Lutz Prager

Cationic RuII Complexes with N‐Heterocyclic Carbene Ligands for UV‐Induced Ring‐Opening Metathesis Polymerization

Metathesis chemistry and, in the context of polymer chemistry, ring-opening metathesis polymerization (ROMP) have gained a strong position in chemistry and materials science. ROMP is strongly associated with two classes of well-defined metal alkylidene based initiators, molybdenumbased Schrock and ruthenium-based Grubbs type initiators. Despite the tremendous achievements in catalyst development, both families of initiators are still experiencing ongoing, vivid development. Most Grubbs type initiators work at room temperature or require only gentle warming to work properly. More recently, an increasing number of reports on latent Ru-based initiators has appeared. Such precatalysts are of particular interest in technical applications of ROMP, since they allow for premixing, that is, the preformulation of a monomer/precatalyst mixture, its storage over a longer period of time even at elevated temperatures (usually less than 45 8C), and, most importantly, the shaping and profiling of such mixtures prior to polymerization (“curing”). Numerous latent Grubbs type initiators have been reported recently; however, all these precatalysts are triggered thermally. By contrast, surface modification and functionalization require UV-triggerable precatalysts. Few such systems have been reported to date. The synthesis of photoactive Schrock type tungsten-based compounds as well as ruthenium and osmium arene compounds of the general formula [Ru(p-cymene)Cl2(PR3)] and [Os(p-cymene)Cl2(PR3)] (R= cyclohexyl, etc.) were first reported by van der Schaaf et al. They also investigated the photoinduced polymerization of different functionalized norbornenes and 7-oxanorbornenes using various [Ru(solvent)n]X2 complexes, (X= tosylate, trifluoromethanesulfonate) as well as Ru half-sandwich and sandwich complexes. Noels and co-workers reported on the visiblelight-induced ROMP of cyclooctene using [RuCl2(IMes)(pcymene)] (IMes= 1,3-dimesitylimidazol-2-ylidene). Some of these systems were also used in ring-closing metathesis reactions. Most of the systems available to date, however, have significant disadvantages. They either show low activity, resulting in low polymer yields (less than 30%) in the photochemically triggered process, or the irradiation wavelength necessary to trigger ROMP is 360 nm or higher. In the latter case, the initiatorsA thermal stability is generally poor, thus discouraging their application in photoinduced ROMP. Thus, none of the systems reported to date was entirely thermally stable above or even at room temperature. Therefore, these systems do not fulfill the requirements of a truly latent photocatalyst. Herein, we report the development of the first thermally stable, truly UV-triggerable precatalysts for ROMP and their application in surface functionalization. We commenced our investigations with [Ru(IMesH2)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-1) and [Ru(IMes)(CF3CO2)(tBuCN)4)] CF3CO2 (PI-2), which were prepared from [Ru(CF3CO2)2(L)(p-cymene)] [31,32] (L= IMes or IMesH2, 1,3-dimesityl-4,5-diyhdroimidazolin-2-ylidene) by reaction with excess tBuCN. Both compounds can be handled in air. H and C NMR spectroscopy data and elemental analysis reveal the presence of one N-heterocyclic carbene (NHC) ligand, two inequivalent trifluoroacetate groups, and four tBuCN ligands, suggesting cationic Ru complexes. The structures of PI-1 and PI-2 were confirmed by X-ray analysis; the structure of PI-1 is shown in Figure 1 (see also the Supporting Information). Upon mixing of either PI-1 or PI-2 with monomers 3–8 (Scheme 1), no reaction was observed at room temperature within 24 h. Even highly reactive (distilled) dicyclopentadiene (4) did not react with PI-1 or PI-2 at room or elevated temperature (RT<T< 45 8C) in the absence of light. Heating a mixture of 8 with PI-1 or PI-2 in 1,2-dichloroethane to 60 8C resulted in the formation of much less than 10% polymer within 24 h. However, exposing mixtures of either PI-1 or PI2 in chloroform with these monomers to 308-nm light at room temperature resulted in the formation of the corresponding polymers. Yields were between less than 5 and 99% (Table 1). Increasing the energy of the light by switching from 308 nm to a 254-nm Hg lamp gave raise to high, in most cases virtually quantitative, yields (Table 1). The molecular weights [*] Dr. D. Wang, Dr. W. Knolle, Dr. U. Decker, Dr. L. Prager, Dr. S. Naumov, Prof. Dr. M. R. Buchmeiser Leibniz-Institut f.r Oberfl/chenmodifizierung e.V. (IOM) Permoserstrasse 15, 04318 Leipzig (Germany) Fax: (+49)341-235-2584 E-mail: michael.buchmeiser@iom-leipzig.de Homepage: http://www.iom-leipzig.de/index_e.cfm

