Photochemical Reactions of the Polymers in Solution

The photochemistry in solution of the polymers with metal-metal bonds in their backbones is qualitatively similar to the reactions of the discrete metal-metal 

SCHEME 4. Synthesis of chain-growth polymers containing metal-metal bonds along their backbones. 

SCHEME 5. Photochemical cleavage of the Re-Re bond and electrochemical formation of the Re-Re bond in poly[(vbpy)Re(CO)3]2. 

The metal radicals produced by photolysis react with radical traps to form monomeric complexes (e.g., eq. 16). 

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The photochemistry of the polymers in solution is analogous to the reactions of the discrete metal-metal bonded organometallic dimers in solution.4,5,6 9 As in the photochemical reactions of the discrete dimers, the photochemical reactions of the polymers can be conveniently monitored by electronic absorption spectroscopy. The quantum yields for the reactions are in the =0.1 to 0.6 range, depending on the specific polymer and the metal-metal bond.6 Sample reactions of the polymers showing the three types of reactivity are shown in Eqs. 7.14-7.16. 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

Polymer solids also work as a carrier of photochemical reaction components. The irradiation of a cellulose paper after adsorbing EDTA, Ru(bpy)3 and MV2+ induced rapid formation of MV4 in the solid phase (Table 2) 45,48). The quenching experiments showed that a photoinduced electron relay of EDTA Ru(bpy)2 - —> MV2+occurs in the solid phase just like in the solution. In this reaction the main path for the MV4 formation is through Ru(bpy)2, and the rate of direct reduction of MV2 by cellulose molecule is very small. Such an electron relay occurred also in a gelatine film 47). 

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

In one study,52 the temperature dependence of quantum yields was investigated using polymer 5 (prepared by the route in equation 26). Thin films of polymer 5 are photochemically reactive ( = 532 or 546 nm) in the absence of oxygen, giving the products shown in Scheme 7.19 The reaction is thus similar to the photochemical radical trapping reactions of the Cp2Mo2(CO)6 dimer that take place in solution in the presence of an alkyl halide, and an analogous mechanism was proposed (Scheme 7). 

Most of the polymers are easily depolymerized photochemically and thermally in solution to the corresponding monomers, as is expected from the ring cleavage reaction of a number of cyclobutane derivatives yielding two olefins. For example, poly-DSP in solution is depolymerized to DSP nearly quantitatively upon photoirradiation for a relatively short period36 or by heating at above 200°C70). 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

In solution, the photochemical properties of the bis-pyridyl polymer (py/Ru 5/1) are related to those of analogous monomers in solution (32). The photochemical loss of pyridine from Ru-22+ a high efficiency reaction which has proven to be... 

Quantitative study of kinetics of radicals accumulation has required solution of auxiliary problem - definition of the rate of photoinitiation Win. In the case of solid-phase reactions there are experimental difficulties in solution of this problem. Measurement of Win according to consumption of inhibitor is complicated by possible photochemical reactions of inhibitor itself and specific solid-phase effects of kinetic stop type and so on [10]. Measurement of Win according to initial rate of radicals accumulation is also tactless in solid polymer, as the latter may be much lower than Wm [164]. 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

Solids can be classified according to their organizational structure and, in the context of photochemical reactions, be classified as glasses, polymers, and crystals. Furthermore, a solid-state photochemical reaction only takes place in a thin layer of the solid surface, while in a dilute solution all the molecules are equally exposed to the radiation. In a tablet, the radiation has been estimated to reach a penetration depth of 0.03 cm, and the faded layer apparently does not increase upon further exposure (Carstensen, 1974). A study of nifedipine in the solid state demonstrated that the degradation rate was inversely proportional to the thickness of the powder bed (Marciniec and Rychcik, 1994). 

The problems of the photolysis and photochemical reactions of polymers themselves are not considered in the subsequent discussion of the photosensitized reactions of polymers in solutions. This subject has already been examined in Chapter 3 and, moreover, in the majority of cases the photosensitized reactions take place with radiation in the range of wavelengths from 3000 A to 8000 A. 

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