Free-radical polymerization with oxygen

High pressure (60—350 MPa) free-radical polymerization using oxygen, peroxide, or other strong oxidizers as initiators at temperatures of up to 350°C to produce low density polyethylene (LDPE), a highly branched polymer, with densities from 0.91 to 0.94 g/cm. ... 

Free radical polymerization (with free radicals from the boron alkyl/ oxygen reaction, see Section 20.1.3.1) is also possible at low temperatures. Poly(thiocarbonyl fluoride) is a good elastomer due to its low glass-... 

Free-radical polymerization processes are used to produce virtually all commercial methaerylie polymers. Usually free-radical initiators tqv > such as a/o compounds or ieroxides are used to initiate the polymerisations. Photochemical and radiation-initiated polymerizations are also well known. At it constant temperature, the initial rate of the hulk or solution radical polymerization of methaerylie monomers is first-order with respect to monomer eoneentration. anil one-half order with respect to the initiator concentration. Methacrylate polymerizations are markedly inhibited by-oxygen therefore considerable care is taken to exclude air during the polymerization stages of manufacturing. 

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. 

Under suitable conditions, anionic polymerization is faster than free-radical polymerization and so can be conducted at lower temperatures. The main reasons are fast initiation by an ionic reaction and absence of an effective termination mechanism. However, the sensitivity to impurities is much greater and choice and control of reaction conditions are more delicate. Water, oxygen, carbon dioxide, and other substances able to react with carbanion chain carriers must be strictly excluded. 

Condensation Nuclei. Many mechanisms have been proposed (9) involving free radical polymerizations of various radicals which could lead to formation of condensation nuclei. It seems that if condensation nuclei are formed by such reactions, the myriad different radicals in a given system would lead to formation of a highly mixed polymer. Noting this, an oversimplified mechanism by which the system NO2 + a-pinene + hv may form condensation nuclei (Figure 1) is for example, reactions of the alkyl peroxy radical formed in Reaction 9 with a-pinene and molecular oxygen ... 

Some chemicals retard or suppress free-radical polymerization by reacting with primary radicals or macroradicals to yield radicals that are very stable to further reaction or yield nonradical products. These materials could be retarders or inhibitors. Retarders slow down the formation of polymer but inhibitors completely eliminate it. Oxygen is one of the most commonly known inhibitors for vinyl polymerization and good practice requires the removal of air from the reactor vessels before the reaction is started. It combines with active radicals to form unreactive peroxy radicals. 

The role of weak links has been considered in all thermal-initiation possibilities (Scott, 1995), and the tendency of many monomers to copolymerise with trace amounts of oxygen in free-radical polymerization (in the case of styrene, to form a 1 1 copolymer) until it has all been scavenged is well known. 

Dr. Bovey was born in Minneapolis in 1918. He received a B.S. degree in chemistry from Harvard in 1940, worked during World War II for the National Synthetic Rubber Corporation, a 3M subsidiary, and entered graduate school at the University of Minnesota as a Rubber Reserve Fellow in 1945. His thesis work, carried out under the direction of 1. M. Kolthoff, dealt with the mechanism of free radical polymerization. During this time he worked out the mechanism of oxygen inhibition and discovered oxygen styrene copolymers. 

In another commercial application of free-radical polymerization, polymerizations may be carried out in industrial coatings in the presence of air to yield a variety of coatings and structures of commercial import. This development is possible. In part, because certain vinyl monomers, particularly the acrylates, are less sensitive to retardation by oxygen compared with other monomers. It is therefore possible to produce radiation-cured coatings. UV-cured printing inks and the photopolymers are important in imaging for printing, photoresist, and related applications. 

The rapidity of the reaction can be seen by the large effect low pressures ( 1 torr) of oxygen can have on the free radical polymerization of a reactive olefin such as styrene [22]. The reaction rate coefficients are expected to be typical for exothermic radical—radical reactions with essentially no activation energy. Thus, if R is alkyl, log(feQ/l mole-1 s-1) would be 9.0 0.5, and be independent of temperature. For simple resonance-stabilized radicals, log(feD/l mole-1 s-1) would be 8.5 0.5. 

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