# Transient Salt-Bridge-Based Supramolecular Polymers: Experiments and Theory

## TL;DR

This paper describes a transient supramolecular polymer formed through salt bridges and hydrogen bonds, which disassembles over time due to decarboxylation.

## Contribution

The study introduces a novel transient supramolecular polymer using activated carboxylic acids as both structural components and stimuli.

## Key findings

- The polymer forms through salt bridges between ammonium and carboxylate groups in solution.
- The degree of polymerization increases with monomer concentration.
- DOSY spectra confirm the formation and disassembly of the polymer over time.

## Abstract

The smooth decarboxylation under basic conditions of
activated
carboxylic acids (ACAs) is exploited to achieve a transient supramolecular
polymer based on hydrogen bonds reinforced by electrostatic interactions.
In particular, it is proved that when the aliphatic α,ω-diamine 3, namely, 1,8-diamino-3,6-dioxaoctane, reacts with an equimolar
amount of the activated dicarboxylic acid 1H2, i.e., a difunctional derivative of 2-cyano-2-phenylpropanoic
acid, a supramolecular polymer of the kind −ABBAAB–
is immediately formed in chloroform solution. The AA and BB
monomers are held together by salt bridges (hydrogen bonds reinforced
by electrostatic interactions) between ammonium and carboxylate functions.
The larger the concentration of the added materials, the higher the
polymerization degree (DP) of the polymer. Under the given experimental
protocol, such a polymer disaggregates over time due to decarboxylation,
and at the end of the process, only diamine 3 and waste
product 4, which cannot interact with one another anymore,
remain in the solutions. DOSY spectra recorded at different reaction
times definitely demonstrate the phenomenology described above. The
trend of the degree of polymerization as a function of monomer concentration
has been clarified in the light of the ring–chain equilibrium
theory. The application of the theory enables the accurate evaluation
of the distribution of linear and cyclic oligomers as well as the
critical concentration, c
crit, above which
polymerization rapidly becomes more extensive due to the saturation
of macrocyclic species. Notably, the ACA is not used just as a stimulus
for a dissipative system, but as one of its structural components.