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Der intensive Pflanzenbau erfordert wirksame Pflanzenschutzmaßnahmen. Eine der wirksamsten Maßnahmen ist die Behandlung des Saatgutes gegen Pathogene. Sie erfolgt bisher fast ausschließlich durch chemische Beizmittel (Pestizide), die Anwender und Umwelt gefährden können. Neue Gesetze und Richtlinien in Deutschland und der Europäischen Union fordern die nachhaltige Einschränkung der Anwendung chemischer Pflanzenschutzmittel und den Ersatz durch neue, alternative Verfahren und Methoden.

Propagation kinetics of isobornyl methacrylate (iBoMA) and methyl methacrylate (MMA) free-radical homopolymerization were studied by the PLP-SEC method, which consists of pulsed laser initiated polymerizations and subsequent polymer analysis by size-exclusion chromatography. The reactions were carried out in bulk and in solution of several organic solvents:  toluene, tetralin, tetrahydrofuran (THF), methyl isobutyrate, and phenethyl isobutyrate. In all polymerization systems, the activation energy of the observed propagation rate coefficient, kp,app, is not affected by either solvent type or solvent concentration. However, kp,app may be significantly different from the corresponding bulk values. In MMA polymerizations, kp,app values in solution were higher than in bulk, with the strongest enhancement of 50% observed for tetralin as solvent. In contrast, a solvent-induced lowering in propagation rate of up to 52% for THF as solvent was seen for iBoMA. The observed changes in propagation rate were assigned to the occurrence of a local monomer concentration in the vicinity of the free-radical chain end rather than to an intrinsic kinetic effect. The variation in local monomer concentrations is suggested to originate from differences in molar volume of the monomer and the solvent. Critical analysis of the results led to a linear correlation between solvent-induced changes in propagation rate and the difference in molar volumes of monomer and solvent. The correlation holds for both monomers.

Pulsed laser initiated polymerizations of butyl methacrylate in butanol show a significant enhancement of the propagation rate coefficient kp compared to polymerizations in bulk. The activation energy EA of kp is reduced due to the presence of butanol:  at a monomer concentration of 0.8 mol·L^-1 kp for polymerizations in butanol at 0°C is about 85% higher than in bulk, the difference reduces to 44% at 80°C. These findings are associated with a reduction in EA of kp from 23.0 kJ·mol^-1 for bulk systems to 20.5 kJ·mol^-1 for polymerizations in butanol. The increase of kp in the presence of an alcohol is explained by the formation of hydrogen bonds between the OH group of the solvent and the carbonyl group of the monomer.

Pulsed laser polymerization (PLP) in conjunction with size-exclusion chromatography (SEC), both being carried out in the aqueous phase, was used to determine propagation rate coefficients, kp, of nonionized acrylic acid (AA) at temperatures between 2 and 25 °C and monomer concentrations, cAA, from 1 to 10 wt %. The product kpcAA,local is the primary experimental quantity deduced via the PLP−SEC technique. Assuming cAA,local to be identical to overall monomer concentration, cAA, yields apparent kp values, which, upon enhancing cAA, first increase and, after passing through a maximum at around 3 wt % AA, significantly decrease. A kp value as high as 180 000 L·mol^-1·s^-1 was determined for 3 wt % AA at 25 °C. The decrease observed toward higher AA concentration is fully consistent with what has been found in a preceding study into kp of nonionized AA at monomer concentrations of 20 and 40 wt %. At constant temperature, variations in apparent kp by about a factor of 3 are seen in the range up to 40 wt % AA. Discussion of the measured rate data suggests that it is primarily cAA,local that changes as a function of monomer concentration rather than kp. As a consequence of strong hydrogen bonds between polymer segments, between polymer segments and AA monomer, and between both these species and water, the AA concentration at the radical site may significantly differ from overall cAA. The assignment of the observed changes in apparent kp to cAA,local is supported by PLP−SEC experiments in which appreciable amounts of propionic acid (PA) have been added to aqueous AA solutions. The addition of PA significantly reduces apparent kp. Addition of NaCl to an aqueous solution of AA in its nonionized form, on the other hand, does not affect apparent kp. Whether the observed changes in kpcAA,local are entirely due to cAA,local differing from cAA or whether also the “true” propagation rate coefficient varies cannot be safely decided on the basis of the presently available data.

