Dendrimers hyperbranched polymers

Instead of the step-wise and tedious procedures used for synthesis of perfect dendrimers, hyperbranched polymers are prepared in a one-step synthetic strategy [87],... 

Hyperbranched polymers differ greatly from linear or moderately branched polymers. For example, the solubility is much higher for hyperbranched polymers but not as high as for dendrimers. Hyperbranched polymers normally exhibit an amorphous, nonentangled behavior, i.e., a Newtonian behavior in the melt. The nonentangled state also makes hyperbranched polymers rather brittle. Several thermoset resin materials have been described where the hyperbranched polymer exhibits a low resin viscosity, thereby reducing the need for solvents to attain the application viscosity. At the same time. 

Compare dendrimers, hyperbranched polymers, and moderately branched polymers with regard to their molecular architecture. What differences in properties of these polymers would be expected in view of their structural differences Describe several alternative methods of synthesizing hyperbranched polymers and the reactions involved in such syntheses. 

These problems might be overcome by using randomly branched polymer structures as supports [ 13,82,112]. In contrast to dendrimers, hyperbranched polymers are easily available in one reaction step. This allows the production of large quantities of material [82]. They contain dendritic, linear and terminal monomer units in their skeleton and hence can be considered as inter me-... 

A graphical comparison of the structure of dendrons, dendrimers, hyperbranched polymers, and dendrigraft polymers is shown in Figure 30.1. The stepwise synthesis of dendrimers involves multiple cycles of protection, condensation, and deprotection reactions to produce strictly... 

Some of the procedures used to synthesize dendritic molecules can be quite involved, and many different methods have been reported. The following three sections provide an overview of some of the pioneering work yielding each of the three main subclasses of dendritic polymers, namely, dendrimers, hyperbranched polymers, and dendrigraft polymers. The general characteristics and properties of these materials are also considered, as well as some of the more recent work including potential applications for these extremely versatile materials. 

Scanning electron microscopy (SEM) is one of the very useful microscopic methods for the morphological and structural analysis of materials. Larena et al. classified nanopolymers into three groups (1) self-assembled nanostructures (lamellar, lamellar-within-spherical, lamellar-within-cylinder, lamellar-within-lamellar, cylinder within-lamellar, spherical-within-lamellar, and colloidal particles with block copolymers), (2) non-self-assembled nanostructures (dendrimers, hyperbranched polymers, polymer brushes, nanofibers, nanotubes, nanoparticles, nanospheres, nanocapsules, porous materials, and nano-objects), and (3) number of nanoscale dimensions [uD 1 nD (thin films), 2 nD (nanofibers, nanotubes, nanostructures on polymeric surfaces), and 3 nD (nanospheres, nanocapsules, dendrimers, hyperbranched polymers, self-assembled structures, porous materials, nano-objects)] [153]. Most of the polymer blends are immiscible, thermodynamically incompatible, and exhibit multiphase structures depending on the composition and viscosity ratio. They have two types of phase morphology sea-island structure (one phase are dispersed in the matrix in the form of isolated droplets, rods, or platelets) and co-continuous structure (usually formed in dual blends). 

INTRODUCTION DENDRIMERS, HYPERBRANCHED POLYMERS, AND SUPRAMOLECULAR CHEMISTRY... 

In addition to dendrimers, hyperbranched polymers have been used by several groups as soluble supports for catalysts [7, 17]. These supports are polydisperse and randomly branched, and, since they are prepared in a single reaction step, are generally much cheaper materials. Nevertheless, it has been shown that catalysts immobilized on hyperbranched polymers may possess similar properties as dendritic systems [18]. Therefore, dendritic catalysts serve as ideal model systems for catalysts attached to hyperbranched polymers. We functionalized several hyperbranched polyethyleneimines (PEIs) employing the peptide coupling protocol in reactions with the pyrphos linker system. The pyrphos-rhodium complexes bound to the hyperbranched polymers were also found to be active catalysts for the hydrogenation ofZ-methyl-a-acetamidocinnamate [16]. As observed for the... 

Many physical properties of hyperbranched aromatic polyesters, such as high solubility and low viscosity, resemble those of dendrimers. However, in contrast to dendrimers, hyperbranched polymers do exhibit a linear dependence of intrinsic viscosity on molecular weight with very low Mark-Houwink constants (0.3-0.4) [13]. In addition, their lack of reactivity toward catalytic hydrogenolysis greatly differs fi om that of the corresponding dendrimers, which are cleanly deprotected under mild conditions rather their reactivity more resembles that of the corresponding linear polymers [59]. As shown in... 

Like dendrimers, hyperbranched polymers are usually amorphous materials with a high degree of surface functionality. They possess characteristics similar to dendrimers, such as low viscosities and high solubility. Their properties are superior to and different from those of their linear counterparts of similar MWs. The high surface functionality permits the modification of the surface groups to produce different polymers of interest. For applications where stmctural perfection can be sacrificed, the less tedious and less costly hyperbranched polymers have supplanted dendrimers that are more difficult and expensive to prepare. For detailed information on hyperbranched polymers, readers may consult the literature. 

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