Fully Recyclable and Remarkably Robust Cross-linked Polyethylene Networks via Direct Free-Radical Copolymerization with Disulfide Dynamic Covalent Bonds

Cross-linked polyethylene (PEX) is a commercially important polyolefin thermoset that suffers from non-recyclability due to its permanent cross-links. Herein, we report the first in-reactor synthesis of dynamic covalently cross-linked polyethylene or polyethylene covalent adaptable networks (PE CANs) via free-radical copolymerization of ethylene and bis(2,2,6,6-tetramethyl-4-piperidyl methacrylate) disulfide (BTMA). BTMA is a bifunctional comonomer containing a dissociative dynamic disulfide bond. These PE CANs exhibit high crystallinity, outstanding dimensional stability, high-temperature creep resistance, and robust thermomechanical properties comparable to commercial PEX. Crucially, our PE CANs demonstrate full recovery of cross-link density and thermomechanical performance after recycling. Stress relaxation studies reveal viscoelastic behavior enabled by cross-link exchange but dominated by polymer chain reptation, resulting in activation energies aligned with those of long-chain-branched PE melts. This work presents a novel, scalable method for free-radical copolymerization of recyclable PE CANs, offering a more sustainable alternative to PEX.