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

Two 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 H-1- and C-13-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. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3674-3687Polymer ScienceSCI(E)EI5ARTICLE173674-36875