Ring-expansion Metathesis Polymerization Research Articles

Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes.

Molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes of the type [Mo(C-p-C6H4Y)(OC(R)(CF3)2)2 (L)(NHC)][B(ArF)4] (Y = OMe, NO2; R = CH3, CF3; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH2), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl2), 1,3-diisopropylimidazol-2-ylidene (IiPr); B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With cis-cyclooctene (cCOE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(cCOE) became available at high monomer concentrations. Also, post-REMP allowed for converting low-molecular-weight cyclic poly(cCOE) into high-molecular-weight cyclic poly(cCOE). Tailored catalysts together with suitable additives offered access to the stereoselective REMP of functional norbornenes providing functional cis-isotactic (cis-it), cis-syndiotactic (cis-st) and trans-it poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.

REMP Initiators with an Unusual Ancillary Ligand

Tethered tungsten‐alkylidenes bearing azoimido ligands (M≡Nγ‐Nβ=NαR) are synthesized, characterized, and tested as initiators for ring expansion metathesis polymerization (REMP). While these ligands are typically unstable and prone to dinitrogen loss, this work demonstrates that tethered alkylidene complexes bearing azoimido ligands are stable enough to be REMP initiators. Moreover, they are more efficient, long‐lived, and stereoselective than their corresponding imido derivatives (M≡NR). Density Functional Theory (DFT) analysis of the azoimido complexes provides insight into their unusual stability.

Ancillary Ligand Lability Improves Control in Cyclic Ruthenium Benzylidene Initiated Ring-Expansion Metathesis Polymerizations.

The synthesis of well-defined cyclic polymers is crucial to exploring applications spanning engineering, energy, and biomedicine. These materials lack chain-ends and are therefore imbued with unique bulk properties. Despite recent advancements, the general methodology for controlled cyclic polymer synthesis via ring-expansion metathesis polymerization (REMP) remains challenging. Low initiator activity leads to high molar mass polymers at short reaction times that subsequently "evolve" to smaller polymeric products. In this work, we demonstrate that in situ addition of pyridine to the tethered ruthenium-benzylidene REMP initiator CB6 increases ancillary ligand lability to synthesize controlled and low dispersity cyclic poly(norbornene) on a short time scale without relying on molar mass evolution events.

Scalable and continuous access to pure cyclic polymers enabled by 'quarantined' heterogeneous catalysts.

Cyclic polymers are topologically interesting and envisioned as a lubricant material. However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recover a used catalyst after the synthesis of cyclic polymers and reuse it. Unfortunately, this is demanding because of the catalyst's vulnerability and inseparability from polymers, which reduce the practicality of the process. Here we develop a continuous circular process, where polymerization, polymer separation and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. It is enabled by introducing silica-supported ruthenium catalysts and newly designed glassware. Different depolymerization kinetics of the cyclic polymer from its linear analogue are also discussed. This process minimizes manual labour, maximizes the security of vulnerable catalysts and guarantees the purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased turnovers (≥415,000) of precious catalysts.

Ring-Expansion Metathesis Polymerization Initiator Design for the Synthesis of Cyclic Polymers

AbstractCyclic polymers are of increasing interest to the synthetic and physical polymer communities due to their unique structures that lack chain ends. This topological distinction results in decreased chain entanglement, lower intrinsic viscosity, and smaller hydrodynamic radii. Many methods for the production of cyclic polymers exist, however, large-scale production of architecturally pure cyclic polymers is challenging. Ring-expansion metathesis polymerization (REMP) is an increasingly promising method to produce cyclic polymers because of the mild and scalable reaction conditions. Herein, a brief history of REMP for the synthesis of cyclic polymers with both ruthenium and non-ruthenium initiators is discussed. Even though REMP is a promising method for synthesizing cyclic polymers, state-of-the-art methods still struggle with poor molar mass control, slow polymerization rates, low conversion, and poor initiator stability. To combat these challenges, our group has developed a tethered ruthenium-benzylidene initiator, CB6, which utilizes design features from ubiquitous Grubbs-type initiators used in linear polymerizations. These structural modifications are shown to improve initiator kinetics, enhance initiator stability, and increase control over the molar mass of the resulting cyclic polymers.1 Introduction2 Ring-Expansion Metathesis Polymerization (REMP) with Ruthenium Initiators3 New Developments in Ruthenium Ring-Expansion Metathesis (REMP) Initiator Design4 Ring-Expansion Metathesis Polymerization (REMP) with Non-Ruthenium Initiators5 Conclusions

