Retardation Effect Induced by Pyridine in the Radical Copolymerization of Styrene and Liquid Sulfur Dioxide

We have found that the radical copolymerization of styrene and liquid sulfur dioxide (liq. SO2) is retarded when a basic solvent such as pyridine is used; we also investigated the mechanism of the retardation induced by pyridine. The polymerization in pyridine was most strongly retarded when the styrene mole fraction in the feed, [St]0/ ([St]0+[SO2]0), was small (below about 0.2) and when the pyridine mole fraction in pyridine-liq.SO2, [Py]0/ ([Py]0+[SO2]0, was 0.4–0.5. Moreover, from the results in the reaction of the phenylsulfonyl radical (the model radical for the propagating sulfonyl radical) in pyridine or in a pyridine - liq. SO2 mixture, it may be concluded that the end unit of the polymer has the pyridinium sulfonate structure (–SO3−HPy+) and that the retardation may occur by means of the reaction of the propagating sulfonyl radical with the pyridine-SO2 complex.