Investigation of the penultimate unit effect in halogen atom chain transfer in free radical copolymerization

Chain transfer constants to carbon tetrachloride (CCl₄) and carbon tetrabromide (CBr₄) were measured for the STY-MMA copolymerization system at 40, 50, and 60 °C, and Arrhenius parameters were determined across this range of temperatures. A value for C_s of CBr₄ in STY (2.8 × 10²) was determined using Bamford's moderated copolymerization method. The chain transfer constants obtained were fitted to a terminal model and a penultimate unit effect (PUE) model, of which only the PUE model provided a realistic fit to the data. The chain transfer reaction of STY-STY-terminated radicals to CCl₄ at 60 °C was found to be 3.3 times faster than that of MMA-STY-terminated radicals. With CBr₄ at the same temperature, STY-STY-terminated radicals reacted 9.4 times faster than MMA-STY-terminated radicals. Subsequently, the effect of temperature on the PUE was quantified. Additional experiments on CCl₄ were performed using dimethylformamide (DMF) as solvent. These experiments showed an increase in the magnitude of both the chain transfer constants and the PUE in DMF compared to the bulk experiments (from a factor of 3.3-7.2 times faster), suggesting the involvement of a polar transition state in transfer to CCl₄ and a polar contribution to the penultimate unit effect. We also discuss the effect that polar stabilization may have in atom transfer radical polymerization.