Photochemical polymerization

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. 

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. 

Moreover, block copolymers with two radically polymerizable monomers can be synthesized with a combination of thermal and photochemical polymerizations. Regarding their utilization in block copolymer synthesis, azocompounds with photoactive benzoin [103,109-111] and azyloximester groups [112] have been described. Two low-molecular weight azo benzoin initiators of the general formula (Scheme 32) were synthe-... 

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... 

Independent estimates of these quantities can be obtained from the temperature coefficients of photopolymerization. If the rate of photoinitiation is assumed to be independent of the temperature, the increase in rate must be due entirely to the change in kp/k] (see Eq. 13), hence the slope of the plot of log Rp against 1/T for the photochemical polymerization should yield Ep — Et/2. Burnett reported the value 5.5 kcal./mole for styrene, and Burnett and Melville found 4.4 kcal./mole for vinyl acetate, in satisfactory agreement with the values given above. 

CF20—)m (—CF2CF20—) . This fluoropolymer has better low-temperature properties than Krytox, but is more expensive. Fomblin Z is made by photochemical polymerization of a mixture of oxygen and tetrafluoroethylene to prepare the random copolymer. The methylene oxide unit (—CF20—) imparts even more extraordinary low-temperature properties than those derived from vibration and free rotation of other perfluoroether linkages. 

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

Ej has a value of about —60 kJ mol-1 for thermal initiator decomposition, and Xn decreases rapidly with increasing temperature. Ej is about the same for a purely thermal, self-initiated polymerization (Fig. 3-16). For a pure photochemical polymerization Ej is positive by approximately 20 kJ mol-1, since Ed is zero and X increases moderately with temperature. For a redox polymerization, Ej is close to zero, since Ed is 40-60 kJ mol-1, and there is almost no effect of temperature on polymer molecular weight. For all other cases, Xn decreases with temperature. 

C3S2, is a red liquid (mp — 0.5° C, bp 60—70°C at 1.6 kPa (12 mm Hg)) produced by the action of an electric arc on carbon disulfide (1—4). The structure has been shown to be S=C=C=C=S on the basis of its reactions to form malonic acid derivatives and on the basis of physical measurements. It is unstable and decomposes in a few weeks at room temperature it decomposes explosively when heated rapidly at 100—120°C with formation of a black polymeric substance (C3S2)x (5,6). Dilute solutions in CS2 are fairly stable, but photochemical polymerization to (C3S2)x occurs. 

Crystalline polydiacetylenes, exhibiting a fully conjugated backbone, can be obtained [113] by thermal or photochemical polymerization of the monomer R C=C C=C R, where R is the organic substituent. The polymeric structures of the type... 

FTIR spectra of the polymeric carbon formed in the channels of the MCM-41 by both the photochemical polymerization of C4I2 and the polymerization of Cmesoporous structure of the MCM-41 protects the reactive polyyne chains. Moreover, the confining of C6HI in the channels of the MCM-41 eliminates the explosive behaviour of the polymeric product. 

Poly(acrylic acid) and Poly(rnethacrylic acid). Poly(acrylic acid) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (widi cross-linker for superadsorber applications) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78°C provides a syndiotactic form that can be hydrolyzed to syndiotactic PAA. 

Photochemical polymerization of dithiophene 325 in acetonitrile in the presence of CC14 and TBABr leads to a thin film with electrochromic properties in the visible region (94JCS(CC)1911 95SM(69)309). Laser flash photolysis in the presence of CC14 showed absorption at 417 nm due to the radical cation. 

Scanning tunneling microscopy of solid films of Cm and C > clearly demonstrate the occurrence of photochemical polymerization of these fullerenes in the solid state. X-ray diffraction studies show that such a polymerization is accompanied by contraction of the unit-cell volume in the case of Cm and expansion in the case of C70. This is also evidenced from the STM images. These observations help to understand the differences in the amotphization behavior of Cm and C70 under pressure. Amorphization of Cm under pressure is irreversible because it is accompanied by polymerization associated with a contraction of the unit cel volume. Monte Carlo simulations show how pressure-induced polymerization is favored in Cm because of proper orientation as well as the required proximity of the molecules. Amorphization of C70, on the other hand, is reversible because Cn is less compressible and polymerization is not favored under pressure. 

Notes A Absorption O Autoxidative/polymerization P Precipitation/gelation R Photochemical/polymerization, ultraviolet cure S Sheet-fed W Web-fed,... 

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