Core-Shell Polymerization

Core-shell polymerization is a seed particle polymerization variation of emulsion polymerization. The seed particles are suspended in the continuous phase. The pre-polymerization mixture of monomer, cross-linker, template and initiator is added to the particle suspension as an emulsion prepared in the continuous phase. The mixture is stirred until the polymerization has completed. The addition of pre-polymerization mixture is repeated several times until the spheres reach the desired size range. The beads formed are composed of a core (i.e. the seed particle) and a shell ofMIP [98, 99], 

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Figure 14.12 A core/shell polymeric particle made of a hydrophobic polystyrene core that is capped with a hydrophilic polyHEMA shell, which contains numerous hydroxyl groups.

The acylation of alcohol-containing monomers, e.g. hydroxyethyl acrylates or vinyl benzyl alcohol with maleic, succinic or sulfosuccinic anhydride leads to bifunctional polymerizable surfactants. A range of such products has been synthesized and tested in batch polymerization and core-shell polymerization of styrene and butyl acrylate [26]. In both cases good stability, high conversion and little burying of the Surfmers were observed. Water rebound was also limited. These advantageous features were however offset by an unacceptable resistance to electrolytes and to freeze-thaw. 

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. 

Norakankom C, Pan Q, Rempel GL et al (2010) Factorial experimental design on synthesis of functional core/shell polymeric nanoparticles via differential microemulsion polymerization. J Appl Polym Sci 116 1291-1298... 

Figure 6.11 Preparation of multifunctional core-shell polymeric nanoparticles from poly (NBE-indo)35-fa-(NBE-HEG),5. (Reprinted from Ref. [76].)...

Rao et al. [82] incorporated doxorubicin into PNBE by a hydrazine linker (Figure 7.8a) and obtained core-shell polymeric nanoparticles soluble in water and biological media. The drug was easily released by hydrolysis of the hydrazine linker at lower acidic conditions of pH = 5.5-6 as compared to the pH = 3.0 in the case of doxorubicin connected to the polymer by an amide linkage. 

Electron spin resonance spectroscopy has been employed, together with the cryogenic quenching technique, by the same group [146] to monitor occluded radical concentrations during methyl methacrylate (MMA)/butyl acrylate (BA) core-shell polymerization reactions. 

A compatibilization technique specifically suitable for emulsion polymerization involves the in-situ polymerization of a polymer in the presence of a previously polymerized polymer. As applied to emulsion polymerization, this is typically referred to as core-shell polymerization. The procedure involves the initial polymerization of seed particles. The addition of other monomers can result in two distinct results. One result involves swelling of the monomer in the particles followed by phase separation once a critical molecular weight is achieved (if the two polymers are thermodynamically immiscible). The other result would involve the... 

Figure 3.3 Core-shell polymerization morphologies (generalized figures represent a slice through the centerofthe particles)...

I 3 Siloxane-Containing Polymers Table 3.8 Core-shell polymerization with PDMS prepared in situ. 

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