Nucleophilic Substitution in Dipolar Aprotic Solvents: Hexamethylphosphoric triamide

The rate of the reaction between ethyl tosylate and chloride as well as bromide ion has been measured in hexamethylphosphoric triamide in the presence of several counterions. Rate constants for the free ions at 25.0° are 14.4 mole^?1 for chloride and 2.0 mole^?1min^?1 for bromide, both independent of the cation used. The equilibrium constants for ion pair dissociation of lithium-, ammonium- and tetra-n-butylammonium chloride are 3.97 × 10^?2, 1.52 × 10^?3 and 6.36 × 10^?3 mole, and for the corresponding bromides 3.30 × 10^?2, 9.65 × 10^?3, and 9.62 × 10^?3 mole.     

Kinetics of Anionic Polymerization of ε-Caprolactone (εCL). Propagation of Poly-ε=CL-K+ Ion Pairs

Kinetics of the anionic polymerization of ε-caprolactone (εCL) initiated with (CH₃)₃SiO^?K⁺ and carried out in THF solution has been studied in the temperature range from 0 to 20°C by using a calorimetric method. From the kinetic results and from conductometric measurements of the dissociation constant of the living Poly-εCL^?K⁺ ion pairs (K_D ²⁰ ? (4 ± 2) × 10^?10 mol/L), we concluded that at the conditions indicated above and for concentrations of active centers ranging from 10^?3 to 3.7 × 10^?2 mol/L, propagation proceeds on the ion pairs and is disturbed neither by dissociation nor by the formation of higher aggregates. For the polymerization of εCL proceeding on the poly-εCL^?K⁺     

Cerium (IV)-2-chloroethanol redox-pair initiated polymerization of acrylamide in aqueous medium

The kinetics of acrylamide polymerization has been investigated by employing cericammoniumnitrate-2-chloroethanol redox pair under nitrogen atmosphere at 30 ± 1°C. The rate of monomer disappearance is directly proportional to the concentration of 2-chloroethanol (1.0 × 10^?2 ? 10.0 × 10^?2 mol. dm^?3) and is inversely proportional to the ceric ion concentration (2.5 × 10^?3 ? 10.0 × 10^?3 mol. dm^?3) but shows square dependence to the concentration of monomer (5.0 × 10^?2 ? 25.0 × 10^?2 mol. dm^?3). The rate of ceric ion disappearance is directly proportional to the initial concentration of ceric ion and 2-chloroethanol but independent of acrylamide concentration. The viscometric average molecular weight (M _v) decreases on increasing the concentration of ceric ion and increases on increasing the concentrations of acrylamide and 2-chloroethanol. A tentative mechanism has been proposed.     

Trityl salt—initiated polymerization of α-methylstyrene

The initiation reaction of the polymerization of α-methylstyrene by trityl tetrachloroferate and tritylhexachloroantimonate in 1,2-dichloroethane at 20°C was studied. The rate constants were 14 × 10^?3 and 27 × 10^?3 L mol^?1s^?1, respectively. The dissociation constants of tritylterachloroferate (K_d = 0.88 × 10^?4M^?1) and tritylhexachloroantimonate (K_d = 2.64 × 10^?4M^?1) was determined. The effect of electron acceptors and donors on the dissociation equilibrium and initiation rate was investigated. It was shown that in strongly dissociated ion pairs such as stable carbenium salts the electron donors and acceptors have no appreciable effect on the magnitude of the dissociation. The temperature dependence of the rate constants in the ?20–+20°C range yielded the following thermodynamic parameters for trityltetrachloroferate: E_i = 8.54 kcal/mol; A = 3.2 × 10⁴ mol^?1s^?1; ΔH* = 8 kcal/mol; and S* = ?39.8 eu.     

