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Photoinduced Radical Polymerization

Radical polymerization is the best-evaluated method to form a polymer by successively adding monomers that have formed radicals. Due to the availability of numerous monomers and the simple process control, radical polymerization is one of the key techniques in large scale industry. According to all chain-growing reactions, the polymerization splits into the main steps of initiation, chain growth, chain termination and dimerization. Several initiator molecules can be used to start the process of growing a polymer chain. The activation energy varies from 45kJmol−1 for thermal to 25 kJ mol−1 for photochemical initiation.[1] Radical formation has to be initiated in situ to avoid immediate deactivation, so reaction control by irradiation offers great advantages for safety and regulation issues. Homolytic dissociation of a photo labile single bond in the initiator gives two starting radicals:radicalpolyeq1

Several peroxides or hydroperoxides, azo compounds and arylketones can undergo this reaction. More specific, popular examples for these photochemically cleavable substance classes are dibenzoyl peroxide (BPO), tert-butyl hydroperoxide, azobisisobutyronitrile (AIBN) and benzoin (see figure 1).

radikalstarter

Figure 1: Some photochemically activatable radical initiators.

Although the photochemical pathway do not depend on temperature directly, some heating to 40C to 70C is required to keep the subsequently ongoing polymerization reaction running.[1] Two exemplary initiation reactions are shown below:radicalpolyeq2

Photochemical initiation is followed by the advancing chain building process and termination. These processes do not differ from the processes initiated by thermal activation.

 

  1. LECHNER, M.; K., G.; NORDMEIER, E.: Makromolekulare Chemie, 4 edn., 2009, Birkhäuser.


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