Starburstr̀ dendrimers — Nanoscopic supermolecules according to dendritic rules and principles

Construction of dendritic macromolecules based on the mimicry of macroscopic branching patterns found in trees is reviewed. From this mimicry, synthetic strategies have been developed for the preparation of precise macromolecular building blocks referred to as Starburstr̀/Cascade dendrimers. These dendrimer constructions involve the amplifications of matter (mass) by organizing monomer units around initiator cores according to geometrically driven mathematical rules and principles. The predictable precision of mass and valency (i.e., number of reactive surface groups) displayed by these dendrimers, as a function of generation, validates their proposed role as fundamental nanoscopic building blocks (i.e., particle sizes of 10 −1000Å). This emerging area of “structure-controlled polymers” is defining a fourth new major class of macromolecular architecture. Ideal, defect free structures of Starburst^r̀ polyamidoamine (PAMAM) dendrimers (e.g., NH₃ core; generation = 2.0, MWt. 2,414) have been synthesized in kilogram quantities with overall yields of 60-70%. The precise masses and surface valencies associated with these dendrimer structures allow one to view these entities as “nanoscopic analogues” to atoms. As such, basic rules of chemical combination between dendrimers to give definite, stoichiometric compositions can be defined much as first noted by Dalton for atoms. The use of these nanoscopic building blocks (i.e., 10–1000Å species) to construct supramolecular/supermolecular structures such as nanoscopic compounds, clusters and macro-lattices will be reviewed. Registered trademark of Dendritech Inc.     

Dendrimer as a versatile platform for biomedical application: A review

Modern chemistry is vastly fascinated by dendrimer chemistry, an area that is rapidly expanding and brimming with potential applications. Dendrimers are highly branched polymers that have multiple peripheral groups, interior cavities and they have many structural properties therefore Dendrimers play a crucial role in the fields of nanotechnology, pharmaceuticals, and medicinal chemistry. The terminal functional groups of dendrimers may be chemically linked to other moieties in order to adjust surface properties for applications such as biomimetic nanodevices. A variety of biologically active agents can be incorporated into dendrimers to create biologically active conjugates, including novel drug carriers, by utilizing the homogeneity of their three-dimensional architecture. The purpose of this review is to provide a brief overview of bio-inspired dendrimer applications, highlighting their use as drug and gene delivery agents, and biomedical diagnostic agents. In addition, the review mentions briefly some dendrimer applications in cosmetics, agrochemicals, and catalyst.     

The chemistry of tri- and high-nuclearity palladium(II) and platinum(II) complexes

This review gives an overview of the progress on tri- and high-nuclearity palladium(II) platinum(II) complexes and discusses recent developments in the chemistry of these complexes. Three or more square-planar metal atoms can be assembled in several ways resulting into myriad geometric forms of the resulting complexes. These square planes may be sharing a corner, an edge and two edges or even separated by ligands having their donor atoms incapable of forming chelates yielding dendrimers and self-assembled molecules. A variety of ligands have been used to stabilize these complexes. The chemistry of these complexes has been dealt based on nuclearity of the complexes. Synthetic, spectroscopic, structural aspects and applications of these complexes are described in this review.     

Liquid-crystalline octopus dendrimers: block molecules with unusual mesophase morphologies

The synthesis and the mesomorphic properties of several new main-chain liquid-crystalline dendrimers, thereafter designated as octopus dendrimers in accordance with their eight sidearms, are reported. In these dendritic systems, the arborescence is ensured by anisotropic segments, acting as branching cells with a double multiplicity, which are incorporated at every node of the dendritic architecture. In such a way, these compounds radically differ from the classical end-functionalized liquid-crystalline dendrimers, the most commonly reported systems. Following our previous report on purely homolithic systems, that is, the building blocks constituting the dendritic matrix are all identical, several heterolithic systems made of different anisotropic blocks have been prepared. The dendritic branches and corresponding dendrimers were synthesized using a modular construction. Polarized optical microscopy and X-ray diffraction studies showed that all of these new octopus dendrimers exhibit either smectic-like or columnar phases with novel morphologies, the nature of the mesophases depending on the number of terminal chains attached to the peripheral groups. The mesomorphism of these heterolithic dendrimers is discussed in terms of their intrinsic architecture and compared to the analogous homolithic octopus systems. Models for the molecular organizations within both the smectic and the columnar phases are proposed on the basis of small Bragg angle X-ray diffraction studies and are supported by molecular modelizations. Moreover, this study showed that the mesophase stability is very sensitive to the nature and to the mutual arrangement (the spatial location) of the mesogenic segments within the dendritic matrix, illustrating the intimate relationships existing between the mesomorphic properties and the molecular architecture of these dendrimers.     