Etude cinetique de la polymerisation cationique du cyclopentadiene amorcee par l'hexachloroantimoniate de triphenylmethyle

A study of cyclopentadiene polymerization, initiated by φ₃C⁺SbCl₆^? in methylene chloride solution, has been carried out at temperatures between ?70 and +20° using a dilatometric method. An overall external second order with respect to monomer has been found. At very low temperature (?70°), the concentration of active centres remains low and roughly constant, in agreement with a quasi-stationary state assumption. Between ?50 and + 10°, experimental determination of (k_p. M^*), obtained from variation of v_p and [M] with time, shows that the concentration of centres goes through a maximum, sharper and more rapidly reached as the temperature is raised. Initiation is slower than propagation and active centres are rapidly destroyed when termination becomes faster than initiation. This explains the partial conversions and the observed maximum for concentration of active centres. Propagation and unimolecular termination rate constants have been determined at each temperature: activation energies are E_p = ?8 ± 0·5 kcal mole^?1 and E_p = ?0·3 ± 0·1 kcal mole^?1. These negative values can be explained by an exothermic process of solvation of active centres, leading to more reactive propagating species.     

Effects of mercaptides on anionic polymerization. III. Polymerization of methyl methacrylate by thiophenoxides in polar aprotic solvents

Sodium thiophenoxide initiated the polymerization of methyl methacrylate in polar aprotic solvents (DMF, DMSO, HMPA). The active species that initiated the polymerization of the monomer was found by spectrophotometric measurements and by the sodium fusion method to be sodium thiophenoxide itself. The activation energy for the polymerization of the monomer in DMF solvent obtained was E = 3.4 kcal/mole below 30°C, and E = ?3.3 kcal/mole above the temperature. The phenomena were reasoned as the result of the formation of two active species: a solvent-separated ion pair and a contact ion pair. The effects of counterions on the reactivity of thiophenoxide increased with increasing electropositivity of the metals: Li < Na < K. Sodium phenoxide, the oxygen analog of thiophenoxide, was also found to initiate the polymerization of the monomer in the solvents. The relative reactivity of thiophenoxide to phenoxide for the monomer in HMPA at 30°C was thus determined: phenyl-SNa > phenyl-ONa. The relative effect of the polar aprotic solvents on the reactivity of thiophenoxide was also as follows: HMPA > DMF > DMSO. The kinetic studies were made by the graphical evaluation of rate constants. The following results were obtained for the monomer at 20°C in DMF solvent: K_p = 3.5 × 10² 1./mole-hr and K_t = 9.8 × 10^?2/hr.     

Aqueous polymerization of acrylamide initiated by acidic potassium permanganate–ascorbic acid redox system

The polymerization of acrylamide initiated by the acidic permanganate–ascorbic acid redox pair has been studied in aqueous media at 30 ± 0.2°C in nitrogen atmosphere. The initial rate of polymerization has been found to be proportional to nearly the first power of the catalyst KMnO₄ concentration within the range 6.0 × 10^?3–14.0 × 10^?3 mole/l. The rate is proportional to the first power of the monomer concentration within the range 4.00 × 10^?2–12.0 × 10^?2 mole/l. However, the rate of polymerization is independent of ascorbic acid concentration within the range 3.0 × 10^?3–6.0 × 10^?3 mole/l., but the further increase of the concentration depresses the rate of polymerization as well as maximum conversion. The initial rate increases but the maximum conversion decreases as the temperature is increased within the range 20–35°C. The overall energy of activation has been found to be 9.8 kcal/mole. The optimum amount of sulfuric acid is essential to initiate the polymerization but its presence in excess produces no effect either on the rate of reaction or the maximum conversion. Water-miscible organic solvents and salts, e.g., CH₃OH, C₂H₅OH, (CH₃)₂CHOH, KCl, and Na₂SO₄, depress the rate. Slight amounts of MnSO₄ · H₂O and a complexing agent NaF increase the rate of polymerization. Cationic and anionic detergents have been found to decrease and increase the rate, respectively, while nonionic surfactants have no effect on the rate of polymerization.     

