Preservation of the Conformational Structures of Single-Polymer Crystals in Solution

Abstract

We have found that the cis-conformational structures in single-polymer crystals obtained by solid-state polymerization are retained in solution. A soluble alternating copolymer was accidentally obtained during our research employing a series of 7,7,8,8-tetrakis­(alkoxycarbonyl)-p-quinodimethanes (1) with alkoxy groups such as methoxy­(1a(Me)), ethoxy­(1b(Et)), propoxy­(1c(Pr)), isopropoxy­(1d(iPr)), butoxy­(1e(Bu)), isobutoxy­(1f(iBu)), pentyloxy­(1g(Pen)), hexyloxy­(1h(Hex)), and dodecyloxy­(1i(Dod)). Thus, 1a(Me), 1b(Et), 1c(Pr), 1e(Bu), and 1g(Pen) with linear alkoxy groups afforded 1:1 charge-transfer complex crystals with 7,7,8,8-tetracyanoquinodimethane (3), while 1d(iPr), 1f(iBu), 1h(Hex), and 1i(Dod) with branching or linear long-chain alkoxy groups did not. The former crystals topochemically underwent photochemical and thermal copolymerizations via a radical mechanism to yield cis-conformational alternating copolymer crystals according to X-ray crystallography. Attractively, the rates of thermal copolymerization were found to increase roughly with an increase in the chain length of the linear alkoxy groups and/or a decrease in the reacting exomethylene carbon distances between 1 and 3. This finding led us to carry out a spontaneous copolymerization of 1g(Pen) with 3 in solution, resulting in the first synthesis of a soluble alternating copolymer during our research. Spectral analyses of the soluble copolymer of 1g(Pen) with 3, together with geometry optimization and spectroscopic simulations, proved that the cis-conformational structure was retained in solution as well as in the solid state. This provides the first spectral observation of a cis-conformational structure in solution with respect to the alternating copolymers of the p-quinodimethane derivatives.

Elucidation of Substituent‐Responsive Reactivities via Hierarchical and Asymmetric Assemblies in Crystalline p‐Quinodimethane Derivatives

“…And also, 1 b (Et) polymerized in solution with a high monomer concentration and in the melt to afford corresponding homopolymers (poly‐ 1 b (Et)) with cis ‐conformation. These cis ‐conformations were confirmed by observation of higher magnetic field shifts for aromatic protons in 1 H NMR spectra and their DFT simulations [18,27] . These findings strongly suggest that p ‐quinodimethane derivatives may construct supramolecular 1D‐columnar assemblies even in solution and melt states, just like crystals.…”

“…Subsequently, any polymerizations begin in the supramolecular 1D columns spontaneously or with radical initiators to afford cis-conformational homopolymers [18,28] and alternating copolymers. [27] We think that such a polymerization mechanism involving supramolecular 1D columnar assemblies might be applicable for previously reported alternating copoly-…”

“…Such 1D assemblies are formed by stacking among the same monomers or alternatingly between acceptor and donor monomers. Subsequently, any polymerizations begin in the supramolecular 1D columns spontaneously or with radical initiators to afford cis ‐conformational homopolymers [18,28] and alternating copolymers [27] . We think that such a polymerization mechanism involving supramolecular 1D columnar assemblies might be applicable for previously reported alternating copolymerization systems between vinyl‐type acceptor monomers and vinyl‐type donor monomers.…”

“…These cis-conformations were confirmed by observation of higher magnetic field shifts for aromatic protons in 1 H NMR spectra and their DFT simulations. [18,27] These findings strongly suggest that p-quinodimethane derivatives may construct supramolecular 1D-columnar assemblies even in solution and melt states, just like crystals.…”