Frechet type poly

Similar results have been presented by Miller and co-workers, who capped the ends of oligo(dihexylfluorene)s and poly(dihexylfluorene)s with Frechet-type dendrons (Figure 7.17C) [73], Annealing experiments coupled with emission studies revealed that G-3 and G-4 dendrons were effective at preventing excimer formation, even when the poly(fluorene) spacer was 50-80 repeat units long. 

In the present chapter, emphasis is placed on the authors recent studies concerning the molecular design and optical/photochemical properties of Frechet-type poly(benzyl ether) dendrimers. 

Experimental Systems. Experimentally, amphiphUic Frechet-type poly(aryl ether) dendrimers displaying poly(ethylene glycol) (PEG) peripheral arms were shown to form monomolecular micellar structures exhibiting solvent-sensitive conformational changes (Fig. 11.7 Gitsov and Frechet 1996). 

Figure 11.7 Solvent-sensitive conformational changes of Frechet-type polyfaryl ether) dendrimers with poly(ethylene glycol) termini.

Commercially available POPAM and PAMAM dendrimers (see Chapter 4) bearing terminal primary amino groups are currently the most commonly used divergently grown dendrimer structures, followed by poly(benzyl ether) den-drons or poly(benzyl ether) dendrimers (Frechet type) as representatives of con-vergently synthesised dendritic molecules. 

Scheme 5.13. The rate of reaction of Frechet-type dendritic wedges with poly(ethylene oxide) [PEO] and poly(ethylene glycol) [PEG] was determined to increase dendron generation and block length increased.

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. 

The poly(benzyl ether) dendrimer synthesis is one of only a few convergent syntheses that can produce dendrons and dendrimers in reasonable yields up to the sixth generation. These dendrons, now frequently referred to as Frechet-type dendrons, have been utilized by a number of groups because they are... 

Wooley et al.234 reported the first dendrimer— fullerene hybrid, 165 (Scheme 21), via the alkylation of a prederivatized fullerene diphenol, 166, with the dendritic bromide 6. Because of purification problems related to impure fullerene starting material, Hawker et al. later investigated the direct cycloaddition235 of a fourth generation azide-functionalized Frechet-type dendron, 167, with Ceo- This dendritic fullerene, 168, could be easily isolated by flash chromatography and showed a significant increase in solubility over the parent fullerene. Avent et al. reported a similar cycloaddition of poly(phenylacetylene) dendrons bearing a focal tosylhydrazone functionality.236... 

Numerous examples of postsynthetic modifications at the dendritic periphery have been explored. In early work, Hawker et al. investigated the hydro-genolysis and subsequent modification of benzyl-protected poly(aryl ester) dendrimers473 and later the saponification of Frechet-type dendrimers with p-methyl benzoate end groups to afford an anionic carboxylated periphery.41 The isophthalate ester end groups proved to be especially versatile (Scheme 28)... 

Dendrimer based-BINAP polymers have been prepared from 124. A series of soluble dendritic mono-BINAP ligands with Frechet-type polyether wedges like 127 and dendritic poly-BlNAP ligands like 128 have been reported Ruthenium complexes of both classes of dendrimer were prepared and found to be comparable both in catalytic activity and enatioselectivity as homogeneous BINAP analogs in asymmetric hydrogenation reactions. 

Dendrons and dendrimers are the most intensely investigated subset of dendritic polymers. In the past decade, over 6,000 literature references have appeared dealing with this xmique class of structure-controlled polymers. The word dendrimer is derived from the Greek words dendri- (tree branch-like) and meros (part of), and was coined by Tomalia et al. about 20 years ago in the first full paper on poly (amidoamine) (PAMAM) dendrimers [47,72]. Since this early disclosure, over 100 dendrimer compositions (families) and 1,000 dendrimer surface modifications have been reported. The two most widely studied dendrimer families are the Frechet-type polyether compositions and the Tomalia-type PAMAM dendrimers. PAMAM dendil-mers constitute the first dendrimer family to be commercialized, and represent the most extensively characterized and best-understood series at this time [46]. 

Parallel studies on Tomalia type PAMAM dendrimers, the Frechet type poly(ether)dendrons, and other dendrimer families have generated an extensive list of unique properties driven by the dendritic state. Figure 42.14 compares several significant physical property differences between the linear and dendritic topologies related to conformations, crystallinity, solubilities, intrinsic viscosities, entanglement, diffusion/mobility, and electronic conductivity. 

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