Simple One-Step Reaction to Achieve Cross-Linked Polymer and Polymer Composites: Models for Rubber Tires That Can Recycled Multiple Times with Robust Recovery of Properties Related to Cross-Link Density

Conventional polymer networks cannot be recycled into high-value products because of permanent, covalent crosslinks. Rubber tires illustrate well the issues ranging from major economic loss to environmental problems that arise with spent, crosslinked polymer. Here, we will discuss a simple, one-step reaction strategy using nitroxide-mediated polymerization (NMP) that we developed to synthesize recyclable crosslinked polymer with dynamic covalent bonds from any monomers or polymers that contain carbon-carbon double bonds that are amenable to free radical polymerization. (See Advanced Materials 2016, 28, 6746.) Using this reaction strategy, we have designed network polymers using polybutadiene and styrene monomer as models for recyclable tires, yielding full property recovery after multiple melt-reprocessing steps. No previous study has achieved direct formation of recyclable networks from commercial monomer/polymer without presynthesis or prefunctionalization and demonstrated full recovery of crosslinks after multiple reprocessing steps. We have likewise demonstrated the effectiveness of this strategy with commercial natural (uncrosslinked) rubber, leading to crosslinked natural rubber that can be effectively recycled and reprocessed multiple times with full recovery of properties associated with crosslink density. Thus, the reaction strategy can be employed with exactly the same polymers currently used in the tire industry and with reaction temperatures comparable to those employed in tire molding operations. As a result, this strategy provides for green reaction engineering not with the use on biobased feedstock but instead by effective, high-value recycling of synthetic materials that have been highly engineered for demanding uses and that are unlikely to be replaced by biobased feedstock. Importantly, we have also shown that this strategy can work well with polymer composites, e.g. networks incorporating carbon black filler exactly of the type used in rubber tires. Additionally, we have also used one-step NMP to produce polymer networks from monomers with very good reprocessabiltiy and relative uniformity of chains between crosslinks. Examples of other novel material development using simple, one-step NMP will also be discussed, time permitting.