Macromolecules, Vol.50, No.9, 3489-3499, 2017
Helical Conformations of Poly(3,5-disubstituted phenylacetylene)s Tuned by Pendant Structure and Solvent
A series of novel cis poly(phenylacetylene)s (PPAs) substituted at the meta-position(s) by both achiral alkoxycarbonyl and chiral alkylamide groups, i.e., rP-I, sP-I to sP-V, or by just a chiral alkylamide group, i.e., rP-VI, were synthesized under catalysis of [Rh(nbd)Cl](2). The dependence of the elongation, screw sense, and stimuli response of helical polyene backbone on the structure and number of substituent was systematically investigated in both solution and solid states. Stretched cis-transoid helices with opposite signs coexisted in the DMF solution of either sP-I or rP-I, but a single handed, contracted cis-cisoid one formed in the mixture of DMF/THF (10/90, v/v). Increasing the substituent size made the polymers sP-III, sP-W, and sP-V to take only single handed stretched cis transoid helical conformations regardless of the solvent polarity. The N-methylation of the amide group in sP-II caused a similar effect. With the removal of achiral methoxycarbonyl substituent, rP-VI took just a stretched cis transoid helical conformation in polar DMF, whereas existed as a mixture in equilibrium of stretched cis transoid and contracted cis-cisoid helices with identical screw sense in less polar solvents such as dioxane, THF, and chloroform. The twisting directions of substituent array and polyene backbone were found to be coincident in a dynamic, contracted helix, but the opposite in a less dynamic, stretched helix. These results suggested that the 3,5-disubstitution, strong intramolecular hydrogen bonding, and small substituent favored the formation of contracted cis cisoid helices for PPAs.