A number of monomers have been polymerized in the presence of catalytic and higher concentrations of SO2. The addition oft-butyl hydroperoxide greatly accelerates the rate of polymerization. The use of35SO2indicates that at catalytic concentrations of SO2(10−2mole/liter), only one or two molecules of SO2are incorporated in the chain, but, at high SO2concentrations, copolymerization of SO2with vinyl monomers occurs.