81
reacts reversibly with the oxidized metal complex. At this point, the halide is
transferred back again from the oxidized metal, giving rise to a dormant species
(P n −X) and the transition metal complex, which will be ready for reactivating any
dormant species.
Apart from alkyl halides traditionally used, there are other ways to conduct
ATRP. Consequently, a conventional free radical initiator such as AIBN is used in
reverse ATRP (Fig. 5.5), which decomposes into two active radicals, which in turn
react with the transition metal complex at highest oxidation state. As a consequence,
the halogen is transferred from the complex to the radicals coming from the decomposition of the initiator, giving rise to a dormant halide species (Matyjaszewski
2018). Propagation takes place through the addition of monomers to active (macro)
radicals before being deactivated by the oxidized metal complex. The fast initiation
and reversible deactivation of active species in the ATRP reactions favor the uniform
growth of all polymer chains, obtaining polymers with narrow MWDs (Banerjee
et al. 2014).
The normal and reverse ATRP aforementioned was combined by Gromada and
Matyjaszewski (2001) in order to overcome common inconveniences associated with
these systems, which were limited to less active catalysts, to avoid too fast termination
reactions, which may arise using too reducing catalysts, i.e. more active. In addition,
combined ATRP was found to be compatible with low polymerization temperatures,
since the system tolerated the slow decomposition of the initiator (Gromada and
Matyjaszewski 2001). Likewise, other ATRP systems were also reported later, based
on initiation mechanisms rooted in non-radical generating reducing agents, which
resulted in activators generated by electron transfer ATRP (AGET) and activators
regenerated by electron transfer ATRP (ARGET). More recently, other ATRP initiation mechanisms have emerged, such as electrochemically, mechanically/ultrasonically and photochemically initiated ATRP (Ribelli et al. 2019).
In general terms, ATRP has been quite likely the most studied CRP technique,
which can be attributed to the high availability of initiators, its applicability to a
large number of monomers such as acrylonitriles, (meth)acrylates and styrenes, and
mild reaction conditions (Wang et al. 2016). In any event, ATRP has not been widely
used in imprinting technology. One reason for this may revolve around the incompatibility of many acidic or hydrogen bonding monomers and/or templates with
ATRP, which are responsible for hindering proper polymerization, since they can
negatively influence the metal-ligand complex (Ramakers et al. 2019).
The first MIPs developed under ATRP were mainly confined to surface imprinting (Wei et al. 2005; Wang et al. 2006; Lu et al. 2009). MIPs with adjustable pores
and uniform structures were obtained through ATRP using 4-vinylpyridine and ethylene glycol dimethacrylate (EDMA) as momoners. Zu et al. (2009) combined for
the first-time precipitation polymerization and ATRP. Based on previous results on
surface imprinting, they demonstrated that both normal and reverse ATRP could
also be used to obtain tailor-made microspheres with improved bonding properties,
compared to MIPs developed under traditional precipitation polymerization by FRP
(Zu et al. 2009). The potential benefits of ATRP for small templates (<1500 Da)
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…
reacts reversibly with the oxidized metal complex. At this point, the halide is
transferred back again from the oxidized metal, giving rise to a dormant species
(P n −X) and the transition metal complex, which will be ready for reactivating any
dormant species.
Apart from alkyl halides traditionally used, there are other ways to conduct
ATRP. Consequently, a conventional free radical initiator such as AIBN is used in
reverse ATRP (Fig. 5.5), which decomposes into two active radicals, which in turn
react with the transition metal complex at highest oxidation state. As a consequence,
the halogen is transferred from the complex to the radicals coming from the decomposition of the initiator, giving rise to a dormant halide species (Matyjaszewski
2018). Propagation takes place through the addition of monomers to active (macro)
radicals before being deactivated by the oxidized metal complex. The fast initiation
and reversible deactivation of active species in the ATRP reactions favor the uniform
growth of all polymer chains, obtaining polymers with narrow MWDs (Banerjee
et al. 2014).
The normal and reverse ATRP aforementioned was combined by Gromada and
Matyjaszewski (2001) in order to overcome common inconveniences associated with
these systems, which were limited to less active catalysts, to avoid too fast termination
reactions, which may arise using too reducing catalysts, i.e. more active. In addition,
combined ATRP was found to be compatible with low polymerization temperatures,
since the system tolerated the slow decomposition of the initiator (Gromada and
Matyjaszewski 2001). Likewise, other ATRP systems were also reported later, based
on initiation mechanisms rooted in non-radical generating reducing agents, which
resulted in activators generated by electron transfer ATRP (AGET) and activators
regenerated by electron transfer ATRP (ARGET). More recently, other ATRP initiation mechanisms have emerged, such as electrochemically, mechanically/ultrasonically and photochemically initiated ATRP (Ribelli et al. 2019).
In general terms, ATRP has been quite likely the most studied CRP technique,
which can be attributed to the high availability of initiators, its applicability to a
large number of monomers such as acrylonitriles, (meth)acrylates and styrenes, and
mild reaction conditions (Wang et al. 2016). In any event, ATRP has not been widely
used in imprinting technology. One reason for this may revolve around the incompatibility of many acidic or hydrogen bonding monomers and/or templates with
ATRP, which are responsible for hindering proper polymerization, since they can
negatively influence the metal-ligand complex (Ramakers et al. 2019).
The first MIPs developed under ATRP were mainly confined to surface imprinting (Wei et al. 2005; Wang et al. 2006; Lu et al. 2009). MIPs with adjustable pores
and uniform structures were obtained through ATRP using 4-vinylpyridine and ethylene glycol dimethacrylate (EDMA) as momoners. Zu et al. (2009) combined for
the first-time precipitation polymerization and ATRP. Based on previous results on
surface imprinting, they demonstrated that both normal and reverse ATRP could
also be used to obtain tailor-made microspheres with improved bonding properties,
compared to MIPs developed under traditional precipitation polymerization by FRP
(Zu et al. 2009). The potential benefits of ATRP for small templates (<1500 Da)
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…