Cooling monomer

Plasma polymerization is usually carried out in a low pressure glow discharge sustained by either a dc or an ac electric field. Examples of the reactors used for this purpose are shown in Fig. 1. The simplest configuration involves a pair of circular parallel plate electrodes mounted inside a glass bell jar. The lower electrode usually serves as the substrate holder and is sometimes heated or cooled. Monomer is introduced through a feed tube and unconsumed monomer and gaseous products are withdrawn through a port in the base plate. 

Several side feeds of deep cooled monomer will lower the base temperature to which the adiabatic temperature rise has to be added. A fraction of the monomer is fed at the beginning of the extruder and has to be heated to the starting temperature of the reaction hence successive side feeds can be metered at much lower temperatures. 

Figure 11. LIF excitation and emission spectra of jet-cooled monomer and clusters of IX [90a-c] top, monomer middle, dimer bottom, van der Waals complexes between IX and acetonitrile. The electronic origins of the excitation spectra of the latter complexes are labelled A (1 2 complex) and B (1 1). Low-intensity features of the 1 3 complex are observed in the red part of the spectrum with the high pressure of acetonitrile. The fluorescence spectra were recorded with a spectral resolution of the detection monochromator of 10-15 nm (the position of excitation is indicated by an asterisk).

Figure 12. Dispersed emission spectra of jet-cooled monomer, dimer and larger homoaggregates upon 0,0 excitation top, BIN bottom, ABN. Reproduced with permission from Ref. [92a].

Ice-cooled monomer was evacuated at the water pump five times at approximately 20 nim Hg with purified nitrogen flushing between evacuation. 

Polymer formation can be followed directly via isolation of the polymer produced. The method has the advantage in that the chemical structure can also be determined. To do this, polyreactions are often stopped by addition of inhibitors or by strongly cooling. Monomer and/or solvent can be removed by distillation, but not all monomer can be distilled off because of the high viscosity. In addition, the initiator or catalyst is not removed by this method. In any case, the distillation must be carried out at very low temperatures, otherwise the polymer may be decomposed or polyreaction may recontinue. 

The ratio of reactants had to be controlled very closely to suppress these impurities. Recovery of the acrylamide product from the acid process was the most expensive and difficult part of the process. Large scale production depended on two different methods. If soHd crystalline monomer was desired, the acrylamide sulfate was neutralized with ammonia to yield ammonium sulfate. The acrylamide crystallized on cooling, leaving ammonium sulfate, which had to be disposed of in some way. The second method of purification involved ion exclusion (68), which utilized a sulfonic acid ion-exchange resin and produced a dilute solution of acrylamide in water. A dilute sulfuric acid waste stream was again produced, and, in either case, the waste stream represented a... 

Several recent patents describe improvements in the basic belt process. In one case a higher soHds polymerization is achieved by cooling the starting monomer until some monomer crystallizes and then introducing the resulting monomer slurry onto the belt as above. The latent heat of fusion of the monomer crystals absorbs some of the heat of polymerization, which otherwise limits the soHds content of the polymerization (87). In another patent a concave belt is described which becomes flat near the end. This change leads to improved release of polymer (88). 

Since acrylic polymerizations liberate considerable heat, violent or mnaway reactions are avoided by gradual addition of the reactants to the kettie. Usually the monomers are added by a gravity feed from weighing or measuring tanks situated close to the kettie. The rate of monomer addition is adjusted to permit removal of heat with full flow of water in the condenser and a partial flow in the cooling jacket. Flow in the jacket can be increased to control the polymerization in cases of erroneous feed rates or other unexpected circumstances. A supply of inhibitor is kept on hand to stop the polymerization if the cooling becomes inadequate. 

The solvent and initiator are charged to the reactor and heated to reflux (ca 80°C). Forty percent of the monomer charge is then added. The remainder of the monomer is added in four equal increments at 24, 50, 79, and 110 min after addition of the initial monomer charge. The reaction mixture is kept at reflux overnight, then cooled and packaged (96). 

Monomer emulsions ate prepared in separate stainless steel emulsification tanks that are usually equipped with a turbine agitator, manometer level gage, cooling cods, a sprayer inert gas, temperature recorder, mpture disk, flame arrester, and various nossles for charging the ingredients. Monomer emulsions are commonly fed continuously to the reactor throughout the polymerisation. 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

The reactor charge is heated to 140°C under a nitrogen atmosphere and the monomer charge and initiator charge are added uniformly over three hours while maintaining 140 2°C. After the additions are complete, this temperature is maintained for two more hours, then the product is cooled and packaged. A clear, viscous solution of about 58% polymer is obtained (63). 

A common procedure for the preparation of vinylated alkyds is as foUows. A base alkyd resin is brought to the desired endpoint. The resin is then cooled to about 160°C and often diluted with aromatic thinner. The desired monomer is added, usually at about 20 —60% based on the final product, foUowed by an appropriate amount of a free radical initiator. Alternatively, a premix of the monomer and the initiator is added at a controUed rate over most of the reaction. The reaction is brought to monomer reflux, until the residual monomer content has fallen below a specified level. Residual monomer, if any, is stripped away before the product is diluted in a solvent, filtered, and packaged. 

Reaction of aHyl chloroformate and diethylene glycol in the presence of alkaU with cooling is another method of preparing the diallyl carbonate ester DADC. The properties of diallyl carbonate monomers are given in Table 1. 

Processes that are essentially modifications of laboratory methods and that allow operation on a larger scale are used for commercial preparation of vinyhdene chloride polymers. The intended use dictates the polymer characteristics and, to some extent, the method of manufacture. Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stabiUty of the polymer. 

---