88
Some authors have tried to improve the stabilization of the particles by means of
the Pickering effect, performing an emulsion polymerization in the presence of
inorganic colloidal particles (Brunier et al. 2018). High yields have also been
obtained through this strategy with a good initial dispersion of vesicles, the choice
of reversible addition-fragmentation chain-transfer polymerization (RAFT), the use
of monomers not too hydrophobic and some crosslinking (Rusli et al. 2018; Siirilä
et al. 2019).
Emulsion polymerization has some drawbacks. On the one hand, the presence of
water as a continuous phase can negatively affect monomer-template interaction
stability, mainly due to the high hydrogen bonding capacity of water. Therefore, the
efficiency of the imprinting process worsens. On the other hand, the surfactant can
also interact with both the monomer and the template, which also influences molecular imprinting. To avoid all these hindrances, Dvorakova et al. (2010) proposed the
synthesis of monodisperse MIP NPs in the absence of water and using a nonionic
polymeric emulsifier. When the polymerization mixture dissolved in water, it is
dispersed in a continuous organic matrix, the process is called inverse emulsion
polymerization. This method was used by Zeng et al. (2009) to imprint polymer
NPs with hydrophilic macromolecules including peptides and proteins.
5.3.2.3 Precipitation Polymerization
Precipitation polymerization consists in a simple strategy for synthesizing polymer
particles in the sub-micrometer range with enough control over the morphology,
thus being used in multiple analytical applications (Gutiérrez-Climente et al. 2016;
Pardeshi and Singh 2016). Usually, this approach is based on a radical polymerization in mixtures having very diluted monomers. The high volumes of porogen prevent coagulation of particles while the polymerization is taking place, preventing
block formation. Precipitation polymerization mainly includes two steps: particle
nucleation and growth. Nucleation starts by aggregation of oligomers to form nuclei
of particles at the beginning of polymerization. With respect to particle growth, it
can happen in two ways: by the absorption of oligomeric and monomeric radicals on
the surface of the particle or by the aggregation of nuclei (homocoagulation). Proper
selection of the solvent is essential to obtain discrete particles, since all the precursors involved in the synthesis and the oligomers that are formed must be solubilized.
Once the polymer reaches a certain mass, the solvent can no longer dissolve it and
precipitates. This method is compatible with a large number of crosslinking agents
and the use of polar aprotic organic solvents (Schirhagl 2014). Nonetheless, it is not
necessary to add stabilizers or other additives, however, when precipitation polymerization is to be carried out in aqueous media, surfactants can be added at a very
low concentration to avoid the use of organic solvents (Wackerlig and Lieberzeit 2015).
Different modifications of this technique have been reported so far in the literature. Wulff et al. (2006) suggested performing a precipitation polymerization using
a post-dilution step. Here, the polymerization is stopped at a point just before macrogelation and then the polymer is widely diluted and the polymerization continues
A. Gómez-Caballero et al.
Some authors have tried to improve the stabilization of the particles by means of
the Pickering effect, performing an emulsion polymerization in the presence of
inorganic colloidal particles (Brunier et al. 2018). High yields have also been
obtained through this strategy with a good initial dispersion of vesicles, the choice
of reversible addition-fragmentation chain-transfer polymerization (RAFT), the use
of monomers not too hydrophobic and some crosslinking (Rusli et al. 2018; Siirilä
et al. 2019).
Emulsion polymerization has some drawbacks. On the one hand, the presence of
water as a continuous phase can negatively affect monomer-template interaction
stability, mainly due to the high hydrogen bonding capacity of water. Therefore, the
efficiency of the imprinting process worsens. On the other hand, the surfactant can
also interact with both the monomer and the template, which also influences molecular imprinting. To avoid all these hindrances, Dvorakova et al. (2010) proposed the
synthesis of monodisperse MIP NPs in the absence of water and using a nonionic
polymeric emulsifier. When the polymerization mixture dissolved in water, it is
dispersed in a continuous organic matrix, the process is called inverse emulsion
polymerization. This method was used by Zeng et al. (2009) to imprint polymer
NPs with hydrophilic macromolecules including peptides and proteins.
5.3.2.3 Precipitation Polymerization
Precipitation polymerization consists in a simple strategy for synthesizing polymer
particles in the sub-micrometer range with enough control over the morphology,
thus being used in multiple analytical applications (Gutiérrez-Climente et al. 2016;
Pardeshi and Singh 2016). Usually, this approach is based on a radical polymerization in mixtures having very diluted monomers. The high volumes of porogen prevent coagulation of particles while the polymerization is taking place, preventing
block formation. Precipitation polymerization mainly includes two steps: particle
nucleation and growth. Nucleation starts by aggregation of oligomers to form nuclei
of particles at the beginning of polymerization. With respect to particle growth, it
can happen in two ways: by the absorption of oligomeric and monomeric radicals on
the surface of the particle or by the aggregation of nuclei (homocoagulation). Proper
selection of the solvent is essential to obtain discrete particles, since all the precursors involved in the synthesis and the oligomers that are formed must be solubilized.
Once the polymer reaches a certain mass, the solvent can no longer dissolve it and
precipitates. This method is compatible with a large number of crosslinking agents
and the use of polar aprotic organic solvents (Schirhagl 2014). Nonetheless, it is not
necessary to add stabilizers or other additives, however, when precipitation polymerization is to be carried out in aqueous media, surfactants can be added at a very
low concentration to avoid the use of organic solvents (Wackerlig and Lieberzeit 2015).
Different modifications of this technique have been reported so far in the literature. Wulff et al. (2006) suggested performing a precipitation polymerization using
a post-dilution step. Here, the polymerization is stopped at a point just before macrogelation and then the polymer is widely diluted and the polymerization continues
A. Gómez-Caballero et al.