Aqueous emulsions, polymerization

Aqueous emulsion polymerization is carried out using a fluorinated emulsifier, a chain-transfer agent to control molecular weight, and dispersion stabilizers such as manganic acid salts and ammonium oxalate (13,14). 

The aqueous emulsion polymerization can be conducted by a batch, semibatch, or continuous process (Fig. 5). In a simple batch process, all the ingredients are charged to the reactor, the temperature is raised, and the polymerization is mn to completion. In a semibatch process, all ingredients are charged except the monomers. The monomers are then added continuously to maintain a constant pressure. Once the desired soflds level of the latex is reached (typically 20—40% soflds) the monomer stream is halted, excess monomer is recovered and the latex is isolated. In a continuous process (37), feeding of the ingredients and removal of the polymer latex is continuous through a pressure control or rehef valve. 

Redox initiation is commonly employed in aqueous emulsion polymerization. Initiator efficiencies obtained with redox initiation systems in aqueous media are generally low. One of the reasons for this is the susceptibility of the initially formed radicals to undergo further redox chemistry. For example, potential propagating radicals may be oxidized to carbonium ions (Scheme 3.44). The problem is aggravated by the low solubility of the monomers (e.g. M VIA. S) in the aqueous phase. 

It was found that 6 (a variant of the latter Grubbs catalyst [34]) would catalyze the living polymerization of norbomene in organic solvents. Although not soluble in water, complex 6 will initiate aqueous emulsion polymerizations. To date, all attempts to solubilize 6 in aqueous solution by the incorporation of sulfonated phosphine ligands have failed to yield active catalysts. 

In a typical aqueous emulsion polymerization, a water-immiscible monomer is polymerized in the presence of a surfactant and a water-soluble initiator, such as potassium peroxodisuUate. A water-soluble radical, e.g. -SOl formed by thermal decomposition of the initiator, grows by addition of monomer dissolved in the... 

Piirma I. Polymeric surfactants as stabilizers in aqueous emulsion polymerization. In Polymeric Surfactants, Surfactant Science Series. Vol. 42, New York Marcel Dekker, 1992 127-164. 

A survey of a variety of polymerizable groups indicated that in the preferred free-radical aqueous-emulsion polymerization system, a per-fluorovinyl ether most nearly fulfills these requirements. In general, the perfluorovinyl ethers copolymerize at a reasonable rate. They change little in rate with variations in chain length or functionality in the per-fluoroalkyl moiety so long as the type of functionality on the perfluoro-alkyl group is consistent with requirements (a) and (b). In addition, such ethers furnish thermally resistant structures in the polymer. 

Although dispersion polymerization is similar to conventional aqueous emulsion polymerization, there are some crucial differences. In dispersion polymerization, the monomer is usually soluble in the organic liquid in which the polymer dispersion is to be made a surfactant is therefore not necessary to maintain the monomer in emulsion form. Since the monomer is in solution, agitation of the polymerizing system is of secondary importance. Furthermore, in contrast to emulsion polymerization, the initiator fragments do not stabilize the resulting dispersion. The initiator to monomer ratio can therefore be used to control the molecular mass of the polymer without affecting the particle size of the dispersion [3.61]. 

To create particles with a hydrophobic core and a hydrophilic shell, preformed hydrophobic polymer particles can be used as substrate for the covalent grafting of hydrophilic polymer chains. Aqueous emulsion polymerization is thus the method of choice for synthesis of core particles with submicrometric diameter (due to the scope of this review article, discussion will be restricted to aqueous emulsion polymerization, although some examples of particle substrates created via suspension polymerization exist however, with much larger diameters). The grafting methods are very close to those used for the surface modification of inorganic materials, such as for instance silicon wafers or colloidal silica particles [46,48-52], Various strategies can be employed such as ... 

PREPARATIVE TECHNIQUES 1. Free radical polymerization in fluorocarbon solvents. 2. Aqueous emulsion polymerization. 

The synthesis of poly(phenylene vinylene) (PPV) nanoparticles using ADMET has also been reported [47]. In this case, an aqueous emulsion polymerization was utilized, such that particle sizes ranging from 200 to 300 nm were observed. The molecular weights of the PPVs in this case were rather low (on the order of 1 kDa), but this was attributed to slow rates of reaction, rather than to the sensitivity of the catalyst species towards aqueous conditions. Despite having low molecular weights, these polymers demonstrated absorption and photoluminescence data that were consistent with those of previously reported PPVs. Some examples of conjugated polymers synthesized via ADMET are shown in Figure 20.3. 

Replacement of organic solvents with water, especially chlorinated solvents Aqueous emulsion polymerization to replace solvent-based polymerization Surface treatment (modification) process development for water-borne... 

Sometimes, the two processes (reversible termination and reversible transfer) can operate simultaneously (example hving radical poljmerizations mediated by organo-cobalt porphjrin complexes reported by Wayland et al. ). Several CRP techniques have been developed in the past thirty years, but their implementation at an industrial scale, especially in heterogeneous aqueous processes which are of major importance nowadays, remains a challenge. In this paper, we report our strategy to set up a new CRP technique based on simple, readily available and economical chemicals and our attempts to control the polymerization in aqueous emulsion polymerizations. Experimental details are given elsewhere.l ... 

However, there are two types of coating in this overall grouping which are based on polymer made originally by aqueous emulsion polymerization. The emulsions are spray-dried and the polymer powder so produced is then redispersed in non-aqueous liquids to produce useful coatings. These coatings are called organosols and plastisols. 

Fluorinated surfactants have been used for decades as processing aids during aqueous emulsion polymerization synthesis of fluoropolymers such as poly (tetrafluoroethylene). The function of the fluorosurfactant is to solubilize both the fluorinated monomer(s) as well as the growing fluoropolymer. Historically, the... 

The elastomers listed in Table 1 are typically prepared by high pressure, free-radical, aqueous emulsion polymerization (5,8,36,37). The initiators are organic or... 

The aqueous emulsion polymerization is conducted by batch, semicontinu-ous, or a continuous process (Fig. 5). In a simple batch process, all the ingredients are charged to the reactor, the temperature raised, and the poljnnerization nm to completion. 

The free radical polymerizations are mostly water based, either as aqueous suspension polymerization (mostly applied for PTFE-polymers) or as aqueous emulsion polymerization in the presence of emulsifiers, most preferably in the presence of fluorinated emulsifiers. The use of such fluorinafed emulsifiers, however, provides fheir own challenges (see chapter 21.3). 

Fluoroemulsifier-Free Polymerizations The various fluoropolymer manufactures are exploring polymerization approaches to eliminate the use of highly fluorinated emulsifiers during the aqueous emulsion polymerizations. In cases where lower molecular weight polymers are generated and in the presence of chain-transfer agents, a couple of new options are feasible ... 

The commercial elastomers are commonly prepared by aqueous emulsion polymerization at elevated temperatures and pressures using a redox initiating system such as a bisulphite-persulphate mixture. Ammonium perfluorooctoate or some other highly fluorinated surfactant is used as a dispersing agent since it is important that any surfactant used does not become involved in transfer reactions. 

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