Wolfgang Knolle

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

Maleimides as electron-transfer photoinitiators: quantum yields of triplet states and radical-ion formation

Abstract The knowledge of quantum yields of triplet states and, moreover, of initiating radicals is a crucial prerequisite to evaluate any novel photoinitiator system. This work provides triplet quantum yields of N -methylmaleimide (0.03±0.01), N -ethylmaleimide (0.07±0.01) and N -propylmaleimide (0.05±0.02) determined by relative actinometry using laser flash photolysis and acetone sensitisation. Unsubstituted maleimide on the other hand shows rapid triplet state tautomerisation, not allowing the application of relative actinometry with triplet sensitisation. The triplet quantum yield was then determined by comparing the transient conductivity of the enolate formed with conductivity actinometry; it is unity. The yield of initiating radicals is only a fraction of the triplet yield, as the electron transfer reaction includes an efficient back donation. The radical ion yields measured span from 2% (thiocyanate) to 28% (allylthiourea) of the triplet quantum yield.

Radiation-induced radical formation and crosslinking in aqueous solutions of N-isopropylacrylamide

To investigate chain-initiating and crosslinking mechanisms, radical formation in dilute aqueous solutions of N-isopropylacrylamide (NIPAAm) and poly-NIPAAM was studied using electron pulse radiolysis with optical detection at room temperature. Several transients of NIPAAm generated by reactions with electrons, hydroxyl radicals and hydrogen atoms were observed. Electron attachment to the carboxyl group (k c = 9.0 x 10 9 dm 3 .mol -1 .s -1 ) forms the radical anion, which undergoes fast and reversible protonation (pK a = 7.8) at the carboxyl oxygen. At pH >> pK a , slow and irreversible protonation of the electron adduct at the vinyl group leads to the α-carboxyalkyl radical CH 3 ( . CH)CONHCH(CH 3 ) 2 , which is also formed by addition of H atoms to NIPAAm (k H = 7.3 x 10 9 dm 3 .mol 1 .s -1 ). Addition of OH radicals (k OH = 5.4 x 10 9 dm 3 .mol -1 .s -1 ) forms CH 2 (OH))( . CH))CONHCH(CH 3 ) 2 . Hydrogen abstraction was not observed in the case of NIPAAm monomer, but it was found for the reaction of OH radicals with thermally polymerized NIPAAm. Semi-empirical quantum chemical calculations support the assignment of the observed spectra to the radicals. A reaction mechanism for the formation of crosslinks is discussed.

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.

Radical formation in electron-irradiated acrylates studied by pulse radiolysis and electron paramagnetic resonance

Electron pulse radiolysis with optical and conductometric detection as well as low-temperature electron paramagnetic resonance (EPR) were used to study the mechanism of electron-irradiation induced polymerisation and cross-linking of acrylates such as triethyleneglycol (TPGDA) and butanediol (BUDA) diacrylate. Irradiation of acrylates with fast electrons generates radical cations and anions that rapidly transforms to radicals. Using butyl chloride or tetrahydrofuran and water as solvents cationic and anionic pathways of radical formation were studied separately. Acrylate radical cations either deprotonate forming vinyl-type radicals or generate covalently bonded or resonance stabilised dimer cations. Vinyl-type radicals as well as dimer cations are able to initiate polymerisation. EPR results show that a structural transformation of TPGDA primary cations takes place by H-transfer from the ester to the positively charged carbonyl group. Addition of solvated electrons to acrylates forms anions. Protonation of the anion at the carbonyl site leads to ketyl-type radicals which are also able to initiate polymerisation. It is concluded that after electron-irradiation of acrylates both cations as well as anions are important precursors of radicals which induce radical polymerisation and cross-linking.

