Dendrimers branched polymers

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

Dendrimers, a relatively new class of macromolecules, differ from traditional Hnear, cross-Hnked, and branched polymers. The conventional way of introducing an active moiety into polymers is to Hnk it chemically into the polymeric backbone or a polymer branch. This synthetic approach results in a topologically complex material. Therefore, a significant effort has to be devoted to improve the structural complexities and functions of the polymers. 

The history of dendrimer chemistry can be traced to the foundations laid down by Flory [34] over fifty years ago, particularly his studies concerning macro-molecular networks and branched polymers. More than two decades after Flory s initial groundwork (1978) Vogtle et al. [28] reported the synthesis and characterization of the first example of a cascade molecule. Michael-type addition of a primary amine to acrylonitrile (the linear monomer) afforded a tertiary amine with two arms. Subsequent reduction of the nitriles afforded a new diamine, which, upon repetition of this simple synthetic sequence, provided the desired tetraamine (1, Fig. 2) thus the advent of the iterative synthetic process and the construction of branched macromolecular architectures was at hand. Further growth of Vogtle s original dendrimer was impeded due to difficulties associated with nitrile reduction, which was later circumvented [35, 36]. This procedure eventually led to DSM s commercially available polypropylene imine) dendrimers. 

Unlike linear or ordinary branched polymers, dendrimers display intrinsically low viscosity, even at high mass. As standard polymeric molecules increase in mass and size, their viscosity normally increases continually. With dendrimers, viscosity increases only up to about the fourth generation, after which it actually begins to decline (Mourey, 1992 Frechet, 1994). In addition, with control over the type of pendent groups that adorn the surface, dendrimers can maintain high solubility regardless of size. 

A prerequisite for the application of filtration methods is a significant difference in molecular size of the catalyst and the reactants / products. Molecular enlargement, i.e. binding the homogeneous catalyst to soluble supports, is often the method of choice. These supports can be dendrimers, hyper-branched polymers or even simple polymers, giving the opportunity to tailor the support according to the given process. 

Several groups have engaged in the study of linear polymer-dendrimer conjugates (i.e. architectural copolymers). These structures combine block copolymer and dendrimer branching features within one molecular architecture [69-71],... 

The synthesis of well-defined LCB polymers have progressed considerably beyond the original star polymers prepared by anionic polymerization between 1970 and 1980. Characterization of these new polymers has often been limited to NMR and SEC analysis. The physical properties of these polymers in dilute solution and in the bulk merit attention, especially in the case of completely new architectures such as the dendritic polymers. Many other branched polymers have been prepared, e.g. rigid polymers like nylon [123], polyimide [124] poly(aspartite) [125] and branched poly(thiophene) [126], There seems to be ample room for further development via the use of dendrimers and hyperbran-... 

Ferrocene-based Branched Polymers (Dendrimers). One of the topics in macromolecular chemistry is constituted by dendrimers, or hyperbranched macromolecules of tridimensional globular structure, the surface of which is characterized by a large number of functional groups, Scheme 4. Such functionalities impart to the molecules solubility, viscosity and thermal properties different from those of the common linear polymers.38c,d 44... 

Fig.1. Schematic description of dendritic polymers comprising dendrimers and hyper-branched polymers...

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. 

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. 

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]. 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

The architecture of hypeibranched polymers and dendrimers is connected with difficulties in determining molar mass. Many of the common characterization techniques—e.g. size exclusion chromatography (SEC)—used for polymers are relative methods where polymer standards of known molar mass and dispersity are needed for calibration. Highly branched polymers exhibit a different relationship between molar mass and hydrodynamic radius than their linear counterparts. 

A hyperbranched polymerization from solid support was developed as a one-pot synthesis for branched polymers that mimic dendrimers (Scheme... 

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