The polymn of vinyl acetate (VA), Me methacrylate (MMA), and Me acrylate (MA) in Et acetate and THF soln. was initiated by the two-component initiating systems Fe(III) 2-ethylhexanoate (I) or Co(II) acetylacetonate (II) and pentamethyldisiloxane as co-initiator. I initiated the polymn. of all three monomers also in the absence of the siloxane, with an activity increasing from VA to MA, II was completely inactive under these conditions. Addn. of the siloxane co-initiator raised the monomer conversions with both transition metal initiators, particularly pronounced for MMA and MA. No.-av. degrees of polymn. for poly(vinyl acetate) and poly(Me acrylate) amounted to some hundred or less, but increased to several thousands for poly(Me methacrylate). ESR spin trap expts. with N-tert-butyl-a-phenylnitrone (PBN) revealed that the transition metal compds. and the siloxane undergo a reaction in which free radicals are produced by oxidative cleavage of the co-initiator to generate hydrogen atoms which initiate the polymn. With I-PBN adducts of carbon-centered radicals are obsd. in the absence and in the presence of the monomers. By way of contrast, with II only the hydrogen atom adduct to PBN is obsd. in the absence of the monomers. II is likely to form a new complex with the co-initiator and the spin-trap mols. The coordinated PBN competes with the two parent ligands of the complex for the liberated hydrogen atoms, however, due to the favorable reaction rate the PBN-H adducts is formed preferably. The PBN adducts of carbon-centered radicals replace the PBN-H adduct when the monomers are added to the initiator soln., i.e. the H atoms are now preferably trapped by the monomer mols.