Reversible Cross-Linking of Ketone-Functionalized Polyethylenes through the Formation of Acetals

Abstract

The recycling of conventional cross-linked polyethylene (XLPE) is hampered by the irreversible nature of cross-link formation. In this work, a ketone-functionalized polyethylene (kf-PE) was reversibly cross-linked through the formation of acetal bridges by using different polyol cross-linkers. The acid-catalyzed acetalization reaction was optimized using two different reaction systems in which water was removed either by azeotropic distillation or through the hydrolysis of an orthoester. para-Toluenesulfonic acid (pTSA) and Bi(OTf)₃ stood out as the most promising acid catalysts. Rheological characterization confirmed the thermoset-like behavior of the cross-linked materials, while solid-state NMR experiments demonstrated that the cross-link density further increased upon an additional heating step. The latter experiments indicate that the acetal cross-links are dynamic and can rearrange at elevated temperature. In this way, cross-linked materials with gel contents up to 80% were obtained. Finally, it was shown that the acetal bridges can be readily removed again through acid-catalyzed hydrolysis, which allows recycling of the original kf-PE.