Photoinitiators-Peresters

Photoinitiation of free-radical reactions.2 Use of thermal initiators for radical sources, such as AIBN or dibenzoyl peroxide, requires temperatures >50°. This perester, in contrast, decomposes at room temperature or below on irradiation at 360 nm. This mode of initiation can be useful when stereoselectivity is enhanced at lower temperatures. 

One of the earliest examples of photodecomposable, but thermally stable, polymeric systems contains pendant perester substituted benzophenone moieties, the photodissociation of the former group being promoted by the benzophenone chromophore which behaves as a triplet sensitizer. Indeed, the homopolymer of 4-vinylbenzoyl-4 -tert-butyl perbenzoate and its copolymers with styrene [poly(VBPE) and poly(VBPE-co-St), respectively] have been prepared and checked as photoinitiators in the polymerization of styrene and MMA by irradiation at 366 nm [79,80]. 

Coumarin Derivatives Coumarins are known as an interesting class of laser dyes. Various structures are able to behave as type II photoinitiators (they work in their first excited singlet state) or to sensitize the photolysis of peresters or bis-aryl imidazoles [210] (see below). 

Among other two eomponent photo initiating systems, eombinations of photoinitiators with peroxy eompounds, like with pyrilliuim peresters, or peroxides with Miehler s ketone were reported to be effeetive. The same is true of eombinations of aromatie ketones with dyes. Further diseussions of the use of dyes in eombination with ketones ean be found in seetions 2.2.4 and 2.2.7. 

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