Marco Drache

Combination of Mechanochemical Degradation of Polymers with Controlled Free‐Radical Polymerization

The result of ultrasound on polymer solutions is the breakage of macromolecular C-C bonds due to cavitation. The fact that termination reactions of mechanoradicals as disproportionation and combination are suppressed in the presence of radical scavengers makes the following method possible. Thus the use of nitroxides acting as chain-terminating agents allows the creation of macroinitiators which can be used in controlled free-radical polymerization. In this work, we investigate the mechanochemical degration of poly(methyl methacrylate) (PMMA) in the presence of OH-TEMPO and the application of the irradiated polymers as macroinitiators in a controlled radical polymerization. The content of OH-TEMPO terminated chains in the degraded product is determined by a computer-aided procedure on the basis of molecular weight distributions.

TEMPO-controlled free radical suspension polymerization

The free radical polymerization controlled by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) was carried out in a water/oil suspension at 120–135°C. Suspension polymerization is a polymerization reaction with high conversion and allows high-molecular-weight polymers to be synthesized. The polymerization was started by a styrene macroinitiator. The block length is determined by the level of conversion and the initial macroinitiator concentration. A kinetic model is introduced which predicts a drastic reduction of polymerization time (at ca. 90% conversion a reduction from ca. 20 h to 5–3 h) and improved polydispersity through selective addition of initiator. The agreements between the simulated results and the experimental findings are surprisingly good. From model calculation, the percentage and molar mass distributions of the dead polymers can also be derived. Die von 2,2,6,6-Tetramethylpiperidin-N-oxyl (TEMPO) kontrollierte radikalische Polymerisation von Styrol wurde in einer Wasser/Ol-Suspension bei 120–135°C durchgefuhrt. Die Suspensionspolymerisation ist eine Polymerisationstechnik fur hohe Umsatze und gestattet, Polymere mit hohen Molmassen zu synthetisieren. Die Polymerisation wurde mit einem Styrol-Makroinitiator gestartet. Die Blocklange wurde durch den Umsatz und die eingesetzte Makroinitiatorkonzentration bestimmt. Mit Modellberechnungen konnen Rezepturen postuliert werden, die bei gezielter Initiatorzugabe zu einer drastischen Reduzierung der Polymerisationszeit (bei ca. 90% Umsatz von ca. 20 h auf 5 bis 3 h) bei Verringerung der Polydispersitat fuhren. Die Ubereinstimmung der simulierten Resultate mit den Experimenten ist uberraschend gut. Aus den Modellrechnungen lassen sich auch Anteil und Molmassenverteilungen der toten Polymeren ableiten.

The influence of charge-transfer complexes on the copolymerization behavior of cyclic maleic acid derivatives with donor monomers

The acceptor monomers maleic anhydride (MSA), itaconic anhydride (ISA), N-phenylmaleimide (NPI) and N-cyclohexylmaleimide (CMI) were combined with the donor monomers styrene (ST), butyl vinyl ether (BV) and N-vinyl-2-pyrrolidone (NVP) and studied by means of UV spectroscopy using CT bands. In this manner, the 1:1 stoichiometry of the CT complexes, the equilibrium constant in the range of 20°C to 60°C, and the thermodynamic equilibrium data were determined. Additionally, Monte-Carlo simulations of the donor-acceptor combinations in the presence of solvent molecules were run. The cohesion energies calculated from these simulations were used to quantify the donor-acceptor interactions and showed good agreement with the UV spectral data. The acceptor strength decreases in the order MSA > NPI > ISA > CMI, the donor strength in the order of NVP > ST > BV. In the free radical solution copolymerization of the systems NVP/NPI, BV/NPI, acrylonitrile (AN) AN/CMI, AN/NPI, MSA/CMI and MSA/NPI, the reaction rate and molecular weight dependence on the monomer combination, as well as the r-values were determined. Using the K c -values, conclusions were drawn concerning the different reaction mechanisms. The properties of the polymers were investigated using differential scanning calorimetry (DSC) and thermogravimetry.

