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

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.