Constitutional isomers of dendrimer-like star polymers:: Design, synthesis, and conformational and structural properties

The design, synthesis, and solution properties of six constitutional isomers of dendrimer-like star polymers is described. Each of the polymers have comparable molecular weights (similar to 80 000), narrow polydispersities (<1.19) and an identical number of branching junctures (45) and surface hydroxyl functionalities (48). The only difference in the six isomers is the placement of the branching junctures. The polymers are constructed from high molecular weight poly(epsilon-caprolactohe) with branching junctures derived from 2,2'-bis(hydroxylmethyl) propionic acid (bis-MPA) emanating from a central core. The use of various generations of dendritic initiators and dendrons coupled with the ring-opening polymerization of epsilon-caprolactones allowed a modular approach to the dendrimer-like star polymer isomers. The most pronounced effects on the physical properties/morphology and hydrodynamic volume was for those polymers in which the branching was distributed throughout the sample in a dendrimer-like fashion. The versatility of this approach has provided the possibility of understanding the relationship between architecture and physical properties. Dynamic light scattering and small-angle X-ray scattering techniques were used to determine the hydrodynamic radius R-h and radius of gyration R-g respectively. The relationship between R-g and molecular weight was indicative of a compact starlike structure, and did not show advanced bias toward either the dense core or dense shell models. The radial density distribution of the isomers was therefore modeled according to a many arm star polymer, and good agreement was found with experimental measures of R-h/R-g.