To examine the polar effect on spin trapping with 2,4,6-tri-tert-butyl-1-nitrosobenzene (BNB), propagating radicals from monomers were allowed to react with BNB to provide anilino radicals and nitroxides of which the amounts were determined by ESR spectroscopy. The proportion of the anilino radical increased with an increase in the steric congestion and electron donating character of the radical. Poly(vinyl ether) radicals exclusively gave the respective anilino radicals. Poly(vinyl ester) radicals yielded large amounts (>50%) of the anilino radicals, although the nitroxides were the main products (>85%) from the poly(alkyl acrylate) radicals. The polymer radicals of methyl methacrylate and ethyl itaconate only yielded the anilino radicals. The polymer radicals from ethyl alpha-chloroacrylate and acrylonitrile reacted preferentially with the respective monomers leading to polymer formation, and the ESR spectra of the adducts were not observed.