Preparation of Novel Nonlinear Optical Polyester with Enhanced Thermal Stability of Dipole Alignment

Abstract

zation of dipole alignment of the NLO chromophore, which was a part of the polymer backbone. In this work we prepared novel polyester containing 2,3-dioxybenzylidenemalononitrile groups as NLO-chromophores. We selected 2,3-dioxybenzylidenemalononitrile groups as NLO-chromophores because they have a large dipole moment and are rather easy to synthesize. Furthermore, 2,3-dioxybenzylidenemalononitrile group constitutes novel Y-type NLO polyester (Fig. 1c), and this Y-type NLO polyester has not been reported in the literature. These mid-type NLO polymers are expected to have the advantages of both main-chain and side-chain NLO polymers: stabilization of dipole alignment and good solubility. After confirming the structure of the resulting polymers we investigated the properties such as solubility, Tg, thermal stability, second harmonic generation (SHG) activity and relaxation of dipole alignment. We now report the results of the initial phase of the work. Results and Discussion 2,3-Di-(2'-vinyloxyethoxy)benzaldehyde (1) was prepared by the reaction of 2-chloroethyl vinyl ether with 2,3dihydroxybenaldehyde. 2,3-Di-(2'-vinyloxyethoxy)benzylidenemalononitrile (2) was prepared by the condensation reaction of 1 with malononitrile. Compound 2 was hydrolyzed to yield acetaldehyde and diol 3. Diol 3 was condensed with terephthaloyl chloride in a dry DMF solvent to yield polyester 4 containing the NLO-chromophore 2,3-dioxybenzylidenemalononitrile group The synthetic route for polyeater 4 is presented in Scheme 1. The polymerization yield was 88-92%. The chemical structures of the resulting polymers were confirmed by 1 H NMR, IR spectra, and elemental analysis. Elemental analysis results fit the polymer structures. 1 H NMR spectra of the polymers showed a signal broadening due to polymerization, but the chemical shifts are consistent with the proposed polymer structures. The IR spectra of the same polymer samples show a strong carbonyl peak near 1722 cm −1 indicating the presence of ester bond. The same polymer sample also shows a strong absorption peak near 2229 cm –1 and 1572 cm –1 due to the nitrile and olefin groups, respectively. These results are consistent with the proposed structures, indicating that the NLO-chromophore remained intact during the polymeri