Dynamic bonds (including dynamic covalent bonds and supramolecular interactions) are a class of chemical bonds capable of being reversibly broken and reformed under certain circumstances (e.g., heating, pH, illumination, and catalyst). Accordingly, polymers carrying dynamic bonds exhibit unique stimuli-responsive properties that are unavailable for conventional polymers. With the introduction of dynamic bonds chemistry, in particular, the boundary between thermoplastics and thermosets no longer exists to a great extent, and an entirely new discipline is evolving out of classic polymer engineering. The current special issue focuses on the latest achievements in the promising field by collecting 12 original research articles and one review article about diverse aspects of dynamic bonds in polymers from a variety of labs. The readers may thus gain updated information about the development trends within a relative short time.
Nowadays the mainstream polymers are usually constructed by irreversible bonds, so that innovative strategies have to be worked out for establishment of dynamic bonds in polymers. Qin et al. (pol.20230383) carefully reviewed a highly efficient synthetic approach in their article, that is, X-yne click polymerization (like thiol-yne, amino-yne, and hydroxyl-yne click polymerizations), and its application in preparation of dynamic covalent polymers. The dynamic exchange of a series of X-yne adducts led to self-healing of injectable polymer hydrogels and functional porous polymeric films, providing a set of attractive tools for creating advanced smart polymers.
As an important step from raw materials to end-use applications, polymer processing accounts for assembling polymer chains into envisaged condensed structures, and the products properties are closely related to the morphologies created during the fabrication. Li and co-workers (pol.20230427) incorporated hydrogen bonds into polydimethylsiloxane (PDMS) elastomer for additive manufacturing of personalized flexible supplies. Not only the disadvantages of existing 3D-printable silicones were overcome, but also self-healing ability was imparted. Meanwhile, Xu and Wu et al. (pol.20230407) synthesized a styrylpyrene-containing photoresponsive liquid crystal polymer (LCP). It was found that thermal treatment can raise orientation degree of the LCP, and dimerization of the styrylpyrenes controlled its crosslinking density. Both of the methods favor adjustment of microstructure and properties.
Besides processing techniques, properties of polymers are often tuned by changing their macromolecular compositions to meet different use needs. The waterborne polyurethane coatings developed by Wang and de Luna et al. (pol.20230133), for example, possessed balanced robustness, corrosion resistance, and self-healing performance. The joint effects of appropriate hard segment contents, ionic interactions, and reversible Diels–Alder bonds played the leading role in this case. Xu et al. (pol.20230455) further demonstrated that side-chain engineering of supramonomers can provide tailorable mechanical properties for cross-linked supramolecular polyureas. As a result, the materials ranging from rigid plastics to elastic materials were easily achievable. Similarly, the location of the dynamic disulfide bonds in liquid crystal elastomers (LCEs) was also found to be critical for affecting the stress relaxation times, by the group of Rowan (pol.20230547), who prepared a few dynamic LCE networks that had similar network characteristics but different placements of the dynamic bonds. Smulders and co-workers (pol.20230446) studied a novel dynamic boronic acid based covalent adaptable network. Molecular tuning via the electron density of meta-positioned ring substituents proved to be an effective means for controlling over relaxation behaviors of the materials.
By taking advantage of the embedded dynamic bonds, crosslinked polymers can be reprocessable, which represents a breakthrough in the development of polymer engineering towards sustainable society. In the paper by Lu et al. (pol.20230411), dense hydrogen bonds and cation–π interactions were introduced into polyurea. The synergistic effect of the dual non-covalent bonds made the resulting film have high strength and can be repeatedly reprocessed. Moreover, its flexibility and transparency were not affected. To improve mechanical properties and reprocessability of PDMS, Zheng's group (pol.20230431) brought Zn(II)–amine coordination bonds into the molecular skeletons. The Zn ions promoted the exchange between the silyl ether linkages and hydroxyl groups, which allowed the authors to achieve their goal.
Another property of the polymers with dynamic bonds closely related to reprocessability is recyclability. Unlike conventional recycling that used to result in properties decay, Ma et al. (pol.20230424) proposed a chain breaking-crosslinking strategy and upcycled thermoplastic polyurethane (TPU) to polyurethane (PU) covalent adaptable networks (CANs). The PU-CANs have acquired enhanced mechanical properties (including creep resistance) and solvent resistance in addition to the reprocessability. Because the experiments were conducted using twin-screw extruder, the proposed technique may serve for mass production in the future.
It is worth noting that opening up new resources is an important way of circular economy. In consideration of the non-renewable nature of fossil oil, Connal et al. (pol.20230005) made use of raspberry ketone as feedstock and produced vitrimer-type elastomers via oxime chemistry. The catalyst-free dynamic oxime transesterification exchange reaction enabled reprocessing of the elastomers, while the shortcomings of catalyst in polyester networks no longer existed. Zeng and co-workers (pol.20230437) prepared carbon nanotubes (CNTs) filled castor oil derived poly(urethane urea) (PPU) composite networks. Pre-polymerization of castor oil with isophorone diisocyanate was the key step of the PPU synthesis. The reprocessability of the materials offered by the built-in dynamic piperidine–urea bonds was retained despite the fact that the stress relaxation rate was reduced with a rise in the CNTs fraction.
Unlike most works in the special issue, which pay attention to macroscopic properties of polymeric materials, the paper by Zhang and Chen et al. (pol.20230058) deals with biocatalysis. The authors developed a new type of glycogen-based dynamic nano-frameworks for activating carbonic anhydrase. The protein–polymer interactions are expected for various biochemical applications.
It is interesting to see from the papers of the special issue that self-healing seems no longer to be the predominant theme of the field, which is different from the case a few years ago, when the first global wave of research on dynamic polymers focused on implementation of self-healing in various ways. Instead, self-healability has become an inherent functionality of this type of materials and less discussion was put on the topic. Such a transition reflects the evolution desire of the investigation of dynamic bonds in polymers from fundamental study to practical usage.
At the end of the editorial, I would like to thank all the authors, reviewers, and editorial staff of Journal of Polymer Science for their significant contributions to this special issue.