Bulky dendrimer

Scheme 4.10. Synthesis of bulky dendrimers via the sequential addition of a 7V-tosylated aminoisophthalate diester building block or monomer.

Mekelburger HB, Rissanen K, Voegtle F. 1993. Repetitive synthesis of bulky dendrimers a reversibly photoactive dendrimer with six azobenzene side chains. Chem Ber 126(5) 1161 1169. 

Dendrimers have also been found useful in the construction of thin films containing isolated fullerenes [85, 86]. Such materials could eventually find practical application in sensors and/or optoelectronic devices [87]. Generally, monolayers prepared from C60 are ill-defined due to the aggregation tendency of fullerenes. The covalent attachement of C60 to bulky dendritic frameworks... 

A rather novel application of the Diels-Alder reaction is the synthesis of dendrimers. Mullen and coworkers116 made cyclopentadienone 161 react with SA AA -tetraethynylbi-phenyl 162. This afforded, after extrusion of carbon monoxide, a first generation dendrimer 163 containing 22 phenyl rings (equation 45). Cyclopentadienone 161 reacted only as a diene, since the bulky triisopropyl groups prevented the ethynyl functions from reacting. 

The fourfold cycloaddition of an excess of cyclopentadienone dendron 27 to the tetraethynyltetraphenylmethane 4 in diphenylether at 200°C affords dendri-mer 2 in 85% isolated yield, respectively (see Scheme 7). Dendrimer 2 corresponds to the second-generation polyphenylene dendrimer made by the divergent method [30]. It should be mentioned that while the addition of dendron 27 to the biphenyl core 9 takes two days the addition to the tetraphenylcore 4 takes one week. This can be explained by the higher mobiUty of the biphenylic core compared to the stiff tetrahedral core, which allows the proper orientation of the ethynyl functions for reactions with the bulky dendrons. 

More complex bulky amines, especially dendrimers, have been developed to enhance the properties of the HTL materials. These include starburst products such as ST638 and componnds with spiro structures (Spiro-TAD) from Covion. These are shown in Figure 3.34. 

When a more globular conformation is enforced by functionaUzing the PPI dendrimers with bulky adamantyl terminal groups, or at higher generations (G4 and G5) of the PAMAM system, the dendrimers behave like nonamphiphUic, hydro-phobic spheroids. 

Our choice was the two series of dendritic polymers 5 and 6, depicted in Figure 4, which have all their open-shell centers (or trivalent carbon atoms) sterically shielded by an encapsulation with six bulky chlorine atoms in order to increase their life expectancies and thermal and chemical stabilities. Indeed, it is very well known that the monoradical counterpart of both series of polyradicals, the perchlo-rotriphenyl methyl radical, shows an astonishing thermal and chemical stability for which the term of inert free radical was coined. The series of dendrimer polymers 5 and 6 differ in the nature and multiplicity (or branching) of their central core unit, N, as well as in their branch-juncture multiplicities, N Thus, series 5 has a hyperbranched topology with = 3 and = 4, while dendrimer series 6 has a lower level of branching with = 3 and = 2, and the topology of a three-coordinated Cayley tree. 

Figure 6.18 Structural formulas of two families of dendrimers containing a 1,3,5-trisubsti-tuted benzenoide core and 9 (A918 + and B918 + ) and 21 (B2142+ and A2142 + ) 4,4 -bipyridinium units in their branches.68 The -type dendrimers are terminated with tetraaryl-methane bulky moieties while the -type dendrimers contain in their periphery less bulky aryloxy groups. For comparison purposes the structural formulas of dendrons A24 + and B24 + and model compound DoV2 + are also reported.

The bulky methylaluminoxane anion stabilizes the coordinatively unsaturated metal cation. Stabilization by noncoordinating anions such as carbosilane dendrimers is also viable.571 Aluminoxanes, however, are required to be used in large excess to be effective. Alternatively, the active catalyst can also be prepared by reacting a metal dialkyl with fluorinated boranes, borate salts or aluminate salts. 

A special type of ABA block copolymer is formed when dendrimers are used as stoppers on rotaxanes. Rotaxanes are linear polymers onto which one or several large macrocycles are threaded. In order to prevent unthreading, sufficiently bulky groups must be attached to the two ends of the rotaxane. The [G-3] po-ly(benzyl ether) dendrimer proved to be highly suited for trapping [128] (see Scheme 16). The linear polymer is made of stiff bipyridinium groups that spontaneously assemble with diparaphenylene-34-crown-10 to form the rotaxane. The dendrimers are attached via quaternation of pyridine with benzyl bromide... 

In order fo furfher invesfigafe fhe influence of fhe sfrucfure of the mesogenic units on the mesomorphic behavior of the dendrimers, bulky units have been introduced at the periphery of the third and fourth generations of amino-terminated PAMAM and PPI (Fig. 39, L3, Le, L9), wifh the aim of achieving cubic mesophases, since the increased volume around the dendritic core could in principle force the dendrimer to adopt a globular conformation [199,222],... 

Fig.40 Molecular model for the LC dendrimers with bulky mesogenic units...

Fig. 41 Variation of the size of the elementary third and fourth generations dendrimer cylinder with bulky mesogenic groups. L3, Lg and Lg are for the bulky groups containing 3, 6 and 9 terminal aliphatic chains, respectively, h is the height of the cylinder, the diameter, and

Jansen, de Brabander-van Den Berg, and Meijer describe the entrapment of molecules in a dendritic box,f ° based on poly(propyleneimine) dendrimers with a chiral shell of protected amino acids (Fig. 7). The resulting dendritic structure, 5 nm in size, possesses a dense shell (as a result of bulky surface... 

Other approaches have involved reduction of interchain interaction either by cross-linking the polymers using vinyl endgroups [ 71 ] or by attaching bulky groups, e.g., dendrimers [72,73] or silsesquioxanes [74,75], as end- or side-groups. There is evidence that reduction of fluorene by calcium cathodes is implicated in the formation of ketone defects during LED operation, and this can be suppressed by introduction of a buffer layer between the polymer and the cathode [76]. 

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