The kinetics and selectivity of the decomposition of diacyl peroxides under high pressure in hydrocarbon solution

The decomposition kinetics of bis(3,5,5-trimethylhexanoyl) peroxide (BTMHP), dioctanoyl peroxide and dibenzoyl peroxide have been monitored under high pressure using time-resolved FTIR spectroscopy. Two experimental methods, one discontinuous the other continuous, were used for BTMHP, and their complementary nature allows measurement of the kinetics in an extended range of decomposition half-lives. Using n-heptane as solvent, investigations were carried out at temperatures from 60 up to 160 degrees C, and in the pressure range 1 to 2500 bar. For BTMHP an activation energy of (131.2+/-6.5) kJ mol(-1) was determined at 1500 bar. The value found at the same pressure for dioctanoyl peroxide is identical within experimental error, whilst that of dibenzoyl peroxide is slightly lower. An activation volume of Delta V-sic = (2.9 +/- 0.1) cm(3) mol(-1) was evaluated for the decomposition of BTMHP, and those of the remaining two peroxides found to be similar. The selectivity of the decomposition of BTMHP was considered by means of GC analysis. Using n-heptane and n-pentadecane as solvents, the effect of pressure on product distributions has been gauged from 1 to 2000 bar at 80 degrees C. For the two main decomposition products, 2,4,4-trimethyl pentane (TMP) and 2,2,4,7,9,9-hexamethyl decane (HMD), pressure was found to enhance the formation of in-cage alkane dimer (HMD) relative to out-of-cage alkane (TMD).