Thermodynamics and Reaction Kinetics of Symmetric Vitrimers Based on Dioxaborolane Metathesis

As a new class of environment-friendly materials, vitrimers have attracted growing attention, but their thermodynamics and reaction kinetics are yet to be better understood. In this study, we examine the thermodynamics and reaction kinetics of symmetric vitrimers that contain two types of precursors, poly­(hexyl methacrylate-random-2,3-dioxaborolanepropyl methacrylate) and poly­(hexyl methacrylate-random-4-vinylphenyl-2,3-dioxaborolanepropyl), which can react with each other through dioxaborolane metathesis reaction to form a vitrimer network. The plateau modulus of the vitrimer samples reflects a degree of cross-linking reaction, while the relaxation of the plateau is controlled by the kinetics of the exchange reaction. For each precursor, we prepared a series of samples with similar degrees of polymerization but different average numbers of functional groups per chain, designated as F A and F B, respectively. When F B is fixed, the plateau modulus increases with increasing F A and saturates for sufficiently large F A, while the relaxation rate of the network first decelerates with cross-linking and then accelerates when F A becomes sufficiently high. A similar trend is observed when changing F B at fixed F A due to the symmetric nature of our model system. The saturation of the modulus can be explained by considering the reaction equilibrium established between the A and B groups, and the transition of the relaxation rate can be explained by considering that the reaction rate constant between the cross-linking sites is much lower (∼2 orders lower) than that between one cross-linking site and one dangling functional group.