Organic templates for the generation of inorganic materials

Mankind's fascination with shapes and patterns, many examples of which come from nature, has greatly influenced areas such as art and architecture. Science too has long since been interested in the origin of shapes and structures found in nature. Whereas organic chemistry in general, and supramolecular chemistry especially, has been very successful in creating large superstructures of often stunning morphology, inorganic chemistry has lagged behind. Over the last decade, however, researchers in various fields of chemistry have been studying novel methods through which the shape of inorganic materials can be controlled at the micro- or even nanoscopic level. A method that has proven very successful is the formation of inorganic structures under the influence of (bio)organic templates, which has resulted in the generation of a large variety of structured inorganic structures that are currently unattainable through any other method.     

Nitrone-mediated radical coupling reactions: a new synthetic tool exemplified on dendrimer synthesis

A synthetic strategy employing nitrones as radical spin traps is presented on the example of the efficient generation of novel dendrimers via a combination of radical and classical 'click' chemistry.     

Discovery of dendrimers and dendritic polymers: A brief historical perspective*

A brief historical perspective relating the discovery of dendrimers and other dendritic polymers is presented. Dendritic polymers are recognized as the fourth major class of macromolecular architecture consisting of four sub‐ classes, namely, (1) random hyperbranched, (2) dendrigrafts, (3) dendrons, and (4) dendrimers. The previous literature is reviewed with anecdotal events leading to implications for dendrimers in the emerging science of nanotechnology. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2719–2728, 2002     

Virus crystals as nanocomposite scaffolds

The generation of long-range three-dimensional nanoscopic patterns is a major goal in materials chemistry. Here we report a strategy for creating such systems using virus crystals as scaffolds which can be infiltrated with metal specifically palladium and platinum. The inorganic component effectively packs within the porous macromolecular crystal architecture, providing a route for patterning these materials on the nanometer length scale. To verify the quality of the metal infiltration, SEM-EDX was used to determine the homogeneous distribution of metal across the crystal, and TEM was used to confirm that the metal was confined within the porous structure of the crystal.     

温度敏感树形聚合物

温度敏感树形聚合物结合了温敏聚合物对温度具有响应行为的特点以及树形聚合物非线形构造的方式、大尺度、结构易于调节和功能化等特征,在智能材料和生物医药等领域有着重要的研究价值和应用前景。此类聚合物可以通过在树形聚合物表面引入温敏基元、控制聚合物结构的亲疏水比例以及采用温敏基元直接构筑聚合物等方式形成,其温敏性可以通过调控聚合物内部或外部基团的亲疏水性、树枝化基元代数、树形构造方式等得以实现与控制。此外,树形聚合物独特的拓扑结构赋予其与线形聚合物不同的温敏行为及脱水机理。本文综述了包括温敏树枝状大分子、温敏树枝化聚合物、温敏超支化聚合物等不同类型温敏树形聚合物近年来的研究进展,重点介绍这些聚合物的合成方法、温敏行为和拓扑结构对温敏行为的影响,以及在纳米材料、生物医用、分子传感器等方面的应用研究。     

