7 research outputs found
Kinetic Investigations of Cu-Mediated ATRP in Aqueous Solution
No full textCu-mediated atom-transfer radical polymerization (ATRP) is studied via on-line vis/NIR spectroscopy in an aqueous solution of a monomer-free model system, with CuBr/2,2-bipyridine acting as the catalyst, and 2-hydroxyethyl 2-bromoisobutyrate as the initiator, at a pressure of up to 2000 bar. Excess NaBr is added to avoid the water-assisted dissociation of the Br-Cu(II)/L bond. The activation-deactivation equilibrium constant, K-model, is measured at different compositions of the water-poly(ethylene glycol) dimethylether (PEO) solvent mixture, in which PEO mimics a water-soluble monomer. K-model increases by about three orders of magnitude in passing from a PEO to a water environment. The change in K-model is essentially due to the effect on the activation rate coefficient. Kinetic analysis of the model system upon variation of NaBr concentration in conjunction with predici simulations shows that the NaBr content has no significant impact on the activation and deactivation rate coefficients and thus on K-model, but on dispersity and on the degree of chain-end functionality.Deutsche Forschungsgemeinschaft; Fonds der Chemischen Industri
Propagation and Termination Kinetics of Poly(Ethylene Glycol) Methyl Ether Methacrylate in Aqueous Solution
No full textThe propagation rate coefficient, k(p), of poly(ethylene glycol) methyl ether methacrylate (M-n approximate to 500 g mol(-1)) has been measured via pulsed-laser polymerization (PLP)-size-exclusion-chromatography in aqueous solution between 5 wt% monomer and bulk polymerization at temperatures from 22 to 77 degrees C. k(p) increases significantly toward higher water content, as is observed for other water-soluble monomers. This entropy-motivated effect enhances the pre-exponential. The activation energy, E-A(k(p)), is more or less identical to the characteristic value of methacrylates. The chain-length-dependent rate coefficient, k(t)(i,i), for termination of two radicals of chain length i has been investigated at low degrees of monomer conversion via the single-pulse-PLP-electron paramagnetic resonance technique. k(t)(i,i) turned out to be adequately represented by the composite model designed by the Russell group. The power-law exponents for the chain-length dependence of small and long radicals are close to the numbers reported for other monomers. The rate coefficient for termination of two radicals of chain length unity scales with the fluidity of the reaction mixture. Viscosity measurements prior to polymerization thus enable estimates of termination rate.Deutsche Forschungsgemeinschaf
Chain-Length-Dependent Termination in Radical Polymerization of Acrylates
No full textThe technique of SP PLP EPR, which is single-pulse pulsed-laser
polymerization (SP PLP) in conjunction with electron paramagnetic resonance (EPR)
spectroscopy, is used to carry out a detailed investigation of secondary (chain-end) radical
termination of acrylates. Measurements are performed on methyl acrylate, n-butyl acrylate
and dodecyl acrylate in bulk and in toluene solution at –40 °C. The reason for the low
temperature is to avoid formation of mid-chain radicals, a complicating factor that has
imparted ambiguity to the results of previous studies of this nature. Consistent with these
previous studies, composite-model behavior for chain-length-dependent termination rate
coefficients, kt
i,i, is found in this work. However, lower and more reasonable values of α
s, the
exponent for variation of kt
i,i at short chain lengths, are found in the present study. Most likely
this is because of the absence of mid-chain radicals, thereby validating the methodology of
this work. Family-type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of
toluene: α
s = 0.79, α
l = 0.21 (long chains) and ic ≈ 30 (crossover chain length). All indications
are that these values carry over to termination of acrylate chain-end radicals at higher, more
practical temperatures. Further, these values largely make sense in terms of what is
understood about the physical meaning of the parameters. Variation of the rate coefficient for
termination between monomeric radicals, kt
1,1, is found to be well described by the simple
Smoluchowski and Stokes-Einstein equations. This allows easy prediction of kt
1,1 for different
alkyl acrylates, solvent and temperature. Through all this the unrivalled power of SP PLP
EPR for measuring and understanding (chain-length-dependent) termination rate coefficients
shines through
Kinetics of Fe-Mesohemin-(MPEG(500))(2)-Mediated RDRP in Aqueous Solution
No full textA speciation analysis for Fe-mesohemin-(MPEG(500))(2)-mediated reversible-deactivation radical polymerization (RDRP) in aqueous solution was carried out by a combination of visible (vis) and Fe-57 Mossbauer spectroscopy. The results were used within kinetic studies of ATRP and OMRP reactions via highly time-resolved EPR spectroscopy. ATRP control was effective with the rate coefficient for deactivation clearly exceeding the one for formation of organometallic species. Deactivation rate coefficients increase by more than 1 order of magnitude in passing from polymerization in 30 to 90 wt % H2O. Media with water contents of and above 70 wt % are well suited for controlled ATRP. The Fe-mesohemin catalyst provides an exceptionally high ATRP equilibrium constant even at ambient temperature, which approaches the one of highly active Cu catalysts
Cu- and Fe-mediated Atom-Transfer Radical Polymerization in Aqueous Solution
No full textThe mechanism and the kinetics of metal-catalyzed radical polymerization were investigated by spectroscopic methods and by PREDICI® simulation. A particular focus was on Cu- and Fe-mediated atom-transfer radical polymerization (ATRP) in aqueous solution of poly(ethylene glycol)ether methacrylate (PEGMA) and on monomer-free model systems.
