A polybutadiene rubber was functionalized with UV-dimerizable cinnamic moieties using thiol-ene click chemistry and the resulting polymer was used to prepare millimeter-thick objects with photo-tunable modulus. The crosslink density was modulated by controlling the irradiation time t under UV. For this purpose, the kinetics of the photo-crosslinking reaction was first investigated via conventional photo-rheology and infrared spectroscopy analyses. In parallel, Raman spectroscopy was used to probe the evolution of the crosslink density as a function of depth from the irradiated surface. These measurements provide insights into the crosslink density's evolution, both over time and as a function of depth within the samples. They reveal gradient profiles with diminishing amplitude over longer irradiation times. After 4 hours of irradiation, the crosslink density becomes mostly homogeneous. The concurrent investigation of the mechanical properties indicates that the elastic modulus of the materials can be varied over almost three orders of magnitude (4 MPa ≤ E ≤ 2 GPa), by varying the exposure time within a relatively small window (5 min ≤ t ≤ 240 min). To challenge the data collected in Raman spectroscopy, Atomic Force Microscopy nanoindentation was used for the local investigation of the modulus, within the materials. The results confirmed the presence of modulus gradients consistent with those of the crosslink density. This unprecedented study of the spatio-temporal evolution of crosslink density in a UV-crosslinkable polymer offers valuable insights for applications requiring soft materials with photo-tunable mechanical properties.