A study of the chain transfer reactions of poly(methyl methacrylate) radicals with substituted phenols

The chain transfer constants (C[lowered s]) of nineteen phenols with poly-(methyl methacrylate) radicals and the extent of retardation (R[lowered e]) of the polymerization reaction by these phenols were determined at 50.0[degrees]C. Attempts were made to correlate both C[lowered s] and R[lowered e] values with previously published antioxidant ratings of the phenols. R[lowered e] values correlated well with antioxidant ratings obtained from the oxidation of petroleum and with the oxidation potentials of the phenols. C[lowered s] values did not correlate well with antioxidant ratings for disubstituted phenols with bulky ortho substituents or with phenols possessing groups with benzylic hydrogens. The poor correlation of Cs values with antioxidant efficiencies appears to be due to the inability of the poly(methyl methacrylate) radicals to abstract phenolic hydrogens from phenols with bulky ortho substituents and to the sluggishness of this radical toward phenolic hydrogen atoms. The latter may result in the abstraction of benzylic and phenolic hydrogen atoms at comparable rates. This premise is supported by a poor correlation of C[lowered s] values with Hammett a substituent values, and by the fact that C[lowered s] values increase as the bond strength of the benzylic hydrogens decrease. Polymerization rates of methyl methacrylate in several phenols and the rates of decomposition of azobisisobutyronitrile in the presence of several phenols were also determined. These investigations showed that phenols affect the rate of decomposition of the initiator, and this may be another factor affecting the correlation of C[lowered s] values with antioxidant ratings and with the Hammett parameter. Inhibition times of the polymerization reaction by phenols in the presence of oxygen correlated well with Hammett a values and indicate that electron donating groups increase the activity of the phenol.