Photoresponsive Gels Prepared by Ring‐Opening Metathesis Polymerization

This communication describes photoresponsive gels, prepared using ring‐opening metathesis polymerization (ROMP), that dissolve upon irradiation with ultraviolet light. Exposure of mixtures of norbornene‐type ROMP monomers and new photoreactive cross‐linkers comprising two norbornene units bound through a chain containing o‐nitrobenzyl esters (NBEs) to well‐known ruthenium carbene catalysts gave cross‐linked polymer networks that swelled in organic solvents or water depending on the structure of the monomer. These gels became homogeneous upon irradiation with UV light, consistent with breaking of the cross‐links through photolysis of the NBE groups. The irradiation time required for homogenization of the gels depended on the cross‐link density and the structure of the photoresponsive cross‐linker.

     

Functional Monolithic Materials for Boronate‐Affinity Chromatography via Schrock Catalyst‐Triggered Ring‐Opening Metathesis Polymerization

Monolithic polymeric materials are prepared via ring‐opening metathesis copolymerization of norborn‐2‐ene with 1,4,4a,5,8,8a‐hexahydro‐1,4,5,8‐exo,endo‐dimethanonaphthalene in the presence of macro‐ and microporogens, that is, of n‐hexane and 1,2‐dichloroethane, using the Schrock catalyst Mo(N‐2,6‐(2‐Pr)₂‐C₆H₃)(CHCMe₂Ph)(OCMe₃)₂. Functionalization of the monolithic materials is accomplished by either terminating the living metal alkylidenes with various functional aldehydes or by post‐synthesis grafting with norborn‐5‐en‐2‐ylmethyl‐4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)benzoate. Finally, boronate‐grafted monolithic columns (100 × 3 mm i.d.) are successfully applied to the affinity chromatographic separation of cis‐diol‐based biomolecules.     

Alternating Ring‐Opening Metathesis Copolymerization by Grubbs‐Type Initiators with Unsymmetrical N‐Heterocyclic Carbenes

A series of Ru^IV–alkylidenes based on unsymmetrical imidazolin‐2‐ylidenes, that is, [RuCl₂{1‐(2,4,6‐trimethylphenyl)‐3‐R‐4,5‐dihydro‐(3H)‐imidazol‐1‐ylidene}(CHPh)(pyridin)] (R=CH₂Ph ( 5 ), Ph ( 6 ), ethyl ( 7 ), methyl ( 8 )), have been synthesized. These and the parent initiators [RuCl₂(PCy₃){1‐(2,4,6‐trimethylphenyl)‐3‐R‐4,5‐dihydro‐(3H)‐imidazol‐1‐ylidene}(CHC₆H₅)] (R=CH₂C₆H₅ ( 1 ), C₆H₅ ( 2 ), ethyl ( 3 )) were used for the alternating copolymerization of norborn‐2‐ene (NBE) with cis‐cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE‐alt‐COE)_n and poly(NBE‐alt‐CPE)_n containing up to 97 and 91 % alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1 – 3 and 5 – 8 were determined by using both a zero‐ and first‐order Markov model. Finally, kinetic investigations using initiators 1 – 3 , 6 , and 7 were carried out. These revealed that in contrast to the 2nd‐generation Grubbs‐type initiators 1 – 3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE‐alt‐COE)_n yielded a fully saturated, hydrocarbon‐based polymer. Its backbone can formally be derived by 1‐olefin polymerization of CPE (1,3‐insertion) followed by five ethylene units and thus serves as an excellent model compound for 1‐olefin polymerization‐derived copolymers.     

