Cascade Cyclopolymerization of 5-Ethynyl-1,8-Nonadiyne Derivatives to Synthesize Low Band Gap Conjugated Polyacetylenes Containing a Fused Bicyclic Structure.

Cyclopolymerization is a powerful method for synthesizing polyacetylenes containing four- to seven-membered rings. However, the structure of the repeat unit only consists of mono-cycloalkene due to the single cyclization of diyne monomers. Herein, we demonstrate a novel cascade cyclopolymerization to synthesize polyacetylenes containing fused bicyclic rings from triyne monomers containing bulky dendrons via sequential cascade ring-closing metathesis. These dendrons provided solubility and stability to the rigid bicyclic polyacetylene backbone. In addition, we controlled the regioselectivity of the catalyst approach by altering its structure and synthesized polymers containing fused bicyclo[4,3,0] or [4,4,0] rings with high molecular weights of up to 120 kg mol-1 . Interestingly, the resulting polymers showed narrower band gaps (down to 1.6 eV) than polymers with mono-cycloalkene repeat units due to the planarization of the conjugated segment resulting from the fused bicyclic structure.

Ru-Catalyzed, cis-Selective Living Ring-Opening Metathesis Polymerization of Various Monomers, Including a Dendronized Macromonomer, and Implications to Enhanced Shear Stability.

An unsaturated polymer's cis/trans-olefin content has a significant influence on its properties. For polymers obtained by ring-opening metathesis polymerization (ROMP), the cis/trans-olefin content can be tuned by using specific catalysts. However, cis-selective ROMP has suffered from narrow monomer scope and lack of control over the polymerization (giving polymers with broad molecular weight distributions and prohibiting the synthesis of block copolymers). Herein, we report the versatile cis-selective controlled living ROMP of various endo-tricyclo[4.2.2.02,5]deca-3,9-diene and various norbornene derivatives using a fast-initiating dithiolate-chelated Ru catalyst. Polymers with cis-olefin content as high as 99% could be obtained with high molecular weight (up to Mn of 105.1 kDa) and narrow dispersity (<1.4). The living nature of the polymerization was also exploited to prepare block copolymers with high cis-olefin content for the first time. Furthermore, owing to the successful control over the stereochemistry and narrow dispersity, we could compare cis- and trans-rich polynorbornene and found the former to have enhanced resistance to shear degradation.

Synthesis of Conjugated Polyenynes with Alternating Six- and Five-Membered Rings via ß-Selective Cascade Metathesis and Metallotropy Polymerization.

Cascade metathesis and metallotropy (M&M) polymerization, which involves sequential olefin metathesis and metallotropic 1,3-shift reactions specifically from multiyne monomers, is the only method reported so far to prepare conjugated polyenynes via the chain-growth mechanism. Using this method, various conjugated polyenynes containing cyclopentene units in the backbone could be synthesized via exclusive α-addition by using the third-generation Grubbs catalyst. Herein, we demonstrate the complete switch of regioselectivity toward ß-addition using a Ru carbene containing a dithiolate ligand, and thus, synthesized unique conjugated polyenynes having alternating cyclohexene and cyclopentene units in the backbone. Furthermore, detailed in situ NMR studies revealed an interesting phenomenon that the adjacent triple bond strongly chelates to the propagating Ru carbene during the polymerization.

Living ß-selective cyclopolymerization using Ru dithiolate catalysts.

Cyclopolymerization (CP) of 1,6-heptadiyne derivatives is a powerful method for synthesizing conjugated polyenes containing five- or six-membered rings via α- or ß-addition, respectively. Fifteen years of studies on CP have revealed that user-friendly Ru-based catalysts promoted only α-addition; however, we recently achieved ß-selective regiocontrol to produce polyenes containing six-membered-rings, using a dithiolate-chelated Ru-based catalyst. Unfortunately, slow initiation and relatively low catalyst stability inevitably led to uncontrolled polymerization. Nevertheless, this investigation gave us some clues to how successful living polymerization could be achieved. Herein, we report living ß-selective CP by rational engineering of the steric factor on monomer or catalyst structures. As a result, the molecular weight of the conjugated polymers from various monomers could be controlled with narrow dispersities, according to the catalyst loading. A mechanistic investigation by in situ kinetic studies using 1H NMR spectroscopy revealed that with appropriate pyridine additives, imposing a steric demand on either the monomer or the catalyst significantly improved the stability of the propagating carbene as well as the relative rates of initiation over propagation, thereby achieving living polymerization. Furthermore, we successfully prepared diblock and even triblock copolymers with a broad monomer scope.

Living Metathesis and Metallotropy Polymerization Gives Conjugated Polyenynes from Multialkynes: How to Design Sequence-Specific Cascades for Polymers.

On the basis of a combined experimental and computational study, a novel method for preparing fully conjugated polyenynes via cascade metathesis and metallotropy (M&M) polymerization of various multialkynes is developed. DFT calculations elucidate the detailed mechanism of the metallotropic 1,3-shift, which is a key process of M&M polymerization. An α,ß-(C,C,C)-agostic interaction stabilizing the metallacyclobutadiene transition state is found to be critically important for the successful polymerization with excellent specificity. The polymerization efficiency displayed by the tetrayne monomer is controlled by the steric demands of its substituents, and more complex hexayne monomers can be successfully polymerized to give access to highly conjugated polyenynes via a series of intramolecular metathesis and metallotropic shift cascade reactions. Furthermore, living polymerization led to the synthesis of block copolymers consisting of fully conjugated polyenyne backbones. The implementation of pentayne monomers provides polyenynes with successive C-C triple bonds via consecutive metallotropic 1,3-shift. In short, the design of multialkynes enables the preparation of diverse conjugated polyenyne motifs via selective M&M cascade reactions.