3,488 research outputs found
SP-PLP-EPR study of chain-length-dependent termination in free-radical polymerization of n-dodecyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate: Evidence of "composite" behavior
No full textThe chain-length dependence of the termination rate coefficient in n-dodecyl methacrylate (DMA), cyclohexyl methacrylate (CHMA), and benzyl methacrylate (BzMA) bulk free-radical homopolymerizations at ambient pressure and at temperatures from -20 to 0 degrees C is deduced using the recently developed technique of SP-PLP-EPR: pulsed-laser polymerization (PLP) in which time-resolved EPR measurement of radical concentration, c(R), is made following each single pulse (SP) of an excimer laser. The decay of c(R) results from termination of radicals of almost identical size. Their chain length, i, increases linearly with time, t, after applying a SP. The rate coefficient, k(t)(i,i), for termination of two radicals of size i is determined by fitting the experimental c(R) vs t data. This process demonstrates that (at least) two power-law exponents are necessary to describe kti,i over the extended chain-length range of i = 1 to 1000. This is consistent with the so-called "composite model", which uses power-law exponents alpha(S) and alpha(L) to describe termination of radicals either shorter or longer, respectively, than a crossover chain length, i(c). The fourth parameter obtained from fitting the SP-PLP-EPR data with this model is k(t)(1,1), the termination rate coefficient for two radicals of degree of polymerization 1. Previous DMA experiments are reanalyzed while new experimental results are reported and analyzed for CHMA and BzMA. The parameter values for CHMA and BzMA termination at 0 degrees C are almost identical - k(t)(1,1) approximate to 3 x 10(7) L mol(-1) s(-1), alpha(s) approximate to 0.50, i(c) approximate to 90, and alpha(L) approximate to 0.21-and they are close to those for DMA at 0 degrees C: k(t)(1,1) approximate to 1 x 10(7) L mol(-1) s(-1), alpha(s) approximate to 0.64, i(c) approximate to 50, and alpha(L) approximate to 0.18. The results fully support the composite model in that the chain-length dependence is more pronounced for shorter than for longer radicals, i.e., alpha(S) > alpha(L). Moreover, the power-law exponent that characterizes termination of long-chain radicals is close to the theoretical value of alpha(L) = 0.16. In fact all parameter values-including the small differences between DMA and CHMA/BzMA-are more-or-less in accord with expectations based on polymer dynamics. Furthermore, our results suggest that termination of methacrylate radicals with large cyclic or long n-alkyl substituents may be affected by steric shielding of the radical functionality
Detailed analysis of termination kinetics in radical polymerization
No full textStudies into the termination step of radical polymerization have mostly been carried out via chain-length-averaged properties. With the advent of pulsed laser polymerization (PLP) induced by single pulses (SPs), the analysis of termination rate coefficients kt has significantly improved. In conjunction with highly time-resolved near-infrared (NIR) detection of monomer consumption, SP-PLP-NIR allows for the measurement of chain-length-averaged termination rate coefficients as a function of temperature, pressure and degree of monomer conversion. With the addition of a suitable reversible addition−fragmentation chain−transfer agent, this technique even allows for the determination of chain-length-dependent termination rate coefficients kti,i, where i refers to the chain length of two terminating radicals of identical size. The decay of kti,i with chain length is more pronounced at small radical size below a crossover chain length ic. At larger chain lengths, kti,i decreases to a weaker extent. Thus, the so-called composite model is perfectly suited for representing kti,i behaviour across the entire chain-length range. A further clear improvement of kti,i determination is provided by the SP-PLP-EPR method, which directly measures the type and concentration of radicals via microsecond time-resolved electron paramagnetic resonance (EPR) spectroscopy. This EPR-assisted technique is particularly valuable for polymerizations where secondary radicals undergo backbiting and thus two types of radicals, secondary and tertiary, are simultaneously present, as is the case with acrylic monomers. © 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry
The Impact of Band Broadening on Molar-Mass Determination of Narrow-Distribution Polymer by Size-Exclusion Chromatography
No full textA systematic study of the effects of band broadening (BB) in size-exclusion chromatography (SEC) on Poisson molar-mass distributions (MMDs) is carried out. Such distributions apply for polymer standards used for SEC calibration, and they are a good model for the MMD from any quasi-living polymerization, including reversible-deactivation radical polymerization (RDRP). BB is simulated using the exponentially modified Gaussian model, which is shown to work excellently for actual polymer standards. A series of simulations with this model revealed how BB parameter values affect number-average degree of polymerization and polydispersity index from SEC. It is shown that the skewness parameter is particularly important in these considerations. Many of the obtained trends echo ones from RDRP, suggesting that BB has been making a hitherto unsuspected-indeed uninvestigated-contribution to deviations from ideal behavior in such systems
SP-PLP-EPR Investigations into the Termination Kinetics of n-Butyl Acrylate Macromonomers
No full textThe definitive version is available at www3.interscience.wiley.comThe termination of model mid-chain radicals (MCRs), which mimic radicals that occur in
acrylate polymerization over a broad range of reaction conditions, has been studied by singlepulse
pulsed-laser polymerization (SP PLP) in conjunction with electron paramagnetic
resonance (EPR) spectroscopy. The model radicals were generated by initiator-fragment
addition to acrylic macromonomers that were preformed prior to the kinetic experiments, thus
enabling separation of termination from the propagation reaction, for these model radicals
propagate sparingly, if at all, on the timescale of SP-PLP experiments. Termination rate
coefficients of the MCRs were determined in the temperature range 0–60 °C in acetonitrile
and butyl propionate solution as well as in bulk macromonomer over 0–100 °C. Termination
rate coefficients slightly below those of the corresponding secondary radicals were deduced,
demonstrating the relatively high termination activity of this species, even when undergoing MCR-MCR termination. For chain length 10, a reduction by a factor of 6 is observed.
