over 10 times faster in THF and DMF than in toluene. Finally, studies targeting Nbb = 500 or 1000 revealed that toluene, EtOAc, and CH2Cl2 demonstrated the highest degree of “livingness” in ROMP. These results will enable the synthesis of complex polymer architectures using ROMP with a high degree of living character.

Metal‐Free Ring‐Opening Metathesis Polymerization with Hydrazonium Initiators**

AbstractThe 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 (Mn≤9.4 kg mol−1) with relatively low dispersities (Đ≤1.4). The optimized conditions showed efficiency comparable to that achieved with Grubbs’ 2nd 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.

Tandem ROMP/Hydrogenation Approach to Hydroxy-Telechelic Linear Polyethylene.

Hydroxy-telechelic polyalkenamers have long been synthesized using ring-opening metathesis polymerization (ROMP) in the presence of an acyclic olefin chain-transfer agent (CTA); however, this route typically requires protected diols in the CTA due to the challenge of alcohol-mediated degradation of ruthenium metathesis catalysts that can not only deactivate the catalysts, but also compromise the CTA. We demonstrate the synthesis and implementation of a new hydroxyl-containing CTA in which extended methylene spacers isolate the olefin and alcohol moieties to mitigate decomposition pathways. This CTA enabled the direct ROMP synthesis of hydroxy-telechelic polycyclooctene with controlled chain lengths dictated by the initial ratio of monomer to CTA. The elimination of protection/deprotection steps resulted in improved atom economy. Subsequent hydrogenation of the backbone olefins was performed by a one-pot, catalytic approach employing the ruthenium complex used for the initial ROMP. The resultant approach is a streamlined, atom-economic, and low-waste route to hydroxy-telechelic linear polyethylene that uses a green solvent, succeeds with miniscule quantities of catalyst (0.005 mol %), and requires no additional purification steps.

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 (Mn ≤9.4 kg mol-1 ) with relatively low dispersities (Đ≤1.4). The optimized conditions showed efficiency comparable to that achieved with Grubbs' 2nd 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.

Star-Shaped ROMP Polymers Coated with Oligothiophenes That Exhibit Unique Emission.

A series of oligo(thiophene)-modified “soluble” star-shaped ring-opening metathesis polymerization (ROMP) polymers were prepared by sequential living ROMP of norbornene and a cross-linking agent using a molybdenum-alkylidene catalyst, followed by Wittig-type coupling for termination with oligo(thiophene) carboxaldehydes. The resultant star-shaped ROMP polymers displayed unique emission properties affected by the core size and arm repeat units as well as the kind of oligothiophene coated. The effects of the thiophene groups on photophysical properties of star-shaped/linear polymers were studied via time-resolved fluorescence spectroscopy. Fluorescence lifetimes were determined in THF as 400, 640, 730, and 820 ps for Star 3TPh, Linear 3TPh, Star 4T, and Linear 4T, respectively. A significant enhancement of the nonradiative rate constants knr in the star-shaped polymers results in relatively lower fluorescence quantum yields and shorter fluorescence lifetimes compared to the corresponding linear polymers.

Composite Correlated Molecular Orbital Theory Calculations of Ring Strain for Use in Predicting Polymerization Reactions.

Strained ring systems play an important role in synthesis and can be characterized by the ring strain energy (RSE). The RSE of 3, 4, 5, and 6 membered saturated and unsaturated ring systems containing N, O, P, and S heteroatoms and H, F, SiMe3 , and SO2 Me substituents were calculated at the G3(MP2) composite correlated molecular orbital theory level using up to 5 models to predict the RSE. Generally, the RSE decreased as ring size increased with a substantial decrease from 4 to 5 membered rings. Replacement of a ring CH2 with P or S reduced the RSE, consistent with less angle strain. The RSE for unsaturated systems were generally greater than for saturated systems due to increased angle strain. No general trends were found with respect to substituent effects. The RSE values suggest that 3-pyrroline and 2-pyrroline and their derivatives may be able to support ring opening metathesis polymerization and warrant further study.

