Vinyl-Addition Homopolymeization of Norbornenes with Bromoalkyl Groups

Vinyl-addition polynorbornenes are of great interest as versatile templates for the targeted design of polymer materials with desired properties. These polymers possess rigid and saturated backbones, which provide them with high thermal and chemical stability as well as high glass transition temperatures. Vinyl-addition polymers from norbornenes with bromoalkyl groups are widely used as precursors of anion exchange membranes; however, high-molecular-weight homopolymers from such monomers are often difficult to prepare. Herein, we report the systematic study of vinyl-addition polymerization of norbornenes with various bromoalkyl groups on Pd-catalysts bearing N-heterocyclic carbene ligands ((NHC)Pd-systems). Norbornenes with different lengths of hydrocarbon linker (one, two, and four CH2 groups) between the bicyclic norbornene moiety and the bromine atom were used as model monomers, while single- and three-component (NHC)Pd-systems were applied as catalysts. In vinyl-addition polymerization, the reactivity of the investigated monomers varied substantially. The relative reactivity of these monomers was assessed in copolymerization experiments, which showed that the closer the bromine is to the norbornene double-bond, the lower the monomer's reactivity. The most reactive monomer was the norbornene derivative with the largest substituent (with the longest linker). Tuning the catalyst's nature and the conditions of polymerization, we succeeded in synthesizing high-molecular-weight homopolymers from norbornenes with bromoalkyl groups (Mn up to 1.4 × 106). The basic physico-chemical properties of the prepared polymers were studied and considered together with the results of vinyl-addition polymerization.

Direct Insertion Polymerization of Ionic Monomers: Rapid Production of Anion Exchange Membranes

The limited number of methods to directly polymerize ionic monomers currently hinders rapid diversification and production of ionic polymeric materials, namely anion exchange membranes (AEMs) which are essential components in emerging alkaline fuel cell and electrolyzer technologies. Herein, we report a direct coordination-insertion polymerization of cationic monomers, providing the first direct synthesis of aliphatic polymers with high ion incorporations and allowing facile access to a broad range of materials. We demonstrate the utility of this method by rapidly generating a library of solution processable ionic polymers for use as AEMs. We investigate these materials to study the influence of cation identity on hydroxide conductivity and stability. We found that AEMs with piperidinium cations exhibited the highest performance, with high alkaline stability, hydroxide conductivity of 87 mS/cm at 80 °C, and a peak power density of 730 mW/cm2 when integrated into a fuel cell device.

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.

Chiral Polymers from Norbornenes Based on Renewable Chemical Feedstocks

Optically active polymers are of great interest as materials for dense enantioselective membranes, as well as chiral stationary phases for gas and liquid chromatography. Combining the versatility of norbornene chemistry and the advantages of chiral natural terpenes in one molecule will open up a facile route toward the synthesis of diverse optically active polymers. Herein, we prepared a set of new chiral monomers from cis-5-norbornene-2,3-dicarboxylic anhydride and chiral alcohols of various natures. Alcohols based on cyclic terpenes ((-)-menthol, (-)-borneol and pinanol), as well as commercially available alcohols (S-(-)-2-methylbutanol-1, S-(+)-3-octanol), were used. All the synthesized monomers were successfully involved in ring-opening metathesis polymerization, affording polymers in high yields (up to 96%) and with molecular weights in the range of 1.9 × 10⁵-5.8 × 10⁵ (M_w). The properties of the metathesis polymers obtained were studied by TGA and DSC analysis, WAXD, and circular dichroism spectroscopy. The polymers exhibited high thermal stability and good film-forming properties. Glass transition temperatures for the prepared polymers varied from -30 °C to +139 °C and, therefore, the state of the polymers changed from rubbery to glassy. The prepared polymers represent a new attractive platform of chiral polymeric materials for enantioselective membrane separation and chiral stationary phases for chromatography.