Phenylenevinylene Block Copolymers via Ring‐Opening Metathesis Polymerization

Fully conjugated block copolymers containing 1,4‐ and 1,3‐phenylenevinylene repeating units can be prepared by the sequential ring opening metathesis polymerization of strained cyclophanedienes, initiated by ruthenium carbene complexes (Grubbs metathesis catalysts). The molecular weight of the constituent blocks can be tightly controlled by changing the catalyst to monomer ratio and the volume fraction of the block copolymers independently tailored by the ratio of the monomers employed. Extensive phase separation between the constituent blocks is observed in thin films of these polymers by atomic force microscopy and efficient energy transfer between blocks containing 1,4‐ and 1,3‐phenylenevinylene units can be seen in the photoluminescence of these materials.

     

Tandem Ring‐Opening–Ring‐Closing Metathesis for Functional Metathesis Catalysts

Use of a tandem ring‐opening–ring‐closing metathesis (RORCM) strategy for the synthesis of functional metathesis catalysts is reported. Ring opening of 7‐substituted norbornenes and subsequent ring‐closing metathesis forming a thermodynamically stable 6‐membered ring lead to a very efficient synthesis of new catalysts from commercially available Grubbs’ catalysts. Hydroxy functionalized Grubbs’ first‐ as well as third‐generation catalysts have been synthesized. Mechanistic studies have been performed to elucidate the order of attack of the olefinic bonds. This strategy was also used to synthesize the ruthenium methylidene complex.     

Sequential Electrografting and Ring‐Opening Metathesis Polymerization: a Strategy for the Tailoring of Conductive Surfaces

Summary: An electrografting technique has been combined with ring‐opening metathesis polymerization (ROMP). Poly(allyl methacrylate) chains have been chemisorbed onto steel and carbon plates under an appropriate cathodic potential in N,N‐dimethylformamide. The allyl moieties have been converted into Ru catalysts active in ROMP of norbornene and its derivatives. Initiation of ROMP from the surface is an efficient strategy to prepare strongly adhering coatings of tunable thickness and hydrophilic/hydrophobic balance, depending on the norbornene derivative polymerized at the surface.

The hydrophobicity of the polymer coating may be controlled by hydrolysis of the polynorbornene derivative.     

Fabrication of Supramolecular Semiconductor Block Copolymers by Ring‐Opening Metathesis Polymerization

ω‐Telechelic poly(p‐phenylene vinylene) species (PPVs) are prepared by living ring‐opening metathesis polymerization of a [2.2]paracyclophane‐1,9‐diene in the presence of Hoveyda–Grubbs 2nd generation initiator, with terminating agents based on N¹,N³‐bis(6‐butyramidopyridin‐2‐yl)‐5‐hydroxyisophthalamide (Hamilton wedge), cyanuric acid, Pd^II–SCS‐pincer, or pyridine moieties installing the supramolecular motifs. The resultant telechelic polymers are self‐assembled into supramolecular block copolymers (BCPs) via metal coordination or hydrogen bonding and analyzed by ¹H NMR spectroscopy. The optical properties are examined, whereby individual PPVs exhibit similar properties regardless of the nature of the end group. Upon self‐assembly, different behaviors emerge: the hydrogen‐bonding BCP behaves similarly to the parent PPVs whereas the metallosupramolecular BCP demonstrates a hypsochromic shift and a more intense emission owing to the suppression of aggregation. These results demonstrate that directional self‐assembly can be a facile method to construct BCPs with semiconducting networks, while combating solubility and aggregation.     

Ring‐Opening Metathesis Polymerization Based Post‐Synthesis Functionalization of Electron Beam Curing Derived Monolithic Media

Monolithic materials were prepared via electron‐beam curing from ethyl methacrylate, trimethylolpropane triacrylate, and norborn‐5‐ene‐2‐ylmethyl acrylate. Reaction of the norborn‐2‐ene groups with either RuCl₂(PCy₃)₂(CHPh) ( 1 ) or RuCl₂(PCy₃)(1,3‐dimesityl‐4,5‐dihydroimidazol‐2‐inylidene)(CHPh) ( 2 ) resulted in the surface attachment of the initiators. The extent of initiator immobilization was found to be substantially higher for 1 than for 2 . Reaction of the surface immobilized initiators with various monomers resulted in the desired surface modification of EB‐derived monoliths. The amounts of grafted monomer were determined by elemental analysis and ICP‐OES.