Synthesis and Micellization of Bottlebrush Poloxamers.

Bottlebrush polymers are characterized by an expansive parameter space, including graft length and spacing along the backbone, and these features impact various structural and physical properties such as molecular diffusion and bulk viscosity. In this work, we report a synthetic strategy for making grafted block polymers with poly(propylene oxide) and poly(ethylene oxide) side chains, bottlebrush analogues of poloxamers. Combined anionic and sequential ring-opening metathesis polymerization yielded low dispersity polymers, at full conversion of the macromonomers, with control over graft length, graft end-groups, and overall molecular weight. A set of bottlebrush poloxamers (BBPs), with identical graft lengths and composition, was synthesized over a range of molecular weights. Dynamic light scattering and transmission electron microscopy were used to characterize micelle formation in aqueous buffer. The critical micelle concentration scales exponentially with overall molecular weight for both linear and bottlebrush poloxamers; however, the bottlebrush architecture shifts micelle formation to a much higher concentration at a comparable molecular weight. Consequently, BBPs can exist in solution as unimers at significantly higher molecular weights and concentrations than the linear analogues.

Main group functionalized polymers through ring-opening metathesis polymerization (ROMP)

The incorporation of main group elements into polymers allows for the generation of new functional materials with hitherto-unattainable physical and chemical properties through the unique molecular geometries, electronic structures, and chemical reactivities of main group element-based functional groups. The synthesis of these novel materials requires highly controllable polymerizations which are tolerant of diverse functionalized monomers. In this review, we highlight recent developments in ring-opening metathesis polymerization (ROMP) as a versatile and advantageous method for the generation of new main group element-functionalized hybrid materials. The unique applications of ROMP-derived main group-functionalized polymers in membranes and polyelectrolytes, resins, composites, elastomers, and other engineering materials, ceramic precursors and flame retardants, polymeric ligands and metallopolymers, polymeric catalysts and reagents, and optoelectronic materials are described.

Ionomers Based on Addition and Ring Opening Metathesis Polymerized 5-phenyl-2-norbornene as a Membrane Material for Ionic Actuators.

Two types of poly(5-phenyl-2-norbornene) were synthesized via ring opening metathesis and addition polymerization. The polymers sulfonation reaction under homogeneous conditions resulted in ionomer with high sulfonation degree up to 79% (IEC 3.36 meq/g). The prepared ionomer was characterized by DSC, GPC, 1H NMR and FT-IR. Polymers for electromechanical applications soluble in common polar organic solvents were obtained by replacing proton of sulfonic group with imidazolium and 1-methylimidazlium. Membranes were prepared using the above-mentioned polymers and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), as well as mixtures with polyvinylidene fluoride (PVDF). Mechanical, morphological, and conductive properties of the membranes were examined by tensile testing, SEM, and impedance spectroscopy, respectively. Dry and air-stable actuators with electrodes based on SWCNT were fabricated via hot-pressing. Actuators with membranes based on methylimidazolium containing ionomers outperformed classical bucky gel actuator and demonstrated high strain (up to 1.14%) and generated stress (up to 1.21 MPa) under low voltage of 2 V.

Mussel Adhesive-Inspired Proteomimetic Polymer.

Herein, a synthetic polymer proteomimetic is described that reconstitutes the key structural elements and function of mussel adhesive protein. The proteomimetic was prepared via graft-through ring-opening metathesis polymerization of a norbornenyl-peptide monomer. The peptide was derived from the natural underwater glue produced by marine mussels that is composed of a highly repetitive 10 amino acid tandem repeat sequence. The hypothesis was that recapitulation of the repeating unit in this manner would provide a facile route to a nature-inspired adhesive. To this end, the material, in which the arrangement of peptide units was as side chains on a brush polymer rather than in a linear fashion as in the natural protein, was examined and compared to the native protein. Mechanical measurements of adhesion forces between solid surfaces revealed improved adhesion properties over the natural protein, making this strategy attractive for diverse applications. One such application is demonstrated, using the polymers as a surface adhesive for the immobilization of live cells.

