Langmuir 2004, 20, 10809-10817
10809
Shell-Cross-Linked Vesicles Synthesized from Block Copolymers of Poly(D,L-lactide) and Poly(N-isopropyl acrylamide) as Thermoresponsive Nanocontainers Michelle Hales, Christopher Barner-Kowollik, Thomas P. Davis, and Martina H. Stenzel* Centre for Advanced Macromolecular Design, School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney, NSW 2052 Australia Received June 29, 2004. In Final Form: September 15, 2004 A polylactide (D,L-PLA) macroRAFT agent was prepared by utilizing a hydroxyl-functional trithiocarbonate as a coinitiator for the ring-opening polymerization. The length of the resultant polymer was controlled by the concentration of the coinitiator leading to the formation of two PLA polymers with Mn ) 12500 g mol-1 (PDI ) 1.46) and Mn ) 20500 g mol-1 (PDI ) 1.38) each with ω-trithiocarbonate functionality. Chain extension of PLA via the RAFT (free radical) polymerization of N-isopropyl acrylamide (NIPAAm) resulted in the formation of amphiphilic block copolymers with the PNIPAAm block increasing in size with conversion. TEM measurements of the aggregates obtained by self-organization of the block copolymers in aqueous solutions indicated the formation of vesicles. The sizes of these aggregates were influenced by the ratio of both blocks and the molecular weight of each block. The lower critical solution temperature (LCST) of the block copolymer was largely unaffected by the size of each block. UV turbidity measurements indicated a higher LCST for the block copolymers than for the corresponding PNIPAAm homopolymers. Stabilization of the vesicles was attained by a cross-linking chain extension of the PNIPAAm block using hexamethylene diacrylate. As the trithiocarbonate group was located between the PLA and PNIPAAm blocks, the chain extension resulted in a cross-linked layer between the core and corona of the vesicles.
Introduction The delivery of drugs using self-organizing block copolymers has received widespread interest over the past decades.1-5 Ringsdorf et al. first proposed the usage of micelles with their core-shell structures as drug carriers of poorly water soluble drugs.6 Self-organizing block copolymers were shown to have several advantages over conventional systems especially as they allow for the possibility of entrapping hydrophobic drugs in the micelle core, while the micelle’s hydrophilic corona confers watersolubility. Furthermore, the size of a micelle (
