1,720,971 research outputs found
Chain-Length-Dependent Termination in n-Butyl Methacrylate and tert-Butyl Methacrylate Bulk Homopolymerizations Studied via SP-PLP-ESR
No full textThe chain-length dependence of the termination rate coefficient, k(t), of bulk homopolymerizations of n-butyl methacrylate (n-BMA) and tert-butyl methacrylate (t-BMA) at ambient pressure and temperatures between -30 and 60 degrees C has been studied via the single pulse-pulsed laser polymerization-electron spin resonance (SP-PLP-ESR) technique. The decay of radical concentration, c(R), after laser SP initiation is monitored with a high time resolution of microseconds by ESR spectroscopy. Radical chain length, i, increases linearly with time t after applying the laser pulse. The experimental k(t)(i,i) values refer to rate coefficients for termination of two radicals of identical chain length i. The variation of k(t)(i,i) with chain length is adequately represented via the composite model proposed by Smith et al., in which two power-law expressions, k(t)(i-i) proportional to i(-alpha), are contained with the exponents alpha(s) and alpha(l) referring to short-chain and long-chain radicals, respectively. The transition between the two regimes occurs at the crossover chain length, i(c). The rate coefficients extrapolated for termination of two radicals of chain length unity. k(t)(l,l), are almost identical for n-BMA and t-BMA with an activation energy of E-A(k(t)(l,l)) approximate to 10 kJ mol(-1). The alpha(s) values are close to each other 0.65 +/- 0.10 (n-BMA) and 0.56 +/- 0.10 (t-BMA). Both alpha(l) values are found to be 0.20 +/- 0.05, which is close to the theoretical value of alpha(l), = 0.16. The crossover chain lengths are i(c) approximate to 50 for n-BMA and i(c) approximate to 70 for t-BMA. The minor differences in composite-model parameter values of n-BMA and t-BMA are assigned to differences in chain mobility
Determination of intramolecular chain transfer and midchain radical propagation rate coefficients for butyl acrylate by pulsed laser polymerization
No full textA novel method to extract individual free-radical polymerization rate coefficients for butyl acrylate intramolecular chain transfer (backbiting), k(bb), and for monomer addition to the resulting midchain radical, k(p)(t), is presented. The approach for measuring k(bb) does not require knowledge of any other rate coefficient. Only the dispersion parameter of SEC broadening has to be determined by fitting measured MWDs or should be available from separate experiments. The method is based upon analysis of the shift in the position of the inflection point of polymer molecular weight distributions produced by a series of pulsed-laser polymerization (PLP) experiments with varying laser pulse repetition rate. The coefficient k(bb) is determined from the onset of the sharp decrease of the apparent propagation rate coefficient (k(p)(app)) with decreasing repetition rate, an approach verified by simulation. With experiments performed between -10 and +30 degrees C, the estimated values are fitted well by an Arrhenius relation with pre-exponential factor A(k(bb)) = (4.84 +/- 0.29) x 10(7) s(-1) and activation energy E-a(k(bb)) = (31.7 +/- 2.5) kJ center dot mol(-1). At low pulse repetition rates, the experimental k(p)(app) values are related to an averaged propagation rate coefficient, k(p)(av), that is dependent on the relative population of chain-end and midchain radicals. Evaluated by comparing simulated and experimental molecular weight distributions, k(p)(av) provides an estimate for k(p)(t). The Arrhenius parameters are: A(k(p)(t)) = (1.52 +/- 0.14) x 10(6) L center dot mol(-1)center dot s(-1) and E-a(k(p)(t)) = (28.9 +/- 3.2) kJ center dot mol(-1)
Chain-length-dependent termination in acrylate radical polymerization studied via pulsed-laser-initiated RAFT polymerization
No full textThe chain-length dependence of the termination rate coefficient, k(t), in methyl acrylate ( MA) and dodecyl acrylate (DA) radical polymerization has been determined via the single pulse pulsed-laser polymerization near-infrared reversible addition-fragmentation chain transfer (SP-PLP-NIR-RAFT) technique. Polymerization is induced by a laser SP and the resulting decay in monomer concentration, c(M), is monitored via NIR spectroscopy with a time resolution of microseconds. A RAFT agent ensures the correlation of radical chain length and monomer-to-polymer conversion. The obtained rate coefficients for termination of two radicals of approximately the same chain length, i, are represented by power-law expressions, k(t)(i, i)alpha i(-alpha). For both monomers, composite model behaviour of k(t)(i, i) showing two distinct chain length regimes is observed. The exponent as referring to short chain lengths is close to unity, whereas the exponent alpha(1), which characterizes the chain-length dependency of large radicals, is slightly above the theoretical value for coiled chain-end radicals. The crossover chain length, i(c), which separates the two regions, decreases from MA(i(c) = 30) to DA(i(c) = 20). The results for MA and DA are consistent with earlier data reported for butyl acrylate. There appears to be a correlation of as and ic with chain flexibility
