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Controlled Synthesis of Polymer Brushes by Atom Transfer Radical Polymerization
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Controlled Synthesis of Polymer Brushes by Atom Transfer Radical Polymerization

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  1. Controlled Synthesis of Polymer Brushes by Atom Transfer Radical Polymerization Jinsheng Zhou Membrane Research Group Department of Chemistry Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  2. Introduction What is Polymer Brushes? Polymer brushes are long-chain polymer molecules attached by one end to a surface or interface with a density of attachment points high enough so that the chains are forced to stretch away from the interface. ----Milner. S.T., Science 1991,251,905 Why are we interested in these systems? Applications: Colloidal stabilization Modification of bulk surfaces and interfaces (to improve adhesion, wetting, and wear properties) * The interface may be a solid substrate, interface between two solvents and between solvent and air. ** But a solid substrate is the subject of today presentation Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  3. Methods of Producing Polymer Brushes • Physical process The binding to surface is noncovalent and desorption of the brushes always occurs. • Chemical process “Grafting to” Serious steric hindrance, hard to achieve a high density of grafting polymer “Grafting from” (Pioneered by Sogah et al, Macromolecules 1990, 23, 1264) Surface-initiated conventional radical, cation, anion or living radical polymn. Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  4. Why choose ATRP? * Accurate control of the thickness of polymer layer on the nanometer scale has great significance in the application for chemical separation, sensor, composite materials. • ATRP permits control of MW of polymers with low MWD and thus precisely control the thickness of polymer brushes. • A wide range of monomers available and allow block polymers to be formed, thus tailoring the properties of polymer brushes over a wide range • Relatively easy to perform, and can be performed at the relatively low temperature. Shah et al. Macromolecules 2000, 33, 7617 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  5. The Mechanism of Atom Transfer Radical Polymerization Matyjaszewski et al. J. Am. Chem. Soc. 1995, 117,5614 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  6. Controlled synthesis of polymer brushes by ATRP (Surface-initiated ATRP) A Two-Step Process 1. Immobilization of initiator on the substrate surface * The current studies focus on two substrates, silicon and gold . 2. Atom transfer radical polymerization Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  7. Silicon Substrate 2-(4-chlorosulfonyl phenyl) ethyl trimethoxysilane Ejaz et al. Macromolecules, 1998,31, 5934 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  8. Gold Substrate Me6TREN Baker et al. J. Am. Chem. Soc. 2000,122,7616 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  9. How control polymer chain growth at the extremely low initiator concentration? • Under typical conditions of LB self-assembly, the concentration of initiator can be no larger than ~51014 molecules/cm2( i.e., 10-9 mol/cm2, or 10-7mol/L). -----Wasserman et al. Langmuir, 1989, 5, 1074 Two initial attempts: ----Husseman et al. Macromolecules, 32, 1424 Standard living radical polymerization conditions (Polymer brushes are formed while no control is observed, ) Lower monomer concentration (polymerization rate reduces dramatically when monomer concentration is less than 25wt%) Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  10. Why is ATRP not well controlled here? Kact A typical ATRP process P-X + CuIX/2LP*+ CuIIX2/2L Activator Deactivator t=0 0 0 t C(P*) C(CuII) The formation of P-P C(P*) C(CuII) Reduction of P* termination High C(CuII) ** In a typical ATRP process, and the concentration of Cu(II) detected by EPR is the range 10-3 mol/L. ** Cu(II) concentration formed by surface-initiator is 10,000 times less than that required for well-controlled ATRP. Kdeact The persistent radical effect Matyjaszeweski et al. Macromolecules 1999, 32,8716 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  11. How to solve this problem? P-X + CuIX/2LP*+ CuIIX2/2L ActivatorDeactivator Synthesis with Free Initiator * Add free untethered initiator in the solution to increase the Initiator (P-X) concentration. Synthesis without Free Initiator * Intentionally add Cu(II) at the beginning of reaction. Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  12. The Results of two Approaches TsCl=2.4mM 1%CuBr(PMDETA) 0.03%CuBr2(PMDETA) 4.8mM Plots of graft-layer thickness vs polymerization time. Synthesis with Free Initiator Synthesis without Free Initiator ---Ejaz et al. Macromolecules, 1998,31, 5934 ---Matyjaszeweski et al. Macromolecules 1999, 32,8716 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  13. Characterization of Grafted polymer chains 1.Direct measurement • Polymer chains are detached from surface and measured by GPC analysis to get MW and MWD. But this method is limited by the too small amount of grafted polymers ( e.g. 0.01mg of polymer corresponds to 100nm thick film grown from a flat surface 1 cm2 ) 2.Indirect measurement • This is the dominating method to study grafted polymer chains. It is generally assumed that the MW of covalently bound polymer chains is related to that of the “bulk” polymer. Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  14. Tailoring of Polymer Brushes ---Shah et al. Macromolecules, 2000,33, 597 ---Matyjaszeweski et al. Macromolecules 1999, 32,8716 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  15. Grafting density and Surface Evenness Ax : a cross-sectional area per chain  : is the mass density NA :is Avogadro’s number t: polymer brushes thickness M: molecular weight of the chain Comparison: Ax: 180Å(ATRP) ~ 900Å(Adsorption) ----Wirth et al. Anal. Chem. 1998, 70, 4023 Baker et al. J. Am. Chem. Soc. 2000,122,7616 Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)

  16. Conclusion • The persistent radical effect plays an important role in surface-initiated ATRP • The thickness of polymer brushes can be effectively controlled by surface-initiated ATRP. • The properties of polymer brushes can be tailored by copolymerization and the selection of monomers • However, how to directly study the grafted polymers is still a challenge (MW, MWD, Chain Conformation). • Surface-initiated ATRP shows a great potential in application for Biocompatible medical materials, nonlinear optical materials, Chemical separation(capillary for electrophoresis), microfabrication. Presentation, Advanced Polymer Chemistry (Dr. H. D. Stover)