72
propagation, and some initiators are not unconsumed at the end of polymerization
(Matyjaszewski 2009). The termination of the growing chains usually arises through
the coupling of two radical species or disproportionation, obtaining in the latter a
double bond and a C-H bond at the end of the chain. In addition, termination can
also occur by chain transfer to monomers, polymers, solvents, template or transfer
agents, by reaction of an active chain end with the initiator or even by reaction of
radical species with impurities, oxygen or polymerization inhibitors.
In FRP, the concentration of radicals is relatively high and they are generated
continuously from the beginning to the end of the polymerization process. In light
of this, propagation and termination reactions are uncontrollable, since radical species with very high reactivity can interact to give dead polymer chains. In this context, FRP provides heterogeneous polymers with a wide molecular weight
distribution (MWD or polydispersity index − PDI = weight-average apparent
molecular weight (M w )/number-average apparent molecular weight (M n )), and the
MIPs developed under these conditions have a more heterogeneous distribution
of binding sites (Ye 2015; DiPasquale and Byrne 2016), also known as polyclonality of cavities (Wulff 2013), which may be responsible for cross-reactivity.
Consequently, this type of MIPs are considered analogous to polyclonal antibodies
being able to recognize not only the target compound (template) but also other
structural analogues which may belong to the same family (Haupt et al. 2012;
Garcia et al. 2015). To overcome the main drawbacks typically associated with
MIPs prepared under FRP conditions, MIP technology adopted new polymerization
strategies based on controlled/living mechanisms, which can be implemented simply by replacing the conventional initiator used in FRP synthesis (Zhang 2013).
5.2.2 Reversible Deactivation Radical Polymerization (RDRP)
Controlled/living Controlled radical polymerization (CRP), namely RDRP as recommended by the International Union of Pure and Applied Chemistry (IUPAC)
(Jenkins et al. 1996), emerged in the 1980s with the objective of minimizing the
bimolecular termination, thereby increasing the shelf life of growing polymer chains
(Beyazit et al. 2016). In RDRP, fast initiation and slow propagation are implicit and
polymerization continues until total monomer consumption (Otsu and Matsumoto
1998), or until the UV/heat source is switched off, being possible to restart it again
at any time. This polymerization approach, unlike FRP, allows for control over
architecture, molecular weight (Mw), polymer size and tacticity, which results in
more efficient polymer networks with more homogeneous structures (Salian and
Byrne 2013).
In this polymerization strategy, a dynamic equilibrium is established during
polymerization between active propagation chains and dormant species, which are
not capable of propagation or termination. Since dormant species are predominate
over propagating chains, the equilibrium is pushed towards the deactivation of
active chains, which leads to an excess of dormant species. This increases the
A. Gómez-Caballero et al.
propagation, and some initiators are not unconsumed at the end of polymerization
(Matyjaszewski 2009). The termination of the growing chains usually arises through
the coupling of two radical species or disproportionation, obtaining in the latter a
double bond and a C-H bond at the end of the chain. In addition, termination can
also occur by chain transfer to monomers, polymers, solvents, template or transfer
agents, by reaction of an active chain end with the initiator or even by reaction of
radical species with impurities, oxygen or polymerization inhibitors.
In FRP, the concentration of radicals is relatively high and they are generated
continuously from the beginning to the end of the polymerization process. In light
of this, propagation and termination reactions are uncontrollable, since radical species with very high reactivity can interact to give dead polymer chains. In this context, FRP provides heterogeneous polymers with a wide molecular weight
distribution (MWD or polydispersity index − PDI = weight-average apparent
molecular weight (M w )/number-average apparent molecular weight (M n )), and the
MIPs developed under these conditions have a more heterogeneous distribution
of binding sites (Ye 2015; DiPasquale and Byrne 2016), also known as polyclonality of cavities (Wulff 2013), which may be responsible for cross-reactivity.
Consequently, this type of MIPs are considered analogous to polyclonal antibodies
being able to recognize not only the target compound (template) but also other
structural analogues which may belong to the same family (Haupt et al. 2012;
Garcia et al. 2015). To overcome the main drawbacks typically associated with
MIPs prepared under FRP conditions, MIP technology adopted new polymerization
strategies based on controlled/living mechanisms, which can be implemented simply by replacing the conventional initiator used in FRP synthesis (Zhang 2013).
5.2.2 Reversible Deactivation Radical Polymerization (RDRP)
Controlled/living Controlled radical polymerization (CRP), namely RDRP as recommended by the International Union of Pure and Applied Chemistry (IUPAC)
(Jenkins et al. 1996), emerged in the 1980s with the objective of minimizing the
bimolecular termination, thereby increasing the shelf life of growing polymer chains
(Beyazit et al. 2016). In RDRP, fast initiation and slow propagation are implicit and
polymerization continues until total monomer consumption (Otsu and Matsumoto
1998), or until the UV/heat source is switched off, being possible to restart it again
at any time. This polymerization approach, unlike FRP, allows for control over
architecture, molecular weight (Mw), polymer size and tacticity, which results in
more efficient polymer networks with more homogeneous structures (Salian and
Byrne 2013).
In this polymerization strategy, a dynamic equilibrium is established during
polymerization between active propagation chains and dormant species, which are
not capable of propagation or termination. Since dormant species are predominate
over propagating chains, the equilibrium is pushed towards the deactivation of
active chains, which leads to an excess of dormant species. This increases the
A. Gómez-Caballero et al.