The reversible addition‐fragmentation chain transfer chain length dependent termination (RAFT‐CLD‐T) technique allows a simple experimental approach to obtain chain‐length‐dependent termination rate coefficients as a function of conversion, k(x). This work provides a set of criteria by which accurate k(x) can be obtained using the RAFT‐CLD‐T method. Visualization of three‐dimensional plots varying all kinetic rate parameters and starting concentrations demonstrates that only certain combinations give an accurate extraction of k(x). The current study provides hands‐on guidelines for experimentalists applying the RAFT‐CLD‐T method.
Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization–electron paramagnetic resonance method. MMA‐d8 has been investigated to enhance the signal‐to‐noise quality of microsecond time‐resolved measurement of radical concentration. Chain‐length‐dependent termination rate coefficients of radicals of identical size, k, are reported for 5–70 °C and up to i = 100. k decreases according to the power‐law expression . At 5 °C, kt for two MMA radicals of chain‐length unity is k = (5.8 ± 1.3) · 108 L · mol−1 · s−1. The associated activation energy and power‐law exponent are: EA(k) ≈ 9 ± 2 kJ · mol−1 and α ≈ 0.63 ± 0.15, respectively.
Unusual difficulties are faced in the determination of propagation rate coefficients (kp) of alkyl acrylates by pulsed‐laser polymerization (PLP). When the backbiting is the predominant chain transfer event, the apparent kp of acrylates determined in PLP experiments for different frequencies should range between kp (propagation rate coefficient of the secondary radicals) at high frequency and k at low frequency. The k value could be expressed from kinetic parameters: , where kfp is the backbiting rate coefficient, kp2 is the propagation rate coefficient of mid‐chain radicals, and [M] is the monomer concentration.
Apparent propagation rate coefficients determined for different frequencies by simulating the PLP of n‐butyl acrylate at 20 °C. Horizontal full lines show the values of kp and k. 相似文献
In 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 °C. The sigmoidal decrease in the apparent propagation rate coefficient, k, towards lower LPRRs is in line with recent predictions. At the highest LPRRs, k approaches the rate coefficient of SPR propagation, k, whereas the limiting value of k at low LPRRs approaches the effective propagation rate coefficient, k, which allows for an estimate of the fraction of MCRs under polymerization conditions, xMCR.
A method that utilizes reversible addition fragmentation chain transfer (RAFT) chemistry is evaluated on a theoretical basis to deduce the termination rate coefficient for disparate length radicals k in acrylate free radical polymerization, where s and l represent the arbitrary yet disparate chain lengths from either a “short” or “long” RAFT distribution. The method is based on a previously developed method for elucidation of k for the model monomer system styrene. The method was expanded to account for intramolecular chain transfer (i.e., the formation of mid-chain radicals via backbiting) and the free radical polymerization kinetic parameters of methyl acrylate. Simulations show that the method's predictive capability is sensitive to the polymerization rate's dependence on monomer concentration, i.e., the virtual monomer reaction order, which varies with the termination rate coefficient's value and chain length dependence. However, attaining the virtual monomer reaction order is a facile process and once known the method developed here that accounts for mid-chain radicals and virtual monomer reaction orders other than one seems robust enough to elucidate the chain length dependence of k for the more complex acrylate free radical polymerization. 相似文献
The average instantaneous shape of an unperturbed polyethylene chain is studied with a Monte Carlo technique. Different short-range interactions in the polyethylene chain are considered. The shape is evaluated as the ratio 〈L〉:〈L〉:〈L〉, where L1≤L2≤L3 are the orthogonal components in the system of principal axes of gyration. Differences are found for different interactions in short- and medium-length chains, while for long chains all ratios converge to a common limit, which is about 1:2.7:12.0 for polyethylene chains. 相似文献
Existing data on the self-reactions of tertiary peroxy radicals RO2 has been reanalyzed and corrected to deduce Arrhenius parameters for both termination and nontermination paths. For R = t-Butyl, these are logkt(M?1sec?1) = 7.1 - (7.0/θ) and logknt(M?1sec?1) = 9.4 - (9.0/θ), respectively, different from those recommended by other authors. The higher magnitudes observed for termination processes of tertiary peroxy radicals like those of cumyl and 1,1-diphenylethyl have been discussed in terms of a much greater cage recombination of cumyloxy radicals as contrasted with t-butoxy radicals. It is shown that for benzyl peroxy radicals, the R—O bond dissociation energy is sufficiently low (18–20 kcal) that reversible dissociation into R˙ + O2 opens a competing second-order path to fast recombination R˙ + RO → ROOR. This path is probably not important for cumyl peroxy radicals under usual experimental conditions but can become important for 1,1-diphenyl ethyl peroxy radicals at (O2) < 10?3M. At very low RO concentrations (<10?5M), in the absence of added O2, an apparent first-order disappearance of RO can occur reflecting the rate determining breaking of the cumyl—O bond followed by the second step above. The thermochemistry of RO is used to show that the reaction of R2O4 → 2RO + O2 must be concerted and cannot proceed via RO which is too unstable and cannot form even from RO˙ + O2. 相似文献
Using combined results of isothermal viscosity measurements and cross-polarized light microscopy on four polyisocyanate/solvent systems, the following were demonstrated: (a) an anisotropic phase appears, associated with a shoulder in the viscosity curve, at a concentration v lower than the peak viscosity at v; (b) the inversion from anisotropic inclusions in an isotropic matrix to isotropic inclusions in an anisotropic matrix, occurs at concentrations v > v and (c) the attainment of a single phase, microscopically anisotropic, occurs at v > v; where the viscosity is decreasing but has not yet reached its minimum. When the experiments were repeated with changes in temperature, the following were observed: (a) within each single phase the viscosity drops with increased temperature; (b) in the biphasic range, the total viscosity η0 remains about constant in the concentration range ≤ and increases with temperature in the range v > v; (c) in the interval v > v of the biphasic range, at constant temperature an increase in concentration decreases η0, and at constant concentration, a decrease in temperature lowers η0. Qualitative explanations of the observations are proposed. 相似文献
Measurements of the translational energy loss accompanying the charge-stripping reactions M++N→M2++N+e− and M2++N→M3++N+e− have been performed for C, C and C, C respectively. The energy nesessary to remove the second electron from Buckminsterfullerene was determined, Q=IE(C→C=12.25±0.5 eV. 相似文献