Study of the mechanism of propagation and transfer reactions in the polymerization of olefins by Ziegler-Natta catalysts, 2. The influence of polymerization temperature on the kinetic characteristics of propagation

By quenching polymerization with radioactive carbon monoxide (¹⁴CO) data on the number of propagation centers (C_p) and the propagation rate constant (K_p) were obtained for the ethylene and propene polymerization in the presence of titanium trichloride at different temperatures. The values of K_p for ethylene and propene polymerization were found to be 8,0·10⁵ e^?13/(RT) and 3,0·10⁵ e^?23/(RT) l mol^?1 S^?1, respectively (activation energies in kJ mol^?1). It was further found that at increasing polymerization temperature the steady-state concentration of propagation centers increased when using Al(C₂H₅)₃ and Al(C₂H₅)₂Cl as cocatalysts, whereas it did practically not change in the case of Al(isobutyl)₃. On the basis of these data several conclusions were drawn on the mechanism of propagation and the role of organoaluminium cocatalysts in this reaction.     

Propylene polymerization on titanium-magnesium catalysts determination of the number of active centers and propagation rate constants

By use of the quenching technique with ¹⁴CO and ¹⁴CO₂ the number of active centers and the propagation rate constants (k_p) were determined for the propylene polymerization on different titanium-magnesium catalysts in the presence and absence of an organoaluminium cocatalyst. The k_p values at 70°C were found to be 500–1000 1·mol^?1·s^?1, which were confirmed by independent data of molecular mass measurements of the isotatic polymer after a short polymerization time (5 s). Similar isotactic and atactic k_pvalues were found. The maximum number of active centers for supported titanium-magnesium catalysts can reach about 10% of the titanium content in the catalyst. The k_p values of ethylene polymerization on catalysts active without an organoaluminium cocatalyst were also determined (≈ 10⁴ l·mol^?1·s^?1 at 70°C).     

Variation of the propagation rate coefficient with pressure and temperature in the free-radical bulk polymerization of styrene

The propagation rate coefficient K_p of the free-radical bulk polymerization of styrene is determined between 30 and 90°C up to a maximum pressure of 2800 bar. The data from pulsedlaser polymerizations and product analyses by gel-permeation chromatography (PLP-GPC) are adequately represented by the expression: The conseaquences of deducing activation volumes and activation energies from k_p/(L · mol^?1 · s^?1) or fromk*_p/(kg · mol^-1 ·. S^?1) are outlined.     

Number of active centers and propagation rate constants in the propene polymerization on supported Ti–Mg catalysts in the presence of hydrogen

Polymerization inhibition with radioactive carbon monoxide was used to determine the number of active centers (C_p) and the propagation rate constant (K_p) in the isospecific polymerization of propene with the catalytic system TiCl₄/diisobutyl phthalate/MgCl₂—AlEt₃—PhSi(OEt)₃. The presence of hydrogen was found to decrease C_p (by a factor of ≈ 2) and to increase k_p (by a factor of ≈ 4). In the absence of hydrogen a considerable part of the active centers (titanium-polymer bonds) reacting with ¹⁴CO seems to be inactive in the propene polymerization for steric reasons. Hydrogen is able to transform such centers into an active state. Thus, k_p ≈ 2,5 · 10³ L/(mol · s) at 70°C obtained in propene polymerization in the presence of the hydrogen seems to be correct, since the portion of centers in the active state is maximum. The influence of polymerization time, external and internal donors on C_p and k_p was studied.     

Solvent effect on the propagation rate constant in the radical polymerization of bis(2-ethylhexyl) itaconate

Polymerization of bis(2-ethylhexyl) itaconate ( 1 ) with dimethyl azobis(isobutyrate) ( 2 ) was carried out at 50°C in various solvents. Polar solvents caused a significant decrease in the polymerization rate (R_p) and the molecular weight of resulting poly( 1 ). The propagating poly( 1 ) radical could be observed as a five-line ESR spectrum in the actual polymerization systems used. The stationary concentration of poly( 1 ) radical was determined by ESR to be 4,2–6,4 · 10^?6 mol · L^?1 at 50°C when the concentrations of 1 and 2 were 1,03 and 3,00 · 10^?2 mol · L^?1. Using R_p the monomer concentration and the polymer radical concentration, the propagation rate constant (K_p) was estimated to be 1,4–6,8 L · mol^?1 · s^?1, depending on the solvents used. The k_p value was smaller in more polar solvents. The solvent effect is explained in terms of the solvent affinity for the propagating polymer chain.     

