Modulation of the cooperativity in the assembly of multistranded supramolecular polymer

Abstract

[eng] It is highly desirable that supramolecular polymers self-assemble following small changes in theenvironment. The degree of responsiveness depends on the degree of cooperativity at play during the assembly. Understanding how to modulate and quantify cooperativity is therefore highly desirable for the study and design of responsive polymers. Here we show that the cooperative assembly of a porphyrin-based, double-stranded polymer is triggered by changes in building blocks and in salt concentration. We develop a model that accounts for this responsiveness by assuming the binding of the salt countercations to the double-stranded polymer. Using our assembly model we generate plots that show the increase in concentration of polymer versus the normalized concentration of monomer. These plots are ideally suited to appreciate changes in cooperativity, and show that, for our system, these changes are consistent with the increase in polymer length observed experimentally. Unexpectedly, wefind that polymer stability increases when cooperativity decreases. We attribute this behaviour to the fact that increasing salt concentration stabilizes the overall polymer more than the nucleus. In other words, the cooperativity factor a, defined as the ratio between the growth constant Kg and the nucleation constant Kn decreases as the overall stability of the polymer increases...