Stirred Bulk Polymerization

Bulk polymerizations that are stirred have been used for various commercial polymers. Some, such as free-radical-initiated polyethylene, are carried out to low conversion so that the unreacted monomer acts as a diluent. These are, in effect, solution polymerizations and are treated in Section 5.3. Equipment that will handle liquids progressively from monomer ( 0.01 poise) to polymer ( 10 poise) with efficient heat removal is usually designed specifically for a given installation. Conventional turbine- or propeller-agitated vessels can handle a limited degree of conversion. With low-molecular-weight condensation polymers, the completed polymer melt may be transferable by gear piunps or merely extruded from the reactor by application of moderate pressme. 

FIGURE 5.4 Twin-screw extruder reactor. (Courtesy of Mack, W. A., and R. Herter, Extruder Reactors for Polymer Production, Werner Pfleiderer Corporation, Waldwick, NJ, 1975.) 

FIGU RE 5.5 Continuous polymerization of Nylon 6,6. Polymer of increasing molecular weight is formed as the acid-rich mixture contacts a countercurrent flow of diamine vapor. (Data from Brearly, A. M. et al., US Patent 5,674,974, E. I. Du Pont de Nemours and Company, 1997.) 

FIGURE 5.6 Continuous hulk polymerization of vinyl ehloride. PVC, poly(vinyl chloride). (Data from Krause, A. Chem. Eng. 72, 117, 1965.) 

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Bulk Polymerization The monomer and initiators are reacted without or with mixing without mixing to make useful shapes direcily, like bakelite products. Because of viscosity hmitations, stirred bulk polymerization is not carried to completion but only to 30 to 60 percent or so, with the remaining monomer stripped out and recycled. A... 

The mixing of reactants and catalyst in this investigation was achieved using a magnetic stirrer (Coming Hot Plate Stirrer PC-351). Empirically, it was found that three poise was the upper limit of viscosity for the stirrer to function well. This limit set a threshold temperature for each diol for magnetically stirred bulk polymerizations. Using the above correlations, the threshold temperatures are 43 C for C7 diol, 55 C for C9 diol, and 70 C for C, diol. 

In bulk polymerization, also called mass or block polymerization, monomer and polymer (and initiator) are the only components. When only part of the monomer charge is converted to polymer, however, the problems encountered are more typical of the solution method, which is discussed next. We can differentiate between quiescent and stirred bulk polymerizations. Both methods are applied to systems where polymer is soluble in monomer and progressively increases in viscosity with conversion. In quiescent systems, gel formation, corresponding to infinite viscosity, can occur. Because of the heat of polymerization and autoacceleration, the reaction rate is difficult to control. Heat removal is impeded by high viscosity and low thermal conductivity. The removal of traces of unreacted monomer from the final product is difficult because of low diffusion rates. Conversion of all monomer is difficult for the same reason. 

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

Step growth polymers, such as polyesters, are often manufactured via bulk polymerization. The reactive species are mixed together in a stirred reactor designed to promote intimate contact between the reactants. Variables such as temperature and pressure are used to control the molecular properties of the final polymer. 

It is not practical to stir all reaction systems, for example, bulk polymerizations, postpolymerization reactions, fixed-bed catalytic reactors, and plug-flow reactors. Although multipoint temperature sensing is often used as a key solution to determine a runaway in nonagitated vessels, the occurrence of hot spots may not always be detected. 

The production of conventional stationary phases in the form of porous polymer particle is based on suspension polymerization. Namely, the polymerization is allowed to proceed in a solvent under vigorous stirring that assures obtaining particles of the desired diameter. Since the particle size is typically in the range of a few micrometers, no problems with heat transfer are encountered. In contrast, the preparation of monoliths requires a so-called bulk polymerization. A polymer mixture consisting of monomers and porogenic solvent is mixed with an initiator. As the temperature is increased, the initiator decomposes and oligomer nu-... 

