G3-dendrons

As was the case for Percec s hybrid structures [25] (see above), studies of the G3-dendronized PPP by scanning force microscopy revealed a high degree of dimensional ordering in which the three-fold symmetry of the substrate surface is effectively recognized by the nanocylinders over several layers of polymer [32], More recently, Schluter extended his study to the preparation of a PPP hybrid... 

Figure 5. SANS scattering curve i(q) vs. q of a G3-dendronized polystyrene, obtained by Forster, Schluter et al. [25], The scattering curve at larger q follows the Porod q law, arising from the scattering at the surface of a cylinder, evidencing the stiff, worm-like character of the macromolecules.

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. 

Finally, the use of a charged double-stranded (ds) DNA with a G3 dendronized polymer to prepare bio-based hybrid macromolecular systems has widened the range of chemistry of the building blocks for this type of molecular construction (Figure 37.13) [41]. 

Another two types of supported dendrons used for generation of dendritic gene delivery devices are the polystyrene-bound polyurea and peptoid dendrons developed by Bradley and coworkers. They demonstrated that these dendrons, once cleaved from the support, complex efficiently plasmid DNA and transfect it successfully into cells. The highest efficiency (comparable or better than that displayed by the commercial transfection agent Superfect) was observed for bis-G3 dendrons capped with lysine. In general, the efficiency increased with the dendron generation (positive effect). 

Scheme 28 PGA-catalyzed fragmentation of a G3-dendron to its building blocks...

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