Different methodologies for the preparation of pH-responsive polymers

In the next section, the most innovated and frequently used methods for the preparation of pH-responsive polymers are described. 

Emulsion polymerization is among the most popular synthetic routes to prepare vinyl-based pH-responsive polymers, especially microgel systems (Rao and Geckeler, 2011). This technique employs a radical chain polymerization methodology to form latexes of narrow particle size distributions. The emulsion polymerization systems are commonly composed of monomer(s), water, water-soluble initiator and surfactant (emulsifier). Colloidal stabilizers may be electrostatic, steric or electrosteric, or display both stabilizing mechanisms. When phase separation occurs, the formation of solid particles takes place before or after the termination of the polymerization reaction. 

A typical formulation used in mini-emulsion polymerization consists of water, monomer mixture, co-stabilizer, surfactant and initiator. Hie key difference between emulsion polymerization and mini-emulsion polymerization is the utilization of a low molecular mass compound as the co-stabilizer and also the use of a high-shear device (ultrasound, etc.). Mini-emulsions are critically stabilized, require a high-shear to reach a steady state and have an interfacial tension much greater than zero (Koul c/ /.,2011 Winkelmann and Schuchmann, 2011). 

Versatile particles have been developed with various co-stabilizers and initiator combinations (Baruch-Sharon and Margel, 2010). Hiese combinations have a predominant influence on the formation and nature of the NPs (Jiang et al., 2010). PAA NPs were synthesized by Kriwet et al. (Kriwet et al, 1998) using a co-emulsifier system consisting of a mixture of Span 80 and Tween 80. The polymerization was initiated by free radicals and the particle size was dependent on the type of radical initiator used. Using water-soluble initiators, such as anunonium persulfate (APS), microparticles were obtained however, NPs were generated almost exclusively with a diameter between 80 and 150 nm when lipophilic radical initiators, such as AIBN, were used. 

Micro-emulsion polymerization is a new and effective approach for preparing nanosized polymer particles and has attracted significant attention. Emulsion and micro-emulsion polymerization differ in the kinetics of the polymerization. Emulsion polymerization exhibits three reaction rate intervals, whereas only two are detected in micro-emulsion polymerization. Both particle size and the average number of chains per particle are considerably smaller in micro-emulsion polymerization (Schork et al., 2005). 

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