Long range chirality transfer in free radical polymerization of vinylterphenyl monomers bearing chiral alkoxy groups

Abstract

ABSTRACTTwo series of chiral bulky vinyl aromatics, 2‐(4'‐alkoxyphenyl)‐5‐(4'‐hexyloxyphenyl)styrene (S‐Am (m = 0, 1, 2, 3)/R‐A0) and 2‐(4'‐hexyloxyphenyl)‐5‐(4'‐alkoxyphenyl)styrene (S‐Bm (m = 0, 1, 2, 3)/R‐B0), were synthesized and radically polymerized (where m denotes the carbon number between stereogenic center and ether oxygen atom). The generation and stereomutation of helical structures as well as chirality transfer from the stereogenic center of the side‐group to the polymer backbone during polymerization were investigated by a combination of 1H‐ and 13C‐NMR, polarimetry, circular dichroism spectroscopy, liquid crystal induced circular dichroism, and computer simulation. All the polymers, except S‐B3, display optical rotations and Cotton effects in the UV‐Vis absorption region of terphenyl pendant, which are quite different from the corresponding model compounds and monomers, suggesting the formation of chiral secondary structures, that is, skewed packing of side‐groups and twisting of polymer backbones with a dominant screw sense. The sign of optical rotation changes alternatively and the strength diminishes when m increases from 1 to 3. However, although the stereogenic centers in S‐A0 and S‐B0 are closer to the vinyl group than those in S‐A1 and S‐B1, separately, a weaker chiral induction power is observed. Moreover, optical rotations of polymers derived from S‐A0/R‐A0 and S‐B0/R‐B0 are opposite in sign and increase with annealing time in tetrahydrofuran. These results are in a sharp contrast with our previous findings in similar vinylterphenyl compounds, where ‐COO‐ rather than ‐O‐ links the chiral alkyl tail to the terphenyl group, manifesting a remarkable effect of small structural variation of side‐group on polymer chiroptical properties and complexity of chirality transfer in helix‐sense‐selective polymerization. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3674–3687