Emulsion initial reaction

An emulsion polymerization reaction follows three conventional steps, namely, initiation, propagation, and termination. These steps can be described by the conventional reactions that are valid for any free radical polymerization. Smith and Ewart [10] proposed that a forming latex particle in an ideal emulsion polymeriza-... 

The function of emulsifier in the emulsion polymerization process may be summarized as follows [45] (1) the insolubilized part of the monomer is dispersed and stabilized within the water phase in the form of fine droplets, (2) a part of monomer is taken into the micel structure by solubilization, (3) the forming latex particles are protected from the coagulation by the adsorption of monomer onto the surface of the particles, (4) the emulsifier makes it easier the solubilize the oligomeric chains within the micelles, (5) the emulsifier catalyzes the initiation reaction, and (6) it may act as a transfer agent or retarder leading to chemical binding of emulsifier molecules to the polymer. 

The water solubilities of the functional comonomers are reasonably high since they are usually polar compounds. Therefore, the initiation in the water phase may be too rapid when the initiator or the comonomer concentration is high. In such a case, the particle growth stage cannot be suppressed by the diffusion capture mechanism and the solution or dispersion polymerization of the functional comonomer within water phase may accompany the emulsion copolymerization reaction. This leads to the formation of polymeric products in the form of particle, aggregate, or soluble polymer with different compositions and molecular weights. The yield for the incorporation of functional comonomer into the uniform polymeric particles may be low since some of the functional comonomer may polymerize by an undesired mechanism. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

Figure C3.1.2 Comparison of lipase activities using an olive oil substrate emulsion prepared with different emulsifying agents. The lipase used in the reaction mixture was from C. rugosa (at 0.06 mg/ml) in the presence of 5% (w/v) each of gum arabic (triangles) or Triton X-100 (diamonds). Reaction progress analysis was obtained using titrimetry. Broken lines represent estimation of initial reaction rates.

The polyazophenylene units are formed from the polyrecombination of the decomposition products from bis(nitrosoacetyl)benzidine. Chain termination can occur by disproportionation of the polymer radicals and by recombination with acetoxy radicals. Despite the rate constant for the recombination of the phenyl and azophenyl radicals being much larger than that of the initiation reaction for isoprene, it is possible to synthesise copolymers from these materials by a careful choice of the various reaction parameters. However, block copolymers could only be obtained using emulsion techniques (see Table 4.11) and not in bulk or in solution. 

The mechanism by which emulsifiers could influence the rate of the thermal initiation reaction is obscure. Most probably the emulsifiers increase the efficiency with which one of the radicals produced in the thermal initiation process escapes into the aqueous phase so that emulsion polymerization may begin. If so those emulsifiers for which exchange between the micelle or the adsorbed layer on a latex particle and true solution in the aqueous phase is most rapid should be most effective in promoting the thermal polymerization. Recently the kinetics of micellization has attracted much attention (29) but the data which is available is inadequate to show whether such a trend exists. 

This study illustrates a particular use of FT-Raman spectroscopy (Section 2.4.2) to monitor an emulsion polymerization of an acrylic/methacrylic copolymer. There are four reaction components to an emulsion polymerization water-immiscible monomer, water, initiator, and emulsifier. During the reaction process, the monomers become solubilized by the emulsifier. Polymerization reactions were carried using three monomers BA (butyl acrylate), MMA (methyl methacrylate), and AMA (allyl methacrylate). Figure 7-1 shows the FT-Raman spectra of the pure monomers, with the strong vC=C bands highlighted at 1,650 and 1,630 cm-1. The reaction was made at 74°C. As the polymerization proceeded, the disappearance of the C=C vibration could be followed, as illustrated in Fig. 7-2, which shows a plot of the concentration of the vC=C bonds in the emulsion with reaction time. After two hours of the monomer feed, 5% of the unreacted double bonds remained. As the... 

The reason for this behaviour is obscure it is not apparently due to the presence of peroxides in the surfactant. However, it is perhaps significant that an emulsion polymerisation reaction in which the rate of polymerisation is first-order with respect to surfactant level is consistent with Smlth-Ewart "Case 2" kinetics for a system in which the surfactant functions as an initiator as well as a micelle generator. 

The initial reaction medium comprises several phases and polymerization occurs in a heterogeneous system, as in emulsion and suspension reactions. 

The effects of emulsifiers in emulsion polymerization systems may be enumerated as follows (l stabilization of the monomer in emulsion, (2) solubilization of monomer in micelles, (3) stabilization of polymer latex particles, (4) solubilization of polymer, (5) catalysis of the initiation reaction, and (6) action as transfer agents or retarders which leads to diemical binding of emulsifier residues in the polymer obtained. 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

Becau% tte emulsion polymerhation obeys a radical mechanism, the way to covalently anchor tte surfactants to the polymer particles is to make them reactive in the radical process. Then one may consider three kinds of reaction the initiation reaction, the propagation reaction and transfer reaction. The termination reaction is carried out between growing polymer radicals and there... 

Fig. 6.5.8 Influence of the water-oil interfacial tension (-y) on the equilibrium product yield and initial reaction rate (v°) for the RAMA-catalyzed aldol addition of DHAP (30 mM) to phenylac-etaldehyde (50 mM) in water/CiaEa/oU 90/4/6w/w gel emulsion systems at 25 °C...

Figure 24, Initial and final reflectance spectra obtained with probe 4, measured from a seeded emulsion polymerization reaction of styrene, concentration is approximately 30 %...

Figure 12 Overall MA mole fraction as a function of conversion for MA-VPV emulsion copolymerization reactions with MU ratios of 0.02 (with initiator concentrations of (o) O.ISgM and (a) 1.2gM), ( ) 0.1, and (A) 0.3. Reprinted from Noel, L. F. J. van AItveer, J. L. Timmermans, M. D. F. German, A. L. J. Polym. Sci., PartA Polym. C/iem. 1996,34,1763-1770. ...

The overall efficacy of microemulsion-based extraction of heavy metals (particularly mercury) from contaminated water involving oleic acid was reported and successfully modeled by Wiencek and coworkers [153,154], who used experimentally determined equilibrium extraction, stripping, and the initial reaction kinetics. This model accurately predicts both the initial extraction kinetics and final mercury extraction equilibrium, Good agreement between theory and experiment on the mechanism of extraction using a microemulsion to that of coarse emulsions has been found. Electrostatic coalescence and butanol addition were evaluated as potential demulsification techniques for recovery of the components from mercury-rich microemulsions [155]. 

There are two stages involved in a typical emulsion polymerization. In the seed stage, a mixture of water, surfactant, and colloid is first heated to the reaction temperature (85-90°C). Next, 5-10% of the monomer mixture with a portion of the initiator is added. At this point the reaction mixture contains monomer droplets stabilized by surfactant, some dissolved monomei the initiator, and surfactant (in solution and in micelles). The initiator breaks down to produce radicals, when heated and these initiate the polymerization of the dissolved monomers. Growing polymer chains eventually enter a micelle, initiating reaction of the monomer inside. If a second growing polymer enters the micelle, termination can occur. 

---