Polybromostryl macromers and their anionic polymerization

The decomposition of polybromostyryl carbanions (PBS^?), obtained by anionic polymerization of 4-bromostyrene in tetrahydrofuran (THF), was investigated in the dark in a temperature range of ?6–?21°C. It was accompanied by the evolution of bromine anions and by the formation of polymeric allylic carbanions (λ_max = 575 nm; ε_max = 6800 eq^?1·L·cm^?1). The reaction mechanism was elucidated. The rate constant of the unimolecular rate-determining step of the process was 1.3 × 10^?5 s^?1 and 9.7 × 10^?5 s^?1 at ?21 and ?6°C, respectively. Its apparent energy of activation E_app = 18.38 Kcal/mol. The polybromostyrenes with allylic carbanions at their ends may decompose further. Their “dark” decomposition yielded 1,3-butadiene-1,3-diphenyl-macromers. The mechanisms of decomposition of the PBS^? carbanions and the dark decomposition of the polybromostyryl allylic carbanions are analogous. The rate constant of the latter process was 2.5 × 10^?6 s^?1 at ?6°C. The anionic polymerization of prepared macromers can be initiated in THF at ?78°C by α-methylstyryl carbanions, which do not react, however, with PBS^? carbanions. “Comblike” polymacromers were prepared in which each branch had a molecular weight of about 50,000. The overall molecular weight of the polymacromer was estimated to be about 1 × 10⁶. It has been assumed that the 2–1 mode of addition to the diene group of the macromer is predominant during its polymerization. The 3–4 mode of addition followed by proton shift represents the termination step. The 4–3 mode of addition was ruled out on the basis of spectroscopic evidence.     

Anionic polymerization with alkaline earth counterions—I. Propagation kinetics of living poly-2-vinylpyridine

Propagation kinetics of the homopolymerization of 2-vinylpyridine initiated with cumyl barium and with difunctional dimeric 1,1-diphenylethyl barium in tetrahydrofuran (THF) are reported. Conductance studies on the monofunctional living polymer solution, at concentrations from 10^?3 to 10^?5 M over the range 15 to ?70°C, revealed the presence of triple ions in thermodynamic equilibrium with free ions and ion pairs. The dissociation constant of ion pairs decreases from 1 × 10^?11 M at 15°C to 0.7 × 10^?11 M at ?70°C with an enthalpy of dissociation of 2.5 kJ/mol. Comparison with the analogous system of monofunctional polystyryl barium led to the conclusion that the living polymer of 2-vinylpyridine is 10 times less dissociated. Occurrence of intramolecular complexation, as observed with sodium and magnesium cations, thus appears much less pronounced in the presence of the barium cation on account of its relatively large radius. Kinetic studies showed that living poly-2-vinylpyridine, irrespective of its functionality, propagates essentially via ion pairs. The corresponding constant of propagation ranges from 294 M^?1 sec^?1 at 22°C to 20.7 M^?1 sec^?1 at ?50°C with an activation energy of 20 kJ/mol. The polymers contain more than 50%, isotactic triads.     

Dissociation rate measurements in SO2 + Ar mixtures behind incident shock waves

Dissociation rates of SO₂ in SO₂ + Ar mixtures at 6%, 11%, 15% and 20% of SO₂ were measured behind incident shock waves over a temperature range 4000–6000 K at initial pressures 1.0 to 2.5 Torr. The recorded laser schlieren signals exhibited two exponentials, the faster one due to vibrational relaxation and the slower one due to dissociation. The initial dissociation rate was calculated from the value of the density gradient at the point of intersection of the two exponentials. A least-squares analysis of the experimental data yielded the following empirical relations: k_SO2Ar = 3.34 × 10¹⁵ exp(?107.6 kcal mole^?1/RT) cm³/mole s, k_SO2SO2 = 5.02 × 10¹⁴ exp(?66.6 kcal mole^?1 kcal mole^?1/RT) cm³/mole s.     

