Highly branched block copolymers:: Design, synthesis, and morphology

Several new approaches to biodegradable dendritic aliphatic block copolymers are described, including hyperbranched and dendrimer-like star structures. The hyperbranched polymers were obtained by the co-condensation of different AB(2) macromonomers. The macromonomers were prepared by ring-opening polymerization (ROP) of either epsilon-caprolactone, L-lactide, or various substituted lactones using the benzyl ester of 2,2'-bis(hydroxymethyl)propionic acid as initiator. Catalytic hydrogenation of the benzyl ester generated the requisite acid functional AB(2) macromonomer. The second route utilizes a new type of molecular architecture, denoted as dendrimer-like star polymers. These block copolymers are described by a radial geometry where the different layers or generations are comprised of high molecular weight polymer emanating from a central core. With this architecture, more control in the placement of the different blocks is afforded over the hyperbranched analogue. As a means of imparting desirable mechanical properties to the dendritic copolymers, a series of new substituted lactones were prepared. The use of such monomers prevents crystallization of the poly(lactone), allowing dendritic polyesters with a range of mechanical properties from thermoplastic elastomers to rubber toughened systems, depending on the relative composition of the two components. The synthesis, characterization, and morphology of these new copolymers are discussed.