Supramolecular Helical Structure of the Stereocomplex Composed of Complementary Isotactic and Syndiotactic Poly(methyl methacrylate)s as Revealed by Atomic Force Microscopy

Abstract

The DNA double helix composed of complementary strands is of fundamental importance for the exquisite functions of DNA, such as the replication and storage of genetic information. Since the discovery of the DNA double helix, [1] the design and synthesis of artificial double helices have attracted significant attention but remain a great challenge in polymer and supramolecular chemistry.[2] In particular, double helices of complementary strands are quite rare. [3] The complex formed from complementary strands of isotactic and syndiotactic poly(methyl methacrylate)s (it-and st-PMMAs) with an it/st stoichiometry of 1:2 is commonly described as a stereocomplex and represents a class of unique, polymer-based supramolecules with an apparent melting point in specific solvents.[4] Although the stereocomplex has been known for half a century, the molecular basis of the structure and the mechanism of complex formation are still under debate, in spite of its availability as advanced materials, such as ultrathin films, [5] thermoplastic elastomers, [6] and dialyzers. [7] It is also a versatile structural motif for stereospecific template polymerization [8] in connection with abiotic replication.In 1989, Schomaker and Challa proposed a reliable model for the PMMA stereocomplex on the basis of X-ray analysis of the stretched fiber, that was the double-stranded helix composed of a 9 1 it-PMMA helix (nine repeating MMA units per turn) surrounded by a 18 1 st-PMMA helix with a helical pitch of 1.84 nm (Figure 1 a). [9] Since then, the doublestranded-helix model has been commonly accepted, because the model could explain rationally 1) the stoichiometry of an asymmetric unit (it/st = 1:2), [4] 2) the template-polymerization phenomena, [8] and 3) the fact that stereocomplexation also took place between it-PMMA and st-poly(methacrylic acid), and even when the methyl ester groups of st-PMMA were replaced by other alkyl groups, whereas the methyl esters of it-PMMA were essential for the stereocomplexation. [4,10] However, because of the limited number of diffuse X-ray diffractions, the complicated structure of the PMMA stereocomplex was difficult to determine by X-ray diffraction, and the proposed double-helix model may require further reconsideration.Although the structural elucidation of helical polymers at a molecular level by X-ray diffraction is a laborious task even now, recent significant developments in microscopic instruments coupled with precise polymerization techniques have made it possible to observe directly the helical structures of certain helical polymers. In fact, we succeeded recently in observing the helical structures of helical poly(phenylacetylene)s and polyisocyanides by high-resolution atomic force microscopy (AFM).[11] These polymers self-assembled into two-dimensional (2D) helix bundles on substrates upon exposure to organic-solvent vapors. This 2D structure enabled the determination of the molecular packing, helical pitch, and handedness (right-or left-handed helix) by AFM. We also visualized successfully b...

Synthesis and structure determination of helical polymers

“…92 The AFM image of the stereocomplex (Figure 13c) prepared from a mixed monolayer of it-and st-PMMA chains spread on a water surface upon compression showed helix- Synthesis and structures of helical polymers E Yashima bundle-like structures, which can be further resolved into individual stripe-patterned chains with a chain-chain lateral spacing of B2.4 nm. 93 In the high-resolution zoomed image (Figure 13d), a number of periodic oblique stripes (green arrows) with a pitch of 0.92 nm can be observed, which are tilted either clockwise or counterclockwise along the stereocomplex main chain (pink lines) (Figure 13d). These results clearly indicate that the stereocomplex is certainly a supramolecular multistranded helical polymer.…”

The transformation of chiral signals into macroscopic properties of materials using chirality-responsive polymers

“…The authors subsequently proposed a triplestranded helix model to explain the poly(methyl methacrylate stereocomplex; their model was supported by direct evidence from AFM phase images. 49 Cheuk et al 50 have reported similar results, in which amphiphilic chains of a chiral polymer self-assembled into a variety of organizational morphologies, including twisting cables, spiral ribbons, spherical vesicles and helical nanotubes. Moreover, Goh et al 51 investigated the relationship between the concentration of a chiral dopant and the helical pitches of a nematic LC induced by the dopant.…”

Photo-induced helix–helix transition of a polystyrene derivative

“…AFM has been shown to be an effective tool in identifying and visualizing polymer helices. 2c, 19 In the case of helical polyisocyanides, 20 polyacetylenes 21 and polymethacrylates, 22 helical shapes of chains have been clearly observed. Fig.…”