Dendrons wedge-shaped

Size and shape Certain steric effects can be achieved using characteristically wedge-shaped dendrons. Thus, self-assembling dendrons have been connected to supramolecular aggregates with defined dimeric or hexameric structures. Such aggregates can form columnar superstructures which reveal liquid-crystalline properties. Spherical superstructures arise from the self-assembly process when conical dendrons are used14 14,161 Similar... 

Figure 5.4 Self-assembly of wedge-shaped molecules (modified from Zeng et al 2004). (a) Dendrons with fewer tethered chains adopt a flat slice-like shape (X is a weakly binding group), (b) The slices stack up and form cylindrical columns, which assemble on a two-dimensional hexagonal columnar (Col ) lattice (c). (d) Dendrons with more end-chains assume a conical shape, (e) The cones assemble into spheres, which pack on three different three-dimensional lattices (f) with different symmetries.

Dendrimers (from the Greek dendron tree) are highly branched, monodisperse (ideally) polymers, based around a central core moiety, from which multiple wedge-shaped dendritic fragments or dendrons spread out [163, 164] (see Fig. 15). 

Supramolecular organization of dendrons can lead to columnar or spherical superstructures, depending on directional and attractive interactions between such wedge-shaped molecules as well as size and shape. For instance, hydrogen-bonding... 

Wedge-shaped dendrons such as those described above form mainly columnar and cubic phases. However, a liquid-crystalline phase with a tetragonal three-dimensional unit cell (Tet phase) containing 30 globular supramolecular dendrimers has been reported recently. Each of these supramolecular dendrimers is the result of the self-assembly of 12 dendrons, as is shown in Fig. 8 [120,121]. Interestingly, the complex tetragonal phase of symme-... 

In one of several important studies on dendronized polymers [4c, 4d]. Schluter and coworkers explored the stiffening of polystyrene chains through the incorporation of Frechet-type dendrons as side chains [28, 29]. While the G-l and G-2 dendrons were not sufficiently bulky to effectively stiffen the polystyrene chain, the G-3 dendron provides enough steric bulk to force the hybrid polymer into adopting a cylindrical shape in solution [28b], In a complementary study, Neubert and Schluter demonstrated that adding charges to the dendritic wedges leads to an expansion of the chains of the hybrid copolymer in aqueous solution [29],... 

Schliiter et alJ65,66 have recently used rod-shaped polymers1673 such as poly(p-phe-nylene)s and poly([l.l.l]propellane)s as cores for the attachment of convergently generated dendrons. These new branched macromolecules possess a rigid backbone wrapped with structural wedges, which become increasingly more dense toward the outer cylindrical surface. The Pd-catalyzed copolymerization of dibromobiphenyl derivatives with the Frechet-type ethereal dendrons with a substituted aryl diboronic acid afforded the dendritic coated poly(p-phenylene) rod. 

Dendrimers usually exhibit spherical (isotropic) shape. However, wedge-like dendrimer fragments ( dendrons ) that have been attached to linear polymers as side groups can be used to create anisotropic nanocylinders , leading to uncoiling and extension of the polymer chains. Synthetic macromolecules of this type can be visualized directly on surfaces and their contour length determined from the images. Unexpected acceleration effects in the self-encapsulated polymerization of dendron monomers used to prepare such polymers as well as the structural consequences of dendritic pieces of cake on linear polymer chains are discussed. 

Scheme 2. Shape-directed self-assembly of cone- and wedge-like dendrons to hexagonal-columnar and cubic superstructures according to Percec et al. [15a].

Structural features of a dendron are the rather soft geometrically defined shape, the definition of an outer rim B and a terminal (inner) functional group A (Figure 30). Whether or to what extent the dendron adopts a wedge like conformation depends only partially on the rigidity of the subunits but mostly on the sterical constraints by the surrounding. 

Figure 30 Second generation dendron in (a) a wedge like conformation and (b) in a more spherical open shape...

As far as the criterion for sectioiung this review article is concerned, it is not straightforward. For conveiuence, we will review 1-D assemblies of molecular rods in terms of molecular architecture. We classify rod assemblers into six molecular categories (1) rod-coil molecifles, (2) macrocyclic molecules, (3) dendron-rod-coil molecules, (4) dumbbell-shaped molecules, (5) wedge-typed molecules, and (6) conjugate rods with lateral chains. We will describe biomimetic or bioconjugate amphiphiUc rods in the last section. 

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