Experimental and theoretical studies of laser ablation of polymers, under various processing conditions, have identified many possible photoexcitation pathways and consequently many likely processes responsible for the onset of ablation. We investigate the role of these processes—namely the thermal, mechanical and chemical processes—occurring in a polymeric substrate during UV irradiation. Molecular dynamics simulations with an embedded Monte Carlo-based reaction scheme were used to study ablation of Poly (methyl methacrylate) at 157 nm. Laser-induced heating and chemical decomposition of the polymeric substrate are considered as ablation pathways. For the heating case, the mechanism of ejection is thermally driven limited by the critical number of bonds broken. This fragmentation process is well reproduced by the existing bulk photothermal ablation model. Alternatively, if the photon energy goes toward direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products leading to near complete decomposition, loss of strength and cohesiveness of the top layers of the polymeric substrate. The ejection of small gaseous molecules weakens and hollows out the substrate, facilitating liftoff of larger fragments of material. These larger clusters are thermally ejected and the photochemical ablation process can be described by the two-step model proposed by Kalontarov.