Free radical polymerization inhibition

One serious fault of these materials is that the presence of an electron-rich phenolic hydroxyl group inhibits free-radical polymerization. Thus, composite resins placed over them do not polymerize completely. 

In addition, Stansbury Brauer (1985), taking advantage of the fact that vanillates do not inhibit free-radical polymerization, incorporated... 

Trifonov and Panayotov (41) attempted to carry out anionic polymerizations of vinyl monomers with semiquinones generated at a cathode. Since semiquinones inhibit free-radical polymerization, anionic polymerization alone should take place in the system. When electrolysis of quinones was conducted in a solution of LiCl or N(CaH6)4I in DMF with mercury cathode, the catholyte turned to red or purple red in accordance with the semiquinones. The presence of free-radical produced on the quinone molecule was proved from the ESR spectrum. When each of the monomers, styrene, acrylonitrile and methyl methacrylate were added to the colored solutions, polymers were obtained. 

Fig. 6-2. Conversion lime plots for normal, retarded and inhibited free radical polymerizations.

This inhibits free-radical polymerization both during the reaction and the final distillation. 

It is well known that molecular oxygen inhibits free-radical polymerization by scavenging the initiator radicals, which not only reduces the polymerization rate but also affects the mechanical, optical, and structural properties of the cured systems. The mechanism of O2 inhibition is represented in Figure 6(a). As a result of a photooxidation reaction, peroxy radicals (or hydroperoxides or alcoxy radicals) are generated, which are less reactive toward monomer to initiate... 

The ability of phenols to inhibit free-radical polymerizations appears to increase with the number of hydroxyl groups on the molecule. The location of these hydroxyl groups on the benzene ring in relationship to each other is important. For instance, catechol is a more efficient inhibitor than resorcinol. 

Oxygen inhibits free-radical polymerization of a-methyl methacrylate. The reaction with oxygen results in formation of low molecular weight polymeric peroxides that subsequently decompose to formaldehyde and methyl pyruvate ... 

Oxygen inhibits free radical polymerization through two pathways. First, an oxygen molecule can quench the active triplet state of the photoinitiator and be excited to singlet state itself The seeond path is scavenging of the photoinitiator and polymer radicals through oxidation with the yield of peroxy radicals. The overall chain reactions of photopolymerization with the presence of O2 is schematically shown as follows... 

Anaerobic adhesives derive their name from the characteristic of requiring a relatively oxygen-free environment for proper cure, such as found in closely mating assemblies. Oxygen inhibits free radical polymerization by the mechanism shown in Eq. (1). The active radical R reacts with molecular oxygen to form an inactive hydroperoxy radical before initiation or chain propogation can occur. Anaerobics can be used as one-part systems, relying on reactions with the active metal surface of the substrate to provide the redox initiation. 

PA exerts an inhibiting action on free radical polymerizations. Bartlett et al (Ref 28) ascribe this to the following reactions ... 

The hazards of a rigid classification of substances which may modify the course of a free radical polymerization are well illustrated by the examples of inhibitors and retarders which have been cited. The distinction between an inhibitor or retarder, on the one hand, and a co-monomer or a transfer agent, on the other, is not sharply defined. Moreover, if the substance is a free radical, it is potentially either an initiator or an inhibitor, and it may perform both functions as in the case of triphenylmethyl. If the substance with which the chain radicals react is a molecule rather than a radical, three possibilities may arise (i) The adduct radicals may be completely unreactive toward monomer. They must then disappear ultimately through mutual interaction, and we have a clear-cut case of either inhibition or retarda-... 

It is commonly known that vinyltin compounds undergo no free-radical polymerization and do not readily copolymerize with various vinyl monomers which can be attributed to their inhibiting effect towards radical reactions48,79. ... 

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. 

Photocurable coatings are widely used for metal, plastics wood and paper. Photoinitiated free-radical polymerization, however, can only be applied to vinyl monomers. The studies of Crivello have broadened the scope of monomers. In addition, photoinitiated cationic polymerization is not sensitive toward oxygen (air). Photoinitiated free-radical polymerization sometimes requires working in inert atmosphere in order to avoid the inhibition through oxygen1). 

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. 

Fig. 1 Conversion vs. time for normal polymerization as well as inhibited and retarded free-radical polymerizations. (From Ref.. )...

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