o-Phthalaldehyde is, to this day, the only aromatic aldehyde that can be homopolymerized through chain-growth polymerization. The product, polyphthalaldehyde (PPA), is a brittle white solid, and, having a polyacetal main chain, presents the ability to depolymerize quite rapidly in the presence of an acid. This review highlights the unique polymerization chemistry of o-phthalaldehyde since its discovery over half a century ago, describing the different methods for the preparation of PPA and its derivatives, how the polymerization chemistry affects the structure and thermomechanical properties of the obtained PPA, and summarizes recent developments in PPA chemistry as a responsive material. Modern material applications such as the use of PPA as photoresists or in thermal-scanning probe lithography, as well as exploration of judiciously end-capped PPA for its use as self-immolative materials are summarized. In addition, the use of PPA blocks in copolymers is described, leading to the development of films with well-defined nanochannels or nanopores that can serve as a template for the preparation of the microorganization of nanomaterials.

Many of the attractive properties in polymers are a consequence of their high molecular weight and therefore, scission of chains due to mechanochemistry leads to deterioration in properties and performance. Intramolecular cross‐links are systematically added to linear chains, slowing down mechanochemical degradation to the point where the chains become virtually invincible to shear in solution. Our approach mimics the immunoglobulin‐like domains of Titin, whose structure directs mechanical force towards the scission of sacrificial intramolecular hydrogen bonds, absorbing mechanical energy while unfolding. The kinetics of the mechanochemical reactions supports this hypothesis, as the polymer properties are maintained while high rates of mechanochemistry are observed. Our results demonstrate that polymers with intramolecular cross‐links can be used to make solutions which, even under severe shear, maintain key properties such as viscosity.

Many of the attractive properties in polymers are a consequence of their high molecular weight and therefore, scission of chains due to mechanochemistry leads to deterioration in properties and performance. Intramolecular cross-links are systematically added to linear chains, slowing down mechanochemical degradation to the point where the chains become virtually invincible to shear in solution. Our approach mimics the immunoglobulin-like domains of Titin, whose structure directs mechanical force towards the scission of sacrificial intramolecular hydrogen bonds, absorbing mechanical energy while unfolding. The kinetics of the mechanochemical reactions supports this hypothesis, as the polymer properties are maintained while high rates of mechanochemistry are observed. Our results demonstrate that polymers with intramolecular cross-links can be used to make solutions which, even under severe shear, maintain key properties such as viscosity.

Flexible sensors can be widely used in future wearable devices to monitor people's health states. However, most of the sensors are sensitive to humidity and bending effects, limiting their application in a real-world environment. A new strategy is proposed for obtaining flexible sensors with good tolerance to humidity. By integrating a hydrophobic layer on the surface of doped polyaniline, a flexible sensor that can resist water response with a concentration up to 350 ppm is developed. Good resilience against mechanical bending is also achieved in this flexible sensor. These results may trigger a renaissance in flexible sensor applications for disease diagnosing by wearable devices.

The mechanochemical stability of a polymer is a fundamental parameter when choosing the ideal material for many different uses where mechanical loading may induce molecular weight reduction. The use of mechanophores has significantly improved the detection of mechanochemical reaction, but their incorporation to different polymers can be synthetically challenging. Alternatively, we return to the old strategy of using spin traps to quantify the radicals produced as a consequence of mechanochemical homolytic bond scission events. Several new spin traps have been developed in recent decades, and pyrenyl nitrones have been shown to effectively bind radicals, providing a spectroscopic methodology to follow radical concentration. Here we demonstrate the use of these probes as excellent tools to follow mechanochemical chain scission.

The mechanical properties of an amorphous polymer are a consequence of the inter- and intramolecular interactions which are typically similar. Yet, polymers can be made with covalent intramolecular cross-links, leading to stronger intramolecular interactions. Here, a study on the effect of this intramolecular cross-linking on the mechanical properties of a glassy polymer is shown. A linear poly­(methyl methacrylate) with defined size was cross-linked at different ratios and assembled into solid samples by solvent casting. Mechanical testing indicates that intramolecular cross-linking does not affect the mechanical properties at the elastic region but does influence these properties at the plastic region. Interestingly, intramolecular cross-linking leads to different effects than “regular” interchain cross-linking. The results are consistent with a reduction in chain entanglement, reducing toughness and to a lesser extent strength.

Addition of intramolecular cross-links to linear polymers significantly improves their resistance to mechanochemical fragmentation, and hence the physical properties of polymer solutions are maintained under shear. However, while fragmentation is suppressed, mechanochemistry of chemical bonds still occurs. In linear polymers, the rate of mechanochemistry has been shown to increase linearly with the degree of polymerisation. Here, we report a systematic study of the mechanochemical fragmentation of a series of polymers with increasing polymer length, linear and intramolecularly collapsed, in order to understand the correlation between destructing and non-damaging mechanochemical events. By comparing the trends of the fragmentation kinetic rate vs. the degree of polymerisation, the effect of intramolecular collapse on fundamental mechanochemistry parameters such as the limiting molecular weight and stabilisation effect can be further understood.

Polymer chains folded with a single disulphide loop are shown to present distinct rates of mechanochemical fragmentation.