POSS基树枝状大分子的合成与应用

树枝状大分子(dendrimer)是一种高度支化、纳米尺度的人工合成大分子,具有独特的物理化学性能和重要的应用前景。利用具有8个可官能化顶点的多面体低聚倍半硅氧烷(POSS)作为树枝状大分子的核心,可在一定程度上简化树枝状大分子繁琐的合成与分离过程,在低代数时就可获得较大的表面官能团密度,并使树枝状分子呈现球形对称结构。POSS基树枝状大分子结合了POSS和树枝状分子结构与性能的优势,是一类极具潜力的有机-无机纳米杂化材料。本文综述了近年来POSS基树枝状大分子的最新研究成果,介绍了具有代表性的POSS基树枝状大分子的合成方法以及它们在催化剂、生物材料、液晶材料和发光材料等领域的应用研究进展,并对该新型材料的发展趋势做了展望。     

Beta-alanine-based dendritic beta-peptides: dendrimers possessing unusually strong binding ability towards protic solvents and their self-assembly into nanoscale aggregates through hydrogen-bond interactions

A series of poly(beta-alanine) dendrimers 1-4 with Boc-carbamate as the surface functionality, beta-alanine as the dendritic branch, 3,5-diaminobenzoic acid as the branching agent, and 1,2diaminoethane as the interior core has been synthesized by a solution-phase peptide-coupling method. The structural identities and purities of the products have been fully characterized by spectroscopic and chromatographic methods. 1H NMR studies on the dendrimers indicated that the Boc-carbamate surface groups exist as a mixture of syn and anti rotamers in solution, and that the dendrimers adopt an open structure in polar solvents; this allows the free interaction of the interior core functionality with solvent molecules. Due to the cooperative effect of a large number of carbamate and amide groups, the dendrimers exhibit an unusually strong binding ability towards protic solvents and behave as H-bond sponges. As a result, the H/D exchange rates of the N-H protons are significantly enhanced in such dendritic structures, as compared to those of nondendritic carbamates and amides. These dendritic peptide dendrimers also exhibit a strong tendency to form nanoscopic aggregates in nonpolar or polar aprotic solvents through intermolecular H-bond interactions.     

A facile convergent procedure for the preparation of triphenylamine-based dendrimers with truxene cores

A simple convergent procedure has been developed for the preparation of triphenylamine dendrons containing an alkene at the center, which can be coupled in a single step to give dendrimers that contain truxene for the core without any protection-deprotection chemistry. These conjugated dendrimers exhibit similar absorption and emission behaviors in solutions and in thin films, which are indicative of the high isolation effect of well-organized three-dimensional dendrimers. They also have high fluorescence quantum yields and high glass transition temperatures, which indicate that these dendrimers are candidates for the application in OLED as light emitting materials.     

Starburst Dendrimers: Molecular-Level Control of Size,Shape, Surface Chemistry,Topology, and Flexibility from Atoms to Macroscopic Matter

Starburst dendrimers are three-dimensional, highly ordered oligomeric and polymeric compounds formed by reiterative reaction sequences starting from smaller molecules—“initiator cores” such as ammonia or pentaerythritol. Protecting group strategies are crucial in these syntheses, which proceed via discrete “Aufbau” stages referred to as generations. Critical molecular design parameters (CMDPs) such as size, shape, and surface chemistry may be controlled by the reactions and synthetic building blocks used. Starburst dendrimers can mimic certain properties of micelles and liposomes and even those of biomolecules and the still more complicated, but highly organized, building blocks of biological systems. Numerous applications of these compounds are conceivable, particularly in mimicking the functions of large biomolecules as drug carriers and immunogens. This new branch of “supramolecular chemistry” should spark new developments in both organic and macromolecular chemistry.     

