Hyperbranched polymers applications

Key words vegetable oil-based hyperbranched polymers, preparation of hyperbranched polymers, structure-property relationships of hyperbranched polymer, applications of hyperbranched polymers. 

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. 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

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. 

For applications of hyperbranched polymers as precursors the polymer networks, the following structural features are important ... 

For the application of hyperbranched polymers as precursors in network formation, the functionality averages are important. The number-average functionality is given by the equation... 

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. 

A shortcoming with condensation polymers, is their sensitivity towards hydrolysis, which might restrict the use of such polymers in certain applications. For that reason, some hyperbranched polymers are synthesized via substitution or ring opening reactions that provide more hydrolytically stable polymers. 

Dendrimers/dendrons are synthesized almost exclusively via elaborate synthetic procedures that are usually more costly than hyperbranched processes. Even though they display many unique properties, their higher costs do not always justify use in some applications. Consequently, hyperbranched polymers may serve as a more cost-effective alternative when optimum properties are not required. 

The first strategies to random hyperbranched polymers involved exclusively step-growth polymerizations. This limited the potential applications for these architectures to areas where only condensation-type polymers are acceptable. Frechet et al. [21] presented the first example of a hyperbranched vinyl polymerization in 1995, ] initiating the birth of a second generation of hyperbranched... 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

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. 

A majority of the hyperbranched polymers reported in the literature are synthesized via the one-pot condensation reactions of A B monomers. Such one-step polycondensations result in highly branched polymers even though they are not as idealized as the generation-wise constructed dendrimers. The often very tedious synthetic procedures for dendrimers not only result in expensive polymers but also limit their availability. Hyperbranched polymers, on the other hand, are often easy to synthesize on a large scale and often at a reasonable cost, which makes them very interesting for large-scale industrial applications. 

Numerous applications have been suggested for hyperbranched polymers but few or none have yet reached full commercial exploitation. Only a few papers have been published that address certain apphcations of hyperbranched polymers. 

The lack of mechanical strength for thermoplastic hyperbranched polymers makes them more suitable as additives in thermoplast applications. Hyperbranched polyphenylenes have been shown to act successfully as rheology modifiers when processing linear thermoplastics. A small amount added to polystyrene resulted in reduced melt viscosity [31]. (Sect> 3.1). 

An important application of polymers in medicine is in advanced drug-delivery systems. These materials control the drug concentration and delivery rate in the body. Hyperbranched polyesters have been suggested for such systems [111]. However, most applications within this field, described in the literature, deal with dendrimers and not with hyperbranched polymers. 

An interesting study that was performed on dendrimers is also applicable to hyperbranched polymers. Roberts et al. [134] studied the effect of the dendrim-er size when used inside the human body. They found that large dendrimers (M ca. 87,000) were passed into the urine and excreted within two days. Smaller dendrimers (M ca. 5,000), on the other hand, accumulated mostly in the liver, kidney and spleen with no urine excretion. Since most hyperbranched polymers are polydisperse, this might create a problem for in vivo applications. 

During recent years a number of papers have been presented where a hyperbranched polymer has been designed for a special application. One of the most... 

For most purposes, polydispersity is not an obstacle, and currently the potentially cheaper hyperbranched polymers are successfully entering industrial research and application. Nevertheless, their success is based on the know-how built up in research on dendrimers. 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

How do dendrimers and hyperbranched polymers compare from an industrial viewpoint Dendrimers offer the potential for producing polymers whose molecular size and structure are more regular and less polydisperse. 1 lyperbranched polymers are easier and cheaper to synthsize—a one-pot synthesis compared to the multipot synthesis for dendrimers. However, not too many AB/ monomers are readily available, and this may modify the overall economics. Hyperbranched polymers will probably find use in larger-scale or commodity applications where lower cost is a necessity and dendrimers in specialty applications where higher cost is justified. 

By modifying the functional groups they can be used,for example, as crosslinkers in high solid or powder coatings and in thermosets. Because of their good miscibility and low melt viscosity, they find applications as melt modifiers and as blend components. Modified hyperbranched polymers, like alkyl chain substituted poiy(ether)s and po-ly(ester)s sometimes exhibit amphiphilic behavior.They can, therefore, be used as carriers for smaller molecules,for example, dyestuff into polypropylene. 

Dendrimers and hyperbranched polymers are two groups of materials resembling each other. The architectural difference is that dendrimers are perfectly branched structures, while hyperbranched polymers contain defects. Dendrimers are mono-dispersed while hyperbranched polymers are more dispersed which can be an advantage in some applications. 

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. 

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