Study of the chain transfer reaction by hydrogen in the initial stage of propene polymerization

The chain transfer reaction by hydrogen in the initial stage of propene polymerization with MgCl₂-supported Ziegler catalyst was studied by means of the stopped-flow polymerization. The yield and molecular weight of polypropene produced in the initial stage were not affected by hydrogen. Thus, the method was successfully applied to find the region in which hydrogen does not act as a chain transfer reagent. On the other hand, a chain transfer reaction proceeded in the initial stage of polymerization by using Zn(C₂H₅)₂. Furthermore, when the catalyst was treated with Al(C₂H₅)₃ before polymerization, the molecular weight of the produced polymer was decreased by using hydrogen, indicating that it acted as a chain transfer agent for the catalyst modified by pre-treatment.     

On the mechanism of thermal oxidation of methane

Using the method of freezing radicals in conjunction with ESR spectroscopic measurements, the kinetics of the thermal oxidation of methane has been studied under atmospheric pressure depending on the temperature, composition of the mixture, and nature of the surface of the reaction vessel. It has been shown that in a reactor treated with boric acid, the intermediates methylhydroperoxide and hydrogen peroxide are responsible for chain branching. It has been established that the leading active centers of the reaction are the HO₂ radicals, while chain branching occurs as a result of the decomposition of peroxy compounds—methylhydroperoxide and hydrogen peroxide. In reactors treated with potassium bromide, the concentrations of radicals and peroxy compounds were found to be lower than the sensitivity of the method of measurement. Computations were performed for the scheme of methane oxidation at 738 K for a reactor treated with boric acid. Satisfactory agreement was found between the experimental and computed kinetic curves of accumulation of main intermediates CH₂O, H₂O₂, CH₃OOH. The influence of their addition on the kinetics of the reaction has been considered. It has been shown that the addition of formaldehyde does not lead to chain branching, however; it contributes to the formation of those peroxy compounds that bring about chain branching. Mathematical modeling confirmed conclusions made on the basis of experimental data concerning the nature of the leading active centers and the products that are responsible for the degenerate chain branching.     

The chain mechanism of ozone destruction by CFCl3 in the 214 nm photolysis of its mixtures with oxygen

The effect of CFCl₃ (0.025–0.200 mbar) addition on the formation of ozone in 214 nm photolysis of oxygen (800–2000 mbar) was investigated. Kinetic analysis of the drastic reduction in ozone formation in the presence of CFCl₃ shows that it proceeds by a chain mechanism with a chain length of 5.07 ± 0.21(2σ). This chain length is independent of CFCl₃ and O₂ pressures as well as incident light intensity and the mechanism of the chain reaction is governed by the Cl generating reactions of ClO radicals. A mechanism based only on the self reaction of these radicals: ClO + ClO → Cl₂ + O₂ (7), Cl + ClO₂ (8), and Cl + OClO (9), followed by fast decomposition of ClO₂ into Cl and O₂, predicts a chain length which is considerably lower than the observed value. Incorporation of the reaction CFCl₂O₂ + ClO → CFCl₂O + ClO₂ (11) in the mechanism satisfactorily accounts for the observed chain length. A lower limit of 3 × 10^?12 cm³ molecule^?1 s^?1 for k₁₁ is estimated.     

Using Crystal Optics to Demonstrate Single‐Layer Localization of a Solid‐State Chain Reaction

Upon warming to 225 K, single crystals of 11‐bromoundecanoyl peroxide (BrUP), in which radicals have been created by photolysis at lower temperature, undergo partial decomposition by a radical chain reaction ca. 40 cycles long. FTIR allowed monitoring two chain products: CO₂ and an α‐lactone that decomposes further at 260 K. When initiation is confined to alternate molecular layers by polarized photoselection, the chain reaction reduces the crystal symmetry from tetragonal to monoclinic. Desymmetrization is easily observed by optical microscopy, although it is difficult to detect by X‐ray diffraction. Accurate monitoring of birefringence using a Sénarmont 1/4‐wave plate, and comparison with FTIR kinetics, proves that the chain reaction occurs within single molecular layers 2 nm thick.     

