Free radical polymerization overall scheme

Another synthetic approach based on pyridium salt photochemistry involves the use of alkoxy radicals which are formed in both direct and sensitized decomposition of pyridinium ions in free radical polymerization [78]. Obviously, polytetrahydrofuran (PTHF), terminated by JV-alkoxy pyridinium ions, can act as macrophotoinitiator for the polymerization of monomers such as methyl methacrylate (MMA) that readily polymerize by a free-radical mechanism. PTHF macrophotoinitiators were prepared by termination of living polymerization of THF by the corresponding IV-oxides, The well-defined macrophotoinitiators with exact functionalities, confirmed by H-NMR, UV-visible and g.p.e. analysis, were obtained. Upon irradiation of macroinitiators at suitable wavelengths, polymeric alkoxy radicals are produced. The overall process is shown for the pyridinium macrophotoinitiator in the following Scheme 21. 

Radical chain polymerization, as noted above, is a chain reaction which involves mainly three steps - initiation, propagation, and termination, taking place in sequence. The overall scheme of the polymerization of a vinyl or related monomer M, initiated by the decomposition of a free-radical initiator I, may be schematically represented as follows (Ghosh, 1990) ... 

The synthesis of macromolecules by the free radical chain polymerization of low molar mass compounds, denoted as monomers, commences with the generation of free radicals, which is conveniently performed through photoreactions of initiator molecules. The subsequent processes, i.e. propagation, including chain transfer, and termination, are thermal (dark) reactions, which are not affected by light The simplified overall mechanism is described in Scheme 10.1. 

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