Novel optically active (S)-2-(3,5-diiodophenyl)oxazoline derivatives ((S)-DIPhROx)) (R¼4-benzyl (1a), 4-phenyl (1b)) were synthesized and polymerized using the Sonogashira-Hagihara coupling reaction to obtain the poly(phenylene-ethynylene)s bearing oxazoline derivatives in the weight range from 1060 to 17 300 in 40-72% yields. The polymers were thermally stable up to 220 1C. All the polymers exhibited higher specific rotation values than the model compound did. Moreover, these polymers also showed intense circular dichroism (CD) signals in the transition region of the conjugated main chain, whereas almost no induced Cotton effect was observed for the model compound. The results of the specific rotation and CD and ultraviolet analyses indicate that the optical activities of the polymers arise not only from the configurational chirality of the optically active oxazoline pendant groups but also from elements of higher-order structure such as the helical conformation.
Optically active poly(m-phenylene)s substituted with chiral oxazoline derivatives have been synthesized by the nickel-catalyzed Yamamoto coupling reaction of optically active (S)-4-benzyl-2-(3,5-dihalidephenyl)oxazoline derivatives (X ¼ Br or I). The structures and chiroptical properties of the polymers were characterized by spectroscopic methods and thermal gravimetric analyses. The polymers showed higher absolute optical specific rotation values than their corresponding monomer, and showed a Cotton effect at transition region of conjugated main chain. The optical activities of the polymers should be attributed to the higher order structure such as helical conformations. Moreover, the helical conformation could be induced by addition of metal salts into polymer solutions. The polymers showed good thermal stabilities, which was attributable to the oxazoline side chains.
Optically active 4-alkyl-2-ethynyloxazoline derivatives (BnEOx) were polymerized with rhodium catalysts. The polymerization in toluene produced polymer with the highest absolute values of specific rotation ([α](D) = -77.3°). The yields, molecular weights, and specific rotations of poly(BnEOx)s were influenced by polymerization conditions. The copolymerization with phenylacetylene (PA) was effective to increase the molecular weight of the copolymer. It is interesting to note that the copolymers exhibited positive specific rotations ([α](D) = +4.7° to +62.5°) despite the fact that [α](D) s of BnEOx and the homopolymer are negative sign. The chiroptical properties were investigated by the chiral/achiral copolymerization of BnEOx with PA. The copolymerizations of BnEOx with PA gave copolymers containing higher order structure such as one-handed helical conformation. Furthermore, induced Cotton effects were observed in the π-π* transition region of conjugated main chain depending a complex of these polymers with zinc triflate salt in tetrahydrofuran solution, indicating the formation of chiral supramolecular aggregates.
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