Scheme 1. Equilibrium between dormant and active species in TEMPO-mediated CRP.

2,2′,5,5′‐tetramethylpyrrolidine‐N‐oxyl (PROXYL) and derivatives bearing one α‐aryl substituent (Ph, p‐CF3Ph, p‐Me2NPh) were prepared and converted to alkoxyamines by reaction with di‐tert‐butyl peroxalate (DTBPO) and excess styrene. Both the nitroxides (plus DTBPO as a radical source) and alkoxyamines were investigated in the controlled radical polymerisation (CRP) of styrene, and their behaviour was compared to that of TEMPO. Polymerisation mediated by each nitroxide displayed a linear relationship between molecular weight and conversion, producing material of low polydispersity indicating a controlled process. However, a comparison of kinetic behaviour was complicated by the unknown concentration of active species present. Polymerisations with preformed alkoxyamines at a known concentration also displayed controlled behaviour (polydispersities 1.25–1.5), and an influence of nitroxide substituent on the overall rate of polymerisation could be determined. PROXYL and the dimethylanilino‐substituted nitroxide gave similar CRP rates to TEMPO, whereas a Ph or (to a lesser extent) p‐CF3Ph substituent resulted in significantly faster reactions. These results are discussed in terms of the likely steric and electronic influence of substituents on the nitroxide radical centre, and the resulting influence on polymerisation rate. It is concluded that the steric bulk of the Ph substituent is the most likely cause of the rate enhancement displayed by the corresponding mediator.