Developing Polypropylene Covalent Adaptable Networks with Complete Cross-Link Density Recovery after Reprocessing by Free-Radical Reactive Processing with Vinyl Aromatic Additives

Free-radical reactive processing of thermoplastic polyethylene or ethylene-containing copolymers with a dynamic covalent cross-linker enables the synthesis of covalent adaptable networks (CANs). While effective for polyethylene, this approach is hindered in polypropylene (PP) due to the propensity of tertiary carbon radicals in PP to undergo β-scission during grafting. We have developed PP-based CANs via one-step, radical-based reactive processing using dicumyl peroxide (a radical initiator), bis(4-methacryloyloxyphenyl) disulfide (BPMA, an aromatic disulfide-based dynamic covalent cross-linker), and, to stabilize the radicals and promote cross-linking, vinyl aromatic additives. Adding 2-vinylnaphthalene (VN) at 2.0 or 4.0 molar equivalents to BPMA effectively suppressed β-scission in PP and enabled robust CAN formation. Divinylbenzene (DVB) at 0.5 molar equivalent to BPMA also enabled PP CAN formation, but due to its additional function as a permanent cross-linker, further increases in DVB level led to the percolation of permanent cross-links and loss of reprocessability. Relative to PP, all PP CANs exhibited significant melt-state creep suppression; the best creep resistance (and highest cross-link density) exhibited by a PP CAN, a factor of 40 better than that of PP, was prepared using a combination of 2.0 molar equivalents of VN and 0.5 molar equivalent of DVB relative to BPMA. Notably, each PP CAN exhibited complete recovery of cross-link density after two reprocessing cycles. Thus, this study represents a successful, one-step, additive-based approach for making robust and reprocessable PP CANs.