Studies on controlled radical polymerisation using 5-membered cyclic PROXYL nitroxides and corresponding alkoxyamines

Abstract

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.

Homolysis and Decomposition of Alkoxyamines Containing PROXYL and TEMPO Residues: A Comparison

“…Compound 4 was synthesized according to a similar procedure; 1 to 3 were prepared from styrene, nitroxide and DTBPO according to a literature procedure. [24] In all cases, characterisation data were in accord with literature values.…”

“…The ESR data alone do not provide an explanation for the differences in polymerisation behaviour of the various alkoxyamines, namely that 3 mediates the polymerisation of styrene significantly faster than that of both 1 [24] and 5 b together with its ability to bring about butyl acrylate polymerisation. [26] In an attempt to rationalize these observations, we examined the decomposition of various alkoxyamines to generate unsaturated species and hydroxylamine (see Scheme 5).…”

“…Previously, we have studied [24][25][26] NMP involving PRO-XYL nitroxides, which are the 5-membered ring analogues of TEMPO. We chose to study these initially because their functionalization, particularly adjacent to the ring N atom, is more accessible synthetically than that of TEMPO.…”

“…[24] More recently, it has been shown that the same mediator can bring about the living (but not controlled) polymerisation of butyl acrylate, which cannot be achieved with TEMPO. [25,26] Therefore, we were keen to investigate the properties of alkoxyamines derived from 2-phenyl-PROXYL and TEMPO to elucidate the reasons for their different performances in NMP.…”

Synthesis of a series of SG1 2‐[N‐tert‐butyl‐N‐(1‐diethoxyphosphoryl‐2,2‐dimethylpropyl)aminoxyl] based alkoxyamines, SG1‐CH(Me)CO2R, and measurement of the homolysis rate constants of the CON bond

“…It has been shown that the activation energy ( E a ) of alkoxyamine homolysis is a good estimate of the value of the bond dissociation energy of the CON bond 32, 33. We28, 32, 34, 35 and others23, 36–42 have shown that the CON bond in alkoxyamines is either strengthened by anomer effects39, 42 due to, for example, a heteroatom bound to the carbon and by polar effects23, 35, 41, 42 due to an electron‐withdrawing group bound to the nitrogen atom or weakened by the steric strain and the polar effects of both alkyl and nitroxyl fragments 23, 27–32, 34, 36. Furthermore, the stabilization of the released alkyl radical and the nitroxyl radical, stabilized by an intramolecular hydrogen bond, also weakens the CON bond 23, 32, 34, 36, 37, 39, 42.…”