Convenient synthesis of poly(2,6-dihydroxy-1,5-naphthylene) by Cu(II)-amine catalyzed oxidative coupling polymerization

A convenient synthesis of regiocontrolled poly(2,6-dihydroxy-1,5-naphthylene) (PDHN) with high molecular weights by oxidative coupling polymerization of 2,6-dihydroxynaphthalene (2,6-DHN) has been developed. Polymerizations were conducted in 2-methoxyethanol in the presence of di-μ-hydroxo-bis[(N,N,N′,N′-tetramethylethylenediamine)copper (II)] chloride (CuCl(OH)TMEDA) as the catalyst under air at 25 °C. To determine the optimum conditions, the effects of the amounts of the catalysts and the solvents were investigated. In the presence of 5 mol% of the catalyst to the monomer in 2-methoxyethanol, polymerization proceeded smoothly, giving PDHN with a number average molecular weight (Mn) of 52,000. PDHN was converted to poly(2,6-dibutoxy-1,5-naphthylene) (PDBN) to improve the solubility. The structure of PDBN was characterized by 1H and 13C NMR spectroscopy and was estimated to consist completely of the 1,5-linkage. The average refractive indices (nAV) of the PDHN and PDBN films were 1.6003 and 1.5815, respectively, and the dielectric constants (ε) estimated from the refractive indices were 2.82 and 2.75, respectively.