Irene Schnöll-Bitai

Laser-flash-initiated polymerization as a tool for evaluating (individual) kinetic constants of free-radical polymerization—5. Complete analysis by means of a single experiment

Abstract Three systems, styrene in bulk (a), styrene-toluene 1:1 (b), and methylmethacrylate in bulk (c) have been subjected to a pseudostationary polymerization initiated by periodic laser flashes at 25° at various pulse separations t0 in the range of 0.1–4 sec. The kinetic analysis, which can be carried out separately and independently for each experiment, is based on molecular mass distributions evaluated by GPC, gravimetric determination of polymer yield and evaluation of P w from the GPC-curves. Rate constants of propagation kp were determined from the points of inflection in the GPC-curves while kp2/kt was calculated from the product of rate of polymerization and P w . Combination of kp and kp2/kt finally yielded kt. In addition, rough estimates could be obtained for the contribution of disproportionation to overall bimolecular termination. The results were: system (a): k p = 77±41 mol −1 sec −1 ; k t =(0.78±0.12)·10 8 l mol −1 sec −1; system (b): k p = 79±51 mol −1 sec −1 ; k t =(0.96±0.09)·10 8 l mol −1 sec −1; system (c): k p = 313±81 mol −1 sec −1 ; k t =(0.39±0.04)·10 8 l mol −1 sec −1

Evaluation of individual rate constants from the chain-length distribution of polymer samples prepared by intermittent (rotating sector) photopolymerization—2. The copolymerization system styrene-methyl methacrylate

Samples of poly(styrene-co-methyl methacrylate) prepared by intermittent photo-initiation (rotating sector) were subjected to GPC-analysis and the effective rate constants of chain propagation kp determined from the position of the point of inflection in the molecular mass distribution curve. The dependence of the kp-values on monomer feed composition is in great disagreement with the terminal model and can be understood in terms of a penultimate model, e.g. the one suggested by Fukuda et al. In addition, (apparent) rate constants of bimolecular termination, kt, were estimated by combining kp-data with kp2/kt-values obtained from rates of (co)polymerization and weight-average degrees of polymerization of the samples. These are best interpreted in the frame of the Walling model with a moderately high ϕ-factor of about 2–3, indicating a slightly favoured cross-termination.

Characterization of the molecular mass distribution of pullulans by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using 2,5-dihydroxybenzoic acid butylamine (DHBB) as liquid matrix.

The performances of several matrices were investigated for the accurate determination of the molecular mass distributions of pullulans by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The ionic liquid matrix (ILM) 2,5-dihydroxybenzoic acid butylamine (DHBB) gave better and more reliable results than the crystalline matrices 2,5-dihydroxybenzoic acid (DHB) and 2,4,6-trihydroxyacetophenone (THAP). With the ILM it was possible to obtain spectra of pullulans up to more than 100 kDa, the highest molar mass reported thus far. Owing to the known advantages of liquid matrices providing better spot-to-spot reproducibility, an almost noise-free spectrum and constant baselines were obtained when working under optimized conditions. In particular, the extent of in-source fragmentation occurring with this group of fragile polymers was considerably and decisively reduced. Thus, a more reliable representation of the true oligomer and polymer distributions is experimentally attainable, especially for distributions with small polydispersity values. The maximum error in the measured distribution associated with fragmentation was estimated by model calculations describing the changes in the polymer distribution upon different probabilities of fragmentation events. These simulation results indicated that the data obtained by MALDI-TOFMS using the liquid DHBB matrix were of high reliability. In particular, the average value of the distributions, M(w), and the polydispersity were obtained with predicted uncertainties of between 3 and 15% depending on the width of the distribution and the mass of the polymers.

Solvent Effects on the Rate Constant of Chain Propagation in Free Radical Polymerization

Summary. Using pseudostationary techniques (pulsed laser polymerization followed by an analysis of the chain-length distribution), the rate constant of chain propagation kp was directly determined for 1:1 mixtures of the monomer and a number of solvents for styrene and methyl methacrylate as well as for a 1:1 comonomer system of these two monomers, thus reducing the bulk monomer concentration to half of its bulk value in all cases. The presence of solvent emerged to be of moderate influence on kp only, the effects never exceeding 20% in either direction. Depressions of kp were more frequent than elevations. The results did not favour one of the existing theories (EDA-complex theory, hot radical theory) over the other. In case of a bad solvent, kp may rather reflect changes in the local monomer concentration at the site of reaction caused by preferential solvation by the monomer which constitutes the better “solvent”.Zusammenfassung. Mit Hilfe einer pseudostationären Technik (Anregung durch periodische Laserpulse und anschließende Analyse der Kettenlängenverteilung) wurde die Geschwindigkeits-konstante des Kettenwachstums kp für 1:1-Gemische des Monomeren mit einer Reihe von Lösungsmitteln bestimmt; als Monomere wurden Styrol und Methylmethacrylat sowie eine 1:1-Mischung dieser beiden (als Comonomere) eingesetzt, sodaß die Monomerkonzentration in allen Fällen die Hälfte der Monomerkonzentration in Substanz betrug. Die Größe von kp wurde durch die Anwesenheit eines Lösungsmittels nur mäßig beeinflußt: die Effekte überschritten in keinem Fall 20%, weder in der einen noch in der anderen Richtung. Herabsetzungen von kp treten dabei häufiger auf als Erhöhungen. Die Resultate der Lösungspolymerisation begünstigen keine der existierenden Theorien (EDA-Komplex-Theorie, hot-radical-Theorie) in eindeutiger Weise. Im Fall eines schlechten Lösungsmittels spiegelt kp eher die Änderung der lokalen Monomerkonzentration wider, die durch präferentielle Akkumulierung des Monomeren (als des besseren, Lösungsmittels“) am Reaktionsort bedingt ist.

