Random hyperbranched polymers

A hyperbranched polymer is often characterized by the degree of branching DB [Holter et al., 1997 Jo and Lee, 2001 Kim, 1998 Lee et al., 2000]. For the hyperbranched polymer produced from AB2, there are three different types of repeat units dendritic, linear, and terminal units defined as units having two, one, and no B groups reacted, respectively. The degree (fraction) of branching (DB) is given by 

B group) units and fewer dendritic (no unreacted B groups) units. The polydispersity in molecular weight and isomerism, the intramolecular cyclization, and the molecular shape variations due to steric crowding contribute to the random nature of hyperbranched polymers. 

Hyperbranched polymers can also be synthesized from the following systems [Jikei and Kakimotoa, 2001]  

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. 

Hyperbranched polymers can also be synthesized by chain polymerization, ring-opening polymerization, and combinations of ring-opening and step polymerization [Kim, 1998 Voit, 2000] (Secs. 3-6e, 3-15b-4, 3-15b-5, 3-15c, 5-4c). 

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. 

Random hyperbranched polymers are generally produced by the one-pot polymerization of ABX monomers or macromonomers involving polycondensation, ring opening or polyaddition reactions hence the products usually consist of broad statistical molecular weight distributions. 

Over the past decade, literally dozens of new AB2-type monomers have been reported leading to an enormously diverse array of hyperbranched structures. Some general types include poly(phenylenes) obtained by Suzuki-coupling [54, 55], poly(phenylacetylenes prepared by Heck-reaction [58], polycarbosilanes, polycarbosiloxanes [59], and polysiloxysilanes by hydrosilylation [60], poly(ether ketones) by nucleophilic aromatic substitution [61] and polyesters [62] or polyethers by polycondensations [63] or by ring opening [64]. 

New advances beyond the traditional AB2 Flory-type branch cell monomers have been reported by Frechet et al [65, 66]. They have introduced the concept 

Further elaboration of these dendrigraft principles allowed the synthesis of a variety of core-shell type dendrigrafts, wherein elemental composition as well as the hydrophobic/hydrophilic character in the core can be controlled independently. 

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Figure 1.9 Polymerization of an AB2 monomer into a random hyperbranched polymer...

Frechet [49, 89] was the first to compare viscosity parameters for (A) linear topologies, as well as (B) random hyperbranched polymers and (C) dendrimers. More recently, we reported such parameters for (D) dendrigraft polymers [111] as shown in Figure 1.19. It is clear that all three dendritic topologies behave differently than the linear. There is, however, a continuum of behavior wherein random hyperbranched polymers behave most nearly like the linear systems. Dendrigrafts exhibit intermediary behavior, whereas dendrimers show a completely different relationship as a function of molecular weight. 

These features are captured to some degree with dendrigraft polymers, but are either absent or present to a vanishing small extent for random hyperbranched polymers. 

High functionality precursors of/ > 10-20 are primary chains if they participate in crosslinking by vulcanization each monomer unit of primary chain is a potential site for crosslinking. Also, dendrimers and random hyperbranched polymers of higher molecular weight, rank among high-functionality precursors. 

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. 

The first strategies to random hyperbranched polymers involved exclusively step-growth polymerizations. This limited the potential applications for these architectures to areas where only condensation-type polymers are acceptable. Frechet et al. [21] presented the first example of a hyperbranched vinyl polymerization in 1995, ] initiating the birth of a second generation of hyperbranched... 

Dendrimer synthesis involves a repetitive building of generations through alternating chemistry steps which approximately double the mass and surface functionality with every generation as discussed earlier [1-4, 18], Random (statistical) hyperbranched polymer synthesis involves the self-condensation of multifunctional monomers, usually in a one-pot single series of covalent formation events [31], Random hyperbranched polymers and dendrimers of comparable molecular mass have the same number of branch points and terminal units, and any application requiring only these two characteristics could be satisfied by either architectural type. Since dendrimer synthesis requires many defined synthetic and process purification steps while hyperbranched synthesis may involve a one-pot synthetic step with no purification, the dendrimers will necessarily be a much more expensive material to produce. 

Figure 11.7 illustrates the three different subtypes of dendritically branched molecules that have been identified within the major architectural class of dendritic polymers. Random hyperbranched polymers, not only exhibit polydispersity in molecular mass between individual molecules, it should also be noted... 

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. 

In spite of this difficult acceptance, it is quite remarkable to note that by the end of 1990 about two dozen publications on dendrimers had appeared in refereed journals. By the end of 1991 the rate of publication of dendrimer papers had started to climb markedly while there still were only three papers on random hyperbranched polymers and two on dendrigraft or arborescent polymers. The courage, persistence and credibility of many key scientists listed in Table 1 during that period, set the stage for the explosive acceptance and recognition of dendritic polymers over the next decade. 

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). 

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],... 

Figure 5.22. Formation of (a) linear and (b) random hyperbranched polymers.

Dendritic Polymer Subclasses Random Hyperbranched Polymers... 

Konkolewicz, D. Thorn-Seshold, O. Gray-Weale, A., Models for Randomly Hyperbranched Polymers Theory and Simulations. J. Chem. Phys. 2008,129, 054901. 

Alexey, V. Lyulin, Adolf, David B., and Davies, Geoffrey R. Computer Simulations of Hyperbranched Polymers in Shear Flows. Macromolecules, 34,3783-3789 (2001). Konkolewicz, D., Thom-Seshold, O., and Gray-Weale,A. Models for randomly hyperbranched polymers Theory and simulation. The Journal of Chemical Physics, 129, 054901 (2008). 

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