A Small Change in Central Linker Has a Profound Effect in Inducing Columnar Phases of Triazine‐Based Unconventional Dendrimers

Four unconventional triazine‐based dendrimers have been prepared and characterized by ¹H and ¹³C NMR spectroscopies, mass spectrometry, and elemental analysis. Based on DSC studies, polarizing microscopy, and powder XRD, two of these dendrimers, containing linkers with an odd number of carbon atoms, were observed to display columnar liquid–crystalline phases during thermal treatment. However, the other two dendritic analogues, containing linkers with an even number of carbon atoms, were not observed to behave correspondingly. Based on computer simulation, we reasonably assume that the dendrimers with an odd number of carbon atoms in their linkers distort their molecular shape and adopt two isomeric structures due to asymmetrical congestion. This reduces the molecular π–π face‐to‐face interaction, which in turn causes the dendrimers to form columnar LC phases during thermal treatment. However, the dendrimers with an even number of carbon atoms in their linkers have more symmetrical skeletons and do not display any liquid–crystalline phase upon thermal treatment. This new strategy should be applicable for eliciting the columnar liquid–crystalline properties of other types of unconventional dendrimers with rigid frameworks.     

The Convergent Route to Globular Dendritic Macromolecules: A Versatile Approach to Precisely Functionauzed Three-Dimensional Polymers and Novel Block Copolymers

The convergent growth approach to dendritic macromolecules is a versatile method for the preparation of globular molecules with highly controlled three-dimensional architectures. The method, inspired from a classical organic disconnection approach, starts growth of the globular dendrimer at what will become its chain-ends and proceeds toward what will become its center. The convergent growth has been applied to the preparation of a number of unconventional dendritic block copolymers as well as hybrid globular-linear copolymers that are not readily accessed by other routes. Control of the chemistry and the precise location of the chain-ends in convergent dendrimers is essential for the preparation of micellar, amphiphilic, dipolar, or other structures that may be useful in applications as varied as drug delivery and molecular devices.     

Supramolecular perspectives in colloid science

Supramolecular chemistry puts emphasis on molecular assemblies held together by non-covalent bonds. As such, it is very close in spirit to colloid science which also focuses on objects which are small, but beyond the molecular scale, and for which other forces than covalent bonds are crucial. We discuss in this review the preparation and properties of new colloidal systems which borrow on the one hand from classical topics in colloid science, such as micellization, and on the other hand from concepts in supramolecular chemistry, such as reversible supramolecular polymers.     

Macromolecular architecture for the twenty first century-new products from old monomers

Due to the high entry costs for new monomers, polymer products for the 21st century will mainly consist of the monomer mix we have now. The demand for new properties will therefore have to be satisfied by changes in macromolecular architecture. Examples which are discussed include: Block polymers (AB, ABA, (AB)_n, ABC, tapered), star polymers (rigid, block, fexible), ladder polymers, graft or comb polymers (from macromonomers), hyperbranched polymers, hypercrosslinked polymers, sequential copolymers. and starburst dendrimers. In addition, control of chain stereo chemistry will increasingly come into play. Synthetic techniques for obtaining these architectural forms are discussed as well as, potential uses for these new products.     

Fluorinated dendrimers

After pioneering works concerning the synthesis of dendrimers and dendrons possessing fluorine atoms in some part of their structure, recent advances are dedicated to the search for uses and applications of these highly branched compounds. Three major fields are concerned: catalysis, materials science and biology to a lesser extend.     

Unicorns in the world of chemical bonding models

The appearance and the significance of heuristically developed bonding models are compared with the phenomenon of unicorns in mythical saga. It is argued that classical bonding models played an essential role for the development of the chemical science providing the language which is spoken in the territory of chemistry. The advent and the further development of quantum chemistry demands some restrictions and boundary conditions for classical chemical bonding models, which will continue to be integral parts of chemistry.     

Copper-containing dendromesogens: the influence of the metal on the mesomorphism

The synthesis and liquid crystalline behaviour of the first and second generations of a dendrimeric structure based on poly(propyleneimine)(DAB-dendr(NH₂)_x) are reported. 4-(4-n-Alkoxybenzoyloxy)salicylaldehydes are used as mesogenic moieties attached at the peripheral amino groups of the dendrimers giving rise to dendromesogens with four and eight mesogenic branches. From these dendromesogens, considered as organic ligands, were prepared six metal-containing dendrimers which incorporate two or four copper atoms in their structures. All the dendrimeric ligands and three of the metal-containing dendrimers exhibit liquid crystalline properties which were studied by optical microscopy, DSC, X-ray diffraction and EPR spectroscopy.