Vitrimers are a new class of cross-linked materials that are capable of network topology alternation through the associative dynamic bond-exchange mechanism, which has recently been invented to solve the problem of conventional cross-linked materials, such as poor recyclability and healability. Thus far, the concept of vitrimers has been applied to various commercial polymers, e.g., polyesters, polylactides, polycarbonates, polydimethylsiloxanes, polydienes, polyurethanes, polyolefins, poly(meth)acrylates, and polystyrenes, by utilizing different compatible bond-exchange reactions. In this review article, the concept of vitrimers is described by clarifying the difference from thermoplastics and supramolecular systems; in addition, the term “associative bond-exchange” in vitrimers is explained by comparison with the “dissociative” term. Several useful functions attained by the vitrimer concept (including recyclability and healability) are demonstrated, and recent molecular designs of vitrimers are classified into groups depending on the types of molecular frameworks. This review specifically focuses on the vitrimer molecular designs with commercial polymer-based frameworks, which provide useful hints for the practical application of the vitrimer concept.

In this research, we demonstrate the preparation and unique functions of elastomeric polyester vitrimer materials with different cross-link densities based on a post-polymerization cross-linking method. First, we synthesize polyesters with thiol (SH) groups at the side groups via melt polycondensation of pentanediol, adipic acid, and thiomalic acid. The SH-attached polyester is reacted with acrylic acid through the Michael addition reaction between the SH side groups of the polyester and the vinyl groups of acrylic acid, providing viscous amorphous polyesters with multiple COOH side groups (PE-COOH). PE-COOH polymers are cross-linked through the reaction with a diepoxy cross-linker (1,4-butanediol diglycidyl ether) in the presence of a trans-esterification catalyst Zn(OAc)₂ at 120 °C. This cross-linking via the COOH–epoxy reaction generates free OH groups in the network, which act as the carriers for trans-esterification with ester bonds in the polyester strands. In this design, the COOH group equivalent weight (M_COOH) in PE-COOH is easily tuned by the initial polycondensation feed ratio between adipic acid and thiomalic acid. This also enables the tuning of the cross-link density and the fraction of free OH groups in the final vitrimer materials as reflected in the tensile and swelling properties. The representative vitrimeric properties, including the softening temperature (T_soft) and stress-relaxation at high temperature, are all affected by the cross-link density (i.e., M_COOH in PE-COOH). Interestingly, a lower T_soft and a shorter relaxation time were attained for the sample with a larger cross-link density. This is because the sample with the larger cross-link density possesses a larger fraction of the bond-exchange carriers, i.e., free OH groups, in the present system, which enables the activation of trans-esterification based bond-exchange with lower temperature and higher frequency.

The article covers the outstanding features of fiber-reinforced vitrimer composites, including their reprocessing, recycling and self-healing properties.

We demonstrate the functionalization of ABA triblock copolymer-based materials by introducing dynamic covalent bonded cross-links. The bond-exchange via trans-N-alkylation was operated in the domains of poly(4-vinylpyridine) end blocks, cross-linked by quaternization with dibromo cross-linkers, thus exhibiting vitrimer-like properties (e.g., softening and great stress relaxation) and other useful functions, such as re-processability, recyclability, and self-adhesion.