Acyclic Diene Metathesis Research Articles

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalysts.

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.

Olefin Metathesis in Confined Geometries: A Biomimetic Approach toward Selective Macrocyclization.

The synthesis of macrocycles is severely impeded by concomitant oligomer formation. Here, we present a biomimetic approach that utilizes spatial confinement to increase macrocyclization selectivity in the ring-closing metathesis of various dienes at elevated substrate concentration up to 25 mM using an olefin metathesis catalyst selectively immobilized inside ordered mesoporous silicas with defined pore diameters. By this approach, the ratio between macro(mono)cyclization (MMC) product and all undesired oligomerization products (O) resulting from acyclic diene metathesis polymerization was increased from 0.55, corresponding to 35% MMC product obtained with the homogeneous catalyst, up to 1.49, corresponding to 60% MMC product. A correlation between the MMC/O ratio and the substrate-to-pore-size ratio was successfully established. Modification of the inner pore surface with dimethoxydimethylsilane allowed fine-tuning the effective pore size and reversing surface polarity, which resulted in a further increase of the MMC/O ratio up to 2.2, corresponding to >68% MMC product. Molecular-level simulations in model pore geometries help to rationalize the complex interplay between spatial confinement, specific (substrate and product) interaction with the pore surface, and diffusive transport. These effects can be synergistically adjusted for optimum selectivity by suitable surface modification.

Ferrocene-Containing Conjugated Oligomers Synthesized by Acyclic Diene Metathesis Polymerization.

A series of conjugated, symmetrical, and ferrocene-containing main-chain monomers was prepared following a gentle coupling reaction. Ferrocene-containing oligomers with all-trans-configured vinylene bonds could be synthesized via acyclic diene metathesis (ADMET) polymerization. These oligomers had a larger Stokes shift (2400 to 2600 cm−1) and both exhibited stable and reversible electrochemistry. Meanwhile, the copolymerization of 1,1’-bis[1-methyl-2-(4-vinylphenyl)ethenyl]ferrocene with 2,7-divinyl-9,9-dioctylfluorene was achieved. The structurally regular copolymers proved their optical and electrochemical properties. The fluorescence intensity of the copolymer gradually enhanced with the increasing number of fluorene units. At the same time, it was also found that the color of the copolymers had a significant change from yellow-green to red.

Bulk Acyclic Diene Metathesis Polycondensation

AbstractBulk acyclic diene metathesis (ADMET) polymerization performed in the melt at temperatures up to 175 °C is completed using a cyclic (alkyl)(amino) carbene (CAAC) ruthenium‐based Grubbs catalyst. High temperature stability is investigated at temperatures well above conventional ADMET chemistry, leading to the highest weight‐average molecular weights ever achieved for any ADMET polymer. A study of isomerization tendency (double bond migration) is done via self‐metathesis of a small molecule model compound using CAAC catalysts. Minimal olefin isomerization is observed using appropriate catalyst ratios and temperatures.

Synthesis of Precision Poly(1,3-adamantylene alkylene)s via Acyclic Diene Metathesis Polycondensation.

Fully saturated, aliphatic polymers containing adamantane moieties evenly distributed along the polymer backbone are of great interest due to their exceptional thermal stability, yet more synthetic strategies toward these polymers would be desirable. Herein, we report for the first time the synthesis of poly(1,3-adamantylene alkylene)s based on α,ω-dienes containing bulky 1,3-adamantylene defects precisely located on every 11th, 17th, 19th, and 21st chain carbon via acyclic diene metathesis polycondensation. All saturated polymers revealed excellent thermal stabilities (452–456 °C) that were significantly higher compared to those of structurally similar polyolefins with aliphatic or aromatic ring systems in the backbone of polyethylene (PE). Their crystallinity increases successively from shorter to longer CH2 chains between the adamantane defects. The adamantanes were located in the PE crystals distorting the PE unit cell by the incorporation of the adamantane defect at the kinks of a terrace arrangement. Precise positioning of structural defects within the polymeric backbone provides various opportunities to customize material properties by “defect engineering” in soft polymeric materials.

