Thermoreversible Reactions on Inorganic Nanoparticle Surfaces: Diels-Alder Reactions on Sterically Crowded Surfaces

Organically surface-functionalized nanoparticles are important cross-linkers for nanocomposites. In the past, many cross-linking reactions were based on simple radical additions. However, novel smart materials require reversible reactions. These reactions, such as the Diels-Alder reaction, often have a specific sterical demand, e.g., a six-centered transition state. In this study, <5 nm silica particles were functionalized with maleimide groups, and their reactivity with regard to Diels-Alder reactions were investigated, applying various techniques. A new method for the surface modification of silica nanoparticles is presented, minimizing agglomeration in organic solvents and thus increasing the accessibility of the functional groups on the particle surface. Kinetic studies of substituted model compounds were carried out to evaluate the reactivity of the maleimide functionality. The Diels Alder reaction between 2,5-dimethylfuran and N-propylmaleimide, N-ethyl(N-propylcarbamato)maleimide, and N-phenylmaleimide was followed by UV/Vis spectroscopy. The reaction rate increases in this order, showing the effect of maleimide substitution. Afterwards N-((3-triethoxysilyl)propyl)maleimide was used to graft maleimidopropyl functional groups onto the nanoparticle surface. 3-Aminopropyltriethoxysilane, which could then be reacted with 1,1'-(methylenedi-4,1-phenylene)bismaleimide, was used to attach phenyl-substituted maleimide functionality to the surface. 3-Isocyanatopropyltriethoxysilane introduced the electron-drawing carbamato functionality into the system. The surface coverage of the samples was characterized applying CHN analysis, TGA-FTIR coupling, and FTIR spectroscopy. All analytical methods revealed that the functional groups are covalently bonded to the silica surface and the maleimide rings remain intact. Diels Alder reactions of the surface groups show that the reactivity of the molecules attached to the particles depends on sterical crowding, but the reaction rate is not significantly changed by surface effects.