Free-radical Polymerization Suspension

Many important polymers are made commerdally via suspension polymerization of vinyl monomers. These include poly(vinyl chloride), poly(methyl methacrylate), expandable polystyrene, styrene-acrylonitrile copolymers and a variety of ion-exchange resins and specialist materials. The annual polymer production from suspension processes is very high. 

---

This polymerization is governed by multiple simultaneous mechanisms such as particle coalescence and break-up, secondary nucleation and the diffusion of monomer to the interface, as Sdnchez-Silva et al. exposed in their study. The collective effect of these mechanisms confers the size, the structure, and the surface properties to the microcapsules. It was developed a method based on a free radical polymerization suspension process to fabricate nonpolar MPCM by Sanchez et al. Also, these authors studied the influence of the temperature in a reaction, stirring rate and the mass ratio of paraffin to styrene on the thermal properties of MPCM. More studies using this process were done by Borreguero et al., who considered two main steps a continuous one with deionized water and the stabilizer (polyvinyl-pyrrolidone, PVP), and a discontinuous one containing the styrene monomer, the paraffin wax, and the benzoyl peroxide. 

Vlayl fluoride undergoes free-radical polymerization. The first polymerization iavolved heating a saturated solutioa of VF ia tolueae at 67°C uader 600 MPa (87,000 psi) for 16 h (24). A wide variety of ioitiators and polymerization conditions have been explored (25—27). Examples of bulk (28,29) and solution (25,28,30,31) polymerizations exist however, aqueous suspension or emulsion methods are generally preferred (26,32—40). VF volatiflty dictates that moderately high pressures be used. Photopolymerizations, usually incorporating free-radical initiators, are also known (26,28,29,35). 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

A novel cross-linked polystyrene-divinylbenzene copolymer has been produced from suspension polymerization with toluene as a diluent, having an average particle size of 2 to 50 /rm, with an exclusive molecular weight for the polystyrene standard from about 500 to 20,000 in gel-permeation chromatography. A process for preparing the PS-DVB copolymer by suspension polymerization in the presence of at least one free-radical polymerization initiator, such as 2,2 -azo-bis (2,4-dimethylvaleronitrile) with a half-life of about 2 to 60 min at 70°C, has been disclosed (78). 

Lignin, brown coal polymer of methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinyl acetate, methyl vinyl ether, ethyl vinyl ether, N-methylmeth-acrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, or 2-acrylamido-N-methylpropane sulfonic acid free radical polymerization of a water-soluble vinyl monomer in an aqueous suspension of coals [705,1847]... 

Sulfonated styrene-maleimide copolymers are similarly active [1073], Examples of maleimide monomers are maleimide, N-phenyl maleimide, N-ethyl maleimide, N-(2-chloropropyl) maleimide, and N-cyclohexyl maleimide. N-aryl and substituted aryl maleimide monomers are preferred. The polymers are obtained by free radical polymerization in solution, in bulk, or by suspension. 

Free-radical polymerization of alkenes has been carried out in aqueous conditions.115 Aqueous emulsion and suspension polymerization is carried out today on a large scale by free-radical routes. Polymer latexes can be obtained as products (i.e., stable aqueous dispersions... 

The principle free radical polymerization techniques are bulk, solution, suspension, and emulsion. Tables 6.5 and 6.6 briefly describe these techniques. 

Free radical polymerization processes [41] are carried out in bulk, solution, suspension, emulsion, or by precipitation techniques. In all cases the monomer used should be free of solvent and inhibitor or else a long induction period will result. In some cases this may be overcome by adding excess initiator. 

A-Vinyl-2-pyrrolidone (1.8 mol), 2,2 -azobis(2-methyl-N-(2-hydroxyethyl)-propio-namide) (0.018 mol), and 2-mercaptoethanol (0.072 mol) were dissolved in 3000 ml of 2-propanol degassed for 1 hour. The free radical polymerization was carried out at reflux while stirring for 44 hours. The mixture was concentrated and then dissolved in 400 ml of 4-methyl-2-pentanone and slowly precipitated into 5000 ml of t-butyl methyl ether. The suspension was filtered and the filter cake washed twice with 200 ml of -butyl methyl ether. The solid was purified by dissolving in 400 ml of... 

At present all commercial polystyrene (with average molecular weights between 100,000 and 400,000) is manufactured by radical polymerization, which yields atactic polymers.476 Peroxides and azo compounds are commonly used initiators. The suspension process (usually as a batch process in water at 80-140°C) produces a product with relatively high residual monomer content.223 More important is the continuous solution process (usually in ethylbenzene solvent at 90-180°C), which yields high-purity product. Styrene can be copolymerized with numerous other monomers.477 One of these copolymers, the styrene-divinylbenzene copolymer produced by free-radical polymerization, has a crosslinked stucture and is used in... 

As with most synthetic plastic materials, they commence with the monomers Any of the common processes, including bulk, solution, emulsion, or suspension systems may be used in the free-radical polymerization or copolymerization of acrylic monomers. The molecular weight and physical properties of the products may be varied over a wide range by proper selection of acrylic monomer and monomer mixes, type of process, and process conditions. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. 

Free-Radical Polymerization in Bulk, Solution, and Suspension. 

In free-radical polymerization in bulk, solution, and aqueous suspension the initiator is dissolved in the monomer. Aqueous suspension polymerization is considered as bulk polymerization in droplets. Some polymers, like poly (vinyl chloride) or polyacrylonitrile, are insoluble in the monomers and precipitate during bulk polymerization. The growth of the precipitated chain, which depends on the number of trapped radicals, is... 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

While polymerization by ionic methods is usually performed in solution, free radical polymerizations can be performed in solution, bulk, suspension, or emulsion.27 Each of these methods is described below. 

Polyvinyl chloride is produced by the free-radical polymerization of vinyl chloride. Bulk, emulsion, solution, and suspension polymerization processes have been used. 

The preparation of polymers through free radical polymerization can be carried out in several ways, both in laboratory and industrial scales. Among the techniques, bulk, suspension, emulsion and solution polymerization methods are of the most practical importance. 

If surfactant is added to a suspension polymerization system, a number of phenomena may occur. If the surfactant is added in small amounts (below the critical micelle concentration or CMC), the reduction in interfacial tension between the organic and aqueous phases will result in smaller monomer droplets, but it has hardly any other effect. If surfactant is added above the CMC, and an oil-soluble initiator is used, the process is commonly termed a microsuspension polymerization. Due to the reduced interfacial tension, the droplet diameter (and hence bead diameter) is reduced to approximately 10-40 pm. Little polymerization takes place in the aqueous phase or in particles generated from surfactant micelles because of the hydrophobic nature of the initiator. However, some smaller particles initiated from surfactant micelles may be found. The kinetics are still essentially those of a bulk free radical polymerization. Microsuspension polymerization is used to produce pressure-sensitive adhesives for repositionable notes. 

SAN copolymers are produced in free-radical polymerizations using solution, suspension or emulsion processes. Industrially useful products are single-... 

---