SYNTHESIS AND CHARACTERIZATION OF PHOTOSENSITIVE POLYIMIDES FROM METHYLTHIOMETHYL-SUBSTITUTED 4,4'-DIAMINODIPHENYLMETHANES AND 3,3',4,4'-BENZOPHENONETETRACARBOXYLIC DIANHYDRIDE

A series of novel polyimides are synthesized by the reaction of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) with four methylthiomethyl-substituted aromatic diamines: 3-methylthiomethyl-4,4'-diaminodiphenylmethane (I), 3,3'-dimethylthiomethyl-4,4'-diaminodiphenylmethane (II), 3,3',5-trimethylthiomethyl-4,4'-diaminodiphenylmethane (III), and 3,3',5,5'-tetramethylthiomethyl-4,4'-diaminodiphenylmethane (IV) in refluxing m-cresol. The polyimide of diamine I and BTDA carrying only one pendant methylthiomethyl group in a repeating unit is readily soluble in m-cresol, chloroform, and polar aprotic solvents. Increasing the number of the pendant group results in higher solubility. These fully imidized polyimides are also intrinsically photosensitive. The fraction of photoreactive benzophenone sites that relates to the rate and degree of completion of photocrosslinking reaction increases systematically with the increase of the pendant group content. As the average number of the pendant group in a repeating unit reaches 3, 63% of benzophenone sites are found to be photoreactive. These methylthiomethyl-substituted polyimides possess moderate tensile strength which falls in the range of 67-81 MPa. As a result of the increase of methylthiomethyl content, this type of polyimide reveals higher glass transition temperature but lower thermal stability due to the considerable dimension of the pendant group and the ready cleavage nature of the C-S bond.