Single‐molecule studies in polymer science

In polymer science, the ability to study single-chain behaviors is important, as phenomena associated with polymers are typically complex and heterogeneous. Over the past decades, single-molecule methods have allowed researchers to probe the intricacies of polymers and unravel their unique structures, dynamics, interactions, and physical properties, thereby providing information where other characterization techniques are often limited. In this special section, we are excited to introduce several contributions that demonstrate how single-molecule studies advance understanding of polymeric systems, as well as two reviews that discuss the history, transformative impact, and future opportunities for single-molecule experiments in polymer science. Qiang and co-workers (pol.20230621) reviewed recent progress using single-molecule fluorescence imaging to monitor chemical reactions in situ with high spatial and temporal resolution. An outline of the working principles and experimental setup of representative techniques will serve as a helpful resource for newcomers to single-molecule fluorescence microscopy. The authors summarized the molecular-scale characterization of model reaction systems, highlighting the quantification of biomolecular activity, catalytic reaction rates, and nanomaterial assembly. For reactive polymer systems, single-molecule imaging led to new insights into local heterogeneities during polymerization, reaction kinetics at catalytic interfaces, and redox states during electrochemical reactions. A brief outlook identifies opportunities to provide new molecular-scale insights into reaction efficacy, structural transitions, and diffusion dynamics in reactive polymer systems. Liu and co-workers (pol.20230688) examined the use of magnetic tweezers to investigate synthetic polymers. An overview of the fundamentals of magnetic tweezers provided technical details for tracking magnetic particles, calibrating and controlling magnetic force, and analyzing data from single chains. The authors emphasized that magnetic tweezers enable real-time, high-speed measurements in liquid-phase environments. Their review of single-chain properties featured static chain conformational changes resulting from different chain topologies and solvent conditions, dynamic conformational changes in response to thermal and mechanical stimuli, and real-time chain growth mechanisms and kinetics during polymerization. Overall, magnetic tweezers provide a powerful toolbox with wide force ranges and high imaging precision, which can reveal structural changes and kinetic behaviors. Polymeric materials can also drive advances in single-molecule measurement science. For example, single-molecule force spectroscopy is a common method to investigate molecular interactions in biophysical and material systems, but its ability to investigate covalent mechanochemistry is challenged by insufficient attachment strengths between the force probe and the analyte. Craig and co-workers (pol.20230051) addressed this limit by introducing a polyelectrolyte handle between an atomic force microscope tip and a polymer analyte. The polyelectrolyte handle was prepared by controlled free radical polymerization, and it provided stronger attachment forces to allow high-force measurements. Moreover, a non-fouling detachment process enabled repeated experiments. Polymer identity played an important role in the force distributions from single-molecule measurements, which were associated with the electrostatic interactions between polymer and tip. This work provided a promising approach to broaden the use of single-molecule force spectroscopy, particularly for investigations of covalent polymer mechanochemistry. Simulations are powerful tools that provide a quantitative understanding of single-chain conformations in various environments, informing fundamental polymer structure–property relationships needed to advance technology design. Xia and co-workers (pol.20230689) employed coarse-grained molecular dynamics simulations to understand the impact of solvent quality, temperature, and chain architecture on the conformational behaviors of conjugated polymers in dilute solutions. Distinct polymer chain conformations emerged under different solvent conditions, including changes from extended structures to aggregated states with reduced polymer–solvent miscibility. This work also revealed that conjugated polymers with longer side chains exhibited both enhanced rigidity and solubility; analysis of the relative shape anisotropy provided insights into the evolving symmetry and dimensionality of polymer chains. Dynamic light scattering and small-angle x-ray experiments validated the simulation results, supporting the use of computational methods to inform the design of polymeric materials. Finally, Klotz and co-workers (pol.20230392) combined graph theory simulations and single-molecule experiments to understand network percolation in catenated DNA structures called kinetoplasts. The connectivity and structure of these networks are complex: minicircles, maxicircles, and a dense edge loop each serve different biological and mechanical functions. This work examined de-percolation processes by tracking the size of connected components upon the removal of minicircles. Graph simulations indicated survival criteria for the edge loop during de-percolation; indeed, fluorescence microscopy revealed that edge loops were the last remaining structures during photocleavage of kinetoplasts. This work provided novel insights into the structure and function of the edge loop and identified kinetoplasts as potential sources of polycatenanes for future single polymer experiments. Looking forward, advances in single-molecule studies enable researchers to further understand complex polymer structures, interactions, and dynamics. These insights will guide the smart, sustainable design of polymeric materials for use in any commercial sector. We hope this special collection inspires and encourages a broader use of single-molecule methods among researchers in the field of polymer science.