Reversible Addition-Fragmentation Chain Transfer-Hetero Diels-Alder (RAFT-HDA) Chemistry as an Efficient Conjugation Technique for Macromolecular Surface Engineering

In the current study - to achieve the modification of solid substrates in an efficient fashion - the next generation of RAFT agents capable of rapidly undergoing HDA reactions under ambient conditions and in the absence catalyst were synthesized. In addition, a range of highly innovative protocols for the orthogonal modification of variety solid substrates were established. The microspheres were also studied to obtain information about the amount of accessible active sites and grafting densities

Polymer Functionalization of Mesoporous Silica Nanoparticles Using Controlled Radical Polymerization Techniques

Mesoporous silica nanoparticles (MSNs) are widely studied and are an interesting material due to its application in wide range of areas, for example, in drug delivery, catalysis, in sensors, and in adsorption and separation. Specifically, MSNs contain high surface area and large pore volume, providing high drug loading capacity, tunable pore size, surface chemistry for accommodation of a variety of guest molecules, and versatile functionalization on the external and internal surface for a broad spectrum of applications. Many new strategies have been developed for the synthesis and functionalization of mesoporous silica-based materials. The functionalization of MSNs is highly important as it leads to the development of new chemical and physical properties. Thus, preparation of these organic/inorganic hybrid structures requires facile and controlled techniques to generate enhanced properties. The grafting of polymers using controlled radical polymerization (CRP) techniques has turned out to be the best suited method to synthesize these well-defined organic-inorganic hybrid MSNs. Most common polymerization techniques are atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide mediated polymerization (NMP). This chapter will be highlighting the state-of-the-art techniques for the synthesis of variety of MSNs, its functionalization using CRP techniques, and application of polymer functionalized MSNs

TEMPO Driven Mild and Modular Route to Functionalized Microparticles

The synthesis of crosslinked polymeric microspheres (3.8–15.0 µm) via (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) initiated thiol‐ene dispersion polymerization under ambient conditions is reported for the first time. The initiating ability of TEMPO for the thiol‐ene reaction is validated by electron paramagnetic resonance (EPR) and 1H nuclear magnetic resonance (NMR) spectroscopy on model reactions between 1‐octadecanethiol and two electron deficient enes, n‐butylacrylate and divinyl sulfone. Critically, the TEMPO resonance observed in the EPR spectra decreases with time when TEMPO is mixed with thiol and an electron deficient ene. The 1H NMR spectra demonstrate formation of up to 90% of thioether under ambient conditions. Based on these model reactions, a variety of crosslinked polymeric microspheres are synthesized with excellent morphological stability using poly(vinyl pyrrolidone) as surfactant. The ability of the microspheres for a second TEMPO initiated thiol‐ene reaction is demonstrated by the ligation of fluorescein‐5‐maleimide (an ene) to the microspheres' surface containing excess of thiol functionality and by ligation of cysteine (containing a thiol group) to the microspheres' surface containing an excess of ene functionality. The synthesized polymeric microspheres are characterized using scanning electron microscopy, differential scanning calorimetry, Fourier‐transform infrared spectroscopy, zeta potential, and X‐ray photoelectron spectroscopy

TEMPO driven mild and modular route to functionalized microparticles

The synthesis of crosslinked polymeric microspheres (3.8–15.0 µm) via (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO) initiated thiol‐ene dispersion polymerization under ambient conditions is reported for the first time. The initiating ability of TEMPO for the thiol‐ene reaction is validated by electron paramagnetic resonance (EPR) and 1H nuclear magnetic resonance (NMR) spectroscopy on model reactions between 1‐octadecanethiol and two electron deficient enes, n‐butylacrylate and divinyl sulfone. Critically, the TEMPO resonance observed in the EPR spectra decreases with time when TEMPO is mixed with thiol and an electron deficient ene. The 1H NMR spectra demonstrate formation of up to 90% of thioether under ambient conditions. Based on these model reactions, a variety of crosslinked polymeric microspheres are synthesized with excellent morphological stability using poly(vinyl pyrrolidone) as surfactant. The ability of the microspheres for a second TEMPO initiated thiol‐ene reaction is demonstrated by the ligation of fluorescein‐5‐maleimide (an ene) to the microspheres' surface containing excess of thiol functionality and by ligation of cysteine (containing a thiol group) to the microspheres' surface containing an excess of ene functionality. The synthesized polymeric microspheres are characterized using scanning electron microscopy, differential scanning calorimetry, Fourier‐transform infrared spectroscopy, zeta potential, and X‐ray photoelectron spectroscopy