Soluble Polymer Precursors via Ring-Expansion Metathesis Polymerization for the Synthesis of Cyclic Polyacetylene

Cyclic polyacetylene (c-PA) is the cyclic derivative of the semiconducting linear polyacetylene. As with the linear derivative, cyclic polyacetylene is insoluble, making its characterization and processing challenging. Herein, we report the synthesis of c-PA via an indirect approach, employing ring-expansion metathesis polymerization of cyclic alkenes to form soluble polymer precursors. Subsequent retro-Diels–Alder elimination through heating provides c-PA. Dilute solution characterizations of the polymer precursors including 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography, and infrared and Raman spectroscopy confirm their cyclic structure and, by inference, the cyclic topology of the resulting c-PA. Solid-state thermal analyses via thermogravimetric analysis and differential scanning calorimetry reveal the chemical and physical transformations occurring during the retro-Diels–Alder elimination step and concurrent isomerization. Freestanding films are attainable via the soluble precursors, and when doped with I2, the films are semiconducting.

A Cyclic Ruthenium Benzylidene Initiator Platform Enhances Reactivity for Ring-Expansion Metathesis Polymerization.

Ring-expansion metathesis polymerization (REMP) has shown potential as an efficient strategy to access cyclic macromolecules. Current approaches that utilize cyclic olefin feedstocks suffer from poor functional group tolerance, low initiator stability, and slow reaction kinetics. Improvements to current initiators will address these issues in order to develop more versatile and user-friendly technologies. Herein, we report a reinvigorated tethered ruthenium-benzylidene initiator, CB6, that utilizes design features from ubiquitous Grubbs-type initiators that are regularly applied in linear polymerizations. We report the controlled synthesis of functionalized cyclic poly(norbornene)s and demonstrate that judicious ligand modifications not only greatly improve kinetics but also lead to enhanced initiator stability. Overall, CB6 is an adaptable platform for the study and application of cyclic macromolecules via REMP.

Tethered Tungsten-Alkylidenes for the Synthesis of Cyclic Polynorbornene via Ring Expansion Metathesis: Unprecedented Stereoselectivity and Trapping of Key Catalytic Intermediates.

This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand. Treating the tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 (1) with isocyanates (RNCO; R = tBu, Cy, and Ph) leads to cycloaddition occurring exclusively at the C═N bond to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery of [(O2CtBuC═)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(tBuOCO)W(≡NR)(tBuCCO)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = tBu (11-tBu) and is isolated and characterized by single-crystal X-ray diffraction. Finally, adding to the short list of catalysts capable of ring expansion metathesis polymerization (REMP), complexes 6-NR and 11-tBu are active for the stereoselective synthesis of cyclic polynorbornene.

Dynamic Memory Effects in the Mechanochemistry of Cyclic Polymers.

Cyclic polymers containing multiple gem-dichlorocyclopropane (gDCC) mechanophores along their backbone were prepared using ring expansion metathesis polymerization. The mechanochemistry of the cyclic polymers was investigated using pulsed ultrasonication. The fraction of gDCC mechanophores that are activated per chain halving event (Φ) was compared to that of linear analogs. For 167 kDa cyclic polymer, Φ = 0.38, vs Φ = 0.62 for 158 kDa linear polymers analogs, even though cyclic chain fragmentation necessarily proceeds through a linear intermediate of comparable composition to the initially linear systems. Ozonolysis of the mechanochemical products further shows that the mechanochemical "activation zone" in the cyclic polymer is less continuous than in the linear polymer. These results suggest that the linear intermediate in cyclic polymer fragmentation undergoes subsequent scission during the same high strain rate extensional event in which it is formed and furthermore retains at least a partial memory of its original cyclic conformation at the time of fragmentation.

