Cross-linking dendritic polymers

Fig. 7.1 Schematic representation of linear versus dendritic polymers linear (left) and hyperbranched (middle) polymers, perfect dendrimer (right). The amount of terminal groups is indicated below each structure. These architectures can also be attached to a cross-linked polymer bead to obtain a high-loading hybrid material.

The affinity of Cgo towards carbon nucleophiles has been used to synthesize polymer-bound Cgo [120] as well as surface-bound Cjq [121]. Polymers involving G q [54, 68, 69] are of considerable interest as (1) the fullerene properties can be combined with those of specific polymers, (2) suitable fullerene polymers should be spin-coatable, solvent-castable or melt-extrudable and (3) fullerene-containing polymers as well as surface-bound Cgo layers are expected to have remarkable electronic, magnetic, mechanical, optical or catalytic properties [54]. Some prototypes of polymers or solids containing the covalently bound Cjq moiety are possible (Figure 3.11) [68,122] fullerene pendant systems la with Cjq on the side chain of a polymer (on-chain type or charm bracelet ) [123] or on the surface of a solid Ib [121], in-chain polymers II with the fullerene as a part of the main chain ( pearl necklace ) [123], dendritic systems III, starburst or cross-link type IV or end-chain type polymers V that are terminated by a fullerene unit For III and IV, one-, two-and three-dimensional variants can be considered. In addition, combinations of all of these types are possible. 

Figure 3.11 Prototypes of polymers involving the Qq tnoiety. Pendant on-chain (la), pendant on-surface (lb), in-chain (II), dendritic (III), cross-link (IV) and end-chain (V) [68, 122],...

Allabashi, R., M. Arkas, G. Hormann, and D. Tsiourvas. 2007. Removal of some organic pollutants in water employing ceramic membranes impregnated with cross-linked silylated dendritic and cyclodextrin polymers. Water Res. 41 476 -86. 

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. 

Dendrimers are a unique class of hyperbranched polymers with well-defined size, shape, and chemical functionality and with properties not found in classical linear and cross-linked polymers. There have been significant developments in the last decade in many areas of dendrimer research, partly due to the commercial availability of dendrimers such as PAMAM, but also the result of the synthesis of several novel dendritic structures. Numerous pharmaceutical applications have been proposed for these hyperbranched polymers, and given the rate of current developments, it is envisaged that dendrimer-based formulations will appear in the next decade. 

There are several key areas that need to be developed in order to be able to prepare supported metal complexes more reliably and better understand their activity. Polymer supports are still expensive to purchase and in order for them to have more widespread use it will be necessary to develop more cost effective routes for their preparation. It is important to understand the role of the polymeric backbone in the catalytic activity of immobilized metal complexes and to realize that very different reactivities and selectivities can be found if, for example, the linker or the degree of cross-linking is altered. When using dendrimer-derived materials, particular attention needs to be focused on dendritic effects. 

Scheme 13. Comparison of Two Polymer-Bound Ti-TADDOLates Generated after Cross-Linking Polymerization of Styrene with Styryl-Substituted TADDOL Monomers (see Scheme 12). Best reproducibility is observed vdth materials of low loading degree (0.10 mmol/g). According to elemental analysis (and rate measurements), ca. 90% of the TADDOL moieties introduced into the polymer (with the monomeric, cross-linking styryl-TADDOLs) carry a Ti-atom [78] [79] and are thus not buried inaccessibly in the cross-linked polymer The dendritic polymer performs better, as far as enantioselectivity of the Et2Zn addition to PhCHO is concerned there is no erratic up and down, and the value obtained in the 20th run is identical to that of the first run (within experimental error) [79].

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

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