Beilstein J. Org. Chem.2023,19, 1580–1603, doi:10.3762/bjoc.19.116
weight and polymerization rate precisely. In order to solve these problems, Hawker et al. [32][33][34] proposed the concept of the unimolecular initiation system. In this system, an alkoxyamine compound is used instead of the original nitroxideradical/initiator combination. These unimolecular initiators
can decompose to produce a stoichiometric pair of the primary initiating radical and a nitroxideradical, thus combining the roles of a conventional initiator and a control agent. The mechanism is shown in Scheme 4 [35].
Due to the steric effect of TEMPO, the dissociation rate constant, kd, of the
acrylates, under milder conditions [37][38][39]. Grimaldi et al. [40] achieved NMP of styrene and n-butyl acrylate using SG1-type nitroxideradical (N-tert-butyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide). Compared with TEMPO, SG1 was considered that it initiated the truly “living”/controlled
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Graphical Abstract
Scheme 1:
Oxidation of catechol and subsequent cross-linking. Scheme 1 redrawn from [3].
Beilstein J. Org. Chem.2013,9, 1487–1491, doi:10.3762/bjoc.9.169
compound; nitroxideradical; TCNQF4; thiourea; Introduction
TEMPO radical (2,2,6,6-tetramethylpiperidinyl-N-oxyl) (1) is a typical nitroxideradical and is persistent because of the steric hindrance of the four neighboring methyl groups of the NO moiety protecting it from attack by various reagents
including oxygen [1]. Sometimes, however, the merit turns out to be a drawback by limiting its properties and reactivity. For example, 1 is inefficient in the oxidation of sterically hindered secondary alcohols.
The AZADO radical (2) [2] is an intriguing nitroxideradical with adamantane-like structure and
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Graphical Abstract
Figure 1:
Structural formula of TEMPO (1), AZADO (2), TCNQF4 (3) and thiourea (4).