Linear Supramolecular Polymers Driven by Anion–Anion Dimerization of Difunctional Phosphonate Monomers Inside Cyanostar Macrocycles

Supramolecular
polymers have enabled far-reaching fundamental science
and the development of diverse macromolecular technologies owing to
the reversible and noncovalent chemical connectivities that define
their properties. Despite the unabated development of these materials
using highly tailorable recognition elements, anion-based polymers
remain rare as a result of the weak interactions they mediate. Here,
we use design rules inspired by cation-driven polymers to demonstrate
a new noncovalent link based on receptor-stabilized anion–anion
interactions that enables the efficient linear polymerization of simple
difunctional phosphonates. The linear main chain connectivity and
molecular topology were confirmed by single crystal X-ray diffraction,
which demonstrates the rare 2:2 stoichiometry between the anionic
phosphonate end groups and a pair of π-stacked cyanostar macrocycles.
The stability of these links enables rapid polymerization of difunctional
phosphonates employing different aliphatic linkers (C6H12, C8H16, C10H20, C12H24). Diphosphonates with greater chain
flexibility (C12H24) enable greater polymerization
with an average degree of polymerization of nine emerging at 10 mM.
Viscosity measurements show a transition from oligomers to polymers
at the critical polymerization concentration of 5 mM. In a rare correlation,
NMR spectroscopy shows a coincident molecular signature of the polymerization
at 5 mM. These polymers are highly concentration dependent, reversibly
polymerize with acid and base, and respond to competitive anions.
They display the design simplicity of metallo-supramolecular polymers
with transfer of the strong 2:2 recognition chemistry to macromolecules.
The simplicity and understanding of this new class of supramolecular
polymer is anticipated to open opportunities in tailoring anion-based
functional materials.