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.

我们发现，通过固态聚合（solid-state polymerization）制备的单聚合物晶体中的顺式构象（cis-conformational）结构在溶液中得以保留。本研究在使用一系列带有不同烷氧基的7,7,8,8-四(烷氧羰基)-对-苯醌二甲烷（7,7,8,8-tetrakis(alkoxycarbonyl)-p-quinodimethanes，编号1）开展实验时，意外获得了一种可溶性交替共聚物；所用烷氧基包括甲氧基（1a(Me)）、乙氧基（1b(Et)）、丙氧基（1c(Pr)）、异丙氧基（1d(iPr)）、丁氧基（1e(Bu)）、异丁氧基（1f(iBu)）、戊氧基（1g(Pen)）、己氧基（1h(Hex)）及十二烷氧基（1i(Dod)）。其中，带有直链烷氧基的1a(Me)、1b(Et)、1c(Pr)、1e(Bu)及1g(Pen)可与7,7,8,8-四氰基对苯醌二甲烷（7,7,8,8-tetracyanoquinodimethane，3）形成1:1的电荷转移络合物（charge-transfer complex）晶体，而带有支链或长直链烷氧基的1d(iPr)、1f(iBu)、1h(Hex)及1i(Dod)则无法形成此类络合物。经X射线晶体学（X-ray crystallography）表征证实，前者的晶体可通过自由基机理（radical mechanism）发生拓扑化学过程介导的光聚合与热聚合反应，生成顺式构象交替共聚物晶体。值得注意的是，热聚合反应速率大致随直链烷氧基链长的增长以及1与3之间反应性亚甲基碳间距的缩短而提升。基于这一发现，我们开展了1g(Pen)与3在溶液中的自发共聚反应，首次在本研究中合成出可溶性交替共聚物。对1g(Pen)与3形成的可溶性共聚物进行光谱分析，并结合几何构型优化（geometry optimization）与光谱模拟（spectroscopic simulations），证实其顺式构象结构在溶液与固态中均得以保留。这也是首次在溶液中通过光谱观测到对-苯醌二甲烷衍生物类交替共聚物的顺式构象结构。