Covalent adaptable networks (CANs) are a promising type of polymers that combine the strength of thermosets, as a result of their crosslinked structure, with recyclability as a result of molecular bond-exchange reactions. The molecular dynamics of these bond-exchange reactions are very dependent on the composition of the polymer network. In this contribution, we demonstrate that the well-known physical-organic concept defined by the Hammett equation, which operates on the molecular level, can also be exploited to achieve quantitatively predictable and delicate chemical control over mechanical properties of vitrimer(-like) materials on the macroscopic level. That is, we present a precise way to tune the dynamic mechanical and thermal properties of dynamic covalent polyimines via the electronic effect of dianiline monomers based on the Hammett equation. This allows for a direct and quantitative correlation of a simple, molecule-based physical parameter (i.e. the monomer’s Hammett parameter, σ) to a range of macroscopic material properties: firstly, the linear correlation of the kinetic activation energy (E_a) for the imine exchange to the σ value, enabled us to tune the E_a from 16 to 85 kJ mol^–1. Furthermore, the creep behaviour (γ), glass transition (T_g) and the topology freezing transition temperature (T_v), all showed a strong, often linear, dependence on the σ value of the dianiline monomer. These combined results demonstrate how dynamic material properties can be directly tuned and designed in a quantitative –and therefore predictable– manner through correlations based on the Hammett equation. Moreover, the dynamic nature of the imine bond enabled not only recycling, but also intrinsic self-healing of the materials over multiple cycles without the need for solvent, catalysts or addition of external chemicals. Finally, the effect of polarity within the polymeric structure on the efficiency of imine exchange will be presented as a further molecular handle to control material properties.

Molecular Control over Vitrimer-like Mechanics

Han Zuilhof

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