Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States.
Department of Chemistry, University of Chicago, Chicago, Illinois 60637, United States.
J Am Chem Soc. 2023 Jun 7;145(22):12315-12323. doi: 10.1021/jacs.3c02837. Epub 2023 May 25.
Crosslinking in polymer networks leads to intrinsic structural inhomogeneities that result in brittle materials. Replacing fixed covalent crosslinks with mobile ones in mechanically interlocked polymers (MIPs), such as in slide-ring networks (SRNs) in which interlocked crosslinks are formed when polymer chains are threaded through crosslinked rings, can lead to tougher, more robust networks. An alternative class of MIPs is the polycatenane network (PCN), in which the covalent crosslinks are replaced with interlocked rings that introduce the unusual catenane's mobility elements (elongation, rotation, and twisting) as connections between polymer chains. A slide-ring polycatenane network (SR-PCN), with doubly threaded rings embedded as crosslinks in a covalent network, combines the mobility features of both the SRNs and PCNs, where the catenated ring crosslinks can slide along the polymer backbone between the two limits of network bonding (covalent and interlocked). This work explores using a metal ion-templated doubly threaded pseudo[3]rotaxane (P3R) crosslinker, combined with a covalent crosslinker and a chain extender, to access such networks. A catalyst-free nitrile-oxide/alkyne cycloaddition polymerization was used to vary the ratio of P3R and covalent crosslinker to yield a series of SR-PCNs that vary in the amount of interlocked crosslinking units. Studies on their mechanical properties show that metal ions fix the rings in the network, leading to similar behavior as the covalent PEG gels. Removal of the metal ion frees the rings resulting in a high-frequency transition attributed to the additional relaxation of polymer chains through the catenated rings while also increasing the rate of poroelastic draining at longer timescales.
聚合物网络中的交联会导致固有结构不均匀，从而导致材料易碎。在机械互锁聚合物（MIPs）中，用可移动的交联取代固定的共价交联，例如在滑动环网络（SRN）中，当聚合物链穿过交联环时形成互锁交联，可以得到更坚韧、更坚固的网络。另一类 MIP 是多轮烷网络（PCN），其中共价交联被互锁环取代，互锁环引入了不寻常的轮烷的移动元件（伸长、旋转和扭曲）作为聚合物链之间的连接。一种滑动环多轮烷网络（SR-PCN），其中双螺旋环作为交联嵌入在共价网络中，结合了 SRNs 和 PCNs 的移动特性，其中交联的轮烷环可以在聚合物主链上沿着网络键合（共价和互锁）的两个极限之间滑动。这项工作探索了使用金属离子模板的双螺旋伪[3]轮烷（P3R）交联剂，结合共价交联剂和链扩展剂，来制备这种网络。使用无催化剂的氮氧化物/炔烃环加成聚合来改变 P3R 和共价交联剂的比例，得到一系列具有不同互锁交联单元数量的 SR-PCN。对它们机械性能的研究表明，金属离子将环固定在网络中，导致与共价 PEG 凝胶相似的行为。金属离子的去除使环自由，导致高频转变归因于聚合物链通过轮烷的额外松弛，同时也增加了更长时间尺度上的多孔弹性排水速率。
