Conventionally cross-linked polymer networks known as thermosets contain permanent cross-links which prevent their recyclability, leading to major sustainability and environmental challenges. To overcome this problem, covalent adaptable networks (CANs) containing...

Reprocessable polymer networks with dynamic covalent bonds exhibit thermoplastic-like properties at elevated processing temperatures while maintaining thermoset responses under service conditions, offering a sustainable solution to the recycling of conventional, permanently cross-linked polymers. Most studies on reprocessable networks that report full cross-link density recovery after recycling have focused on step-growth polymer networks; no study has previously reported full cross-link density recovery of reprocessable networks prepared directly from only monomers via addition polymerization. Here, we report the utilization of dialkylamino disulfide chemistry as a fast, robust dynamic chemistry in the synthesis of reprocessable networks from monomers and/or polymers with carbon−carbon double bonds that are amenable to free-radical polymerization. In particular, we have employed a simple one-step method to design a bifunctional bis(dialkylamino) disulfide cross-linker. With this dynamic cross-linker, we synthesized a catalystfree, reprocessable polymethacrylate network that exhibits full property recovery (within error) associated with cross-link density after multiple reprocessing steps. This achievement could allow for the facile development of chemically recyclable versions of common, commercially important addition-type polymer networks.

Thermosets and thermoset composites constitute an extraordinary challenge for recycling and participation in a circular economy because their permanent covalent cross-links prevent spent thermosets from being melt processed into new products. With annual world-wide production in the tens of billions of kilograms, the inability to recycle thermosets into high-value products represents major economic and sustainability losses. While recent research into polymer networks with dynamic covalent cross-links has indicated promise for reprocessability at common melt-state processing temperatures, a crucial shortcoming has been identified: such reprocessable networks and network composites commonly exhibit creep at use conditions due to their dynamic nature, which may prevent their use in applications that require long-term dimensional stability. Here, we use a strategy based on nitroxide-mediated polymerization to synthesize reprocessable networks and network composites containing alkoxyamine dynamic bonds. The resulting networks, including those synthesized from lab-grade polybutadiene and industrial-grade natural rubber/carbon black composites, exhibit full cross-link density recovery and essentially no creep at 80 °C, where alkoxyamine cross-links are nearly static, after multiple molding cycles at 140/160 °C, where alkoxyamine cross-links are dynamic. This capability to "turn on" and "arrest" dynamic chemistry over a relatively narrow temperature window is attributed to the high activation energy (∼120 kJ/mol) and thus strong temperature dependence of the alkoxyamine dissociation reaction. With this key element of high activation energy for the dissociation reaction in systems undergoing dynamic reversion or the dynamic exchange reaction in vitrimers, it is possible to design covalent network materials with acute temperature response allowing for reprocessability with outstanding elevatedtemperature creep resistance.

Conventional cross-linked polymers cannot be reprocessed because of the presence of permanent covalent cross-links, preventing reuse and recycling. Covalent adaptable networks (CANs) employ dynamic covalent bonds that undergo dynamic reactions under external stimulus, allowing recyclability of these network materials. Hindered urea chemistry is one of the recently discovered dissociative dynamic chemistries. While hindered urea bonds have traditionally been exploited in the synthesis of step-growth type CANs, the use of hindered urea bonds in the synthesis of chain-growth-type dynamic networks has only been narrowly explored. Here, we present a simple, catalyst-free, fast method to synthesize a hindered-urea-based dynamic cross-linker that can undergo a free radical polymerization with vinyl-type monomers or polymers to form reprocessable CANs. Using this cross-linker, we developed dynamic polymethacrylate networks that can be (re)­processed at 80 °C. These dynamic covalent networks exhibit full recovery of cross-link density after multiple recycling steps; they are only the second chain-growth network synthesized directly and exclusively from carbon–carbon double bond monomers to demonstrate such recovery. Unlike other dissociative dynamic polymer networks, polymethacrylate networks that contain dissociative dynamic hindered urea bonds do not flow and maintain their network structure even at high temperature (300 °C). Despite its relatively fast reprocessability, the network showed delayed and extremely slow stress relaxation at the processing temperature. This work offers a simple approach to obtain reprocessable addition-type networks based on hindered urea bonds while revealing the limitations of stress relaxation experiments in relationship to the processability of some dynamic polymer networks.