Kinetics of the Copolymerization of Methyl Vinyl Ketone and Acrylamide Initiated by the Adduct of Methyl Vinyl Ketone and Imidazole

N-(Butyl-3-one)imidazole acts as an initiating adduct which is formed in the anionic polymerization of methyl vinyl ketone (MVK) induced by imidazole (Im) and is directly formed from Im and the MVK monomer. The kinetics of the anionic homopolymerization of MVK and acrylamide (AAm) under argon in the presence of the adduct were investigated in tetrahydrofuran (THF). The rate of polymerization for the MVK system is expressed as R_p = k[Adduct] [MVK], where k = 3.1 × 10^?6 L/(mol·s)in THF at 30°C. The overall activation energy, E_a , was found to be 5.34 kcal/mol. The R_p for the AAm system is expressed as R_p = k[Adduct] [AAm], where k = 6.8 × 10^?6 L/(mol·s) in THF at 30°C, with E_a 7.78 kcal/mol. The mechanism of the polymerization induced by the initiator adduct is discussed on the basis of these results.     

Determination of kinetic parameters for the reaction between phenol and formaldehyde by differential scanning calorimetry

The kinetic parameters of the complex reaction between phenol and formaldehyde in the presence of sodium hydroxide (NaOH) have been obtained by differential scanning calorimetry (DSC). The two dominant reactions appear to be addition of formaldehyde to phenol with formation of o-hydroxymethyl-phenol and subsequent condensation of the latter. For both reactions, the activation energy (E_a), reaction order and rate constants at different temperatures have been determined. E_a for addition changes from 23·7 to 19·3 kcal mole^?1 and for condensation from 22·9 to 19·1 kcal mole^?1 when the amount of NaOH is increased from 0·25 to 1·00 per cent. The reaction order for addition is 2 and for condensation 1. Thus DSC appears useful for studying the kinetics of more complex polymerization reactions.     

Electric conductivity of polybutadienyl- and polyisoprenyl-lithium in tetrahydrofurane and dimethoxyethane

The electric conductivity of polybutadienyl-lithium and polyisoprenyl-lithium has been investigated in tetrahydrofuran and dimethoxyethane solutions over wide temperature and concentration ranges (0 to ?70°; 7·10^?2 to 1·10^?7 and 5·10^?3 to 1·10^?7 mol/l). Dissociation constants, enthalpy and entropy of dissociation of ion pairs, relative constants for formation of ion triplets, and interionic distances were calculated for each system investigated. The nature of “living” chains under these conditions is considered.     

Thermal decomposition kinetics of bisthiourea cadmuim(II) tetrathiocyanato diammine chromate(III)

Cadmium thiourea reinickate undergoes two-stage thermal decomposition on heating. The DTG peak temperatures are 291 and 469°C and the corresponding DTA temperatures are 255 and 490°C. The kinetic parameters for the first stage decomposition are E* ≈ 120kJ mole^?1; Z ≈ 1.2 × 10⁸ cm³ mole^?1 sec^?1 and ΔS* ≈ ?95 J mole^?1 K^?1. For the second stage, E* ≈ 133 kJ mole^?1; Z ≈ 6.1 × 10⁵ cm^?1 mole^?1 sec^?1 and ΔS* ≈ ?142 J mole^?1 K^?1.     

Thermolysis of pentasubstituted 3-hydroxy-1,2-dioxolanes

A series of pentasubstituted 3-hydroxy-1,2-dioxolanes, la-e , was synthesized by oxygen trapping of β-keto radicals formed during α-azo hydroperoxide decomposition. Thermolysis of the pentasubstituted 3-hydroxy-1,2-dioxolanes (hemiperketals) in benzene proceeded cleanly and yielded pairs of ketones and carboxylic acids. Two of the hemiperketals yielded only one pair of products while the others produced two sets of products. One of each pair of fragmentation products had undergone skeletal rearrangement. Only methyl migrations were observed when in competition with phenyl groups from the same position. The activation parameter data for ld [ΔH? = 24.3 kcal/mol, ΔS? = -8.4 eu, ΔG? = 27.1 kcal/mol, k_60°. = 3.1 × 10^?5 s^?1] were consistent with 0.0 bond scission as the rate-determining-step. A likely mechanism for this thermolysis is initial peroxy bond homolysis to the 1,5-oxygen diradical followed by β-scissions with rearrangements.     

