Hyperbranched polymers structural features

The interest in hyperbranched polymers arises from the fact that they combine some features of dendrimers, for example, an increasing number of end groups and a compact structure in solution, with the ease of preparation of hn-ear polymers by means of a one-pot reaction. However, the polydispersities are usually high and their structures are less regular than those of dendrimers. Another important advantage is the extension of the concept of hyperbranched polymers towards vinyl monomers and chain growth processes, which opens unexpected possibilities. 

Owing to multi-functionahty, physical properties such as solubihty and the glass transition temperature and chemical functionahty the hyperbranched (meth) acrylates can be controlled by the chemical modification of the functional groups. The modifications of the chain architecture and chemical structure by SCV(C)P of inimers and functional monomers, which may lead to a facile, one-pot synthesis of novel functionahzed hyperbranched polymers, is another attractive feature of the process. The procedure can be regarded as a convenient approach toward the preparation of the chemically sensitive interfaces. 

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

The search for an efficient and non-toxic gene transfection vector has led to the design and synthesis of a great variety of macromolecular scaffolds. An extensive analysis of the key features for the efficient and safe delivery of genes in vivo and in vitro has led to the conclusion that hyperbranched polymers are potential candidates for further development. In this chapter we have presented a detailed analysis of the different hyperbranched polymer scaffolds commonly used in gene delivery applications. Several structural modifications toward the development of an optimal gene vector have been analyzed. 

Figure 5.23. The general strategy (a) and examples (It-c) of hyperbranched polymer network structures featuring a siloxane crosslinkage. Reproduced with permission from Meijer, D. Dvornic, R R. Fall 2005 ACS meeting, Midland, Ml.

Recently the development of dendritic and hyperbranched polymers (HBPs) has attracted much attention (Tomalia, 1985, Newkome et al, 1985, Webster, 1991, Chu and Hawker, 1993, Wooley et al, 1994, Feast and Stanton, 1995, Malmstrom et al, 1995, Kim, 1998). The key features of the macromolecular architecture of dendrimers and HBPs are given in Section 1.2, and their synthesis by stepwise polymerization is discussed in Section 1.2.1. Dendrimers and HBPs are globular macromolecules that have a highly branched structure with multiple reactive chain ends (shell), which converge to a central focal point (core) see Figure 5.1, where I is the core, 11 is the structure and 111 is the shell. 

Hyperbranched polymers also possess a dendritic architecture, but with imperfect branching. The basic structural features present in these molecules are the same as in dendrimers, namely, a core surrounded by layers of BC capped with terminal units. The one-pot syntheses used to create these treelike stmctures also rely upon AB -type monomers (Scheme 30.1), but without protecting groups preventing simultaneous condensation reactions. The resulting polymers typically have broad MWD ( ) > 2), with multiple isomers and geometries. Because they are created in a single reaction step, hyperbranched polymers are more economical to produce than dendrimers as their synthesis is less time and resource intensive. This trait represents a major draw for industry and the development of commercial applications for dendritic polymers... 

Here, the main challenge derives from the eponymous structural feature of these polymers - the hyperbranching - which is closely connected to the occurrence of branching subunits in the polymer structure. 

Vegetable oil-based hyperbranched polymers possess unique structural features which give them a wide number of potential applications in different fields. These include nanoscopic size, spheroidal surface, high... 

Precisely branched polymers include hyperbranched polymers, dendrimers and den-drons. Dendrimers and dendrons are characterized by perfectly controlled structures in three dimensions such as tree branch architecture, and they have attractive features such as a well-ordered chemical structure, molecular mass, size and configuration of polymers [5], Although the precise order of shape of hyperbranched polymers is less than that of dendrimers and dendrons, hyperbranched polymers have unique properties such as low viscosity attributed to the lack of entanglanent of polymer segments, and the possibility of chemical modification in terminal functional groups such as in dendrimers [1-3]. 

In this chapter we consider the properties of synthetic polymers. First, the main techniques of polymer synthesis are outlined (Section 2.2). Then the conformation of polymer molecules is discussed in Section 2.3. We move on to a summary of the main methods for characterization of polymeric materials in Section 2.4. Then the distinct features of the main classes of polymer are considered, i.e. solutions (Section 2.5), melts (and glasses) (Section 2.6) and crystals (Section 2.7). Then the important properties of plastics (Section 2.8), rubber (Section 2.9) and polymer fibres (Section 2.10) are related to microscopic structure and to rheology. Polymer blends and block copolymers form varied structures due to phase separation, and this is compared and contrasted for the two types of system in Section 2.11. Section 2.12 is concerned with dendrimers and hyperbranched polymers. Section 2.13 and 2.14 deal with polyelectrolytes and (opto)electronic polymers respectively. 

There are a number of examples of macromolecules with very complex structural features such as stars, dendrimers, hyperbranches, and crosslinks. Each of fliese classes of materials comes with its own advantages and disadvantages in terms of synthesis, processability, and properties. The crosslinked polymer 64 was prepared by the thermal copolymerization of ferrocenophanes. The swelling properties of the crosslinked polymers allowed for determination of the solubility parameter of the analogous linear homopolymer. The crosslinked polymers were foimd to possess increased thermal stabihty relative to their linear analoges. 

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). 

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