Other Hyperbranched Polymers

The degree of branching was initially reported to be close to 0.8 [1], but was recently reevaluated after it was shown that the hydroxy-functional hyperbranched polyesters undergo facile acetal formation. The acetal formation was catalyzed by residual trace amounts of acid remaining in the sample. After reevaluation in DMSO the degree of branching was close to 0.45 which is in accordance with most other hyperbranched polymers. (1. Malmstrom, E., Johansson, M. and Hult, A. Macromolecules, 28, 1698 (1995) 2. Malmstrom, E., Trollsas, M., Hawker, C.J., Johansson, M. and Hult, A. Polym. Mat. Sci. Eng., 77,151 (1997). 

Other hyperbranched polymers showed similar absorption and luminescence properties. Upon photoexcitation, the hb-PA solutions emitted deep-blue to blue-green lights, whose intensities were higher than that of poly(l-phenyl-l-octyne), a well-known highly emissive polyene. The PL efficiencies of the polymers varied with their molecular structures. Polymers hb-P(38-VI), hfo-P(45-V), hb-P(48-VI), M>-P(50-VI), fcfo-P(50-VII) and hb-P(59-VI) exhibited (P values higher than 70%, with hfc-P(50-VII) giving the highest value of 98%. 

On the other hand, hyperbranched polyimides not only have the features of other hyperbranched polymers (e.g. low viscosity, good solubility) but also possess high thermal... 

AB2 reacts selectively with only one antagonist function of a second polyfunctional molecule, the other ones being protected81 (Fig. 5.16). The perfect hyperbranched molecules obtained according to that step-by-step process are called dendrimers. The degree of branching characterizes the structure of a hyperbranched polymer and has been defined by Hawker et al.82 as... 

Often in hyperbranched polymers obtained via SCVP, it is not possible to determine the DB directly via NMR analysis. Therefore, other methods, for example, viscosity measurements and light-scattering methods have to be used to confirm the compact structure of a hyperbranched polymer. Such characterizations of hyperbranched (meth)acrylates will be discussed in the next section. 

Since that time, synthetic chemists have explored numerous routes to these statistically hyperbranched macromolecular structures. They are recognized to constitute the least controlled subset of structures in the major class of dendritic polymer architecture. In theory, all polymer-forming reactions can be utilized for the synthesis of hyperbranched polymers however, in practice some reactions are more suitable than others. 

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. 

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. 

The dilution properties of hyperbranched polymers also differ from those of linear polymers. In a comparison between two alkyd resin systems, where one was a conventional high solid alkyd and the other based on a hyperbranched aliphatic polyester, the conventional high solid alkyd was seen to exhibit a higher viscosity [113]. A more rapid decrease in viscosity with solvent content was noted for the hyperbranched alkyd when the polymers were diluted. 

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. 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

The AB2 + AB system is equivalent to AB2 except that AB2 units are separated from each other by AB units. The AB2 + B3 system modifies the AB2 system by using B3 as a central core from which polymerization radiates and offers greater control of molecular shape. The A2 + B3 system is one of the standard systems used to produce crosslinked polymers (Sec. 2-10). It is useful for synthesizing hyperbranched polymers only when crosslinking is minimized by limiting conversion and/or diluting the reactants with solvent. 

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. 

In the beginning, the term dendrimer , which was established by Tomalia in 1985 [42,43], described all types of dendritic polymers. Later a distinction based on the relative degree of structural control present in the architecture was drawn. Nowadays, many other types of dendritic architectures are known, even if most of them, however, have not yet been widely investigated and fully characterized. The term dendritic polymer involves four substructures (Fig. 2), namely dendrimers themselves, dendrons, random hyperbranched polymers, and dendrigraft polymers [44, 45],... 

The third International Dendrimer Symposium took place at Berlin Technical University in 2003. Interdisciplinary lectures demonstrated the extent to which dendritic molecules branch ouf into other areas of science, such as physics, biology, medicine, and engineering. The possibilities of functionalisation and resulting applications in industry were at the focus of this symposium. For example, nano-dimensioned dendrimer-based contrast agents were presented as multilabels for visualisation of blood vessels (see Chapter 8). Potential applications of dendritic materials as luminescence markers in diagnostics attracted lively interest (see Chapter 8). Consideration of the differences between dendrimers and hyperbranched polymers from the viewpoint of their cost-favourable application was also a topic of discussion [18]. 

Calculations at the MP2/6-31G //MP2/6-31G level indicates that the C2h fully planar structure is about 8 kcal/mol more stable than the C2V perpendicular form (a transition state). Likewise, condensation reactions with isatins have suggested the involvement of the superelectrophile 86 (eq 21).40 This superelectrophilic chemistry has been successfully applied in the preparation of hyperbranched polymers and other macromolecules 40b d Other... 

Notably, while the most common complexes contain the two components in a 1 1 ratio, other ratios are possible depending on relative concentrations of the components along with steric requirements. This is demonstrated by the 1 2 complex 78 <2003PCA4669> and the extended hyperbranched polymer 79 <2004JA14738>. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

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