Fully Recyclable and Remarkably Robust Cross-Linked Polyethylene Networks via Direct Free-Radical Copolymerization with Disulfide Dynamic Covalent Bonds
Logan M. Fenimore, Mathew J. Suazo, Sarah Mitchell, Hadi Mohammadi, Kimberly Miller McLoughlin, Cassandra Gallaschun, Patryk Sewruk, Markus Busch, John M. Torkelson
Abstract
Cross-linked polyethylene (XLPE) is a commercially important polyolefin thermoset that suffers from nonrecyclability 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 XLPE. 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 XLPE.