Graft copolymerization with perester side groups

Abstract

AbstractThe kinetics of the graft copolymerization of styrene onto methyl acrylate (MA) and methyl methacrylate copolymers (MMA) containing tert‐butyl perester groups have been studied. The synthesis of these perester copolymers was carried out in two steps: (1) copolymerization of acrylyl chloride with methyl (meth)acrylate, and (2) reaction of the acrylyl chloride copolymer with tert‐butyl hydroperoxide. Further, these perester copolymers were usually treated with diazomethane in order to methylate any carboxylic group which might be present. The graft copolymers were separated from both hompolymers by progressive precipitation on addition of methanol to the chloroform solution of the copolymers. The stability of these perester copolymers has been considered and found to depend on whether there are acid groups in the copolymer. The first‐order rate constants and the activation energy of decomposition in the absence of acid groups have been evaluated. These values depend on the molar composition of the copolymer; an induced decomposition of peresters is favored by a high perester content, which is responsible for a decrease in Ea (26.8 instead of 29.6 kcal./mole). In the presence of free carboxylic groups, some of the perester groups decompose very rapidly, the energy of activation of their decomposition being only 15 instead of 25.6 kcal./mole for the slow step. The degree of conversion attained during the rapid step is a function of the stereochemical structure of the copolymer, and the rate of decomposition is much higher than that produced by the addition of external acid. With the MA perester the rate of grafting is proportional to the square root of the perester concentration while it is first order with respect to the monomer concentration. These dependences were found to hold also for the overall rate of polymerization (from the total weight increase) as well as for the grafting rate (from the isolated graft copolymer and its styrene content). The styrene content of the graft copolymer increases with monomer concentration (e.g., 53.8 and 75.3 for monomer concentrations of 2.6 and 6.1 moles/l., respectively). At low monomer concentration, grafting is greatly favored over homopolymerization; it is also enhanced by the viscosity of the solution. With the MMA perester also, the grafting rate obeys the square root law for the perester concentration and the first‐order dependence on monomer. However, the rate of grafting is practically independent of the styrene concentration over a large range. The relative importance of grafting over homopolymerization is in this case much less than with MA‐TBP, except at high dilution. Different interpretations are suggested in order to explain the very high and sometimes very low values of Rg/Rh, the ratio of the rate of grafting to the rate of hompolymerization. The different behaviors of the MA and MMA peresters are discussed.