Functional polydiynes prepared by metathesis cyclopolymerization of 1,7-dihalogen-1,6-heptadiyne derivatives

The MCP route used for the polymerization of 1,6-heptadiynes was successfully applied to the polymerization of 1,7-dihalogen-1,6-heptadiynes, and the target polymers were obtained in high yield with high molecular weight and unique UCST behavior.

Synthesis of Functional Polyacetylenes via Cyclopolymerization of Diyne Monomers with Grubbs-type Catalysts

Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor β-addition and the formation of 6-membered rings in the polymer backbone. While modest β-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers' catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ring-closing steps.

Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures

Monomers possessing two functionalities suitable for polymerization are often designed and utilized in syntheses directed to the formation of cross-linked macromolecules. In this review, we give an account of recent developments related to the use of such monomers in cyclopolymerization processes, in order to form linear, soluble macromolecules. These processes can be activated by means of radical, ionic, or transition-metal mediated chain-growth polymerization mechanisms, to achieve cyclic moieties of variable ring size which are embedded within the polymer backbone, driving and tuning peculiar physical properties of the resulting macromolecules. The two functionalities are covalently linked by a "tether", which can be appropriately designed in order to "imprint" elements of chemical information into the polymer backbone during the synthesis and, in some cases, be removed by postpolymerization reactions. The two functionalities can possess identical or even very different reactivities toward the polymerization mechanism involved; in the latter case, consequences and outcomes related to the sequence-controlled, precision synthesis of macromolecules have been demonstrated. Recent advances in new initiating systems and polymerization catalysts enabled the precision syntheses of polymers with regulated cyclic structures by highly regio- and/or stereoselective cyclopolymerization. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures, which can hardly be obtained by conventional polymerization methods.

Bulk self-assembly and ionic conductivity of a block copolymer containing an azobenzene-based liquid crystalline polymer and a poly(ionic liquid)

A block copolymer containing a liquid crystalline polymer and a poly(ionic liquid) self-assembles and can be used as a solid electrolyte.