Irreversible photoactivation

Figure 32 Structures of three common photocross-linking moieties. Benzophenone is photoactivated to a reactive diradical that can relax back to the starting material. Aryl azides and diazirines both irreversibly lose N2 to react through nitrene and carbene species, respectively.

Photons of energies 2—8eV incident on gas/solid interfaces may produce, in addition to the photophysical processes considered above, the rupture and/or formation of chemical bonding within adsorbates or between them on the surface. These photochemical processes at the interface may be further subdivided into (i) photoassisted surface reactions yielding products which remain at the interface and so irreversibly alter its chemical composition and reactivity in the selected reaction conditions and (ii) photocatalytic processes wherein the products from photoinitiated reactions at the interface are continuously removed to the gas phase in the reaction conditions (e.g. by thermally assisted or photo-assisted desorption) with the result that the active surface is continuously regenerated and can become responsible for high turnover accomplished per photoactivated surface site (t.a.p.s. ). 

As living polymerization implies that during the process, side reactions such as irreversible termination and transfer reactions are virtually absent, in free radical polymerization it can be achieved by a reversible termination by reaction between the active center and another radical, using photoactivation (Otsu and Kuriyama, 1984) or thermal activation (Bledzki et al., 1983 Crivello et al., 1986 Otsu et al., 1987). An example of the latter is provided by stable nitroxyl radicals like TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) (I). 

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