The present thesis aims at new approaches towards the preparation of functional polymer materials with tunable mechanical properties. By introducing photoreactive crosslinks either into the polymer backbone or in the interface between inorganic fillers and fibers and the polymer matrix, smart composite materials with photoswitchable material properties are obtained. Functional polynorbornenes bearing anthracene molecules in their side chain were synthesized by ring opening methathesis polymerization (ROMP). UV irradiation leads to a crosslinking of the pendant anthracene molecules due to the [4π+4π] photocycloaddition resulting in a decrease in solubility of the UV illuminated areas. A thermally induced decrosslinking was achieved, revealing the reversibility of this photoreaction. The influence of the flexibility of the macromolecules, on i.e. the mobility of the anthracene groups in the polymeric material, the conversion rate, the reversibility and the crosslinking behavior was studied by means of spectroscopy and sol-gel analysis. The present work further focuses on the design of reversibly crosslinkable epoxy-based networks that undergo well-defined bond formation and bond cleavage in response to external stimuli. A new anthracene epoxy monomer was synthesized which can be used as crosslinker in epoxy-based resin formulations. Upon UV irradiation a crosslinking of the resin is accomplished via the photodimerization of the anthracene moieties, whilst a subsequent thermal step leads to a cleavage of the dimer. The crosslinked resin samples were characterized using mechanical testing methods over several photochemically triggered repair cycles. The results evidence the ability of the photo-healable epoxy-based networks to restore their mechanical properties after the inflicting of mechanical cracks at the same fracture point. In another approach a new epoxy monomer bearing UV cleavable o-nitrobenzyl ester groups was synthesized and characterized. Due to the photoinduced cleavage reaction of the o-nitrobenzyl ester groups a distinctive decrease of the storage modulus and the glass transition temperature of the cured resin is observed upon UV illumination. In terms of composite materials, the o-nitrobenzyl ester derivatives were also covalently attached onto glass fibers which were used as inorganic fillers in a thermosetting resin. Fiber pull-out tests were performed for characterization of the adhesion strength between cured resin and functional filler, and it is evident that the photocleavable matrix makes the formation of composite materials with enhanced recyclability feasible.