Donut-Shaped Nanoparticles Templated by Cyclic Bottlebrush Polymers

Cyclic bottlebrush polymers were synthesized and used as single molecular templates to prepare donut-shaped hybrid nanoparticles with organo-silica cross-linked and gold nanocluster coordinated internal structures. “Grafting-onto” strategy was adopted to prepare cyclic bottlebrush polymers by combining ring-expansion metathesis polymerization (REMP), reversible addition–fragmentation chain transfer (RAFT) polymerization, and triazolinedione (TAD)-diene click reaction. In this approach, cyclic poly(norbornene imide) backbones with diene side groups (C-PNB-diene) were synthesized based on REMP technique. TAD-terminated diblock copolymer side chains were produced from RAFT polymerization including TAD-terminated poly(3-(triethoxysilyl)propyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PTEPM-b-POEGMA) and TAD-terminated poly(glycidyl methacrylate)-block-poly(oligo(ethylene glycol) methacrylate) (TAD-PGMA-b-POEGMA). The cyclic bottlebrush polymers 1 and 2 were then prepared by virtue of ...

Introducing "Ynene" Metathesis: Ring-Expansion Metathesis Polymerization Leads to Highly Cis and Syndiotactic Cyclic Polymers of Norbornene.

Tungsten alkylidynes [CF3-ONO]W≡CC(CH3)3(THF)2 (1) and [(t)BuOCO]W≡CC(CH3)3(THF)2 (3) react with ethylene. Complex 1 reacts reversibly with ethylene to give the metallacyclobutene (2). Complex 3 reacts with ethylene to form the tethered alkylidene (4) featuring a tetraanionic pincer ligand. Complexes 1 and 3 initiate the polymerization of norbornene at room temperature. The polymerization of norbornene by 1 is not stereoselective, whereas 3 generates a highly cis and syndiotactic cyclic polynorbornene. Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology.

Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst.

The tungsten alkylidyne [(t)BuOCO]W≡C((t)Bu) (THF)2 (1) reacts with CO2, leading to complete cleavage of one C═O bond, followed by migratory insertion to generate the tungsten-oxo alkylidene 2. Complex 2 is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly cis-syndiotactic cyclic polynorbornene.

Emulsion Templating Cyclic Polymers as Microscopic Particles with Tunable Porous Morphology.

Cyclic polymers are a particular class of macromolecules without terminal groups. Most studies has involved their physical properties and polymer composition, while attention has rarely been paid to their emulsification in an oil-water system. Herein we synthesized a cyclic polymer with polystyrene side chains via ring-expansion metathesis polymerization and click-chemistry. This cyclic polymer was compared with linear polystyrene in order to investigate the effect of cyclic topology on preparing porous particles by emulsion templating methods. The contribution of cyclic topology to emulsification originates from the formation of hollow microspheres with the use of cyclic polymer while linear polymer only afforded solid microspheres. With addition of hexadecane as soft template, both cyclic polymer and linear polymer emulsions were successfully converted into porous particles. Superior to linear polymer, cyclic polymer enables the stabilization of emulsion droplets and the tuning of porous morphology. It is revealed that cyclic polymer with nanoring shape tends to assemble at the interfacial area, leading to the Pickering effect that decelerates the macrophase separation. Furthermore, the unique porous feature of polymer particles affords a convenient application for the detection of trace explosive.

