Dendron Generation Research Articles

Extremely Long Dendronized Polymers: Synthesis, Quantification of Structure Perfection, Individualization, and SFM Manipulation This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 448, TPs 1 and 5) and the Fonds der Chemischen Industrie; SFM=scanning force microscopy.

Moving individual molecules: Up to 250-nm long polystyrene chains with fourth generation dendrons at each repeat unit were synthesized, individualized on highly oriented pyrolytic graphite (HOPG), and manipulated into a stretched linear conformation by a scanning force microscope tip (see images, arrow indicates direction of movement).

Chiral dendritic polymers for photonic applications

We present chiral dendrons of different generations accomplished by reacting the hydroxyl groups at the chain ends with (−)menthoxyacetic acid. Subsequent deprotection of the carboxylic acid rendered acid functional chiral dendrons. The acid-functionalized chiral dendrons were doped with divalent cations Cu 2+, Fe 2+ and Zn 2+, and trivalent lanthanide cations Nd 3+ and Pr 3+. We present results on their optical rotatory power along with circular dichroism spectroscopy and results of paramagnetic resonance. The chiral dendrons were shown to influence the electronic transitions of the metal ions (CD spectra). Attempts to characterize the circularly polarized luminescence of the Nd-dendrimer failed due to low quantum yield. The luminescence efficiency was found to be at least one order of magnitude lower than that of a fluorinated and non-chiral dendrimer structure of similar size and coordination structure.

Neutron Reflectivity of Linear−Dendritic Diblock Copolymer Monolayers

Specular neutron reflectivity has been used to investigate the structure of monolayers formed from linear-dendritic diblock copolymers at an air-water interface. The dendritic block copolymers consist of a linear poly(ethylene oxide) (PEO) block of 2000 molecular weight linked to a third or fourth generation polyamidoamine (PAMAM) dendron. The dendritic end groups were functionalized with deuterated stearic acid to make the dendritic block hydrophobic, resulting in a macroamphiphile. Results indicate that stable monolayers are formed with PEO resting on the subphase and stearate groups extending into the air. In general, at low surface concentrations the PEO block intermixes with the PAMAM dendron, whereas at high surface concentrations the PAMAM forms a distinct layer above the PEO. The ordering of the stearate groups functionalized on the dendrimer was dependent on generation. Stearate groups form a distinct ordered layer which is separate from the third generation PAMAM dendron, whereas the stearate groups are intermixed with fourth generation PAMAM segments due to the curvature of the higher generation dendron. The PEO block becomes intermixed with the water subphase if the monolayer is held at constant area for at least 10 h. These finding are consistent with earlier published studies of pressure-area isotherms of these systems on the Langmuir-Blodgett trough.

Functionalization via hydrosilylation of linear and cyclic siloxanes with appendent first generation dendrons containing electronically communicated ferrocenyl units

The synthesis, structural characterization and redox-properties of a new family of siloxane-based poly(ferrocenyl) dendronized molecules are described. Hydrosilylation reaction of the vinyl-functionalized silicon-bridged biferrocene, CH 2CHSiMe[Fe(η 5-C 5H 4)(η 5-C 5H 5)] 2 ( 1) with SiH-containing poly(siloxane) backbones afforded the novel copolymer 6 and homopolymer 7. To understand the properties of these polymers more thoroughly, we also synthesized the well-defined, closely related tetraferrocenyl-functionalized disiloxane and octaferrocenyl-functionalized cyclotetrasiloxane model compounds 4 and 5. Solution voltammetric studies of the siloxane-based poly(ferrocenyl) derivatives 4– 7 showed two reversible redox processes with a wave separation (Δ E) of 190–160 mV, indicating that significant electronic communication exists between the ferrocenyl moieties bridged by a pendant silicon atom. The neutral cyclotetrasiloxane-based octaferrocenyl 5, as well as polymers 6 and 7 become insoluble upon complete oxidation to the corresponding polycationic species, yielding electroactive films onto platinum and glassy-carbon electrode surfaces. The well-defined and persistent redox waves of electrode coated with these films are characteristic of electrochemically stable, surface-confined reversible redox couples. The microstructure of these multimetallic electroactive films has been analyzed by scanning electron microscopy.

Synthesis and catalytic antibody functionalization of dendrimers.