Dual Covalent Cross-Linking Networks in Polynorbornene: Comparison of Shape Memory Performance

In this work, tetrakis(dimethyllamino)ethylene (TDAE) plasticized polynorbornene (PNB) was used as the matrix, sulfur (S) and dicumyl peroxide (DCP) were simultaneously used as crosslinking agents to construct dual covalent cross-linking networks in PNB. The effects of different amounts of cross-linkers on the crosslinking degree, mechanical property, glass transition temperature, and PNB shape memory performance were investigated. Two crosslinking mechanisms were examined by Fourier transform infrared spectrometer and Raman spectrometer. The results showed that sulfur-rich cross-linked PNB exhibited a higher crosslinking degree, tensile strength, and slightly higher glass transition temperature than the DCP-rich system. Cross-linked PNB presented better shape memory performance than the uncross-linked one. Sulfur-rich cross-linked PNB showed even better shape memory behavior than the DCP-rich system, both with a shape fixation ratio of over 99% and a shape recovery ratio of over 90%. The reaction mechanism of sulfur and DCP in cross-linking PNB was different. Sulfur reacted with the α-H in PNB to form monosulfide bonds, disulfide bonds, and polysulfide bonds in PNB and the number of polysulfide bonds increased with increased amounts of sulfur. DCP reacted with the double bonds in PNB to form C-C covalent bond crosslinking networks. The crosslinking mechanism revealed that the sulfur-containing cross-linked bonds, especially polysulfide bonds, were more flexible and bore large deformation, which gave the PNB excellent mechanical properties and ensured a higher shape entropy elastic recovery ratio.

An Efficient Ring-Closure Method for Preparing Well-Defined Cyclic Polynorbornenes

An efficient bimolecular ring-closure method is developed to prepare the well-defined cyclic polynorbornenes by combining the living ring-opening metathesis polymerization (ROMP) with the self-accelerating double strain-promoted azide-alkyne cycloaddition (DSPAAC) reaction. In this method, ROMP is used to synthesize the well-defined linear polynorbornenes with both azide terminals by virtue of a N-hydroxysuccinimide-ester-functionalized Grubbs initiator following the modification of polymer end groups. DSPAAC click reaction is then used to ring-close the linear polymer precursors and prepare the corresponding well-defined cyclic polynorbornenes using the sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) as small linkers. The self-accelerating DSPAAC ring-closing reaction facilitates this method to efficiently prepare pure cyclic polynorbornenes in the presence of a molar excess of DIBOD small linkers to the linear polynorbornene precursors. This is the first report to prepare well-defined polynorbornenes with cyclic topology based on the ring-closure strategy for cyclic polymers.

Polyvinylnorbornene Gas Separation Membranes

Polynorbornenes are already used in a wide range of applications. They are also considered materials for polymer gas separation membranes because of their favorable thermal and chemical resistance, rigid backbone and varied chemistry. In this study, the use of 5-vinyl-2-norbornene (VNB), a new monomer in the field of gas separations, is investigated by synthesizing two series of polymers via a vinyl-addition polymerization. The first series investigates the influence of the VNB content on gas separation in a series of homo and copolymers with norbornene. The second series explores the influence of the crosslinking of polyvinylnorbornene (pVNB) on gas separation. The results indicate that while crosslinking had little effect, the gas separation performance could be fine-tuned by controlling the VNB content. As such, this work demonstrates an interesting way to significantly extend the fine-tuning possibilities of polynorbornenes for gas separations.