Macroion-pairs and macroions in the kinetics of polymerization of hexamethylene oxide (oxepane)

Cationic polymerization of oxepane (hexamethylene oxide) ( 1 ) in CH₂Cl₂ and C₆H₅NO₂ as solvents was initiated with 1,3-dioxolan-2-ylium hexafluoroantimonate ( 2 ). Dissociation constants (K_D) of the ion-pairs of polyoxepane into ions were measured: K_D (in CH₂Cl₂, T = 25°C) = 2,8·10^?5 mol·l^?1 [ΔH_D = ?3,8 kJ·mol^?1 (?0,9 kcal·mol^?1), ΔS_D = ?98 J·mol^?1·K^?1 (?23,4 cal·mol^?1·K^?1)]; K_D (in C₆H₅NO₂, T = 25°C) = 1,6·10^?3 mol·l^?1 [ΔH_D = ?7,1 kJ·mol^?1 (?1,7 kcal·mol^?1), ΔS_D = ?78 J·mol^?1·K^?1 (?18,7 cal·mol^?1·K^?1)]; these values are close to those of the ion-pairs of polytetrahydrofuran. Rate constants k_p⁺ and k_p^±, determined from the kinetic measurements for degrees of dissociation of macroion-paris ranging from 0,02 to 0,21 (in CH₂Cl₂) and from 0,09 to 0,7 (in C₆H₅NO₂), were found to be identical within an experimental error of kinetic measurements. The activation parameters of propagation were measured and their dependences on the polarity of the polymerization mixtures are discussed.     

Study on the NdCl3-supported Ziegler-Natta catalyst for olefin polymerization

The catalyst NdCl₃/EtOH/TiCl₄ (NSC) was prepared using NdCl₃ as carrier. The composition of the NSC was analyzed by IR, XPS, GC and titration. This catalyst and MgCl₂/EtOH/TiCl₄ catalyst (MSC) were employed for the polymerization of ethylene and propene. The titanium component in NSC exists basically as Ti(OEt)Cl₃ but both as TiCl₄ and Ti(OEt)Cl₃ in MSC. NSC combined with organoaluminium compounds polymerizes ethylene with fairly high productivity. Both the activity and the concentration of active centres C^* in ethylene polymerization with NSC-AlEt₃ catalyst system decrease with increasing temperature, while the propagation rate constant k_p increases. The difference in the kinetic behavior for polymerization of ethylene between the NSC-AlEt₃ and the MSC-AlEt₃ catalyst system would arise mainly from the different stability and reactivity of the titanium active center. The activity for polymerization of propene by using NSC in comparison to MSC is very low.     

Capacity flow method for determination of propagation and termination rate constants in radical polymerization

The possibility of determination of the propagation and termination rate constants k_p and k_t, resp., as well as their ratios k_p/k_t and k_p/k in homogeneous radical polymerization is shown using the capacity flow method. A theoretical analysis is carried out and relatively simple equations are introduced. The essential point is that the life time of the growing polymer chain is obtained graphically from the residence time of the reactants in the reactor vessel, which is a given quantity. As an experimental example the polymerization of methyl methacrylate initiated by benzoyl peroxide in benzene is investigated in a flow reactor with perfect mixing at 80°C. It is characteristic that the process can be followed by means usually applied for studying slow reactions. The degree of conversion is measured turbidimetrically and gravimetrically, whereas the initiation rate is analysed iodometrically. Thus, through a numerical linear approximation by the method of least squares k_p/k_t = (2,28±0,45)·10^?5, k_p/k = (1,50±0,22). 10^?1 are found from the experimental data, and hence k^p = (9,95±0,83)·10² l mol^?1 s^?1 and k_t = (4,36±0,49)·10⁷ l mol^?1 s^?1 are obtained.     

Propene polymerization with a magnesium chloride-supported ziegler catalyst, 1. Principal kinetics

The main kinetic behavior of the slurry polymerization of propene with a MgCl₂-supported TiCl₄/C₆H₅COOC₂H₅ catalyst, activated by Al(C₂H₅)₃, was studied, Examination of the dependence of the polymerization rate on temperature and concentrations of Al(C₂H₅)₃ and of propene resulted in a Langmuir-Hinshelwood rate law with the number of polymerization centers dependent on time. The Polymerization rate as function of the polymerization temperature shows a maximum, which is compatible with the rate law. The analysis of the phenomenon of an optimum temperature gave 15 KJ. mol^?1 and 36 KJ. Mol ^?1 for the activation energy of the rate determining step and the adsorption energy of Al(C₂H₅)₃, respectively. Examination of the rapid decay of the polymerization rate showed that the main part of the decay is represented by a second order decay independent of the amount of polymer produced, which can be understood by a second order decay of surface sites by Al(C₂H₅)₃. The number of active centers of the catalyst in gas phase polymerization was estimated applying the inhibition method with carbon monoxide. The results show a constant value for the propagation rate constant, K_p, during the second order rate decay. The observed polymerization kinetics strongly suggest the existence of two kinds of polymerization centers (isotactic and atactic).     