Antibody 38C2 catalyzed a retro-aldol process upon dendritic modified aliphatic polyesters. This catalytic system was studied in detail and displayed rate enhancements, k(cat)/k(uncat), of greater than 10(6). These antibody-catalyzed reactions took place in a stepwise manner yielding partially modified aldol-dendrimers until a fully substituted aldehyde dendrimer was formed. The catalytic antibody 38C2 only reacted with surface-exposed aldol moieties and did not significantly interact with the core groups for dendrons 4 and 8. For a higher generation dendron 8 the rate of unmasking slightly decreased presumably due to steric crowding of the aldol functionalities. In addition, catalytic antibody 38C2 was able to selectively differentiate block-hybrid dendrons and was regiospecific in the retro-aldol reaction of dendron 21. This is an inaugural report of a catalytic antibody utilizing dendrimers as substrates and suggests that antibodies could be used as selective catalysts for the controlled release and activation of specific molecules attached to biodegradable polymeric materials. Furthermore, this is the first example of catalytic antibody 38C2 displaying regioselectivity on a multifunctional aldol substrate. Important for synthetic applications is the antibody's ability to selectively differentiate regions on dendritic substrates and produce partly aldol functionalized dendrons under conditions mild enough to avoid beta-elimination.

Synthesis and characterization of higher generation dendrons based on melamine using p-aminobenzylamine. Evidence for molecular recognition of Cu(II)

The difference in nucleophilicity between the amines of p-aminobenzylamine can be used to synthesize dendrimers based on melamine using a convergent, orthogonal approach. The resulting molecules comprise groups that donate and accept hydrogen-bonds, limiting synthetic accessibility due to intermolecular aggregation. Dendrons with molecular weights exceeding 10 kDa can be prepared by this strategy, but yields drop off significantly at the highest generation (D5) due to difficulties in purification. The addition of Cu(II) reduces the extent of aggregation as evident in SEC traces. UV–vis spectroscopy confirms recognition.

Synthesis and polymerization of functionalized dendritic macromonomers

AbstractThe synthesis of dendritic building blocks (dendrons) of the first generation (G1) and the second generation, which carry differently protected amine groups in the periphery, is reported. The dendrons are used for the synthesis of the corresponding acrylic and methacrylic macromonomers. Their polymerization behavior under radical conditions is investigated. The G1 dendronized polymers are decorated at their peripheral amino groups, that is, with the chiral amino acid L‐phenylalanine by the attach‐to approach. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1940–1954, 2001

Dendron-Controlled Nucleation and Growth of Gold Nanoparticles.

Passivation of the metal surface by dendrons bearing a focal 4-pyridone functionality (the second-generation dendron is shown; C: gray, N: blue, O: red) allows controlled nucleation and growth of gold nanoclusters. The particle size is a direct function of the generation number of the dendritic ligands, with higher generation dendron producing larger particles.

Novel dendron-stabilized gold nanoparticles with high stability and narrow size distribution

Dendron-stabilized gold nanoparticles synthesized with Fréchet-type dendrons possessing a single thiol group at the focal point have small cores (average diameters of 2.4–3.1 nm) with narrow size distribution and remarkably high stability; in particular, the nanoparticles synthesized with the second generation dendron has an almost monodisperse core (2.4 ± 0.2 nm).

Synthesis and Surface Functionalization of Aliphatic Polyether Dendrons

The convergent synthesis and surface modification of a novel family of aliphatic polyether dendrons is described. The synthesis utilizes methallyl dichloride as the monomeric building block along with peripheral benzyl ether and ketal protected hydroxy functionalities. The allylic nature of the electrophilic moieties of methallyl dichloride facilitates their nucleophilic displacement by 2 equiv of the appropriate terminal alcohol group. Having served its activating function during the coupling step, the alkene focal point can then be efficiently transformed into a primary alcohol via hydroboration−oxidation leading to a nucleophile that may then be coupled again to additional monomer. Repetition of these coupling and activation steps affords higher generation dendrons. Taking advantage of the versatility of the convergent approach and the rugged nature of the aliphatic ether building blocks, dendrimers with tailored solubility properties and well-defined unsymmetrical surfaces can be obtained.

Constitutional Isomers of Dendrimer-like Star Polymers: Design, Synthesis, and Conformational and Structural Properties

The design, synthesis, and solution properties of six constitutional isomers of dendrimer-like star polymers is described. Each of the polymers have comparable molecular weights (∼80 000), narrow polydispersities (<1.19) and an identical number of branching junctures (45) and surface hydroxyl functionalities (48). The only difference in the six isomers is the placement of the branching junctures. The polymers are constructed from high molecular weight poly(e-caprolactone) with branching junctures derived from 2,2‘-bis(hydroxylmethyl) propionic acid (bis-MPA) emanating from a central core. The use of various generations of dendritic initiators and dendrons coupled with the ring-opening polymerization of e-caprolactones allowed a modular approach to the dendrimer-like star polymer isomers. The most pronounced effects on the physical properties/morphology and hydrodynamic volume was for those polymers in which the branching was distributed throughout the sample in a dendrimer-like fashion. The versatility of...

