Amphiphilic star-block copolymers based on a hyperbranched core: Synthesis and supramolecular self-assembly

Novel amphiphilic star-block copolymers, star poly(caprolactone)-blockpoly[(2-dimethylamino)ethyl methaerylate] and poly(caprolactone)-block-poly(methacrylic acid), with hyperbranched poly(2-hydroxyethyl methacrylate) (PHEMA-OH) as a core moiety were synthesized and characterized. The star-block copolymers were prepared by a combination of ring-opening polymerization and atom transfer radical polymerization (ATRP). First, hyperbranched PHEMA-OH with 18 hydroxyl end groups on average was used as an initiator for the ring-opening polymerization of epsilon-caprolactone to produce PHEMA-PCL star homopolymers [PHEMA = poly(2-hydroxyethyl methacrylate); PCL = poly(caprolactone)]. Next, the hydroxyl end groups of PHEMA-PCL were converted to 2-bromoesters, and this gave rise to macroinitiator PHEMA-PCL-Br for ATRP. Then, 2-dimethylaminoethyl methacrylate or tert-butyl methacrylate was polymerized from the macroinitiators, and this afforded the star-block copolymers PHEAIA-PCL-PDMA [PDMA = poly(2-dimethylaminoethyl methacrylate)] and PHEMA-PCL-PtBMA [PtBMA = poly(tert-butyl methacrylate)]. Characterization by gel permeation chromatography and nuclear magnetic resonance confirmed the expected molecular structure. The hydrolysis of tert-butyl ester groups of the poly(tert-butyl methacrylate) blocks gave the star-block copolymer PHEMA-PCL-PMAA [PMAA = poly(methacrylic acid)]. These amphiphilic star-block copolymers could self-assemble into spherical micelles, as characterized by dynamic light scattering and transmission electron microscopy. (c) 2005 Wiley Periodicals, Inc.