Abstract: A free radical polymerization process for the preparation of thermoplastic resin or resins comprising:heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound;adding under pressure a supercritical fluid, and mixing with sulfur dioxide;polymerizing said polymerizable monomer to form said thermoplastic resin or resins with a high monomer to polymer conversion;cooling said mixture;optionally isolating said thermoplastic resin or resins; andoptionally washing and drying said thermoplastic resin, or resins.