Dendritic units

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

J(linear units) + (dendritic units) + (terminal units)... 

The properties of these rotaxane dendrimers are quite different from those of the individual rotaxanes or dendrimers and often a blend of both. In view of the versatile characteristics that a dendron or dendrimer can manifest, several new properties can be imparted to the rotaxanes. For example, the solubility of rotaxanes in organic solvents as well as in water can be significantly improved when large dendrimer units are appended enhancing the prospects of their use as molecular machines. The dendritic units can also influence the photo/electro-chemical properties of the rotaxanes. Employing photo-receptive dendron units, photo chemically driven molecular machines may be developed, where the dendrons act as antenna for photo-harvesting [62]. 

Figure 8, The differently incorporated bis-MPA units in the hyperbranched skeleton, assuming 100% conversion of carboxylic acid groups. A is the dendritic unit, B the linearly incorporated unit, and C the terminal unit.

In addition, taper-shaped and dendritic units were connected either covalently (at the apex or at the periphery) [133,135, 383,405] or by means of self assembly (e.g., hydrogen bonding [399]) to a polymer backbone. For example, polymers based on poly[4-(3-methacryloylpropyl]pyridine] form hydrogen bonded complexes with partially fluorinated mono-, di-, or trisubstituted benzoic acids (see complex 221 in Fig. 72). The type of mesophase formed by these complexes varies from smectic via columnar to cubic, most probably micellar cubic, with rising... 

Scheme I Synthesis of hyperbranched poly(ethylene imine) (PEI) via acid catalyzed ring-opening polymerization of aziridine. Each color represents a different branching unit blue for linear units (L), black for dendritic units (D), red for terminal units (T). PEI has a degree of branching (DB) of 62-73%, the depicted structure represents only a small idealized fragment. Reproduced with permission from [91]...

Rule 2 A dendritic structure consists of a core unit and a dendritic unit... 

The graft-to method of attaching dendrons numbers among the polymer-analogous synthetic strategies which start from a functionalised polymer backbone to which convergently preconstructed dendritic units of the desired generation can be fixed in a dense sequence (Fig. 2.16). 

Balzani, Vogde et al. demonstrated for dendritic cyclam compounds that signalling units are dendritically amplified by increasing the number of peripheral fluorophores. Opportunities for these PET sensor systems exist in the combination of selective coordination sites with the dendritic architecture. The mechanism resembles that described above, but can be influenced by the introduction of dendritic units of different generations into the sensor system. The distance between the terminal naphthalene groups of the dendron and the receptor (ion as guest in the cyclam core) is of crucial importance for electron transfer (ET) (Fig. 8.15) [53],... 

The degree of branching has been defined122 as the sum of dendritic units plus terminal units divided by the sum of dendritic units plus terminal and linear units, and in this case was determined to be ca. 50%. Also, the acetoxy monomer was copolymerized with various AB-type monomers, e.g., 3-(4-acetoxyphenyl)propionic acid. 

Because of the one-step polymerization procedure, hyperbranched polymers often contain not only D and T but also L repeating units. This can be expressed by DB, which is an important structural parameter of hyperbranched polymers. DB is estimated as the sum of the D and T units divided by the sum of all the three structural units, that is, D, T and L [41]. By definition, a linear polymer has no dendritic units and its DB is zero, while a perfect dendrimer has no linear units and its DB is thus unity. Frey has pointed out that DB statistically approaches 0.5 in the case of polymerization of AB2 monomers, provided that all the functional groups possess the same reactivity [42]. The structures of the hb-PYs could be analyzed by spectroscopic methods such as NMR and FTIR. The DB value of the phosphorous-containing polymer hb-F21, for example, was estimated to be 53% from its 31P NMR chemical shifts (Chart 1). 

Kim et al. (294) reported the synthesis of a small, 6-member array of discretes L25 and of a small 27-member pool indole library L26 using a commercially available, polyamide-based dendrimer support (8.107, Fig. 8.50) in which eight amino reaction sites per dendritic unit are present. The synthetic route for the preparation of the indole nuclei is reported in Fig. 8.51. Dendrimer 8.107 was first coupled to the base-labile... 

Based on the data above we propose that the vesicles of the various dendritic molecules have a stracture in which the amphiphilic dendrimers are aligned with their hydrophilic, protonated dendritic unit facing the aqueous phase and in which the aliphatic chains are forming a bilayer (Figure 23). 

Boltom H40 dendritic molecules were covalently linked in this work to make a network with aliphatic 1,6-hexamethylene diisocyanate (HDI). The molar NCO/OH ratio was varied for the reactants from 10 to 50% to prepare networks with different degrees of connectivity of dendritic units. The network samples are designated as H40/Z where Z stands for NCO/OH ratio expressed as a percentage. The network formation reaction of H40 with HDI was carried out in N,N-dimethylformamide (DMF) of 99.8% purity at 90 C. No catalyst was added. Isocyanates react readily with moisture to form urea linkages, therefore special precautionary measures were implemented to prevent moisture uptake either by HDI or DMF. Observation of the reaction vessel was maintained over the course of the reaction to monitor the viscosity of the solution. As viscosity of the solution increased to the desired level, suggesting that the gelation point was near, the solution was cast onto a glass plate which was immediately placed... 

Sengar and Basu have reported and characterized some dioxo-MoVI complexes of formula [Mo02(Tp )(p-SCgH4Dn)] (Dn = dendritic unit).138... 

The spectral parameters for component A again coincide with those of TEMPO in pure water ( a 48.3 MHz), i.e., this spin probe is located in a strongly hydrated, hydrophilic environment. The observed decrease of ai o by 3.7 MHz for species B ( hnal at 65 °C) is indicative of much more hydrophobic and less hydrated surroundings for these spin probes (comparable to chloroform or tert-hutyl alcohol [83]). At temperatures below the collapse temperature Tq, only the hydrophilic spectral component A is observed since all dendritic units are water-swollen. Above the critical temperature of 33 °C an increasing fraction of hydrophobic species B is observed with increasing temperature. The dehydration of the dendritic units thus leads to a local phase separation with the formation of hydrophobic cavities. Unlike in PNIPAAM hydrogels (Sect. 3.1), here hydrophobic regions are not observed below the macroscopic collapse temperature. 

---