Reactions of adjacent substituents on polymer chains: Combinatorial consequences of the mechanism

This paper deals with a statistical problem arising from the pairwise reaction of immediately adjacent substituents along the backbone of a linear polymer chain. The possibility arises that a given substituent becomes unavailable for reaction as a result of the reaction of its nearest neighbors on either side. Previous treatments of this problem have not explicitly taken the consequences of the reaction mechanism into account. In this paper, it is shown that the expected number of unreacted substituents remaining after exhaustive pairwise reaction is a function of the reaction mechanism. For example, in the case of the chain model adopted here, which corresponds to a perfectly regular head-to-tail vinyl halide (-CH₂-CHX-), we show that the fraction of halide atoms remaining after exhaustive removal is equal to 0.1233. The result is compared with results obtained from previous work.     

The effect of solvents on asymmetric induction cationic copolymerization of l-menthyl vinyl ether with indene

Cationic copolymerization of l-menthyl vinyl ether (l-MVE) with indene (IN) was carried out in several solvents with BF₃OEt₂ as catalyst at 0°C. The solvents used in this study were selected toluene (Tol), chloroform (CHCl₃), chlorobenzene (BzCl), 1,2-dichloroethane (EtCl₂), and nitrobenzene; (BzNO₂)/Tol = 65/35 mixture solvent. l-Methyl residue, which is an optically active side chain of copolymer produced by cationic copolymerization, was removed with dry hydrogen bromide gas by ether cleavage reaction. The copolymer [vinyl alcohol(VA)–lN], produced by the ether cleavage reaction, also showed optical rotation. From this result, therefore, it was concluded that asymmetric induction takes place in the copolymer main chain. The efficiency of asymmetric induction was determined by the measurement of optical rotation of VA–IN copolymer after the ether cleavage reaction. The efficiency of asymmetric induction in the copolymer main chain developed from the variation on polymerization solvents; the order was Tol > EtCl₂ > BzCl > CHCl₃ > BzNO₂/Tol (65/35) mixture solvent.     

Specific features of the radiolysis of water and aqueous solutions of H2 and O2 by mixed n,γ-radiation with a high portion of the neutron component

A previously unknown feature of the kinetics of the radiolysis of water and hydrogen, oxygen, and hydrogen peroxide solutions has been discussed. By calculation, it has been revealed that concentration oscillations of the radiolysis products can appear during irradiation of the solution with fast neutrons or mixed n,γ-radiation with a high portion of the neutron component. The period and amplitude of the oscillations depend on the temperature, the dose rate, and the ratio of n/γ radiation components. It has been shown that oscillations cannot be excited during γ-radiolysis under any conditions. It is suggested that the mechanism of the oscillations is similar to the Belousov-Zhabotinsky reaction mechanism. A chain reaction proceeds in the irradiated system, in which the reactants H₂O₂ (“reducing agent”), “oxidizing agent” OH radicals initiating the chain, and the “catalyst” are introduced from the outside. Hydrogen molecules produced by the action of radiation play the role of the “catalyst”, and H₂O molecules formed in the secondary reactions are the “deactivated form of the catalyst”. Hydrogen atoms and hydrated electrons propagate the chain. Oxygen formed in both spurs and the secondary reactions is the “inhibitor” terminating the chain reaction.     

Pyrolysis of 1‐chloro‐1,1‐difluoroethane: Considerations about its molecular nature

A new insight into the contested origin of the absence of radical reaction in the pyrolysis of CF₂ClCH₃ is given. This chain reaction would be too slow compared with the molecular reaction because of a too slow homogeneous initiation leading to a too fast wall recombination of the Cl atom chain carriers. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 283–289, 1999     

The active species of the chromium (phillips type) catalysts for the polymerization of ethylene: A unifying approach

From literature data it is concluded that the rate of the reduction, alkylation, polymer chain growth, and chain transfer reactions of three chromium(II) and one chromium(III) surface species all increase with decreasing electron density at the chromium ion. This electron density has previously been measured by the IR shift of the stretching vibration of one CO molecule terminally adsorbed on these chromium ions. It is observed that the reduction half time decreases proportional to the increasing Lewis acidity and that the rate of the polymer chain growth reaction increases exponentially for three chromium surface species with increasing CO stretching vibration. Due to the large difference of the polymer chain growth rates for the two chromium (II) species (A_d and C_d), common in the normal Phillips catalyst, both contribute almost equally to the polymer product, although the A_d species outnumbers the C_d one by more than 3 to 1.     

