Spherical polymeric particles polymerization techniques

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

The aerosol technique can also be used to produce polymer colloids by addition polymerization. Thus, when droplets of toluene-2,4-diisocyanate (TD1) or 1,6-hexa-methylene diisocyanate (HDI) were brought into contact with ethylenediamine (EDA) vapor (in the apparatus shown in Fig. 1.5.3) spherical polyurea particles with modal diameters of 1-3 p,m were formed. The entire process, i.e., the formation of droplets and the polymerization, was carried out at moderate temperatures (<80°C)... 

These imprinted micro spheres can then be packed more efficiently into chromatography columns or into solid-phase extraction (SPE) cartridges than the particles prepared by bulk polymerization techniques. Larger spherical imprinted polymer particles can be prepared by modification of preformed latex particles either by reswelling with a secondary polymerization mixture or by coating a spherical core particle with an imprinted polymer shell. 

Spherical beads possess better hydrodynamic and diffusion properties than irregularly shaped particles. It is, hence, desirable to apply MIPs in a spherical bead format, especially for flow-through applications. Methods to synthesize spherical polymer beads are often classified according to the initial state of the polymerization mixture (i) homogeneous (i.e. precipitation polymerization and dispersion polymerization) or (ii) heterogeneous (i.e. emulsion polymerization and suspension polymerization). In addition, several other techniques have been applied for the preparation of spherical MIP beads. The techniques of two-step swelling polymerization, core-shell polymerization, and synthesis of composite beads will be detailed here. 

Suspension and emulsion polymerization are two classical polymerization techniques to produce spherical polymeric particles. Larger particles (usually larger than 50 tim) with an appreciable size distribution are produced by suspension polymerization. Submicron polymeric particles (usually smaller than 0.1 im) with extremely uniform in size are obtained by conventional emulsion polymerization processes. Recent techniques, such as swollen emulsion polymerization, dispersion polymerization, etc. give micron-size (usually between 1-50 im) monosize polymeric particles (23). 

The predominant method used to prepare a crosslinked polymer support is to polymerize a monovinylcompound with a small amount of a divinylcompound by radical initiators. This enables the use of suspension techniques and spherical particles are therefore available. 

Polymerization in microemulsion has developed into a powerful technique for the preparation of strictly spherical micronetworks [1,2]. The final size of the polymerized particles is solely governed by the ratio of surfactant to monomer concentration, i.e., the fleet ratio S. To predict the final particle size at full conversion, two simple models for the polymerization in microemulsion have been proposed which differ only in some minor details. One of the models considers variable headgroup contributions to the particle radius [3]. This calculation finally arrives at Eq. 1. 

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