Reprocessable and Recyclable Chain-Growth Polymer Networks Based on Dynamic Hindered Urea Bonds

Abstract

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.

Neighboring Group Participation in Ionic Covalent Adaptable Networks

“…Based on ideal rubber elasticity theory, the rubbery plateau E ′ is inversely related to the molecular weight between cross-links ( M x ) and thus proportional to the cross-link density . Accordingly, the rubbery plateau E ′ serves as a proxy for assessing the recovery of cross-link density after each reprocessing iteration …”

“…55 Accordingly, the rubbery plateau E′ serves as a proxy for assessing the recovery of cross-link density after each reprocessing iteration. 56 The E′ following the transition region does not reach a constant plateau and instead gradually decreases as a function of temperature (Figure 5b). This temperature-dependent decay in E′ after T g is a feature frequently observed in CANs that undergo a transient reduction in cross-link density during a dissociative bond-exchange process, such as the S-BCN network (Figure 1).…”

Dynamic covalent polymers enabled by reversible isocyanate chemistry

“…Commodity thermoplastics, such as poly(methyl methacrylate) (PMMA) and polystyrene (PS) can be engineered more durable, robust, and environmentally resistant via chemical crosslinking with HUBs, which simultaneously endow them with malleability upon heating. [162][163][164][165] Commercial n-butyl methacrylate (BMA) reacted with 2-isocyanatoethyl methacrylate (IEM) and the N,N-t-butyl-ethyl diamine crosslinker in a one-pot manner to produce PMMA CANs with a high Young's modulus. 162 The self-healing capacities of the system were verified with repeated scratch-healing and bulk-healing tests and the mechanical performances were partially recovered.…”

“…162 The self-healing capacities of the system were verified with repeated scratch-healing and bulk-healing tests and the mechanical performances were partially recovered. Rusayyis et al 163 synthesized a methacryloyl-capped HUB crosslinker, with which poly( n -hexyl methacrylate) networks were synthesized via radical polymerization. These samples exhibited the full restoration of the crosslinking density after multiple recycling experiments.…”

Functional Epoxy Elastomer Integrating Self-Healing Capability and Degradability for a Flexible Stretchable Strain Sensor

“…Recently, many self-healing elastomers have been prepared by introducing noncovalent bonds (hydrogen bonds, , host–guest interaction, , metal–ligand coordination, , etc.) and reversible covalent bonds (hindered urea bonds, , disulfide bonds, , Schiff base, , etc.) into polymer networks.…”