Methyl methacrylate bulk polymerization

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Uses. The azobisnitriles have been used for bulk, solution, emulsion, and suspension polymerization of all of the common vinyl monomers, including ethylene, styrene vinyl chloride, vinyl acetate, acylonitrile, and methyl methacrylate. The polymerizations of unsaturated polyesters and copolymerizations of vinyl compounds also have been initiated by these compounds. 

Another example of the macroinitiator approach to making block copolymers is shown in Scheme 8.4. Since methyl methacrylate (MMA) polymerization cannot effectively be initiated by TEMPO-based alkoxyamine initiators, a poly(methyl methacrylate) macroinitiator (XIII) was prepared using conventional free radical polymerization [16]. However, the azo initiator was functionalized with a TEMPO-based alkoxyamine. Since the main mechanism of termination during bulk MMA polymerization is by radical coupling, most of the MMA polymer chain-ends are functionalized with alkoxyamine groups. 

Thus the values shown in Table 1.8 are for standard conditions and represent just one of a series of ceiling temperatures for various monomer concentrations above which polymer formation is not favoured. Thus, in a bulk polymerization reaction the ceiling temperature may change with conversion in such a way that complete conversion is not achieved. For example, if methyl methacrylate is polymerized at 110°C the value of [M]c calculated from the above equation is 0.139M and this will be the monomer concentration in equilibrium with the polymer. The polymer, when removed from the monomer, will have the expected ceiling temperature as given in Table 1.8 and will depolymerize only if there is a source of free radicals to initiate the depolymerization (Section 1.4.1)... 

Almost simultaneously, Aldridge et al. [110] used NIRS to monitor monomer conversion in situ and in real time in bulk methyl methacrylate (MMA) polymerizations performed inside a mold. The authors used a dispersive instrument to observe the spectral changes at 868 and 890 nm, related to the disappearance of the third overtone of the CH stretch of the vinyl group and the concomitant appearance of the CH of the methylene group. Spectral changes were monitored with the help of the second-derivative spectra of the samples and posterior subtraction of the initial monomer spectrum. Simple linear calibrations were performed and shown to be in good agreement with conversion data obtained with independent procedures. 

Dispersion polymerizations can be carried out either in bulk or in the presence of a solvent. Dispersion bulk polymerizations are industrially utilized to polymerize vinyl chloride, acrylonitrile, and vinylidene chloride. Polymerizations requiring the presence of a solvent are those whose monomer is solid in the polymerization conditions a solvent that favors phase separation can also be useful for example, styrene and methyl methacrylate are polymerized in dispersion in the presence of an alcohol inducing phase separation. When an adequate suspending agent (hydrox-ypropylcellulose, etc.) is added in the reaction medium, dispersion polymerization affords particles of imiform size (about 3-30 pm). 

Because the polymerization occurs totally within the monomer droplets without any substantial transfer of materials between individual droplets or between the droplets and the aqueous phase, the course of the polymerization is expected to be similar to bulk polymerization. Accounts of the quantitative aspects of the suspension polymerization of methyl methacrylate generally support this model (95,111,112). Developments in suspension polymerization, including acryUc suspension polymers, have been reviewed (113,114). 

Polymethacrylates. Poly(methyl methacrylate) [9011-14-7] is a thermoplastic. Itis the acryUc resin most used in building products, frequendy as a blend or copolymer with other materials to improve its properties. The monomer is polymerized either by bulk or suspension processes. Eor glazing material, its greatest use, only the bulk process is used. Sheets are prepared either by casting between glass plates or by extmsion of pellets through a sHt die. This second method is less expensive and more commonly used. Peroxide or azo initiators are used for the polymerization (see Methacrylic polymers). 

Acrylic is a generic name for derivatives of acrylic acid, of which methyl methacrylate is the most important. Polymerization is controlled to produce chain length of 800 to 3,000 monomer units. A small amount of plasticizer such as dibutyl phthalate may be added before bulk polymerization to assist in deep molding. The outstanding property of polymethyl metliacrylate is 0 transparency resistance to ultraviolet radiation from fluorescent lamps and ability to be... 

In the literature there is only one serious attempt to develop a detailed mechanistic model of free radical polymerization at high conversions (l. > ) This model after Cardenas and 0 Driscoll is discussed in some detail pointing out its important limitations. The present authors then describe the development of a semi-empirical model based on the free volume theory and show that this model adequately accounts for chain entanglements and glassy-state transition in bulk and solution polymerization of methyl methacrylate over wide ranges of temperature and solvent concentration. 

Some typical examples of this autoacceleration are (Figure 5) Norrish and Smith ( 2) polymerized methyl methacrylate in bulk and in the presence of various precipitants and measured the polymerization rates dilatometrically. They determined that autoacceleration of the precipitation polymerizations was larger than that observed for the Trommsdorf effect in bulk polymerization. 

Figure 5. Effect of autoacceleration on the precipitation polymerization of methyl methacrylate (2). The curves, from left to right, are for the diluents cyclohexane t-hutylsterate heptane and bulk.

Polymerization of methyl methacrylate to Plexiglas is done in the bulk process. High pressure polymerization of ethylene is done this way also. But other addition polymerizations frequently become too exothermic and without adequate heat removal system, the reaction tends to run away from optimum conditions. 

Bulk polymerization of a liquid monomer such as methyl methacrylate is relatively simple in the absence of oxygen where small bottles or test tubes can be used as the reaction vessel. 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

The heat of an emulsion polymerization is the same as that for the corresponding bulk or solution polymerization, since AH is essentially the enthalpy change of the propagation step. Thus, the heats of emulsion polymerization for acrylic acid, methyl acrylate, and methyl methacrylate are —67, —77, and —58 kJ mol-1, respectively [McCurdy and Laidler, 1964], in excellent agreement with the AH values for the corresponding homogeneous polymerizations (Table 3-14). 

Some effect of viscosity on r has been observed [Kelen and Tudos, 1974 Rao et al., 1976]. Copolymerization of styrene (Mil-methyl methacrylate (M2) in bulk leads to a copolymer containing less styrene than when reaction is carried out in benzene solution [Johnson et al., 1978]. The gel effect in bulk polymerization decreases the mobility of styrene resulting in a decrease in r and an increase in r%. 

Polymerization of Methyl Methacrylate with 2,2 -Azobisisobutyronitrile in Bulk... 

Bulk Polymerization. This involves only monomer, initiator, and perhaps chain-transfer agent. It gives the greatest polymer yield per unit of reactor volume and a very pure polymer. However, in large-scale batch form, it must be run slowly or in continuous form with a lot of heat-transfer area per unit of conversion to avoid mnaway. Objects are conveniendy cast to shape using batch bulk polymerization. Poly(methyl methacrylate) glazing sheets are produced by batch bulk polymerization between glass plates. They are also made by continuous bulk polymerization between polished stainless steel... 

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