Hydroxocobalamin reduced with ascorbic acid acts as a spin trap for free radical polymerization in water. The reaction mechanism is dependent on monomer type. An acrylic olefin such as 2-hydroxyethyl acrylate (HEA) is polymerized at pH 7 and in the presence of a conventional radical source mainly through reversible deactivation by a coupling mechanism. In contrast, a methacrylate like polyethylene glycol methacrylate undergoes catalytic chain transfer oligomerization.