Synthesis of Thermo-Controlled Cyclic Olefin Polymers via Ring Opening Metathesis Polymerization: Effect of Copolymerization with Flexible Modifier

Synthesis of Thermo-Controlled Cyclic Olefin Polymers via Ring Opening Metathesis Polymerization: Effect of Copolymerization with Flexible Modifier

Exploring the Structural Diversity of DNA Bottlebrush Polymers Using an Oligonucleotide Macromonomer Approach.

Herein, we demonstrate that macromonomers consisting of organics-soluble, chemically protected oligonucleotides (protDNA) and poly(ethylene glycol) (PEG) chains can be converted into bottlebrush polymers of distinct architectures via ring-opening metathesis polymerization (ROMP). Using a custom norbornene-containing phosphoramidite, two types of macromonomers were obtained: a linear norbornene-protDNA-PEG structure and a Y-shaped structure where the polymerizable norbornene group is situated at the junction where protDNA and PEG meet. With this strategy, the PEG chains can be placed either near the backbone of the bottlebrush or on its periphery, and in principle anywhere between these two extremes by adjusting the norbornene location, which makes this strategy attractive for constructing architecturally sophisticated oligonucleotide-containing copolymers.

Turnaround in use of enol ether opens the door to degradable plastics

While ring-opening metathesis polymerization (ROMP) is a robust method to construct polymers, monomer systems for degradable ROMP polymers are limited. Existing systems suffer from lengthy synthesis and/or uncontrolled polymerization. Xia and coworkers demonstrated the synthesis of degradable ROMP copolymers from the commercially available monomers in a controlled manner. While ring-opening metathesis polymerization (ROMP) is a robust method to construct polymers, monomer systems for degradable ROMP polymers are limited. Existing systems suffer from lengthy synthesis and/or uncontrolled polymerization. Xia and coworkers demonstrated the synthesis of degradable ROMP copolymers from the commercially available monomers in a controlled manner.

Poly(ionic liquid)s with Dicationic Pendants as Gas Separation Membranes.

Poly(norbornene)s and poly(ionic liquid)s are two different classes of attractive materials, which are known for their structural tunability and thermal stabilities, and have been extensively studied as gas separation membranes. The incorporation of ionic liquids (ILs) into the poly(norbornene) through post-polymerization has resulted in unique materials with synergistic properties. However, direct polymerization of norbornene-containing IL monomers as gas separation membranes are limited. To this end, a series of norbornene-containing imidazolium-based mono- and di-cationic ILs (NBM-mIm and NBM-DILs) with different connectivity and spacer lengths were synthesized and characterized spectroscopically. Subsequently, the poly(NBM-mIm) with bistriflimide [Tf2N−] and poly([NBM-DILs][Tf2N]2) comprising homo-, random-, and block- (co)polymers were synthesized via ring-opening metathesis polymerization using the air-stable Grubbs second-generation catalyst. Block copolymers (BCPs), specifically, [NBM-mIM][Tf2N] and [NBM-ImCnmIm] [Tf2N]2 (n = 4 and 6) were synthesized at two different compositions, which generated high molecular weight polymers with decent solubility relative to homo- and random (co)polymers of [NBM-DILs] [Tf2N]2. The prepared BCPs were efficiently analyzed by a host of analytical tools, including 1H-NMR, GPC, and WAXD. The successfully BCPs were cast into thin membranes ranging from 47 to 125 μm and their gas (CO2, N2, CH4, and H2) permeations were measured at 20 °C using a time-lag apparatus. These membranes displayed modest CO2 permeability in a non-linear fashion with respect to composition and a reverse trend in CO2/N2 permselectivity was observed, as a usual trade-off behavior between permeability and permselectivity.

