Synthesis and characterization of poly(pyridine–imide)s with tert- butyl substituents

The aromatic diamine monomer containing a pyridine heterocyclic group and a tert-butyl substituent, 4-[(4′-tert-butyl)phenyl]-2,6-bis(4-aminophenyl)pyridine (BPAP), was successfully synthesized by a modified Chichibabin reaction of 4-nitro-acetophenone and 4-tert-butyl benzaldehyde, followed by a reduction of the resulting dinitro compound 4-[(4′-tert-butyl)phenyl]-2,6-bis(4-nitrophenyl)pyridine with palladium/carbon and hydrazine monohydrate, successively. A series of poly(pyridine–imide)s were prepared from the diamine BPAP with the alicyclic anhydride 1,2,3,4-cyclobutanetetracarboxylic dianhydride, aromatic anhydride pyromelitic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, and 4,4′-oxydiphthalic anhydride in N,N-dimethylformamide (DMF) via the conventional two-step method and further chemical imidization to obtain poly(pyridine-imide) powders. Due to the introduction of tert-butyl group, the polyimides (PIs) derived from BPAP were amorphous and displayed good solubility in amine-type polar solvents, such as DMF, dimethyl sulfoxide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone. Because of the presence of rigid triphenylpyridine units and bulky substituents, the final PIs held a good thermal stability with 10% weight loss temperatures above 504°C and left more than 55% residue even at 700°C under nitrogen atmosphere. The effect of tert-butyl substituents on the properties of the PIs was studied by comparing with 4-phenyl-2,6-bis(4-nitrophenyl)pyridine. These PIs exhibited some fluorescent properties because of the triphenylpyridine units, and the chemical structure of dianhydrides took an effect on the fluorescent property of these PIs.