Hyperbranched materials polymer properties

The hydroxyfunctional hyperbranched polyesters have been characterized with respect to their mechanical and theological properties, both as thermoplastics and in cross-linked networks. The high number of terminal groups in hyperbranched polymers has a large impact on the properties, and also makes it easy to functionalize the polymers for various applications. One option is to attach reactive groups at chain ends, forming a cross-linkable polymer. Variations in functionality and the type of functional groups will affect both the polymer properties and the final cross-linked material properties. 

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 general area of dendritic and hyperbranched polymers has received remarkable attention over the past decade. New properties not available with linear polymers have been demonstrated. For example, evidence has been provided that supports the existence of considerable space for the encapsulation of small molecules, and this has led to the idea of a dendritic box" (69j. A severe problem with dendrimers is their timesynthetic methods that form hyperbranched materials that may exhibit many of the advantageous properties of dendritic macromolecules have been receiving significant attention (70j. 

In order to compare general properties of hyperbranched polymers and dendrimers, Wooley et al. examined a model hyperbranched polyester and corresponding dendrimer. Pol)miers prepared from 3-hydroxy-5-( eri-butyldi-methylsiloxy)benzoic acid, as branching point, showed that thermal properties, such as Tg and those shown by thermogravimetic analysis (TGA), were independent of pol)mier architecture. However, the dendritic and hyperbranched materials demonstrated comparative solubilities that were much greater than that found for the linear polymer [99]. Their conclusions on the thermal properties may contradict some other findings. For examples, the of hyper-... 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Dendrimers with terminal functional groups represent mode compact precursors that are spherical and almost monodisperse, with reactive groups placed on their periphery. Their synthesis, structure and properties have been reviewed in monographs and review articles often together with hyperbranched polymers (cf., e.g. [15-20]), as well as in this book. Application of dendrimers as precursors for conventional materials is limited at this time by their relatively high cost. 

Use of dendritic molecules (mainly hyperbranched polymers) as precursors of a crosslinked (engineering) material in order to modify its processing and materials properties. 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

Despite the simplicity of chemistries presently employed for the commercially available hyperbranched polymers, these materials are still relatively high priced compared to traditional commodity polymers. This is undoubtedly related to the early stage, low volume demands for these products. Hyperbranched polymers will evolve as substitutes for traditional polymers as their unique properties are used to greatly enhance products on a cost-performance basis. In summary, the enhanced used of hyperbranched polymers in engineered products will depend upon many of the following prerequisites ... 

The lower cost of synthesizing hyperbranched polymers allows them to be produced on a large scale, giving them an advantage over dendrimers in applications involving large amounts of material, although the properties of hyperbranched polymers are intermediate between those of dendrimers and linear polymers [33]. 

We will focus on the variety of different hyperbranched polymers that have been synthesized, on the specific properties that hyperbranched polymers exhibit, and hopefully stimulate the reader to find new and unique areas where these novel materials can find future applications. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

The urge of polymer scientists to develop new materials is driven by society s wish to substitute conventional materials by plastics and thereby gain in performance. One reason for the emerging interest in hyperbranched polymers and other macromolecular architectures is their different properties compared to conventional, linear polymers. 

Hyperbranched polymers is a young and rapidly growing area within the field of macromolecules. The special properties of these polymers are now clearly described and a number of interesting appUcations of the hyperbranched polymers will bring them to the marketplace. The future looks bright for these materials. 

Very recently, highly regular, highly controlled, dense branching has been developed. The resulting dendrimers often have a spherical shape with special interior and surface properties. The synthesis and properties of dendrimers has been reviewed (see e.g. G.R. Newkome et al. Dendritic Molecules , VCH, 1996). In this series, a chapter deals with the molecular dimensions of dendrimers and with dendrimer-polymer hybrids. One possible development of such materials may be in the fields of biochemistry and biomaterials. The less perfect hyper-branched polymers synthesized from A2B-type monomers offer a real hope for large scale commercialization. A review of the present status of research on hyperbranched polymers is included. 

Finally, first attempts were made to extend the concept of soluble hyperbranched polymers to dendronized sohd-phase materials. Recently, the first dendronized sohd phase, accessible in only one reaction step was reported (Fig. 7.4) [36]. The coupling of hyperbranched polyglycerol to Merrifield resin yields a new type of high-loading sohd-phase hybrid material with loading capacities of ca. 3 mmol g and good swelling properties even in protic solvents (see also Section 3.4). 

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]. 

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