Über einen neuen Typ der Lysolecithin-Acylierung

A new Batch microcalorimeter was employed to investigate the existence of a relationship between the heat evolved during lecithin hydrolysis with phospholipase A (EC 3.1.1.4.) and the reversibility of the reaction. Factors, which allow the reacylation of lysolecithin, give rise to ΔH =?0.39 kcal · mole^?1 during the hydrolysis of lecithin. Whereas, a strongly exothermic reaction results either with desoxycholate (ΔH =?8.85 kcal · mole^?1) or with be venom phospholipase A (ΔH =?5.04 kcal · mole^?1) in the reaction system. The applied microcalorimetric method is fully described.     

Etude de la reactivite de la vinyl-2 Pyridine en polymerisation anionique—I: Cinetique de l'homopolymerisation AVEC les contre-ions Na+ et Cs+ Dans le tetrahydrofuranne

A kinetic study of the anionic polymerization of 2VP in T.H.F. solution has shown that, for Na⁺ as counter-ion, experimental results can be interpreted by the existence of thermodynamic equilibrium between three types of active centres (contact ions pairs, free ions and positive triple ions) the reactivities of which were estimated. When the counter-ion is Cs⁺, the system can be described by a single equilibrium between ion pairs and free ions. The reactivity of free anions has been measured at various temperatures; at 0°, for example the rate constant of propagation k_(?) = 105.000 l mole^?1 sec^?1.     

Polymerization of acrylamide initiated by the Ce4+/thiourea redox system

The polymerization of acrylamide (M) initiated by the Ce⁴⁺/thiourea (TU) redox system has been studied in an aqueous sulfuric acid medium at 35 ± 0.2°C under nitrogen atmosphere. The rate of polymerization is governed by the expression The activation energy is 5.9 kcal deg^?1 mol^?1 in the investigated temperature range 30–50°C. The molecular weight is directly proportional to the concentration of monomer and inversely proportional to the catalyst concentration. With increasing concentration of DMF molecular weight decreases. The range of concentrations for which these observations hold at sulfuric acid concentration of 2.5 × 10^?2 mol/L are [monomer] = 5.0 × 10^?2–3.0 × 10^?1, [catalyst] = (5.0–15.0) × 10^?4, and [activator] = (1.0–6.0) × 10^?3 mol/L.     

The acetyl radical studied by flash photolysis and kinetic spectroscopy

The acetyl radical absorption spectrum is a broad band with maximum decadic extinction coefficient of (1.0 ± 0.1) × 10⁴ ? mole^?1 cm^?1 at 215 nm and an oscillator strength of 0.23 ± 0.03. Absolute rate constants were estimated as 4.5 × 10¹⁰ ? mole^?1 s^?1 for the mutual interaction of acetyl radicals, and as 7.5 × 10¹⁰ ? mole^?1 s^?1 for the cross interaction of acetyl and methyl radicals.     

Properties of strong hydrogen-bonded systems: The formation of hydrogen-bonded ion pair in amine·HCL systems

The hydrogen-bonded complexes between CH₃NH₂ and (CH₃)₂NH with HCl have been studied by the ab initio molecular orbital method using the 4–31G basis set. Calculations show that the proton potential curve has a single minimum near to the nitrogen atom in both complexes. This means that the proton has been transferred from HCl to the amine. ΔE and the dipole moment of the complexes studied are as follows: ?18.2 kcal mol^?1, 10.3 D for methylamine ·HCl, and ?21.7 kcal mol^?1 11.1 D for the corresponding dimethylamine complex. Other properties of the hydrogen-bonded ion pairs are discussed.