Site-Specific Functionalization of Hyperbranched Polymers Using “Click” Chemistry

2009-06-23T00:00:00Z (GMT) by Animesh Saha S. Ramakrishnan
Different strategies for functionalization of the core region and periphery of core−shell type hyperbranched polymers (HBP) using the “click” reaction have been explored. For achieving peripheral functionalization, an AB<sub>2</sub> + A−R<sub>1</sub> + A−R<sub>2</sub> type copolymerization approach was used, where A−R<sub>1</sub> is heptaethylene glycol monomethyl ether (<b>HPEG-M</b>) and A−R<sub>2</sub> is tetraethylene glycol monopropargyl ether (<b>TEG-P</b>). A very small mole fraction of the propargyl containing monomer, <b>TEG-P</b>, was used to ensure that the water-solubility of the hyperbranched polymer is minimally affected. Similarly, to incorporate propargyl groups in the core region, a new propargyl group bearing B<sub>2</sub>-type monomer was designed and utilized in an AB<sub>2</sub> + A<sub>2</sub> + B<sub>2</sub> + A−R<sub>1</sub> type copolymerization, such that the total mole fraction of B<sub>2</sub> + A<sub>2</sub> is small and their mole-ratio is 1:1. Further, using a combination of both the above approaches, namely AB<sub>2</sub> + A<sub>2</sub> + B<sub>2</sub> + A−R<sub>1</sub> + A−R<sub>2</sub>, hyperbranched structures that incorporate propargyl groups both at the periphery and within the core were synthesized. Since the AB<sub>2</sub> monomer carries a hexamethylene spacer (C-6) and the periphery is PEGylated, all the derivatized polymers form core−shell type structures in aqueous solutions. Attempts were made to ascertain and probe the location of the propargyl groups in these HBP’s, by “clicking” azidomethylpyrene, onto them. However, the fluorescence spectra of aqueous solutions of the pyrene derivatized polymers were unable to discriminate between the various locations, possibly because the relatively hydrophobic pyrene units insert themselves into the core region to minimize exposure to water.