**2. Components of heterogeneous emulsion polymerization**

The main components of emulsion polymerization media involve monomer(s), dispersing medium, emulsifier, and water-soluble initiator [5, 17–19]. The dispersion medium is water in which hydrophobic monomers is emulsified by surface-active agents (surfactant). When surfactant concentration exceeds critical micelle concentration (CMC) it aggregate in the form of spherical micelles, so surface tension at the surface decrease, as a result hydrophobic monomers enter in to the vicinity of micelle and reaction continue until all monomer droplets are exhausted and micelle containing monomers increase in size. Typical micelles have dimensions of 2–10 nm, with each micelle containing 50–150 surfactant molecules [5]. Water-soluble initiators enter into the micelle where free radical propagation start. In general, monomer droplets are not effective in competing with micelles in capturing free radicals generated in the aqueous phase due to their relatively small surface area [1], so the micelle act as a meeting site of water-soluble initiators and hydrophobic vinyl monomers. As polymerization continue inside micelle, the micelle grow by monomer addition from monomer droplets outside and latex are formed. Schematic representation of emulsion polymerization shown in **Figure 1**. Emulsion polymerization carried out through three main intervals as shown in **Figure 2**.

There is a separate monomer phase in intervals I. The particle number increases with time in interval I and particle nucleation occurs in interval I. At the end of this stage most of surfactants are exhausted (i.e. micelles are exhausted) [5]. About one of every 102–103 micelles can be successfully converted into latex particles [1]. Particle nucleation process is greatly affected by surfactant concentration, which in turn affect particle size and particle size distribution of latex [1]. The lower the surfactant concentration, the lower the nucleation period the narrow the particle size distribution. At interval II (Particle growth stage), the polymerization continue and polymer particles increase in size until monomer droplets exhausted. Monomer droplets act as reservoirs to supply the growing particles with monomer and surfactant species. At interval

**Figure 1.** Schematic representation of emulsion polymerization.

III, the polymer size increase as latex particles become monomer-starved and the concentration of monomer in the reaction loci continues to decrease toward the end of polymerization [1].

#### **2.1. Initiators**

Systems of emulsion polymerization involve (1) conventional emulsion polymerization, in which a hydrophobic monomer emulsified in water and polymerization initiated with a watersoluble initiator [5]. (2) Inverse emulsion polymerization [7], where organic solvents of very low polarity as paraffin or xylene used as a polymerization media to emulsify hydrophilic monomers [5], then initiation proceed with the aid of hydrophobic initiator [5]. These two polymerization types known as oil-in-water (o/w) and water-in-oil (w/o) emulsions [5]. (3) Mini emulsion polymerization involves systems with monomer droplets in water with much smaller droplets than in emulsion polymerization and characterized by monomer droplet =50–1000 nm, surfactant concentration < critical micelle concentration (CMC), water insoluble co stabilizer as hexadecane to prevent Ostwald ripening, polymer particle size equal monomer droplet size = 50–1000 nm, and both water soluble and oil soluble initiator used [4, 8]. (4) Microemulsion polymerization with very much smaller monomer droplets, about 10–100 nm, and characterized by surfactant concentration > CMC, polymer particles = 10–50 nm, water-soluble initiator are commonly used [9, 10]. Miniemulsion, microemulsion and conventional emulsion polymerizations show quite different particle nucleation and growth mechanisms and kinetics [1]. Many articles discuss different types of emulsion polymerization found in literature [1, 11–16].

4 Recent Research in Polymerization

**2. Components of heterogeneous emulsion polymerization**

The main components of emulsion polymerization media involve monomer(s), dispersing medium, emulsifier, and water-soluble initiator [5, 17–19]. The dispersion medium is water in which hydrophobic monomers is emulsified by surface-active agents (surfactant). When surfactant concentration exceeds critical micelle concentration (CMC) it aggregate in the form of spherical micelles, so surface tension at the surface decrease, as a result hydrophobic monomers enter in to the vicinity of micelle and reaction continue until all monomer droplets are exhausted and micelle containing monomers increase in size. Typical micelles have dimensions of 2–10 nm, with each micelle containing 50–150 surfactant molecules [5]. Water-soluble initiators enter into the micelle where free radical propagation start. In general, monomer droplets are not effective in competing with micelles in capturing free radicals generated in the aqueous phase due to their relatively small surface area [1], so the micelle act as a meeting site of water-soluble initiators and hydrophobic vinyl monomers. As polymerization continue inside micelle, the micelle grow by monomer addition from monomer droplets outside and latex are formed. Schematic representation of emulsion polymerization shown in **Figure 1**. Emulsion polymerization carried out through three main intervals as shown in **Figure 2**.

