Radical polymerization of styrene falls under control when conducted at 100 degreesC, in the presence of an ortho-quinone, e.g., phenanthrenequinone (PhQ): and a catalytic amount of cobalt(II) acetylacetonate. Criteria for a controlled chain polymerization are fulfilled at least until 50% of monomer conversion, i.e., molar mass increasing with monomer conversion. molar mass predictable from the styrene/ quinone molar ratio, linear time dependence of ln([M](o)/[M]), and effective resumption of Styrene polymerization by preformed oligopolystyrene. A tentative mechanism is proposed for this new System designated as quinone transfer radical polymerization (QTRP).