Noncovalent Hydrogen Bonds Tune the Mechanical Properties of Phosphoester Polyethylene Mimics.

Polyethylene mimics of semicrystalline polyphosphoesters (PPEs) with an adjustable amount of noncovalent cross-links were synthesized. Acyclic diene metathesis copolymerization of a phosphoric acid triester (M1) with a novel phosphoric acid diester monomer (M2) was achieved. PPEs with different co-monomer ratios and 0, 20, 40, and 100% of phosphodiester content were synthesized. The phosphodiester groups result in supramolecular interactions between the polymer chains, with the P–OH functionality as an H-bond donor and the P=O group as an H-bond acceptor. A library of unsaturated and saturated PPEs was prepared and analyzed in detail by NMR spectroscopy, size exclusion chromatography, differential scanning calorimetry, thermogravimetry, rheology, and stress–strain measurements. The introduction of the supramolecular cross-links into the aliphatic and hydrophobic PPEs showed a significant impact on the material properties: increased glass-transition and melting temperatures were observed and an increase in the storage modulus of the polymers was achieved. This specific combination of a flexible aliphatic backbone and a supramolecular H-bonding interaction between the chains was maximized in the homopolymer of the phosphodiester monomer, which featured additional properties, such as shape-memory properties, and polymer samples could be healed after cutting. The P–OH groups also showed a strong adhesion toward metal surfaces, which was used together with the shape-memory function in a model device that responds to a temperature stimulus with shape change. This systematic variation of phosphodiesters/phosphotriesters in polyethylene mimics further underlines the versatility of the phosphorus chemistry to build up complex macromolecular architectures.

Synthesis and self-assembly of PMMA-b-(u)PE-b-PMMA copolymers: study the aggregate morphology in toluene vapor

Well-defined poly(methyl methacrylate)-b-polyethylene-b-poly(methyl methacrylate) triblock copolymers with saturated and unsaturated PE as the central block (PMMA-b-(u)PE-b-PMMA) were synthesized by combining acyclic diene metathesis (ADMET) with thiol-ene click chemistry and atom transfer radical polymerization (ATRP). The mass ratios of (u)PE/PMMA in the copolymers are 1:1 and 1:2 respectively, while the molecular weight of the (u)PE block remained constant (22 k). The characterization, thermal properties and crystallization of copolymers were determined by 1H/13C NMR, IR, TGA, DSC and XRD. As a result, the PMMA blocks can form stereocomplex crystal, which was affected by copolymer composition as well as the crystallization of PE middle block. Under solvent vapor annealing (SVA) conditions, amorphous or weak crystallization block copolymers assembled into worm-like network aggregates, while the aggregates evolution to fibrous or banded and fabric-like network covering particles morphology driving by PE nuclear crystallization and PMMA shell crystallization respectively.

Olefin metathesis of fatty acids and vegetable oils

The article reviews various olefin metathesis reactions namely self-metathesis (SM), cross-metathesis (CM), acyclic diene metathesis (ADMET) polymerization, ring closing metathesis (RCM) and ring opening metathesis (ROM) reactions for the synthesis of a variety of platform chemicals with potential industrial applications from vegetable oils and fatty acids. Different fatty acids employed for various metathesis reactions were oleic, linoleic, linolenic, eicosenoic, erucic, petroselinic, sterculic, undecenoic and ricinoleic acids, while vegetable oils used were Helianthus (sunflower), Brassica napus (rapeseed), Glycine max (soybean), Hevea brasiliensis (rubber), Butea monosperma (palash), Nicotiana tabacum (tobacco) and Sterculia foetida (jangli badam). Even though Grubbs’ catalysts were employed for most of the reactions, other catalysts such as rhenium, molybdenum and tungsten based have also been used for selective reactions of vegetable oils and fatty acids. The article reviews some of the mechanistic pathways involved in the generation of unusual intermediates from fatty acids and triglycerides. Various olefin metathesis reactions namely self-metathesis (SM), crossmetathesis (CM), acyclic diene metathesis (ADMET) polymerization, ring closing metathesis (RCM) and ring opening metathesis (ROM) reactions for the synthesis of a variety of platform chemicals with potential industrial applications from vegetable oils and fatty acids are discussed in this article.

