Rhodium-Catalyzed Oxidative Polycoupling of Phenylpyrazole and Internal Diynes: A New Polymerization Route for Atom-Econ
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Rhodium-Catalyzed Oxidative Polycoupling of Phenylpyrazole and Internal Diynes: A New Polymerization Route for Atom-Economical Synthesis of Poly(pyrazolylnaphthalene)s Yajing Liu, Meng Gao, Jie Li, and Ben Zhong Tang Department of Chemistry, Institute for Advanced Study and Division of Biomedical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China. Email: [email protected] ABSTRACT A new route for atom-economical synthesis of functional polymers was developed. Oxidative polycoupling of 3,5-dimethyl-1-phenylpyrazole with 4,4’-(α,ω-alkylenedioxy) bis(diphenylacetylene)s and 1,2-diphenyl-1,2-bis[4-(phenylethynyl)phenyl]ethene, respectively, were catalyzed by [Cp*RhCl2]2, 1,2,3,4-tetraphenylcyclopenta-1,3-diene and copper(II) acetate in dimethylformamide under stoichiometric imbalance conditions, affording soluble poly(pyrazolylnaphthalene)s in satisfactory yields (isolation yield up to 82%) with high molecular weights (Mw up to 35700). All the polymers were thermally stable, losing little of their weight at high temperatures of 323–422 oC. They possessed good film-forming property and their thin solid films showed high refractive indices (RI = 1.747–1.593) in a wide wavelength region of 400−1000 nm. The polymer carrying tetraphenylethene unit displayed a phenomenon of aggregation-induced emission and showed enhanced light emission in the aggregated state. INTRODUCTION Development of new methodology for the synthesis of functional polymers is an important research area in macromolecular science. Olefins have been the main sources of monomers, whose addition polymerization yield polymers with electronically saturated backbones that are commonly used as commodity materials. Polymerization of acetylenic monomers can generate polymers with S-conjugated backbones that are expected to be electronically active. Indeed, polyacetylene was found to exhibit metallic conductivity upon doping by Shirakawa, MacDiarmid and Heeger in the 1970s.1 This seminal discovery has triggered great efforts in utilizing alkynes as building blocks to construct functional polymers. As a result, a large number of S-conjugated polymers has been synthesized from alkyne monomers.2 Among various employed methods, the polycyclotrimerization or Diels- Alder reaction could afford polymers with stable benzene rings (Scheme 1),3 and hence broad applications in organic light-emitting diodes,4 photovoltaic cells5 and sensors.6 Highly substituted naphthalene derivatives are well-known for their high thermal stability and unique electro- and photochemical properties, as well as their potential uses as organic semiconductors and luminescent materials.7 Recently, we found that the rhodium-catalyzed oxidative polycoupling of arylboronic acids and internal diynes proceeded smoothly under stoichiometric imbalance-promoted conditions,8 affording poly(naphthalene)s with moderate molecular weights in satisfactory yields (Scheme 1).9 Thanks to their high aromatic content, the resulting polymers showed high thermal stabil
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