Kinetics of block polymerizations

Abstract

AbstractThe chain decomposition of tert‐butyl hydroperoxide in the presence of copperoctoate producestert‐butoxide and tert‐butyl peroxy radicals which are both able to initiate the polymerization of styrene. Consequently, half the number of the endgroups of the polymer molecules will be present, as peroxybutyl group, which can initiate a second‐step polymerization. In the first part of this paper, the kinetics of polymerization of styrene with t‐butyl hydroperoxide (t‐BHP) and copper octoate have been studied. In the absence of monomer, the chain decomposition is relatively short on account of the reaction of the Cu1+ ion with tert‐butyloxide radical and the precipitation of the copper salt; contrarily, in the presence of styrene the primary radicals add preferentially monomer, and no copper salt precipitation occurs. In both cases the overall energy of activation of decomposition t‐BHP/Cu2+ octoate is equal to above 21 kcal/mole. At constant Cu(oct)2 concentrations the square root law relationship between the rate of polymerization and t‐BHP concentration is only valid for the low concentrations; at higher values, it increases much slower up to an asymptotic value. Similar conclusions arise from the rate dependence on the copper octoate concentrations at constant t‐BHP and monomer concentrations.In the second part of this paper, the polystyrene carrying peroxy‐t‐butyl end‐groups has been used as initiator of the polymerization of a second monomer (methyl methacrylate and vinyl acetate) for the synthesis of block polymers at 97°C. This second step polymerization is a first order reaction with respect to the monomer concentration, and 0.5 order with respect to the initiator. Some thermal polymerization of methyl methacrylate must be taken into account for the evaluation of the amounts of second homopolymer. The relative reactivity of both radicals has been evaluated from their ability of further monomer addition; by measuring the amounts of homopolymer RH and block polymer RB by fractional precipitation and determination of their styrene content, it was possible to determine the efficiency of block polymerization.In the case of methyl methacrylate the ratio RB/RH is equal to 1.2 in agreement with the chain termination reaction; therefore the efficiency of both radicals is admitted to be equal, and consequently independent of their size.In the case of vinyl acetate the much lower value of RB/RH (0.26) is not a measure of the relative efficiency but a consequence of chain transfer reactions on monomer and solvent, which increase the amount of homopolymer.