Special Topic 6.13 Polymer photodegradation

Photodegradation of polymers (photoageing), involving chain scission and/or cross-linking, occurs by exposure to solar or artificial radiation and causes structural modifications, usually accompanied by a dramatic deterioration of the physical and mechanical properties of the polymer.954 Typically, radical intermediates are formed upon excitation, which initiate subsequent (dark) degradation. The presence of other species, such as oxygen, water, organic solvents or additives, and also mechanical stress and heat, may enhance the efficiency of these processes. 

Case Study 6.21 Asymmetric synthesis in crystals - application of chiral auxiliaries 

Molecular motions in confining chiral crystal lattice media are known to be severely restricted. Therefore, high photoreaction selectivities in the solid state can be achieved (see also Special Topic 6.5).172,173 In the present case, it was shown that salts of achiral 

5 Carbonyl Compounds Photocyclization Following n, 1-Hydrogen Abstraction 

R1 = alkyl, aryl R2 = various groups X = various atoms n =1-6 

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The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

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