Department of Chemistry , Duke University , Durham , North Carolina 27708 , United States.
J Am Chem Soc. 2020 Feb 5;142(5):2105-2109. doi: 10.1021/jacs.9b13359. Epub 2020 Jan 22.
Degradable polymers are desirable for the replacement of conventional organic polymers that persist in the environment, but they often suffer from the unintentional scission of the degradable functionalities on the polymer backbone, which diminishes polymer properties during storage and regular use. Herein, we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli-mechanical and acid triggers-in an "AND gate" fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of an acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the time scale of 100 ms is approximately 2 nN.
可降解聚合物是替代在环境中持久存在的传统有机聚合物的理想选择，但它们通常会遭受聚合物主链上可降解功能的无意断裂，这会在储存和常规使用过程中降低聚合物的性能。在此，我们报告了一种策略，通过将两种常见的降解刺激——机械和酸触发——以“与门”的方式结合在一起，来对抗聚合物中的非预期降解。环丁烷（CB）机械敏感基团被用作机械门，来调节在酸降解聚合物中广泛应用的酸敏感缩酮。这种门控缩酮进一步被整合到聚合物主链中。单独存在酸触发时，原始聚合物保留其主链完整性，仅通过超声处理施加高机械力会将聚合物降解至表观极限分子量 28 kDa。然而，超声处理后再进行酸处理，会导致分子量进一步降低 11 倍，降至 2.5 kDa。实验和计算证据进一步表明，未门控的缩酮具有与传统聚合物主链相当的机械强度。单分子力谱（SMFS）表明，在 100 ms 的时间尺度上激活 CB 分子门所需的力约为 2 nN。