Styrene−methyl methacrylate−glycidyl methacrylate terpolymers to be used as binders in powder coating materials were synthesized in a homogeneous fluid phase containing 20 wt % CO2. Reactions carried out at 120 °C and 350 bar yield almost complete monomer conversion. The molecular weights were around 3000 g·mol^-1 with a polydispersity index of 1.7. Molecular weight control was achieved by n-dodecyl mercaptan or by a catalytic chain-transfer agent. Simulations of the cumulative and differential copolymer composition are in excellent agreement with experimentally derived values.

Polymer capacitance sensors were assessed in this study for measuring soil gas relative humidity and total suction in soils that are too dry for thermocouple psychrometers. Capacitance of the sensors varies as the relative humidity of the gas surrounding the sensor is varied. Soil gas relative humidity is inferred through a calibration curve relating soil gas relative humidity and capacitance. Tests conducted with six finer textured soils and three sands showed that the capacitance sensors can be used to obtain unbiased estimates of relative humidity for a broad variety of soils with a precision of approximately ±3%. Tests also showed that the sensors are non-hysteretic and relatively insensitive to temperature, except when the temperature approaches the freezing point of water. An example is provided illustrating how capacitance sensors can be used to monitor soil gas relative humidities in alternative earthen final covers that employ passive flow of atmospheric air to remove water.

Experiments directed toward the determination of free-radical propagation rate coefficients kp from pulsed laser initiated polymerizations (PLP) in conjunction with size-exclusion chromatography (SEC) were modeled using the program PREDICI. The simulations were carried out in order to investigate whether an experimentally observed variation of kp with the initiating laser pulse repetition rate is necessarily due to a chain-length dependence of kp. The modeling was performed for laser pulse repetition rates ranging from 0.5 to 50 Hz and primary radical concentrations being varied by more than 2 orders of magnitude. The resulting molecular weight distributions were very different in shape. The kp data derived from the simulated molecular weight distributions showed a variation of about 30% with chain lengths, although the same propagation rate coefficient was introduced into all the simulations. The experimental finding of kp being dependent on chain length induced by using different laser pulse repetition rates is not necessarily due to a chain-length dependence of kp. The modeling results show the importance of reporting all experimental details together with kp data derived from PLP−SEC experiments.

Free-radical polymerization of styrene in homogeneous phase of supercritical carbon dioxide (scCO2) has been studied at temperatures between 40 and 80 °C and pressures between 300 and 1500 bar. Applying pulsed-laser polymerization in conjunction with size-exclusion chromatography yields propagation rate coefficients, kp. Within the extended pressure range under investigation, kp for solution polymerization in carbon dioxide, at CO2 contents up to 46 wt %, is identical to kp of styrene bulk polymerizations. The studies with CO2 suggest that solvent effects on kp occur in systems where the solvent quality for the polymer is not very high and where intrasegmental interactions are important.

The propagation kinetics of vinyl acetate (VAc) free-radical homopolymerization in bulk and in solution of 40 wt % CO2 have been studied by the PLP−SEC method at pressures up to 1500 bar. The molecular weight distributions of the polymer samples show well-resolved pulsed-laser-induced structure with at least two, in most cases even three, distinct inflection points. At the p and T conditions of this work, no difference is seen in propagation rate coefficients, kp, of VAc in solution of carbon dioxide and in bulk. The activation volume for kp at 25 °C in both reaction media is ΔV^# = −(10.2 ± 0.9) cm³ mol^-1.

Pulsed laser polymerization (PLP) in conjunction with analysis of the resulting polymer by size-exclusion-chromatography (SEC) was used to measure propagation rate coefficients, kp, of acrylic acid (AA) in aqueous solution at concentrations of 20 and 40 wt % and temperatures between 2.6 and 28.5 °C. Under these conditions, more than 99% of acrylic acid exists in the nonionized form. The molecular weight distribution (MWD) of poly(AA) is directly measured by aqueous SEC. The kp values at 20 wt % are about 60% higher than at 40 wt %. The activation energy of kp is close to 12.0 kJ·mol^-1 at both concentrations. At 25 °C and 20 wt %, kp of AA in aqueous solution exceeds 100 000 L·mol^-1·s^-1 which is the highest kp value determined by the PLP−SEC technique so far.

