Functional Polymers from Bicyclic Dienes
Various polymer precursors were synthesized via radical polymerization of bicyclic dienes. These polymers can be converted to a variety of functional materials based on their different chemical structures. First, a new thermal cross-linking strategy based on retro Diels-Alder reaction of bicyclic diene polymers was developed. 2,3-Dimethylene-norbornane (1) and its derivatives polymerized under free-radical conditions predominantly via the 1,4-addition mode. Nitroxide-mediated polymerization of 1 at 110 °C provided bicyclic diene polymers with controlled molecular weights and narrow molecular weight distributions, as well as block copolymers of 1 and methyl acrylate. Poly(1) underwent the retro Diels-Alder reaction with a concurrent release of ethylene upon heating to 245 °C, which ultimately led to the formation of cross-linked structures. Block copolymers of 1 and methyl acrylate phase separated into lamellae and cylindrical microstructures when annealed at 180 °C. Upon further heating to 245 °C, the poly(1) domain in the block copolymer cross-linked to provide rigid polymer nanostructures. The morphology formed by block copolymer phase separation was preserved during the cross-linking process. Secondly, we designed and synthesized a new bicyclic diene with an epoxy substituent, which can be polymerized by free radical polymerization with 1-butanthiol as the radical transfer agent. Mixing this oligomer with a polyamine at 80 °C led to the formation of a cross-linked network. Degradation occurred when the temperature went up to 160 °C because of the retro Diels-Alder reaction with the formation of aromatic furan ring. Meanwhile, no volatile side products were produced during this process according to the Thermal Gravimetric Analysis (TGA). This new epoxy glue could be used for temporary adhesive since it can be easily degraded by heating without producing any volatile toxic chemicals. Thirdly, a new bicyclic diene containing a benzofuran group was synthesized by palladium(0) catalyzed heteroannulation of bicyclic alkene and subsequent 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) aromatization. 1,4-polymerization of this monomer resulted in a polymer precursor that underwent retro Diels-Alder reaction at 140 °C. Much lower retro Diels-Alder temperature compared to previous polymers was due to the formation of an aromatic ring and a concurrent release of ethylene gas. The obtained dibenzofuran-containing polymer exhibited microporous structure, which could enable the use of this polymer as a gas separation membrane. Finally, we developed a new method for the fabrication of soluble polyacetylene derivatives based on bromination-dehydrobromination of bicyclic diene polymers. High molecular weight polymer precursors were synthesized by radical 1,4-polymerization of corresponding dienes. Regioselective elimination from the brominated polymer afforded a polyacetylene derivative containing bicyclic substituents, which was readily soluble in common organic solvents. The polymer bandgap was in the range of 1.4–1.7 eV, which was much lower than the values reported for polyacetylene derivatives with a substituent on every second carbon. Low bandgap values were attributed to the conformational inflexibility of the bicyclic substituent forcing coplanar orientation of the backbone double bonds. Solid state conductivity of the produced polymer in the undoped form was measured to be 1.5×10 −5 S/m. This new synthetic method allows for the chain-growth-production of polyacetylene derivatives that possess favorable electronic properties and superior solubility characteristics to pristine polyacetylene.