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PLP-SEC study into free-radical propagation rate of nonionized acrylic acid in aqueous solution
No full textPulsed 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, k(p), of nonionized acrylic acid (AA) at temperatures between 2 and 25 degreesC and monomer concentrations, c(AA), from 1 to 10 wt %. The product k(p)c(AA,local) is the primary experimental quantity deduced via the PLP-SEC technique. Assuming c(AA,local) to be identical to overall monomer concentration, c(AA), yields apparent kp values, which, upon enhancing c(AA), 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 degreesC. The decrease observed toward higher AA concentration is fully consistent with what has been found in a preceding study into k(p) 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 c(AA,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 c(AA). The assignment of the observed changes in apparent kp to c(AA,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 k(p). Addition of NaCl to an aqueous solution of AA in its nonionized form, on the other hand, does not affect apparent k(p). Whether the observed changes in k(p)c(AA,local) are entirely due to c(AA,local) differing from c(AA) or whether also the "true" propagation rate coefficient varies cannot be safely decided on the basis of the presently available data
PLP-SEC study into the free-radical propagation rate coefficients of partially and fully ionized acrylic acid in aqueous solution
No full textPropagation rate coefficients, kappa(p), for acrylic acid (AA) polymerization at 6 degreesC in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, alpha, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous-phase size-exculsion chromatography (SEC). Strictly speaking, the reported kappa(p) 's are "apparent" propagation rate coefficients deduced from the PLP-SEC data under the assumption that the local monomer concentration at the radical site is identical to overall monomer concentration. At an AA concentration of 0.69 mol.L-1, the apparent kappa(p) decreaes from 111 000 L.mol(-1).s(-1) at alpha = 0 to 13 000 L.mol(-1).s(-1) at alpha = 1.0. The significant lowering of kappa(p) with higher alpha is attributed to the repulsion between both monomer molecules and macroradicals becoming negatively charged. Addition of up to 10 mol-% (with respect to AA) sodium hydroxide to the fully ionized aqueous AA solution leads to an enhancement of kappa(p) up to 57 000 L.mol(-1).s(-1)
Aqueous phase size-exclusion-chromatography used for PLP-SEC studies into free-radical propagation rate of acrylic acid in aqueous solution
No full textPulsed 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 degreesC. 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 7( are about 60% higher than at 40 wt %. The activation energy of hp is close to 12.0 kJ . mol(-1) at both concentrations. At 25 degreesC and 20 wt %, k(p) 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
Pressure and temperature dependence of the propagation rate coefficient of free-radical styrene polymerization in supercritical carbon dioxide
No full textFree-radical polymerization of styrene in homogeneous phase of supercritical carbon dioxide (scCO(2)) has been studied at temperatures between 40 and 80 degreesC and pressures between 300 and 1500 bar. Applying pulsed-laser polymerization in conjunction with size-exclusion chromatography yields propagation rate coefficients, k(p). Within the extended pressure range under investigation, k(p) for solution polymerization in carbon dioxide, at CO2 contents up to 46 wt %, is identical to k(p) of styrene bulk polymerizations. The studies with CO2 suggest that solvent effects on k(p) occur in systems where the solvent quality for the polymer is not very high and where intrasegmental interactions are important
Free-radical propagation rate coefficient of nonionized methacrylic acid in aqueous solution from low monomer concentrations to bulk polymerization
No full textThe propagation rate coefficient, k(p), for free-radical polymerization of nonionized methacrylic acid (MAA) in aqueous solution has been studied via pulsed laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion chromatography (SEC). The PLP-SEC experiments were carried out between 20 and 80 degrees C within the entire concentration range from dilute solution containing 1 wt % MAA up to bulk MAA polymerization. The k(p) values which are determined under the assumption that the relevant monomer concentration at the radical site is identical to the known overall MAA concentration decrease by about 1 order of magnitude between 1 and 100 wt % MAA. This significant lowering is almost entirely due to a reduction in the Arrhenius preexponential factor, A(k(p)), whereas the activation energy, E-A(k(p)), stays essentially constant. The decrease in A(k(p)) is assigned to intermolecular interactions between the transition state (TS) structure for MAA propagation and an MAA environment being significantly stronger than the ones between this TS structure and an H2O environment. In an MAA-rich environment, the barrier to rotational motion of the relevant degrees of motion of the TS thus experiences enhanced friction, which is associated with a lowering of the preexponential factor and thus of k(p)
Chain Transfer to 2-Mercaptoethanol in Methacrylic Acid Polymerization in Aqueous Solution
No full textThe chain-transfer constant, CS = ktr/kp, of 2-mercaptoethanol (ME) for methacrylic acid (MAA) polymerization in aqueous solution has been measured at MAA concentrations between 5 and 30 wt% to be 0.12 +/- 0.01 at 50 degrees C. Analysis has been carried out via both the Mayo and the chain-length distribution (CLD) methods. No change of CS with monomer concentration is observed. The chain-transfer rate coefficient, ktr, thus exhibits the same strong dependence on monomer concentration as the propagation rate coefficient, k(p)
Free-radical polymerization kinetics of 2-acrylamido-2-methylpropanesulfonic acid in aqueous solution
No full textThe SP-PLP-NIR technique, which combines Pulsed laser polymerization (PLP) initiated by a single pulse (SP) with time-resolved monitoring of the resulting monomer conversion via near-infrared (NIR) spectroscopy, was used to investigate the kinetics in aqueous solution of 2-acrylamido-2-methylpropanesulfonic acid (AMPS). For initial AMPS concentrations of 2.79 mol(.)L(-1) (50 wt% AMPS) and 1.04 mol(.)L(-1) (20 wt% AMPS), the ratio of (chain length averaged) termination and propagation rate coefficients, /k(p), was measured up to almost complete monomer conversion at temperatures between 10 and 40 degrees C and ambient pressure. Up to 80% monomer conversion, /k(p) is only slightly lowered, whereas there is a clear decrease upon further increasing conversion. Variation of temperature and of pH does not significantly affect /k(p). For estimating individual rate coefficients, and k(p), in addition chemically initiated polymerizations have been carried out, in which AMPS conversion was monitored via in-line FT-NIR spectroscopy. The resulting and k(p) values, for 40 degrees C and an initial AMPS concentration of 2.79 mol(.)L(-1), are 2 x 10(7) L(.)mol(-1.)s(-1) and 1 x 10(5) L(.)mol(-1.)s(-1), respectively. Both rate coefficients are significantly higher at the lower AMPS content of 1.04 mol(.)L(-1). at this lower AMPS content may be understood in terms of termination Occurring under reaction diffusion control. The lowering in rate coefficients measured at the higher AMPS content is indicative of a reduced poly(AMPS) chain mobility
Radical Polymerization of Alkali Acrylates in Aqueous Solution
No full textMonomer conversion versus time profiles for the radical polymerization of ionized acrylic acid (AA), 0.6 to 0.8 mol L-1 in aqueous solution, are measured at 50 degrees C via inline near-infrared spectroscopy mostly on fully ionized AA, i.e., of alkali acrylates. Ionization reduces polymerization rate with the size of this effect being dependent on the type and on the concentration of the alkali counter cation, which is varied from lithium to rubidium. Polymerization rate becomes very small upon shielding the cation by complexation, whereas an enhancement of cation concentration, by addition of alkali chloride, may enhance polymerization rate such as to even reach the high rates observed with the radical polymerization of nonionized AA
Propagation 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
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