     

One‐Step Ring Opening Metathesis Block‐Like Copolymers and their Compositional Analysis by a Novel Retardation Technique

Using a one‐step synthetic route for block copolymers avoids the repeated addition of monomers to the polymerization mixture, which can easily lead to contamination and, therefore, to the unwanted termination of chain growth. For this purpose, monomers ( M1 – M5 ) with different steric hindrances and different propagation rates are explored. Copolymerization of M1 (propagating rapidly) with M2 (propagating slowly), M1 with M3 (propagating extremely slowly) and M4 (propagating rapidly) with M5 (propagating slowly) yielded diblock‐like copolymers using Grubbs’ first ( G1 ) or third generation catalyst ( G3 ). The monomer consumption was followed by ¹H NMR spectroscopy, which revealed vastly different reactivity ratios for M1 and M2 . In the case of M1 and M3 , we observed the highest difference in reactivity ratios (r₁=324 and r₂=0.003) ever reported for a copolymerization method. A triblock‐like copolymer was also synthesized using G3 by first allowing the consumption of the mixture of M1 and M2 and then adding M1 again. In addition, in order to measure the fast reaction rates of the G3 catalyst with M1 , we report a novel retardation technique based on an unusual reversible G3 Fischer‐carbene to G3 benzylidene/alkylidene transformation.     

A Precision Ethylene‐Styrene Copolymer with High Styrene Content from Ring‐Opening Metathesis Polymerization of 4‐Phenylcyclopentene

Ring‐opening metathesis polymerization of 4‐phenylcyclopentene is investigated for the first time under various conditions. Thermodynamic analysis reveals a polymerization enthalpy and entropy sufficient for high molar mass and conversions at lower temperatures. In one example, neat polymerization using Hoveyda–Grubbs second generation catalyst at −15 °C yields 81% conversion to poly(4‐phenylcyclopentene) (P4PCP) with a number average molar mass of 151 kg mol⁻¹ and dispersity of 1.77. Quantitative homogeneous hydrogenation of P4PCP results in a precision ethylene‐styrene copolymer (H₂‐P4PCP) with a phenyl branch at every fifth carbon along the backbone. This equates to a perfectly alternating trimethylene‐styrene sequence with 71.2% w/w styrene content that is inaccessible through molecular catalyst copolymerization strategies. Differential scanning calorimetry confirms P4PCP and H₂‐P4PCP are amorphous materials with similar glass transition temperatures (T_g) of 17 ± 2 °C. Both materials present well‐defined styrenic analogs for application in specialty materials or composites where lower softening temperatures may be desired.

     

Improved Molecular Weight Control in Ring‐Opening Metathesis Polymerization (ROMP) Reactions with Ru‐Based Olefin Metathesis Catalysts Using N Donors and Acid: A Kinetic and Mechanistic Investigation

