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

The thermal decomposition in solution of n-heptane of several aliphatic tert-butyl peroxyesters RC(O)OOTB: TB peroxypropionate, TB peroxy-n-butyrate, TB peroxy-iso-butyrate, TB peroxy-2-methylbutyrate, TB peroxy-2-ethylhexanoate, TB peroxy-3,5,5-trimethylhexanoate, and TB peroxy-neo-decanoate has been investigated at pressures up to 2500 bar and temperatures up to 185°C. The experiments were carried out in a tubular reactor at residence times up to 140 s. Peroxide concentration was monitored via quantitative FT-IR spectroscopic analysis of the pressurized medium under continuous flow conditions. For each peroxide first-order decomposition kinetics were observed over several half-lives. The type of carbon atom (in the R moiety) that is in α -position to the carbonyl group controls decomposition kinetics. The observed first-order rate coefficient, k_obs, is smallest in situations where this particular C-atom is primary and is largest when it is tertiary. The ´´primary´´ TB peroxyesters are associated with the higher activation energies, around 145 kJ mol^-1. The activation energies, E_A, of the ´´tertiary´´ and ´´secondary´´ TB peroxyesters are around 120 und 130 kJ mol^-1, respectively, with the individual E_A´s being proportional to the stability of the particular free-radical species R^·. The activation volumes, Δ V^≠_obs, of the ´´secondary´´ and ´´tertiary´´ TB peroxyesters are in the narrow range 3.0±1.5 cm³ mol^-1. Δ V^≠_obs of the ´´primary´´ TB peroxyesters is above 10 cm³ mol^-1 and, in addition, is slightly pressure-dependent. Both observations demonstrate that the ´´primary´´ aliphatic TB peroxyesters decompose via single-bond scission with this result being supported by modelling dissociation dynamics in the compressed fluid state. The ´´secondary´´ and ´´tertiary´´ TB peroxyesters decompose via concerted two-bond scission associated with the immediate formation of CO₂. These results are fully consistent with evidence from the literature and from our preceding detailed study into TB peroxyacetate and TB peroxypivalate dissociation kinetics. The mode of primary bond dissociation, via single-bond or via concerted two-bond scission, largely affects the initiation efficiency of TB peroxyesters in free-radical polymerization. For each TB peroxyester, rate expressions for k_obs(p,T) are presented. This data for decomposition in compressed n-heptane should be highly suitable for simulation and optimization of technical high-pressure polymerization processes.