Propylene-Ethylene Copolymer Covalent Adaptable Networks Synthesized by Resonance-Stabilized, Radical-Based Reactive Processing with Excellent Elevated-Temperature Creep Resistance

Propylene–ethylene copolymer (PEC) elastomers with low crystallinity are prone to creep in both the melt and semicrystalline states. To enhance their creep resistance while retaining reprocessability, we introduced dynamic covalent cross-links via one-step, radical-based reactive processing to form PEC covalent adaptable networks (CANs). During reactive processing, it is essential to suppress the β-scission pathway associated with propylene-repeat-unit-derived macroradicals. We achieved this by promoting the formation of stabilized macroradical intermediates through resonance stabilization. We enabled such stabilization by replacing a methacrylate-based dynamic cross-linker, bis(4-methacryloyloxyphenyl) disulfide (BiPheS methacrylate, BPMA) with a phenylacrylate-based cross-linker, bis(4-phenacryloyloxyphenyl) disulfide (BiPheS styrene, BPST) as well as by incorporating vinyl aromatic additives such as styrene (ST) and divinylbenzene (DVB) with the dynamic cross-linker. We found that, in the absence of vinyl aromatic additives, the use of BPST (but not BPMA) resulted in the formation of percolated PEC CANs, while the addition of one (ST) or two (ST and DVB) vinyl aromatic additives significantly reduced the disparity in cross-linking capability between BPMA and BPST by compensating for differences in resonance stabilization. The resulting PEC CANs exhibited markedly improved creep resistance at elevated temperatures compared to neat PEC. Notably, relative to the thermoplastic PEC elastomer from which the PEC CANs were made, the best-performing PEC CAN made by BPST and a combination of ST and DVB suppressed >99% of viscous creep at 160 °C (melt state) over 600 s and >98% at 100 °C (semicrystalline state) over 10,000 s. This best-performing PEC CAN was reprocessable via both compression molding and twin-screw extrusion, with full recovery of cross-link density and tensile properties within experimental uncertainty. These results demonstrate a promising one-step strategy to produce reprocessable PEC CANs with enhanced creep resistance in both melt and semicrystalline states, addressing key limitations of low-crystallinity polyolefins.