Random hyperbranched structures

Figure 1.8 Branch cell structural parameters (a) branching angles, (b) rotation angles, (/) repeat units lengths, (Z) terminal groups and dendritic subclasses derived from branches (IVa) random hyperbranched, (IVb) dendrigrafts and (IVc) dendrons/dendrimers...

Flory was the first to hypothesize concepts [28,52], which are now recognized to apply to statistical, or random hyperbranched polymers. However, the first purposeful experimental confirmation of dendritic topologies did not produce random hyperbranched polymers but rather the more precise, structure controlled, dendrimer architecture. This work was initiated nearly a decade before the first examples of random hyperbranched4 polymers were confirmed independently in publications by Odian/Tomalia [53] and Webster/Kim [54, 55] in 1988. At that time, Webster/Kim coined the popular term hyperbranched polymers that has been widely used to describe this type of dendritic macromolecules. 

Dendrimers, the most precise subset of structure-controlled dendritic polymers preceeded by nearly half a decade, the more recent attention focused on hyperbranched polymers. It is notable that literature reports describing dendrimers far exceed the number of investigations published on random hyperbranched polymers. 

Another important issue is the difference between various branching types such as random hyperbranched [31], dendrigrafts and dendrimers. The complexity of synthesis requirements manifested by the statistical dendritic polymers versus the more structurally controlled dendrimers could make the former orders of magnitude more expensive than hyperbranched. Are the structures as significantly unique and of sufficient value effectiveness to justify the higher costs ... 

An interesting novel approach to the synthesis of (metallo)dendrimer catalysts could be the use of random hyperbranched polymers [38]. Obviously, these hyperbranched polymers have comparable but less defined structures, but to arrive at dendrimers with similar sizes, a larger number of preparative steps are required, which may be an economic disadvantage. Furthermore, materials involving heterogeneous supports with well-defined metallodendritic subunits [15] can be a promising future direction giving rise to new types of supramolecu-lar catalysts that can easily be recovered from production streams. 

Some 17 years later, many of these predictions are turning into experimental reality as many of these questions are being answered in each new publication or patent that appears on dendritic architecture. Presently, dendritic polymers are recognized as the fourth major class of polymeric architecture consisting of three subsets that are based on degree of structural control, namely (a) random hyperbranched polymers, (b) dendrigraft polymers and (c) dendrimers (Figure 6). 

Fig. 28. The ratio A2M [rj] at large for star molecules (symbols) and randomly branched structures [25,26,108,130,131]. The shaded area indicates the range of the experimental findings with randomly and hyperbranched samples [144]. The line was drawn to guide the eye...

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],... 

Fig. 2 Structurally controlled polymers (a) dendrimer, (b) dendron, (c) random hyperbranched, and (d) dendrigrafts. The metabolism of the polyamine is still unclear. Reproduced with permission from [37]. Copyright 2001 Elsevier...

Fig. 9 Branch cell structural parameters a branching angle, b rotation angle, I repeat unit length, Z terminal group, I molecular reference marker or core. Dendritic subclasses derived from branches IVa random hyperbranched, IVb dendrigrafts, and IVc dendrons/dendrimers [93]. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

As mentioned in Chap. 1, randomly hyperbranched chains are even more complicated than dendrimers. It has not been completely clear whether they are fractal objects [11, 12] and whether those previously reported M-dependent intrinsic viscosities from an on-line combination of the size exclusion chromatograph (SEC) with viscosity and multi-angle laser light scattering (MALLS) detectors actually captured its structure-property relationship [11, 13]. In the next section, we ll discuss our experimental results in detail. 

Hyperbranched polymers are randomly branched structures, similar to dendrimers, but containing imperfections in the branching points due to their traditional one-pot synthesis (Voit and Lederer, 2009 Carlmark et al, 2009 Feng et al, 2005 Kitajyo et al, 2007). These hyperbranched polymers, which can be synthesized by condensation reactions, cationic procedures, free radical polymerizations and ring opening polymerization of AB or (A + By)-type monomers, are the least challenging from a synthetic point of view as they do not require any protection/deprotection procedures or tedious workups (Scheme 8.2). 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch 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). 

Relationships between dilute solution viscosity and MW have been determined for many hyperbranched systems and the Mark-Houwink constant typically varies between 0.5 and 0.2, depending on the DB. In contrast, the exponent is typically in the region of 0.6-0.8 for linear homopolymers in a good solvent with a random coil conformation. The contraction factors [84], g=< g >branched/ <-Rg >iinear. =[ l]branched/[ l]iinear. are another Way of cxprcssing the compact structure of branched polymers. Experimentally, g is computed from the intrinsic viscosity ratio at constant MW. The contraction factor can be expressed as the averaged value over the MWD or as a continuous fraction of MW. 

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