The mechanism of the addition of tetrachloromethane to oct-1-ene catalysed by [Mo2(CO)6(η-Cp)2]

A study of the reaction of CCl₄ with oct-1-ene in the presence of catalytic amounts of [Mo₂(CO)₆(η-Cp)₂] has shown that in the early stages it proceeds by a redox-catalysed mechanism. However, gradual decomposition of the catalyst leads to the intervention of a radical chain pathway.     

Carbonylation reactions

Conclusions From the UV spectra it follows that the chain reaction for the carbonylation of piperidine, initiated by CuCl₂, proceeds via the Intermediate formation of charge-transfer complexes, in which the Cu²⁺ ion undergoes oxidation-reduction reaction with the free electron pair of the amine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2705–2708, December, 1973.     

Catalytic synthesis of styryl‐capped isotactic polypropylenes

Bis‐styrenic molecules, 1,4‐divinylbenzene (DVB) and 1,2‐bis(4‐vinylphenyl)ethane (BVPE), were successfully combined with hydrogen (H₂) to form consecutive chain transfer complexes in propylene polymerization mediated by an isospecific metallocene catalyst (i.e., rac‐dimethylsilylbis(2‐methyl‐4‐phenylindenyl)zirconium dichloride, I ) activated with methylaluminoxane (MAO), rendering a catalytic access to styryl‐capped isotactic polypropylenes (i‐PP). The chain transfer reaction took place in a unique way where prior to the ultimate chain transfer DVB/H₂ or BVPE/H₂ caused a copolymerization‐like reaction leading to the formation of main chain benzene rings. A preemptive polymer chain reinsertion was deduced after the consecutive actions of DVB/H₂ or BVPE/H₂, which gave the styryl‐terminated polymer chain alongside a metal‐hydride active species. It was confirmed that the chain reinsertion occurred in a regio‐irregular 1,2‐fashion, which contrasted with a normal 2,1‐insertion of styrene monomer and ensured subsequent continuous propylene insertions, directing the polymerization to repeated DVB or BVPE incorporations inside polymer chain. Only as a competitive reaction, the insertion of propylene into metal‐hydride site broke the chain propagation resumption process while completed the chain transfer process by releasing the styryl‐terminated polymer chain. BVPE was found with much higher chain transfer efficiency than DVB, which was attributed to its non‐conjugated structure with much divided styrene moieties resulting in higher polymerization reactivity but lower chain reinsertion tendency. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3709–3713, 2010     

Cationic Polymerization of Cyclic Sulfides

Abstract

The mechanism of the cationic polymerization of several thietanes and of propylene sulfide under the influence of triethyloxonium tetrafluoroborate in methylene chloride is described. The thietane polymerizations stop at limited conversions because of a termination reaction occurring between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms of the polymer chain. The maximum conversions obtained under identical conditions differ markedly for the different monomers. Ratios of rate constants of propagation (k_p) to rate constants of termination (k_t) have been calculated. The differences in k _p/k_t. values for the different monomers are explained in terms of differences in basicity and differences in steric hindrance of the monomers compared to the corresponding polymers. In the case of propylene sulfide it is proposed that the main termination reaction is the formation of 12-membered ring sulfonium salts by an intramolecular reaction of the third sulfur of the growing polymer chain with the reactive chain end (three-membered ring sulfonium salt). This terminated polymer is able to reinitiate the polymerization, for example, by reaction of a monomer molecule at the exocyclic carbon atom of the sulfonium salt function. The cyclic tetramer of propylene sulfide is formed in this reaction. After complete polymerization, formation of cyclic tetramer continues, probably via a backbiting mechanism. In methylene chloride as solvent, the absolute value of the rate constant of propagation for 3,3-dimethylthietane changes with changing concentration of initiator and by adding different amounts of indifferent electrolyte to the reaction mixture. From these changes, and assuming that the value of the dissociation constant of the growing chain-ends is close to values of dissociation constants of low molecular weight sulfonium salts, separate rate constants for propagation via free ions and ion-pairs were calculated. The propagation constant of free ions is about 70 times higher than that of ion pairs in methylene chloride at 20°C. Free ions and ion pairs are nearly equally reactive in nitrobenzene.     

