Double-Stranded Helical Polymers Consisting of Complementary Homopolymers

Abstract

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.

Chiral amplification in double-stranded helical polymers through chiral and achiral amidinium–carboxylate salt bridges

“…Upon mixing equimolar amounts of poly-A x (x ¼ 0.2, 0.4, 0.6, and 0.8) and poly-C, the positive cotton effects, which are completely different from those of the chiral/achiral amidine copolymers, appeared in the absorption regions of the p-diethynylphenylene linkers, and the CD intensities gradually increased with time and reached constant values after ca. 9-24 h (Figure 3b), as observed for the double-helical poly-A 1 Á poly-C, 29 which indicates that the poly-A x Á poly-C duplexes (x ¼ 0.2, 0.4, 0.6, and 0.8) most likely formed an excess single-handed double-helical structure through amidinium-carboxylate salt bridges. Interestingly, the poly-A x Á poly-C duplexes showed an additional positive cotton effect in the longer wavelength regions, and their intensities tended to increase with the increasing achiral amidine contents.…”

“…Chiral and achiral amidines and carboxylic acid monomers ((R)-A, A, and C, respectively) were prepared according to previously reported methods. 29 …”

“…29 The polymerization results are summarized in Table 1. Except for poly-C, all of the homopolymers and copolymers were obtained in moderate yields (449%) and had relatively high molecular weights (M n 4ca.…”

“…9,15,18,19 We also reported on mterphenyl-based conjugated polymers, which contain optically active amidine groups (poly-A 1 ) and achiral carboxylic groups (poly-C), that folded into an intertwined double-stranded helical structure through the chiral amidinium-carboxylate salt bridges. 29 In this study, we synthesized a series of m-terphenyl-based random copolymers containing chiral and achiral amidines (poly-A x ) and their complementary homopolymers containing achiral carboxylic acids (poly-C), and investigated the effect of the chiral/achiral amidine contents on the amplification of the helical chirality 30,31 during the complementary double-helix formation ('the sergeants and soldiers effect') 3,4,32,33 (Figure 1) using absorption and circular dichroism (CD) spectroscopies. Such a unique amplification of the helical chirality along the polymer backbones assisted by a small chiral unit has been proven to be applicable to some stiff single-stranded helical polymers 3,7,9,[31][32][33][34][35][36][37][38][39] and supramolecular helical systems.…”

“…9,15,18,19 We also reported on mterphenyl-based conjugated polymers, which contain optically active amidine groups (poly-A 1 ) and achiral carboxylic groups (poly-C), that folded into an intertwined double-stranded helical structure through the chiral amidinium-carboxylate salt bridges. 29 In this study, we synthesized a series of m-terphenyl-based random copolymers containing chiral and achiral amidines (poly-A x ) and …”