Brad S Lokitz

Publications (6)13.45 Total impact

• Article: In Situ Formation of Gold-“Decorated” Vesicles from a RAFT-Synthesized, Thermally Responsive Block Copolymer§

  Yuting Li, Adam E. Smith, Brad S. Lokitz, Charles L. McCormick
  10/2007;
• Article: Aqueous RAFT Synthesis of Micelle-Forming Amphiphilic Block Copolymers Containing N-Acryloylvaline. Dual Mode, Temperature/pH Responsiveness, and “Locking” of Micelle Structure through Interpolyelectrolyte Complexation†

  Brad S. Lokitz, Adam W. York, Jonathan E. Stempka, Neil D. Treat, Yuting Li, William L. Jarrett, Charles L. McCormick
  [show abstract] [hide abstract]
  ABSTRACT: Temperature- and pH-responsive, micelle-forming, amphiphilic block copolymers were prepared from N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPAM), and N-acryloylvaline (AVAL) utilizing aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. A series of block copolymers were synthesized by employing DMA as a macro-chain transfer agent to mediate the statistical copolymerization of NIPAM with AVAL. Structural organization and solution behavior were investigated utilizing dynamic light scattering, two-dimensional NMR spectroscopy, and transmission electron microscopy. It has been demonstrated that the critical micellization temperature for the block polymers can be tuned to range from ≈10 to 36 °C by adjusting the solution pH. Micelles with apparent hydrodynamic diameters from 45 to 86 nm are formed between pH 2 and 5. Above pH 5, a sufficient number of the AVAL units are deprotonated which prevents micellization. The extent of pH and temperature changes on the apparent hydrodynamic diameters have been illustrated via 3-D plots. Significantly, micelles assembled within a specified range of pH and temperature can be “locked” by interpolyelectrolyte complexation of anionic AVAL segments with those of a cationic polymer, in this case a RAFT-generated poly([ar-vinylbenzyl]trimethylammonium chloride) (PVBTAC). When the temperature is lowered to room temperature, the polymeric micelles remain “locked” in their multimeric structures which remain dispersed in water. Addition of 0.3 M NaCl to the aqueous solution results in dissociation of the complexes into the respective water-soluble components.
  07/2007;
• Article: Responsive Nanoassemblies via Interpolyelectrolyte Complexation of Amphiphilic Block Copolymer Micelles†

  Brad S. Lokitz, Anthony J. Convertine, Ryan G. Ezell, Andrew Heidenreich, Yuting Li, Charles L. McCormick
  [show abstract] [hide abstract]
  ABSTRACT: Shell “locked” nanoassemblies ranging in size from 34 to 78 nm have been prepared from interpolyelectrolyte complexation of block copolymer micelles of poly[(N,N-dimethylacrylamide)-b-(N-acryloylalanine)-b-(N-isopropylacrylamide)] with the homopolymer poly(ar-vinylbenzyl)trimethylammonium chloride above the unimer to micelle phase transition temperature of the block copolymer in water. Of technological significance is the reversibility of the shell cross-linking by addition of 0.4 M NaCl, allowing micelle dissociation below the lower critical solution temperature of the copolymer micelles. Poly(N-acryloylalanine) (AAL) and block copolymers were prepared directly in water via controlled reversible addition fragmentation chain transfer (RAFT) polymerization utilizing mono- and difunctional poly(N,N-dimethylacrylamide) macroCTAs.
  11/2006;
• Article: Thermally responsive vesicles and their structural "locking" through polyelectrolyte complex formation.

  Yuting Li, Brad S Lokitz, Charles L McCormick
  Angewandte Chemie International Edition 10/2006; 45(35):5792-5. · 13.45 Impact Factor
• Article: Synthesis of Reversible Shell Cross-Linked Micelles for Controlled Release of Bioactive Agents†

  Yuting Li, Brad S. Lokitz, Steven P. Armes, Charles L. McCormick
  03/2006;
• Article: RAFT Synthesis of a Thermally Responsive ABC Triblock Copolymer Incorporating N-Acryloxysuccinimide for Facile in Situ Formation of Shell Cross-Linked Micelles in Aqueous Media†

  Yuting Li, Brad S. Lokitz, Charles L. McCormick
  [show abstract] [hide abstract]
  ABSTRACT: A poly(ethylene oxide) (PEO)-based macromolecular chain transfer agent (macro-CTA) was employed to demonstrate the controlled polymerization of N,N-dimethylacrylamide (DMA) in anhydrous 1,4-dioxane at 70 °C using reversible addition−fragmentation chain transfer polymerization. This macro-CTA was then used to mediate the statistical copolymerization of DMA and the reactive monomer N-acryloxysuccinimide (NAS), forming a diblock copolymer of PEO-b-(DMA-s-NAS). Subsequent chain extension with N-isopropylacrylamide (NIPAM) yielded a PEO-b-(DMA-s-NAS)-b-NIPAM thermally responsive triblock copolymer. In aqueous solution at room temperature the triblock copolymer chains exist as unimers but form micelles when the solution temperature is raised above the lower critical solution temperature (LCST) of the NIPAM block. The hydrodynamic dimensions and micellization temperatures depend on the length of the NIPAM block. Incorporation of the NAS units into the triblock copolymer allows for facile formation of uniform shell cross-linked micelles by reaction with difunctional primary amines in aqueous media. These shell cross-linked micelles swell when the solution temperature is lower than the LCST of the NIPAM block.
  Macromolecules. 39(1):81-89.