Self-healing nanocomposites were studied consisting of surface-functionalized silica nanoparticles and polymers with different glass transition temperatures. The silica nanoparticles were synthesized using the Stöber process and afterwards surface-functionalized with the coupling agents 2-furyl-(undecenyl)-11-triethoxysilane or 3-maleimidopropyltriethoxysilane. The obtained nanoparticles were studied as cross-linking agents in thermally triggered self-healing nanocomposites based on Diels-Alder (DA) chemistry. Cross-linking of the nanoparticles with modified poly(butyl methacrylates) containing DA groups revealed that the rigid thermoplasts show only low tendency to undergo the cross-linking reactions. Contrary, similarly modified polysiloxanes display thermally reversible cross-linking in DSC as well as in rheology measurements. The results reveal that a specific extent of mobility and flexibility of the polymer backbones is crucial for a reasonable fast self-healing process if Diels-Alder groups are used as crosslinking agents.