Controlling the ring mobility in palladium-based slide-ring gels

A mechanical bond is not shared between atoms, instead, it develops between molecular entities entangled in space and provides higher structural integrity. The molecules resulting from the mechanical bonds are called mechanically interlocked molecules (MIMs), and the most common MIMs are catenanes and rotaxanes. These mechanically interlocked structures have been exploited in a wide range of research fields, in particular molecular machines and polymeric materials. The slide-ring gels is one of those research areas where the mechanically interlocked molecules or mechanical bonds are incorporated in a three-dimensional polymeric network. The aim of this work is to study the ring chain interactions in slide-ring gels based on the variation of the strength between the ring and the polymer chain by using palladium-based slide-ring networks. Due to the presence of palladium-based metal-ligand interactions in the network, the ring mobility can be controlled by removing or keeping the metal-ligand interactions between the ring and the thread. Thus, by using a single system, three different types of ring mobilities; from immobile, to partially mobile, to completely free mobility of the rings can be achieved without changing any other parameter. The palladium-based slide-ring gel systems are developed by two different approaches, named as polyrotaxane approach and one-pot approach. In the polyrotaxane approach, a polyrotaxane is first synthesised and then cross-linked to form the slide-ring gel, while in the one-pot approach, a gel is directly synthesised by using a tetra functional bis-pseudorotaxane and other precursors. The precursors used in both the approaches are roughly the same and utilised polyurethane chemistry to obtain the polymeric materials. The palladium-based pseudorotaxane is the most important precursor in this work. A number of pseudorotaxanes are synthesised to explore different strategies. In polyrotaxane approach, several trials were made to develop the slide-ring gels and eventually, a stable mechanically interlocked network was obtained by using a copper(I)-catalyzed alkyne-azide cycloaddition mediated cross-linking method. On the other hand, a completely stable one-pot slide-ring gel was never formed. The stepwise rheological study of the synthesised slide-ring gels proved that all the steps of slide-ring gel formation are accomplished. Moreover, the rheological results of slide-ring gel with palladium complex (fixed rings) and without palladium (movable rings) complex in different solvents (tetrachloroethane and dimethylacetamide) successfully showed the effect of three different ring mobilities. Finally, the hand stretchability test on all these different slide-ring gel samples showed good agreement with the rheological results.