Mechanochromic polymers are macromolecular materials that change their color in response to deformation, either on account of altered absorption or reflection. A broader definition that we apply for this special issue of Macromolecular Rapid Communications includes polymers in which other optical characteristics change upon application of mechanical force, notably photoluminescent materials that change their emission properties. We also include mechanochromic polymer systems, whose mechanoresponsiveness results from the integration of multiple components. Mechanochromic materials are (potentially) useful for a broad range of applications that range from pressure‐sensing films to tamper‐evidencing packaging films. The wealth of possible applications is certainly one of the reasons why research on such materials is booming.

The first examples of (commercially successful) mechanochromic materials go back a century. For example, photoelasticity of celluloid specimens was reported on January 1 in 1921,^[1] i.e., one hundred years before online publication of this special issue. The technology to create carbonless copy paper was first invented in the 1950s,^[2] and reports on mechanically tunable distributed‐Bragg‐reflectors based on multilayer polymers date back to the late 1970s.^[3] Around the same time, poly(diacetylene)s were reported to show piezochromic^[4] and thermochromic^[5] properties, and somewhat later demonstrated to show a mechanochromic response when employed as stress‐reporters in polyurethanes.^[6] “Mechanochromic polymers” became a term in the early 1990s, when Nallichieri and Rubner reported mechanochromic behavior of segmented polyurethanes containing chromogenic poly(diacetylene)s,^[7] and Kim and Reneker observed this effect in polyurethane elastomers containing azobenzenes that had been converted to the cis‐form.^[8] These examples illustrate that mechanically induced color changes in polymers can be achieved by very different mechanisms that include physical effects, chemical transformations, or engineering approaches that may involve combinations of both. In the three decades since, research on mechanochromic polymers has developed into a vibrant interdisciplinary field whose exponential growth reflects the significant scientific interest in, and technological usefulness of, materials that translate mechanical inputs into optical outputs. The range of transduction principles that can be utilized to impart polymers with mechanochromic behavior has been greatly expanded. Tremendous progress has been made in understanding and controlling relationships of between macroscopic forces, molecular, microscopic, and macroscopic changes, and optical changes in materials developed on these schemes.

Collectively, the 16 articles that make up this special issue of Macromolecular Rapid Communications attempt to provide a current account of the state of this rapidly emerging, interdisciplinary field. We are delighted that both established researchers who have contributed to the development of the field for a long time, and emerging investigators, who in some cases are just starting their independent careers, have contributed to this issue. It is heartening to see that despite worldwide Covid‐19 related campus and workplace closures in this past year, a large number of authors have been able to contribute and we extend our gratitude for the extra efforts that they have made.

The three reviews, three feature articles, and ten communications cover relevant aspects that span a wide range of transduction principles and cover effects at all length scales. At the molecular level, spiropyrans have emerged as the most widely used mechanochromic mechanophore type; in these motifs, a (reversible) color change originates from a mechanochemical ring‐opening reaction. In his feature article Michael Sommer discusses how substituent effects can be used to control spiropyran‐merocyanine equilibria.^[9] In their communication, Stephen L. Craig and co‐workers report how the strain rate affects the activation of such spiropyrans in silicone elastomers.^[10] Guillaume De Bo and co‐workers report a new mechanoresponsive fluorescent hydrogen‐bonded rotaxane based on a maleimide dye and demonstrate its force‐sensing properties in a synthetic model of living tissue.^[11] Ester Verde‐Sesto, José A. Pomposo and co‐workers review on techniques that can be used to manipulate and mechanically activate single‐chain molecules and thus allow one to elucidate information on the stress‐induced response of individual mechanophores and polymer molecules.^[12]

Several communications address investigations of structure‐property relations of polymer systems based on mechanochromic mechanophores. Stephen L. Craig's group shows that mechanophores based on coumarin dimers exhibit a strength that is comparable to that of sulfur‐sulfur bonds that represent the weakest bonds in vulcanized rubbers and allowed them to investigate how macroscopic mechanical stress is transferred at the molecular scale in such polymers.^[13] Hideyuki Otsuka's team demonstrate that multicolor mechanochromism in mixtures of two mechanochromic polystyrene samples containing different mechanochromophores allows one to detect the duration mechanical stimulation.^[14] In their communication on semi‐interpenetrating elastomer network nanocomposites containing Janus nanoparticles and a mechanoluminescent motif, Yulan Chen and co‐workers further raise the bar with respect to complexity.^[15] They show that the mechanophore is useful to study the stress transfer between the polymer and Janus nanoparticles and thereby the toughening mechanism and the failure process of complex polymer nanocomposites with high spatial and temporal resolution. Finally, Harald Rupp and Wolfgang H. Binder present mechanochromic 3D‐printed composites that rely on the compression‐activated generation of triazole‐based luminophores.^[16]

