Van Lijsebetten Filip, Maes Stephan, Winne Johan M, Du Prez Filip E
Polymer Chemistry Research Group, Centre of Macromolecular Chemistry (CMaC) and Laboratory of Organic Synthesis, Departement of Organic and Macromolecular Chemistry, Faculty of Sciences, Ghent University Krijgslaan 281-S4 Ghent 9000 Belgium
Chem Sci. 2024 Mar 26;15(19):7061-7071. doi: 10.1039/d4sc00417e. eCollection 2024 May 15.
Acid-base catalysis is a common strategy to induce covalent bond exchanges in dynamic polymer networks. Strong acids or strong bases can promote rapid network rearrangements, and are simultaneously preferred catalysts for chemical reactions where maximum efficiency at the lowest possible temperature is aimed for. However, within the context of dynamic polymer networks, the incorporation of highly active catalysts can negatively affect the longer term application potential. Network dynamicity can diminish through catalyst ageing or quenching and highly active catalysts may prematurely activate bond exchanges, leading to dimensional instability and thus low creep resistance of the polymer networks. Herein, we present several examples where we explicitly explored weak acids (carboxylic acids) as catalysts for dynamic bond exchanges, using vinylogous urethanes (VU) as a well-understood protic acid catalysed vitrimer chemistry. Surprisingly, we have found that the sought-after long-term stability offered by a weak acid does not necessarily bring lower activity at high temperature. In fact, the weak acids show a remarkable thermoswitchable catalytic behaviour, going from an inactive hydrogen bonded state to an active state where the polymer matrix is protonated, with a profound impact on the network reactivity and rheology. Carboxylic acids with different electronic or steric environments show clear reactivity trends and their fine-tuning resulted in the most thermally responsive VU vitrimers studied to date. Our findings point out that catalyst choice and design for vitrimers is only poorly informed by catalyst performance in more traditional chemical reactions (in solvent), and that a more tailored catalyst design holds great promise for the field of vitrimers.
酸碱催化是在动态聚合物网络中诱导共价键交换的常见策略。强酸或强碱可促进网络的快速重排，同时也是旨在在尽可能低的温度下实现最大效率的化学反应的首选催化剂。然而，在动态聚合物网络的背景下，引入高活性催化剂可能会对其长期应用潜力产生负面影响。网络动态性可能会因催化剂老化或猝灭而降低，高活性催化剂可能会过早激活键交换，导致尺寸不稳定，从而使聚合物网络的抗蠕变性较低。在此，我们展示了几个实例，其中我们明确探索了弱酸（羧酸）作为动态键交换的催化剂，使用乙烯基脲（VU）作为一种已被充分理解的质子酸催化的玻璃态转化体化学体系。令人惊讶的是，我们发现弱酸所提供的人们所追求的长期稳定性并不一定会在高温下带来较低的活性。事实上，弱酸表现出显著的热开关催化行为，从无活性的氢键状态转变为聚合物基质被质子化的活性状态，这对网络反应性和流变学有深远影响。具有不同电子或空间环境的羧酸表现出明显的反应性趋势，对其进行微调得到了迄今为止研究的最具热响应性的VU玻璃态转化体。我们的研究结果指出，玻璃态转化体的催化剂选择和设计在很大程度上并未受到其在更传统化学反应（在溶剂中）中催化剂性能的影响，而且更具针对性的催化剂设计在玻璃态转化体领域具有巨大潜力。
