Highly branched block copolymers: Design, synthesis, and morphology

Several new approaches to biodegradable dendritic aliphatic block copolymer s are described, including hyperbranched and dendrimer-like star structures . The hyperbranched polymers were obtained by the co-condensation of differ ent AB(2) macromonomers. The macromonomers were prepared by ring-opening po lymerization (ROP) of either epsilon-caprolactone, L-lactide, or various su bstituted lactones using the benzyl ester of 2,2'-bis(hydroxymethyl)propion ic acid as initiator. Catalytic hydrogenation of the benzyl ester generated the requisite acid functional AB(2) macromonomer. The second route utilize s a new type of molecular architecture, denoted as dendrimer-like star poly mers. These block copolymers are described by a radial geometry where the d ifferent layers or generations are comprised of high molecular weight polym er emanating from a central core. With this architecture, more control in t he placement of the different blocks is afforded over the hyperbranched ana logue. As a means of imparting desirable mechanical properties to the dendr itic copolymers, a series of new substituted lactones were prepared. The us e of such monomers prevents crystallization of the poly(lactone), allowing dendritic polyesters with a range of mechanical properties from thermoplast ic elastomers to rubber toughened systems, depending on the relative compos ition of the two components. The synthesis, characterization, and morpholog y of these new copolymers are discussed.