Methoxy-Group Control of Helical Pitch in Stereoregular Poly(2-ethynylmethoxynaphthalene) Prepared by Rhodium Complex Catalyst.

Yasuteru Mawatari,Kai Huang,Yoshiaki Yoshida,Masayoshi Tabata

doi:10.3390/polym11010094

Abstract

The position of the methoxy group in a poly(n-methoxy-2-ethynylnaphthalene) (PnMeO2EN) was found to control the helical pitch of the π-conjugated polymer in the solid state. These PnMeO2ENs were stereoregularly synthesized using an Rh-complex catalyst in ethanol or toluene as the solvent. The helical structure in the solid phase was confirmed by conventional analytical methods, namely diffuse reflective ultraviolet–visible light (UV–Vis) and Raman spectroscopies, X-ray diffraction, and 13C cross-polarization magic angle spinning NMR spectroscopy, together with molecular mechanics calculations, because the as obtained polymers were insoluble in common solvents. The color of poly(6-methoxy-2-ethynylnaphthalene) (P6MeO2EN) (yellow or red) depended on the polymerization solvent, whereas no such dependency was observed for the yellow-colored P7MeO2EN and P8MeO2EN. The helical structures energetically optimized by molecular mechanics indicate that the red- and yellow-colored P6MeO2ENs form contracted and stretched helices, respectively. Due to the relatively unconstrained rotations of the 6-methoxynaphthyl moieties, the methoxy groups in P6MeO2EN are less sterically hindered along the helical axis. On the contrary, P7MeO2EN and P8MeO2EN have stretched helices due to the considerable steric hindrance imparted by their methoxy groups. The thermal cis-to-trans isomerization of P6MeO2EN in the contracted-helix form required a somewhat higher temperature than that of the stretched helix.

Highlights

The unique shape and functionality of helical-structured polymers and supramolecules make them the object of keen research interest
We reported the selective preparation of Poly(phenylacetylene) derivatives (PPAs) and its derivatives with different helical pitches, i.e., with stretched and contracted helices, and described the drastic color changes accompanying the stretched-to-contracted transition [22,23,24,25]
Rh-catalyzed polymerization of ortho-substituted phenylacetylenes to afford PPAs is known to induce partial cis-to-trans isomerization and, result in helical structure destruction [39], since 1-ethynylnaphthalene (1EN) monomers can be regarded as equivalents of ortho-substituted phenylacetylene monomers [40,41]

Summary

Introduction

The unique shape and functionality of helical-structured polymers and supramolecules make them the object of keen research interest. Numerous reports and reviews recently focused on the helix sense of such helical structures, describing methods for inducing/confirming the formation of one-handed helices, dynamic helix inversion caused by external stimuli, and sergeant/soldier roles for Polymers 2019, 11, 94; doi:10.3390/polym11010094 www.mdpi.com/journal/polymers. Poly(phenylacetylene) derivatives (PPAs) are popular π-conjugated helical polymers that attracted considerable attention in the last two decades [6,7,8,9,10,11,12,13,14,15]. We reported the selective preparation of PPA and its derivatives with different helical pitches, i.e., with stretched and contracted helices, and described the drastic color changes accompanying the stretched-to-contracted transition [22,23,24,25]. The above transition was shown to smoothly occur upon heat treatment [22] and contact with appropriate solvents [24,25]

Methods

Results

Conclusion