Chemically and Mechanically Recyclable Polyolefins Incorporating Covalent Adaptable Networks

We report on the synthesis and characterization of mechanically and chemically recyclable polypropylene-based crosslinked polyolefins (CR-POs) incorporating ester-containing covalent adaptable networks (CANs). To obtain the material, polypropylene (PP, Mn ≈ 3900 g/mol) modified with maleic anhydride (MA) is crosslinked via epoxy-anhydride polymerization using 1,4-butanediol diglycidyl ether (BDE) in the presence of ZnCl₂ catalyst. Variation of the anhydride-to-epoxy (A/E) ratio shows that A/E ≈ 0.5 is the most effective formulation. The resulting crosslinked networks have a gel-fraction of ∼75% yet remain melt-processable through extrusion and compression molding due to CANs. CR-POs are semicrystalline materials with Tm ≈ 140°C and crystallinity of ∼25%. The first-generation material's flexural modulus (520 MPa) and stress at 5% strain (22 MPa) were intermediate between HDPE and LDPE. The CR-PO materials can be mechanically reprocessed for at least five compression-molding cycles without significant degradation of thermal or mechanical properties. CR-POs can be depolymerized via transesterification to a low-molecular-weight polyolefin and repolymerized using BDE after modification with MA. The second-generation chemically recycled networks exhibited a flexural modulus of 390 MPa and stress at 5% strain of 13 MPa. This work shows a practical route to mechanically and chemically recyclable polyolefin thermosets, advancing circular design for polyolefins.