Colorless polyimides with low coefficient of thermal expansion derived from alkyl-substituted cyclobutanetetracarboxylic dianhydrides

Alkyl-substituted cyclobutanetetracarboxylic dianhydrides (CBDAs) were synthesized by photo-dimerization of alkyl-substituted maleic anhydrides to obtain novel colorless polyimides (PIs). Dimethyl-substituted CBDA (DM-CBDA) showed much higher polymerizability with various diamines than conventional cycloaliphatic tetracarboxylic dianhydrides and led to high molecular weights of PI precursors. Polyaddition of non-substituted CBDA and trans-1,4-cyclohexanediamine (t-CHDA) was completely inhibited by salt formation in the initial reaction stage. The use of DM-CBDA allowed the formation of a homogeneous/viscous PI precursor solution by overcoming the salt formation problem. The prominent substituent effect probably reflects how the methyl substituents of DM-CBDA contributed to increasing the salt solubility. Some of the thermally imidized DM-CBDA-based systems simultaneously possessed non-coloration, low coefficient of thermal expansion (CTE), very high Tg exceeding 300 °C and very low dielectric constant. Copolymerization was very effective for improving the solubility of DM-CBDA-based PIs. The copolyimide cast films prepared via chemical imidization displayed a further decreased CTE without sacrificing other target properties, suggesting that the present materials can be useful as plastic substrates in display devices. The mechanism of self-chain orientation behavior during solution casting is also discussed. A potential application of the copolyimide systems as optical compensation film materials in liquid crystal displays is proposed. © 2013 Society of Chemical Industry.