Herein, a green approach to crosslinked polymer microspheres with undoped methacrylate monomers is presented. The protocol is set up by means of photoinitiated precipitation polymerization of a trifunctional crosslinker, i.e. trihydroxymethyl propane trimethyl acrylate (TMPTMA) and methyl methacrylate (MMA) for the purpose of achieving pure and crosslinked microspheres. The evolution of photoinitiated precipitation polymerization is tracked and the formation mechanism of poly(TMPTMA–MMA) microspheres is also elucidated. Finally, the potency of so-obtained crosslinked microspheres is explored to serve as organic fillers in dental restorative materials.