Synthesis and characterization of phosphorus- and sulfur-containing aromatic polyimides for high refractive index

A new diamine, bis(4-((4-aminophenyl)thio)phenyl)(phenyl)phosphine oxide (BATPO), containing phosphine oxide and thioether bonds was prepared via the Grignard reaction to dichlorophenylphosphine oxide with 4-fluorophenylmagnesium bromide, followed by an aromatic substitution reaction with 4-aminothiophenol. The polyimides (PIs) were synthesized by the conventional two-step polycondensation from several aromatic dianhydrides, such as 4,4’-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA), 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4′-oxydiphthalic anhydride (ODPA) and pyromellitic dianhydride (PMDA) with the BATPO. The synthesized PIs exhibited outstanding thermal properties including high thermal decomposition temperatures (>500 °C) and glass transition temperature (T_g) in the range of 212–295 °C, and optical transparency at 450 nm of over 80% for films of ca. 10 μm thickness. In particular, 3SDEA-BATPO exhibited a high refractive index (1.725) with low birefringence (0.0087) due to the heavy atom effect of the high sulfur and phosphorous contents in the polymer main-chain.