Propagation kinetics of butyl acrylate (BA) free-radical homopolymerization in fluid CO2 has been studied by the PLP−SEC (pulsed laser polymerization−size-exclusion chromatography) method at pressures between 200 and 2000 bar and temperatures between −9 and 20 °C. The CO2 content was close to 45 wt % in all experiments. As a consequence of the poor chain-transfer activity in solution of CO2, the molecular-weight distributions of the polymer samples show well-resolved PLP structures with at least two, but in most cases three, distinct inflection points. At the p and T conditions of this work, the observed propagation rate coefficients (kp) of BA in fluid carbon dioxide at CO2 contents around 45 wt % are found to be about 40% below the corresponding bulk polymerization values. The activation volume and activation energy of kp are, however, identical for bulk and solution (in CO2) free-radical BA polymerizations within experimental accuracy. The origin of the solvent influence on the propagation reaction is discussed. The observed lower values of kp in solution of CO2 are assigned to the poor solvent quality of carbon dioxide, presumably resulting in a local monomer concentration at the site of the propagating radical which is smaller than the overall monomer concentration in the system.

Free-radical termination kinetics of butyl acrylate (BA) homopolymerization in supercritical CO2 (scCO2) and in bulk have been studied by single pulse−pulsed laser polymerization (SP−PLP) at temperatures and pressures up to 120 °C and 2500 bar, respectively. Polymerization induced by a single laser pulse is monitored via microsecond time-resolved near-infrared (NIR) spectroscopy. From individual SP−PLP experiments performed at several stages during the course of a polymerization, the ratio of termination to propagation rate coefficients, kt/kp, is obtained as a function of monomer conversion. With kp being available from pulsed laser polymerization in combination with size exclusion chromatography (PLP−SEC) experiments in both solution of scCO2 and in bulk, the kt/kp data yield individual (chain-length averaged) kt. Irrespective of polymerization pressure and temperature, kt is larger in scCO2 than in bulk polymerizations. At the highest CO2 content of 46 wt %, this enhancement amounts to about 350%. The activation volumes of kt are similar for polymerizations in scCO2 and in bulk, whereas the activation energies are clearly dissimilar:  In solution of scCO2 an apparent activation energy is found which is negative. In bulk polymerization, on the other hand, EA(kt) is positive. The experimental observations indicate that thermodynamic contributions strongly affect termination rate in scCO2 solution.

Using pulse lasers for initiation in conjunction with online spectroscopic detection of monomer conversion and with size exclusion chromatographic analysis of polymer molecular weight distribution has enormously improved the knowledge about free-radical polymerizations up to high pressure. The novel procedures which partly include modeling of the full molecular weight distribution are outlined. The survey primarily addresses propagation and termination rate coefficients of homopolymerizations up to 3 kbar with special emphasis on the conversion dependence of termination rate. In addition, recent results on reactivity ratios in ethene- acrylic ester copolymerizations and on initiator decomposition rates are presented.

Free-radical propagation rate coefficients (kp) for three cycloalkyl methacrylates and two functional methacrylates were measured at temperatures between 30 and 110 °C. The data were obtained by analyzing molecular weight distributions of polymer produced by pulsed-laser polymerization − the PLP/MWD technique. Arrhenius parameters were compared with recent results reported for linear and branched alkyl methacrylates, including methyl methacrylate (MMA) and n-dodecyl methacrylate (DMA). The highest kp value, for 2-hydroxypropyl methacrylate, is more than twice the MMA value, and is 50% higher than the DMA value at 50 °C. The monomer with the lowest kp value, isobornyl methacrylate, has a coefficient of similar magnitude to DMA. Values for the other three monomers − glycidyl, cyclohexyl, and benzyl methacrylates − are intermediate between these two levels.

