Dendritic structures hybrid

The extension of Rouse s approach from linear chains to other polymer systems is quite straightforward and leads eventually to the concept of generalized Gaussian structures (GGS), which are the subject of this review. In the framework of the GGS approach, a polymer system is modeled as a collection of beads (subject to viscous friction), coimected to each other by means of elastic springs in a system-spedfic way. Initially, the GGS concept was inspired by the study of cross-linked polymer networks however, its applications have turned out to cover large classes of substances, such as dendritic polymers, hybrid polymers, and hierarchically-built structmes. 

More recently Frechet and Gitsov [130] used a similar approach as above and synthesized a novel series of dendritic copolymers derived from a central penta-erythritol core unit. These hybrid star molecules behaved as unimolecular micelles with different core-shell conformational-structures as a response to the polarity of the solvent used. 

The properties of the hybrid diblock structures can be altered drastically by simply taking advantage of the high terminal functionality of the dendritic block. For example unusual diblock structures useful for the modification of surfaces have been prepared by ATRP of polystyrene (PS) initiated from the benzylic halide focal point of Frechet-type dendrons with terminal isophthalate ester groups [9b], Well-defined copolymers with narrow molecular weight distributions were obtained and excellent agreement was observed between calculated... 

Since this work was performed significant advances have been made in the area of living radical polymerization with the introduction of novel, better controlled, initiators as well as reaction conditions that enable the use of lower polymerizations temperatures with a broader choice of monomers. It is clear that these advances could easily be applied to the preparation of a broader array of well-defined hybrid dendritic-linear structures. 

As this brief overview demonstrates, novel copolymers obtained by hybridization of the linear and globular architectural states are readily prepared through a variety of synthetic approaches. In general the dendritic components of the hybrid copolymers are well defined, with unique molecular and structural characteristics. In contrast, all the linear components prepared polymerization are less precisely defined and are polydisperse. Only the very short linear components, themselves prepared by stepwise synthesis just like the dendrons, are monodisperse and can be used to prepare well-defined, monodisperse hybrids. While architectural and structural precision may be of great importance for the determination of ultimate properties, some degree of structural variation is quite acceptable for practical applications in many areas including, for example, surface modification, sensing, or encapsulated delivery. 

Fig. 7.1 Schematic representation of linear versus dendritic polymers linear (left) and hyperbranched (middle) polymers, perfect dendrimer (right). The amount of terminal groups is indicated below each structure. These architectures can also be attached to a cross-linked polymer bead to obtain a high-loading hybrid material.

Dendritic hybrid architectures of the two dendrimer types - POPAM and PAMAM - designated by Majoros et al. as POMAM dendrimerf [17] are structural rarities. One such dendrimer of this type was assembled with PAMAM branching units starting from a POPAM core unit [18]. Vogtle et al. developed POPAM/PAMAM hybrid dendrimers up to the third generation (Fig. 4.8) [19]. 

In this review, the recent progresses of functional macromolecules comprised with the dithiafulvene moiety, e.g., dithiafulvene/ -conjugated polymer hybrid systems, dendritic dithiafulvenes, polyrotaxane structures, and intramolecular CTs are highlighted. 

When dendritic fragments are attached to polymer chains, the conformation of the polymer chain is strongly affected by the size and chemical structure of the dendritic wedges attached. Dense attachment of dendritic side chain converts a linear polymer into a cylindrically shaped, rigid and nanoscopic dimension. Frechet and Flawker [70] were one of the first to recognize these hybrid architectures . 

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