ACS Macro Lett. 2022 Apr 19;11(4):568-574. doi: 10.1021/acsmacrolett.2c00045. Epub 2022 Apr 6.
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.
传统的交联聚合物由于存在永久的共价交联而无法再加工，从而阻止了它们的再利用和回收。共价适应性网络（CANs）采用在外部刺激下发生动态反应的动态共价键，允许这些网络材料的可回收性。受阻脲化学是最近发现的一种离解动态化学之一。虽然受阻脲键传统上被用于逐步增长型 CANs 的合成，但受阻脲键在链增长型动态网络合成中的应用仅得到了有限的探索。在这里，我们提出了一种简单、无催化剂、快速的方法来合成基于受阻脲的动态交联剂，该交联剂可以与乙烯基型单体或聚合物进行自由基聚合，形成可再加工的 CANs。使用这种交联剂，我们开发了可以在 80°C 下（再）加工的动态聚甲基丙烯酸酯网络。这些动态共价网络在多次回收步骤后可以完全恢复交联密度；它们是仅有的第二个直接且完全由碳-碳双键单体合成的链增长网络，展示了这种恢复能力。与其他离解动态聚合物网络不同，含有离解动态受阻脲键的聚甲基丙烯酸酯网络不会流动，并在高温（300°C）下保持其网络结构。尽管其再加工性相对较快，但该网络在加工温度下表现出延迟和极其缓慢的应力松弛。这项工作提供了一种简单的方法来获得基于受阻脲键的可再加工加成型网络，同时揭示了一些动态聚合物网络的加工性能与应力松弛实验之间的局限性。
