Polymers photocleavage

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KIP, by contrast, due to the formation of a polymer-anchored benzoyl radical (Scheme 22), caimot release benzaldehyde after photocleavage, thus allowing production of non-yellowing and low-odour coatings [89,91], especially required in the field of food packaging. 

The application of a triazene motif to the development of a photolabile linker was reported by Enders in 2004 [63]. The reaction of a secondary amine with polymer-bound diazonium salt 45 produced the triazene moiety 46. It was stable to a wide range of non-acidic conditions, suitable for other organic reactions. Photocleavage was carried out with 355 nm UV laser irradiation (3co Nd-YAG) in methanol-diethyl ether (Scheme 17.22). 

Initiator activities (a) together with average values for polymer activities (A) are presented in Table II. In marked contrast to the earlier reports (10, 11), it is seen that polymer activities are less than that of the corresponding initiator in each case. Unfortunately, we can offer no rationale for this discrepancy. However, the present data are readily interpreted in terms of benzoin ether photocleavage into benzoyl and benzyl ether radicals (eqn 1), which initiate MMA polymerization. The incorporation data may be analyzed by eqns 2-4, wherein aj, a2 and a3 refer to the specific activities of initiators BEx, BE2 and BE3, respectively, Ax, A2 and A3 represent the corresponding polymer activities, while Band E are the number... 

Sensitizers can be employed for agricultural purposes as herbicides and insecticides, or for medical purposes as antibacterial and antiviral agents. Moreover, sensitizer-based methods serve as tools for the analysis of the interaction faces of polymer complejKs and the sequence-selective photocleavage of double-stranded DNA. The ways in which photosensitized reactions are utilized are illustrated by the following typical examples. The first case relates to the photochemotherapy of cancer cells in superficial solid tumors [48]. The sophotodynamic therapy, PDT, is based on the selective incorporation of a photosensitizer into tumor cells, followed by exposure to light (commonly at A=600 nm). Cytotoxic products, namely singlet oxygen, O, and superoxide radical anions,... 

Photoinitiators are classified according to the type of photopolymerization system they initiate (i.e., radical or ionic). The basic photochemical routes that produce radicals are photocleavage (type I), intermolecular hydrogen abstraction (type II), and electron transfer followed by proton transfer. Schematics of these photochemical routes are given in Scheme 5. The efficiency of these routes directly determines the monomer conversion, molecular weight of the polymer, and degree of polymerization, and hence the structural, physical, and mechanical properties of the final product. The general relationships of Pp, Pi, and DP were provided in the previous section. 

Low molecular free radicals formed from the photocleavage of external impurities (RH) may abstract hydrogen from the polymer molecule (PH) giving polymer alkyl radicals (P ) ... 

Hydrogen abstraction by polymer alkoxy (PO ) and hydroxyl (HO ) radicals from the photocleavage of polymer hydroperoxy groups (POOH) in polypropylene occurs with 50% probabiUty at methyl groups. This means that up to 25% of the polymer peroxy radical (POO ) pairs produced in the initiation process may be pairs of primary peroxy radicals and the remaining 75% would be pairs between secondary, tertiary and mixed peroxy radicals. Due to the high radical concentration within such pairs of immediately adjacent radicals, the termination processes would be favoured over the slow hydrogen abstraction reaction of these radicals. 

Radicals of different types, formed from photocleavage of the photoinitiator, have different reactivities with polymer molecules. 

More effective and rapid degradation of a polymer due to more radicals produced from the photocleavage of a photoinitiator. 

Knowledge of the photochemistry of solvents is necessary in order to understand all the reactions which may be involved in the photodegradation of polymers in solutions. Free radicals formed from photocleavage of solvents may abstract hydrogen from polymer molecules (PH) and produce polymer alkyl radicals (P ). 

Studies on photoresists, based on methacryloyethyl-phenylglyoxylate, show that, in aprotic solvents, the main reaction mechartism is a Norrish type II photolysis leading to chain scission. In aprotic solvents, the polymer is photoreduced and crosslinks are formed. Polarity of the solvent determines quantum yields in polyirtrides." The most efficient photocleavage was in medium-polar solvents. The selection of solvent may change the chemical mechanism of degradation and the associated products of such reactions. 

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