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.
The synthesis, polymerization, and copolymerization of a new cyclic ester, γ-(2-bromo-2-methyl propionyl)--caprolactone (3), containing a pendent-activated alkyl bromide functional group is described. This new compound serves as both a monomer for "living" ring-opening polymerization (ROP) as well as an initiator for the controlled atom transfer radical polymerization (ATRP). Three distinctive routes to poly( -caprolactone)-graft-poly(methyl methacrylate) copolymers were surveyed by either sequential or concurrent living polymerization procedures. The first approach involves the ROP of -caprolactone with various compositions of 3, followed by the polymerization of methyl methacrylate via ATRP from the activated alkyl bromide sites along the polyester backbone. Alternatively, R-lactone functional methyl methacrylate macromonomers were prepared by ATRP of methyl methacrylate initiated from 3. The macromonomers were copolymerized with -caprolactone via ROP to form the target graft copolymers. Finally, the graft copolymers could be prepared in a simple one-step approach by the concurrent polymerization of -caprolactone, 3, and methyl methacrylate together with the appropriate initiator for the ROP and the catalyst for the ATRP. The new monomer enabled new graft copolymers having an aliphatic polyester backbone with poly(methyl methacrylate) grafts of controlled molecular weight and narrow polydispersities (∼1.3). IntroductionThe renewed interest in the macromolecular engineering of poly(lactones) and related polyesters stems from the discovery that many organometallic compounds are effective initiators/catalysts for controlled ring-opening polymerization (ROP), 1 allowing the preparation of functional oligomers, random and block copolymers, and polymers with unique topology or architectural control. 1-4 For instance, asymmetric functionality can be introduced in a controlled way through the use of aluminum alkoxide initiators bearing functional alkoxide groups [Et 2 Al(ORX), where X is any functional group]. After hydrolytic deactivation of the active aluminum alkoxide growing species, R-X-ω-hydroxy poly-(lactone) telechelic chains are quantitatively and selectively recovered. The coupling of the telechelic macromonomers through block copolymerization provides a precise methodology to controlled macromolecular architecture. 2 Alternatively, dual "living" controlled polymerizations from a single initiating molecule without intermediate activation or transformation steps has been demon-
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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