Quenched Instationary Polymerization Systems, 6. The Direct Determination of Axial Dispersion in Size Exclusion Chromatography by Comparison of Theoretical and Ideal Peak Widths

For the direct determination of axial dispersion in size exclusion chromatography a simple method is presented which makes use of the measured and ideal peak widths. The peak width can be defined in two ways: either absolute as the difference of successive points of inflection or relative as the ratio of these points. If the absolute peak width is invariant for the number, molar mass and hyper distribution then this distribution can unambiguously be classified as Poissonian. The relative peak width for such distributions is strictly determined by the experimental parameters. It is demonstrated that axial dispersion only leads to an additive increase in the peak variances for peaks with a relative peak width smaller thai 1.25. Thus, it is possible to determine directly the axial dispersion of an experimental size exclusion chromatography set-up by the use of Poisson distributions prepared by quenched instationary polymerization techniques or any other technique leading to ideal Poisson distributions.

The Quenched Instationary Polymerization System, 5. The Peak Broadness as an Invariant Quantity of the Number, Molar Mass and Hyper Distributions

The total number, molar mass and hyper distributions generated by quenched instationary polymerization techniques are dominated by the radical chain length distribution (RCLD) whereas the contribution from the polymer chain length distribution (PCLD) is in most cases negligible. For the determination of the rate constant of propagation (k p ) the location of different extraordinary points of the distribution curves is determined by the use of the first and second derivatives. For the number, molar mass and hyper distributions these points are related in an unambiguous way to k p [M]tx and can be used to extract k p . The choice of t x (duration of the dark period, or an initiation period. or the sum of different periods) depends on the experimental conditions (δ-pulse, incomplete preeffect, combination of periods differing in initiation extent) and is essential for the proper determination of k p . The broadness of appearing peaks (introduced as the difference between two succesive points of inflections) turned out to remain the same inespective which type of distribution curve was analyzed. Analytical expressions for the peak broadness were derived for different types of quenched instationary polymerization conditions. For δ-pulse initiation the broadness of the Poisson peak depends simply on the number of propagation steps that occurred whereas for non-δ-pulse initiation conditions the peak broadness is governed by the corresponding duration of the initiation period.

The quenched instationary polymerization system, 2. Derivation of the radical and polymer chain length distributions for an incomplete pre‐effect

An instationary polymerization system with constant chain initiation is prevented from attaining a final stationary state by the occurrence of a highly efficient quench reaction which deactivates all radicals present at a certain time t m . The set of differential equations was used to derive the radical spectrum as a function of polymerization time. These, the numerically calculated polymer chain length distributions and the convolutes of both were analyzed in order to determine the chain lengths of the extrema of the first and the second derivatives. These values were found to be proportional to k p [M]t in . The values were recalculated by using already derived correction factors and the deviations from the input parameter were in most cases smaller than 1% but became more pronounced as soon as the total polymer concentration was bigger than the total radical concentration.

Instationary polymerization technique : basic principles and first results

Abstract Basic principles of a new polymerization technique and the subsequent analysis of the resultant molar mass distribution for the direct determination of the rate constant for chain propagation are introduced. First results are presented for the bulk polymerization of methyl methacrylate at room temperature. It is demonstrated, that the correlation between the active life time of a radical chain and the number of propagation steps, L act = k p [M] t act is valid in the case of instationary polymerization conditions too.

The importance of the molecular weight distribution for the direct determination of the kinetic constants of free radical polymerization

Abstract The distribution curves obtained by stationary, pseudostationary and instationary techniques are surveyed in rough outlines. Special emphasis is laid on the possibilities to determine directly kinetic quantities from both theoretical and experimental point of view. A simple method for the direct determination of axial dispersion is also presented based on the comparison of ideal and experimental absolute peak widths.

Tailoring of the molecular weight distribution by nonstationary (free radical) polymerization conditions

The choice of quenched instationary polymerization (QUIP) conditions will influence the shape and the moments of the polymers formed. As the quench reaction deactivates all radicals present at a given time, the total molecular weight distribution will always be a superposition of the (quenched) radical chain length distribution (RCLD) and the polymer chain length distribution (PCLD). Model calculations were carried out taking into account the different types of initiation conditions. In principle, distributions with extremely low polydispersities (Poisson-type) can be obtained as well as broad structured or non-structured distributions. Polymerization conditions will also determine whether the radical spectrum dominates a certain region of the distribution.