Photochemical polymers

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

Two crosslinks per macromolecule are needed to make a polymer insoluble. Quantum yield of a diazide photolysis is ca. 0.5, therefore, the critical dose of the photochemical polymer insolubilization should be about 50mJ/cm. ... 

Jung, O.-S., and Pierpont, C. G., Photochemical polymers. Synthesis and characterization of a polymeric pyrazine-bridged cobalt semiquinonate-catecholate complex, J. Am. Chem. Soc., 116, 2229-2230 (1994). 

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Subliming ablators are being used in a variety of manufacturing appHcations. The exposure of some organic polymers to pulsed uv-laser radiation results in spontaneous ablation by the sublimation of a controUed thickness of the material. This photoetching technique is utilized in the patterning of polymer films (40,41) (see PHOTOCHEMICAL TECHNOLOGY). 

Photochemical technology Photoconductive polymers Photography Printing processes Radioactive tracers Radiopaques... 

Free-radical polymerization processes are used to produce virtually all commercial methacrylic polymers. Usually free-radical initiators (qv) such as azo compounds or peroxides are used to initiate the polymerizations. Photochemical and radiation-initiated polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of methacrylic 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 methacrylic monomers initiated with 2,2 -azobisisobutyronitrile [78-67-1] (AIBN) have been deterrnined and are shown in Table 8. 

A review covers the preparation and properties of both MABS and MBS polymers (75). Literature is available on the grafting of methacrylates onto a wide variety of other substrates (76,77). Typical examples include the grafting of methyl methacrylate onto mbbers by a variety of methods chemical (78,79), photochemical (80), radiation (80,81), and mastication (82). Methyl methacrylate has been grafted onto such substrates as cellulose (83), poly(vinyl alcohol) (84), polyester fibers (85), polyethylene (86), poly(styrene) (87), poly(vinyl chloride) (88), and other alkyl methacrylates (89). 

Multiphoton processes are also undoubtedly involved in the photodegradation of polymers in intense laser fields, eg, using excimer lasers (13). Moreover, multiphoton excitation during pumping can become a significant loss factor in operation of dye lasers (26,27). The photochemically reactive species may or may not be capable of absorption of the individual photons which cooperate to produce multiphoton excitation, but must be capable of utilising a quantum of energy equal to that of the combined photons. Multiphoton excitation thus may be viewed as an exception to the Bunsen-Roscoe law. 

DADC HomopolymeriZation. Bulk polymerization of CR-39 monomer gives clear, colorless, abrasion-resistant polymer castings that offer advantages over glass and acryHc plastics in optical appHcations. Free-radical initiators are required for thermal or photochemical polymerization. 

Sulfurized olefins (S2CI2 plus isobutene) are further reacted with S and Na2S to give products useful as extreme pressure lubricant additives (144,145). The reaction of unsaturated natural oils with sulfur monochloride gives resinous products known as Factice, which are useful as art-gum erasers and mbber additives (146,147). The addition reaction of sulfur monochloride with unsaturated polymers, eg, natural mbber, produces cross-links and thus serves as a means for vulcanizing mbber at moderate temperatures. The photochemical cross-linking of polyethylene has also been reported (148). 

Specialized training is an absolute requirement for technical service personnel. A typical example is a person involved in supporting a polymer for which the use is the manufacture of rotationaHy molded consumer products. The technical service person is expected to be reasonably familiar with topics such as polymer rheology evaluations, gel-permeation chromatography, rotational mol ding, color science, regulatory requirements for use, mechanical and photochemical behavior of the pigmented polymer, optics, and so forth. Expertise of this variety caimot be expected to be obtained without careful... 

Functional dyes (1) of many types are important photochemical sensitizers for oxidation, polymerization, (polymer) degradation, isomerization, and photodynamic therapy. Often, dye stmctures from several classes of materials can fulfiH a similar technological need, and reviewing several dye stmctures... 

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