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The inhibitory effect of oxygen on the radical photopolymerization is well known. Oxygen reacts with carbon —centered propagating radicals at the diffusion-controlled limit to form peroxy radicals much less reactive towards double bond, thereby reducing the efficiency of initiation. This generally leads to significant retardation (or even inhibition) of the polymerization 1,2]. In open systems,' polymerization will only occur if the generation of radicals can compete successfully with the replenishment of oxygen. However, the peroxy radicals readily abstract a hydrogen atom from a polymer or monomer molecule regenerating a reactive radical, which also can react with oxygen. Such a chain oxidation process accelerates oxygen consumption in the polymerizing system 3]. The radical scavenging by oxygen is retarded by a rise in viscosity and will be severely restricted in the gelled and vitrified state.' Temperature rise also reduces oxygen inhibition due to decrease in solubility of oxygen in polymerizing medium 4]. Polymerization of multivinyl monomers occurs with significant radical trapping 1,5,6], thus the radicals can be eliminated from the reaction in two ways: in the first-order process involving only one polymer radical (monomolecular termination) characterized by the monomolecular termination rate coefficient
kf' and'in the second order process - the usual bimolecular interaction of polymer radicals
(bimolecular termination) characterized by the bimolecular termination rate coefficient k/'. Therefore, the termination process may be considered to occur according to one of three possible termination mechanisms: monomolecular, bimolecular and mixed 7-il]. The competition of the propagation 'process and radical scavenging by oxygen in association with further reactions of peroxy radicals highly affects the polymerization kinetics. The influence of oxygen on the photopolymerization kinetics was considered in two ways. Kloosterboer 1] suggested that the
chain oxidation process induces enhanced mobility of the radical sites in the network and leads to
enhanced bimolecular termination; this is reflected by reduction of the overall rate of polymerization. The influence of oxygen on the rate of polymerization first increases with conversion. This indicates that termination by direct combination is gradually replaced by termination via oxygen induced hydrogen transfer, which is caused by strong reduction of the mobility of macroradicals. Towards the end of the reaction, the rate reduction by oxygen is progressively suppressed by the vitrification of the sample. Thus, we may expect that reaction of oxygen with trapped radicals will reduce the contribution of the monomolecular termination to the overall termination process. On the other hand, in other works 8,9], reaction of carbon-centered radicals with oxygen with the formation of relatively unreactive peroxy radicals was indicated as a pseudo first-order termination process.