Dendritic hyperbranched polymer

It has been an aim of this chapter to indicate that dendrimers have very significant potential for application in separation technologies. Nevertheless, at present it is difficult to predict whether their use will be limited to specific applications or whether they will be introduced more widely. However, dendritic hyperbranched polymers possessing a polydisperse and irregular structure can be synthesized more easily via a one-step procedure and these appear to be an economical alternative to dendrimers [34] it may be noted that their use in particularly interesting applications, with significant benefits for the separation of azeotropic mixtures by extractive distillation or solvent extraction, are at present under consideration [35]. 

Toughened epoxy resins and epoxy nanocomposite systems were synthesized using DGEBA resin, diethylene famine hardener, octadecylammonium modified montmorillonite and epoxy functional dendritic hyperbranched polymer (Boltorn El, Perstorp Spedahty Chemicals, Sweden) with an epoxy equivalent weight of 875 g/eq and a molecular weight of 10500 g/mol. 

Blends of PLA with a dendritic hyperbranched polymer (DHP) and starch were studied by Zhang et al. They showed that addition of DHP and starch... 

Boogh, L, Pettersson, B. and Manson, J. A. E. Dendritic hyperbranched polymers as tougheners for epoxy resins. Polymer (1999). 

Hirao, A., Sugiyama, K., Matsuo, A. et al. (2(X)8) Synthesis of weU-defined dendritic hyperbranched polymers by iterative methodologies using living/controlled polymerizations. Polymer International, 57,554—570. 

Mezzenga R, Boogh L, Manson JAE (2001) A review of dendritic hyperbranched polymer as modifiers in epoxy composites. Compos Sci Technol 61 787-795... 

Zhang, J. F. and Sun, X. Mechanical properties and crystallization behavior of poly(lactic acid) blended with dendritic hyperbranched polymer , Polym Int 53, 716-722 (2004). 

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 ... 

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). 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

Figure 5.17 (a) Linear, (b) hyperbranched, and (c) dendritic aromatic polymers. 

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). 

Hyperbranched polymers are generally composed of branched (dendritic), Hn-ear, and terminal units. In contrast to AB2 systems, there are two different types of linear units in SCVP one resembles a repeat unit of a polycondensate (----A -b----) and one a monomer unit of a vinyl polymer (--a(B )---). 

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. 

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. 

There are two aspects of linking the dendritic or hyperbranched polymers into a network ... 

Use of dendritic molecules (mainly hyperbranched polymers) as precursors of a crosslinked (engineering) material in order to modify its processing and materials properties. 

Since that time, synthetic chemists have explored numerous routes to these statistically hyperbranched macromolecular structures. They are recognized to constitute the least controlled subset of structures in the major class of dendritic polymer architecture. In theory, all polymer-forming reactions can be utilized for the synthesis of hyperbranched polymers however, in practice some reactions are more suitable than others. 

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 polymerization of AB -functional vinyl monomers is fundamentally different from the step-growth polymerization of AB2-monomers. Condensation of AB2-monomers results immediately in the formation of hyperbranched polymers since the reactivity of the end-groups are the same, regardless of what type of repeat unit (linear or dendritic) that is formed. 

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