Dendrimers and hyperbranched polymers: Two families of three-dimensional macromolecules with similar but clearly distinct properties

Abstract

Dendrimers and hyperbranched polymers are globular macromolecules that are characterized both by a highly branched structure, in which all bonds converge to a focal point or core, and a multiplicity of reactive chain-ends. Because of the obvious similarity of their building blocks, many assume that the properties of these two families of dendritic macromolecules are almost identical and that the terms "dendrimer" and "hyperbranched polymer" can be used interchangeably. This assumption is incorrect because only regular dendrimers have a precise end-group multiplicity and functionality and exhibit properties that are totally unlike those of all other families of macromolecules. For example, regular dendrimers display a maximum in the relationship between their intrinsic viscosity and molecular weight, but hyperbranched polymers do not. Because of their uniformity in shape, size, and structure, only regular dendrimers spread uniformly at the air-water interface. The chemical reactivity of the chain-ends of dendrimers as well as the inner nanoenvironment that their regularly branched structure provide are unique features of these three-dimensional globular macromolecules. However, the price that has to be paid for these special properties is one of accessibility. Today, regular dendrimers can only be prepared using rather tedious, multistep syntheses that require intermediate purifications. Though processes involving little intermediate purification have been claimed to afford "dendrimers," the molecules that are obtained have varying degrees of chemical and structural regularity and may not possess the ultimate properties of regular dendrimers. In contrast, hyperbranched polymers are easily obtained using a variety of one pot procedures, some of which mimic, but do not truly achieve, regular dendritic growth. As a result, hyperbranched polymers also exhibit fascinating properties such as low solution and melt viscosities, but these properties tend to be intermediate between those of the fully regular dendrimers and those of branched or star-shaped macromolecules.