pH- and Metal Ion- Sensitive Hydrogels based on N-[2-(dimethylaminoethyl)acrylamide]

Smart hydrogels are promising materials for actuators and sensors, as they can respond to small changes in their environment with a large property change. Hydrogels can respond to a variety of stimuli, for example temperature, pH, metal ions, etc. In this article, the synthesis and characterization of polyampholyte hydrogels based on open chain ligands showing pH and metal ion sensitivity are described. Copolymer and terpolymer gels using different mixtures of monomers i.e., N-[2-(dimethylaminoethyl)acrylamide] (DMAEAAm), N,N-dimethylacrylamide (DMAAm), acrylic acid (AA) and 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS), have been synthesized. The effect of copolymer composition, i.e., the ratio and amount of ionic monomers and the degree of crosslinking on the swelling characteristics, was evaluated as a function of pH. On this basis, metal ion sensitivity measurements were performed at selected pH values. The metal ion sensitivity was measured by varying the concentration of Cu2+, Zn2+ and Ag+ ions under acidic pH conditions

Accessing quantitative degrees of functionalization on solid substrates via solid-state NMR spectroscopy

The development of a solid-state nuclear magnetic resonance (NMR) method allowing the quantification of active sites (i.e., residual vinyl groups) accessible for chemical functionalization on the surface of poly(divinyl benzene) microspheres is presented. Residual vinyl groups of poly(divinyl benzene) microspheres (PDVB55 and PDVB80) were quantified via solid-state (13)C cross-polarization magic-angle spinning (CP-MAS) NMR spectroscopy. In addition, (13)C CP-MAS NMR spectroscopy allows the comparison of core and grafted microspheres functionalization on the same (arbitrary) scale in a short measuring time. This scale was calibrated by an extended absolute quantification of the vinyl groups using (13)C single pulse excitation (SPE) MAS NMR spectroscopy. The degree of cross-linking of the microspheres was calculated to be 30 and 50% for PDVB55 and PDVB80 microspheres, respectively. The number of active groups per nominal surface area is 110 and 179 groups per mm(2) for PDVB55 and PDVB80 microspheres, respectively. The loading capacities of the microspheres (e.g., 0.61 and 0.65 mmol . g(-1)) are not too far removed from those found in Merrifield resins of comparable sizes

Efficient and mild modification of Si surfaces via orthogonal hetero Diels-Alder chemistry

An efficient and mild modification of Si surfaces through orthogonal hetero Diels-Alder (HDA) chemistry, was investigated. The modification of Si wafers was achieved by the silanization of pretreated Si surfaces using 3-(N-strylmethyl-2-aminoethylamino)-propyl trimethoxy silane (SM-TMS). The RAFT-HDA reaction between the PSDTF functionalized PiBA and the styryl functionalized Si surface proceeds in the absence of catalysts at 60°C. The coupling reaction between Si surfaces modified with a styrene linker and PiBA-PSDTF was performed at 60° C in chloroform solution within 12 h. The conjugation leads to the formation of a 3,6-dihydro-2H-thiopyran ring, the stability of which has been investigated and found to be withstanding relatively harsh conditions of pH and heat. The surface modification was also confirmed by measuring the thickness through ellipsometry of the Si wafers after hydrolsis

