Highly branched architecture

Dendritic polymers are most often reported to be amorphous, which can be anticipated from their highly branched architecture. However, some exceptions are presented in the literature. Percec et al. [34,35] reported on liquid crystalline (LC) hyperbranched polymers where the LC-phase was achieved by conformational isomerism. Various repeat units of A2B type have been used where a flex-... 

Alternative approaches involve the reaction together of preformed blocks, such as in telechelic polymers and coupling reactions. The latter allow highly branched architectures such as star polymers to be formed by linking living anionic chains to a multifunctional core such as SiCl4 (Cowie, 1989a). 

Alditols polyols are readily renewable, inexpensive and harmless to the environment. By incorporation of polyols into aliphatic polyesters, functional linear or hyperbranched polymers can be prepared with specific biological activities and/or that respond to environmental stimuli. Polyesters with carbohydrate or polyol repeat units in chains have been prepared by chemical methods. " In some cases, the reaction conditions led to hyperbranched polymers (HBPs). The highly branched architecture of HBPs leads to unusual mechanical, rheological and compatibility properties. " These distinguishing characteristics have garnered interest for their use in numerous industrial and biomedical fields. Chemical routes to linear polyol-polyesters require elaborate protection-deprotection steps ". Furthermore, condensation routes to hyperbranched polymers generally require harsh reaction conditions such as temperatures above 150 C and highly acidic catalysts ". ... 

Trees, roots, snowflakes, river deltas, corals, circulation systems, and many other natural structures all demonstrate a characteristic highly branched architecture in the macro world. Descending into the nanoscale, we find... 

Starch is an extremely abundant edible polysaccharide present in a wide variety of tubers and cereal grains. In most of its manifestations, it is composed of two macromolecules bearing the same structural units, 1,4-D-glucopyran-ose, in linear (amylose. Fig. 11(a)) and highly branched architectures (amylopectin. Fig. 11(b)), present in different proportions according to the species that produces it. The utilization of starch or its derivatives for the production... 

The free-radical polymerization of ethylene produces low-density PE (LDPE). The reaction is carried out by chain propagation radical chemistry producing a high branching content where 30% are usually butyl groups. PE obtained by firee-radical polymerization presents little control over the primary polymer structure. While free-radical polymerization of ethylene produces highly branched architectures, polymerization of ethylene using... 

Molecular encapsulation in dendritic and highly branched polymers has received increasing attention recently. It was shown for dendrimers that the site isolation of a fimaional core unit in a dendritic scaffold is of great interest with respect to optical properties, catalysis, and other future applications. Detailed studies on dendrimers have revealed that at some critical dendrimer generation, the core is encapsulated by the sterically aowded and densely packed highly branched architecture. Within this context, Frechet et al introduced l-(N,N-dimethylamino)-4-nitrobenzene as a solvatochromic chromophore at the focal point of a poly(benzyl ether) dendrimer. In other works, manganese and zinc porphyrins... 

LCB polymers can be formed by chemically linking preformed polymers (arm first or polymer first method) or by growing polymer chains from a multifunctional initiatior (core first method). In both cases living polymerization techniques are preferred because they provide better control over MW, MW distribution and the final branching architecture. However, highly selective coupling reactions e.g. with multifunctional isocyanates, or dicyclohexylcar-bodiimide (DCC) coupling, have also been successful. 

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