A new type of macromolecular architecture, denoted as dendrimer-like star block copolymers, is reported. These block copolymers are described by a radial geometry where the different generations or layers are comprised of high molecular weight polymer emanating from a central core. A hexahydroxyl functional core was used as an initiator for the “living” ring opening polymerization (ROP) of ε-caprolactone producing a hydroxyl terminated six arm star polymer with controlled molecular weight and narrow polydispersities (PD < 1.1). Capping these chain ends with dendrons containing activated bromide moieties produced “macro-initiators” for atom transfer radical polymerization (ATRP). Methyl methacrylate was polymerized from these “macro-initiators” in the presence of an organometallic promoter to produce the requisite dendrimer-like star polymers. High molecular weight was obtained with low polydispersities (<1.2). Alternatively, amphiphilic character could be introduced by designing the different layers or generations to be either hydrophobic or hydrophilic. For example, methyl methacrylate (MMA) with either hydroxyethyl methacrylate (HEMA) or methacrylate functional ethylene oxide macromonomers (EO) were polymerized from these “macro-initiators” to provide a hydrophilic outer layer. The use of macromolecular building blocks allows rapid attainment of high polymer in a limited number of steps with purification between transformation requiring only polymer precipitation.

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 ringopening polymerization (ROP) of either -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 AB2 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.

Novel hyperbranched poly(ε-caprolactone)s have been synthesized. The versatile synthesis utilizes AB2 macromonomers and allows the thermo-physical properties of the polymers to be tailored. The ε-caprolactone-based AB2 macromonomers were synthesized through living ring opening polymerization, using aluminum benzyloxide as the initiator. The aluminum benzyloxide initiated polymers were then functionalized with benzylidene-protected 2,2‘-bis(hydroxymethyl) propionic acid and subsequently deprotected to form the α-carboxylic-ω-dihydroxy functional AB2 macromonomers. The AB2 polyesters were condensed into hyperbranched polymers through a room-temperature esterification synthesis using dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridinium 4-toluenesulfonate (DPTS). All polymers were characterized by 1H NMR, SEC, and DSC. The degree of branching in the hyperbranched polymers was found, by 1H NMR, to be 0.37 ± 0.03, and to be independent of the macromonomer used. Significant flexibility exists in the new approach since the molecular weight and the type of macromonomer can be varied.

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