Microimaging of Transient Concentration Profiles of Reactant and Product Molecules during Catalytic Conversion in Nanoporous Materials

Microimaging by IR microscopy is applied to the recording of the evolution of the concentration profiles of reactant and product molecules during catalytic reaction, notably during the hydrogenation of benzene to cyclohexane by nickel dispersed within a nanoporous glass. Being defined as the ratio between the reaction rate in the presence of and without diffusion limitation, the effectiveness factors of catalytic reactions were previously determined by deliberately varying the extent of transport limitation by changing a suitably chosen system parameter, such as the particle size and by comparison of the respective reaction rates. With the novel options of microimaging, effectiveness factors become accessible in a single measurement by simply monitoring the distribution of the reactant molecules over the catalyst particles.

Vacuum‐UV Irradiation‐Based Formation of Methyl‐Si‐O‐Si Networks from Poly(1,1‐Dimethylsilazane‐co‐1‐methylsilazane)

The vacuum-UV (VUV)-induced conversion of commercially available poly(1,1-dimethylsilazane-co-1-methylsilazane) into methyl-Si-O-Si networks was studied using UV sources at wavelengths around 172, 185, and 222 nm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS), X-ray photo electron spectroscopy (XPS), and Fourier transform infrared (FTIR) measurements, as well as kinetic investigations, were carried out to elucidate the degradation process. First-order kinetics were found for the photolytically induced decomposition of the Si-NH-Si network, the subsequent formation of the methyl-Si-O-Si network and the concomitant degradation of the Si-CH(3) bond, which were additionally independent of the photon energy above a threshold of about 5.5 eV (225 nm). The kinetics of these processes were, however, dependent on the dose actually absorbed by the layer and, in the case of Si-O-Si formation, additionally on the oxygen concentration. The release of ammonia and methane accompanied the conversion process. Quantum-chemical calculations on methyl substituted cyclotetrasilazanes as model compounds substantiate the suggested reaction scheme. Layers <100 nm in thickness based on mixtures of poly(1,1-dimethylsilazane-co-1-methylsilazane) and perhydropolysilazane (PHPS) were coated onto polyethylene terephthalate (PET) foils by a continuous roll to roll process and cured by VUV irradiation by using wavelengths <200 nm and investigated for their O(2) and water vapor-barrier properties. It was found that the resulting layers displayed oxygen and water vapor transmission rates (OTR and WVTR, respectively) of <1 cm(3) m(-2) d(-1) bar(-1) and <4 g m(-2) d(-1), respectively.

222 nm Photo-induced radical reactions in silazanes. A combined laser photolysis, EPR, GC-MS and QC Study

The initiation mechanism of the VUV-induced conversion of polyorganosilazanes into methyl-Si-O-Si networks was studied by means of model disilazane compounds. A combined experimental approach was chosen to determine the primary radicals and their properties (lifetimes, spectra) as well as the major final products. It was verified that both Si-N and Si-CH(3) cleavage occur in the condensed phase, the former with higher yield. The lifetime of the primary Si- and N-centred radicals in de-oxygenated n-hexane solution is less than </=10 mus. N-centred radicals transform into amines by H abstraction, the availability of weakly bonded H as in the case of tetramethyldisilazane accelerates the reaction considerably. In rigid matrix (frozen solutions) CH(3), silyl radicals and methylene radicals CH(2)R are trapped. In the presence of oxygen, peroxyl radicals are formed and serve as precursors of the subsequent oxidative conversion. Product analysis by GC-MS reveals linear R-(Si-O)(n)- chains rather than branched compounds as the initial products of the oxidative conversion of tetramethyldisilazane. It was shown that reactive silylene intermediates do not play a role in the conversion process. Quantum chemical calculations assist in the interpretation.