Strongly electron deficient sulfonyldithioformate based RAFT agents for hetero Diels-Alder conjugation: Computational design and experimental evaluation

The synthesis of a novel class of reversible addition-fragmentation chain transfer (RAFT) agents-based on quantum-chemical ab initio calculations-having methylsulfonyl and phenylsulfonyl moieties as Z-group is described. The resulting C-sulfonyldithioformate transfer agents [benzyl methylsulfonyldithioformate (PSDTF) and benzyl phenylsulfonyldithioformate (PSDTF)] feature extremely strong electron deficient C=S double bonds and should thus be suitable to undergo rapid hetero Diels-Alder (HDA) reactions with variable dienes under mild (i.e., ambient and catalyst free) reaction conditions. It can be demonstrated via a series of model reactions, whose outcome is monitored via electrospray ionization mass spectrometry (ESI-MS), that C-sulfonyldithioformate based RAFT agents undergo HDA reactions with a series of diene-capped macromolecules (10 min\ reaction time\ 24 h) at ambient temperatures (T ≈25°C) with reaction times ranging from 24 h (for open chain dienes) to a few minutes (for cyclopentadiene) in the absence of any catalysts. Concomitantly, PSDTF is able to efficiently mediate the polymerization of isobornyl acrylate (iBA) with living characteristics (2300\ Mn (g/mol)\ 16,000, 1.08\ PDI \ 1.31). In a subsequent step, it is demonstrated that conjugates of poly(iBA) and polystyrene can be constructed under mild reaction conditions (reaction time \ 10 min, T ≈25°C, Mn≈ 6000 (g/mol), PDI≈1.3, no catalyst). In addition, we highlight that sulfonyldithioformate type RAFT agents are such effective dienophiles that they can undergo HDA reactions with certain monomers, including styrene. While such a strong HDA activity limits their use in polymerizations, it opens an avenue for catalyst free efficient surface modification reactions under mild conditions with variable dienes. © 2009 Wiley Periodicals, Inc

Strongly Electron Deficient Sulfonyldithioformate Based RAFT Agents for Hetero Diels-Alder Conjugation: Computational Design and Experimental Evaluation

The synthesis of a novel class of reversible addition-fragmentation chain transfer (RAFT) agents-based on quantum-chemical ab initio calculations-having methylsulfonyl and phenylsulfonyl moieties as Z-group is described. The resulting C-sulfonyldithioformate transfer agents [benzyl methylsulfonyldithioformate (PSDTF) and benzyl phenylsulfonyldithioformate (PSDTF)] feature extremely strong electron deficient C=S double bonds and should thus be suitable to undergo rapid hetero Diels-Alder (HDA) reactions with variable dienes under mild (i.e., ambient and catalyst free) reaction conditions. It can be demonstrated via a series of model reactions, whose outcome is monitored via electrospray ionization mass spectrometry (ESI-MS), that C-sulfonyldithioformate based RAFT agents undergo HDA reactions with a series of diene-capped macromolecules (10 min\ reaction time\ 24 h) at ambient temperatures (T ≈25°C) with reaction times ranging from 24 h (for open chain dienes) to a few minutes (for cyclopentadiene) in the absence of any catalysts. Concomitantly, PSDTF is able to efficiently mediate the polymerization of isobornyl acrylate (iBA) with living characteristics (2300\ Mn (g/mol)\ 16,000, 1.08\ PDI \ 1.31). In a subsequent step, it is demonstrated that conjugates of poly(iBA) and polystyrene can be constructed under mild reaction conditions (reaction time \ 10 min, T ≈25°C, Mn≈ 6000 (g/mol), PDI≈1.3, no catalyst). In addition, we highlight that sulfonyldithioformate type RAFT agents are such effective dienophiles that they can undergo HDA reactions with certain monomers, including styrene. While such a strong HDA activity limits their use in polymerizations, it opens an avenue for catalyst free efficient surface modification reactions under mild conditions with variable dienes