Chain‐length‐dependent termination rate coefficients of the bulk free‐radical polymerization of styrene at 80 °C are determined by combining online polymerization rate measurements (DSC) with living RAFT polymerizations. Full kt versus chain‐length plots were obtained indicating a high kt value for short chains (2 × 109 L · mol−1 · s−1) and a weak chain‐length dependence between 10 and 100 monomer units, quantified by an exponent of −0.14 in the corresponding power law 〈kti,i〉 = kt0 · P−b.
Double logarithmic plots of 〈kti,i〉 versus P, evaluated from experimental time‐resolved Rp data according to the procedure described in the text, for different CPDA and AIBN concentrations. The best linear fit for (10 < P < 100) is indicated as full line. 相似文献
In nitroxide‐mediated radical polymerization, the polymerization times decrease with the increasing re‐formation rate constant of the C? ON bond (→ alkoxyamine) between the growing polymer chain and the nitroxide radical. The factors influencing the re‐formation rate constant are of considerable interest, but up to now, the polar/stabilization effects have not been addressed thoroughly. The combination of new data with previously reported data now showed that the re‐formation rate constant kc increases with the increasing polar character of the substituents attached to the nitroxide moiety. The polar/stabilization effects are weaker for the re‐formation than for the homolysis of the C? ON bond, and may be mainly attributed to the relocation of the odd electron onto the O‐atom of the N? O moiety, i.e., the stabilization of the nitroxide moiety. Hence, it is possible to predict the values of kc by combining both the polar/stabilization (σI) and steric effects (E ), i.e., log(kc/M ?1 s?1) = 9.86 + 0.57 ? σI + 0.40 ? Es. 相似文献
Below −60° and without catalyst, 1,2‐dimethylidenecyclopentane ( 16 ), 1,2‐dimethylidenecyclohexane ( 13 ), 1,2‐dimethylidenecycloheptane ( 17 ), and 1,2‐dimethylidenecyclooctane ( 18 ) add to sulfur dioxide in the hetero‐Diels‐Alder mode, giving the corresponding sultines 4,5,6,7‐tetrahydro‐1H‐cyclopent[d][1,2]oxathiin 3‐oxide ( 19 ), 1,4,5,6,7,8‐hexahydro‐2,3‐benzoxathiin 3‐oxide ( 14 ), 4,5,6,7,8,9‐hexahydro‐1H‐cyclohept[d][1,2]oxathiin 3‐oxide ( 20 ), and 1,4,5,6,7,8,9,10‐octahydrocyclooct[d][1,2]oxathiin 3‐oxide ( 21 ), respectively. Above −40°, the sultines are isomerized into the corresponding sulfolenes 3,4,5,6‐tetrahydro‐1H‐cyclopenta[c]thiophene 2,2‐dioxide ( 22 ), 1,3,4,5,6,7‐hexahydrobenzo[c]thiophene 2,2‐dioxide ( 15 ), 3,4,5,6,7,8‐hexahydro‐1H‐cyclohepta[c]thiophene 2,2‐dioxide ( 23 ), and 1,3,4,5,6,7,8,9‐octahydrocycloocta[c]thiophene 2,2‐dioxide ( 24 ). Kinetics and thermodynamics data were collected for these reactions. The sultines are ca. 10 kcal/mol Diels‐Alder additions (ΔH≠( 16 −36±3 cal mol−1 K−1) in agreement with third‐order rate laws that imply that two molecules of SO2 intervene in the transition states of these cycloadditions. Similar observations were made for the cheletropic additions of SO2. Attempts to simulate the thermodynamics and kinetics parameters of the reactions of SO2 with dienes 16 and 13 by density‐functional theory (DFT) suggest that the calculations require an appropriate number of polarization functions in the basis set employed. A satisfactory recipe to compute the SO2 additions to large dienes can be: B3LYP/6‐31G(d) geometry optimizations followed by B3LYP/6‐31+G(2df,p) single‐point calculations or G2(MP2,SVP) estimates on the B3LYP/6‐31G(d) geometries. 相似文献
Summary A kinetic study of the oxidation of chromium(III) by N-bromosuccinimide (NBS) in aqueous solutions and H2O-MeOH solvent mixtures were performed. The kinetics in aqueous solutions obeyed the rate law: d[CrVI]/dt = {k4KhK2[NBS][CrIII]T}/[+]{1 + Kh/[H+] + (K1 + KhK2/[H+][NBS])} where Kh, K1 and K2 are the hydrolysis constant of [CrIII(H2O)6]3+, and pre-equilibrium formation constants for the protonated and deprotonated precursor complexes, respectively. An innersphere
mechanism is proposed. An argument based on isokinetic correlations among activation parameters for the oxidation of a series
of cobalt(II) and chromium(III) complexes including [Cr(H2O)6]3+ is presented in support of a common mechanism for these reactions.
Abstracted from the Ph.D. Thesis (Ain Shams University) of A. E.-D. M. Abdel-Hady. 相似文献
The ionic liquid (IL) trihalogen monoanions [N2221][X3]− and [N2221][XY2]− ([N2221]+=triethylmethylammonium, X=Cl, Br, I, Y=Cl, Br) were investigated electrochemically via temperature dependent conductance and cyclic voltammetry (CV) measurements. The polyhalogen monoanions were measured both as neat salts and as double salts in 1-butyl-1-methyl-pyrrolidinium trifluoromethane-sulfonate ([BMP][OTf], [X3]−/[XY2]− 0.5 M). Lighter IL trihalogen monoanions displayed higher conductivities than their heavier homologues, with [Cl3]− being 1.1 and 3.7 times greater than [Br3]− and [I3]−, respectively. The addition of [BMP][OTf] reduced the conductivity significantly. Within the group of polyhalogen monoanions, the oxidation potential develops in the series [Cl3]−>[BrCl2]−>[Br3]−>[IBr2]−>[ICl2]−>[I3]−. The redox potential of the interhalogen monoanions was found to be primarily determined by the central halogen, I in [ICl2]− and [IBr2]−, and Br in [BrCl2]−. Additionally, tetrafluorobromate(III) ([N2221]+[BrF4]−) was analyzed via CV in MeCN at 0 °C, yielding a single reversible redox process ([BrF2]−/[BrF4]−). 相似文献
Air and moisture sensitive K5[CuO2][CO3] was prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, KN3, KNO3 and K2CO3. According to the single‐crystal X‐ray analysis of the crystal structure [P4/nbm, Z = 2, a = 7.4067(5), c = 8.8764(8) Å, R1 = 0.053, 433 independent reflections] K5[CuO2][CO3] represents an ordered superstructure of Na5[NiO2][CO3]. The structure contains isolated [CuO2]3– dumbbells and CO32– anions, with the latter not connected to the transition element. Raman spectroscopic measurements confirm the presence of CO32– in the structure. 相似文献
Summary: A novel method for measuring termination rate coefficients, kt, in free‐radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator‐derived radicals. During subsequent polymerization, radical concentration is monitored by time‐resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain‐length dependence of kt to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.
Two straight lines provide a very satisfactory representation of the chain‐length dependence of kt over the entire chain‐length region (cR = radical concentration). 相似文献