C4-C14-alkyl nitrates as organic trace compounds in air

The long chain alkyl nitrates (C _n >5) form a complex spectrum of natural and anthropogenic organic trace compounds in air. HRGC/ECD and HRGC/MSD using 56 amu as the signal reveal a standard pattern of isomeric n-alkyl nitrates in semi-rural air. This is regulated by the input of the corresponding alkanes, their rate constants for the reaction with OH, the rate constant of the alkylperoxy radicals for the reaction to alkyl nitrates, the atmospheric concentrations of NO/NO₂ and by the rate constants of the alkyl nitrates for the reaction with OH radicals as the major removal reaction. The complex pattern of signals given by the ECD in the retention index range between 700 and 2000 has been observed before but this is the first time that it has been assigned to a defined group of chemical compounds.The environmental impact of the occurrence of the different groups of alkyl nitrates has yet to be evaluated. Their general property as organic stabilizers for NO/NO₂ and therefore as precursors of NO ₃ ^- ions in rain and their biological potentials are also known. The long chain alkyl nitrates act as lipophilic carriers for nitric acid.     

Kinetics of the liquid-phase oxidation of cyclooctene in the presence of MnO2

The kinetics of the liquid-phase oxidation of cyclooctene by molecular oxygen in the presence of a MnC₂ catalyst is studied. It is found that MnC₂ is an initiator of this reaction and has no effect on the steps of chain propagation and termination. The oxidation occurs by a radical chain mechanism with the escape of radicals to the bulk of the reaction mixture. Radicals are formed by the interaction of the olefin with the solid catalyst surface. The kinetic parameters of the reaction are calculated.     

Kinetics of photosensitized chain reaction of 1,2-dichloroethane induced by a cw CO2 laser

The photosensitized chain reaction of 1.2-dichloroethane induced by a cw CO₂ laser produced vinyl chloride with high purity. The initiation of the chain reaction is strictly dependent on the irradiated laser frequency. Kinetics of the chain reaction has been studied on the basis of the effects of laser frequency, laser incident power and the pressure of sensitizer on reaction rate. Arrhenius plot shows an activation energy of 21.8 kcal per mole. Generating the active centers by infrared laser can allow the reactions of chain propagation with low activation energy to proceed at a lower temperature, which is of significance in cutting down the energy expenditure and formation of by-products in the process.     

Controlled Chain‐Scission of Polybutadiene by the Schwartz Hydrozirconation

Controlled chain‐scission of polybutadiene (PB), polyisoprene, and poly(styrene‐co‐butadiene), induced by bis(cyclopentadienyl) zirconium hydrochloride (Cp₂ZrHCl), was revealed at room temperature. The chain‐scission reaction of linear PB was studied by means of GPC, NMR spectroscopy, and MALDI‐TOF‐MS. It was confirmed that the molecular weights of degraded products were quasi‐quantitatively controlled by Cp₂ZrHCl loading, irrespective of the starting PB, whereas the microstructure of PB chains was crucial to the scission reaction. The hydrozirconation of model molecules indicated that the existence of an internal double bond in compounds with multiple double bonds was essential for chain cleavage. The chain‐cleavage mechanism was proposed to involve hydrozirconation of internal double bonds in PB chains and β‐alkyl elimination. Furthermore, metallocene‐catalyzed chain‐scission by a chain‐transfer reaction was developed. It is believed that the reported chain scission offers a promising pathway for end‐group functionalization by chain cleavage and presents a new application of Schwartz’s reagent.     