Unusually high activation energies were found for the termination rate coefficient in these
systems, with 35 kJ mol⁻¹ being determined for bulk macromonomer
Russell Gulch mining district
No full textScale not given.Blueline.Shows underground workings located in Sec. 18, T.3 S., R.72 W., Gregory and Russell Gulch mining districts
Language and theology in St Gregory of Nyssa
No full textThis MA thesis focuses on the work of one of the most influential and authoritative theologians of the early Church: St Gregory of Nyssa (†396). My topic of research consists in the relationship between language and theology, as it shaped in Gregory’s polemical works against the radical Arians, in particular against Eunomius of Cyzicus (†395).The first chapter tackles the historical side of the controversy and provides the chronology of the dogmatic disputes on the dogma of Trinity following the Council of Nicaea (325). The second chapters illustrate the conflict being at stake between two theological methodologies: Gregory's grammar of thought is scriptural, whereas Eunomius' theology is much more philosophical and inflexible in its terms. Eunomius claimed that one can know God by his essence in the concept of 'ingenerate'. On the contrary, for Gregory of Nyssa, God 'is above all names'. For him, language and sexuality are realitites of the post-lapsarian world, which made human mind opaque and the exercise of interpretation indispensable. Gregory included also the episode of Babel in the genealogy of our linguistic finitude. The third and the fourth chapters focus on the relationship between language and theological knowledge in St Gregory's third book Contra Eunomium. All words used in human language - including Eunomius' concept of agennetos – have complementary meanings, since no one can describe the essence of an object or of any part of reality. On this basis, Gregory develops his 'theory of relativity' of names, which can never befit God's majesty and glory. In the last chapter, under the heading 'Pragmatics of Language', I investigate the immediate consequences of Gregory's 'theory of relativity'. Speech is treated as a sphere, which resembles the creative power of the hypostatic Word. Therefore, rhetoric becomes the perfect tool for his pastoral concern in doing theology. By choosing rhetoric, Gregory is free to start his theological argument from anywhere, since theology is a discourse about God's redemptive economy. In conclusion, I try to emphasise the actuality of Gregory's theory of names and its importance for the contemporary debates in the Church on thorny issues as Trinitarian theology or gender. I also evaluate Gregory of Nyssa's self-consistency in positive terms
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
Determination of the Mode of Termination in Radical Polymerization via Mass Spectrometry
No full textWe comprehensively explore the use of mass spectrometry (MS) for quantitative determination of the mode of termination, i.e., the extent of disproportionation versus combination, in radical polymerization. Development of the underlying kinetic theory forms a major portion of our endeavors. What emerges from this theory is that the ratio of polymer from disproportionation to combination is a function of chain length and of fundamental kinetic parameters such as the rate of initiation and the rate coefficients for propagation and termination. Therefore, all this information must be at hand in order to determine lambda, the fraction of termination by disproportionation. We implement our findings using electrospray-ionization MS results for poly(methyl methacrylate) that we synthesized at 85 degrees C in benzene employing bis(3,5,5-trimethylhexanoyl) peroxide as initiator, chosen because it has the advantage of yielding only one primary radical species. Our results are well fitted by theory and are found to vary with chain length and initiator concentration as predicted. We also address the issue that signal intensities from macromolecular MS techniques may not give the true molecular weight distribution of a sample because of variation of ionization efficiency with species mass. We devise a way of obtaining lambda from data for hypothetical combination and disproportionation species of identical mass. Our final result is lambda = 0.63 +/- 0.10, suggesting that disproportionation is not as rampant in methyl methacrylate systems as commonly thought.Deutsche Forschungsgemeinschaft [BU 426/10
Critically evaluated rate coefficients in radical polymerization - 8. propagation rate coefficients for vinyl acetate in bulk
No full textPropagation rate coefficient values, <i>k</i><sub>p</sub>, reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed-laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in <i>k</i><sub>p</sub> is observed as laser pulse repetition rate is increased from low (25–100 Hz) to high (300–500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5–70°C. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 1.35 × 10<sup>7</sup> L mol<sup>−1</sup> s<sup>−1</sup> and an activation energy of 20.4 kJ mol<sup>−1</sup>, with 95% confidence ellipsoids for the parameters also presented
Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods
No full textThe knowledge of accurate rate coefficients for individual steps of free-radical polymerization (FRP) is of scientific interest and of application-oriented importance. For a wide variety of homopolymerizations and for many copolymerizations, reliable propagation rate coefficients, k p, are accessible via the IUPAC-recommended method of PLP-SEC (pulsed laser polymerization - size-exclusion chromatography). For termination rate coefficients, kt, the situation is less favorable. Even for very common monomers, no kt benchmark data sets are available. Moreover, instead of having one recommended technique for measuring kt, there are a plethora of such methods. Seventeen of the most prominent approaches for measuring kt are here reviewed, including innovative ones that have emerged over the last decade. The methods have been subdivided into two categories: (i) 'Kinetic methods', in which analysis of the time dependence of concentrations is essential, and (ii) 'MWD methods', in which the analysis of the molecular weight distribution plays the dominant role. The methods are evaluated with respect to their potential for providing routine access to measuring kt as a function of monomer conversion and of free-radical chain length. Moreover, it has been considered whether expensive instrumentation or highly demanding analysis is required for a particular method and whether a method is applicable to many types of monomers. A table summarizes all these evaluations in a readily accessible form. The use of kinetic methods appears to be generally preferable over MWD-based methods. The largest potential is currently seen for methods in which polymerization is induced by a single laser pulse and where the subsequent time evolution of either monomer concentration or free-radical concentration is measured. © 2005 Elsevier Ltd. All rights reserved
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