Dendrigraft polymers

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. 

Dendritic polymers, the fourth major architectural class of macromolecules, can be divided into three subclasses. These subclasses may be visualized according to the degree of structural perfection attained, namely (1) hyperbranched polymers (statistical structures, Chapter 7), (2) dendrigraft polymers (semi-controlled structures, reviewed in this chapter) and (3) dendrimers (controlled structures, Chapter 1). 

Figure 9.1 Generic synthetic route to dendrigraft polymers...

Preparation of dendrigraft polymers has been achieved by three methods, namely (a) grafting onto , (b) grafting from , and (c) grafting through . 

Synthesis of higher generation dendrigraft polymers is achieved by repetition of the deprotection and grafting cycles. 

The synthesis and characterization of a series of dendrigraft polymers based on polybutadiene segments was reported by Hempenius et al. [15], The synthesis begins with a linear-poly(butadiene) (PB) core obtained by the sec-butyllithium-initiated anionic polymerization of 1,3-butadiene in n-hexane, to give a microstructure containing approximately 6% 1,2-units (Scheme 3). The pendant vinyl moities are converted into electrophilic grafting sites by hydrosilylation with... 

Scheme 6 Representative examples of hyperbranched base polymers used for sialic acid attachment and subsequent inhibition of flu virus infection. 14 Poly(ethyleneimine) (PEI) backbone 15 comb-branch polymer 16 dendrigraft polymer 17 PAMAM scaffolded on PEI...

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],... 

The graft copolymers with poly (H-1) arms have multiple carbon—halogen bonds that can initiate copper-catalyzed radical polymerization to give highly branched dendrigraft polymers.432 The products with styrene or nBMA had relatively narrow MWDs because each chain was prepared by living polymerization. 

A graphical comparison of the structure of dendrons, dendrimers, hyperbranched polymers, and dendrigraft polymers is shown in Figure 30.1. The stepwise synthesis of dendrimers involves multiple cycles of protection, condensation, and deprotection reactions to produce strictly... 

Some of the procedures used to synthesize dendritic molecules can be quite involved, and many different methods have been reported. The following three sections provide an overview of some of the pioneering work yielding each of the three main subclasses of dendritic polymers, namely, dendrimers, hyperbranched polymers, and dendrigraft polymers. The general characteristics and properties of these materials are also considered, as well as some of the more recent work including potential applications for these extremely versatile materials. 

Figure 30.1 Structure of four types of dendritic polymers (a) dendron, (b) dendrimer, (c) hyperbranched polymer, and (d) dendrigraft polymer. (See insert for the color representation of the figure.)...

Divergent Grafting-Onto Strategy The first dendrigraft polymer syntheses were reported independently by two research groups in 1991. Gauthier and Mdller [12]... 

The branched structure of dendrigraft polymers obtained by the convergent grafting-through method can be described... 

Considering the versatility of dendrigraft polymer syntheses, it is relatively easy to introduce features in the molecules of interest for specific applications. Since the chain segments of dendrigraft polymers are covalently bonded, copolymers with amphiphilic character behave like unimolecular micelles that provide an interesting basis for comparison with regular micelles. The self-assembly of... 

The dendritic polymer literature reviewed herein provides compelling evidence that these materials are a unique and versatile class of branched polymers. The synthesis of dendritic macromolecules can be accomplished by numerous methods allowing for specific tailoring of the characteristics of the polymer, to yield desired properties or functionality. While some of the procedures reported are quite intricate, requiring multiple cycles of synthetic steps and work-up, one-pot syntheses have also been developed for hyperbranched and dendrigraft polymers, making these materials more viable for (large-scale) commercial production and industrial applications. 

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