Polymeric Antioxidant via ROMP of Bioderived Tricyclic Oxanorbornene Based on Vanillin and Furfurylamine

Herein, a bioderived antioxidant oxanorbornene lactam monomer bearing a vanillyl moiety has been successfully synthesized. The monomer was synthesized via reductive amination of two biobased molecules, vanillin and furfurylamine, followed by tandem Diels–Alder and lactamization reactions with maleic anhydride. For comparison, an inactive monomer was prepared using benzaldehyde instead of vanillin. Rapid controlled polymerization of the tricyclic strained monomers has been conducted using ring opening metathesis polymerization. The resultant polymers were fully characterized, and their antioxidant potentials had been assessed. The monomer with the vanillyl moiety and its corresponding polymer showed exceptional antioxidant activities compared to the reference analogue with a benzyl pendant group. Indeed, the developed antioxidant polymer shows promising potential in the sustainable polymer industry for a smaller ecological footprint.

Synthesis of styrene-norbornene diblock copolymers via ring-opening metathesis polymerization and nitroxide-mediated radical polymerization

Diblock copolymers containing polynorbornene (PNB) and polystyrene (PS) were synthesized by ring-opening metathesis polymerization (ROMP) and nitroxide-mediated radical polymerization (NMRP) approach. PNB containing nitroxide free radical was prepared by ROMP in presence of Grubbs 2nd generation catalyst and served as a macro-initiator for NMRP of styrene to achieve the targeting diblock copolymers. The effects of the chain transfer agent (CTA) concentration, catalyst amount, reaction time and reaction temperature on the synthesis of macro-initiator as well as the effects of mole ratio of monomer to macro-initiator and reaction time on the synthesis of diblock copolymers were studied. As a result, the diblock copolymers with Mn ranging from 5 kDa to 10 kDa and PDI ranging from 1.5 to 1.8 were obtained successfully. The desired diblock copolymers have broad applications in polymer electrolytes, nanoporous membranes and many other fields.

Microreactor-based chemo-enzymatic ROP-ROMP platform for continuous flow synthesis of bottlebrush polymers

Microreactor-based enzymatic ring-opening polymerization (ROP) and metal-catalytic ring-opening metathesis polymerization (ROMP) cascade reaction platform was developed for graft through synthesis of bottlebrush polymers. Single and integrated microreactor system were assembled for the evaluation of ROP, ROMP, and ROP-ROMP, respectively. By applying a continuous flow chemo-enzymatic ROP-ROMP strategy, a series of well-defined poly(norbornene)-graft-poly(ε-caprolactone) (PNB-g-PCL) bottlebrush polymers were successfully prepared with fast kinetics, exemption of macromonomer isolation and purification, the absence of macromonomer residue in the product, controlled molecular weight and narrow distribution. This work would provide inspiration for the chemo-enzymatic synthesis, microflow chemistry, and bottlebrush polymers.

ROMP of norbornene and oxanorbornene derivatives with pendant fluorophore carbazole and coumarin groups

A series of new (oxa)norbornene dicarboximide monomers, decorated with carbazole and coumarin as pendant fluorophore groups, were synthesized and characterized. Their behaviour in ring-opening metathesis polymerization (ROMP) was explored in the presence of commercial Grubbs’ third generation catalyst G3 and cis-selective catalyst GZ. Norbornene dicarboximide (NDI) monomers were successfully polymerized by both the catalysts, while oxanorbornene dicarboximide (ONDI) analogues showed scarce reactivity toward polymerization promoted by GZ. The nature of the spacer (ethylene or p-xylene) between the NDI unit and the chromophore seems to not affect the outcome of the polymerization. Moving from G3 to GZ, it was possible to synthetize NDI-based polymers with cis-olefin content increasing from 47 to 89%, depending on the nature of the pendant fluorophore group. The obtained polymers showed photoluminescence in solution and solid state owing to the carbazole or coumarin fragment, regardless of the nature of the spacer and cis-olefin amount. Interestingly, a significant excimer emission due to the overlap of carbazole groups was observed for a carbazole-functionalized polymer film, as a result of the combined effect of two factors, the stiffness of the spacer (p-xylene) linking the NDI moiety to the fluorophore and the high stereoregularity degree (83% cis) of the main chain.