Synthesis of cyclopolyolefins via ruthenium catalyzed ring-expansion metathesis polymerization

Abstract Polymers exhibiting cyclic topology have attracted great interest over the past 30 years. Macrocycles or cyclopolymers with more than 20 repeating monomer units are considered exceptional candidates for thermoplastic engineering application due to unique properties in comparison to their linear analogues including large hydrodynamic radii and functional group density, heat resistance, good insulating ability and low intrinsic viscosity. Cyclic polymers are thus expected to exhibit improved physical and mechanical properties for certain applications due to the absence of end groups. Although synthetic challenges have historically limited research on cyclic polymers, recent developments in ruthenium catalyzed ring-expansion metathesis polymerization (REMP) have enabled the synthesis and the preliminary investigation of structure-property relationships of high molecular weight macrocycles. In REMP reactions the polymer formation is proposed to proceed through a transient macrocyclic complex in which both ends of the growing polymer chain remain attached to the Ru center. This article summarizes the recent discoveries on the field of REMP assisted cyclopolymer based material development.

Cyclic polymers as a building block for cyclic brush polymers and gels

Cyclic polymers, as one of the oldest topological polymers, are undergoing resurgence. This is largely ascribed to the significant achievements in modern polymer chemistry. The novel ring-expansion techniques have conveniently produced varied cyclic polymers with highly topological purity and on large scales, which should facilitate their use in the near future. Beyond the monocyclic molecular conformations, the combination of controlled polymerization techniques and click chemistry have established a robust strategy for preparing cyclic polymers with more complex architectures, such as theta, eight, and tadpole shapes. This diversification in cyclic polymer composition and conformation significantly broadens interest in the cyclic polymers. However, compared to the synthesis achievements, the exploration of cyclic polymer property and application are lagging behind. Recently, we explored the ring-expansion metathesis polymerization on various functional ring-strained olefin monomers to produce cyclic functional polymers, which were then used as the building blocks to fabricate cyclic brush polymers and cyclic gel materials and will be discussed here.

Metallo-Supramolecular Cyclic Polymers

Cyclic brush polymers represent an exciting new macromolecular topology. For the first time, this new topology has been combined with metallo-supramolecular interactions to construct novel cyclic brush polymers. Here, ring-expansion metathesis polymerization was used to synthesize a universal cyclic template with a polynorbornene backbone, which was further modified with the metal-chelating synthon terpyridine. The terpyridine side chains served as the key supramolecular unit for the creation of cyclic polymer brushes and gels. This metallo-supramolecular functionality allowed direct visualization of the cyclic brush polymers by transmission electron microscopy for the first time. This demonstration should open a new area in which supramolecular interactions are used to build an array of novel cyclic brush copolymers as well as other cyclic-polymer-based architectures generating new materials.

Cyclic Brush Polymers by Combining Ring-Expansion Metathesis Polymerization and the "Grafting from" Technique.

A novel synthetic route to cyclic brush polymers was developed by combining ring-expansion metathesis polymerization (REMP) and the "grafting from" technique. In this approach, ultrahigh molecular weight cyclic polymers with hydroxyl side groups were synthesized first by REMP to form the cyclic macroinitiators. Polyester side chains were then grown from these cyclic macroinitiators by virtue of a triazabicyclodecene-catalyzed cyclic ester ring-opening polymerization to produce the cyclic brush polymers.

Templates for Cyclic Polymers

Ring-expansion metathesis polymerization (REMP) can be used to prepare perfect cyclic polymer chains. Activated ester template A was prepared from 1 using Grubbs cyclic ruthenium-alkylidene catalyst. This template was transformed into a copper(I) click-ready template B. Both templates were post-functionalized to create cyclic gels (e.g., cyclic gel A and B) and cyclic polymer brushes (A, B, B′).

Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers

Convenient route to cyclic polymers: Ultrahigh molecular weight cyclic brush polymers were synthesized through ring-expansion metathesis polymerization of various macromonomers. Atomic force microscopy was used to visualize toroidal shapes and large opening pores, along with linear chains, which may result from high sensitivity of brush polymers to mechanical degradation.

Universal Cyclic Polymer Templates

Two unique molecular templates for generating polymeric materials with a cyclic molecular architecture were developed by combining ring-expansion metathesis polymerization and click chemistry. These two universal cyclic polymers were used in three examples to demonstrate the wide range of potential materials enabled. They include functional cyclic polymers, cyclic polymer brushes, and cyclic gels.