This was derived assuming uniform concentration because good mixing is important for this relationship to hold. It also assumes a constant temperature. Both these assumptions are only approached in most batch systems. Further, stirring becomes more difficult as conversion increases so that both control of localized temperature and concentration become more difficult. In reality, this relationship holds for only a few percentage points of conversion. Overall, temperature is a major concern for vinyl polymerizations because they are relatively quite exothermic. This is particularly important for bulk polymerizations. This, coupled with the general rapid increase in viscosity, leads to the Trommsdorff-like effects. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

In a 100 ml three-necked flask with stirrer and thermometer 1 mol% of p-toluenesul-fonic acid methyl ester (methyl tosylate) are added to 3 g (0.03 mol) of anhydrous 2-methyl-2-oxazoline.The reaction mixture is stirred under nitrogen at 100-120 °C.The bulk polymerization sets in immediately. After 30 min. the viscous polymer melt is poured in a dish where it solidifies within minutes. After cooling to room temperature about 2 grams are dissolved in 20 ml ethanol and precipitated in 500 ml THF.The collected precipitate is dried under vacuum. 

Bulk Polymerization. Monomer and polymer (with traces of initiator) are the only constituents in bulk polymerizations. Obviously, the monomer must be soluble in the polymer for this type of process to effectively proceed. Bulk polymerization, also called mass or block polymerization, can occur in stirred-tank reactors, or can be unstirred, in which instance it is called quiescent bulk polymerization. The primary difficulty with bulk polymerizations is that as the polymerization proceeds and more polymer is formed, the viscosity increases, thermal conductivity decreases, and heat removal becomes difficult. 

A bnlk polymerization reactor can be as simple as a tube into which the reactants are fed and from which the polymer mixture emerges at the end it can be more of a traditional, continnons stirred-tank reactor (CSTR), or even a high-pressure autoclave-type reactor (see Figure 3.21). A bulk polymerization process need not be continuous, but it should not be confnsed with a batch reaction. There can be batch bnlk polymerizations jnst as there are continnons bulk polymerizations processes. 

A continuous bulk polymerization process with three reaction zones in series has been developed. The degree of polymerization increases from the first reactor to the third reactor. Examples of suitable reactors include continuous stirred tank reactors, stirred tower reactors, axially segregated horizontal reactors, and pipe reactors with static mixers. The continuous stirred tank reactor type is advantageous, because it allows for precise independent control of the residence time in a given reactor by adjusting the level in a given reactor. Thus, the residence time of the polymer mixtures can be independently adjusted and optimized in each of the reactors in series (8). 

Because of the highly exothermic nature of acrylonitrile polymerization, bulk processes arc not normally used commercially. Howevei. a commercially feasible process lor bulk polymerization in a continuous stirred lank reactor has been developed. The heat nl reaction is controlled hy operating at relatively low conversion levels and supplementing the normal jacket cooling with reflux condensation of umcaclcd monomer... 

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

Free radical polymerization of neat monomer in the absence of solvent and with only initiator present is called bulk or mass polymerization. Monomer in the liquid or vapor state is well mixed with initiator in a heated or cooled reactor as appropriate. The advantages of this method are that it is simple, and because of the few interacting components present, there is less possibility for contamination. However, vinyl-type polymerizations are highly exothermic so that control of the temperature of bulk polymerization may be difficult. Also, in the absence of a solvent viscosities may become very high toward the end of a polymerization, which could make stirring difficult, and add to the difficulty of heat removal from the system. The advantages of this system, however, are sufficiently attractive for this to be used commercially for the free radical polymerization of styrene, methyl methacrylate, vinyl chloride, and also for some of the polymerization processes of ethylene [7]. 

Free radical polymerization of a monomer in solution is often more versatile and more amenable to temperature control than bulk polymerization. The presence of the solvent avoids any potential viscosity or stirring problems. Also the exotherm of the polymerization is moderated by the lower monomer concentrations under these conditions, which slows down rates, and by the thermal mass contributed by the solvent. Sometimes the polymerization temperature will be conveniently controlled by the reflux temperature of the solvent used. 

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