Dendrimer polymer conformation

Abstract. The synthesis and study of dendrimers has been truly dramatic in the last ten years. This review gives a brief introduction to some of the key concepts and main synthetic strategies in dendrimer chemistry. The focus of the chapter is a survey of modern analytical techniques and physical characterization of dendrimers. Results of model calculations and experiments probing the dimensions and conformation of dendrimers are reviewed. In the final sections the experimental work on dendrimer-polymer hybrids is highlighted. The dense spherical conformation of dendrimers has been combined with the loose random-coil conformation of ordinary polymers to form new hybrids with potentially interesting new properties. 

Keywords. Dendrimer, Dendrimer-polymer hybrid, Conformation, Branched polymers... 

The title that was ultimately chosen allowed for the inclusion of polymers conforming to the preceding definition and also the new class of supramolecular polymers that may be stabilized by covalent bonds but nevertheless exhibit novel supramolecular features. The latter class (Part III) is exemplified by covalent chains based on 2-concatenane units, dendrimers, monolayers, and some engineered assemblies (Part IV). 

X-ray diffraction patterns from dendrimers tend to lack sharp features and are similar to those from amorphous linear polymers. This suggests a molecular arrangement in dendrimers that is fairly disordered. There is also the problem that these molecules can exist in a large number of energetically equivalent conformations and that in solution there can be rapid interchange between these conformations. This contributes to the overall amorphous structure of dendrimers. 

Lockman JW, Paul NM, Parquette JR. The role of dynamically correlated conformational equilibria in the folding of macromolecular structures. A model for the design of folded dendrimers. Prog Polym Sci 2005 30 423-452. 

Chemical programming of polymers has proven to be a crucial tool in mimicking virus-like features. Thus, alterations in polymer structure and conformation can be correlated to transfection efficiency and cytotoxicity. It has to be considered that the better polymer structure is defined, the more detailed study of structure-activity relationships is possible. First approaches towards better defined polymer structure with narrow molecular weight distribution have been made by PAE and pseudo-dendrimer synthesis already discussed in Sect. 4.1. 

In conclusion it has turned out to be difficult to polymerize all but the first generation dendritic macromonomer. Higher generation dendrimers need to be attached through a long series of bonds to the polymer backbone. This in turn prevents any strong conformational influence on the backbone and no interesting new properties have been detected so far. 

The attachment of several chromophores to a rigid polymeric backbone can also lead to a significant enhancement of the overall hyperpolarizability and dipole moment of the polymer. The three polymeric conformations that have been shown to affect the overall nonlinear response are schematically depicted in Figure 6. They include I) polymers with a helical backbone and pendant groups that have a fixed orientation relative to the axis of the central helix, II) dendrimers with pendant chromophores attached in the outer shell, and III) main-chain polymers with a rigid backbone. 

The SEC partition coefficient [6] (.K sec) was measured on a Superose 6 column for three sets of well-characterized symmetrical solutes the compact, densely branched nonionic polysaccharide, Ficoll the flexible chain nonionic polysaccharide, pullulan and compact, anionic synthetic polymers, carboxylated starburst dendrimers. All three solutes display a congruent dependence of K ec on solute radius, R. In accord with a simple geometric model for SEC, all of these data conform to the same linear plot of i sEc versus R. This plot reveals the behavior of noninteracting spheres on this column. The mobile phase for the first two solutes was 0.2M NaH2P04-Na2HP04, pH 7.0. In order to ensure the suppression of electrostatic repulsive interactions between the dendrimer and the packing, the ionic strength was increased to 0.30M for that solute. 

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