Thermally stable and flame retardant low dielectric polymers based on cyclotriphosphazenes

Abstract

We prepared thermally stable, low dielectric polymers by thermal polymerization of cyclotriphosphazenes bearing acetylene or styrene groups. Homosubstituted cyclotriphosphazene 1, containing acetylene groups, was synthesized by reaction of hexachlorocyclotriphosphazene with sodium salts of 4′-trimethylsilylethynyl-1,1′-biphenyl-4-ol and subsequent removal of trimethylsilyl groups under basic conditions. Heterosubstituted cyclotriphosphazene 2 was prepared using 4-phenylphenol as a cosubstituent. Cyclotriphosphazene 3 with styrene groups was prepared by reaction of hexachlorocyclotriphosphazene with sodium salts of 4-hydroxystyrene. Highly cross-linked polymers were obtained by thermal polymerization of the cyclotriphosphazenes at 200 °C for 1 and 2 and at 150 °C for 3. The 5% weight loss temperatures of the cross-linked polymers of compounds 1–3, measured by TGA, were above 470 °C. The flame-retardant property of the polymers was evaluated by measuring LOI (limiting oxygen index) values. The polymers of cyclotriphosphazenes 1–3 showed very high LOI values of 52, 50 and 49, respectively. We also prepared copolymers of cyclotriphosphazene 3 with styrene by UV irradiation in the presence of a photoinitiator (2,2-dimethoxy-2-phenylacetophenone). In the TGA thermograms, the copolymers showed good thermal stability, nearly comparable to the homopolymer of 3 when the composition of 3 was higher than 60%. The dielectric properties of the polymers were measured by a metal–insulator–metal method. The polymer obtained from acetylene-containing cyclotriphosphazene 1 showed dielectric constant and dielectric loss values of about 3.12 and 0.0016, respectively, at 1 GHz. The polymer of cyclotriphosphazene 3 showed a dielectric constant of 2.4 and a dielectric loss of 0.0014 at 1 GHz.