FREE-RADICAL INITIATION MECHANISMS IN THE POLYMERIZATION OF METHYL-METHACRYLATE AND STYRENE WITH 1,1,3,3-TETRAMETHYLBUTYL PEROXYPIVALATE - ADDITION OF NEOPENTYL RADICALS

The reactions of 1,1,3,3-tetramethylbutyl (tert-octyl) peroxypivalate (1) with methyl methacrylate (MMA) and styrene in the presence of the free radical scavenger 1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-ylo xyl (2) have been studied at 60 degrees C. tert-Butyl and tert-octylox yl radicals (3) were generated from the thermolysis of 1. The predomin ant unimolecular reactions of 3, that is, beta-scission to form neopen tyl radicals (14b) and a 1,5-H shift to form 4-hydroxy-2,2,4-trimethyl pentyl radicals (14c), were observed in both monomer systems. The resu lting alkyl radicals underwent selective addition to the two monomers. The relative reactivities of the alkyl radicals toward addition to th e monomers were obtained from competitive addition/trapping reactions. The absolute rate constants for the addition of alkyl radicals 14b an d 14c to the two monomers at 60 degrees C were estimated to be 9.5 x 1 0(5) and 2.6 x 10(5) M-1 s(-1) to MMA and 4.5 x 10(5) and 0.7 x 10(5) M-1 s(-1) to styrene, respectively. The low reactivities of 3 and 14c toward addition to MMA and styrene were attributed to steric effects. Steric effects were also responsible for the low rate of the 1,5-H shi ft in 3.