Quantum yield chain scission

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 limitation of this method is the relatively low quantum yield of radical formation by chain scission for most polymers. It will take high doses at short wavelengths (< 300 nm) to produce enough initiating radicals for a complete surface coverage of grafted... 

To perform the dissociation of the hydrocarbon to alkyl radicals with C—C bond scission, a hydrocarbon molecule should absorb light with the wavelength 270-370 nm. However, alkanes do not absorb light with such wavelength. Therefore, photosensitizers are used for free radical initiation in hydrocarbons. Mercury vapor has been used as a sensitizer for the generation of free radicals in the oxidized hydrocarbon [206-212], Nalbandyan [212-214] was the first to study the photooxidation of methane, ethane, and propane using Hg vapor as photosensitizer. Hydroperoxide was isolated as the product of propane oxidation at room temperature. The quantum yield of hydroperoxide was found to be >2, that is, oxidation occurs with short chains. The following scheme of propane photoxidation was proposed [117] ... 

The absolute sensitivity was defined as the number of main chain scissions occurring in one photon molecule when one photon was irradiated on a unit surface (1 cm2) of the film. If the spectral dependence of the quantum yield has been obtained, the absolute sensitivity is calculated following the formula (4). The results obtained are shown in Fig. 9. The practical sensitivity will be the integral of the product of the absolute intensity of the irradiated light and the absolute sensitivity in Fig. 9 over all the wavelength of the spectra. 

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... 

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

The radiation chemical yields are expressed in terms of G-values. G(scission), G(s), equals the number of main chain scissions produced per 100 eV of energy absorbed and G (cross-linking), G(x), the number of crosslinks formed per 100 eV absorbed. The G-value is a structure dependent constant similar to quantum efficiency in photochemistry. 

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). 

A number of workers have looked at the effect of photooxidation and photodynamic sensitizers on DNA. Rose Bengal photosensitizes strand breaks in double-stranded, supercoiled, pBR322 DNA the effect follows first-order kinetics with respect to light fluence and dye concentration. The reaction is substantially more efficient in the absence of oxygen, but the quantum yield of strand breaks in air is only 10 8. The results are consistent with the initiation of chain scission by Rose Bengal triplet, with some additional mechanism coming into play in the presence of oxygen. 

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). 

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. 

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