The review of Stephen Schrettl and co‐workers lays out that chromic effects can also be achieved without covalent bonds scission, but instead exploiting mechanically induced conformational or morphological changes of polymers containing chromogenic moieties.^[17] In their feature article, Ben Zhong Tang and co‐workers further elaborate on one particular aspect and effect—aggregation‐induced emission (AIE)—that can be used to impart polymers with mechanochromic fluorescent behavior.^[18] A new mechanophore—8‐(2‐hydroxyethoxy)pyrene‐1,3,6‐trisulfonate—whose mechanochromic luminescent behavior is driven by the dissociation of molecular aggregates and the mechanochromic behavior of polyurethanes that carry this motif is reported in the communication by Rint Sijbesma and co‐workers.^[19] In a similar vein, the communication of Andrea Pucci, Giacomo Ruggeri and their team revolves around the mechanochromic response of blends of polyethylene and a perylene bisimide derivative, whose dichroic absorption is related to the distinct anisotropic polarizability of the chromophores.^[20]

Moving to another length scale and different operating principle, Jess Clough et al. review the current state of structurally colored polymeric materials.^[21] Two communications deal with specific new materials platform based on such photonic structures. Markus Gallei and co‐workers report dye‐containing mechanically and pH responsive elastomeric opal films,^[22] whereas Luyi Sun and co‐workers developed stretchable bilayers with mechanical‐responsive wrinkled surfaces that exhibit a considerable transparency change.^[23]

Addressing, finally, the microscopic scale, the feature article by Céline Calvino focuses on capsule‐release systems and provides an overview of the different encapsulating approaches that have been employed to prepare mechanochromic polymers, with a focus on the containers and the chromic operating principles used for this purpose.^[24]

As exemplified by the excellent contributions to this themed issue, the fast progress in the field of mechanochromic polymers will no only offer tremendous possibilities to tackle fundamental and application‐oriented questions in smart materials science, but also impact a broad range of fields including catalysis, drug‐release technologies, sensors, and many others. We hope that this issue will provide you with new and stimulating insights, encourage fruitful scientific discussions, and introduce new ideas and expertise to this exciting emerging area.

机械致变色聚合物是大分子材料，由于吸收或反射的改变，其响应变形而改变其颜色。我们适用于本期《大分子快速通讯》的更广泛定义包括在施加机械力后其他光学特性会发生变化的聚合物，特别是会改变其发射特性的光致发光材料。我们还包括机械变色聚合物系统，其机械响应性是由多个组件的集成产生的。机械变色材料在压力传感器薄膜到篡改包装薄膜等广泛的应用中很有用。大量可能的应用程序无疑是对此类材料的研究蓬勃发展的原因之一。

（商业上成功的）机械变色材料的第一个例子可以追溯到一个世纪之前。例如，赛璐oid样品的光弹性报告于1921年1月1日，^{[ 1 ]，}即该特刊在线发表前一百年。创建无碳复写纸的技术最早于1950年代发明^{[ 2 ]}，有关基于多层聚合物的机械可调分布式布拉格反射镜的报道可追溯到1970年代后期。^{[ 3 ]}大约在同一时间，据报道聚（二乙炔）显示出变色^{[ 4 ]}和热变色^{[ 5 ]}性能，后来证明在用作聚氨酯的应力报告剂时显示出机械变色响应。^{[ 6 ]} “机械致变色聚合物”在1990年代初成为一个术语，当时Nallichieri和Rubner报告了含有发色聚二乙炔的链段聚氨酯的机械致变色行为，^{[ 7 ]} Kim和Reneker观察到这种效果在含有偶氮苯的聚氨酯弹性体中已转换为顺式形式。^{[ 8 ]}这些示例说明，可以通过非常不同的机制来实现聚合物中机械诱导的颜色变化，这些机制包括物理效果，化学转化或可能涉及这两者的组合的工程方法。此后的三十年间，对机械变色聚合物的研究已发展成为一个充满活力的跨学科领域，其指数增长反映了人们对将机械输入转换为光学输出的材料的重大科学兴趣和技术实用性。可用于赋予聚合物机械变色行为的转导原理范围已大大扩展。在理解和控制宏观力，分子，微观和宏观变化之间的关系方面取得了巨大进展，

总的来说，构成本期《大分子快速通信》的16篇文章试图提供当前对该快速发展的跨学科领域的现状的最新描述。我们感到高兴的是，为该领域的发展做出了长期贡献的知名研究人员，以及在某些情况下刚刚开始独立职业的新兴研究人员，都为这一问题做出了贡献。令人鼓舞的是，尽管在过去的一年中Covid‐19在全球范围内关闭了相关的校园和工作场所，但仍有大量的作者做出了贡献，我们对他们所做的额外努力表示感谢。

