Dendritic and Hyperbranched Polymers

Dendrimers are a relatively new class of macromolecules different from the conventional linear, crosslinked, or branched polymers. Dendrimers are particularly interesting because of their nanoscopic dimensions and their regular, well-defined, and highly branched three-dimensional architecture. In contrast to polymers, these new types of macromolecules can be viewed as an ordered ensemble of monomeric building blocks. Their tree-like, monodispersed structures lead to a number of interesting characteristics and features globular, void-containing shapes, and unusual physical properties [107-111]. 

Spontaneous, Noncentrosymmetric Organization of NLO Chromophores in Dendritic Structures (NLO Dendritic Effect at the Moiecuiar Levei) 

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Star and miktostar polymers have been synthesized by living cationic polymerization using dienes and trienes as coupling agents and/or multifunctional initiators [Faust and Shaffer, 1997 Hadjichristidis, et al., 1999 Hadjikyriacou and Faust, 2000 Kennedy and Jacob, 1998 Puskas et al., 2001 Sawamoto, 1991]. Multifunctional halides such as hexaiodo-methylmelamines have also been used to obtain star and comb polymers [Ryu and Hirao, 2001 Zhang and Goethals, 2001], Hyperbranched and dendritic polymers have also been studied. 

Dendritic macromolecules exhibit compact globular structures which lead to their low viscosity in the melt or in solution. Furthermore, dendritic macromolecules are characterized by a very large number of available functional groups, which lead to unprecedented freedom for changing/tuning/tailoring the properties of these multivalent scaffolds via complete or partial derivatization with other chemical moieties. All these features have contributed to multidisciplinary applications of these unique macromolecular structures in recent years 6, 7). The development of efficient synthetic routes in recent years has given rise to a virtually unlimited supply of commercially available dendritic polymers, at very affordable price. The transport properties of hyperbranched and dendritic polymers have recently attracted attention as potentially new barrier and membrane materials 8-9). 

This cutting-edge reference supplies the very latest advances in research on star, hyperbranched, and dendritic polymers—providing design strategies needed for a wide variety of industrial applications. 

Hyperbranched and dendronized polymers such as 40, 41, and 42 have also been synthesized using the transition metal coupling strategies in recent years.32 These polymers are fundamentally different from those traditional linear polymers. They possess dendritic arms within die polymer or along the polymer backbone. It is believed that they possess interesting properties and have potential applications in many fields such as nanotechnology and catalysis ... 

While it can be expected that a number of physical properties of hyperbranched and dendritic macromolecules will be similar, it should not be assumed that all properties found for dendrimers will apply to hyperbranched macromolecules. This difference has clearly been observed in a number of different areas. As would be expected for a material intermediate between dendrimers and linear polymers, the reactivity of the chain ends is lower for hyperbranched macromolecules than for dendrimers [125]. Dendritic macromolecules would therefore possess a clear advantage in processes, which require maximum chain end reactivity such as novel catalysts. A dramatic difference is also observed when the intrinsic viscosity behavior of hyperbranched macromolecules is compared with regular dendrimers. While dendrimers are found to be the only materials that do not obey the Mark-Houwink-Sakurada relationship, hyperbranched macromolecules are found to follow this relationship, albeit with extremely low a values when compared to linear and branched polymers [126]. 

Reichert and Mathias prepared related branched aramids, to those of Kim,t5-34] from 3,5-dibromoaniline (23) under Pd-catalyzed carbonylation conditions (Scheme 6.7). These brominated hyperbranched materials (24) were insoluble in solvents such as DMF, DMAc, and NMP, in contrast to the polyamine and polycarboxylic acid terminated polymers that Kim synthesized, which were soluble. This supports the observation that surface functionality plays a major role in determining the physical properties of hyperbranched and dendritic macromolecules J4,36 A high degree of cross-linking could also significantly effect solubility. When a four-directional core was incorporated into the polymerization via tetrakis(4-iodophenyl)adamantanc,1371 the resultant hyperbranched polybromide (e.g., 25) possessed enhanced solubility in the above solvents, possibly as a result of the disruption of crystallinity and increased porosity. 

The use of hyperbranches as primers or as crosslinking agents for the epoxy and PU adhesives resulted in improved shear strength and higher adhesion durability (wedge test results). The modification of PU and Epoxy systems with the hyperbranched or dendritic polymers was effective using small amounts. HB polymers were more effective than the dendrimers for shear strength improvement. 

The dendritic polymer literature reviewed herein provides compelling evidence that these materials are a unique and versatile class of branched polymers. The synthesis of dendritic macromolecules can be accomplished by numerous methods allowing for specific tailoring of the characteristics of the polymer, to yield desired properties or functionality. While some of the procedures reported are quite intricate, requiring multiple cycles of synthetic steps and work-up, one-pot syntheses have also been developed for hyperbranched and dendrigraft polymers, making these materials more viable for (large-scale) commercial production and industrial applications. 

Due to their high molecular masses, macromolecular substances (polymers) show particular properties not observed for any other class of materials. In many cases, the chemical nature, the size, and the structure of these giant molecules result in excellent mechanical and technical properties. They can display very long linear chains, but also cyclic, branched, crosslinked, hyperbranched, and dendritic architectures as well. The thermoplastic behaviour or the possibility of crosslinking of polymeric molecules allow for convenient processing into manifold commodity products as plastics, synthetic rubber, films, fibres, and paints (Fig. 1.1). 

Aerts, J. Prediction of intrinsic viscosities of dendritic, hyperbranched and branched polymers. Computational and Theoretical Polymer Science, 8,49-54 (1998). 

Comprehensive reviews of nomenclature for structure-based and source-based representations for dendritic, hyperbranched, and hypercrosslinked polymers, " " and for star and star-block polymers, have been completed recently. 

The liquid crystalline unit-containing polymers begin a timid entry in the TPE field. The liquid crystalline sequences can be part of the backbone or may be present as side chains. Nair et al. [200,201] carried out an abundant and very valuable work with both fundamental and applied aspects. These polymers are prepared by chain polymerization or by polycondensation and are discussed in Chapter 2. Some of them are blends and the morphology and properties of Rodrun LC3000 were the subject of two articles [202,203]. The first applications and patented products are very promising and their development should enjoy a rapid increase. This holds also for the introduction of hyperbranched and dendritic segments in block copolymers [204]. 

A comparison of the physical properties of hyperbranched and dendritic macromolecules with linear polymers and the linear analogs of these 3-dimensional polymers is presented. It is found that thermal properties, such as glass transition temperature and degradation, are the same regardless of the macromolecular architecture but are very sensitive to the number and nature of chain end functional groups. However, other properties, such as solubility, melt viscosity, chemicd reactivity, intrinsic viscosity were found to be very dependent on the macromolecular architecture. 

If stars or dendrimers contain functional groups on the terminals of the arms, they react much faster than linear polymers to form thermoset coatings. Figure 9 shows schematic drawings of linear, branched, star, and hyperbranched (or dendritic) polymers. (Dendritic polymers are special hyperbranched materials with very ordered stractures.)... 

Synthesis of complex polymeric molecules such as random and block copolymers, star and graft polymers, hyperbranched and dendritic structures by NMP and other CRP techniques has been reviewed several times. Good overviews of telechelic polymers or the coupling of NMP with other polymerization techniques are available. Synthesis of bioconjugates through CRDRP, including NMP, is treated by Nicolas et... 

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