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

Abstract

A single-step method that produces percolated, dynamic covalent cross-links integrated into the 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 phenyl acrylate-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 phenyl acrylate-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 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.