这三篇评论，三篇专题文章和十篇来文涵盖了涉及广泛转导原理的相关方面，并涵盖了所有长度范围的效应。在分子水平上，螺吡喃已经成为使用最广泛的机械致变色机理。在这些图案中，（可逆的）颜色变化源自机械化学的开环反应。迈克尔·索默（Michael Sommer）在他的专题文章中讨论了如何利用取代基效应控制螺吡喃-花青素的平衡。^{[ 9 ]} Stephen L. Craig及其同事在通讯中报告了应变速率如何影响有机硅弹性体中此类螺并吡喃的活化。^{[ 10 ]}Guillaume De Bo及其同事报告了一种基于马来酰亚胺染料的新型机械响应型荧光氢键轮烷，并在生物组织的合成模型中证明了其力感特性。^{[ 11 ]} Ester Verde-Sesto，JoséA. Pomposo及其同事对可用于操纵和机械激活单链分子的技术进行了综述，从而使人们能够阐明有关单个机电体和聚合物在应力诱导下的响应的信息。分子。^{[ 12 ]}

基于机械致变色机理的聚合物体系结构-性质关系的若干通讯研究。Stephen L. Craig的研究小组表明，基于香豆素二聚体的机械载体的强度与硫-硫键相当，后者代表了硫化橡胶中最弱的键，并允许他们研究在这样的分子尺度上宏观机械应力如何转移聚合物。^{[ 13 ]} Hideyuki Otsuka的研究小组证明，在两种含有不同机械发色团的机械致变色聚苯乙烯样品的混合物中，多色机械致变色可以检测机械刺激的持续时间。^{[ 14 ]}在与包含Janus纳米粒子和机械发光图案的半互穿弹性体网络纳米复合材料的交流中，陈玉兰及其同事进一步提高了复杂性的门槛。^{[ 15 ]}他们表明，该机理有助于研究聚合物与Janus纳米颗粒之间的应力转移，从而研究具有高时空分辨率的复杂聚合物纳米复合材料的增韧机理和破坏过程。最后，Harald Rupp和Wolfgang H. Binder提出了机械致变色3D打印复合材料，该复合材料依赖于基于三唑基发光体的压缩活化生成。^{[ 16 ]}

对Stephen Schrettl及其同事的评论表明，在没有共价键断裂的情况下，也可以实现变色效应，而是利用含有生色部分的聚合物的机械诱导构象或形态变化。^{[ 17 ]} Ben Zhong Tang及其同事在他们的专题文章中进一步阐述了一个特定的方面和效果-聚集诱导发射（AIE）-可用于赋予聚合物以机械变色荧光行为。^{[ 18 ]}Rint的通讯中报道了一种新的机械载体，即8-（2-羟基乙氧基）py-1,3,6-三磺酸盐，其机械致变色发光行为是由分子聚集体的解离驱动的，带有该基序的聚氨酯的机械致变色行为Sijbesma和同事。^{[ 19 ]}同样，安德里亚·普奇（Andrea Pucci），贾科莫·鲁杰里（Giacomo Ruggeri）及其团队之间的交流也围绕着聚乙烯与per双酰亚胺衍生物共混物的机械变色响应而展开，其二向色吸收与发色团的独特各向异性极化有关。^{[ 20 ]}

转向另一种长度刻度和不同的工作原理，Jess Clough等人。回顾结构彩色聚合物材料的当前状态。^{[ 21 ]}两次通信处理基于此类光子结构的特定新材料平台。马库斯·加莱（Markus Gallei）及其同事报道了含有染料的机械响应和pH响应的弹性蛋白石膜，^{[ 22 ]}而孙鲁毅及其同事开发了具有机械响应皱纹表面的可拉伸双层膜，该膜表现出相当大的透明度变化。^{[ 23 ]}

最后，在微观层面上，CélineCalvino的专题文章重点介绍了胶囊释放系统，并概述了用于制备机械变色聚合物的不同封装方法，重点介绍了容器和所用的变色操作原理。以此目的。^{[ 24 ]}

正如对这一主题问题的杰出贡献所证明的那样，机械变色聚合物领域的飞速发展不仅为解决智能材料科学中的基础和面向应用的问题提供了巨大的可能性，而且还影响了包括催化，药物释放技术，传感器等。我们希望本期杂志将为您提供新的，令人兴奋的见解，鼓励富有成果的科学讨论，并为这个令人兴奋的新兴领域引入新的想法和专业知识。