1,721,095 research outputs found
Dependence of termination kinetics in methyl acrylate-dodecyl acrylate free-radical copolymerization on comonomer composition, pressure, temperature and conversion
No full textThe termination kinetics of the free-radical bulk copolymerization of dodecyl acrylate (DA) and methyl acrylate (MA) has been investigated at various monomer mole fractions between 15 and 50°C and up to 2000 bar. The ratio of termination to propagation rate coefficients, (kt/kp)copo, is measured via the single pulse-pulsed laser polymerization (SP-PLP) technique. Chain-length averaged kt,copo are deduced from (kt/kp)copo in conjunction with kp,copo data that are estimated via a simplified version of the implicit penultimate unit effect (IPUE) model. At low and moderate degrees of monomer conversion extended ranges of almost constant kt are observed where termination is controlled by segmental diffusion with important contributions of steric effects. These plateau kt values are significantly - by almost two orders of magnitude - different for MA and DA. The increase with MA content of kt,copo is adequately described by a penultimate unit model which uses the geometric mean approximation for estimating rate coefficients of cross-termination between radicals of different free-radical terminus. The model applies within the entire pressure and temperature range of the present study. At high degrees of monomer conversion, at and above 50%, homo-kt and kt,copo are almost insensitive toward the composition of the monomer mixture. The termination rate under these conditions is essentially controlled by reaction diffusion
Initiation and termination kinetics in fluid-phase free-radical polymerization up to high pressure
No full textRate coefficients of peroxyester decomposition in solution of n-heptane have been measured as a function of temperature and pressure. The data is used to determine initiator efficiencies for the ethene high-pressure polymerization. The efficiency strongly depends on the structure of the peroxyester. Free-radical termination rate coefficients of (meth)acrylate systems have been studied up to about 50 % monomer conversion. The reaction is controlled by segmental diffusion in the early period and by reaction diffusion at later stages of the polymerization
Recent studies into initiation, propagation, and termination steps of radical polymerization
No full textModeling termination kinetics of non-stationary free-radical polymerizations
No full textPulsed-laser induced polymerization is modeled via an approach presented in a previous paper. An equation for the time dependence of free-radical concentration is derived. It is shown that the termination rate coefficient may vary significantly as a function of time after applying the laser pulse despite of the fact that the change in monomer concentration during one experiment is negligible. For the limiting case of t ≫ c-1 (kpM)-1, where c is a dimensionless chain-transfer constant, kp the propagation rate coefficient and M the monomer concentration, an analytical expression for kt is derived. It is also shown that time-resolved single pulsed-laser polymerization (SP-PLP) experiments can yield the parameters that allow the modeling of kt in quasi-stationary polymerization. The influence of inhibitors is also considered. The conditions are analyzed under which M (t) curves recorded at different extents of laser-induced photo-initiator decomposition intersect. It is shown that such type of behavior is associated with chain-length dependence of kt
Elementary steps of initiation and termination processes in radical polymerization
No full textInvestigations into the kinetics of primary radicals produced in photochemically and thermally induced decomposition of peroxides of type R1C(O)O-OR2 are presented. The correlation of peroxide structure with decomposition rate and with initiator efficiency in radical polymerizations is discussed. Termination rate coefficients, k(t), as a function of temperature, pressure, polymer content, and of chain length may be deduced from two types of time-resolved experiments in which, after applying an excimer laser pulse, either monomer conversion is measured via near-infrared spectroscopy or the decay in radical concentration is monitored via electron spin resonance
Initiation, propagation and termination in fluid phase free-radical polymerization
No full textFree-radical polymerizations are carried out in extended ranges of temperature, pressure, and conversion. The precise knowledge of individual rate coefficients of initiation, propagation, termination, and chain-transfer is essential for the modelling and optimization of monomer conversion and of polymer microstructure in technical polymerizations. In addition to the application-oriented interest, this data is of fundamental importance for the detailed understanding of reaction mechanisms of such free-radical-molecule, free-radical-free-radical, and unimolecular decomposition processes. Even for the polymerization of rather common monomers at moderate temperatures and ambient pressure such information is scarce. The present paper illustrates some recent advances in measuring, within wide ranges of pressure and temperature; propagation and termination rate coefficients of free-radical homo- and copolymerizations and also peroxyester decomposition rate coefficients
Propagation rate coefficients of acrylate-methacrylate free-radical bulk copolymerizations
No full textCopolymerization propagation rate coefficients, kp,copo, have been measured for the binary systems methyl acrylate (MA)-dodecyl methacrylate (DMA), butyl acrylate (BA)-methyl methacrylate (MMA), dodecyl acrylate (DA)-DMA, and DA-MMA at 40 °C and 1000 bar by the pulsed laser polymerization (PLP)-size-exclusion chromatography (SEC) technique. These acrylate-methacrylate systems are interesting because of the significant difference, by more than 1 order of magnitude, between the homopropagation rate coefficients of the two families. Reactivity ratios, ri, are determined from monomer feed compositions and the NMR spectroscopically measured copolymer compositions. The resulting ri values for the four acrylate-methacrylate copolymerizations agree within experimental accuracy. Moreover, these ri data are surprisingly close to reactivity ratio data estimated from individual addition rate coefficients to MA and MMA, respectively, of appropriate small (meth)acrylate-type free radicals. Such addition rate coefficients have been determined via EPR in liquid solution by the Hanns Fischer group. The terminal model allows for excellent individual fits of composition and of kp,copo for each of the four systems. The implicit penultimate unit effect (IPUE) model (and the explicit penultimate unit effect (EPUE) model) are capable of simultaneously fitting composition and rate data for the MMA-BA and DMA-MA systems whereas both models fail to provide a satisfactory representation of the two DA-containing systems. The data suggest that, with DA being one of the comonomers, individual propagation rate coefficients are not adequately described by consideration of only terminal and penultimate units at the free-radical terminus. On the other hand, ratioing individual propagation rate coefficients of free radicals with the same penultimate units seems to eliminate most of the impact of the penultimate units. For this reason the resulting and widely used "terminal model" reactivity ratios are reasonable and meaningful kinetic quantities although penultimate effects on the individual propagation rate coefficients undoubtedly operate
