Dendronized polymers objects

This chapter draws a comprehensive picture of what has been done in the field of dendrimers with polymeric cores putting emphasis first on synthetic issues and then on experiments investigating the aggregation behavior of these intruiging macromolecules both in the solid state and on surfaces. Additionally, experiments will be described which show that some of these dendrimers can be considered cylindrical molecular objects. The macromolecules treated in this chapter may be considered as either dendrimers with polymeric core or alternatively dendronized polymers (or polymers with appendent dendrons) depending on whether one sees them from the vantage point of an organic or macromolecular chemist. 

Scheme 9. Cartoons of some dream structures of dendronized polymers and cylindrical objects...

Frauenrath H. Dendronized polymers—building a new bridge from molecules to nanoscopic objects. Prog Polym Sci 2005 30 325-384. Available at ETH D-MATL. www.polychemc. mat. ethz. ch/frauem-ath/index/. 

Dendronized Polymers An Approach to Single Molecular Objects... 

As noted above, dendronized polymers are rigid cylindrical objects of which, urdike conventional polymers, the typical diameters are on the order of a few nanometers rather than a few angstroms. Consequently, the persistence length of these materials is on the order of few tens of nanometers. Yet, these remarkable features of dendronized polymers might invoke some interesting questions ... 

Suzuki polycondensation of a dendronized maaomonomer with a comonomer has been used to generate cylindrical dendronized brushes. One objective of research on dendronized polymers is to use the decoration with dendrons as a means to stiffen the backbone to the point that it is fully stretched. This goal should be reached when the individual... 

Dendronized polymers (denpols) were proved to be ideal objects for nanomanipulations and nanoreactions. In... 

Encapsulation of any of these c/s-conformers into libraries of columnar supra-molecular dendrimers eliminates the intramolecular electrocyclization and replaces the hehx-coil transition with an unprecedented helix-helix transition and a reversible transition from cw-transoidal to cis-cisoidal. When the repeat unit of the dendronized polymer also contains a stereocenter, this reversible process can be monitored by circular dichroism (CD) and visualized by different methods [104-111]. This concept was used to elaborate molecular machines that were interfaced for the first time with the real world to lift heavy objects [111]. 

Following A. D. Schliiter s question of whether one can create a molecular object, i.e., a molecular system that does not respond to its surrounding, by making a polymer thicker and thicker [93], shape-persistent dendronized polymers in solution were studied by advanced pulse EPR methods. As expected, DEER spectroscopy yields the size (thickness) of different generations of charged cylindrical dendronized polymers in solution [94]. Moreover, a combination of CW EPR and a modified isotopolog-specific DEER variant provides a better understanding of how amphiphilic molecules can be loaded into and released upon external stimulation from these thick polymers [95]. 

Macromolecules that contain two or more topologically distinct components are complex architectures that can lead to emergent properties or behaviors that are different to those of either of the individual molecular architectures. Dendronized polymers [16-19] are examples of such complex molecular architectures and are composed of a linear polymer backbone and perfectly branched dendritic side chains on each repeat unit (Scheme 1). The molar masses of such polymers emphasize the shift in thinking brought about by Staudinger s concept of macromolecules [1,2]. Individual dendronized polymers are nanoscopic objects [20-25] whose organization in bulk is determined by hierarchical processes that occur on a different set of length scales compared to conventional polymers [16,26]. By virtue of the size and shape of dendronized polymers, interest in this complex macromolecular architecture has moved toward how to extract functionality from these nanoscale molecular objects. 

Scheme 1 Dendronized polymers contain dendrons attached to each repeat unit of a polymer that can enforce a helical conformation of the backbone and yield a nanoscale cylindrical object...

Macroscopic changes in the extruded fibers of dendronized polymers can be further harnessed to perform work. As the polymer fiber elongates, a force is exerted at the ends of the fiber. That force resulted in the displacement of an object of much greater mass than the dendronized polymer fiber (Fig. 3). This example is one of a few cases where molecular motion in self-organized liquid crystalline... 

Percec and coworkers have shown that the oriented fibers of dendronized polyacetylenes can undergo thermally induced mechanical motion [38]. In this system (Figure 37.12), the dendrons around the polymer backbone become self-organized and thus facilitate a reversible ciscoid-to-transoid conformational isomerism of the polymer backbone. This isomerism is then converted into unidirectional macroscopic motion along the fiber axis. Remarkably, this mechanical motion could be used to displace objects with a mass of up to 250-fold that of the polymer fiber. 

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