Photochemical reaction radical polymerization

Cyanoacrylate esters readily polymerize by several different mechanisms. Anionic polymerization is the most common and the most important method with respect to their use as adhesives. Typically this process is initiated by the nucleophilic contaminants found on the surfaces being bonded. The other routes by which cyanoacrylates polymerize or copolymerize involve radically initiated and photochemically initiated reactions. Radical polymerization or copolymerization of cyanoacrylates usually does... 

The initiator in radical polymerization is often regarded simply as a source of radicals. Little attention is paid to the various pathways available for radical generation or to the side reactions that may accompany initiation. The preceding discussion (see 3.2) demonstrated that in selecting initiators (whether thermal, photochemical, redox, etc.) for polymerization, they must be considered in terms of the types of radicals formed, their suitability for use with the particular monomers, solvent, and the other agents present in the polymerization medium, and for the properties they convey to the polymer produced. 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

As has already been indicated, a thorough discussion of the mechanism of metal, metal alkyl-, oxygen- or peroxide-induced polymerizations is beyond the scope of this article. Chiefly for purposes of comparison, it seems worthwhile to present a brief outline of the free radical type of mechanism which best explains these reactions and which probably is valid also for the photochemical and thermal polymerizations, particularly in the lower temperature range. 

Photochemical or photoinitiated polymerizations occur when radicals are produced by ultraviolet and visible light irradiation of a reaction system [Oster and Yang, 1968 Pappas, 1988]. In general, light absorption results in radical production by either of two pathways ... 

Other modes of copolymerization, like photochemically or chemically initiated free radical polymerization, ROMP, or polycondensation reactions, in the presence of inert diluents are, however, supposed to be comparable with respect to the formation of support porosity. 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

Most c/s-azoalkanes do this on heating, or on irradiation with shorter-wavelength radiation, and photolysis of azoalkanes provides a convenient source of certain radicals the photochemical process can be more readily controlled than the thermal reaction. Radicals produced in this way may be employed as initiators for the polymerization of alkenes. and a widely used compound for this purpose is azobisisobutyronitrile (ABIN, 5.20). 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

A more elegant procedure is to use a photochemical reaction to dissociate groups from the polymer chains and form radicals capable of polymerization with an added monomer. 

Cjc/o-sulfur rings are yellow or pale orange, low-melting solids (Table 6) of variable stability. For example, S9 decomposes above 0°C, while Se and Sy are only moderately stable at room temperature. Their thermal stability decreases in the order S12, Sig, S20 (years) > Se, S9, Sio, Su, Si3 (days) Sy (minutes). Their interconversion reactions (S Sm, m f n) have been studied in some detail. Thermal as well as photochemical reactions are possible, and a number of mechanisms have been proposed (Figure 7). (1) Radical, consisting of ring opening (S" - " S ), polymerization (S ham and depolymerization (S" " mixture... 

The most common type of chain-growth polymerization is free-radical polymerization. An initiator or a photochemical reaction produces a free radical that attaches itself to a monomer molecule, creating a group with odd-electron configuration (reactive center) at which monomer molecules are added until two such centers react with one another or, more rarely, a center is deactivated by some other process. This is a mechanism much like that of ordinary chain reactions (see Chapter 9 the term "chain" in chain growth refers to that kind of mechanisms, not to the growing molecular chain of repeating units in the polymer.)... 

The most widely studied addition-polymerization reaction for crosslinking is the free-radical polymerization of difunctional monomers (such as divinyl benzene and ethylene glycol dimethacrylate) in the presence of the corresponding monofunctional monomer. This may be thermally or photochemically initiated, and the latter application is widely used for coatings and dental composites. This is shown in Figure 1.26. 

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