FREE-RADICAL POLYMERIZATION OF ACRYLATES AND VINYL-ACETATE INITIATED BY TRANSITION-METAL HYDROSILANE 2-COMPONENT INITIATION SYSTEMS

The polymerization of vinyl acetate (VA), methyl methacrylate (MMA), a nd methyl acrylate (MA) in ethyl acetate and tetrahydrofuran solution was initiated by the two-component initiating systems Fe(III) 2-ethylh exanoate or Co(II) acetylacetonate and pentamethyldisiloxane as co-ini tiator. Whereas Fe(III) 2-ethylhexanoate initiated the polymerization of all three monomers also in the absence of the siloxane, with an act ivity increasing from VA to MA, Co(II) acetylacetonate was completely inactive under these conditions. Addition of the siloxane co-initiator raised the monomer conversions with both transition metal initiators, particularly pronounced for MMA and MA. Number-average degrees of pol ymerization for poly(vinyl acetate) and poly(methyl acrylate) amounted to some hundred or less, but increased to several thousand for poly(m ethyl methacrylate). Electron spin resonance (ESR) spin trap experimen ts with N-tert-butyl-alpha-phenylnitrone (PEN) revealed that the trans ition metal compounds and the siloxane undergo a reaction in which fre e radicals are produced by oxidative cleavage of the co-initiator to g enerate hydrogen atoms which initiate the polymerization. With Fe(III) 2-ethylhexanoate PEN adducts of carbon-centered radicals are observed in the absence and in the presence of the monomers. By way of contras t, with Co(II) acetylacetonate only the hydrogen atom adduct to PBN is observed in the absence of the monomers. Co(acac)(2) is likely to for m a new complex with the co-initiator and the spin-trap molecules. The co-ordinated PEN competes with the two parent ligands of the complex for the liberated hydrogen atoms, however, due to favorable reaction r ate the PBN-H adduct is formed preferably. The PEN adducts of carbon-c entered radicals replace the PBN-H adduct when the monomers are added to the initiator solution, i.e. the H atoms are now preferably trapped by the monomer molecules.