Role of Oxygen and BIS(N-arylsalicylaldimino)- Cobalt(II) complexes in radical polymerization.

Abstract

The technique of DSC has been successfully applied to analyze the exact role of oxygen (either as an initiator or as a retarder) during the radical polymerization of acrylamide. It is found that in the melt polymerization of acrylamide, oxygen interferes in the propagation step and produces peroxy radicals that propagate very slowly compared to the normal propagation reaction. Consequently, the retardation of the overall polymerization takes place in the presence of oxygen. The Okamoto–Oka reaction has been successfully extended to the case of the tetrahedral cobalt–Schiff base complexes–NaBH? redox system in carrying out the oxidation of styrene to 1?phenylethanol. Thus, the formation of yet another novel oxidation product of the vinyl radical, apart from the usual peroxide and epoxide products, is demonstrated in the present study. The present investigation has also enabled us to carry out the radical polymerization of styrene at room temperature. The use of tetrahedral cobalt complexes in this reaction is particularly important because their applications as initiators for radical polymerization are hitherto unknown. The formation of a non?polymeric product (1?phenylethanol) via the radical intermediate produced during the redox reaction of the cobalt complex, NaBH?, and styrene is a rare demonstration leading to the elucidation of the mechanism of oxygen inhibitor action during a radical polymerization. The dilatometric study of the kinetics of the bulk polymerization of styrene and MMA has been carried out in the presence of the additives A and B. Thermal as well as AIBN?initiated melt polymerization of acrylamide has been studied in the presence of the additives A–D using DSC. The additives impart a nominal induction period in the styrene polymerization, but it is significant in the MMA and acrylamide polymerizations. A difference in the reactivity of the propagating radical towards the additives is responsible for this behaviour. Styrene polymerization is nominally retarded by the additives, while an unexpected acceleration is brought about by the same additives in MMA polymerization. Decomposition of the CoH? complex, formed during the reaction of the propagating radicals of MMA with the additives, provides an additional initiation reaction. This leads to an increase in the rate of initiation. Consequently, the overall rate of the MMA polymerization is accelerated in the presence of the additives. The participation of the monomer in this extra initiation step is responsible for the reaction order higher than unity for the monomer, as observed experimentally. These changes are significant and are brought about by the tetrahedral cobalt complexes only because of the coordinately unsaturated nature of the central metal ion, which is responsible for the reaction of the radicals with the additives. This once again proves that the coordinately unsaturated nature of the central metal ion is responsible for the variation in radical polymerization observed in the presence of transition?metal additives. The additives bring about a significant inhibition of the thermal polymerization of acrylamide, while they only delay the AIBN?initiated acrylamide polymerization. A large difference in the concentration of the radicals results in such variations. The effect of the additive concentration on the rate of polymerization of AIBN?initiated MMA polymerization is found to be very complex and does not show any definite trend.