Highly thermostable rigid-rod networks constructed from an unsymmetrical bisphthalonitrile bearing phthalazinone moieties

Abstract

Phthalazinone rigid-rod networks with excellent thermostability have been prepared by the polyaddition of an unsymmetrical phthalonitrile-functional phthalazinone, namely 4-(4-(4-(3,4-dicyanophenoxy)phenyl)-1-oxophthalazin-2(1H)-yl)phthalonitrile (5) in the presence of promoting agents. 5 was readily prepared via nucleophilic displacement of 4-(4-hydroxylphenyl)(2H)-phthalazin-1-one (1) with 4-nitrophthalonitrile (3) under mild conditions. The substitution of N–H in phthalazinone was accomplished via a novel N–C coupling reaction in an essentially quantitative yield, confirmed by spectroscopic studies as well as a model reaction of 4-(4-tolyl)(2H)-phthalazin-1-one (2). Green prepolymers (7a–7c) were obtained by the polymerization of 5 in its melt phase with the charge of aromatic diamines (6a–6c), and with concurrent cyclization a mixture of poly(iminoisoindolenine) oligomers was produced. The obtained amorphous 7a–7c show T_gs ranging from 180 to 190 °C depending on the amine nature. Both 5 and 7a–7c have a non-coplanar conformation and exhibit sufficiently good solubility in common solvents (e.g., DMSO, DMF, DMAc, NMP, CHCl₃, etc.), and hence can be processed either from their melt or from solution. The insoluble networks (8a–8c) prepared by direct curing of 5 or by post-curing of 7a–7c exhibit excellent thermal properties together with superior long-term thermo-oxidative stabilities.