The effect of the addition of H₃PO₄ on the ROMP activity of cyclooctene (COE) with first‐ [Cl₂(PCy₃)₂Ru?CHPh] and second‐generation [(H₂IMes)Cl₂(PCy₃)Ru?CHPh] Grubbs’ catalysts 1 and 4 (Cy=cyclohexyl, Ph=phenyl, Mes=2,4,6‐trimethylphenyl (mesityl)), their inhibited mixtures with 1‐methylimidazole (MIM), as well as their isolated bis‐N,N′‐dimethylaminopyridine (DMAP) derivatives [Cl₂(PCy₃)(DMAP)₂Ru?CHPh)] ( 5 b ) and [Cl₂(H₂IMes)(DMAP)₂Ru?CHPh] ( 7 b ) (DMAP=dimethylaminopyridine), a novel catalyst, has been investigated. The studies include the determination of their initiation rates, as well as a determination of the molecular weights and molecular weight distributions of the polymers obtained with these catalysts and catalyst mixtures from the exo‐7‐oxanorbornene derivative 11 . The structure of catalyst 7 b was confirmed by means of X‐ray diffraction. All N‐donor‐bearing catalysts or N‐donor‐containing catalyst mixtures not only exhibited elevated activity in the presence of acid, but also increased initiation rates. Using the reversible inhibition/activation protocol with MIM and H₃PO₄ enabled us to conduct controlled ROMP with catalyst 4 producing the isolated exo‐7‐oxanorbornene‐based polymer 12 with predetermined molecular weights and narrow molecular weight distributions. This effect was based on fast and efficient catalyst initiation in contrast to the parent catalyst 4 . Hexacoordinate complex 5 b also experienced a dramatic increase in initiation rates upon acid‐addition and the ROMP reactions became well‐controlled in contrast to the acid‐free reaction. In contrast, complex 7 b performs well‐controlled ROMP in the absence of acid, whereas the polymerization of the same monomer becomes less controlled in the presence of H₃PO₄. The closer evaluation of catalysts 5 b and 7 b demonstrated that their initiation rates exhibit a linear dependency on the substrate concentration in contrast to catalysts 1 and 4 . As a consequence, their initiation rates are determined by an associative step, not a dissociative step as seen for catalysts 1 and 4 . A feasible associative metathesis initiation mechanism is proposed.     

Antimicrobial Polymers Prepared by Ring‐Opening Metathesis Polymerization: Manipulating Antimicrobial Properties by Organic Counterion and Charge Density Variation

The synthesis and characterization of a series of poly(oxanorbornene)‐based synthetic mimics of antimicrobial peptides (SMAMPs) is presented. In the first part, the effect of different organic counterions on the antimicrobial properties of the SMAMPs was investigated. Unexpectedly, adding hydrophobicity by complete anion exchange did not increase the SMAMPs’ antimicrobial activity. It was found by dye‐leakage studies that this was due to the loss of membrane activity of these polymers caused by the formation of tight ion pairs between the organic counterions and the polymer backbone. In the second part, the effect of molecular charge density on the biological properties of a SMAMP was investigated. The results suggest that, above a certain charge threshold, neither minimum inhibitory concentration (MIC₉₀) nor hemolytic activity (HC₅₀) is greatly affected by adding more cationic groups to the molecule. A SMAMP with an MIC₉₀ of 4 μg mL^?1 against Staphylococcus aureus and a selectivity (=HC₅₀/MIC₉₀) of 650 was discovered, the most selective SMAMP to date.     

A Hybrid Ring‐Opening Metathesis Polymerization Catalyst Based on an Engineered Variant of the β‐Barrel Protein FhuA

A β‐barrel protein hybrid catalyst was prepared by covalently anchoring a Grubbs–Hoveyda type olefin metathesis catalyst at a single accessible cysteine amino acid in the barrel interior of a variant of β‐barrel transmembrane protein ferric hydroxamate uptake protein component A (FhuA). Activity of this hybrid catalyst type was demonstrated by ring‐opening metathesis polymerization of a 7‐oxanorbornene derivative in aqueous solution.     

Facile Synthesis of Brush Poly(phosphoamidate)s via One‐Pot Tandem Ring‐Opening Metathesis Polymerization and Atom Transfer Radical Polymerization

Poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA)‐based brush poly(phosphoamidate)s are successfully synthesized by a combination of ring‐opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP) following either a commutative two‐step procedure or a straightforward one‐pot process using Grubbs ruthenium‐based catalysts for tandem catalysis. Compared with the traditional polymerization method, combining ROMP and ATRP in a one‐pot process allows the preparation of brush copolymers characterized by a relatively moderate molecular weight distribution and quantitative conversion of monomer. Moreover, the surface morphologies and aggregation behaviors of these polymers are studied by AFM and TEM measurements.

