The reactions of tert-pentyl peroxypivalate (1b) and tert-hexyl peroxypivalate (1c) with methyl methacrylate (MMA) in the presence of the free radical scavenger 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxy (2) have been studied at 60 degrees C. tert-Pentyloxyl radicals (3b) and tert-hexyloxyl radicals (3c) were formed in the thermolysis of Ib and le. The major reaction pathways of the alkoxyl radicals were beta-scission and rearrangement (1,5-H shift) to generate alkyl radicals, which occurred to the extent of 83% (3b) and 90% (3c), respectively. The resulting alkyl radicals underwent selective addition to MMA. The overall ratios of addition to H-abstraction in the reactions of 3b and of 3c with MMA were 13:1 and 24:1, respectively. From the competitive addition/trap reactions of alkyl radicals, relative reactivities of alkyl radicals toward addition to MMA were in the ratio : Et-.:n-Pr-.:4-hydroxy-4-methylpentyl radical:Bu-t .=0.39:0.48:0.51:1.00.