Tailoring the material properties of gelatin hydrogels by high energy electron irradiation

Natural hydrogels such as gelatin are highly desirable biomaterials for application in drug delivery, biosensors, bioactuators and extracellular matrix components due to strong biocompatibility and biodegradability. Typically, chemical crosslinkers are used to optimize material properties, often introducing toxic byproducts into the material. In this present work, electron irradiation is employed as a reagent-free crosslinking technique to precisely tailor the viscoelasticity, swelling behavior, thermal stability and structure of gelatin. With increasing electron dose, changes in swelling behavior and rheology indicate increasing amounts of random coils and dangling ends as opposed to helical content, a result confirmed through Fourier transform infrared spectroscopy. Gel fraction, rheology and swelling measurements at 37 °C were used to verify thermal stability in biological conditions. Scanning electron microscopy images of dried gelatin samples support these conclusions by revealing a loss of free volume and apparent order in the fracture patterns. The degree of crosslinking and mesh size are quantified by rubber elasticity theory and the Flory-Rehner equation. Overall, precise control of material properties is demonstrated through the interplay of concentration and irradiation dose, while providing an extensive parameter-property database suitable for optimized synthesis.

Radiation-induced reactions of benzoyl chloride and acrylates in solution. A pulse radiolysis study

Using electron pulse radiolysis with optical detection, the radiation-induced reactions of benzoyl chloride and acrylates were studied in tetrahydrofuran and acetonitrile solution at room temperature. In both solvents electron transfer leads to the formation of transient radical anions of acrylates (k≈2–3×1010 dm3 mol−1 s−1) and of benzoyl chloride (k≈3×1010 dm3 mol−1 s−1). The latter dissociates into chloride ion and the benzoyl radical (k=3×106 s−1), whereas the acrylate anion transforms by protonation into a ketyl-type radical. In mixed solutions a fast electron transfer from acrylate anions to benzoyl chloride is found (k≈1×1010 dm3 mol−1 s−1). The benzoyl chloride anion reacts with the monomer (k=2.8×109 dm3 mol−1 s−1) with simultaneous release of Cl−, forming species which may initiate polymerization. In the presence of oxygen the formation of benzoylperoxy radicals (k=1.6×109 dm3 mol−1 s−1), showing a strong absorption band in the near-UV (λmax=400 nm), is observed.

Direct initiation of the photopolymerization of acrylates by short-wavelength excimer UV radiation

Abstract Investigations on the direct initiation of the photopolymerization of acrylates by irradiation with short-wavelength monochromatic UV light using the 222 nm emission of a KrCl * excimer lamp are reported. The reactivity of various acrylates was studied by real-time FTIR-ATR spectroscopy. Laser photolysis experiments and accompanying quantum chemical calculations were performed in order to propose a possible mechanism of initiation.

Electron-beam derived polymeric cryogels

In this study we publish a novel attempt to macroporous, polymeric cryogels (MPCs) by electron-beam (EB) initiated free radical crosslinking polymerization of (meth)acrylates in frozen aqueous media. The EB-process is an environmentally benign fast process (radiation and reaction time about 10–20 min) which allows the initiator-free polymerization of double-bond containing monomers. The MPCs were thoroughly characterized by swelling experiments, SEM, detailed XPS studies, DMA, Hg intrusion porosity and ESEM. The processes during the reaction were studied using ESR experiments. The influences of the reaction parameters like dose, radiation time, freezing conditions, chemical composition and (meth)acrylate concentration on the properties of the MPCs were investigated. The MPCs were synthesized in situ in capillaries or in tubes up to a diameter of 25 mm. The porous structure of the MPCs is mainly controlled by the freezing temperature and the macromonomer/crosslinker concentration and consists of large interconnected pores in the range of 10–70 μm. The crosslinker concentration directly affects the stiffness and the Tg of the materials. The MPCs were successfully modified with poly(allylamine hydrochloride). This straightforward method provides a toolbox for the synthesis of manifold MPCs from various unsaturated substances in large quantities and scale.

Distonic dimer radical cation of 2,3-dihydrofuran: Quantum chemical calculations and low-temperature EPR results

Abstract The distonic dimer radical cation of the 2,3-dihydrofuran was radiolytically generated in Freon matrix and studied by low-temperature EPR spectroscopy. The structure of the dimer radical cation was also investigated using different quantum chemical methods. The BH&HLYP, similarly as ab initio HF and MP2 methods, predict in agreement with the experiment a distonic dimer radical cation, with strongly separated spin and charge. The widely used DFT B3LYP and B3PW91 methods fail in this case and lead to a wrong (delocalized) structure.