High-pressure tuned polymerization kinetics have been examined to elucidate the (photon-assisted) polymerization mechanism of phenylacetylene (PA, C6H5C≡CH3) under different stimuli including high-pressure, physical state, and UV radiation effects by using in situ FTIR spectroscopy. The pressure-induced glass-forming and crystalline states of PA are found to be formed at different compression rates. The experimental results suggest that the polymerization is induced in the glass-forming state at a very low pressure in contrast to that in the crystal phase where much higher threshold pressure is required. The measured rate constants were found to strongly depend on the pressure, the physical state, and UV radiation. In particular, the rate constant reduced to different extents either by UV irradiation or upon phase transition. The derived activation volumes from the rate constants allow the direct comparison and elucidation of photoactivation and environmental effects on the polymerization. Finally, the diffusion-controlled 1D polymer growth processes were suggested for the glass-forming state or the crystal state at specific pressures. Overall, the mechanistic insights of this work provide guidance of optimizing the multiplexed reaction conditions for the production of the conducting poly(PA).