Highly Thermally Stable, Cross-Linked, All-Aromatic Polyketone From Polycyclotrimerization of Bis(Ethynylketo)phenylene in Bulk

To develop highly heat-resistant resins that are processable at low temperatures, we investigated polycyclotrimerization of m-bis(ethynylketo)benzene (1) in bulk, leading to a cross-linked, all-aromatic polyketone. The cyclotrimerization of 1-phenyl-2-propyn-1-one (3) with a catalytic amount of secondary amine was first examined in bulk as a model reaction. When 30 mol% of piperidine, which is a liquid at room temperature, was added dropwise to 3 in bulk, the reaction proceeded explosively with fuming. However, solid piperazine, instead of piperidine, could be mixed with 3 without fuming. The reaction of 1 with 2.5 mol% of piperazine in bulk at 110°C for 24 h afforded 1,3,5-tribenzoylbenzene in 83% yield. Accordingly, the polycyclotrimerization of 1 with 2.5 mol% of piperazine was carried out in bulk at 130°C, 150°C, and then 180°C for 120 min at each temperature. The conversion of the ethynyl group gradually increased to 82%, 92%, and 96%, respectively. Thermogravimetric analysis (TGA) of cured 1 showed that T _d5 was 426°C, which is higher than that of conventional cyanate ester resin (T _d5 = 410°C) or epoxy resin (T _d5 = 360°C). Dynamic mechanical analysis (DMA) revealed that the storage modulus of cured 1 was as high as 3.7 × 10⁹ Pa at 40°C in the glassy flat region and was 1.8 × 10⁹ Pa even at 380°C; T _g was not observed till 400°C, in contrast to the case of cyanate ester resin (T _g = 280°C) or epoxy resin (T _g = 145°C). Thus, the resin obtained by polycyclotrimerization of 1 at 130°C–180°C has greater chemical and physical heat resistance than conventional cyanate ester resin or epoxy resin.