(36d) Dynamic Chemistry Leading to Full Property Recovery Associated with Crosslink Density in Reprocessed Polymer Networks and Network Composites: Network Synthesis By Step-Growth Reactions and By Nitroxide-Mediated Polymerization

Conventional polymer networks are unable to be reprocessed in the melt state and recycled into high-value products because of permanent covalent crosslinks. Reprocessable networks, or covalent adaptable networks, are crosslinked polymers that contain sufficient reversible covalent bonds for network reconfiguration under proper conditions. Such networks contain dynamic linkages that can dissociate or exchange with others at the reprocessing condition. We focus on two reprocessable polymer networks that do not require pre- or post-polymerization functionalization but instead involve simple one-step chemistry with reaction of monomer and/or polymer. In one case, we show how novel use of nitroxide-mediated polymerization (NMP) involving reactions of a multifunctional radical initiator and a polymerizable monomer incorporating a stable nitroxide radical with polybutadiene and styrene lead to models for crosslinked tire rubber. (See Adv. Mater. 2016, 28, 6746.) The strongly temperature-dependent reversible capping/uncapping step in NMP provides crosslink reversibility. Based on the rubbery plateau modulus from DMA and tensile properties, these networks can be reprocessed multiple times in the melt state with the product exhibiting full recovery of properties related to crosslink density. Robust recovery is also observed in polymer composite networks made by NMP with filler such as carbon black. This one-step NMP method also yields polymer network structures that are possible with only a limited few other reaction methods or have yet to be obtained by any other method. In the second case, a step-growth polymer network has been synthesized and also exhibits full property retention after multiple reprocessing steps. In this case, the type of polymer that we employed has been described in literature as a vitrimer. We show that this particular network polymer is not a true vitrimer because both reversible and exchange reactions are present at reprocessing conditions. It is important to appreciate that reversible reactions may accompany exchange reactions in some vitirimers, because the extent of reversible reactions will alter the required reprocessing time. Also, if reversible reaction products are volatile and lost during reprocessing, it will be impossible for the reprocessed material to achieve full property recovery.