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

Abstract

Two series of chiral bulky vinyl aromatics, 2-(4'-alkoxyphenyl)-5-(4'-hexyloxyphenyl)styrene (S-Am (m 5 0, 1, 2, 3)/R-A0) and 2-(4'-hexyloxyphenyl)-5-(4'-alkoxyphenyl)styrene (S-Bm (m 5 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 1 H-and 13 C-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.

Synthesis and properties of novel helical 3-vinylpyridine polymers containing proline moieties for asymmetric aldol reaction

“…The CD spectrum of P(S)-PhVPyA presents one intensive and two weak negative peaks at 220 nm, 250 nm and 315 nm, and an intensive positive peak at 285 nm, suggesting the side groups were arranged in a skewed way. By analogous to the previously 50 reported polymers, 46, 51 the formation of chiral secondary structure was proposed for P(S)-PhVPyA. The CD curve of P(S)-PhVPyA had mirror symmetry with that of P(R)-PhVPy, indicating again that the prevailing twist sense of polymer backbone was determined by the configuration of stereocenter.…”

“…Three Waters Styragel columns with a 10 µm bead size were connected in series. Their effective molecular weight ranges were 100-10,000 Dalton for Styragel 50 HT2, 500-30,000 Dalton for Styragel HT3, and 5000-600,000 Dalton for Styragel HT4, separately. The pore sizes were 50, 100, and 1000 nm for Styragel HT2, HT3, and HT4, respectively.…”

“…Furthermore, 50 it seemed that the configurational chirality of side groups and the conformational chirality of polymer main chain exerted competitive influence. Water and Brønsted acid are known to enhance the catalytic performance of proline amide based aldolases.…”

“…Some well-defined polymer 50 aggregates, which could be switched by solvent, 31 pH, 34 and temperature, 35,36 were prepared to promote aldol reaction performance. Dendrimers instead of linear or cross-linked polymers were also used as the matrix to immobilize aldolases.…”

Optically Active Helical Vinylterphenyl Polymers: Chiral Teleinduction in Radical Polymerization and Tunable Stereomutation

“…It is interesting that the long‐range chirality transfer in radical polymerization of vinylterphenyl monomers greatly relies on the structure of the linkage that connects the chiral tail to the terphenyl group. When the –COO– group in 4a – e and 5a – e is replaced with an –O– group, the resulting monomers, 2‐(4′‐alkoxyphenyl)‐5‐(4′‐hexyloxyphenyl)styrene ( 6a – e ) and 2‐(4′‐hexyloxyphenyl)‐5‐(4′‐alkoxyphenyl)styrene ( 7a – e ), display distinct teleinduction behavior (Figure ) [35]. All of the polymers, except for that from 7d , display optical rotations and Cotton effects in the UV–vis absorption region of the terphenyl pendant, which are quite different from the corresponding model compounds and monomers, suggesting the formation of chiral secondary structures, i.e., skewed packing of side groups and twisting of polymer backbones with a dominant screw sense.…”