Synthesis and dental aspects of acrylic phosphonic acids

Phosphoric acid esters of 2-hydroxyethyl methacrylate are favorable as components of dentin adhesives because they are able to remove the smear layer on dentin and achieve a strong bond between the restorative material and the tooth substance. However, one disadvantage of these polymerizable phosphoric acids is their low hydrolytic stability. This problem should be overcome with monomers 1 (R2,3=H) containing more hydrolytically stable bonds between the polymerizable methacrylic group and the strong acidic phosphorus group.

4-Oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO)- and 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido-TEMPO)-controlled free radical polymerization of styrene

The controlled free radical polymerization of styrene initiated with benzoyl peroxide (BPO) in the presence of 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO) as well as 4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl (4-acetamido-TEMPO) at 135°C was studied. The obtained results were compared with those of the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) controlled free radical polymerization of styrene. Up to a nitroxide/BPO molar ratio of 1.5:1, the conversions in the 4-oxo-TEMPO/BPO-system are higher than the conversions in the other systems. The conversion in the 4-acetamido-TEMPO/BPO-system is lower than that in the 4-oxo-TEMPO/BPO-system, but higher than the conversion in the TEMPO/ BPO-system under the same reaction conditions. The shortest induction periods were observed in the 4-oxo-TEMPO/BPO-system, while the longest induction periods were observed in the TEMPO/BPO-system. Except in the lower region of nitroxide/BPO molar ratio (1:1), the number-average molecular weight (M n ) increases linearly with conversion in all systems. Interestingly, the growth of the molecular weight (M n ) is independent of the nitroxide/BPO molar ratio in the 4-oxo-TEMPO-controlled free radical polymerization of styrene, while M n in the 4-acetamido-TEMPO/BPO- and the TEMPO/BPO-system decreases with increasing nitroxide/BPO molar ratio. The polydispersities (M w /M n ) of the obtained polymers are below 1.4 except those of low nitroxide/BPO molar ratios, which indicates a controlled polymerization process.

Mechanochemical Modification of Polystyrene and Polymethylmethacrylate

The goal of this work was the suitability of the polymers polystyrene and polymethylmethacrylate for the selective mechanochemical degradation and to determine which parameters influence the average molar weight and the molar weight distribution.

Conformation analysis of poly(styrene-alt-maleic anhydride) in solvents

Conformation analyses of polymers lead to special difficulties in molecular modelling. Usually, the size of the structures to be modelled allows only the application of the field force method, in which the electrostatic interactions are usually only unsatisfactorily considered. Furthermore, a conformation analysis in vacuum leads to overestimation of intramolecular stabilizations. In the case of styrene-maleic anhydride presented in this work, strong charge separations occur in the polymer chain. Furthermore, strong polymer-solution interactions occur which influence the stability of the conformations. To model the solvate shell, an atomistic Monte Carlo process was applied. Using the EVOCAP (Excluded Volume Constrained Assembly Packing) method the copolymer molecules were completely covered with solvent molecules. The partial charge was calculated using the charge equilibration method. This method is based on a simple quantum mechanical ab initio approach and calculates the partial charges over an electronegativity balance. The solvate shell affects the electronegativity of single atoms within the polymer chain and so their partial charges change against the vacuum. Beside the conformation energy in vacuum, the stabilization through the solvate shell is thereby also included. The energy calculation was done in the MMFF94 force field. The stabilization of the polymer conformations in the solvents tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were each calculated from 600 solvatation trials at 300 K for stretched and coiled vacuum conformations. The simulation confirmed the stronger extension of the polymer chain in DMF as experimentally found through viscosimetric measurements. Konformationsanalysen von Polymeren stellen eine besondere Schwierigkeit im Molecular Modeling dar. Die Grose der modellierten Strukturen last meist nur Kraftfeldverfahren zum Einsatz kommen. Dabei konnen die elektrostatischen Wechselwirkungen oft nur wenig zufriedenstellend berucksichtigt werden. Desweiteren fuhrt eine Konformationsanalyse im Vakuum zu einer Uberbewertung intramolekularer Stabilisierungen. Im vorliegenden Fall der untersuchten Styrol-Maleinsaureanhydrid-Copolymeren treten starke Ladungsseparationen in der Polymerkette auf. Desweiteren sind starke Polymer-Losungsmittel-Wechselwirkungen vorhanden, welche einen grosen Einflus auf die Stabilitat der Konformationen besitzen. Zur Modellierung der Solvathulle wurde ein atomistisches Monte-Carlo-Verfahren eingesetzt. Mittels der als EVOCAP (Excluded VOlume Constrained Assembly Packing) bezeichneten Methode werden die Copolymermolekule mit Losungsmittelmolekulen vollstandig umhullt. Die Partialladungen werden mit der Charge-Equilibration-Methode berechnet. Diese Methode beruht auf einem einfachen quantenmechanischen ab initio-Ansatz und berechnet die Partialladungen uber einen Elektronegativitatsausgleich. Die Solvathulle beeinflust die Elektronegativitat der einzelnen Atome der Polymerkette, und damit verandern sich deren Partialladungen gegenuber dem Vakuum. Neben der Konformationsenergie im Vakuum konnte damit die Stabilisierung durch die Solvathulle erfast werden. Die Energieberechnung erfolgte im MMFF94-Kraftfeld. Die Stabilisierung der Polymerkonformationen in den Losungsmitteln Tetrahydrofuran (THF) und N,N-Dimethylformamid (DMF) wurden aus je 600 Solvatationsversuchen bei 300 K fur gestreckte und geknauelte Vakuumkonformationen berechnet. Die Simulation bestatigt die durch visokosimetrische Messungen ermittelte starkere Aufweitung der Polymerkette in DMF.

