Dependence of O₂ Depletion on Transition Metal Catalyst in Radical Polymerization of Cross-Linking Alkene Resins

Cobalt(II) carboxylates show broad reactivity with peroxides and O₂ and are the industry standard catalyst for the activation of peroxide initiators for the radical polymerization of alkenes under ambient conditions. Curing alkene-based resins containing cross-linking units, i.e., monomers containing two or more alkene units, is important in forming hard protective coatings and materials. The activation of peroxide initiators produces the propagating chain end radicals needed for polymerization. Since polymerization progress depends on the rate of initiator activation and the concentration of propagating radicals, interception of radicals by O₂ can inhibit curing. Cobalt(II) carboxylates are used due to their reactivity in the presence of oxygen, even in resin coatings. Alternative catalysts based on manganese and iron are desirable. Hence, the impact of O₂ on their performance in resin curing is of interest. Here, we use NIR emission and time-resolved spectroscopy, employing the O₂-sensitive probe [Ru(ph₂phen)₃]²⁺, to determine the concentration of dissolved [O₂] in alkene resins during curing with three representative catalysts, Co(II)(2-ethylhexanoate)₂, Fe(II)-bispidine, and Mn(II)(neodecanoate)₂. The rate of depletion of O₂ is highly dependent on the catalyst used, but in all cases, it is well before the onset of the autoacceleration of polymerization in cross-linking resins.