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solid particles. This polymerization strategy can be considered as a small-scale form
of bulk polymerization. However, it must be fulfilled that the monomers and the
initiator are completely insoluble in the continuous phase and that the stabilizer
does not participate in the radical reactions. In addition, in a turbulent flow, the
monomer drops can break up and coalesce with each other as polymerization progresses within the droplets. This fact can influence the particle size distribution of
polymers, which is one of the most important issues in suspension polymerization
(Hashim and Brooks 2004). The final polymer particles usually range between 0.1
and 5 mm in diameter, at least ten times larger than those obtained by emulsion
polymerization.
One of the main drawbacks associated with this polymerization approach is that
the non-covalent interactions between monomers and the template are weakened,
due to the high-water content in the polymerization mixture. These interactions are
crucial for proper imprinting, since they determine the homogeneity of the imprinted
sites created in the resulting polymers. Thus, weak monomer-template interactions
lead to weak prepolymerization complexes, and therefore, a heterogeneous distribution of the binding site, which would be responsible, in part, for cross-reactivity. To
overcome this, some authors have recommend working with continuous nonaqueous phases, specifically inverse suspension polymerization. This aspect appears
to be particularly significant when polymers are synthesized with printed ions
(Meouche et al. 2012). In the inverse suspension approach, the polymerization mixture, dissolved in water, is dispersed in a continuous organic phase. Here, perfluorocarbons are generally used as dispersing phase, since it does not interfere with the
functional monomer and template interactions. In addition, they are immiscible in
almost all organic solvents, being compatible with a wide variety of solvents and
monomers (Pérez-Moral and Mayes 2006).
5.3.2.2 Emulsion Polymerization
Emulsion polymerization is a radical polymerization process which allows to obtain
spherical NPs with sizes up to nanometers. In this process, the crosslinker, the
functional monomer(s) and the template are emulsified in an aqueous phase containing a surfactant. The mixture is generally stirred and sonicated resulting in a
polymer colloid, which comprises a discrete phase of colloidally stable particles
dispersed in a continuous aqueous phase. In some micelles, polymerization takes
place, while others are inactive and constitute a source of monomer. As the reaction proceeds, inactive micelles supply monomer to the active ones, which grow to
form the solid polymer. Virtually, all polymerization occurs within these micelles
that act as nanoreactors. This process is called mini-emulsion and, at the end of the
reaction, homogeneous drop-shaped NPs are obtained, having a diameter between
50 nm and 1 μm. Since radicals are compartmentalized within colloidal particles,
this approach results in polymerization yields and Mws that cannot normally be
achieved in solution.
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…
solid particles. This polymerization strategy can be considered as a small-scale form
of bulk polymerization. However, it must be fulfilled that the monomers and the
initiator are completely insoluble in the continuous phase and that the stabilizer
does not participate in the radical reactions. In addition, in a turbulent flow, the
monomer drops can break up and coalesce with each other as polymerization progresses within the droplets. This fact can influence the particle size distribution of
polymers, which is one of the most important issues in suspension polymerization
(Hashim and Brooks 2004). The final polymer particles usually range between 0.1
and 5 mm in diameter, at least ten times larger than those obtained by emulsion
polymerization.
One of the main drawbacks associated with this polymerization approach is that
the non-covalent interactions between monomers and the template are weakened,
due to the high-water content in the polymerization mixture. These interactions are
crucial for proper imprinting, since they determine the homogeneity of the imprinted
sites created in the resulting polymers. Thus, weak monomer-template interactions
lead to weak prepolymerization complexes, and therefore, a heterogeneous distribution of the binding site, which would be responsible, in part, for cross-reactivity. To
overcome this, some authors have recommend working with continuous nonaqueous phases, specifically inverse suspension polymerization. This aspect appears
to be particularly significant when polymers are synthesized with printed ions
(Meouche et al. 2012). In the inverse suspension approach, the polymerization mixture, dissolved in water, is dispersed in a continuous organic phase. Here, perfluorocarbons are generally used as dispersing phase, since it does not interfere with the
functional monomer and template interactions. In addition, they are immiscible in
almost all organic solvents, being compatible with a wide variety of solvents and
monomers (Pérez-Moral and Mayes 2006).
5.3.2.2 Emulsion Polymerization
Emulsion polymerization is a radical polymerization process which allows to obtain
spherical NPs with sizes up to nanometers. In this process, the crosslinker, the
functional monomer(s) and the template are emulsified in an aqueous phase containing a surfactant. The mixture is generally stirred and sonicated resulting in a
polymer colloid, which comprises a discrete phase of colloidally stable particles
dispersed in a continuous aqueous phase. In some micelles, polymerization takes
place, while others are inactive and constitute a source of monomer. As the reaction proceeds, inactive micelles supply monomer to the active ones, which grow to
form the solid polymer. Virtually, all polymerization occurs within these micelles
that act as nanoreactors. This process is called mini-emulsion and, at the end of the
reaction, homogeneous drop-shaped NPs are obtained, having a diameter between
50 nm and 1 μm. Since radicals are compartmentalized within colloidal particles,
this approach results in polymerization yields and Mws that cannot normally be
achieved in solution.
5 Plastic Receptors Developed by Imprinting Technology as Smart Polymers Imitating…