Polypropylene covalent adaptable networks with full cross-link density recovery after reprocessing: Development by free-radical reactive processing with resonance-stabilized, aromatic disulfide cross-linkers

A single-step method that produces percolated, dynamic covalent cross-links integrated into PP homopolymer has not been previously demonstrated. Here, we synthesized covalent adaptable networks (CANs) from polypropylene (PP) homopolymers using 180 °C, radical-based, reactive processing with a free-radical initiator, dicumyl peroxide (DCP), and resonance-stabilized, aromatic disulfide cross-linkers, one methacrylate-based and another phenylacrylate-based. Both cross-linkers yielded networks when reactively processed at 4 wt% with relatively high molecular weight (MW) PP (melt flow index (MFI) = 12) and 4 wt% DCP. The phenylacrylate-based cross-linker also yielded PP networks at other studied DCP/cross-linker concentrations and with relatively low MW PP (MFI = 35). Notably, our highest cross-link density PP CAN exhibited full recovery of cross-link density and tensile properties after three reprocessing steps by compression molding; that PP CAN also exhibited full cross-link density recovery within experimental uncertainty after reprocessing by melt extrusion.