Redox copolymerization, emulsion

FLOYD Emulsion Redox Copolymerization of Vinyl Ferrocene... 

FLOYD Emulsion Redox Copolymerization of Vinyl Ferrocene TABLE III. TLC OF FERROCENE-SPIKED POLYMER... 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

These superabsorbents are synthesized via free radical polymerization of acrylic acid or its salts in presence of a crosslinker (crosslinking copolymerization). Initiators are commonly used, water-soluble compounds (e.g., peroxodi-sulfates, redox systems). As crosslinking comonomers bis-methacrylates or N,hT-methylenebis-(acrylamide) are mostly applied. The copolymerization can be carried out in aqueous solution (see Example 5-11 or as dispersion of aqueous drops in a hydrocarbon (inverse emulsion polymerization, see Sect. 2.2.4.2). 

The emulsion copolymerization of BA with PEO-MA (Mw=2000) macromonomer was reported to be faster than the copolymerization of BA and MMA, proceeding under the same reaction conditions at 40 °C [100]. Polymerizations were initiated by a redox pair consisting of 1-ascorbic acid and hydrogen peroxide in the presence of a nonionic surfactant (p-nonyl phenol ethoxylate with 20 moles ethylene oxide). In the macromonomer system, the constant-rate interval 2 [9,10] was long (20-70% conversion). On the other hand, the interval 2 did not appear in the BA/MMA copolymerization and the maximum rate was lower by ca. 8% conversion min 1 and it was located at low conversions. 

The mechanism of particle formation at submicellar surfactant concentrations was established several years ago. New insight was gained into how the structure of surfactants influences the outcome of the reaction. The gap between suspension and emulsion polymerization was bridged. The mode of popularly used redox catalysts was clarified, and completely novel catalyst systems were developed. For non-styrene-like monomers, such as vinyl chloride and vinyl acetate, the kinetic picture was elucidated. Advances were made in determining the mechanism of copolymerization, in particular the effects of water-soluble monomers and of difunctional monomers. The reaction mechanism in flow-through reactors became as well understood as in batch reactors. Computer techniques clarified complex mechanisms. The study of emulsion polymerization in nonaqueous media opened new vistas. 

Redox systems are used for polymerizations at lower temperatures. Many of these redox initiator couples were developed for the emulsion copolymerization of butadiene and styrene, since the 5-10"C cold recipe yields a better rubber than the hot SO C emulsion polymerization. 

Free-radicals generated in many oxidation-reduction (or redox) reactions can be used to initiate chain poymerization. An advantage of this type of initiation is that, depending on the redox system used, radical production can occur at high rates at moderate (0-50°C) and even lower temperatures. Redox systems are generally used in polymerizations only at relatively low temperatures, a significant commercial example being the production of styrene-butadiene rubber by emulsion copolymerization of butadiene and styrene at 5-10°C ( cold recipe ). 

Redox emulsion copolymerization of vinyl acetate and vinyl propionate [235]. 

The most successful method developed for the production of a general-purpose synthetic rubber was the emulsion copolymerization of butadiene and styrene (SBR), which still represents the main process in use today (Blackley, 1975 Hofmann, 1989 Blow, 1971 Brydson, 1981 Bauer, 1979 Sun and Wusters, 2004 Demirors, 2003). The general principles of copolymerization will be discussed in a later section, but it is instructive at this point to examine the other main features of this system. The types of recipes used are seen in Table 2.5 (Bauer, 1979). The recipes shown are to be considered only as typical, as they are subject to many variations. It should be noted that the initiator in the 50°C recipe (hot rubber) is the persulfate, whereas in the 5°C recipe (cold mbber) the initiator consists of a redox system comprising the hydroperoxide-iron(II)-sulfoxylate-EDTA. In the latter case, the initiating radicals are formed by the reaction of the hydroperoxide with the ferrous iron, whose concentration is... 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

Copolymers [69418-26-4] of acrylamide and AETAC (see Table 5, footnote b) are the most important flocculants because of a uniform sequence distribution of comonomers (140,141). Reactivity ratios obtained under very different free-radical copol5unerization conditions can agree very well. For example, in one case, a free-radical copol5unerization was initiated using potassium persulfate (KPS) [7727-21-1] in aqueous solution at pH 6.1 (141), while in the other case the copolymerization was initiated using a TBHP/MBS redox pair in an inverse emulsion stabilized with sorbitan monooleate (SMO) at pH 3.5 (140). The surfactant in an inverse emulsion may alter the reactivity of both AMD and AETAC. For... 

