Determination of the relative rates of addition of styrene and acrylonitrile to the 1-(1,3-diphenyl)propyl and 1-(3-cyano-1-phenyl)propyl radicals. Evidence for a penultimate effect in radical copolymerization

1,1’-Azobis( 1,3-[ 1-^(13)C]diphenylpropane) (1b) and 4,4’-azobis(4-phenyl[4-^(13)C]butyronitrile) (1C) were prepared overall yields of 10% and 1.2%, respectively, both starting from Ba^(13)CO_3 (99 atom %). Analysis of end-group concentrations styrene-acrylonitrile (SAN) copolymers prepared with 1b as initiator allows accurate determination of the relative rates addition of these monomers (k_s/k_A) to the 1-( 1,3-diphenylpropyl) radical (2b); similarly, analysis of SAN copolymers prepared IC allows determination of k_s/k_A for the 1-(3-cyano-l-phenylpropyl) radical (2c). Significantly different preferences the radicals for addition of styrene and acrylonitrile were observed; we obtain k_s/k_A = 0.21 f±0.01 for 2b and k_s/k_A = 0.52 ± 0.03 for 2c. These results show that k_s/k_A for 1-(1-phenylalkyl) radicals is sensitive to substitution y to the radical and provide strong support for the existence of a penultimate unit effect in SAN copolymerization. The results are consistent with the penultimate model analysis of SAN copolymerization by Hill, O’Donnell, and OSullivan.