Borane chain transfer reaction in olefin polymerization using trialkylboranes as chain transfer agents

This article discusses a new borane chain transfer reaction in olefin polymerization that uses trialkylboranes as a chain transfer agent and thus can be realized in conventional single site polymerization processes under mild conditions. Commercially available triethylborane (TEB) and synthesized methyl‐B‐9‐borabicyclononane (Me‐B‐9‐BBN) were engaged in metallocene/MAO [depleted of trimethylaluminum (TMA)]‐catalyzed ethylene (Cp₂ZrCl₂ and rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂ as a catalyst) and styrene (Cp*Ti(OMe)₃ as catalyst) polymerizations. The two trialkylboranes were found—in most cases—able to initiate an effective chain transfer reaction, which resulted in hydroxyl (OH)‐terminated PE and s‐PS polymers after an oxidative workup process, suggesting the formation of the B‐polymer bond at the polymer chain end. However, chain transfer efficiencies were influenced substantially by the steric hindrances of both the substituent on the trialkylborane and that on the catalyst ligand. TEB was more effective than TMA in ethylene polymerization with Cp₂ZrCl₂/MAO, whereas it became less effective when the catalyst changed to rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂. Both TEB and Me‐B‐9‐BBN caused an efficient chain transfer in the Cp₂ZrCl₂/MAO‐catalyzed ethylene polymerization; nevertheless, Me‐B‐9‐BBN failed in vain with rac‐Me₂Si(2‐Me‐4‐Ph)₂ZrCl₂/MAO. In the case of styrene polymerization with Cp*Ti(OMe)₃/MAO, thanks to the large steric openness of the catalyst, TEB exhibited a high efficiency of chain transfer. Overall, trialkylboranes as chain transfer agents perform as well as B? H‐bearing borane derivatives, and are additionally advantaged by a much milder reaction condition, which further boosts their applicability in the preparation of borane‐terminated polyolefins. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3534–3541, 2010     

Molecular weight distributions in polymerizing systems with different coexisting active centers

Polymerization of acrylonitrile initiated by organomagnesium compounds Bu₂Mg or BuMgCl leads to almost monodisperse polymers. The molecular weight distribution (MWD) of these polymers is so narrow that it cannot be measured by means of the ultracentrifuge. It remains narrow till high conversions. On the other hand, initiation by complex Bu₃Mg₂I leads to polymers with a broad MWD. From kinetic measurements we know that this reaction is free of chain termination and chain transfer. Also the initiation of chains is practically immediate. The reason for broad MWD in this particular case is the coexistence of different growing centers of polymerization. The propagation constants for these centers are obviously different. If these different active centers can periodically undergo transition from one type into the other the MWD of resulting polymer chains will broaden. A very simple semiquantitative theory for this phenomenon was developed. It shows that the fewer exchanges between the coexisting centers that occur during the reaction time, the broader will be the MWD of the polymer chains. Hence it was concluded that the broadest MWD of products must be found at low conversions. With an increase of reaction time and conversion the MWD becomes more narrow. This peculiar prediction was corroborated by experiment.     

Effect of enthalpy of polar modifiers interaction with n-butyllithium on the reaction enthalpy,kinetics and chain microstructure during anionic polymerization of 1,3-butadiene

The influence of interaction enthalpy (ΔH_MOD/BuLi) of μ, σ, σ+μ and σ-μ complexing polar modifiers with n-butyllithium on the 1,3-butadiene anionic polymerization enthalpy (ΔH_BD), polymerization reaction rate (k_p) and polybutadiene microstructure was studied. It has been found that enthalpy of interaction depends on complex type, molar ratio of polar modifier to n-butyllithium (MOD/BuLi) and temperature. For the first time it has been proven that with increasing ΔH_MOD/BuLi the content of 1,2 butadiene (vinyl) units in the chain increases individually for each complex type but the value of ΔH_BD decreases similarly for all complexes containing σ-donor groups, exhibiting linear nature. Since ΔH_MOD/BuLi controls the content of butadiene isomeric structures in the chain its value was compared with polymerization reaction rate, ranging from ∼46 to ∼5500 moL/L·min, and discussed on mechanistic level.