Conventional polymer networks cannot be recycled into high-value products because of permanent, covalent crosslinks. Rubber tires illustrate well the issues ranging from economic loss to environmental problems and sustainability issues that arise with spent, crosslinked polymers. (More than a billion rubber tires are discarded annually worldwide.) A nitroxide-mediated polymerization (NMP) strategy has been developed that allows for one-step synthesis of recyclable crosslinked polymeric materials from any monomers or polymers that contain carbon-carbon double bonds amenable to radical polymerizaation. The resulting materials possess dynamic alkoxyamine crosslinks that undergo reversible decrosslinking as a function of temperature. As demonstrated with polybutadiene as starting material, and styrene, an appropriate nitroxide molecule and bifunctional initiator for initial crosslinking, a model for tire rubber can be produced by reaction at temperatures comparable to those employed in tire molding. Upon cooling, the crosslinks are made permanent due to the extraordinarily strong temperature dependence of the reverisible nitroxide capping and uncapping reaction. Based on thermomechanical property characterization, when the original crosslinked model rubber is chopped into bits and later remolded into the melt state into a well-consolidated material, the resulting material properties at temperature below 373 K indicate full recovery of properties associated with crosslink density after multiple reprocessing steps. Additionally, this simple one-step strategy allows for robust, relatively sustainable recyclability of crosslinked polymers and design of relatively uniform networks for advanced technologies. A more complete description of this stategy and outcomes is provided in a recent publication. Other strategies for melt-state reprocessing of crosslinked polymers will complete or nearly complete recovery of properties will also be discussed.

Recyclable crosslinked polymer networks via one-step controlled radical polymerization: Full property recovery after multiple melt-reprocessing steps of models for tire rubber and other polymer networks

253rd National Meeting (2017)

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