Photocrosslinkable polynorbornene-based block copolymers with enhanced dielectric and thermal properties

Block copolymers poly(endo-N-3,5-bis(trifluoromethyl)biphenyl-norbornene-pyrrolidine)-block-poly(exo-N-(cinnamoyloxyethyl)-7-oxanorborn-5-ene-2,3-dicarboximide) (endo-PTNP-b-exo-PCONBI) and poly(exo-N-3,5-bis(trifluoromethyl)biphenyl-norbornene-pyrrolidine)-block-poly(exo-N-(cinnamoyloxyethyl)-7-oxanorborn-5-ene-2,3-dicarboximide) (exo-PTNP-b-exo-PCONBI) were synthesized by ring-opening metathesis polymerization. The endo- or exo-PTNP served as the high dielectric functional chain, and exo-PCONBI acted as the crosslinking segment. The endo-PTNP-b-exo-PCONBI, in which endo-PTNP has a high content of trans double bond and adopts isotactic configuration, shows a dielectric constant (ε) of 15.5, whereas exo-PTNP-b-exo-PCONBI, in which exo-PTNP has 67% trans double bonds and atactic microstructure, displays relatively low ε of 7.1. The cinnamate groups in exo-PCONBI were crosslinked to form three-dimensional network by cycloaddition reaction under UV irradiation. Exposed to UV-light for 10 min, the cinnamate group in polymer films has a crosslinking conversion of 36%, as determined by UV-Vis absorption measurements. By photocrosslinking, the polymer film has an increased ε of 16.6, a dielectric loss of 0.03, an elevated glass-transition temperature of 137 °C, and an enhanced decomposition temperature of 405 °C, compared to those of polymer films without irradiation.