Determination of the Thickness of Silazane-Based SiOx Coatings in the Submicrometer Range by Near-Infrared Reflection Spectroscopy

The thickness of thin silica layers in the submicrometer range, i.e., between about 150 and 700 nm, was determined by near-infrared (NIR) reflection spectroscopy. Silica layers were prepared by spin-coating of perhydropolysilazane (PHPS) on silicon wafers or poly(ethylene terephthalate) (PET) foil and subsequent conversion of the PHPS into SiOx by vacuum ultraviolet (VUV) irradiation at 172 nm. Since the NIR spectra of the inorganic layers do not show overtone and combination bands, analysis is based on tiny differences in reflectance of samples provided with layers of different thicknesses. Quantitative investigations were carried out by use of chemometric approaches on the basis of the partial least squares (PLS) algorithm. Optimization of the chemometric models was achieved by systematic variation of the preprocessing of the spectra before application of the PLS regression. The root mean square error of prediction (RMSEP) and the coefficient of determination R2 were used for the evaluation of the various pretreatment strategies. Reference data for the calibration procedures were obtained by means of gravimetry. The maximum error for the determination of the thickness was estimated to be on the order of 20%. The method was used to monitor the homogeneity of the thickness of silica layers made by use of a pilot scale coating machine. Thickness profiles recorded by NIR spectroscopy showed clear differences between layers with uniform or non-uniform quality of the application. Moreover, a close correlation of the profiles with the average coating weights determined by gravimetry was found.

Ultraviolet Light and Electron Beam Induced Degradation of Trichloroethene

Chlorinated hydrocarbons such as trichloroethene (TCE) and tetrachloroethene are widely used as solvents and degreasing agents. Emission of these volatile compounds causes considerable environmental pollution. In the extreme case of spillage by accident or criminal negligence, they quickly penetrate into the soil and the groundwater table on account of their relatively high specific weight compared to water and their fluidity. In the soil under anaerobic conditions, these compounds undergo bacterial degradation, prominent products being cis-1,2-dichloroethene and vinyl chloride whose carcinogenic potential is known (Nerger and Mergler-Volkl, 1988).

Wetting Dynamics on Superhydrophilic Surfaces Prepared by Photonic Microfolding

The wetting dynamic on microrough and perfectly wetting (superhydrophilic) acrylates was studied. These surfaces were achieved by coating polymer films made of poly(ethyleneterephthalate) (PET) with a hydrophilic acrylate based on hydroxypropylacrylate and polyethyleneglycolmonoacrylate, which was then cured and microroughened by photonic microfolding. The high transparency of the thin acrylate layers and polymer films allowed us to record the spreading of an applied water droplet through the film samples. Subsequently, the dynamic radius of the spreading pattern rc(t) was determined from the video recording. Various models for the wetting dynamics of superhydrophilic surfaces, namely, Tanners law and a roughness-modified derivation published by McHale et al. in 2009, were then compared to the experimental results. Basically, the development of rc(t) in time was found to be in good agreement with McHales model. Data analysis showed, however, that the initial phase of the spreading, that is, for t < 1 s, was not predicted well. This differing behavior relates well to a theory published by Cazabat and Cohen Stuart, who proposed that, on rough surfaces, spreading follows a power law in three time regimes. In this model, the (very) initial spreading is expected to be similar to the spreading on a smooth surface.

Electron Beam Degradation of Chlorinated Hydrocarbons Air-stripped from Polluted Ground Water: a Laboratory and Field Study

Abstract Pilot-plant studies with strip-gas from a ground water remediation site at Dusseldorf, Germany (gas throughput 1000 m3 h-1, generated from 100 m3 h-1 water), demonstrated that vapour-phase electron-beam treatment of chlorinated hydrocarbons (CHC) such as cis-1,2-di-, tri- and tetrachloroethene, vinyl chloride, 1,1,1-trichloro- ethane, tri- and tetrachloromethane, which are released into the gas phase by soil-vapour extraction or by airstripping of contaminated ground water, is a very cost-efficient pollution-abatement technique. Laboratory studies have shown that for the chloroethenes, an OH-radical-induced chain reaction constitutes the main degradation pathway. Even at a relatively low dose of 2.5 kGy, electron-beam irradiation of CHC-polluted strip-gas (CHC total 3 ppmv, mainly cis-1,2-di-, tri- and tetrachloroethenes) achieves a higher than 90% decomposition of CHC with more than 80 % mineralization.