Propylene Ziegler‐Natta Polymerization: Numbers and Propagation Rate Constants for Stereospecific and Non‐Stereospecific Centers

The data on the number of active centers (C_p) and values of propagation rate constants (K_p) have been obtained for stereospecific and non‐stereospecific centers at propylene polymerization with catalytic systems of various composition: TiCl₃‐AlEt₃ (I), TiCl₄/D/MgCl₂‐AlEt₃ (II), TiCl₄ /D/MgCl₂‐AlEt₃‐TES (III), where D – dibutyl phthalate, TES – tetraethoxysilane. The C_p and K_p values were determined by ¹⁴CO quenching method at the polymerization in the absence and the presence of hydrogen. The C_p and K_p values were determined for three PP fractions: boiling pentane soluble fraction (atactic PP), boiling heptane soluble fraction (stereoblock PP) and boiling heptane insoluble fraction (isotactic PP). The following results are obtained. (i) The K_p values increase in the following order: atactic centers < stereoblock centers < isotactic centers (from ≈ 200 to ≈ 3·10³ l/mol·s, 70°C) for all catalysts studied. (ii) The respective K_p values do not differ essentially with the change of the catalyst composition (systems I, II, III). (iii) As the external donor is introduced in TMC (system III), the portion of stereospecific active centers increases (from 34 to 62%), but the total number of active centers changes insignificantly. Most likely the non‐stereospecific active centers transform into stereospecific ones in the presence of silane.     

Determination of absolute rate constants for radical polymerization and copolymerization of ethyl α-cyanoacrylate in the presence of effective inhibitors against anionic polymerization

The absolute rate constants of propagation k_p and of termination k_t of ethyl α-cyanoacrylate (ECNA) were determined in bulk at 30°C by means of the rotating sector method under conditions to suppress anionic polymerization; k_p = 1 622 1 · mol^?1 · s^?1 and k_t = 4,11 · 10⁸ 1 · mol^?1 · s^?1 for the polymerization in the presence of acetic acid, and k_p = 1610 1 · mol^?1 · s^?1 and k_t = 4,04 · 10⁸ l · mol^?1 · s^?1 for the polymerization in the presence of 1,3-propanesultone. The magnitude of k/k_t determined was 6,39 · 10^?3 l · mol^?1 · s^?1. The absolute rate constants for cross-propagation in ECNA copolymerizations were also evaluated. Quantitative comparison of the rate constants with those of common monomers and polymer radicals shows that the strong electron-withdrawing power of the ethoxycarbonyl and cyano groups enable the poly(ECNA) radical to add to monomers as fast as the other polymer radicals. The relatively high reactivity of ECNA, regardless of the type of attacking polymer radical, is interpreted by a transition state greatly stabilized by both the ethoxycarbonyl and the cyano groups.     

Anionic polymerization of one-ended “living” strontium-polystyrene in tetrahydropyran in the presence of tetraglyme and in benzene

The anionic polymerization of the strontium salt of one-ended living polystyrene (SrS₂) was investigated at 20°C in tetrahydropyran (THP) in the presence of two different concentrations of added tetraglyme. Similarly to BaS₂ in tetrahydrofuran (THF) and to SrS₂ in THF and in pure THP, the observed pseudo-first-order rate constant of propagation, k_obs, was nearly independent of the total concentration of salt, their values being 7,5.10^?3 s^?1 and 9 · 10^?3 s^?1, respectively, i. e. about 100 to 120 times higher than in pure THP. This indicates that the propagation occurs mainly via an increased but constant amount of free S^? anions resulting from the two already known equilibria SrS₂ ? (SrS)⁺ + S^?(K₁) and 2 SrS₂ ? (SrS)⁺ + (SrS₃)^? (K₂) and the equilibrium of glymation (SrS⁺) + G ? G, (SrS)⁺ (K_i). A small not exactly determinable contribution of glymated ion-pairs and/or triple ions, whose rate constants would then probably be of the order of 18 l · mol^?1 · s^?1 and 80 l · mol^?1 · s^?1, respectively, could not be excluded. The glymation constant K_i was found to be about 3 · 10⁶ 1 · mol^?1, i.e., approximately 17 times greater than for the Na⁺ cation. Finally, a kinetic experiment with SrS₂ at 20°C in pure benzene (contaminated, however, with some remaining THP from the preparation of SrS₂) indicated that propagation by ion-pairs is possible with a bimolecular apparent rate constant K_app = 1,1 · 10^?1 l · mol^?1 · s^?1.     

