  We report here a study of reversible pressure-induced structural
transformation between two amorphous forms of SO$_2$: molecular at pressures
below 26 GPa and polymeric above this pressure, at temperatures of 77 - 300 K.
The transformation was observed by Raman spectroscopy and x-ray diffraction in
a diamond anvil cell. The same phenomenon was also observed in ab initio
molecular dynamics simulations where both forward and reverse transitions were
detected, allowing to analyze in detail the atomic structure of both phases.
The high-pressure polymeric amorphous form was found to consist mainly of
disordered polymeric chains made of 3-coordinated sulfur atoms connected via
oxygen atoms, and few residual intact molecules. The simulation results are in
good agreement with experimental data. Our observations suggest a possible
existence of molecular liquid - polymeric liquid transition in SO$_2$ and show
a case example of polyamorphism in system consisting of simple molecules with
multiple bonds.
