Dendrimers and Dendritic Polymers In recent years, a new structural class of macromolecules, the dendritic polymers, has attracted the attention of the scientific community. These nanometer-sized, polymeric systems are hyper-branched materials having compact hydrodynamic volumes in solution and high surface, functional group content. They may be water-soluble but, because of their compact dimensions, they do not have the usual rheological thickening properties that many polymers have in solution. Dendrimers, the most regular members of the class, are synthesized by step-wise convergent or divergent methods to give distinct stages or generations.
The Three Components that Define DendrimersDendrimers are defined by their three components: a central core, an interior dendritic structure (the branches), and an exterior surface (the end groups). Over 50 compositionally different families of these nanoscale macromolecules, with over 200 end-group modifications, have been reported. They are characterized by nearly spherical structures, nanometer sizes, large numbers of reactive end group functionalities, shielded interior voids, and low systemic toxicity.
Potential Industry Applications for DendrimersThis unique combination of properties makes them ideal candidates for nanotechnology applications in both biological and materials sciences. The spate of reports in the current literature has been directed toward their applications in a broad range of fields, including materials engineering, industrial, pharmaceutical, and biomedical applications. Specifically, nanoscale catalysts, novel lithographic materials, rheology modifiers, and targeted drug delivery systems, MRI contrast agents, and bioadhesives represent some of the potential applications.
Polyhedral Silsesquioxanes - or Inorganic-Organic Hybrid Nanoparticles Hybrid inorganic-organic composites are an emerging class of new materials that hold significant promise. Materials are being designed with the good physical properties of ceramics and the excellent choice of functional group chemical reactivity associated with organic chemistry. New silicon-containing organic polymers, in general, and polysilsesquioxanes, in particular, have generated a great deal of interest because of their potential replacement for, and compatibility with, currently employed, silicon-based inorganics in the electronics, photonics, and other materials technologies.
Silsesquioxanes - Network Polymers or Polyhedral ClustersHydrolytic condensation of tri-functional silanes yields network polymers or polyhedral clusters, having the generic formula (RSiO1.5)n. Hence they are known by the "not quite on the tip of the tongue" name, silsesquioxanes. Each silicon atom is bound to an average of one and a half (sesqui) oxygen atoms and to one hydrocarbon group (ane). Typical functional groups that may be hydrolyzed/condensed include alkoxy- or chlorosilanes, silanols, and silanolates.
Controlling Molecular Scale Regularity and External Morphology in Inorganic/Organic HybridsSynthetic methodologies that combine pH control of hydrolysis/condensation kinetics, surfactant-mediated polymer growth, and molecular templating mechanisms, have been employed to control molecular scale regularity as well as external morphology in the resulting inorganic/organic hybrids (from transparent nanocomposites, to mesoporous networks, to highly porous and periodic organosilica crystallites), all of which have the silsesquioxanes (or RSiO1.5) stoichiometry.
The Improved Material Properties that Silsesquioxanes Can OfferThese inorganic-organic hybrids offer a unique set of physical, chemical, and size dependent properties that could not be realized from just ceramics or organic polymers alone. Silsesquioxaness are therefore depicted as bridging the property space between these two component classes of materials. Many of these silsesquioxanes hybrid materials also exhibit an enhancement in properties such as solubility, thermal and thermomechanical stability, mechanical toughness, optical transparency, gas permeability, dielectric constant, and fire retardancy, to name just a few.