Quantum yield for chain scission

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

Quantum yields for chain scission, 0°, were determined from the Initial slopes of plots of the number of scissions, s, per average chain against the quanta absorbed by the average chain or from plots of s against the quanta, q, absorbed per g of polymer and calculated from the relation = (A/M ) (s/q). The course of the scission process was followed vlscometrlcally and s was estimated from ([tj]°/[t)]) -1 an approximation of a was taken as 0.65 for all solutions In DMM. 

Figure 8. Quantum yields for chain scission in PS-PVK (VII) in solid films above and below the glass transition (29). Reprinted with permission from E. Dan and J. E. Guillet, Photochemistry of ketone pol)nners. X. Chain scission reactions in the solid state. Macromolecules, 6, 230 (1973). C<9yright 1973, American Chemical Society. 

Fig. 10. Action spectra for photodegradation of polycarbonate at ambient temperature. Change in yellowness index from [105] change in absorbance from [104] change in quantum yield for chain scission from [96]...

The Norrish type II reaction of vinyl ketone copolymers also leads to scission of the backbone of polymer chain. Typical temperature dependence of the quantum yield for chain scission due to the Norrish type II reaction is shown in Fig. 27 for the copolymer of styrene and phenyl vinyl ketone There is a relatively small ittcrease in the quantum yield below T owing to the presence of local mode rdaxation in the main chain, but a drastic increase tak place in the region of glass transition temperature. Above T the quantum yield for the type II process is identical to that observed in solution. 

Fig. 15.14. Quantum yield for chain scission (-o-), efficiency of photo-Fries rearrangement (- -), and absorption spectrum (—A-) of bisphenol-A polycarbonate [34],...

Table II summarizes the quantum yield of chain-scission in the solid state and in solution for both PMIPK and PIPTBK.

One important exception to this rule is vinylketone polymers in which the Norrish type II reaction is responsible for the decrease of the molecular weight of irradiated polymers (see section 4). In contrast to polymethylmethacrylate, polymethylacrylate becomes insoluble on irradiation with 253.7 nm in vacuo [82]. In air, no visible insoluble material is formed and an apparent quantum yield of chain scission of 1.3 x 10-2 has been determined by viscosity measurements [82]. However, a qualitative comparison of sedimentation patterns of initial and irradiated samples indicates that crosslinking also occurs in the presence of air even if gelation is retarded by oxygen. This makes the above-mentioned value meaningless. Photolysis of thin polymethylacrylate films at 253.7 nm in vacuo has also been studied by measuring the insoluble fraction as a function of dose as described in section 2. Quantum yields of 1.9 x 10-3 have been estimated for both the chain scission and the crosslinking processes [83]. 

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. 

The quantum yield of chain scission (< s) has been determined for many polymers (Table 1.3). Methods of its determination are given in section 10.8. 

The quantum yield for the chain scission of poly(methylphenylsilane) in solution is 0 = 0.97, whereas in a thin film, 0 = 0.17 [2135]. The lower quantum yield of chain scission in a film, in comparison to that in solution, can be explained by the cage effect (in the solid), which hinders free motion and favours recombination of reactive sites so formed. 

The / -scission of the tertiary radical IX so produced provides another, potentially efficient, method of causing main-chain scission in the polymeric solid phase. Similar high quantum yields for the Norrish type I process were... 

This paper will describe the synthesis of PIPTBK and report the chain scission quantum yield, ( ) for this material both in thin films and in solution. For comparison, the chain scission quantum yield for PMIPK was measured under similar conditions. 

The same mechanism, without depolymerization, however, has been proposed to explain the decrease in the molecular weight observed on irradiation of polymethylisopropenylketone with 253.7 nm radiation at room temperature in the presence of air [ 59]. In this case a quantum yield of 0.22 has been estimated. It must be remembered that the Norrish type I reaction is not considered to yield main chain scission in the case of polymethylvinylketone irradiated at 313 nm [11, 55], but the difference might be due to the higher energy of the incident radiation. Another possibility is that the main chain scission of polymethylisopropenylketone occurs through a seven-membered ring transition state, as postulated for... 

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