Pressure and temperature dependence of the decomposition rate of aliphatic tert-butyl peroxyesters
No full textThe thermal decomposition in solution of n-heptane of several aliphatic tert-butyl peroxyesters RC(O)OOTB: TB peroxypropionate, TB peroxy-n-butyrate, TB peroxy-iso-butyrate, TB peroxy-2-methylbutyrate, TB peroxy-2-ethylhexanoate, TB peroxy-3,5,5-trimethylhexanoate, and TB peroxy-neo-decanoate has been investigated at pressures up to 2500 bar and temperatures up to 185 degrees C. The experiments were carried out in a tubular reactor at residence times up to 140 s. Peroxide concentration was monitored via quantitative FT-IR spectroscopic analysis of the pressurized medium under continuous flow conditions. For each peroxide first-order decomposition kinetics were observed over several half-lives. The type of carbon atom (in the R moiety) that is in ex-position to the carbonyl group controls decomposition kinetics. The observed first-order rate coefficient, k(obs), is smallest in situations where this particular C-atom is primary and is largest when it is tertiary. The "primary" TB peroxyesters are associated with the higher activation energies, around 145 kJ mol(-1). The activation energies, E-A, of the "tertiary" and "secondary" TB peroxyesters are around 120 und 130 kJ mol(-1), respectively, with the individual E-A's being proportional to the stability of the particular free-radical species R-.. The activation volumes, Delta V-obs(not equal) of the "secondary" and "tertiary" TB peroxyesters are in the narrow range 3.0 +/- 1.5 cm(3) mol(-1). Delta V-obs(not equal), of the "prinnary" TB peroxyesters is above 10 cm(3) mol(-1) and, in addition, is slightly pressure-dependent. Both observations demonstrate that the "primary" aliphatic TB peroxyesters decompose via single-bond scission with this result being supported by modelling dissociation dynamics in the compressed fluid state. The "secondary" and "tertiary" TB peroxyesters decompose via concerted two-bond scission associated with the immediate formation of CO2. These results are fully consistent with evidence from the literature and from our preceding detailed study into TB peroxyacetate and TB peroxypivalate dissociation kinetics. The mode of primary bond dissociation, via single-bond or via concerted two-bond scission, largely affects the initiation efficiency of TB peroxyesters in free-radical polymerization. For each TB peroxyester, rate expressions for k(obs)(p,T) are presented. This data for decomposition in compressed n-heptane should be highly suitable for simulation and optimization of technical high-pressure polymerization processes
High-pressure free-radical copolymerization of ethene with methacrylic acid and ethene with acrylic acid, 2 - Ethene reactivity ratios
No full textFree-radical copolymerizations of ethylene (E) with methacrylic acid (MAA) and of E with acrylic acid (AA) were carried out in a continuously operated tank reactor at 2000 bar and temperatures up to 280 degrees C. The (M)AA content of the polymer, F-(M)AA, that may be reached by polymerization in homogeneous phase, e.g., at 260 degrees C, is below 10 mol-% and the associated (M)AA content of the monomer mixture is below 1 mol-%. Under such conditions, the ethene and the comonomer reactivity ratios, r(E) and r((M)AA), respectively, can not be simultaneously derived by fitting the measured F-(M)AA and f((M)AA) data according to the classical procedure. With r((M)AA), however, being available from a preceding C-13 NMR study(1)), r(E) may be calculated from each pair of experimental F-(M)AA and f((M)AA) values. F-(M)AA values are determined by means of elemental analysis of carbon and oxygen with consistency checks of copolymer composition brine performed via FT-IR spectroscopy on polymeric films. f((M)AA) is deduced from the measured monomer feed fluxes and from the amount and composition of the copolymer produced within given intervals of stationary polymerization. The r(E) data for the E/MAA and E/AA systems are presented and discussed. At identical polymerization pressure and temperature, the measured r(E) values of both systems agree within about +/-10%. The r(E) values, in addition, are very close to the ethene reactivity ratios that were recently measured for several E/(meth)acrylate copolymerizations. Arguments for this family-type behavior of r(E) are discussed
Termination kinetics of free-radical methyl methacrylate-dodecyl acrylate and dodecyl methacrylate-methyl acrylate copolymerizations
No full textCopolymerization termination rate coefficients (k(t)) of the methyl methacrylate-dodecyl acrylate (MMA-DA) and dodecyl methacrylate-methyl acrylate (DMA-MA) systems at 40degreesC and 1000 bar have been measured using the single pulse (SP)-pulsed laser polymerization (PLP) technique. Plateau regions of k(t) are observed in the initial polymerization period. The region of constant k(t) increases with the size of the alkyl ester group, e.g. it extends up to at least 60% monomer conversion in DA and DMA homopolymerizations. The plateau k(t) values of MA and MMA are significantly above the corresponding DA and DMA values. The k(t) penultimate unit effect model, which uses the so-called geometric mean approximation, is well suited for representation of the dependence on monomer composition of the plateau k(t) values. The dependence of k(t) on monomer composition is quite different at low and at high degrees of monomer conversion. The reason behind this is seen in different types of diffusion control being operative at low and at high degrees of monomer conversion. The plateau-type behaviour is assigned to segmental diffusion control, whereas high-conversion k(t) is controlled by reaction diffusion
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