The propagation kinetics of PEGMA in aqueous solution were determined between 20 and 77 °C via PLP–SEC. The significant dependence of kp on monomer content is due to the difference in the degree by which internal rotations of the transition state for propagation are hindered.
Chain-length-dependent termination was analyzed in terms of the composite model for PEGMA in aqueous solution via SP–PLP–EPR. The termination rate coefficient for two radicals of chain-length unity, kt1,1, scales with the inverse viscosity of the solution prior to polymerization. The composite-model parameters for the short-chain and long-chain regime, αs and αl, respectively, are independent of water content, whereas the crossover chain length, ic, decreases toward higher dilution.
Cu-mediated ATRP in aqueous solution of the monomer-free model systems and of PEGMA polymerizations with CuBr/2,2’-bipyridine acting as the catalyst and 2-hydroxyethyl 2-bromoisobutyrate (HEMA Br) as the initiator were studied via online Vis/NIR spectroscopy. In the monomer-free model system, PEGMA was replaced by poly(ethylene glycol) dimethylether (PEO) to mimic an ATRP situation. The SP–PLP–EPR technique was used for the first time to measure an ATRP deactivation rate coefficient, kdeact, in aqueous solution. Excess NaBr has been added to the polymerization system to avoid water-assisted dissociation of the Br-Cu species.
The activation–deactivation equilibrium constant, KATRP, was measured at different water concentrations. In both the model system and the PEGMA polymerization, KATRP increases by about three orders of magnitude in passing from the bulk monomer toward a water environment. Since kdeact is independent of water content, the change in KATRP is essential due to the effect of the aqueous environment on the activation rate coefficient, kact.
Kinetic analysis of the model system in conjunction with PREDICI® simulation under variation of NaBr concentration shows that NaBr does not affect kact and kdeact, and thus has no impact on KATRP. PREDICI® simulation of the ATRP systems however tells that the concentrations of water and NaBr influence dispersity and the degree of chain-end functionality. Addition of at least five equivalents salt with respect to the total catalyst concentration are essential for carrying out successful ATRP experiments in aqueous solution.
Fe-mediated RDRP studies were performed with the bio-inspired protoporphyrin IX containing a ferric ion catalyst with an additional axial bromide ligand, Fe/Br mesohemin-(MPEG500)2. The catalyst was kindly provided by the Matyjaszewski group.[1] The Fe-catalyst was studied by combined Mössbauer and online Vis/NIR spectroscopic analysis for the relevant Fe species. The interplay between ATRP and an organometallic reaction (OM), which includes the reaction of propagating radicals with FeII, may occur depending on the ratio of FeII/FeIII concentrations.
The SP–PLP–EPR method was also applied to measure kdeact for the FeIII/Br-mesohemin-(MPEG500)2 catalyst in aqueous solution. Toward higher water content, kdeact increases by about one order of magnitude from 30 to 90 wt% H2O, which is beneficial for ATRP control in diluted aqueous solution.
The activation–deactivation equilibrium and the addition of radicals to the FeII catalyst, kadd,Fe, were measured for the Fe/Br-mesohemin-(MPEG500)2 complex via UV/Vis spectroscopy in combination with stopped-flow injection. KATRP was found to be insensitive toward water content in the concentration range between 50 and 70 wt% H2O, whereas kadd,Fe exhibits an increase by a factor of five. It could be shown that kdeact exceeds kadd,Fe by almost one order of magnitude, and that the control operates exclusively by ATRP.