     

Effect of Cocatalysts on the Catalytic Activities of Tantalum‐ and Niobium‐Based Catalysts for Ring‐Opening Metathesis Polymerization of Norbornene

A series of pentavalent tantalum and niobium complexes with aryloxy ligands was prepared, and their catalytic behavior for the ROMP of norbornene was studied in the presence of an alkylaluminum cocatalyst. Tantalum complexes 1 – 4 showed very high activity for the ROMP of NBE in combination with ⁱBu₃Al to give high‐molecular‐weight polymers. In contrast, the niobium complexes 5 and 6 , as well as NbCl₅, exhibited very high activity upon activation with Me₃Al to give high‐molecular‐weight polymers.

     

Ring‐Opening Metathesis Polymerization Based Pore‐Size‐Selective Functionalization of Glycidyl Methacrylate Based Monolithic Media: Access to Size‐Stable Nanoparticles for Ligand‐Free Metal Catalysis

Monolithic polymeric supports have been prepared by electron‐beam‐triggered free‐radical polymerization using a mixture of glycidyl methacrylate and trimethylolpropane triacrylate in 2‐propanol, 1‐dodecanol, and toluene. Under appropriate conditions, phase separation occurred, which resulted in the formation of a porous monolithic matrix that was characterized by large (convective) pores in the 30 μm range as well as pores of <600 nm. The epoxy groups in pores of >7 nm were hydrolyzed by using poly(styrenesulfonic acid) (M_w=69 400 g mol^?1, PDI=2.4). The remaining epoxy groups inside pores of <7 nm were subjected to aminolysis with norborn‐5‐en‐2‐ylmethylamine ( 2 ) and provided covalently bound norborn‐2‐ene (NBE) groups inside these pores. These NBE groups were then treated with the first‐generation Grubbs initiator [RuCl₂(PCy₃)₂(CHPh)]. These immobilized Ru–alkylidenes were further used for the surface modification of the small pores by a grafting approach. A series of monomers, that is, 7‐oxanorborn‐5‐ene‐2,3‐dicarboxylic anhydride ( 3 ), norborn‐5‐ene‐2,3‐dicarboxylic anhydride ( 4 ), N,N‐di‐2‐pyridyl‐7‐oxanorborn‐5‐ene‐2‐carboxylic amide ( 5 ), N,N‐di‐2‐pyridylnorborn‐5‐ene‐2‐carboxamide ( 6 ), N‐[2‐(dimethylamino)ethyl]bicyclo[2.2.1]hept‐5‐ene‐2‐carboxamide ( 7 ), and dimethyl bicyclo[2.2.1]hept‐5‐en‐2‐ylphosphonate ( 8 ), were used for this purpose. Finally, monoliths functionalized with poly‐ 5 graft polymers were used to permanently immobilize Pd²⁺ and Pt⁴⁺, respectively, inside the pores. After reduction, metal nanoparticles 2 nm in diameter were formed. The palladium‐nanoparticle‐loaded monoliths were used in both Heck‐ and Suzuki‐type coupling reactions achieving turnover numbers of up to 167 000 and 63 000, respectively.     

Tandem Ring‐Opening Metathesis – Vinyl Insertion Polymerization: Fundamentals and Application to Functional Polyolefins

The accomplishments in the copolymerization of ethylene with cyclic olefins such as norborn‐2‐ene or cis‐cyclooctene via tandem ring‐opening metathesis polymerization (ROMP) – vinyl insertion polymerization (VIP) are outlined. This approach provides polyolefins with high molecular weight (600,000 < M_n < 4,500,000 g mol⁻¹) and substantial amounts of double bonds along the polymer main chain. Olefinic moieties in ROMP‐derived polymers can be converted into hydroxyl, amino, silyl, ester, or carboxylate groups by different means including controlled radical polymerization‐based grafting. The underlying concept for the switch in polymerization mechanism, the resulting pre‐catalyst requirements, limitations and challenges and the chemistry developed for functionalizing unsaturated polymers are outlined in detail.