Bulk polymerization Exothermic

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. 

The term suspension polymerization refers to the polymerization of macroscopic droplets in an aqueous medium. The kinetics is essentially that of a bulk polymerization with the expected adjustments associated with carrying out a number of bulk polymerizations in small particles more or less simultaneously and in reasonably good contact with a heat exchanger (i.e., the reaction medium) to control the exothermic nature of the process. Usually, suspension polymerizations are characterized by the use of monomer-soluble initiators and the use of suspending agents. 

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... 

This is one of the simplest methods of polymerization. It is often used in the polymerization of step-growth polymers.28 In these types of systems the viscosity remains low for a large portion of the reaction and heat transfer is easily controlled. Chain-growth polymers are more difficult to polymerize by this method due to the rapid and highly exothermic reactions. As the viscosity increases, thermal control becomes more difficult and may result in thermal runaway or localized hot spots. Commercial use of bulk polymerization for vinyl polymers is rather limited for... 

In the case of bulk polymerization, the initiator is dissolved in the monomer and the viscosity of the system increases with progressing polymerization from liquid, through the state of gel ("gel-effect") to solid polymer. This polymerization technique has many disadvantages, among others the transfer of exothermic reaction heat from the system is very complicated. The reaction heat reaches values as high as 85 kj/ mol, and because polymers are poor heat conductors, this may cause the temperature of the system to reach the boiling point of monomer and consequently the polymer is foamed by vapours of monomer. 

Other variables can also be used to influence the thermodynamics of a polymerization. For example, most step polymerizations involve equilibrium reactions, which may be driven to completion by removing the small molecule by-product in an open system. Addition polymerizations are influenced by the solvent used. That is, [ML depends on both the nature of the solvent and on [M]o- For example, the equilibrium monomer concentration of THF increases as the acidity of the solvent increases due to complex formation. In other cases, solvation of a polymer segment may be more exothermic than monomer solvation, resulting in a more exothermic AHP compared to the bulk polymerization. 

Free radical polymerization may be carried out in various media. Bulk polymerization is the simplest, but while the reactants (monomers) are most often liquid, the product (polymer) is solid. This leads to problems when removing the polymer from the reactor. In addition, since most free radical polymerizations are highly exothermic, the high viscosity of the monomer/polymer mix inhibits the removal of the heat of reaction. Solution polymerization will reduce, to some extent, the viscosity of the polymerizing mass, but it brings with it the environmental and health issues of organic solvents. In addition, the solvent reduces the monomer concentration, and hence the rate of polymerization. Finally, recovery and recycling of the solvent can add substantially to the cost of the process. Nevertheless, solution polymerization of vinylic monomers is used in a number of commercial processes. 

Crystal polystyrene is produced by thermally initiated (Section 6.5.4) bulk polymerization of styrene at temperature of I20°C or more. (The term crystal refers to the optical clarity of products made from this polymer, which is not crystalline.) The rate of polymerization would decrease with increasing conversion and decreasing monomer concentration if the reaction were carried out at constant temperature. For this reason, the polymerization is performed at progressively increasing temperatures as the reaction mixture moves through a series of reactors. The exothermic heat of polymerization is useful here in raising the reaction temperature to about 250°C as the process nears completion. 

Bulk polymerization. Bulk polymerization is the simplest and most direct method (from the standpoint of formulation and equipment) for converting monomer to polymer. It requires only monomer (and possibly monomer-soluble initiator or catalyst), and perhaps a chain transfer agent for molecular weight control, and as such gives the highest-purity polymer. However, extra care must be taken to control the process when the polymerization reaction is very exothermic and particularly when it is run on a large scale. Poly(methyl methacrylate), polystyrene, or low-density (high pressure) polyethylene, for example, can be produced from... 

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]. 

Incidentally, bulk polymerization is also the chief method used for commercial polycondensations. Polycondensations are not as exothermic as free radical catalyzed vinyl-type polymerizations, so thermal control is less of a problem. Bulk polycondensation also favors formation of linear polymer rather than the cyclic products that are favored by dilute solution polymerization, particularly if AB-type monomers are being used. Finally, since a high degree of polymerization (i.e., high DP, or high molecular weight) is only... 

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. 

