Polymerization initiators water-soluble

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. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Why are some initiators water soluble and others monomer soluble Which type is used for suspension polymerization Emulsion polymerization ... 

The living radical polymerization process is also valid for the polymerization of water-soluble monomers. The polymerization of sodium styrenesulfonate in aqueous ethylene glycol (80%) in the presence of TEMPO using potassium per-sulfate/sodium bisulfite as the initiator at 125 °C gave a water-soluble polymer with well-controlled molecular weight and its distribution [207]. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

The initiators used in emulsion polymerization are water-soluble initiators such as potassium or ammonium persulfate, hydrogen peroxide, and 2,2 -azobis(2-amidinopropane) dihydrochloride. Partially water-soluble peroxides such a succinic acid peroxide and f-butyl hydroperoxide and azo compounds such as 4,4 -azobis(4-cyanopentanoic acid) have also been used. Redox systems such as persulfate with ferrous ion (Eq. 3-38a) are commonly used. Redox systems are advantageous in yielding desirable initiation rates at temperatures below 50°C. Other useful redox systems include cumyl hydroperoxide or hydrogen peroxide with ferrous, sulfite, or bisulfite ion. 

Kinetically, each bead acts as a small independent reactor there is little exchange of material between the beads. Since there is no solvent present at the locus of polymerization, the kinetics are those of bulk polymerization, with the molecular weight distribution (MWD) characteristics similar to those of bulk or solution polymerizations. If water-soluble initiator is used in a suspension polymerization, very little polymerization will occur, since few free radicals will reach the locus of polymerization in the monomer beads. 

Claverie et al. [325] have polymerized norbornene via ROMP using a conventional emulsion polymerization route. In this case the catalyst was water-soluble. Particle nucleation was found to be primarily via homogenous nuclea-tion, and each particle in the final latex was made up of an agglomeration of smaller particles. This is probably due to the fact that, unlike in free radical polymerization with water-soluble initiators, the catalyst never entered the polymer particle. Homogeneous nucleation can lead to a less controllable process than droplet nucleation (miniemulsion polymerization). This system would not work for less strained monomers, and so, in order to use a more active (and strongly hydrophobic) catalyst, Claverie employed a modified miniemulsion process. The hydrophobic catalyst was dissolved in toluene, and subsequently, a miniemulsion was created. Monomer was added to swell the toluene droplets. Reaction rates and monomer conversion were low, presumably because of the proximity of the catalyst to the aqueous phase due to the small droplet size. 

Polymerization of vinylidene fluoride by emulsion or suspension polymerization in water is conducted at conditions of 10-130 °C and 10-200 bar. In the emulsion polymerization, either water-soluble peroxides or monomer-soluble peroxy or organic peroxides are used as initiators [ 17]. Fluorinated surfactants, such as ammonium perfluorooctanoate, are used as dispersing agents. Chain transfer agents, such as acetone, chloroform, or trichlorofluoromethane, may be... 

The suspension polymerization is conducted using monomer-soluble per-oxy initiators. Water-soluble polymers, such as poly(vinyl alcohol), are typically used as suspending agents to reduce the coalescence of the polymer particles [ 17]. A slurry of polymer particles 30-100 pm in diameter is formed during the polymerization. The particles are washed and dried before further processing. 

The conditions which have been defined for the formation of effective microemulsions (nature of the oil, Ro and HLB values) are also required for obtaining clear and stable microlatices after polymerization. Various water-soluble monomers have been polymerized by a free radical process in anionic and nonionic microemulsions, either under U.V. irradiation or thermally with AIBN as the initiator (11,14,22,29). Total conversion to polymer was achieved in less than 20 minutes (a few minutes in some cases). Series of experiments have been performed in various oils. Table III summarizes some of the results and emphasizes the importance of the formulation. A good chemical matching between oils and emulsifiers (G 1086 -t Arlacel 83, Isopar H) leads to stable latices, a poor matching (G 1086 Arlacel 83, heptane) leads to unstable latices which settle within a few hours to a few days (22). 

In aqueous-based emulsion polymerizations using water-soluble initiators, the surface groupings formed are frequently determined the nature of the initiator used (Otlewill et aK 1967 van den Hul et at., 1970 Goodwin et aU 1973) and the following have been reported ... 

Usually, monomer droplets are bdieved not to play any role in emulsion polymerization other than as a source of monomer. Ugelstad and associates have shown, however, that in cases with very small monomer droplets, these may become an important, or even the sole, loci for particle nucleation. The system may then be regarded as a microsuspension polymerization with water-soluble initiators. It has therefore been pointed out (Hansen and Ugelstad, 1979c) that particle nucleation mndels should include all three initiation mechanisms— micellar, homogenous, and droplet—since all these mechanisms may compete and coexist in the same system, even if one of them usually dominates. 

