Polyethers dendritic

A structure resembling that of the dumbbell polymers was made by Frechet et al. In this case the connector is linked with polyether dendritic groups [272, 273]. The synthetic approach involved the preparation of a difunctional polystyrene chain in THF using potassium naphthalenide as initiator. The living polymer was end-capped with 1,1-diphenylethylene (DPE) to reduce its nucle-ophilicity and avoid side reactions with benzylic halomethyl groups. Addition of the fourth generation dendrimer [G-4] -Br led to the final product (Scheme 100). 

Functionalisation of these templates with phosphine ligands, followed by complexation with Co or Pd precursors, led to the demonstration of remarkable dendritic effects on the activity and selectivity of the catalytic systems in the Pauson-Khand [63] and Heck reactions (Scheme 5) [64]. The advantage of the weakly coordinating polyether dendritic backbone, as compared with the coordinating dendritic backbones (e.g. polyamide), was demonstrated for the Heck reaction [65]. 

Scheme 3.30 Aza-Morita-Baylis-Hillman reactions catalysed by polyether dendritic phosphine.

As an alternative approach, flexible coil parts in rod-coil molecules can be structurally modihed into dendritic shapes. In comparison to linear-type coils, dendritic coils would cause a larger steric repulsion at the rod/coil interface, which influences rod-packing structures. Lee et al. devised a dumbbellshaped molecule (12) based on an elongated dodeca-para-phenylene rod block and aliphatic polyether dendritic coils with chiral carbon centers, and investigated the self-assembly behavior in aqueous solution [54]. In remarkable contrast to ordinary one-dimensional fibers, the aggregate structures... 

Fig. 21. Dendritic polyrotaxanes with mechanical branching units containing covalently linked bis-dendrons and a core imit fused to polyether macrocycles...

Porphyrin complexes are particularly suitable cores to construct dendrimers and to investigate how the behavior of an electroactive species is modified when surrounded by dendritic branches. In particular, dendritic porphyrins can be regarded as models for electron-transfer proteins like cytochrome c [42, 43]. Electrochemical investigation on Zn-porphyrins bearing polyether-amide branches has shown that the first reduction and oxidation processes are affected by the electron-rich microenvironment created by the dendritic branches [42]. Furthermore, for the third generation compound all the observed processes become irreversible. 

Dendritic molecules with a single triethylene amine core surrounded by hyperbranched polyether sectors catalyze the nitro-aldol reaction between aromatic aldehydes and nitroalkanes (Eq. 3.5).15 The activity of the catalysts decreases when the generation number increases. No significant changes in stereo-control are observed on passing from lower- to higher-generation dendrimers. 

Mourey, T.H., Turner, S.R., Rubenstein, M., Frechet, J.M.J., Hawker, C.J., and Wooley, K.L. (1992) Unique behavior of dendritic macromolecules Intrinsic viscosity of polyether dendrimers. Macromolecules 25, 2401-2406. 

The advantages that heterogeneous catalysts have is that they are easily separable from the product, and can be recycled. A number of studies have been conducted in which ligands have been attached or bound to polymeric material to provide an immobilized ligand, and these include polyacrylate and silica [27], polyurea [28], polythiourea [29], polyether [30, 31] and dendritic [32] systems. Upon metal coordination, the immobilized catalysts have retained most of the activity and selectivity, but they now provide the advantage of simple separation and recycling. For exam-... 

Parallel studies on PAMAM dendrimers, the Frechet type polyether den-drons, and other dendrimer families have generated an extensive list of unique properties driven by the dendritic state/ Figure 1.18 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. 

It is also possible to introduce polymerizable groups at the focal point of dendritic macromolecules and a number of authors have demonstrated the synthesis of novel hybrid dendritic-linear block copolymers by this methodology [69-73]. Initial experiments in the use of the dendritic macromonomers involved the preparation of a series of polyether dendrimers, such as the third generation derivative, 37, which contains a single styrene at its focal point [74]. Copolymerization of 37 with styrene then affords a hybrid block copolymer, 38, in which... 

One immediate effect of being able to construct dendritic macromolecules with a wide variety of different chain ends is that the solubihty characteristics of the dendrimer can be finely tuned. For example, polyether dendrimers based on 3,5-dihydroxybenzyl alcohol as the repeat unit can be prepared with dodecyl... 

A wide range of other monomer units have been employed in the synthesis of hyperbranched macromolecules and the range of structures obtained is nearly as diverse as those for dendritic macromolecules. For example, hyperbranched polyphenylenes [93], polyesters [104, 107-109], polyethers [110-112], polyamides [113], polysilanes [114], polyetherketones [100], polycarbazoles [115], etc. [116-118] have been prepared. Interestingly, a number of groups have also used growth processes other than condensation chemistry to prepare hyper-... 

Cho B-K, Jain A, Nieberle J, Mahajan S, Wiesner U, Gruner SM, Tuerk S, Raeder HJ. Synthesis and self-assembly of amphiphihc dendrimers based on ahphatic polyether-type dendritic cores. Macromolecules 2004 37 4227-4234. 

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