Chain-scission yield, methacrylate

Unlike the polyalkyl acrylates, which are thermally degraded by random chain scission, polyalkyl methacrylates unzip when heated, and excellent yields of the monomers are produced when the polymers of the lower homologues are heated. When higher homologues are heated, there is also some thermal degradation of the alkyl substituents. 

Chain-Scission Yields of Methacrylate Copolymers Under Electron Beam Radiation... 

The degradation reactions of polymers have been widely reviewed 525). In the absence of air, thermal reactions are the important degradation route. They may involve reactions of functional groups on the chain without chain scission, typified for example by the dehydrochlorination of PVC, or reactions involving chain scission, often followed by depropagation and chain-transfer reactions to yield complex mixtures of products. This latter route would be typical of the degradation of poly(methyl methacrylate), which depolymerizes smoothly to its monomer, and of polystyrene, which produces a wide range of tarry products. 

Silylated methacrylates are also useful in positive, chain scission, bilevel deep-UV resists (30, 31). However, the silyl substituent must be carefully selected to avoid an excessive decrease in Tg. For example, pentamethyl-disiloxypropyl methacrylate has limited utility in resist applications, because it decreases the Tg (30). Copolymerization with other monomers may yield glassy, oxygen-resistant materials, but syntheses requiring controlled polymerization of three or more constituents may be unduly complicated. 

Further confirmation of the important effect of solid-phase transitions in polymer photochemistry was reported by Dan and Guillet (29). They studied the quantum yields of chain scission, c >s, as a function of temperature in thin solid films of vinyl ketone homo- and copolymers. For polymers where the Norrish type-II mechanism was possibici large increases in n were observed at and above the glass transition T. Figure 8 shows this effect in a styrene copolymer containing about 5% phenyl vinyl ketone (PVK). Below Tg, )s is about 0.07, but at Tg it rises to about 0.3, a value similar to that observed for photolysis in solution at 2S°C. A similar effect was observed with poly (methyl methacrylate-co-methylvinyl ketone) (PMMA-MVK) and PVK homopolymer. 

There has been a slight increase in activity in this area compared with that in the previous two year period. For the polymeric esters of acrylic, methacrylic acids, and related polymers the simplest reaction, apart from thermal depolymerization, is hydrolysis, and one or two papers on this subject have appeared. One of these concerns a comparison of the kinetics of hydrolysis of a number of methacrylate esters and a further two deal with the formation of copolymers containing carboxylic acid functions. Methyl trifluoroacrylate forms alternating copolymers with cE-olefins (ethylene, propylene, isobutylene) and these are readily hydrolysed in boiling aqueous methanolic sodium hydroxide to yield hydrophilic fluoropolymers. Hydrolysis is reported to be nearly quantitative with no chain scission. An alternating copolymer is also formed by radical polymerization of maleic anhydride with A-vinyl succinimide. On hydrolysis this copolymer is... 

The triplet lifetime and the quantum yield of the chain scission ( J of copolymers of phenyl isopropenyl ketone and methyl methacrylate are independent of copolymer composition, indicating that the photolysis of the phenyl isopropenyl ketone moieties is not influenced by neighbouring methyl methacrylate units. 

Poly(methyl methacrylate) has no tendency to form crosslinks when irradiated as a film, or in solution. The quantum yield for random chain scission of poly(methyl methacrylate) in benzene solution is independent of... 

To study the structural sensitivity of poly silanes to ionizing radiation, a number of samples were irradiated with a calibrated Co source, and the degraded materials were analyzed by GPC in a manner similar to that described for the determination of photochemical quantum yields (59). In radiation processes, the slopes of the plots of molecular weight versus absorbed dose yield the G values for scissioning, G(s), and cross-linking, G(x), rather than the respective quantum yields. These values, which represent the number of chain breaks or cross-links per 100 eV of absorbed dose, are indicative of the relative radiation sensitivity of the material. The data for a number of polysilanes are given in Table IV. Also included in Table IV for comparison is the value for a commercial sample of poly(methyl methacrylate) run under the same conditions. The G(s) value of this sample compares favorably with that reported in the literature (83). 

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