There is a separate monomer phase in intervals I. The particle number increases with time in interval I and particle nucleation occurs in interval I. At the end of this stage most of surfactants are exhausted (i.e. micelles are exhausted) [5]. About one of every 102–103 micelles can be successfully converted into latex particles [1]. Particle nucleation process is greatly affected by surfactant concentration, which in turn affect particle size and particle size distribution of latex [1]. The lower the surfactant concentration, the lower the nucleation period the narrow the particle size distribution. At interval II (Particle growth stage), the polymerization continue and polymer particles increase in size until monomer droplets exhausted. Monomer droplets act as reservoirs to supply the growing particles with monomer and surfactant species. At interval Initiator act to generate free radicals by thermal decomposition, or redox reactions. The initiators may be; (1) water-soluble initiators like 2,2-Azobis(2-amidinopropane) dihydrochloride, K<sup>2</sup> S2 O8 , APS (Ammonium persulfate) and (H<sup>2</sup> O2 ) hydrogen peroxide. (2) Partially water-soluble peroxides like t-butyl hydroperoxide and succinic acid peroxide and azo compounds such as 4,4-azobis(4-cyanopentanoic acid) [14]. (3) Redox systems such as persulfate with ferrous ion, cumyl hydroperoxide or hydrogen peroxide with ferrous, sulfite, or bisulfite ion [5, 20]. Other initiators such as surface active initiators which "inisurfs," for example; bis[2-(4′-sulfophenyl)alkyl]-2,2′-azodiisobutyrate ammonium salts

**Figure 2.** Emulsion polymerization intervals.

and 2,2′-azobis(N-2′-methylpropanoyl-2-amino-alkyl-1-sulfonate). These initiators initiates emulsion polymerization without the need of stabilizers.

#### **2.2. Surfactants**

Act to decrease interfacial tension between monomer and aqueous phase, stabilize the latex and generate micelles in which monomers emulsified and nucleation reaction proceed. Surfactants increase particle number and decrease particle size, these surfactants may be (1) Anionic surfactants such as fatty acid soaps (sodium or potassium stearate, laurate, palmitate), sulfates, and sulfonates (sodium lauryl sulfate and sodium dodecylbenzene sulfonate); (2) Nonionic surfactants such as poly (ethylene oxide), poly (vinyl alcohol) and hydroxyethyl cellulose; (3) Cationic surfactants such as dodecylammonium chloride and cetyltrimethylammonium bromide [5, 21]. For ionic surfactants, micelles formed only at temperatures above the Krafft point. For a nonionic surfactant, micelles formed only at temperatures below the cloud point. Emulsion polymerization carried out below the cloud temperature and above the Krafft temperature [5]. Polymerizable surfactants (surfactants with active double bond) such as sodium dodecyl allyl sulfosuccinate [13, 22–24] also used to produce latexes with chemically bound surface-active groups [5, 25–30, 31]. Polymerized surfactants (surfactants with active double bond) consist of amphipathic structure comprising hydrophobic tail and hydrophilic head group [32], in addition to polymerized vinyl groups [33] in their molecular structure, which acquire them unique physicochemical properties other than traditional surfactants moieties [34] such as;


Freedman et al., [38] reported about the first synthesis of vinyl monomers which serve as emulsifying agents [39]. Active vinyl groups comprise vinyl, allyls, acrylates, methacrylates, styryl and acrylamide [40]. Polymerized groups may be "H-type" where, i.e. located in the hydrophilic head group, or "T-type" where, i.e. located in the hydrophobic tail have a profound effect on surfactant self-assembly and properties [41]. All kinds of polymerizable traditional surfactants, including cationic [41], anionic [42] and nonionic [43] have been synthesized to study the influence of the molecular structure on the properties and application. Anionic polymerizable surfactants seem to be the most promising for utilizing in coatings, adhesives and enhanced oil recovery.

#### **2.3. Dispersion medium**

Water is the frequently used dispersion medium in emulsion polymerization as it is cheap and environmentally friendly. It represents the medium of transfer of monomer from droplets to particles and as a solvent for emulsifier, initiator, and other ingredients.

