Soluble and cross-linkable polyimides from a vanillin-derived diamine: preparation, post-polymerization and properties†

A novel trifluorovinyl-ether (–OCFCF₂, TFVE) functionalized diamine (DA-TFVE) with a triarylmethane structure was successfully obtained from renewable vanillin through a Brønsted acidic ionic liquid catalyzed Friedel–Crafts reaction. Subsequently, traditional two-step condensations between DA-TFVE and dianhydride (6FDA and CBDA) were used to prepare two linear polyimides (PI-1 and PI-2, respectively) containing TFVE side groups. Both polyimides exhibited excellent film-forming ability and good solubility in common organic solvents. Upon heating, the linear polyimides could be transformed into cross-linked polyimides (cPI-1 and cPI-2) with perfluorocyclobutyl (PFCB) ether linkers through thermal [2π + 2π] post-polymerization of TFVE groups. Surprisingly, both cPI-1 and cPI-2 exhibited high mechanical strength with a storage modulus of more than 2.0 GPa at room temperature, as well as outstanding thermal stability with glass transition temperatures of 402 °C and 374 °C, respectively. The polymer films before and after thermal crosslinking showed high transparency. In particular, a transmittance of 73% at 450 nm and 81% at 500 nm was observed for the colorless polyimide PI-2 with aliphatic segments. X-ray diffraction results of the obtained PIs showed that the average inter-chain distances (d-spacing) were 0.44–0.59 nm, indicating the hindered chain packing caused by PFCB linkers between the polymer chains. All these results suggested their potential application as candidate materials in high-performance packaging and flexible electronics.