Z-Selective and Syndioselective Ring-Opening Metathesis Polymerization (ROMP) Initiated by MonoAryloxidePyrrolide (MAP) Catalysts

Influence of solvent on cyclic polynorbornene tacticity

Tacticity is critical to polymer properties. The influence of solvent on tacticity in the catalytic synthesis of cyclic polynorbornene (c-PNB) is reported. In toluene cis,syndiotactic c-PNB forms; in THF, cis,syn/iso c-PNB forms.

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.

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

A Fischer-Type Ruthenium Carbene Complex as a Metathesis Catalyst for the Synthesis of Enol Ethers

The Grubbs G-I or G-II catalyst gives the ruthenium ethoxy carbene complex, which catalyzes ring-opening cross metathesis (ROCM) of a strained cyclic alkene to give a diene where one of the two alkene moieties in the product contains an ethoxy substituent. No polymeric products are detected. Hydrocarbons such as parent norbornene or substituted cyclopropenes can proceed with the reaction smoothly. Tertiary amines, N-alkylimides, esters, and aryl or alkyl bromides remain intact under the reaction conditions. In addition to vinyl ethers, vinylic esters can also be used. The time required to reach a 50% yield of the ROCM product t₅₀ varies from 0.01 to 140 h depending on the strain and nucleophilicity of the double bond. Anchimeric participation of an electron-rich group would result in significant enhancement of the reactivity, and the t₅₀ could be as short as several minutes. A similar substrate without such a neighboring group shows a much slower rate. An exo-norborne derivative reacts much faster than the corresponding endo-isomer. Alkenes with poor nucleophilicity are less favored for the ROCM process, so is less strained cyclooctene.

Polymerization of Cyclopropenes: Taming the Strain for the Synthesis of Controlled and Sequence-Regulated Polymers

Cyclopropenes (CPEs) are highly strained cyclic olefins, yet there are surprisingly limited examples leveraging their high strain energy for polymerization. In the past, attempts had been made to polymerize CPEs via cationic and insertion polymerization, but side reactions often gave uncontrolled polymers with mixed backbone structures. Ring-opening metathesis polymerization (ROMP) represents an ideal strategy for polymerizing CPEs to access new types of polymers. The proximity of substituents to the olefin in the small framework of CPEs offers a modular handle to tune the kinetic barrier to propagation by the modulation of the substituents. While the first few studies focused on the homopolymerization of simple alkyl or phenyl disubstituted CPEs, we recently explored the metathesis of a wide range of CPEs with different substituents using Grubbs catalysts and discovered surprising and diverse reactivities that are contingent on the positions, sterics, and electronics of substituents. The observed reactivities ranged from living homopolymerization to catalyst deactivation to single addition to the catalyst without homopropagation. In particular, the exclusively single addition reactivity found in two families of CPEs, with either bis(methanol ester) or phenyl and methanol ester substituents at the allylic position, is unusual for any monomer and perhaps counterintuitive for highly strained cycles. These single-addition CPEs could, however, be copolymerized with low-strain cyclic olefins to generate perfectly alternating copolymers with controlled molecular weights and low dispersity and to introduce degradable backbone linkages. A single equivalent (relative to the active chain end) of such CPEs could also be added to the active chain end of living ROMP polymers to install functional terminal groups or during living ROMP to place single units of functional moieties or side chains at any desired chain locations in narrow-disperse homopolymers and block copolymers. This account summarizes the polymerization of CPEs with a focus on our journey to uncover the rich and unique metathesis reactivities of CPEs and their utility in synthesizing well-controlled and sequence-regulated polymers. It provides the first collective structure-metathesis reactivity relationships for CPEs in the context of polymer chemistry and an understanding of the interactions between the catalyst and the substituents of appended ring-opened CPEs. It may become clear from this Account that the exploration of strained cycles in polymer chemistry can be quite fruitful in discovering new chemistry and accessing new types of polymer materials.

Cationic tungsten imido alkylidene N-heterocyclic carbene complexes for stereospecific ring-opening metathesis polymerization of norbornene derivatives

Cationic W alkylidene N-heterocyclic carbene complexes allow for the stereospecific ring-opening metathesis polymerization of norbornenes with high trans-iso- and high cis-syndiospecificity, respectively.

