75
macroradical, which can then be combined with the dormant radical, giving rise to
a dormant polymer chain. Alternatively, propagation macroradicals can also react
with iniferter molecules or dormant polymer chains, thus promoting chain transfer
(Otsu and Matsumoto 1998).
MIP materials synthesized using iniferter polymerization have been developed
almost exclusively through photochemical initiation (Beyazit et al. 2016). Table 5.1
includes different iniferters used so far in imprinting technology, as well as their
main features.
5.2.2.2 Nitroxide Mediated Polymerization
Georges et al. (1993) reported an FRP process conducted by benzoyl peroxide but
in the presence of a stable radical, namely 2,2,6,6-tetramethyl-1-piperidynyloxy
(TEMPO), to synthesize poly(styrene) (PS). The polymers were synthesized by
thermal initiation at 123 °C and presented polydispersities below 1.3 (Georges et al.
1993). This publication made by NMP has gained considerable attention.
NMP or aminoxyl-mediated radical polymerization (as recommended by the
IUPAC) is a stable free radical mediated polymerization in which the deactivation
of free radicals involves a reversible coupling with aminoxyl radicals (Jenkins et al.
1996), also known as nitroxides. These are capable of reversibly deactivating active
propagation (macro)radicals, to produce a (macro)alkoxyamine as the predominant
species (Nicolas et al. 2013), which allows the establishment of a rapid equilibrium
between the active and dormant species. This equilibrium is responsible for providing the polymerization with a living character since, otherwise, radicals can undergo
a natural termination.
Initially, NMP involved a bicomponent pathway, in which a conventional thermal initiator such as 2,2′-azobis isobutyronitrile (AIBN) or benzoyl peroxide (BPO)
is used in the presence of a nitroxide such as TEMPO. Alternatively, unimolecular
initiators, such as alkylated TEMPO (Hawker et al. 2002), which decompose under
temperature can also be used, resulting in an initiator radical and a nitroxide (1:1).
These compounds are known as alkoxyamine initiators and allow better control over
Mw and architecture of resulting polymers compared to those mentioned above
(Benoit et al. 1999; Nicolas et al. 2013). A summarized polymerization mechanism
for NMP is illustrated in Fig. 5.4.
Despite its potential benefits, NMP has been little used for imprinting so far. To
the authors’ knowledge, only one article describes NMP in imprinting technology.
In light of this, Boonpangrak et al. (2006) developed a MIP for cholesterol by NMP
and demonstrated that it had higher selectivity than MIPs prepared by traditional
FRP, probably due to a better ordered polymer structure. In any case, the MIP synthesis by NMP has not attracted much attention, which can be attributed to the high
temperatures required for activation-deactivation of alkoxyamines, which can be
counterproductive when MIPs are synthesized using the non-covalent approach
(Haupt et al. 2012). Usually, temperatures above 100 °C are employed in this strategy. However, the latest trends on NMP point towards alkoxyamines capable of
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…
macroradical, which can then be combined with the dormant radical, giving rise to
a dormant polymer chain. Alternatively, propagation macroradicals can also react
with iniferter molecules or dormant polymer chains, thus promoting chain transfer
(Otsu and Matsumoto 1998).
MIP materials synthesized using iniferter polymerization have been developed
almost exclusively through photochemical initiation (Beyazit et al. 2016). Table 5.1
includes different iniferters used so far in imprinting technology, as well as their
main features.
5.2.2.2 Nitroxide Mediated Polymerization
Georges et al. (1993) reported an FRP process conducted by benzoyl peroxide but
in the presence of a stable radical, namely 2,2,6,6-tetramethyl-1-piperidynyloxy
(TEMPO), to synthesize poly(styrene) (PS). The polymers were synthesized by
thermal initiation at 123 °C and presented polydispersities below 1.3 (Georges et al.
1993). This publication made by NMP has gained considerable attention.
NMP or aminoxyl-mediated radical polymerization (as recommended by the
IUPAC) is a stable free radical mediated polymerization in which the deactivation
of free radicals involves a reversible coupling with aminoxyl radicals (Jenkins et al.
1996), also known as nitroxides. These are capable of reversibly deactivating active
propagation (macro)radicals, to produce a (macro)alkoxyamine as the predominant
species (Nicolas et al. 2013), which allows the establishment of a rapid equilibrium
between the active and dormant species. This equilibrium is responsible for providing the polymerization with a living character since, otherwise, radicals can undergo
a natural termination.
Initially, NMP involved a bicomponent pathway, in which a conventional thermal initiator such as 2,2′-azobis isobutyronitrile (AIBN) or benzoyl peroxide (BPO)
is used in the presence of a nitroxide such as TEMPO. Alternatively, unimolecular
initiators, such as alkylated TEMPO (Hawker et al. 2002), which decompose under
temperature can also be used, resulting in an initiator radical and a nitroxide (1:1).
These compounds are known as alkoxyamine initiators and allow better control over
Mw and architecture of resulting polymers compared to those mentioned above
(Benoit et al. 1999; Nicolas et al. 2013). A summarized polymerization mechanism
for NMP is illustrated in Fig. 5.4.
Despite its potential benefits, NMP has been little used for imprinting so far. To
the authors’ knowledge, only one article describes NMP in imprinting technology.
In light of this, Boonpangrak et al. (2006) developed a MIP for cholesterol by NMP
and demonstrated that it had higher selectivity than MIPs prepared by traditional
FRP, probably due to a better ordered polymer structure. In any case, the MIP synthesis by NMP has not attracted much attention, which can be attributed to the high
temperatures required for activation-deactivation of alkoxyamines, which can be
counterproductive when MIPs are synthesized using the non-covalent approach
(Haupt et al. 2012). Usually, temperatures above 100 °C are employed in this strategy. However, the latest trends on NMP point towards alkoxyamines capable of
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…