Pressure and Temperature Dependence of the Decomposition Rate of Aliphatic tert-Amyl Peroxyesters

Abstract

Thermal decomposition of several aliphatic tert-amyl (TA, 1,1-dimethylpropyl) peroxyesters, RC(O)OOTA, has been studied in dilute solution of n-heptane at pressures up to 2500bar and temperatures up to 195°C. The peroxides under investigation were: tert-amyl peroxyacetate (TAPA), tert-amyl peroxy-n-butanoate (TAPnB), tert-amyl peroxy-iso-butanoate (TAPiB, TA peroxy-2-methyl propionate), tert-amyl peroxy-2-ethylhexanoate (TAPO), and tert-amyl peroxypivalate (TAPP, TA peroxy-2,2-dimethyl propionate). The experiments were carried out in a tubular reactor at residence times up to 140s. Peroxide concentration was monitored under continuous flow conditions via quantitative FT-IR spectroscopic analysis in an optical high-pressure cell positioned behind the tubular reactor. First-order decomposition kinetics over several half-lives were observed. For each TA peroxyester, rate equations for the first-order rate coefficient, k_obs(p,T), are presented. k_obs values for TA peroxyester decomposition are by 23 ± 9 per cent above the corresponding numbers reported for associated tert-butyl (TB) peroxyesters, with the same R group and at identical p, T, and solvent environment. The carbon atom of the R moiety that is in α-position to the carbonyl group controls decomposition kinetics. k_obs is smallest in cases where this particular C-atom is a “primary” one and is largest when it is “tertiary”. The “primary” TA peroxyesters are associated with activation energies around 140 kJ mol^-1 whereas the E_A′s of the “secondary” and “tertiary” TA peroxyesters are around 130 und 120 kJ mol^-1, respectively. The activation volumes, ΔV^‡_obs, of the “secondary” and “tertiary” TA peroxyesters are in the narrow range of ΔV^‡_obs = 3.0 ± 1.5cm³mol^-1. The ΔV^‡_obs′s of the “primary” TA peroxyesters are above 8cm³mol^-1. Within the limits of experimental accuracy, the activation parameters of TA peroxyester decomposition are identical to the corresponding quantities reported for the associated TB peroxyesters. The activation parameters suggest that “primary” aliphatic peroxyesters decompose via single-bond scission whereas the “secondary” and “tertiary” TA peroxyesters undergo concerted two-bond scission or extremely fast successive scission of the two bonds which latter two processes can not be distinguished on the timescale of the experiments. The mode of primary bond dissociation, via single-bond or via concerted two-bond scission, largely affects initiator efficiency in free-radical polymerization. The peroxide decomposition rate data measured in dilute solution of compressed n-heptane are useful for simulation and optimization of technical high-pressure ethene polymerizations.