Helix-sense-selective radical polymerization of bulky styrenic monomers: chiral induction and liquid crystallinity

Eight chiral vinylterphenyl monomers, (+)-2,5-bis{4′-(S)-1″-methylpropyloxy]phenyl}styrene (Ia), (+)-2,5-bis{4′-(S)-2″-methylbutyloxy]phenyl}styrene (Ib), (+)-2,5-bis{4′-(S)-3″-methylpentyloxy]phenyl}styrene (Ic), (+)-2,5-bis{4′-(S)-4″-methylhexyloxy]phenyl}styrene (Id), (?)-2,5-bis{4′-(R)-1″-methylpropyloxy]phenyl}styrene (Ie), (+)-2-{4′-(S)-1″-methyl-propyloxy]phenyl}-5-{4′-(R)-1″-methylpropyloxy]phenyl}styrene (IIa), (?)-2-{4′-(R)-1″-methylpropyloxy]phenyl}-5-{4′-(S)-1″-methylpropyloxy]phenyl}styrene (IIb), and (+)-2-{4′-(S)-2′′-methylbutyloxy]phenyl}-5-{4′-(S)-1″-methylpropyl-oxy]phenyl}styrene (III), were synthesized and radically polymerized. These molecules were designed to further understand long-range chirality transfer in radical polymerization and to possibly tune the chiroptical properties of the polymers by varying the spatial configuration, position, and various combination of the stereogenic centers at the ends of p-terphenyl pendants. The resultant polymers adopted helical conformations with a predominant screw sense. When the stereogenic centers ran away from the terphenyl group as in Ib?d, the corresponding polymers changed the direction of optical rotation in an alternative way and showed no obvious stereomutation upon annealing in tetrahydrofuran. The two stereogenic centers of IIa, IIb, and III acted concertedly in chiral induction, whereas those of Ia and Ie played a counteractive role. The five polymers derived from Ia, Ie, IIa, IIb, and III underwent stereomutation when annealed in tetrahydrofuran. The polymers PIa-e had good thermal stability and high glass transition temperatures (T _gs). They generated liquid crystalline phases at above T _gs that could be kept upon cooling, with the exception of PIe. This result was consistent with the extended helical structures.