Chain propagation rate constants for gas-phase polymerization of propene and 1-butene with Ziegler-Natta catalysts

¹³C NMR analysis of propene/1-butene block copolymers obtained by gas-phase polymerization with Ziegler-Natta catalysts allows the determination of the propagation rate constants for the homopolymerization of the two monomers. They are very similar for the Solvay type catalyst δ-TiCl₃/AlMe₃ and the supported catalyst TiCl₄/MgCl₂/phthalate/AlMe₃. The constant of propene polymerization is three times higher than that of 1-butene polymerization. The high value of the constant found for propene polymerization is in agreement with the literature value determined by the stopped flow polymerization method.     

TiCl4/MgH2-supported Ziegler-type catalyst system, 2. Effect of Ti concentration,Ti content and surface area of TiCl4/MgH2 catalyst on the concentration of active sites in ethylene polymerization

The determination of active centres concentration in ethylene polymerization using various TiCl₄/MgH₂-Al(C₂H₅)₃ catalytic systems, by the ¹⁴CO radio-tagging method, is reported. It is found that with increasing the absolute titanium amount the concentration of active centres, [C^*], increases, whereas the propagation rate constant, k_p, decreases. In addition, using various TiCl₄/MgH₂ catalysts in ethylene polymerization, it is found that the lower the Ti content, the higher is the surface area of the catalyst and the higher is the polymerization activity. Determination of [C^*] shows consclusively that the decrease in the polymerization activity observed to occur with increasing the Ti content, and thus decreasing the surface area, is unequivocally due to a reduction in the concentration of active centres but not to any fundamental change in the value of the propagation rate constant.     

Effect of SnCl4 on the radical-initiated polymerization of diethyl itaconate: Kinetical and ESR studies

The effect of SnCl₄ on the polymerization of diethyl itaconate ( 1 ) with dimethyl 2,2′-azoisobutyrate ( 2 ) in benzene was investigated kinetically and ESR spectroscopically. The polymerization rate (R_p) at 50°C shows a flat maximum on varying the SnCl₄ concentration. The molecular weight of the resulting polymer decreases with increasing SnCl₄ concentration. The overall activation energy of the polymerization is lowered from 52 to 33 kJ · mol^?1 by the presence of SnCl₄, (0,342 mol · L^?1). An NMR study revealed that 1 and SnCl₄ form 1:1 and 2:1 complexes with a large stability constant in benzene. The propagating polymer radicals in the absence and presence of SnCl₄ are ESR-observable as a five-line spectrum under the actual polymerization conditions. The complexed polymer radicals show further three-line splitting due to two methylene hydrogens of the ethyl ester group. The polymer radical concentration increases with the SnCl₄ concentration. The rate constant (k_p) of propagation was determined using R_p and the polymer radical concentration. k_p (6,3–2,9 L · mol^?1 · s^?1 at 50°C) decreases with increasing SnCl₄ concentration. The presence of SnCl₄ (0,342 mol · L^?1) reduces the activation energy of propagation from 29 to 21 kJ · mol^?1. The rate constant (k_t) of termination was estimated from the decay curve of the polymer radicals, k_t (3,1–1,1 · 10⁵ L · mol^?1 s^?1) also decreases with the SnCl₄ concentration. The activation energies of termination in the absence and presence of SnCl₄ (0,342 mol · L^?1) are 30 and 24 kJ · mol^?1, respectively. Suppression of propagation and termination by SnCl₄ seems to be explicable in terms of an entropy factor.     

Determination of rate constants of initiation and propagation in the cationic polymerization of α-methylstyrene with CF3SO3H and H2SO4

The polymerization of α-methylstyrene with CF₃SO₃H and H₂SO₄ as initiators was studied at 30°C in dichloroethane by the stopped-flow/rapid-scan spectroscopic technique. The propagating cation shows λ_max at 336–340 nm. The initiation process and the early stage of propagation were analyzed kinetically on the basis of the cation formation and monomer consumption by taking into account the equilibrium monomer concentration. The rate of initiation with CF₃SO₃H was found to be proportional to the concentrations of CF₃SO₃H and monomer, but the initiation with H₂SO₄ is not proportional to the H₂SO₄ concentration. The rate constant of initiation was estimated to be (2 ± 1)·10³ 1.mol^?1.s^?1 with CF₃SO₃H, and similar values were found with H₂SO₄. The rate constant of propagation is 3.10⁴ 1.mol^?1.s^?1 with CF₃SO₃H as initiator and 10⁶ ? 10⁷ 1.mol^?1.s^?1 with H₂SO₄ as initiator. These k_p values are close to that obtained previously in the radiation polymerization. Finally, λ_max values of the propagating cation, obtained by the stopped-flow technique, were collected.     

