Stereospecific Synthesis of Poly(methylene-E-vinylene) by Ring Opening Metathesis Polymerization of Substituted Cyclopropene Using Grubbs Catalysts

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.

An Ion-Pairing Approach to Stereoselective Metal-Free Ring-Opening Metathesis Polymerization

Stereochemistry can have a profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP. Ion-pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98 %. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (-78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.

Ring-Opening Metathesis Polymerization of Norbornene-Based Monomers Obtained via the Passerini Three Component Reaction

Ring-opening metathesis polymerization is a robust method to synthesize a variety of polymers by using ring-strained molecules as monomers, e.g., norbornenes. However, the synthesis of monomers with multiple functional groups remains a challenge, albeit peptide functional norbornenes have previously been used. Here, the Passerini three component reaction is exploited to synthesize norbornenes with two variable functional groups varying in bulkiness and distance from the polymerizable alkene. The results indicate that the functional groups do not affect the kinetics of the polymerization, whereas the length of the linker has a minor effect. Furthermore, a diblock-type copolymer is synthesized in a one-pot fashion, also indicating good control of the polymerization process. The thermal properties of all polymers are evaluated, highlighting the effect of monomer composition. This synthetic approach can be transferred to a variety of compounds, thus promising highly diverse polymers with complex compositions and architectures.

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.

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.