Polysiloxane dendrimer

Masamune et al. constructed a polysiloxane dendrimer with an oligosilane core unit [98]. A repetitive synthetic strategy, consisting in catalytic oxidation of the terminal SiH to SiOH groups and substitution of the hydroxyl groups by a spacer, provided access to polysiloxane dendrimers of up to the third generation. 

Scheme 4.21. Construction of polysiloxane dendrimers prepared by an iterative silane hydroxylation and chloride displacement at silicon.

Scheme 5.38. The convergent preparation of polysiloxane dendrimers also employing silicon as a branching center.

Polysiloxane dendrimers T ris-[(phenyldimethylsiloxy) dimethylsiloxy]methylsilane core bis[(phenylimethylsiloxy)methyl siloxy] dimethylsilanol as the building block Three generations [r]]-M relationship/Mark-Houwink constants, NMR, molecular diameter discussed [276]... 

As mentioned above, selection of proper synthons with active functions and high-yield coupling reactions for the chain extension is quite important in dendrimer synthesis. Siloxane bonds are usually prepared by the reaction between nucleophilic silanols and electrophilic silanes (Si. Lithe divergent synthesis of the polysiloxane dendrimers, the protection of Si was employed instead of the protection of silanol [9]. Conversion from the synthon to active Si species (deprotection) had to be carried out under neutral conditions, because siloxanes are easily hydrolyzed under both acidic and basic conditions. It was found that one of the most suitable synthons for is phenylsilane, which reacts with bromine to form bromobenzene andbromosilane. Surprisingly, siloxanes are stable with bromine for short periods of time at room temperature. 

Preparation of polysiloxane dendrimers by the divergent method is depicted in Scheme 8. The core GO-Ph was converted to activated core GO-Br by reaction with bromine. Next, GO-Br was reacted with diethylamine to convert the terminal bromosilane to aminosilane, which affords siloxane in higher yields by the reaction with silanols. Then, the chain was extended by the reaction with the building block B, which possessed two phenylsiliuies and one silanol at its terminal positions. The obtained Gl-Ph possessed six phenylsi-lanes at the terminal positions. Gl-Ph was then successively reacted with bromine, diethylamine, and the building block to form molecules of higher generations. Repeat of these operations afforded G2-Ph and G3-Ph, which had 12 and 24 terminal phenylsilanes, respectively. 

The polysiloxane dendrimers, thus obtained, were colorless oils, and were purified by gel permeation chromatography (GPC). As shown in Table 1, they had very narrow molecular weight distributions, and values of molecular weight of the purified dendrimers obtained by GPC were smaller than those of the calculated formula number. The Mark-Houwink shape factor (a) was calculated as 0.21, which was nearly identical to factors reported for Tomalia s soft, spherical Starburst dendrimers. 

Table 1 Molecular Weight and Related Values of Polysiloxane Dendrimers...

Repetitive silicon-based transformations were then employed for dendritic construction. Palladium-catalyzed silane hydroxylation of the core 71 afforded triol 73, which was then treated with three equivalents of monochloropolysiloxane 72 to generate the hexa-hydrido, first generation dendrimer 74. Further application of the Pd-mediated hydroxylation, followed by attachment of monochloro-monomer 72, led to the second (75) and third (76) generation polysiloxane cascades. 

The mesogenic behavior of 43 emphasizes the role played by the oc-tasilsesquioxane core when comparing with the related side-chain liquid-crystalline polysiloxanes [95,96]. It is remarkable that the thermal stability of the chiral nematic phase is similar in both the polymer and the dendrimer, suggesting that the cubic core does not perturb significantly the associations between the mesogens necessary to support the chiral nematic phase. 

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

PAMAM dendrimers-polysiloxane containing 3-chloropropyl groups-SR CB/SR... 

Ihe literature " describes a number of dendrimers and the closely related star-like 120-123 polysiloxanes. Ihe hyperbranched polysiloxanes are the primary example of more random structures. Although the emphasis has been on synthesis and characterization,i i modeling on hyperbranched polymers has also been carried out. Some of the most interesting species involve polysiloxane chains. Star polymers, some with nanosized silica cores, have also been synthesized.i °-i i Hyperbranched polysiloxanes have been prepared with controllable molecular weights and polydispersities,i -1 - with epoxy terminal groups some are UV-curable ° and some serve as a source of molecular silica. Hyperbranched polysiloxanes have also been used in the sol-gel preparation of polypropylene/silica nanocomposites. ... 

In some applications it is useful to have hydrophilicity in the bulk of the polymer instead of just at the surface. One way of doing this is by simultaneously end linking hydrophilic poly(ethylene glycol) (PEG) chains and hydrophobic PDMS chains. Another way is to make a PDMS network with a trifunctional organosilane R Si(OR)3 end linker that contains a hydrophilic R side chain, such as a polyoxide. Treating only the surfaces is another possibility, for example, by adding hydrophilic brushes by vapor deposition/hydrolysis cycles.Such hydrophilic polysiloxanes can also serve as surfactants. It has also been possible to make radially layered copolymeric dendrimers with hydrophilic polyamidoamine interiors and hydrophobic organosilicon exteriors. " ... 

The hydrosilylation processes of carbon-carbon multiple bonds, applied in the field of polymer chemistry and materials science, are described in part three (section 4) of this article. It includes functionalization (and cross-linking) of polysiloxanes and (poly)silsesquioxanes as well as hydrosilylation polymerization of difunctional organosilicon monomers besides the application of hydrosilylation in the synthesis of silicon-containing dendrimers and hybrid organic-inorganic materials. 

Unusual polysiloxane structures that were covered at the symposium are stiffened chains (Van Dyke, Zhang, Lauter), cyclics (Semiyen, Dagger), ladders and cages (41) (Crivello, Lichtenham, Feher, Rebrov, Carpenter, Rahimian, Haddad),hyperbranched structures (Moller, Herzig, Muzafarov, Vasilenko), dendrimers (42,43) (Dvornic, Owen, Vasilenko, Sheiko, Rebrov), and sheets and tubes (Kenney, Katsoulis) (26),... 

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