Long-Range Chirality Transfer in Free Radical Polymerization of Bulky Vinyl Monomers Containing Laterally Attached p-Terphenyl Groups

Two series of chiral bulky vinyl monomers, 2-(4'-hexyloxyphenyl)-5-(4'-alkoxycarbonylphenyl)styrene and 2-(4'-alkoxycarbonylphenyl)-5-(4'-hexyloxyphenyl)styrene (abbreviated as S-(+)-HexMm/ R-(-)-HexM0 and S-(+)-MmHex/R-(-)-M0Hex (m = 0, 1, 2, 3), respectively), were designed and synthesized. The former differed the latter only by the relative location of vinyl group, i.e., orientation against or toward the stereocenter. Except for S-(+)-HexM1 and S-(+)-HexM3, all the monomers readily underwent radical polymerization to yield the polymers displaying optical rotations and Cotton effects in the UV-vis absorption region of side groups distinct to the corresponding model compounds and mononmers, implying the formation of main chain chirality, most probable helicity. The influences of the configuration, the distance to p-terphenyl, and the position relative to vinyl group of stereocenter on the chiroptical properties of the resultant polymers were systematically studied. It wits found that inverting the absolute spatial configuration of asymmetric center or changing the parity of methylene number between stereocenter and p-terphenyl alternated the direction of optical rotation of polymer, similar to the odd-even alternation behavior observed in the cholesteric phase of achiral 2,5-bis(4'-hexyloxyphenyl)styrene doped with the corresponding monomers. This parallel indicated an identical driving force for the formation of chiral mesophase and the asymmetric secondary structure of the polymer. When the spatial configuration of stereocenter was Fixed, its distance to p-terphenyl rather than vinyl group played a dominant role in the induction of an excess helical sense. However, the stereogenic center at the ortho position of vinyl group exhibited a larger induction power than those at the meta position.