Tuning the Length of Cooperative Supramolecular Polymers under Thermodynamic Control

Abstract

In the field of supramolecular (co)polymerizations, the ability to predict and control the composition and length of the supramolecular (co)polymers is a topic of great interest. In this work, we elucidate the mechanism that controls the polymer length in a two-component cooperative supramolecular polymerization and unveil the role of the second component in the system. We focus on the supramolecular copolymerization between two derivatives of benzene-1,3,5-tricarboxamide (BTA) monomers: a-BTA and Nle-BTA. As a single component, a-BTA cooperatively polymerizes into long supramolecular polymers, whereas Nle-BTA only forms dimers. By mixing a-BTA and Nle-BTA in different ratios, two-component systems are obtained, which are analyzed in-depth by combining spectroscopy and light-scattering techniques with theoretical modeling. The results show that the length of the supramolecular polymers formed by a-BTA is controlled by competitive sequestration of a-BTA monomers by Nle-BTA, while the obvious alternative Nle-BTA acts as a chain-capper is not operative. This sequestration of a-BTA leads to short, stable species coexisting with long cooperative aggregates. The analysis of the experimental data by theoretical modeling elucidates the thermodynamic parameters of the copolymerization, the distributions of the various species, and the composition and length of the supramolecular polymers at various mixing ratios of a-BTA and Nle-BTA. Moreover, the model was used to generalize our results and to predict the impact of adding a chain-capper or a competitor on the length of the cooperative supramolecular polymers under thermodynamic control. Overall, this work unveils comprehensive guidelines to master the nature of supramolecular (co)polymers and brings the field one step closer to applications.

Scheme 1. Chemical Structures of Benzene-1,3,5-tricarboxamides a-BTA and S-Nle-BTA (A); Schematic Molecular Structure of the Helical Stack Formed by a-BTA (B) and the Dimeric Hydrogen-Bonded Structure Formed by S-Nle-BTA (C)

Figure 1

(A) CD spectra of the mixture a-BTA and S-Nle-BTA at c_tot = 50 μM in MCH at 20 °C. (B–C) Temperature-dependent UV (B) and CD (C) spectra of solutions containing different ratios of S-Nle-BTA/a-BTA probed at λ_max with c_tot = 50 μM in MCH (cooling rate = 2 K·min^–1). (D) Normalized CD and UV cooling curves probed at λ_max with c_tot = 50 μM in MCH at the ratio of S-Nle-BTA/a-BTA 1/9.

Figure 2

CD spectra at the ratio of S-Nle-BTA/a-BTA 1/9 (A) and 3/1 (B) at c_tot = 50 μM in MCH at different temperatures.

Figure 3

(A) CD cooling curves calculated with the intercalation model containing the competitive formation of dimers/trimers at different ratios of S-Nle-BTA/a-BTA. (B–F) Calculated concentrations of BTAs in the various species types (P helical aggregate, M helical aggregate, dimers and trimer) and concentration of free monomers as a function of the temperature for several fractions of sergeants S-Nle-BTA 4% (B), 10% (C), 25% (D), 50% (E), and 75% (F) at c_tot = 50 μM (in the legend, A is a-BTA and Nle is S-Nle-BTA).

Figure 4

(A) Normalized calculated mean polymer length (number of monomers per stack) as a function of the S-Nle-BTA content predicted with the competitor model (red trace) and the chain-capper model (black trace). (B) Measured weight-average length of the supramolecular polymer as a function of the S-Nle-BTA content (red trace) compared with the corresponding dilution (black trace) determined via SLS at c_tot = 0.5 mM in MCH at 20 °C.

Scheme 2. Schematic Representation of the Supramolecular Copolymers Formed by a-BTA and S-Nle-BTA at Room Temperature as a Function of the Composition

For clarity, only the predominant species are represented.

Figure 5

Calculated weight-average mean length of the cooperative P-polymers over the energy difference ΔH_A/B^diff with the addition of 10% (A) and 25% (B) of a chain-capper (red trace) and a competitor forming dimers and trimers (green trace) at 293 K, c_tot = 50 μM.