Wolfram Schnabel

Bio: Wolfram Schnabel is an academic researcher from Istanbul Technical University. The author has contributed to research in topics: Flash photolysis & Cationic polymerization. The author has an hindex of 24, co-authored 99 publications receiving 1682 citations. Previous affiliations of Wolfram Schnabel include RÖHM GmbH.

Phenyl-2,4,6-trimethylbenzoylphosphinates as water-soluble photoinitiators. Generation and reactivity of OṖ(C6H5)(O−) radical anions

Abstract: Lithium- and magnesium phenyl-2,4,6-trimethylbenzoylphosphinates (TMPPL and TMPPM) are effective water-soluble photoinitiators for the free-radical polymerization of appropriate monomers such as acrylamide (AA) and methacrylamide (MAA) in aqueous solution. They are also capable of initiating the polymerization of other olefinic compounds such as styrene (St), methyl methacrylate (MMA) or acrylonitrile (AN) in water-containing solvent mixtures such as 1:1 water-acetonitrile mixtures. This is due to the fact that TMPPL and TMPPM undergo α-scission with a rather high quantum yield (ϕ(α) ≈ 0,35) resulting in the formation of 2,4,6-trimethylbenzoyl radicals and OṖ (C6H5)(O−) radical anions. The latter are very reactive toward olefinic monomers. Bimolecular rate constants kR+M/(L/(mol · s)) were determined by flash photolysis at room temperature, e. g. in neat water: 3,8 · 108 (MAA), 2,2 · 108 (AA), and in H2O/CH3CN (1:1, v/v): 1,8 · 107 (St), 1,2 · 108 (MMA), 8,4 · 107 (AN).

On the reactivity of phosphonyl radicals towards olefinic compounds.

Abstract: Phosphonyl radicals of the structure OṖR1 (R2) with R1 and R2: C6H5(1); C6H5, OCH(CH3)2(2); CH3, OCH3(4) and OC2H5(5) were generated by UV-photolysis of appropriate acylphosphine oxides or acylphosphonates which are effective initiators of the free radical polymerization of olefinic compounds. All five radicals are very reactive towards monomers such as methacrylonitrile, styrene and methyl methacrylate (k = 5 · 107 to 2 · 108 1 · mol−1 · s−1), as was found by employing laser flash photolysis techniques. In the cases of acrylonitrile, methyl acrylate, butyl vinyl ether, and vinyl acetate k ranged from 106–107 1 · mol−1 · s−1. The high reactivity of the radicals 1 – 5 is due to their tetrahedral structure. The Q-e scheme of Alfrey and Price proved useful to recognize trends in the dependence of k on the chemical nature of both phosphonyl radicals and monomers.

New aspects on the photoinitiated free radical promoted cationic polymerization

Abstract: General aspects of the photoinitiated free radical promoted cationic polymerization are discussed. The applicability of pyridinium salts, a new class of photoinitiators, of appropriate redox potential Ered1/2 in oxidizing carbon-centered free radicals to carbocations capable of initiating the polymerization of various compounds is demonstrated. Moreover, the use of polysilanes as source for readily oxidizable free radicals is described. Furthermore, possibilities for preparing block copolymers by free radical promoted cationic polymerization are pointed out.

Photoinitiated Polymerization: Advances, Challenges, and Opportunities

Abstract: The use of photoinitiated polymerization is continuously growing in industry as reflected by the large number of applications in not only conventional areas such as coatings, inks, and adhesives but also high-tech domains, optoelectronics, laser imaging, stereolithography, and nanotechnology. In this Perspective, the latest developments in photoinitiating systems for free radical and cationic polymerizations are presented. The potential use of photochemical methods for step-growth polymerization is also highlighted. The goal is, furthermore, to show approaches to overcome problems associated with the efficiency, wavelength flexibility, and environmental and safety issues in all photoinitiating systems for different modes of activation. Much progress has been made in the past 10 years in the preparation of complex and nano-structured macromolecules by using photoinitiated polymerizations. Thus, the new and emerging applications of photoinitiated polymerizations in the field of biomaterials, surface modific...

3D-Printed Microfluidics.

Abstract: The advent of soft lithography allowed for an unprecedented expansion in the field of microfluidics. However, the vast majority of PDMS microfluidic devices are still made with extensive manual labor, are tethered to bulky control systems, and have cumbersome user interfaces, which all render commercialization difficult. On the other hand, 3D printing has begun to embrace the range of sizes and materials that appeal to the developers of microfluidic devices. Prior to fabrication, a design is digitally built as a detailed 3D CAD file. The design can be assembled in modules by remotely collaborating teams, and its mechanical and fluidic behavior can be simulated using finite-element modeling. As structures are created by adding materials without the need for etching or dissolution, processing is environmentally friendly and economically efficient. We predict that in the next few years, 3D printing will replace most PDMS and plastic molding techniques in academia.

Photoinduced Electron Transfer Reactions for Macromolecular Syntheses

Abstract: Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications ...

A Versatile Synthetic Extracellular Matrix Mimic via Thiol‐Norbornene Photopolymerization

Abstract: Synthetic hydrogels with engineered, cell-mediated degradation sites are an important category of biomimetic materials. Here, hydrogels are synthesized by a step-growth reaction mechanism via a radically mediated thiol-norbornene (thiol-ene) photopolymerization. This reaction combines the advantages of ideal, homogeneous polymer network formation, facile incorporation of peptides without post-synthetic modification, and spatial and temporal control over the network evolution into a single system to produce proteolytically degradable poly(ethylene glycol) (PEG) peptide hydrogels. Using a thiol-ene photopolymerization, rapid gelation times are achieved, while maintaining high cell viability for cell encapsulation. The enzyme- and cellresponsive characteristics are demonstrated by tailoring the rate of spreading of human mesenchymal stem cells (hMSCs) through both the selection of proteolytically degradable crosslinkers and the density of the adhesion peptide RGDS. Furthermore, cellular function is manipulated spatially within the thiol-ene hydrogels through biochemical photopatterning. The high degree of spatial and temporal control over gelation, combined with robust material properties, makes thiol-ene hydrogels an excellent tool for a variety of medical and biological applications.