Boronic Acid Esters and Anhydrates as Dynamic Cross-Links in Vitrimers

Abstract

Growing environmental awareness imposes on polymer scientists the development of novel materials that show a longer lifetime and that can be easily recycled. These challenges were largely met by vitrimers, a new class of polymers that merges properties of thermoplastics and thermosets. This is achieved by the incorporation of dynamic covalent bonds into the polymer structure, which provides high stability at the service temperature, but enables the processing at elevated temperatures. Numerous types of dynamic covalent bonds have been utilized for the synthesis of vitrimers. Amongst them, boronic acid-based linkages, namely boronic acid esters and boroxines, are distinguished by their quick exchange kinetics and the possibility of easy application in various polymer systems, from commercial thermoplastics to low molecular weight thermosetting resins. This review covers the development of dynamic cross-links. This review is aimed at providing the state of the art in the utilization of boronic species for the synthesis of covalent adaptable networks. We mainly focus on the synthetic aspects of boronic linkages-based vitrimers construction. Finally, the challenges and future perspectives are provided.

N-Coordinated Organoboron in Polymer Synthesis and Material Science

“…The excellent biocompatibility and high transesterification efficiency make boronic acid/ester polymers have an outstanding research value and potential in reprocessable and healable materials. , However, the boronic acid/ester covalent network inevitably faces two flaws: (1) both boronate ester and boroxine are easily hydrolyzed and (2) the lower determined activation energy ( E a ) of the boronate ester transesterification reaction affects the rigidity of materials. Therefore, researchers hope to coordinate with the boron atom by introducing a Lewis acid–base pair into the polymer to improve the reliability of the boronate ester.…”

Impact absorption in shear-stiffening materials

“…Boronic acids have gained a lot of attention in the past few years in the field of supramolecular chemistry due to the unique reversibility of the B-O covalent bond 44,152,153 . In the presence of diol boronic acid could form an ester upon dehydration; this reaction is fully reversible and can be dynamic when in equilibria.…”

“…Upon dehydration, in the absence of diols, boronic acids can yield cyclic trimers called boroxines 44,156 (Figure 2.18, b). There are two commonly known pathways of boroxine formation in a dry solvents/solventless environmentvia direct dehydration or Lewis base ligand facilitated dehydration 44,152,156 . Boroxines can also be broken down/reformed in a dissociative fashion through hydrolysis/dehydration.…”

“…Boric acid is a mild Lewis acid and, therefore, is able to change its hybridization from sp 2 to sp 3 in the presence of Lewis base. Upon this change, BA undergoes the transition from trigonal to tetragonal state where it can form a fourth bond (dative) with the atom donating electrons (such as oxygen or nitrogen) 44,45,152 . The ability of boron forming dative bonds with oxygen was proposed as another explanation of dynamic crosslinking in PBS alongside H-bonding.…”

“…Several mechanisms of dynamic crosslinking were proposed during these 70 years of research, such as hydrogen bonding between borono end-groups 28,126,127 , dative bonding between boron and oxygen 42,43,143 , boroxine formation, and oxygen ligands exchange 29 . In parallel, a whole new brunch of boron chemistry research has been developed, which brought new insight into the possible mechanism of boron reversible crosslinking in polymers 44,152,156 . Some of the most elaborated mechanisms are the reversible formation of boronic esters and boroxines.…”