Polymerization of Azo Monomers

Although the living anionic polymerization technique is the most acceptable method for synthesis of well-defined azo BCs because of the living nature of the end groups, this technique has some disadvantages too  

Because of the aforementioned difficulties, many commercial polymers and copolymers (almost 50% of synthetic polymers) are prepared by radical processes. The major success of radical polymerization is the large number of monomers that can undergo free-radical polymerization, the convenient temperature range, and the 

Besides the ATRP method, other controlled radical polymerization techniques such as reversible addition/fragmentation chain transfer polymerization (RAFT) (Zhang et al., 2007) and nitroxide-mediated polymerization (NMP) (Yoshida and Ohta, 2005), have also been explored to synthesize azo BCs. 

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Living polymerization of azo monomers is one of the most effective ways to prepare well-defined azo BCs. Generally, a monodispersed macroinitiator should be prepared first. It is then used as an initiator for the subsequent polymerization of azo monomers. Finkelmann and Bohnert (1994) first reported the synthesis of LC-side chain AB azo BCs by direct anionic polymerization of an azo monomer. As shown in Scheme 12.1, the polymerization of polystyrene (PS)-based diblock copolymers was carried out from a PS-lithium capped with 1,1-diphenylethylene (DPE), whereas the poly(methyl methacrylate) (PMMA)-based diblock copolymers were prepared by addition of methyl methacrylate (MMA) monomers to the living azo polyanion, obtained by reaction of l,l-diphenyl-3-methylpentylithium (DPPL) with the azo monomer in tetrahydrofuran (THF) at lower temperature. By this method, a series of well-defined azo BCs were obtained with controlled molecular weights and narrow polydispersities (Lehmann et al., 2000). 

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