Macromonomer synthesis, addition processes

Scheme 59 Synthesis of macromonomers through addition-fragmentation processes...

Various macromonomers made from an addition-fragmentation process have been employed as precursors of graft copolymers [292,297-300]. But Krstina et al. [301] also characterized the use of such macromonomers in the synthesis of block copolymers. They explained that for macromonomers based on methacrylic monomers (Scheme 60,1), fragmentation of the adduct (Scheme 60, 2) (formed by addition of the methacrylate monomer onto the methacrylate macromonomer) always dominates over reaction with the monomer. This fragmentation leads to block copolymers and graft copolymerization does not occur. 

Frechet et al. described the first synthesis of such a linear dendronized block copolymer 10 (Figure 37.6) [23]. For this purpose, a setof G2 and G3 aryl ether-based dendrons equipped with a norbornene unit was synthesized. Unfortunately, only the G3-exo-isomer could be polymerized, using a Grubbs third-generation catalyst the endo-isomer failed to polymerize, possibly due to a shielding effect of the G3 dendron. The obtained polydispersity index (PDI) of 1.03 indicated a good control over the polymerization process. However, when this G3 polymer was used as a macroinitiator for polymerization of the G2 monomer, the system failed to produce any diblock copolymer. Gonsequently, the order of monomer addition was reversed, such that the G2 macromonomer was polymerized first to complete conversion. 

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