Philipp Becker, Michael Buback, Johannes Sandmann
Initiator Efficiency of Peroxides in High-Pressure Ethene Polymerization
Macromol. Chem. Phys. 203, 2113-2123 (2002)

The initiator efficiency of three organic peroxides, tert-butyl peroxypivalate (TBPP), tert-butylperoxy-2-ethylhexanoate (TBPEH), and di-tert-butyl peroxide (DTBP), has been measured in a stirred tank reactor (CSTR) which is part of a continuously operated miniplant device. The experiments were carried out at 2 000 bar and temperatures between 150 and 250 °C. Initiator efficiency f is defined as the fraction of primary (initiator-derived) free-radical species that add to monomer molecules. The reported data refer to ideal CSTR mixing conditions. The analysis of f is based on results of separate measurements of (i) rate coefficients of peroxide decomposition carried out in dilute solution of heptane and of (ii) ethene propagation and termination rate coefficients from laser single-pulse experiments. The novel quantity reduced peroxide-induced monomer conversion defined as the amount of monomer polymerized per square root of the amount of decomposed initiator turns out to be very suitable for deducing initiator efficiency. At the low and moderate ethene conversions, up to 10%, which were applied in the present study, f appears to be independent of temperature. The values of f obtained for DTBP, TBPEH, and TBPP are close to 1.0, 0.64, and 0.42, respectively. The relative low efficiency in TBPP polymerization is primarily assigned to in-cage cross-disproportionation of the tert-butoxy radical with the tert-butyl radical that is instantaneously produced, together with carbon dioxide, during TBPP decomposition. The primary fragments from DTBP decomposition cannot undergo cross-disproportionation, as hydrogen atoms in -position to the oxygen-centered free-radical site are not available.