A method of curing a free-radically polymerizable composition includes contacting a curable composition with at least one metal oxide selected from the group consisting of magnesium oxide, ferrous metal oxides, aluminum oxide, nickel oxide, silver oxide, and combinations thereof. The curable composition includes: free-radically polymerizable compound; and a beta-dicarbonyl compound represented by the formula (I) or a salt thereof, wherein: X1 and X2 independently represent a covalent bond, wherein each R4 independently represents H or alkyl having from 1 to 18 carbon atoms; R1 and R2 independently represent a hydrocarbyl or substituted-hydrocarbyl group having from 1 to 18 carbon atoms, and R3 represents hydrogen, or a hydrocarbyl or substituted-hydrocarbyl group having from 1 to 18 carbon atoms, or taken together any two of R1, R2, or R3 form a five-membered or six-membered ring; an organic peroxide; and a quaternary ammonium halide. Articles including the cured compositions are also disclosed.