For the large majority of heterocyclic monomers polymerization is exothermic due to the release of the ring strain. The entropy of the system usually also decreases (AS < 0) due to the loss of translational entropy. Thus, for a system with AH < 0 and AS < 0 polymerization is possible only below a certain temperature T = AH / AS , called the ceiling temperature. This temperature is highest for bulk polymerization and decreases with the fmitial monomer concentration ... 

Polymerization in bulk, that is, of undiluted monomer, minimizes any contamination of the product. Bulk polymerization is difficult to control, however, due to the high exothermicity and high activation energies of free-radical polymerization and the tendency toward the gel effect in some cases. 

Polymers can be classified according to the techniques used during the polymerization of the monomer. In bulk polymerization, only the monomer (and possibly eatalyst and initiator, but no solvent) is fed into the reactor. The monomer undergoes polymerization, at the end of whieh a (nearly) solid mass is removed as the polymer product. As we shall see later, bulk polymerization is employed widely in the manufacture of condensation polymers, where reactions are only mildly exothermic and viscosity is mostly low thus enhancing ready mixing, heat transfer, and bubble elimination. Solution polymerization involves polymerization of a monomer in a solvent in which both the monomer (reactant) and polymer (product) are soluble. Suspension polymerization refers to polymerization in an aqueous medium with the monomer as the dispersed phase. Consequently, the polymer resulting from such a system forms a solid dispersed phase. Emulsion polymerization is similar to suspension polymerization but the initiator is located in... 

As a result of its highly exothermic nature, bulk polymerization of vinyl acetate poses problems at high conversions. The properties of the resulting polymer are susceptible to deterioration due to chain branching. Therefore, bulk polymerization of vinyl acetate is usually stopped at 20 to 50% conversion. Thereafter, the unreacted monomer is either distilled off or the polymer precipitated with a suitable solvent (methanol, ethanol). Poly (vinyl acetate) is manufactured primarily by free-radical-initiated emulsion and, sometimes, solution polymerization. 

Poly (methyl methacrylate) for molding or extrusion is produced commercially by fiee-radical-initiated suspension or bulk polymerization of methyl methacrylate. To minimize polymerization reaction exotherm and shrinkage, bulk polymerization, which is used in the production of sheets, rods and tubes, is carried out with a reactive syrup of partially polymerized methyl methacrylate, which has a viscosity... 

Bulk polymerization is used in the production of polystyrene, poly(methyl-methacrylate), and poly (vinyl chloride). The reaction mixture contains only monomer and initiator, but because the reaction is exothermic, hot spots tend to develop when heat removal is inefficient. Auto-acceleration occurs in the highly viscous medium, making control difficult and impeding efficient monomer conversion. To overcome some of the disadvantages, low conversions are used, after which the... 

The bulk polymerization cannot be scaled-up. The process consists of an exothermal radical reaction that can be controlled only when the amount of polymerization mixture does not exceed a few hundredths of milliliters. After that, heat dissipation becomes difficult and dangerous overheating of the sample can occur, with significant risks for operators. 

Such polymerizations often exhibit a second, discontinuous phase. They frequently exhibit high exothermicity, but this is more characteristic of the reaction mechanism than of bulk polymerization as such. Bulk polymerizations of the free-radical variety are most common, although commercially important condensation processes also have a highly viscous continuous phase. 

PVC can be made by either bulk polymerization, suspension polymerization, or emulsion polymerization of the VCM. Most of the resin (over 90%) is now made by free-radical-initiated suspension polymerization of VCM. The reactor used for emulsion polymerization of the monomer is also about the same as that for suspension polymerization except that stripping is usually carried out under vacuum. Polymerization of VCM is an exothermic reaction (+410 cal/g) and removal of heat from the system is an important consideration in large-scale manufacture. Controlling the temperature of the reaction is important as it dictates the average molecular weight and the polydispersity of the resin product. This is relatively easier to achieve in suspension polymerization compared to bulk polymerization processes because the former is carried out in a water medium. 

Methacrylate polymerization is a highly exothermic reaction, releasing, in the particular case of methacrylate monoliths, around 190 J/g of heat. Since preparation of the monoliths proceeds through bulk polymerization, the heat generated cannot be dissipated fast enough ... 

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