Kinetic and ESR studies of the radical-initiated polymerization of N-(2,6-dimethylphenyl)itaconimide

The polymerization of N-(2,6-dimethylphenyl)itaconimide (1) with azoisobutyronitrile (2) was studied in tetrahydrofuran (THF) kinetically and spectroscopically with the electron spin resonance (ESR) method. The polymerization rate (R_p) at 50°C is given by the equation: R_p = K [2] ^0,5 · [1] ^2,1. The overall activation energy of the polymerization was calculated to be 91 kJ/mol. The number-average molecular weight of poly (1) was in the range of 3500–6500. From an ESR study, the polymerization system was found to involve ESR-observable propagating polymer radicals of 1 under the actual polymerization conditions. Using the polymer radical concentration, the rate constants of propagation (k_p) and termination (k_t) were determined at 50°C. k_p (24–27 L · mol^?1 · s^?1) is almost independent of monomer concentration. On the other hand, k_t (3,8 · 10⁴–2,0 · 10⁵ L · mol^?1 · s^?1) increases with decreasing monomer concentration, which seems mainly responsible for the high dependence of R_p on monomer concentration. Thermogravimetric results showed that thermal degradation of poly (1) occurs rapidly at temperatures higher than 360°C and the residue at 500°C was 12% of the initial polymer. For the copolymerization of 1 (M₁) with styrene (M₂) at 50°C in THF the following copolymerization parameters were found; r₁ = 0,29, r₂ = 0,08, Q₁ = 2,6, and e₁ = +1,1.     

Rate coefficients of the free-radical polymerization of 1,3-butadiene

Rate coefficients of termination and transfer in the free-radical polymerization of 1,3-butadiene in chlorobenzene were determined in the temperature range 318 K < T < 333 K. On the basis of an earlier published temperature dependence of the rate coefficient of propagation, for the termination reaction the Arrhenius equation K_t = 1,13 · 10¹⁰ · exp(? 711 K/T) L · mol^?1 · s^?1 was obtained. For the transfer to monomer the experiments yielded the Arrhenius equation K_tr,M = 4,22 · 10⁶ · exp(? 5140 K/T) L · mol^?1 · S^?1 and for the transfer to the solvent K_tr,S = 2,25 · 10⁸ · exp(? 7050 K/T) L · mol^?1 · S^?1.     

PLP‐SEC Study into the Free‐Radical Propagation Rate Coefficients of Partially and Fully Ionized Acrylic Acid in Aqueous Solution

Summary: Propagation rate coefficients, k_p, for acrylic acid (AA) polymerization at 6 °C in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, α, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous‐phase size‐exclusion chromatography (SEC). Strictly speaking, the reported k_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 k_p decreases from 111 000 L · mol^?1 · s^?1 at α = 0 to 13 000 L · mol^?1 · s^?1 at α = 1.0. The significant lowering of k_p with higher α 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 k_p up to 57 000 L · mol^?1 · s^?1.

Dependence of apparent k_p values on the degree of ionization of acrylic acid (a) and on pH (b) for aqueous polymerizations of acrylic acid.     

The propagation rate-constants for the cationic polymerisation of acenaphthylene and styrene in nitrobenzene

Rate measurements of the polymerisation of acenaphthylene in nitrobenzene at 298 K by PhCO⁺ SbF₆^? gave k_p⁺ = 23,2 ± 2 dm³ · mol^?1 · s^?1 which confirmed our previous value with yet another initiator. Under the same conditions we obtained k_p⁺ = 188 ± 9 dm³ · mol^?1 · s^?1 for styrene. The completeness of initiation was indicated by a count of carbonyl groups by spectroscopy. The conductivity measurements indicated that [P_n⁺] is independent of time and that there is no ion-pairing. The DPD of the polymers was unimodal and the DP independent of m₀ and c₀. Exploratory experiments showed that HClO₄ and CF₃SO₃H in nitrobenzene are not useful for k_p⁺ determinations. In an Appendix a detailed treatment is given of how the impurity content of solvent and monomer can be determined by kinetic measurements.