Dendrigraft

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...

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. 

Figure 1.10 Comparison of dendrimer and dendrigraft architecture Generation 0-2...

Figure 1.11 Comparison of degree of polymerization as a function of topology and growth process (a) dendrigraft, (b) dendrimer, (c) non-linear straight chain and (d) linear...

Figure 1.12 Proposed mechanism for conversion of ethylene monomer to dendrigraft polyethylene with Brookhart catalyst at low pressure...

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. 

Unique features offered by the dendritic state , that have no equivalency in the linear topologies, are found almost exclusively in the dendron/dendrimer subset or to a slightly lesser degree in the dendrigrafts. They include ... 

Figure 1.19 Comparison of intrinsic viscosities (log (f/)) versus molecular weight (log M) for (A) linear, (B) random hyperbranched, (C) dendrimers and (D) dendrigraft topologies. Data for A, B, C adapted from Frechet etal.. Ref. 49.

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. 

Figure 1.23 Intermediary of (III) branched and (IV) dendritic architecture in the conversion of (I) linear thermoplastics to (II) crosslinked thermoset polymers. Intermediary of (IVb) dendrigrafts and (IVc) dendrimers in the formation of megamers...

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... 

The synthetic approach used for dendrigraft-po y(butadicncs) has the potential to provide control over the composition and architecture of the molecules. The branch molecular weight is easily varied with the amount of initiator used in the polymerization reaction. Solvent polarity control in the polymerization allows variation of the proportion of 1,2-units in the side chains, and hence the branching density. 

The solution properties of dendrigraft polybutadienes are, as in the previous cases discussed, consistent with a hard sphere morphology. The intrinsic viscosity of arborescent-poly(butadienes) levels off for the G1 and G2 polymers. Additionally, the ratio of the radius of gyration in solution (Rg) to the hydrodynamic radius (Rb) of the molecules decreases from RJRb = 1.4 to 0.8 from G1 to G2. For linear polymer chains with a coiled conformation in solution, a ratio RJRb = 1.48-1.50 is expected. For rigid spheres, in comparison, a limiting value RJRb = 0.775 is predicted. 

DENDRIGRAFT (ARBORESCENT)-POLY(STYRENE)-GRAFT-POLY(ISOPRENE) COPOLYMERS... 

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