Synthesis of precisely diphenyl ether-functionalized polyethylene via acyclic diene metathesis polymerization

We demonstrated the facile synthesis of a new class of functionalized polyethylene containing precisely sequenced diphenyl ether moieties via acyclic diene metathesis (ADMET) polymerization of a series of α, ω-diene monomers (M1-M3) with central diphenyl ether segments and subsequent exhaustive hydrogenation. 1H and 13C NMR spectroscopy have provided unambiguous evidence for precise primary architectures of the synthesized monomers and the resulting polymers. The thermal behaviors of all polymers were systematically investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results indicate that the introduction of diphenyl ether segments significantly enhanced the thermal stability of the polyethylenes. Ring-opening insertion metathesis polymerization (ROIMP) of M1-M3 with cis-cyclooctene was also performed, and after exhaustive hydrogenation, a series of irregularly sequenced copolymers with less regular chain structure and lower diphenyl ether content were yielded. Those copolymers possess a broad and ill-defined melting transition compared with the precisely-defined saturated homopolymers.

Synthesis of Ether‐Diols with Low Polarity from Long‐Chained Fatty Alcohols for Use in Block Copolymers

AbstractDiol‐terminated polyethers are important intermediates for the manufacturing of block copolymers, but only a few polyethers other than polyethylene glycols are available on a technical scale. Most of them are highly polar. Natural fatty alcohols—converted to similar polyethers—should have a grossly reduced polarity because of a large separation of the oxygen atoms in the C18 chain. A synthesis sequence of polyethers with a longer carbon chain compared to the ethylene glycol‐derived ethers based on commercially available fatty alcohols was designed for a future examination of the hydrophobic properties. Palladium‐catalyzed cleavage of olefinic dialkyl carbonates results in carbon dioxide elimination and subsequent formation of ethers from an allyl‐palladium cation. It could be shown that this process with fatty alcohols like undec‐10‐en‐1‐ol or oleyl alcohol can be run with appreciable yield. Although carbonates were obtained here using expensive chloroformates as starting materials, transesterification of dimethylcarbonates can be used similarly. Ruthenium‐catalyzed acyclic diene metathesis (ADMET) at both chain terminations then was applied to polymerize the ethers. Depending on the alkenyl chain, short oligomers with a degree of polymerization (DP) of about 5–8 seem to be formed according to gel permeation chromatography (GPC).

Application of olefin metathesis in the synthesis of functionalized polyhedral oligomeric silsesquioxanes (POSS) and POSS-containing polymeric materials.

This mini-review summarizes the applications of olefin metathesis in synthesis and functionalization of polyhedral oligomeric silsesquioxanes (POSS) and POSS-containing polymeric materials. Three types of processes, i.e., cross metathesis (CM) of vinyl-substituted POSS with terminal olefins, acyclic diene metathesis (ADMET) copolymerization of divinyl-substituted POSS with α,ω-dienes and ring-opening metathesis polymerization (ROMP) of POSS-substituted norbornene (or other ROMP susceptible cycloolefins) are discussed. Emphasis was put on the synthetic and catalytic aspects rather than on the properties and applications of synthesized materials.

Aliphatic Long-Chain Polypyrophosphates as Biodegradable Polyethylene Mimics.