Determination of the Orientation of Pendants on Rigid-Rod Polymers

Bis-norbornene and bis-cyclobutene with different kinds of linkers have been extensively used for the synthesis of double stranded ladderphanes under ruthenium- or molybdenum-catalyzed ring opening metathesis polymerization (ROMP) conditions. The key to the success relies on the selective formation of comb-like polynorbornenes or polycycloubtenes, where pendants are all aligned towards similar direction. This minireview summarizes various methods (chemical methods, spectroscopic means, and nonlinear optical measurements) for determining the comb-like conformations of pendants on these rigid-rod polymers. The approach is based on the proximal relationship between adjacent pendants. Interactions between these adjacent pendants would enable a change in chemical reactivity.

Molecular-level insight in supported olefin metathesis catalysts by combining surface organometallic chemistry, high throughput experimentation, and data analysis

Combination of high-throughput experimentation, surface organometallic chemistry and data analysis helps to extract key structural parameters that facilitate olefin metathesis.

A combination of high-throughput experimentation (HTE), surface organometallic chemistry (SOMC) and statistical data analysis provided the platform to analyze in situ silica-grafted Mo imido alkylidene catalysts based on a library of 35 phenols. Overall, these tools allowed for the identification of σ-donor electronic effects and dispersive interactions and as key drivers in a prototypical metathesis reaction, homodimerization of 1-nonene. Univariate and multivariate correlation analysis confirmed the categorization of the catalytic data into two groups, depending on the presence of aryl groups in ortho position of the phenol ligand. The initial activity (TOF_in) was predominantly correlated to the σ-donor ability of the aryloxy ligands, while the overall catalytic performance (TON_{1 h}) was mainly dependent on attractive dispersive interactions with the used phenol ligands featuring aryl ortho substituents and, in sharp contrast, repulsive dispersive interactions with phenol free of aryl ortho substituents. This work outlines a fast and efficient workflow of gaining molecular-level insight into supported metathesis catalysts and highlights σ-donor ability and noncovalent interactions as crucial properties for designing active d⁰ supported metathesis catalysts.

cis-Specific ring opening metathesis polymerisation (ROMP) of cyclic olefins using (pentafluorophenylimido)vanadium(v)-alkylidene, V(CHSiMe3)(NC6F5)[OC(CF3)3](PMe3)2

Highly cis-specific (Z selective) ring opening metathesis polymerisation of cycloheptene has been demonstrated using V(CHSiMe₃)(NC₆F₅)[OC(CF₃)₃](PMe₃)₂.

Diblock brush-arm star copolymers via a core-first/graft-from approach using γ-cyclodextrin and ROMP: a modular platform for drug delivery

Water-soluble diblock brush-arm star copolymers using γ-CD-based core-first ring-opening metathesis polymerization, allowing for anticancer drug delivery via host–guest interaction.

Tuning the Reactivity of Cyclopropenes from Living Ring-Opening Metathesis Polymerization (ROMP) to Single-Addition and Alternating ROMP

Ring-opening metathesis polymerization (ROMP) has become one of the most important living polymerizations. Cyclopropenes (CPEs) remain underexplored for ROMP. Described here is that the simple swap of 1-methyl to 1-phenyl on 1-(benzoyloxymethyl)CPEs elicited strikingly different modes of reactivity, switching from living polymerization to either selective single-addition or living alternating ROMP. The distinct reactivity stems from differences in steric repulsions at the Ru alkylidene after CPE ring opening. Possible olefin or oxygen chelation from ring-opened CPE substituents was also observed to significantly affect the rate of propagation. These results demonstrate the versatility of CPEs as a new class of monomers for ROMP, provide mechanistic insights for designing new monomers with rare single-addition reactivity, and generate a new functionalizable alternating copolymer scaffold with controlled molecular weight and low dispersity.

Polypentenamer Renaissance: Challenges and Opportunities

This Viewpoint highlights the viability and increasing variety of functionalized polypentenamers as unique and valuable materials created through enthalpy-driven ring-opening metathesis polymerization (ROMP) of low ring strain cyclopentene monomers. The terms "low ring strain" and "enthalpy-driven" are typically conflicting ideologies for successful ROMP; however, these monomers possess a heightened sensitivity to reaction conditions, which may be leveraged in a number of ways to provide performance elastomers with good yield and precise functional topologies. Over the last several years, a rekindled interest in these systems has led to a renaissance of research aimed at improving their synthesis and exploring their potential. Their chemistry, applications, and future outlook are discussed.

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.

Synthesis of vanadium–alkylidene complexes and their use as catalysts for ring opening metathesis polymerization

Synthesis of vanadium–alkylidene complexes and some reactions have been reviewed; highly active, thermally robust, cis specific ROMP of cyclic olefins has been attained by ligand modification in (imido)vanadium(v)–alkylidene catalysts.