Dendritic, 1,1′-Binaphthalene-Derived Cleft-Type Receptors (Dendroclefts) for the Molecular Recognition of Pyranosides

Two series of optically active, cleft-type dendritic receptors (dendroclefts) for carbohydrate recognition were prepared by attaching Fréchet-type dendrons via ethynediyl linkers to a core consisting of one or two 1,1′-binaphthalene-2,2′-diyl phosphate moieties. Sugar substrates were expected to bind via bidentate ionic H-bonding of two OH groups to the phosphodiester core and, additionally, to undergo van der Waals and CH⋅⋅⋅π interactions with the aromatic rings of the surrounding dendritic wedges. The synthesis of the dendritic receptors G-1-(S)-1, G-2-(S)-2, and G-3-(S)-3 (Fig. 1; G-x=dendritic generation) with a single binaphthalene core started from 3,3′-diethynylated MOM-protected (MOM=methoxymethyl) 1,1′-binaphthalene-2,2′-diol (S)-13 to which the Fréchet-type dendrons of generations 1 – 3 were attached via Sonogashira cross-coupling (Scheme 3). MOM-Ether deprotection followed by phosphodiester formation and ion exchange provided the targeted receptors. By a similar route, receptor G-1-(S)-23 with dendritic wedges capped with oligoether groups was obtained (Scheme 4). In receptor G-1-(S)-26, the ethynediyl linker was omitted, and, in its synthesis, the dendritic wedges were attached to MOM-protected 3,3′-diiodo-1,1′-binaphthalene-2,2′-diol by Suzuki cross-coupling (Scheme 5). The synthesis of the dendritic receptors G-2-(S,S)-42 and G-1-(S,S)-43 with two binaphthalene moieties at the core (Fig. 3) started from diethynylated (S,S)-39 and (S,S)-33, which contain two MOM-protected 1,1′-binaphthalene-2,2′-diol moieties bridged by p-phenylene or buta-1,3-diynediyl linkers, respectively, and was completed by attachment of the dendritic wedges by Sonogashira coupling, MOM-ether deprotection, phosphodiester formation, and ion exchange (Schemes 9 and 10). By an alternative route, the C-frame of receptor G-2-(S,S)-41 was prepared by coupling the dendron to dialkynylated 1,1′-binaphthalene (S)-44, followed by oxidative Glaser-Hay coupling (Scheme 8). For control studies, the non-dendritic reference receptors (S)-4 and (S)-5 (Fig. 1) with one and (S,S)-31 and (S,S)-32 (Fig. 2) with two 1,1′-binaphthalene-2,2′-diyl phosphate moieties were also prepared. 1H-NMR Complexation studies with the dendritic receptors containing one binaphthalene core and octyl glycosides 53 – 55 in CD3CN and CDCl3 (Tables 2 – 4) revealed that ionic H-bonding between the phosphodiester core in the dendritic receptors and the sugar OH groups provides the major driving force for stoichiometric 1 : 1 host-guest association. A smaller, yet significant contribution to the binding free enthalpy was also provided by interactions between the sugar guests and the dendritic wedges. Binding selectivity was weak in all cases, and only small changes in association strength were observed as a function of dendritic generation. In studies with the dendritic receptors, which contain two binaphthalene moieties at the core, higher-order complex stoichiometries prevented the determination of quantitative binding data. As a result of unfavorable steric interactions between the dendritic wedges, these flexible receptor systems apparently avoid adopting the `syn'-conformation with convergent phosphodiester sites that is required for efficient 1 : 1 host-guest complexation.

A General Entry into Glycopeptide “Dendrons”

Successive generations of novel glycopeptide dendrons (see picture) were obtained from an orthogonally protected AB2-type building block by an iterative reaction sequence of deprotection and peptide coupling. The carbohydrate-containing dendrons so prepared may be considered as glycocalyx mimetics.

Dendronized Polymers: Increasing of Dendron Generation by the Attach-to Approach

Dendronization with the first generation (G1) building block 10b of the polystyrenes 13c and 14c carrying first (G1) and second generation (G2), amino-terminated appendant dendrons is reported. Comparison of the 1H and 13C NMR spectra of the resulting G2 and G3 polystyrenes (15, 17a) with those recorded of the same polymers independently prepared from the corresponding G2 and G3 macromonomers 16 and 12, respectively, proved the degree of coverage to be virtually 100%. This is the first example of a convergent synthesis of a G3 polymer, which will considerably widen the synthetic access into this class of macromolecules.

Dendrimers as Solubilizing Groups for Conducting Polymers: Preparation and Characterization of Polythiophene Functionalized Exclusively with Aliphatic Ether Convergent Dendrons

The first conducting polythiophene, having only dendritic solubilizers, has been prepared using a Stille coupling approach. The use of second and third generation aliphatic ether convergent dendrons as a solubilizing platform facilitated the preparation of dendrimer-oligothiophene hybrid macromonomers having minimal substitution. Polymers were then prepared via an A2 + B2 step-growth polymerization between an oligothiophene−dibromide macromonomer and 2,5-bis(trimethylstannyl)thiophene using Pd(PPh3)2Cl2 as the catalyst. Both the dendrimer size and the dendrimer to thiophene unit ratio were variables that required optimization in order to obtain a solution-processable conducting polymer. Conductivities as high as 200 S/cm were measured for iodine doped thin films of the polythiophene with six thiophene repeat units for each third generation dendron.