Equations, which are also applicable to suspension, solution, and bulk polymerization, form an extension of the Smith-Ewart rate theory. They contain an auxiliary parameter which is determined by the rate of initiation, rate constant of termination, and volume of the porticles. The influence of each variable on the kinetics of emulsion polymerization is illustrated. Two other variables are the number of particles formed and monomer concentration in the particles. Modifications of the treatment of emulsion polymerization are required by oil solubility of the initiator, water solubility of the monomer, and insolubility of the polymer in the monomer. 

After examination of the role of each reactant implied in the polymerization of water-soluble A-alkylacrylamide and Ai-alkylmethacrylamide monomer in the presence of the water-soluble crosslinker agent and radical initiators, the polymerization mechanism of this system in the preparation of thermally sensitive microgel submicron particles can be presented and detailed as follows (Figure 12.16). 

The remaining two examples of metathesis chemistry in water are related to the synthesis of polymers. In the first case, the solubility and stability of the ruthenium catalysts in water is exploited in the emulsion polymerization of norbor-nenes and cyclooctadiene. In emulsion polymerization, a water-soluble initiator is required. Claverie [58] used complex 11 or the related complex RuC12-(TPPTS)2(=CHC02Et) (where TPPTS = tris(3-sulfonatophenyl)phosphine, sodium salt). These two complexes were used with standard surfactants to product well... 

Initiation. Water-soluble initiators are normally used in emulsion polymerization, and droplet initiation can only take place when a waterborne oligomer diffuses into the monomer droplet. Although such diffusion does take place, in most emulsion polymerization systems the bulk of the Initiation and propagation occurs in the particles. Oil-soluble initiators... 

Problems exist with the chemical and structural purity of the inisurfs especially from the colloidal point of view. One must always bear in mind that impurities are present in most systems investigated. Nevertheless, the results known so far clearly show the pecularities of inisurfs compared to conventional initiators for emulsion polymerizations like water-soluble peroxides or AIBN. 

With the work by Grubbs et al. [27] and Herrmann et al. [28], the use of ruthenium carbene complexes as homogeneous catalysts for the ROMP (Ring-Opening Metathesis Polymerization) of olefins was estabhshed (see Section 2.4.4.3). The development of catalysts that can catalyze hving polymerization in water was an important goal to achieve, especially for applications in biomedicine. In this context, two water-soluble ruthenium carbene complexes (3 and 4) have been reported that act as initiators for the living polymerization of water-soluble monomers in a quick and quantitative manner [29]. 

The urea-formaldehydes (UFs) are the most important and most used class of amino resin adhesives. Amino resins are polymeric condensation products of the reaction of aldehydes with compounds carrying aminic or amidic groups. Formaldehyde is by far the primary aldehyde used. The advantage of UF adhesives are their (1) initial water solubility (this renders them eminently suitable for bulk and relatively inexpensive production), (2) hardness, (3) nonflammability, (4) good thermal properties, (5) absence of color in cured polymers, and (6) easy adaptability to a variety of curing conditions [1,2]. 

A priori, it seems logical to apply the accepted concepts of conventional emulsion polymerization (with water-soluble initiators) to inverse emulsions using oil-soluble initiators. In fact, only few attempts have been made to apply the Smith-Ewart theory [26,36-38]. The determination of n is difficult here because of the ill-defined stages of the reaction, the unusual kinetics and the broad particle size distribution. The kinetic studies of Vanderhoff et al. [26,29] and Visioli [37] are examples of applying the Smith-Ewart theory to the polymerization of acrylamide and p-vinylbenzenesulfonate in xylene initiated with benzoyl peroxide. The data unexpectedly followed Smith-Ewart Case 1 (n 0.S). It was postulated that radicals were generated in, or enter particles pairwise due in the enhanced water solubility of the benzoyl peroxide by the presence of monomo-. 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

The preparation of microparticles by emulsion polymerization, originally developed in the synthetic rubber industry (Whitby and Katz 1933), allows for the formation of microparticles with a narrow distribution of sizes. In this technique, a monomer is dispersed in a solution of surfactant and water where the surfactant creates micelles in the water. Low solubility of the monomer in water is required, such that the addition of the polymer creates large droplets of the monomer within the water. Small amounts of monomer diffuse through the water into the surfactant micelles. To begin the polymerization process, water-soluble initiator is added to the solution, propagating the monomer to form... 

Keywords aqueous-phase polymerization free-radical polymerization methacrylic acid PLP-SEC propagation rate coefficients pulsed-laser initiation water-soluble monomers... 

There are in principle three methods for surface modification to generate a diffuse structure. 1. Binding of water-soluble polymer chains to the substrate surface like enzyme immobilization 2. Polymerization of water-soluble monomer by initiating in the substrate surface and propagating toward the outside. 3. Coating of the polymer. 

Projea No. 2004-034-1-400 Critically evaluated propagation rate coefficients for free-radical polymerization of water-soluble monomers polymerized in the aqueous phase Projea No. 2009-050-1-400 Critically evaluated rate coefficients associated with initiation of radical polymerization... 

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