Commodity Rubber Material with Reversible Cross-linking Ability: Application of Boroxine Cross-links to Ethylene-Propylene Rubber

Ethylene-propylene rubber (EPM) efficiently and reversibly cross-linked by boronic acid functionalities was developed. The introduction of boronic acids into EPM was achieved by the direct terpolymerization of alkenylboronic acid comonomers masked with 1,8-diaminonaphthalene (dan) with ethylene and propylene. Cross-linking by thermal dehydration-condensation of boronic acid can significantly proceed above 80 degrees C, indicated by the IR and NMR spectroscopy and viscoelastic measurements of the terpolymer immediately after isolation. An efficient cross-linking behavior is indicated by the swelling test and the mechanical property of the molded sample. Generally, boron-cross-linked EPM showed higher tensile strength than sulfur-vulcanized ethylene-propylene-diene rubber (EPDM) with a higher amount of cross-linking reagent. An efficient de-cross-linking of the boron moiety in the terpolymer was achieved by alcoholysis using a chelating reagent like N-methyl diethanolamine. The completion of the de-cross-linking reaction led to the terpolymer being soluble in toluene, showing that most boroxine cross-linking points were cleaved in a short time. The novel rubber with reversible cross-linking behavior would be a candidate alternative to the conventional vulcanized rubber, enabling the development of a closed-loop process in material recycling.