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

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

Simultaneously Antimicrobial, Protein-repellent and Cell-compatible Polyzwitterion Networks: More Insight on Bioactivity and Physical Properties

A poly(oxanorbornene)-based polyzwitterion with primary ammonium and carboxylate groups (PZI) has been reported previously as the first simultaneously antimicrobial and protein-repellent polyzwitterion. Here, additional physical and biological properties of three poly(oxanorbornene)-based polyzwitterions with different functional groups (PZI, the polycarboxybetaine PCB, and the polysulfobetaine PSB) are compared to understand the molecular origins of this unusual bioactivity. Additionally, the three polyzwitterions and the antimicrobial, polycationic SMAMP are exposed to proteins, bacteria suspensions, human plasma and serum. These interactions are investigated by surface plasmon resonance spectroscopy. In protein adhesion studies, neither fibrinogen nor lysozyme adhere irreversibly to PZI, yet reversible interaction with lysozyme is observed at pH 7 and 8. In the presence of bivalent cations, reversible fibrinogen adhesion on PZI and PSB is observed, but not on PCB. This might explain why mammalian cells grow on PZI and PSB, but not on PCB. PZI does not show human plasma adhesion, while PCB and PSB have 0.27 and 0.48 ng mm^-2 adhered plasma, and SMAMP even 6.3 ng mm^-2. Both PZI and SMAMP show strong serum adhesion, while no serum adhered to PCB, and only little to PSB. This could be related to the pH difference between serum and plasma, to which the pH-responsive primary ammonium groups are susceptible, while the permanently charged NR₄ ⁺ groups are unaffected. Both PZI and PCB showed none or only little bacterial adhesion. PCB is also intrinsically antimicrobial against E. coli and S. aureus bacteria and thus is also simultaneously protein-repellent and antimicrobially active. Thus, while the carboxylate groups of PZI and PCB seems to be a prerequisite for the dual antimicrobial activity and protein-repellency, the pH-responsiveness of the primary ammonium group seems to make the PZI molecule vulnerable for protein adhesion in fluids that are slightly out of the physiological range.

Design of Thermochromic Polynorbornene Bearing Spiropyran Chromophore Moieties: Synthesis, Thermal Behavior and Dielectric Barrier Discharge Plasma Treatment

A new class of thermochromic polynorbornene with pendent spiropyran moieties has been synthesized. Functionalization of norbornene monomers with spirobenzopyran moieties has been achieved using Steglich esterification. These new monomeric materials were polymerized via Ring Opening Metathesis Polymerization (ROMP). In spite of their poor solubility, polynorbornenes with spirobenzopyran exhibited thermochromic behavior due to the conversion of their closed spiropyran moieties to the open merocyanine form. Moreover, these polymers displayed bathochromic shifts in their optical response, which was attributed to the J-aggregation of the attached merocyanine moieties that were associated with their high concentration in the polymeric chain. The surface of the obtained polymers was exposed to atmospheric pressure air Dielectric Barrier Discharge (DBD) plasma system, which resulted in the reduction of the surface porosity and converted some surface area into completely non-porous regions. Moreover, the plasma system created some areas with highly ordered J-aggregates of the merocyanine form in thread-like structures. This modification of the polymers’ morphology may alter their applications and allow for these materials to be potential candidates for new applications, such as non-porous membranes for reverse osmosis, nanofiltration, or molecular separation in the gas phase.

A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic nature of the monomer is conserved. The resulting polymer, polynorbornene, has a very high glass transition temperature, Tg, and interesting optical and electrical properties. However, the polymerization of functional norbornenes by this mechanism is complicated by the fact that the endo substituted norbornene monomer has, in general, a very low reactivity. Furthermore, the separation of the endo substituted monomer from the exo monomer is a tedious task. Here, we present a simple protocol for the polymerization of substituted norbornenes (endo:exo ca. 80:20) bearing either a carboxylic acid or a pendant double bond. The process does not require that both isomers be separated, and proceeds with low catalyst loadings (0.01 to 0.02 mol%). The polymer bearing pendant double bonds can be further transformed in high yield, to afford a polymer bearing pendant epoxy groups. These simple procedures can be applied to prepare polynorbornenes with a variety of functional groups, such as esters, alcohols, imides, double bonds, carboxylic acids, bromo-alkyls, aldehydes and anhydrides.