Biodegradable polyethylene mimics have been synthesized by the introduction of pyrophosphate groups into the polymer backbone, allowing not only hydrolysis of the backbone but also further degradation by microorganisms. Because of cost, low weight, and good mechanical properties, the use of polyolefins has increased significantly in the past decades and has created many challenges in terms of disposal and their environmental impact. The durability and resistance to degradation make polyethylene difficult or impossible for nature to assimilate, thus making the degradability of polyolefins an essential topic of research. The biodegradable polypyrophosphate was prepared via acyclic diene metathesis polymerization of a diene monomer. The monomer is accessible via a three-step synthesis, in which the pyrophosphate was formed in the last step by DCC coupling of two phosphoric acid derivatives. This is the first report of a pyrophosphate group localized in an organic polymer backbone. The polypyrophosphate was characterized in detail by NMR spectroscopy, size exclusion chromatography, FTIR spectroscopy, differential scanning calorimetry, and thermogravimetry. X-ray diffraction was used to compare the crystallization structure in comparison to analogous polyphosphates showing poly(ethylene)-like structures. In spite of their hydrophobicity and water insolubility, the pyrophosphate groups exhibited fast hydrolysis, resulting in polymer degradation when films were immersed in water. Additionally, the hydrolyzed fragments were further biodegraded by microorganisms, rendering these PE mimics potential candidates for fast release of hydrophobic cargo, for example, in drug delivery applications.

Impact of side-chain extension on physical and electronic properties of cross-conjugated Poly(thienylene vinylene)s (PTVs)

Poly(thienylene vinylene)s (PTVs) are close analogs to the extensively studied polythiophenes and the insertion of one double bond between every two thiophene units along the main-chain of PTV backbone eliminates steric repulsion and potentially allows installment of conjugated side-chains without significantly distorting the main-chain planarity, offering a facile way of fine-tuning the polymer electronic properties through main-chain/side-chain cross-conjugation. We describe here a new methodology that combines acyclic diene metathesis and post-polymerization modification methods for the access of a series of cross-conjugated PTVs bearing aromatic side-chains connected to the main-chains through alkyne spacers. Compared with unfunctionalized PTVs, these cross-conjugated PTVs showed broader absorption ranges, reduced bandgaps, and rarely observed fluorescence. Organic solar cells employing the cross-conjugated PTVs displayed up to double the efficiency improvement over those employing pristine PTVs. For the first time, these PTVs were applied as sensitizers in composites with g-C3N4 for photocatalytic hydrogen production. It was found that the relative energy level positions between the polymers and g-C3N4 and polymer crystallinity likely played decisive roles in determining the hydrogen production efficiencies, which gave us useful guidelines on future optimization of polymer structures and selection of photocatalytic systems.

Synthesis of new polyesters by acyclic diene metathesis polymerization of bio-based α,ω-dienes prepared from eugenol and castor oil (undecenoate).

Synthesis of new polyesters by acyclic diene metathesis (ADMET) polymerization of α,ω-diene, 4-allyl-2-methoxyphenyl 10-undecenoate (M1), prepared from bio-renewable eugenol and castor oil (undecenoate), have been demonstrated. Ruthenium-carbene (called second generation Grubbs) catalyst afforded polymers with unimodal molecular weight distributions (Mn = 12 700, Mw/Mn = 1.85). The polymerization in the presence of a triarm cross-linker, 5-formylbenzene-1,2,3-triyl tris(undec-10-enoate), also afforded polymers with certain uniform network structures.

Redox-switchable olefin cross metathesis (CM) reactions and acyclic diene metathesis (ADMET) polymerizations

We show that redox-switchable catalysis may be used to control acyclic diene metathesis (ADMET) polymerizations and related cross metathesis (CM) reactions.