Termination and Transfer Kinetics of Butyl Acrylate Radical Polymerization Studied via SP-PLP-EPR
No full textButyl acrylate (BA) solution polymerization (1.5 M in toluene) was investigated via single-pulse pulsed laser polymerization in conjunction with electron paramagnetic resonance spectroscopy (SP-PLP-EPR) with emphasis on the termination and transfer kinetics of the system in which two distinctly different types of radicals, secondary chain-end radicals (SPRs) and midchain radicals (MCRs), are present. MCRs are produced by intramolecular hydrogen transfer (backbiting). They may react back to SPRs by monomer addition. The evolution of SPR and MCR concentrations after photoinitiation with an intense laser pulse was measured via highly time-resolved EPR at temperatures between 40 and +60 degrees C. At very low temperatures the MCR concentration is negligible, enabling the chain-length-dependent rate coefficient of SPR termination, k(t)(ss)(i,i), to be directly determined. At higher temperatures it was necessary to use PREDICI simulation of the radical concentration vs time traces, a process which yields the chain-length-dependent rate coefficient of SPR termination for monomeric radicals, k(t)(ss)(1,1), as well as the rate coefficients for backbiting, k(bb), for monomer addition to an MCR, k(p)(t), and for SPR-MCR cross-termination, k(t)(st). The composite model adequately represents k(t)(ss)(i,i), with the power-law exponents alpha(s) = 0.85 +/- 0.09 and alpha(1) = 0.16 +/- 0.07 for short-chain and long-chain radicals, respectively, and a crossover chain length between short-chain and long-chain behavior at around i(c) = 30. The activation energy for both k(t)(ss)(1,1) and k(t)(st)(1,1) is found to be as one would expect for translational diffusion of small molecules
PLP Labeling in ESR spectroscopic analysis of secondary and tertiary acrylate propagating radicals
No full textPropagation Rate Coefficient and Fraction of Mid‐Chain Radicals for Acrylic Acid Polymerization in Aqueous Solution
No full textIn acrylate polymerizations both SPRs and tertiary MCRs occur. Via pulsed laser polymerization, using a wide range of LPRRs, in conjunction with aqueous-phase size-exclusion chromatography the polymerization of 1.35 mol-L-1 acrylic acid in aqueous solution has been investigated at 6 degrees C. The sigmoidal decrease in the apparent propagation rate coefficient, k(p)(app), towards lower LPRRs is in line with recent predictions. At the highest LPRRs, k(p)(app) approaches the rate coefficient of SPR propagation, k(p)(SPR), whereas the limiting value of k(p)(app) at low LPRRs approaches the effective propagation rate coefficient, k(p)(eff), which allows for an estimate of the fraction of MCRs under polymerization conditions, x(MCR). {graphics
Determination of the Mode of Radical Termination from Pulsed Laser Polymerization Experiments in the Presence of Retardation and Chain Transfer to Agent
No full textThe pulsed laser polymerization-high termination rate limit-size-exclusion chromatography (PLP-HTRL-SEC) technique is used to estimate the mode of termination () for n-butyl methacrylate (n-BMA) polymerization at 30 degrees C. It is found that molecular mass distributions measured in these experiments are influenced by an unknown side reaction such as retardation or chain transfer that results in a marked decrease of intensity of the PLP peak. A new approach is developed to evaluate , with numerical experiments used to demonstrate the robustness of the methodology in the presence of retardation or chain transfer to agent. Application to the experimental n-BMA polymerization at 30 degrees C leads to an estimate for of 0.60 +/- 0.03
‐Butyl Acrylate Radical Polymerization
No full textVia electron paramagnetic resonance (EPR) spectroscopy, concentrations of secondary propagating radicals (SPRs) and tertiary mid-chain radicals (MCRs) in n-butyl acrylate solution polymerization were measured. The EPR spectrum is dominated by the 4-line spectrum of SPRs at -50 degrees C and by the 7-line spectrum of MCRs at +70 degrees C. At intermediate temperatures, a third spectral component is seen, which is assigned to an MCR species with restricted rotational mobility. The MCR components are produced by 1,5-hydrogen shift (backbiting) of SPRs. The measured ratio of MCRs to SPRs U allows for estimating the rate coefficient k(p)(t) for monomer addition to a mid-chain radical. For 70 degrees C, k(p)(t) is obtained to be 65.5 L . mol(-1) . s(-1)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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