The rate coefficients determined within this thesis allow for the prediction of dispersity, chain-end functionality and conversion vs time profiles for Cu- and Fe-mediated ATRP of PEGMA in bulk and aqueous solutions with the investigated catalysts and with catalysts of similar reactivity
Kinetics of Fe–Mesohemin–(MPEG<sub>500</sub>)<sub>2</sub>‑Mediated RDRP in Aqueous Solution
No full textA speciation
analysis for Fe–mesohemin–(MPEG500)2-mediated reversible-deactivation radical polymerization
(RDRP) in aqueous solution was carried out by a combination of visible
(vis) and 57Fe Mössbauer spectroscopy. The results
were used within kinetic studies of ATRP and OMRP reactions via highly
time-resolved EPR spectroscopy. ATRP control was effective with the
rate coefficient for deactivation clearly exceeding the one for formation
of organometallic species. Deactivation rate coefficients increase
by more than 1 order of magnitude in passing from polymerization in
30 to 90 wt % H2O. Media with water contents of and above
70 wt % are well suited for controlled ATRP. The Fe–mesohemin
catalyst provides an exceptionally high ATRP equilibrium constant
even at ambient temperature, which approaches the one of highly active
Cu catalysts
About the Dutch method of wax-resin lining of canvas paintings in Croatia
No full textOsmišljena i prvi put primijenjena sredinom 19. stoljeća u Amsterdamu, nizozemska metoda dubliranja slika na platnu širila se svijetom razmjenom između restauratora sve do kasnog 20. stoljeća. Premda su opseg i putovi širenja metode još uvijek predmet proučavanja, prema istraživanju autorice, u Hrvatsku ju je 1916. godine uveo naš prvi restaurator Ferdo Goglia. U Splitu se sustavno primjenjivala u radionici Regionalnog zavoda za zaštitu spomenika kulture od 1955. godine. Uvid u digitalizirani i uređeni dio arhiva Hrvatskog restauratorskog zavoda otkriva 834 slike koje su na ovaj način tretirali stručnjaci djelatni u Zagrebu i Splitu od 1916., odnosno 1955,. do 1990-ih godina, uz naznaku znatno veće brojke. Osim velikog dijela povijesti primjene voštano-smolne smjese u Hrvatskoj, u radu se iznose i rezultati prvih kemijskih analiza voštano-smolne smjese s nekoliko slika, a autorica u tekstu iznosi i osobna zapažanja o fizičkom stanju umjetnina tretiranih opisanom metodom.Although the Dutch method was introduced in Croatia at the beginning of the 20th century and used through local variations almost until its end, the contemporary history of painting restoration techniques in Croatia has also been shaped by other foreign influences. Various different traditional and contemporary lining techniques have been used, from the Austrian starch paste of the 19th century, as well as earlier and later anonymous amateur linings using glue or a mixture of glue and flour, through Dutch wax-resin lining and Florentine pastes, to lining techniques with BEVA glue and cold processes using acrylate resins. Terminology in the field of restoration of canvas paintings in Croatia mainly follows foreign linguistic trends, adapting them to the characteristics and achievements of the Croatian language. According to previous research, Croatia is the second country in the region (the former Yugoslavia and part of Central and Eastern Europe) where the Dutch method arrived, 5 years after it appeared in Slovenia. The Dutch method of lining canvas paintings was invented in the mid-19th century in Amsterdam and spread around the world through visits by experts and exchanges between restorers until the late 20th century. Although its scope and the way it spread around the world and Croatia are still subjects of research, according to the author\u27s research, it was brought to Croatia in 1916 by our first restorer, Ferdo Goglia, while he was self-employed, and later when working for the Archaeological Department of the National Museum in Zagreb. He probably acquired the technique during his apprenticeship in Vienna or Budapest in 1916, but it is also possible that he learned about it from Eduard Gerisch, Viennese painter and restorer. Gerisch also worked in Dalmatia, Istria and Slovenia, and from 1896 used the same recipe as Goglia: a simple mixture of wax and Venetian turpentine, according to art historian Theodor Frimmel. However, this claim, made in 1904, has yet to be proven. The method was later used by Zvonimir Wyroubal, restorer of the Museum of Arts and Crafts, later an employee of the Conservation Institute of the Yugoslav Academy of Sciences and Arts, as well as his successors in that institution and institutions that were founded later, all the way to the Croatian Conservation Institute in the present day. In Split, from 1955, it was used in the workshop of the Regional Institute for the Protection of Cultural Monuments. Insight into the digitized and edited part of the archives of the Croatian Conservation Institute reveals 834 paintings that were lined using this technique by experts working in Zagreb and Split from 1916, or from 1955 to the 1990s, with an indication of a significantly higher number. The paper also presents the results of chemical analyses of wax-resin samples from several paintings collected during research within participation in the international workshop in the Netherlands, The Dutch Method Unfolded. In the text, the author presents her personal observations about the physical condition of the works of art on which the method has been used. The options that become available through new research and a different perspective through modern conservation concepts by applying less invasive conservation and restoration approaches are focused on minimal interventions and monitoring conditions. There is always room for testing innovations and your own ideas. This is how every restoration method was created, including wax-resin lining of canvas paintings in the Amsterdam workshop of Nicolaas Hopmann. It was the result of his reflections, ideas and experiments
