Redox emulsion polymerization, vinyl chloride

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

The production of vinyl chloride monomer is only a part of PVC production. Polymerization of the monomer completes the process. Commercially, it is a batch operation by one of three methods suspension, emulsion, or bulk. In all three methods, the chemical reaction is a free radical-initiated chain reaction. Peroxides or redox systems generally are used to provide the initial free radicals. 

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. 

The earliest polymerization processes were either batch mode or semibatch. The semibatch method was used for products, where the two monomers differed greatly in reactivity, as in Union Carbide s early Dynel, acrylonitrile-vinyl chloride, process. Bulk, solution, and emulsion polymerization processes have also been developed for acrylonitrile and its copolymers. However, in recent years nearly every major acrylic fiber producer has used a continuous aqueous suspension process, employing a redox catalyst, followed by a series of steps, which includes slurry filtration and polymer drying. 

Emulsion polymerizations of vinyl chloride are usually conducted with redox initiation. Such reactions are rapid and can be carried out at 20 C in one to two hours with a high degree of conversion. Commercial poly(vinyl chloride)s range in molecular weights from 40,000-80,000. The polymers are mostly amorphous with small amounts (about 5%) of crystallinity. The crystalline areas are syndiotactic. ... 

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]. 

The emulsion polymerization of vinyl chloride is usually carried out by the thermochemical initiation with water-soluble initiators such as peroxodisulfate, hydrogen peroxide and/or the redox systems. 

Mork and Ugelstadt [85] investigated the emulsion polymerization of vinyl chloride initiated by peroxodisulfate/bisulphite/Cu /citrate redox system. The rate of polymerization was found to be of first order with respect to the concentration of bisulphite and independent of the peroxodisulfate concentration. 

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. 

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