Oxygen polymerization inhibition

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Transfer tissne to polyethylene-embedding capsnles or gelatin capsules, fill the capsules with complete resin, and cap. Oxygen will inhibit polymerization. 

Transfer tissue to polyethylene-embedding capsules or gelatin capsules, fill the capsules with 100% LR Gold + initiator, and cap. Oxygen will inhibit polymerization. Polymerize the plastic with UV light (366 nm, two 15-W Sylvania F15T8/BLB bulbs) placed 10 cm from the capsules. Expose the capsules to the UV light for 24 h at -20°C. 

Another kind of inhibitor often present in photopolymer systems is dissolved oxygen, which inhibits by producing stable species, incapable of initiation, after reaction with radicals. If inhibitors are present, either deliberately or adventitiously they must be depleted before normal polymerization can proceed. This results in observation of an inhibition period before onset of steady state polymerization. 

One unusual and surprising characteristic of tertiary aromatic amines is that in addition to acting as accelerators, the same compounds in low concentration in the presence of oxygen and an initiator may act as inhibitors of polymerization (41, 42). This behavior has also been attributed to the ability of the amine to engage in charge-transfer complex formation. Oxygen also inhibits radical polymerization and results in uncured films at the surface of dental sealants (42). 

The polymerization mixture is thoroughly degassed with an inert gas [generally N2) in order to remove oxygen dissolved in this mixture as oxygen can inhibit the polymerization process. The degassing step is normally carried out in an ice bath in order to minimize the possible evaporation of any of the components present, especially those at the lowest concentrations. 

Let us consider methods of decreasing the edge stresses of a system based on unsaturated compounds. For this purpose the capacity of oxygen to inhibit the process of radical polymerization of unsaturated compounds is exploited. In addition to other factors, the location of the layer under study determines the oxygen concentration in the system. 

This is essentially a redox reaction (see Section 20.1.4.). That free radicals are produced is established by the fact that the dimers (e.g., octane) can be isolated. Because of reaction (20-6), for polymerization starts less than the stoichiometric amount of boralkane with respect to oxygen must be used. An excess of oxygen, however, inhibits polymerization (see Section 20.1.5). The boralkane/oxygen system induces polymerization at temperatures as low as — 100°C. The half-life of peroxyborane in heptane at — 80°C is about a week. 

Molecular oxygen can inhibit or accelerate polymerization, depending on the conditions of the experiment. In kinetic studies, therefore, oxygen must be excluded, i.e., its concentration must lie below 2 ppm (parts per million). 

Scheme of radical polymerization and oxygen polymerization anti-inhibition by thiol RSH. 

A photosensitizer has the function of improving the sensitivity of a sensitizing dye to active radiation rays, or preventing oxygen from inhibiting polymerization of the monomer (52). 

Molecular oxygen contains two unpaired electrons and has the distinction of being capable of both initiating and inhibiting polymerization. It functions in the latter capacity by forming the relatively unreactive peroxy radical ... 

Acrylate polymerizations are markedly inhibited by oxygen therefore, considerable care is taken to exclude air during the polymerization stages of manufacturing. This inhibitory effect has been shown to be caused by copolymerization of oxygen with monomer, forming an alternating copolymer (81,82). 

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