Mechanistic investigation of the ring opening metathesis polymerisation of alkoxy and alkyl substituted paracyclophanedienes

This paper discusses the living nature of the ring opening metathesis polymerisation (ROMP) of alkoxy and alkyl substituted [2.2] paracyclophane-1,9-dienes (M1 and M2), initiated with Grubbs’ second and third generation catalysts (G2 and G3).

Synthesis and Structural Characterization of Lithium and Titanium Complexes Bearing a Bulky Aryloxide Ligand Based on a Rigid Fused-Ring s-Hydrindacene Skeleton

The bulky aryl alcohols, (Rind)OH (1) [Rind = EMind (a) and Eind (b)], based on the rigid fused-ring 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacene skeleton were prepared by the reaction of (Rind)Li with nitrobenzene followed by protonation. The treatment of 1 with (n)BuLi affords the lithium aryloxide dimers [(Rind)OLi(THF)]2 (2) or trimers [(Rind)OLi]3 (3), depending on the employed solvents (THF = tetrahydrofuran). The salt metathesis reaction of [(EMind)OLi(THF)]2 (2a) with TiCl4(THF)2 leads to the formation of the mononuclear diamagnetic mono- and bis(aryloxide) Ti(IV) complexes, [(EMind)O]TiCl3(THF) (4a) and [(EMind)O]2TiCl2 (5a). We also isolated a trace amount of the tris(aryloxide) Ti(IV) complex, [(EMind)O]3TiCl (6a). The reaction between 2a and TiCl3(THF)3 resulted in the isolation of the mononuclear paramagnetic mono- and bis(aryloxide) Ti(III) complexes, [(EMind)O]TiCl2(THF)2 (7a) and [(EMind)O]2TiCl(THF)2 (8a). The discrete monomeric structures of the titanium complexes 4a, 5a, 6a, 7a, and 8a were determined by X-ray crystallography.

Small molecule activation by mixed methyl/methylidene rare earth metal complexes

Diverse reactivity patterns of mixed tetramethyl/methylidene rare-earth complexes bearing bulky benzamidinate coligands with PhCN, alkynes, and CS₂ have been established and fully characterized.

Ring-opening metathesis polymerization of 1,2-disubstituted cyclopropenes

We report the first study of ROMP of 1,2-CPs, which yielded narrowly dispersed polymers but exhibited a surprising slow termination reaction.

Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf)2] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)2] (4), [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf){OCH(CF3)2}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH2)(CHCMe2Ph)(OTf)](+)BArF(-) (6), [Mo(N-2,6-Cl2C6H3)(IMesH2)(CHCMe3)(OTf)2] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)2] (8) are reported (IMesH2=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF(-)=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3(-)). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω-diynes. In the RCM of α,ω-dienes, in the homometathesis of 1-alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100,000, 210,000 and 30,000, respectively, were achieved. With I1 and I2, virtually Mo-free products were obtained (<3 ppm Mo). With 1,6-hepta- and 1,7-octadiynes, catalysts 3, 4, and 5 allowed for the regioselective cyclopolymerization of 4,4-bis(ethoxycarbonyl)-1,6-heptadiyne, 4,4-bis(hydroxymethyl)-1,6-heptadiyne, 4,4-bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne, 4,4,5,5-tetrakis(ethoxycarbonyl)-1,7-octadiyne, and 1,6-heptadiyne-4-carboxylic acid, underlining the high functional-group tolerance of these novel Group 6 metal alkylidenes.

Living Alternating Ring-Opening Metathesis Polymerization Based on Single Monomer Additions

By judiciously modulating the ring strain and sterics, we developed a class of disubstituted cyclopropenes that selectively underwent single monomer addition in ring-opening metathesis but readily underwent alternating ring-opening metathesis polymerization with low-strain cyclic olefins in a living fashion. The substituents on cyclopropenes effectively inhibited homoaddition and prevented secondary metathesis on the polymer backbone. The resulting polymers had controllable molecular weights and end groups, very low dispersities, and high regularity in microstructure under optimized conditions. (1)H and (13)C NMR spectroscopy and MALDI-TOF MS showed a rigorously alternating sequence. Interestingly, disubstituted cyclopropenes were found to present zero-order kinetics, indicating their rapid single addition and the rate-determining ring opening of the low-strain olefin.