Apart from the fluoro monomers vinyl fluoride (VF), vinylidene fluoride (VF2), and tetrafluoroethylene (TFE), only chlorofluoroethylene has found commercial use as homopolymer. It is applied as thermoplastic resin based on its vapor-barrier properties, superior thermal stability (Tdec > 350 °C), and resistance to strong oxidizing agents [601]. Chlorofluoroethylene is homo- and copolymerized by free-radical-initiated polymerization in bulk [602], suspension, or aqueous emulsion using organic and water-soluble initiators [603,604] or ionizing radiation [605], and in solution [606]. For bulk polymerization, trichloroacetyl peroxide [607] and other fluorochloro peroxides [608,609] have been used as initiators. Redox initiator systems are described for the aqueous suspension polymerization [603,604]. The emulsion polymerization needs fluorocarbon and chlorofluorocarbon emulsifiers [610]. 

The polymerization of 1 can be started thermically, with radicals, or by light [385,390,391]. However, since only oligomers were observed, those homopolymerizations are of academic interest only. 1 has been copolymerized with vinyl chloride and vinyl acetate [392], initiated by redox initiators in emulsion. Copolymers of this monomer are also available by hydrolysis of copolymers containing derivatives of 1-alkenylphosphonic acid, such as dichlorides [392-394] or diesters [395]. Copolymers are also described with acrylonitrile, acrylic amide, N-vinylacetamide, and N-vinylpyrrolidone they are particularly interesting for textile dying, tanning techniques and water separating membranes [396-399]. 

Xu and Chen [32] prepared two polymerizable surfactants, sodium 4-((o-acryloyloxyalkyl)oxy benzene sulfonate with the alkyl chain length equal to 8 or 10, and used them to stabilize the semibatch emulsion copolymerization of butyl methacrylate. A redox initiator system of ammonium persulfate and tetramethylethylenediamine was used to start the polymerization at room temperature. The latex particle size increases continuously, whereas the number of particles per unit volume of water remains relatively constant with the progress of polymerization. This is attributed to the predominant micellar nucleation mechanism. X-ray photoelectron spectroscopy data show that polymerizable surfactant molecules are preferably located near the latex particle surface layer. 

Three general methods of copolymerization were attempted (1) radical solution polymerization in an evacuated sealed glass tubes, (2) redox emulsion polymerization in water solution, and (3) radical polymerization (either in solution or neat) cast between mylar sheets to form a thin membrane film and initiated imder a uv lamp. The third method proved to be the best for forming membrane materials that could easily be tested for proton conductivity in an in-plane conductivity cell. This method was used most extensively and produced a large number of viable membrane materials. 

For industrial production, the polymerization is carried out in bulk, in emulsion or in suspension. Bulk polymerization, like that of vinyl chloride, gradually leads to precipitation of polymer after its appearance (dispersion polymerization). Emulsion polymerization is initiated by redox systems at relatively low temperature ( 50°C). Suspension polymerization (like that in bulk) is initiated by means of organic peroxides (lauryl peroxide, etc.). The vinylidene chloride is also copolymerized with vinyl chloride to give a material whose Tg is higher than that of PVC. 

Solution Copolymerization of Glycidyl Methacrylate and Styrene 41-8. Redox Emulsion Polymerization of Ethyl Acrylates 41-9. Preparation of Isotactic Poly(methyl methacrylate)... 

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