Kinetic Monte Carlo Simulation Based Detailed Understanding of the Transfer Processes in Semi-Batch Iodine Transfer Emulsion Polymerizations of Vinylidene Fluoride

Semi-batch emulsion polymerizations of vinylidene fluoride (VDF) are reported. The molar mass control is achieved via iodine transfer polymerization (ITP) using IC4F8I as chain transfer agent. Polymerizations carried out at 75 °C and pressures ranging from 10 to 30 bar result in low dispersity polymers with respect to the molar mass distribution (MMD). At higher pressures a significant deviation from the ideal behavior expected for a reversible deactivation transfer polymerization occurs. As identified by kinetic Monte Carlo (kMC) simulations of the activation–deactivation equilibrium, during the initialization period of the chain transfer agent already significant propagation occurs due to the higher pressure, and thus, the higher monomer concentration available. Based on the kMC modeling results, semi-batch emulsion polymerizations were carried out as a two pressure process, which resulted in very good control of the MMD associated with a comparably high polymerization rate.

Propagation Kinetics of Isoprene–Glycidyl Methacrylate Copolymerizations Investigated via PLP–SEC

Pulsed-laser polymerization combined with polymer analysis by NMR and size-exclusion chromatography is used to study the radical copolymerization kinetics of isoprene (IP) with glycidyl methacrylate (GMA). The copolymer is characterized by a close-to-alternating microstructure, with the addition of IP leading to a significant decrease in the composition-averaged propagation rate coefficient. A rigorous fitting strategy is developed to fit a mixed penultimate model to the data, with the selectivity of the IP, but not the GMA, macroradical dependent on the penultimate unit.

Simulation of Polymer Aggregates as a Method to Investigate the Solvent Effect on Blend Complexation and the Relative Strength of H‐Bonding Groups in Blends

Simulations of polymer-solvent and polymer-polymer aggregates, in which the study of hydrogen bonding plays an important role, have been carried out with two blend systems. The aim was to examine the influence of the solvent on blend complexation and to compare the strength of different hydrogen bonds in a blend system. We quantified the strength of one hydrogen bond in the blend environments. For this we used the EVOCAP software, developed by our institute. It allows the building of large molecular aggregates with realistic and homogeneous densities, with an implemented positioning algorithm of the molecules under consideration and their excluded volume, and a charge equilibration method for the partial charge calculation. In the simulated aggregates the specific interaction energy of the hydrogen atom forming the hydrogen bond was a useful indicator for our studies. Through a direct correlation of this specific-interaction energy with the strength of the hydrogen bond, we supported the experimental result that, in toluene, complex formation between poly(methyl methacrylate) (PMMA) and PSOH, a hydroxyl-modified polystyrene, is possible, but not in tetrahydrofuran. Varying the proton-donor polymer, also a hydroxyl-modified polystyrene, in blends of poly(vinyl methyl ether) (PVME) with groups of different donor strength, we reconstructed the experimental row of increasing hydrogen-bond strengths.