Thermoplastic Silicone Elastomers through Self-Association of Pendant Coumarin Groups

Although there are many benefits associated with thermoplastic elastomeric silicones, very few examples exist: silicone elastomers are normally thermoset materials. We have discovered that the simple incorporation of coumarin groups on linear silicone polymer backbones creates physical silicone polymeric networks that exhibit thermoplastic elastomeric properties in the absence of covalent cross-links. A range of materials was prepared by incorporating four different concentrations of coumarin along the silicone backbone using thermal azide/alkyne cycloaddition reactions: higher coumarin concentrations lead to more tightly cross-linked, higher modulus materials. Intermediate properties could be obtained by mixing silicones with different coumarin loadings in the melt. Physical cross-links arise from 1:1 coumarin complexes. As a consequence, it is possible to reduce cross-link density by adding silicones bearing a single coumarin to an elastomer. The physical interactions between coumarin-triazoles on the silicone polymers could be temporarily overcome thermally as shown by tensile, rheometry and thermal remolding experiments. The simple expedient of grafting coumarin groups, which cross-link reversibly through head-to-tail π-stacking, to silicone chains allows one to tailor the mechanical properties of these thermoplastic elastomers, enhancing their utility.