Synthesis and mesophase behaviors of 2,5-disubstituted styrene-based random copolymers: Effect of difference in side-group length on liquid crystallinity

Five series of binary copolymers, poly[2,5-di(ROOC)styrene-co-2,5-di(R′OOC)styrene]s (R = n-C3H7-, R′ = C2H5-, n-C4H9-, and n-C5H11-; R = n-C6H13-, R′ = n-C4H9-, and n-C5H11-), were synthesized via free radical polymerization. The random nature of the copolymers was expected on the basis of the assumed similar reactivities of the analogous monomers and proved by 13C NMR analysis. It was also implied by the smoothly varying glass transition temperatures and further supported by the monotonous variation in d spacings for the liquid crystalline copolymers, where both corresponding homopolymers are liquid crystals. Subtle difference of R and R′ was found to have significant impact on the mesomorphic properties of the copolymers. When the pair of the corresponding homopolymers has the same mesogenic structure, a difference of one carbon atom in the alkyl chain can be tolerated over the whole copolymer composition range; however, the liquid crystalline structure soon disappears with the incorporation of a co-unit of an homopolymer that is not liquid crystal. For the copolymers with alkyl chain length differing by two carbon atoms, the liquid crystalline structure is lost with the incorporation of relatively low co-unit content despite the pair of corresponding homopolymers having the same mesogenic structure.