Photochemical radical polymerization

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

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. 

The kinetics and mechanism of the thermal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel. The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon2 and Sheppard.50 The general chemistry of azo-compounds has also been reviewed by Koga et cr/./11 Koenig,3 and Smith.3J... 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Vinyl chloride Free radical polymerization in bulk or emulsion rapid in presence of peroxides susceptible to photochemical polymerization —CH2—CH— 75 Largely amorphous, except when highly oriented by stretching. Hard. Soluble in ketones and esters... 

Since crosslinking of the polymer occurs as the result of the initial formation of silyl radicals, the siloxane polymer containing both phenyldisilanyl units and functional groups which undergo radical polymerization should produce solid material whatever the thickness of the films. To ascertain this, we have examined the photochemical behavior of the polymers 2-4. 

Moore and Hemmens [119] studied the photosensitization of primaquine and other antimalarial agents. The drugs were tested for in vitro photosensitizing capability by irradiation with 365 nm ultraviolet light in aqueous solutions. The ability of these compounds to photosensitize the oxidation of 2,5-dimethylfuran, histidine, trypotophan, or xanthine, and to initiate the free radical polymerization of acrylamide was examined in the pH range 2 12. Primaquine does not have significant photochemical activity in aqueous solution. 

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. 

Methacrylate monoliths have been fabricated by free radical polymerization of a number of different methacrylate monomers and cross-linkers [107,141-163], whose combination allowed the creation of monolithic columns with different chemical properties (RP [149-154], HIC [158], and HILIC [163]) and functionalities (lEX [141-153,161,162], IMAC [143], and bioreactors [159,160]). Unlike the fabrication of styrene monoliths, the copolymerization of methacrylate building blocks can be accomplished by thermal [141-148], photochemical [149-151,155,156], as well as chemical [154] initiation. In addition to HPLC, monolithic methacrylate supports have been subjected to numerous CEC applications [146-148,151]. Acrylate monoliths have been prepared by free radical polymerization of various acrylate monomers and cross-linkers [164-172]. Comparable to monolithic methacrylate supports, chemical [170], photochemical [164,169], as well as thermal [165-168,171,172] initiation techniques have been employed for fabrication. The application of acrylate polymer columns, however, is more focused on CEC than HPLC. 

Photocatalysis of organometallics, 19 114-117 free-radical polymerization, 19 117 Photochemical formation of M-M bonds, 19 142, 143... 

The polymerizations of methyl methacrylate and methyl acrylate in the presence of triethyl aluminum are photochemical. The initiator forms a complex with the monomer which absorbs light at 400 mfi. The polymerzaition shows different behaviour from normal stationary radical polymerization. Firstly, the decay of polymerization after extinction of illumination is much slower than the decay theoretically expected for normal radical polymerization. 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical and radiation-initiated polymerizations are also well known. Methods of radical polymerization include bulk, solution, emulsion, suspension, graft copolymerization, radiation-induced, and ionic with emulsion being the most important. 

When radical generation is stopped abruptly, polymerization dies out at a rate proportional to the decreasing concentration of radicals. This is called the post or after effect. In a defined manner it can be brought about only in photochemically initiated polymerizations simply by switching off the light. In the course of polymerization decay, radicals are consumed by mutual termination... 

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.)... 

After the template-monomer complexes have been formed, an azo initiator (usually azo-V,M-bis-isobutyro-nitrile, AIBN) is added to the polymerization mixture. Free-radical polymerization is initiated by heating at 40-60°C or by photochemical homolysis by ultraviolet (UV) radiation (0-15°C). MIPs prepared at lower temperatures (0°C) by photopolymerization have been found to exhibit better molecular recognition. It is theorized that the template-monomer complexes are more stable at lower temperatures thus, the imprints are more homogeneous and better defined in the resulting MIPs. 

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