Values of free-radical propagation rate coefficients (kp) for two new methacrylate monomers&sbd;2-ethylhexyl and isodecyl methacrylates&sbd;have been measured in the temperature range 10−90 °C by analyzing molecular weight distributions of polymer produced by pulsed-laser polymerization (the PLP/MWD technique). In addition, new PLP/MWD results for n-butyl and n-dodecyl methacrylates are reported, and previously published data for ethyl and isobutyl methacrylates are reanalyzed. The temperature dependence of kp for these six monomers is compared to that recently reported for methyl methacrylate. A consistent increase of kp with size of the ester group is observed, with values for n-dodecyl methacrylate 50−60% higher than those for methyl methacrylate. Although this increase is statistically significant, it is less certain whether it is attributable to a decrease in the activation energy or an increase in the frequency factor. The experimental results favor the former explanation.

Propagation rate coefficients kp have been determined for free radical polymerizations of methacrylic acid (MAA) using pulsed-laser polymerizations (PLP) combined with size exclusion chromatography (SEC). The temperature dependence of kp has been studied for polymerizations of 33 vol % monomer in methanol in the temperature range from 20 to 60 °C. kp is described by the following Arrhenius relation:  ln[kp/L·mol^-1·s^-1] = (13.31 ± 0.49) − (2132 ± 133)K/T. Variations in MAA concentration and the type of solvent do not influence kp significantly. Any distinctions elicited in kp by these variations are of the order of the experimental uncertainty and are much smaller than the relative changes in the MAA monomer−dimer equilibrium observed by FTIR spectroscopy. The magnitude of kp and its variation with temperature are similar to results reported for methyl methacrylate and the rest of the methacrylate family.

Analysis of the molecular weight distributions (MWDs) of polymer produced by pulsed-laser polymerization (PLP) is an effective method for the determination of free-radical propagation rate coefficients (kp). The PLP technique has been successfully applied to many slowly propagating monomers, but kp determination by PLP for faster propagating systems such as the acrylates is more difficult. Experimental conditions for the polymerization must be selected so that laser pulses are the dominant chain-starting and chain-stopping events. Interference from excessive chain-transfer reactions, or from termination reactions which are either too fast or too slow, results in MWDs which are not suitable for kp determination. Guidelines for design of successful experiments are outlined using an extensive n-butyl acrylate data set, from which kp has been determined over the temperature range 5−30 °C; limited data for n-dodecyl acrylate and 2-ethylhexyl acrylate are also presented.

An extensive study of free-radical depropagation kinetics has been performed for several methacrylates at high temperatures in bulk and solution using a pulsed-laser technique. As expected from theory, the relative importance of depropagation on rate increases with increasing temperature and with decreasing monomer concentration. The temperature dependence of n-dodecyl methacrylate depropagation has been quantified. The resulting estimates for enthalpy (−50 to −60 kJ/mol) and entropy (−105 to −127 J/(mol·K)) of polymerization, although highly correlated, are in good agreement with available literature data. Moreover, the best-fit values (−53.8 kJ/mol and −113 J/(mol·K)) provide a good representation of high-temperature propagation/depropagation kinetics measured for n-butyl, cyclohexyl, isobornyl, and 2-hydroxypropyl methacrylates, suggesting that a single set of universal values may be employed when modeling high-temperature polymerizations of all methacrylates.

The PLP/MWD technique, based upon analysis of the molecular weight distribution (MWD) of a polymer produced by pulsed-laser polymerization (PLP), has proven to be a powerful probe for the examination of free-radical homopolymerization kinetics. The same method is applicable to the study of copolymerization propagation kinetics provided that proper calibrations are established for the measurement of copolymer MWDs, as illustrated by an examination of the methyl methacrylate (MMA)/n-butyl acrylate (nBA) system. Statistical analysis indicates that although MMA/nBA copolymer composition is well described by the terminal model, propagation kinetics are not. Like many other systems recently examined, the data are well represented by the “implicit penultimate unit effect” model of Fukuda and co-workers. However, whereas previous systems studied have propagation rates that are lower than those predicted by the terminal model, it is found that the propagation rate of MMA/nBA is greater than the terminal model predictions. The implications of these results to theoretical interpretations of the penultimate model are discussed.

No abstract.