#### **2.4. Monomer**

and 2,2′-azobis(N-2′-methylpropanoyl-2-amino-alkyl-1-sulfonate). These initiators initiates

Act to decrease interfacial tension between monomer and aqueous phase, stabilize the latex and generate micelles in which monomers emulsified and nucleation reaction proceed. Surfactants increase particle number and decrease particle size, these surfactants may be (1) Anionic surfactants such as fatty acid soaps (sodium or potassium stearate, laurate, palmitate), sulfates, and sulfonates (sodium lauryl sulfate and sodium dodecylbenzene sulfonate); (2) Nonionic surfactants such as poly (ethylene oxide), poly (vinyl alcohol) and hydroxyethyl cellulose; (3) Cationic surfactants such as dodecylammonium chloride and cetyltrimethylammonium bromide [5, 21]. For ionic surfactants, micelles formed only at temperatures above the Krafft point. For a nonionic surfactant, micelles formed only at temperatures below the cloud point. Emulsion polymerization carried out below the cloud temperature and above the Krafft temperature [5]. Polymerizable surfactants (surfactants with active double bond) such as sodium dodecyl allyl sulfosuccinate [13, 22–24] also used to produce latexes with chemically bound surface-active groups [5, 25–30, 31]. Polymerized surfactants (surfactants with active double bond) consist of amphipathic structure comprising hydrophobic tail and hydrophilic head group [32], in addition to polymerized vinyl groups [33] in their molecular structure, which acquire them unique physicochemical properties other than traditional sur-

**A.** They have surface activity like ordinary surfactants and polymerized vinyl group like vinyl monomers, so they have the ability to undergo polymerization reactions.

**B.** Due to their amphoteric structure and polymerization ability, they serve to synthesize inorganic/organic nanocomposite, and applied to emulsion polymerizations as polymerized emulsifiers, to stabilize the formed latexes, to prepare novel water-soluble hydrophobically associating polymers with strong thickening properties [35] so, they greatly

**C.** Allow developing hybrid Nano sized reaction and templating media. Moreover surfmer serve as hydrophilic monomer to copolymerize with acrylamide derivatives (AM) forming hydrophobically associating polyacrylamide (HAPAM), which acquire wide applica-

Freedman et al., [38] reported about the first synthesis of vinyl monomers which serve as emulsifying agents [39]. Active vinyl groups comprise vinyl, allyls, acrylates, methacrylates, styryl and acrylamide [40]. Polymerized groups may be "H-type" where, i.e. located in the hydrophilic head group, or "T-type" where, i.e. located in the hydrophobic tail have a profound effect on surfactant self-assembly and properties [41]. All kinds of polymerizable traditional surfactants, including cationic [41], anionic [42] and nonionic [43] have been synthesized to study the influence of the molecular structure on the properties and application. Anionic polymerizable surfactants seem to be the most promising for utilizing in coatings,

tion in improved oil recovery coats and paintings and drilling fluids [37].

emulsion polymerization without the need of stabilizers.

applied in the field of enhanced oil recovery [36].

**2.2. Surfactants**

6 Recent Research in Polymerization

factants moieties [34] such as;

adhesives and enhanced oil recovery.

Emulsion polymerization require free radical polymerizable monomers. Generally, vinyl monomers are used in this type of polymerization such as acrylamide, acrylic acid, butadiene, styrene, acrylonitrile, acrylate ester and methacrylate ester monomers, vinyl acetate, and vinyl chloride [1] and many other vinyl derivatives [25]. Depending on monomer solubility in aqueous phase, there is three categories of typical emulsion polymerization monomers which comprise (1) monomers of high solubility such as acrylonitrile, (2) monomers of medium solubility as methyl methacrylate and monomers insoluble in aqueous phase such as butadiene and styrene [44].

#### **2.5. Other constituents**

Other components involve emulsion polymerization medium that is generally deionized water. Antifreeze additives which involve inorganic electrolytes, ethylene glycol, glycerol, methanol, and monoalkyl ethers of ethylene glycol to allow polymerization at temperatures below 0°C. Sequestering agents which used to solubilize the initiator system or to deactivate traces of hardness elements (Ca+2, Mg+2 ions) such as ethylene diamine tetra acetic acid or its alkali metal salts. Buffers used to stabilize the latex toward pH changes such as phosphate or citrate salts [5, 20]. Chain transfer agents like mercaptans.
