Particle in emulsion polymerization

Propagation constants for butadiene and isoprene were determined from rate of polymerization per particle in emulsion polymerization. 

According to the first method, each micelle in an emulsion behaves like a separate micro-continuous reactor which contains all the components, i.e. monomers and radicals from the aqueous phase. Thus, analogous to the latex particles in emulsion polymerization, microgels formed by emulsion polymerization are distributed in the whole available volume. 

Polymerizable surfactants capable of working as transfer agents include thiosulfonates, thioalkoxylates and methyl methacrylate dimer/trimer surfactants. Thioalkoxylates with 17-90 ethylene oxide units were produced from ethoxylated 11 bromo-undecanol by replacing the bromine with a thiol group via the thiazonium salt route [8]. In the presence of water-soluble azo initiator the thio ended Transurfs (used at a concentration above the CMC) gave monodispersed latex particles in emulsion polymerization of styrene. However, the incorporation of the Transurf remained low, irrespective of the process used for the polymerization (batch, semibatch, seeded). The stability of the lattices when the surfactant and the transfer function were incorporated in the same molecule was better than when they were decoupled. 

Polymerizations of the monomer emulsions were carried out with oil-soluble initiators. Oil-soluble initiators have often been employed in emulsion polymerization recipes and are generally used in suspension polymerization. Whereas in the latter case the initiation naturally takes place in the monomer droplets, the locus of initiation and growth of particles in emulsion polymerization with oil-soluble initiators has been open to some doubt. However, the fact that the particle size and size distribution is not very different from the results with water-soluble initiators and that the particles are generally much smaller than the droplets in the monomer emulsions indicates that with... 

C p maintains its saturation value during intervals I and n provided that the interfacial area of monomer droplets is high enough to allow the monomer transport to the growing particles where it replaces the monomer consumed swelling the polymer formed [146]. However, on the basis of Monte-Carlo simulations, Tauer and Hernandez [147] have claimed that latex particles in emulsion polymerization never experience either a period of saturation with monomer or a constant monomer concentration during interval II, as frequently assumed. 

In dispersion polymerization, the monomer and the initiator are soluble in the continuous solvent phase, the polymer phase separates but is stabilized as a colloid with stabilizer additives. Polymerization proceeds to high degrees of polymerization and the end product is recovered as spherical particles. In emulsion polymerization, the initiator is preferentially dissolved in the continuous phase and not the monomer phase, and the monomer does not have high solubility in the continuous phase. 

A very widespread issue in polymer production is the existence of nano-, micro-, and macroscopic particulates. In some cases, these particulates are integral to the process, such as bacteria in bioreactors, cell fragments in bioextraction, or latex particles in emulsion polymerization. In most cases, however, the particulates are unwanted and can be detrimental both to the product and the reactors. Such particulates include physical aggregates, branched and cross-linked microgels, microcrystals, impurity particles, and macroscopic coagula. This topic is covered in depth in Section 14.3. 

Santos AF, Pinto JC, McKenna TFL.On-Une monitoring of the evolution of number of particles in emulsion polymerization by conductivity measurements. Part I. Model formulation. J Appl Polym Sci 2003 90 1213-1226. 

One potential method developed to predict the concentration of monomer in polymer particles in emulsion polymerization is the thermodynamic approach [77-80], Morton et al. [77] proposed the following equation for calculating the equilibrium concentration of monomer in the polymer particles ... 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Initia.tors, The initiators most commonly used in emulsion polymerization are water soluble although partially soluble and oil-soluble initiators have also been used (57). Normally only one initiator type is used for a given polymerization. In some cases a finishing initiator is used (58). At high conversion the concentration of monomer in the aqueous phase is very low, leading to much radical—radical termination. An oil-soluble initiator makes its way more readily into the polymer particles, promoting conversion of monomer to polymer more effectively. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

It may also be possible to crosslink the acrylic PSA with the help of multifunctional acrylates or methacrylates [87], These monomers can simply be copolymerized with the balance of the other monomers to form a covalently crosslinked network in one step. Since the resulting polymer is no longer soluble, this typ)e of crosslinking is typically limited to bulk reactions carried out as an adhesive coating directly on the article or in emulsion polymerizations where the crosslinked particles can be dried to a PSA film. 

Suspension polymerization produces beads of plastic for styrene, methyl methacrviaie. viny l chloride, and vinyl acetate production. The monomer, in which the catalyst must be soluble, is maintained in droplet fonn suspended in water by agitation in the presence of a stabilizer such as gelatin each droplet of monomer undergoes bulk polymerization. In emulsion polymerization, ihe monomer is dispersed in water by means of a surfactant to form tiny particles held in suspension I micellcsK The monomer enters the hydrocarbon part of the micelles for polymerization by a... 

A key factor in doing a successful suspension polymerization is the composition of the aqueous phase or stabilizer. Too much stabilizer results in emulsion polymerization, which produces small particles (less than 1 /cm). Too little stabilizer results in bulk polymerization. For the production of GPC gels, the ratio of aqueous phase to organic phase should be about 2 1. 

Medvedev et al. [57] extensively studied the use of nonionic emulsifiers in emulsion polymerization. The emulsion polymerizations in the presence of nonionic emulsifiers exhibited some differences relative to those carried out with the ionic ones. Medvedev et al, [57] proposed that the size of latex particles remained constant during the reaction period, but their number increased continually with the increasing monomer conversion. The use of nonionic emulsifiers in emulsion polymerization usually results in larger sizes relative to those obtained by the ionic emulsifiers. It is possible to reach a final size value of 250 nm by the use of nonionic emulsifiers in the emulsion polymerization of styrene [58]. 

Preparation of uniform seed particles Soapless emulsion polymerization is usually preferred for the preparation of uniform seed particles since this technique provides emulsifier-free, larger, and highly uniform micropheres relative to those that can be obtained by the conventional emulsion recipes including emulsifiers and various additives. The size of uniform seed particles with the soapless emulsion procedure is in the range of 0.6-1.2 tm depending on the polymerization conditions [75,108]. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

Even though the chemical reactions are the same (i.e. combination, disproportionation), the effects of compartmentalization are such that, in emulsion polymerization, particle phase termination rates can be substantially different to those observed in corresponding solution or bulk polymerizations. A critical parameter is n, the average number of propagating species per particle. The value of h depends on the particle size and the rates of entry and exit. 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Sulfosuccinamates like DTSM or TTSM are used in emulsion polymerization [92-94]. TTSM imparts small particle sizes ( 0.02-0.06 pm) in many systems except vinyl acetate. It can be used in all monomer systems including... 

Particle morphology, of fillers, 11 303-304 Particle nucleation, in emulsion polymerization, 14 713-714... 

Many water-soluble vinyl monomers may be polymerized by the emulsion polymerization technique. This technique, which differs from suspension polymerization in the size of the suspended particles and in mechanism, is widely used for the production of a number of commercial plastics and elastomers. While the particles in the suspension range from 10 to 1000 nm, those in the emulsion process range from 0.05 to 5 nm in diameter. The small beads produced in the suspension process may be separated by filtering, but the latex produced in emulsion polymerization is a stable system in which the charged particles cannot be recovered by ordinary separation procedures. 

Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves the polymerization of monomers in the form of emulsions (i.e., colloidal dispersions). The process bears a superficial resemblance to suspension polymerization (Sec. 3-13c) but is quite different in mechanism and reaction characteristics. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. 

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