Properly Assembled Dendrons Can Be Immobilized into Dendrimers by in situ Cross-link

Abstract Aggregates obtained from saccharide-containing dendrons were immobilized by in situ cross-link with 1,3-phenylene diisocyanate. Characterizations of the resultant oligomers by dynamic light scattering and MALDI-TOF Mass have established that the higher generation dendrons which tend to aggregate into a spherical structure result in a dendritic polymer whereas the lower generation dendrons which tend to aggregate into a disordered large particle cannot result in such a dendrimer-shaped oligomer.

Starlike Polymeric Architectures by Atom Transfer Radical Polymerization: Templates for the Production of Low Dielectric Constant Thin Films

The synthesis of novel poly(methyl methacrylate)s with starlike architectures by controlled radical polymerization starting from dendritic 2-, 4-, 6-, and 12-arm multifunctional initiators is described. The more highly functionalized initiators were obtained by coupling a bromo-functionalized bis(hydroxymethyl)propionic acid (bis-MPA) first generation dendron to hydroxyl-functionalized precursors. The controlled free radical character of the atom transfer radical polymerization is supported by the remarkably low polydispersities of the polymers and the close correspondence between the calculated and measured molecular weights. NMR studies on the polymers were consistent with initiation occurring from all initiator arms for the 2-, 4-, and 6-arm derivatives. Initial studies on deuterated polymers derived from 12-arm initiator suggest that in some cases initiation was not quantitative. In addition, starlike random copolymers of methyl methacrylate containing varying amounts of hydroxyethyl methacrylate with...

Preparation of Poly[(ether)-(ether ether ketone)] Dendrimers by the Convergent Method

New highly branched poly[(ether)-(ether ether ketone)] dendrimers were synthesized by the convergent approach using two building blocks, 3, 5-dihydroxybenzyl alcohol, 1, and 3, 5-dihydroxy-4′-(4-fluorobenzoyl)diphenylether, 2, alternatively. The reaction of p-fluorobenzonitrile and 1 gave 3, 5-di(4-cyanophenoxy)benzylalcohol (the first generation dendron, G1-OH). After G1-OH was converted to 3, 5-di(4-cyanophenoxy)benzylbromide (G1-Br), the resultant benzylic bromide functionality was allowed to react with 2 to yield the second-generation dendron (G2-F). The third-generation dendron (G3-OH) was obtained by the reaction of G2-F and 1, and the fourth-generation dendron (G4-F) was obtained by reaction of G3-Br derived from G3-OH, with 2. Finally, dendrimers, Den-(G2)3 and Den-(G4)3 were synthesized by reaction of G2-F and G4-F with trifunctional core, respectively. Analysis was made of these structurally unambiguous dendrons and dendrimers by gel permeation chromatography (GPC) and matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectroscopy. 1H and 13C NMR spectra were consistent with the structures of these dendrons and dendrimers.

Rapid Synthesis of Phosphorus-Containing Dendrimers with Controlled Molecular Architectures: First Example of Surface-Block, Layer-Block, and Segment-Block Dendrimers Issued from the Same Dendron

A third generation dendron possessing one activated vinyl group linked to the core and 16 chlorines linked to the surface has been functionalized with various groups at the core (primary amine, phosphine, or azide), and on the surface (aldehyde, tertiary amine, or nitrile). These functionalized dendrons are used as building blocks for the synthesis of several complex dendritic architectures, obtained through core−core, core−surface, or core−core−surface−core couplings using very simple one-step reactions between these functions, such as Michael additions between vinyl and primary amino groups or Staudinger reactions between phosphines and azides. These reactions finally induce the formation of layer-block dendrons, a surface-block dendrimer, a layer-block dendrimer, a layered surface-block dendrimer, and a segment-block dendrimer, all issued from the same original dendron.

Synthesis and structure of intramolecularly hydrogen bonded dendrons

[reaction: see text] A convenient synthesis of monodendrons whose conformation is restricted through the intervention of intramolecular hydrogen bonding and repulsive electrostatic interactions is described. X-ray crystal structure analysis of the second generation dendron shows the presence of a propeller-type secondary structure and indicates that the dendrons have assembled into a symmetrically interdigitated dimer in the solid state. 1H NMR and IR spectral data are in agreement with the presence of intramolecular hydrogen bonding between the amides and the pyridine N throughout the dendron structure in solution.