Oligodeoxynucleosides with Olefin Bridges

The synthesis and self-assembly studies of DNA/RNA-mimic artificial polymers have drawn extensive scientific attention since synthetic chemists can manipulate the functions of these modified nucleic acid molecules by varying their macromolecular backbone and nucleoside unit structures. In this paper, we report two types of modified nucleic acids with olefin bridges prepared by acyclic diene metathesis polymerization (ADMET). The 3′-OH and 5′-OH groups of deoxythymidine and deoxyadenosine analogue monomers are functionalized with either two terminal allyl units or allyl and acryloyl units, respectively. The consequent ADMET approaches provide either regio-uncontrolled oligodeoxynucleosides with poor E/Z selectivity or well-defined oligodeoxynucleosides built in a highly ordered head-to-tail addition manner. These two different macromolecular structures markedly influence the self-assembly morphologies of the corresponding oligodeoxynucleosides, which show spherical and filamentous helix nanostructures, respectively.

Fatty Acid–Derived Aliphatic Long Chain Polyethers by a Combination of Catalytic Ester Reduction and ADMET or Thiol‐Ene Polymerization

AbstractAliphatic long‐chain polyesters are a common class of renewable polymers and can be obtained by various synthetic routes, most notably from renewable fatty acids. In contrast, for aliphatic long chain polyethers only a few examples are known. Recently, the GaBr3‐catalyzed reduction of esters to ethers has been introduced as convenient post‐polymerization modification to obtain polyethers directly from polyesters. Here, this reduction is applied to synthesize fatty acid–based ω,ω′‐unsaturated diene diether monomers, which are polymerized afterward by thiol‐ene or acyclic diene metathesis (ADMET) polymerizations in the “green” solvents methyl‐THF (2‐methyltetrahydrofuran) and polarclean. After polymerization, the thiol‐ene or ADMET polymers are modified by either oxidation or hydrogenation, respectively.

Polymacrocycles Derived via Ugi Multi-Component Reactions.

The synthesis of macrocycles based on the Ugi-4CR has been thoroughly explored by Wessjohann and coworkers, while polymerizations utilizing the Ugi-4CR are already patented by Ugi and recently studied more in detail, developing a new trend in polymer chemistry. Here, the combination of both, that is, the synthesis of polymacrocycles, is demonstrated. As diverse functional groups can be easily introduced in a macrocycle via Ugi-4CR, a straightforward design of polymacrocycles is achieved in a two-step procedure. First, the Ugi-4CR of 10-undecenoic acid, a diamine, a diisocyanide, and an aldehyde results in diversely substituted macrocycles having two terminal double bonds. Subsequently, these macrocycles are polymerized by ADMET (acyclic diene metathesis) or thiol-ene polymerization to generate polymacrocycles with potential application in coordination chemistry as, for example, sensors, filters, or phase-transfer catalysts. Moreover, the setup of the literature-known Ugi macrocyclization is simplified by systematic reaction screening.

Synthesis of Cinchona Alkaloid Derived Chiral Squaramide Polymers by ADMET Polymerization and Their Application to Asymmetric Catalysis

The synthesis of cinchona alkaloid-derived chiral squaramide polymers using acyclic diene metathesis (ADMET) polymerization over Hoveyda-Grubbs 2nd generation catalysts (HG2) is reported herein. Th...

Functional Precision Polymers via Stereo- and Regioselective Polymerization Using Group 6 Metal Alkylidene and Group 6 and 8 Metal Alkylidene N-Heterocyclic Carbene Complexes.

The concepts of functional precision polymers and the latest accomplishments in their synthesis are summarized. Synthetic concepts based on chain growth polymerization are compared to iterative synthetic approaches. Here, the term "functional precision polymers" refers to polymers that are not solely hydrocarbon-based but contain functional groups and are characterized by a highly ordered primary structure. If insertion polymerization is used for their synthesis, olefin metathesis-based polymerization techniques, that is, ring-opening metathesis polymerization (ROMP), acyclic diene metathesis (ADMET) polymerization, and the regio- and stereoselective cyclopolymerization of α,ω-diynes are almost exclusively applied. Particularly with regio- and stereospecific ROMP and with cyclopolymerization, the synthesis of tactic polymers and copolymers with high regio-, stereo-, and sequence control can be accomplished; however, it requires carefully tailored transition metal catalysts. The fundamental synthetic concepts and strategies are outlined.