Single-Crystal Polycationic Polymers Obtained by Single-Crystal-to-Single-Crystal Photopolymerization

The
efficient preparation of single-crystalline ionic polymers
and fundamental understanding of their structure–property relationships
at the molecular level remains a challenge in chemistry and materials
science. Here, we describe the single-crystal structure of a highly
ordered polycationic polymer (polyelectrolyte) and its proton conductivity.
The polyelectrolyte single crystals can be prepared on a gram-scale
in quantitative yield, by taking advantage of an ultraviolet/sunlight-induced
topochemical polymerization, from a tricationic monomera self-complementary
building block possessing a preorganized conformation. A single-crystal-to-single-crystal
photopolymerization was revealed unambiguously by in situ single-crystal
X-ray diffraction analysis, which was also employed to follow the
progression of molecular structure from the monomer, to a partially
polymerized intermediate, and, finally, to the polymer itself. Collinear
polymer chains are held together tightly by multiple Coulombic interactions
involving counterions to form two-dimensional lamellar sheets (1 nm
in height) with sub-nanometer pores (5 Å). The polymer is extremely
stable under 254 nm light irradiation and high temperature (above
500 K). The extraordinary mechanical strength and environmental stabilityin
combination with its impressive proton conductivity (∼3 ×
10–4 S cm–1)endow the
polymer with potential applications as a robust proton-conducting
material. By marrying supramolecular chemistry with macromolecular
science, the outcome represents a major step toward the controlled
synthesis of single-crystalline polyelectrolyte materials with perfect
tacticity.