Interlocked Macromolecules

In the Rowan group, we are interested in creating new polymer architectures and applying them as advanced materials. Some of the most exciting new materials being studied today use mechanically interlocking polymers to access extraordinary properties such as self-healing, impact resistance, exceptional toughness, high extensibility and recoverability, to name a few. These unique polymers contain two or more components that are not covalently bound to each other, rather they cannot be separated without breaking a covalent bond. For instance, polyrotaxanes have a “rings-on-a-string” structure with bulky stopper groups to prevent dethreading, and polycatenanes are composed of interlocking ring molecules. Through the synergy of organic chemistry, materials science, and polymer physics, the Rowan group synthesizes and studies these new mechanically interlocked polymers with the aim of designing next-generation materials.

The chemical synthesis of interlocking molecules is a major challenge and requires highly sophisticated techniques; in fact, the 2016 Nobel Prize in Chemistry recognized achievements in this field by Prof. Rowan’s postdoctoral advisor Sir J. Fraser Stoddart, among others. In the Rowan group, we use organic chemistry and molecular engineering principles to design templating motifs that facilitate the assembly of interlocked polymers in high yield. In 2017, we used this approach to synthesize the world’s first linear poly[n]catenane, which is composed entirely of interlocking cyclic molecules and is the molecular equivalent of a macroscopic chain. Current efforts are focussed on refining the synthesis of poly[n]catenanes, and applying these templating techniques to the synthesis of polyrotaxanes. The long term aim of these projects is to understand the structure-property relationship of materials that incorporate these sliding components.

Beyond their potential as high-performance materials, mechanically interlocked polymers also serve as model systems for studying the effects of topology in polymer systems. These effects are also important in other systems of scientific interest, for instance knotted polymers, proteins, ring polymer solutions/melts, and cellular chromatin. In collaboration with the de Pablo group, we use theory and simulation to explore the fundamental physics of interlocking polymers and related systems. For example, we recently demonstrated that mechanical bonds dramatically alter the dynamics of linked rings compared to their unlinked counterparts and showed how these effects can be related to entanglement in traditional linear polymer systems.

Selected Publications

Abstract:

As the macromolecular version of mechanically interlocked molecules, mechanically interlocked polymers are promising candidates for the creation of sophisticated molecular machines and smart soft materials. Poly[n]catenanes, where the molecular chains consist solely of interlocked macrocycles, contain one of the highest concentrations of topological bonds. We report, herein, a synthetic approach toward this distinctive polymer architecture in high yield (~75%) via efficient ring closing of rationally designed metallosupramolecular polymers. Light-scattering, mass spectrometric, and nuclear magnetic resonance characterization of fractionated samples support assignment of the high–molar mass product (number-average molar mass ~21.4 kilograms per mole) to a mixture of linear poly[7–26]catenanes, branched poly[13–130]catenanes, and cyclic poly[4–7]catenanes. Increased hydrodynamic radius (in solution) and glass transition temperature (in bulk materials) were observed upon metallation with Zn2+.

Link to paper
Abstract:

Poly[n]catenanes are mechanically interlocked polymers consisting of interlocking ring molecules. Over the years, researchers have speculated that the permanent topological interactions within the poly[n]catenane backbone could lead to unique dynamical behaviors. To investigate these unusual polymers, molecular dynamics simulations of isolated poly[n]catenanes have been conducted, along with a Rouse mode analysis. Owing to the mechanical bonds within the molecule, the dynamics of poly[n]catenanes at short length scales are significantly slowed and the distribution of relaxation times is broadened; these same behaviors have been observed in melts of linear polymers and are associated with entanglement. Despite these entanglement-like effects, at large length scales poly[n]catenanes do not relax much slower than isolated linear polymers and are less strongly impacted by increased segmental stiffness.

Link to paper
Mechanically interlocked polymers (MIPs), such as polyrotaxanes and polycatenanes, are polymer architectures that incorporate a mechanical bond. In a polyrotaxane, the mechanical bond is the result of a linear dumbbell component threaded through a ring, while in a polycatenane, it is the consequence of interlocked ring components. The interlocked nature of these architectures can result in high degrees of conformational freedom and mobility of their components, which can give rise to unique property profiles. In recent years, the synthesis and studies of a range of MIPs has allowed researchers to build an initial understanding of how incorporating mechanical bonds within a polymer structure impacts its material properties. This Review focuses on the understanding of these structure–property relationships with an outlook towards their applications, specifically focusing on four main classes of MIPs: polyrotaxanes, slide-ring gels, daisy-chain polymers and polycatenanes.

Link to paper

Selected Publications

Poly[n]catenanes: Synthesis of molecular interlocked chains

Topological Effects in Isolated Poly[n]catenanes: Molecular Dynamics Simulations and Rouse Mode Analysis

Material properties and applications of mechanically interlocked polymers