Rate Constants Of Chain Transfer Research Articles

Possibilities For Using Ionic Liquids In The Polymerization Process

Abstract The paper investigates the polymerization of various methacrylate esters in different ionic liquids (ILs) and compares the results with those obtained in traditional organic solvents, particularly benzene. The study highlights the influence of ILs on the polymerization process, focusing on factors such as polymer yield, average molecular weight, and thermal stability. The results indicate a relatively higher degree of polymerization in ILs compared to benzene under identical conditions. Additionally, polymers synthesized in ILs exhibit higher average molecular weights and thermal stability. The observed differences are attributed to the unique properties of ILs, such as low chain transfer rate constants and the ability to stabilize active radicals during polymerization. The findings underscore the potential of ILs as promising reaction media for polymer synthesis, offering improved efficiency and polymer properties compared to conventional organic solvents.

Ethylene Polymerization over Supported Vanadium-Magnesium Catalysts with Different Vanadium Content: The Effect of Hydrogen on Molecular Weight Characteristics of the Produced Bimodal Polyethylene

Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (Mw ≥ 106 g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (KtrH) and polymer chain propagation (Kp) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.

Geometrically Constrained Polymerization of Styrene Over Heterogeneous Catalyst Layer in Silica Nanotube Reactors

Styrene has been polymerized to syndiotactic polystyrene (sPS) over a layer of heterogeneous Cp*Ti(OCH3)3/MAO catalyst immobilized onto the surfaces of silica nanotube reactor (SNTR) arrays of 60–200 nm in diameter. The polymer produced in the SNTR arrays has been found to have the molecular weights much larger than the polymers synthesized by a liquid slurry polymerization over silica‐supported catalysts. A dynamic reactor model that consists of diffusion and reaction terms has been derived and solved to quantify the kinetics of styrene polymerization in a single nanotube reactor. The two‐site kinetic model applied to the silica nanotube reactor model shows that the experimentally observed high polymer molecular weight can be fitted if the chain transfer rate constants for monomer and β‐hydride elimination are reduced significantly. The simulation results suggest that the presence of dense crystalline sPS nanofibrils filling the nanotubes constrain the molecular movements of polymer chain ends in the proximity of catalyst sites to limit the chain transfer reactions. POLYM. ENG. SCI., 60:700–709, 2020. © 2020 Society of Plastics Engineers

Sol–Gel Transition-Accompanied Radical Polymerization Initiated by Two Primary Radicals

When irradiation generates two radicals capable of initiating chain growth with different rate constants for chain transfer, two polymerization routes with different average chain length emerge. These pathways are not mixed in the initial part of the ballistic growth. Their mixing in chain transfer events is accompanied by transition from ballistic to diffusion growth. In the sol–gel transition region, the routes are spatially separated if the chain transfer depends on the rotational mobility of the reactants and decreases with increasing viscosity as the resulting oligomers accumulate. As an example, the polymerization of tetrafluoroethylene in solutions of silanes and oxysilanes is considered.

Theoretical investigation of thiol-ene click reactions: A DFT perspective

In this study, a detailed quantum chemical investigation of the contribution of phenyl thiol derivatives in thiol-ene reaction mechanism has been carried out for the first time. DFT calculations have been used to investigate the role of substitution in thiol-ene reactions. It is well known that the reaction mechanism is strongly controlled by the kP/kCT ratio, where kP is the propagation rate constant of the thiyl radical’s addition to the alkene and kCT is the rate constant of chain transfer to a thiol. The electrophilic nature of the phenylthio radicals and the singlet-triplet (S-T) gap of alkenes are mainly responsible for the variation of the activation barriers for the propagation reaction, this demonstrates the importance of the ene functionality on the propagation reaction. A correlation between the radical stabilization energy of the carbon centered radical intermediate and the chain transfer activation energy could not be established. The transition structures of the chain transfer reactions were shown to be stabilized by intramolecular interactions, which have lowered the activation barriers. In this study, we underlie the kP/kCT ratio which is highly dependent not only on the alkene functionality, but also on the thiol functionality. Tailor-made polymers can be obtained by altering the substituents or their positions, and the computational procedure described herein is expected to guide the synthesis.

Steric and Solvation Effects on Polymerization Kinetics, Dormancy, and Tacticity of Zr-Salan Catalysts

1-Hexene polymerization using four zirconium-based amine bis(phenolate) catalysts, Zr[2-R-4-tBu-ONNO]Bn2 (where R = methyl (1), ethyl (2), isopropyl (3), and tert-butyl (4)), was examined to establish the effect of ligand sterics on kinetics and stereocontrol of olefin polymerization. For each catalyst, a rich data set was obtained including monomer consumption, molecular weight evolution, end-group analysis, and active site counts. A set of reaction specific rate constants was determined for catalysts 2–4. In addition to the standard elementary reaction steps of initiation, propagation, mis-insertion, and chain transfer, these catalysts undergo isomerization leading to dormancy. This isomerization is particularly prominent in bromobenzene and for less sterically hindered complexes. Across the series 2–4, ligand steric properties have a systematic effect on both insertion and chain transfer rate constants. This relationship was rationalized using the Sterimol steric parameters. The rates of propagation an...

Theoretical study of chain transfer to solvent reactions of alkyl acrylates.

This computational and theoretical study deals with chain transfer to solvent (CTS) reactions of methyl acrylate (MA), ethyl acrylate (EA), and n-butyl acrylate (n-BA) self-initiated homopolymerization in solvents such as butanol (polar, protic), methyl ethyl ketone (MEK) (polar, aprotic), and p-xylene (nonpolar). The results indicate that abstraction of a hydrogen atom from the methylene group next to the oxygen atom in n-butanol, from the methylene group in MEK, and from a methyl group in p-xylene by a live polymer chain are the most likely mechanisms of CTS reactions in MA, EA, and n-BA. Energy barriers and molecular geometries of reactants, products, and transition states are predicted. The sensitivity of the predictions to three hybrid functionals (B3LYP, X3LYP, and M06-2X) and three different basis sets (6-31G(d,p), 6-311G(d), and 6-311G(d,p)) is investigated. Among n-butanol, sec-butanol, and tert-butanol, tert-butanol has the highest CTS energy barrier and the lowest rate constant. Although the application of the conductor-like screening model (COSMO) does not affect the predicted CTS kinetic parameter values, the application of the polarizable continuum model (PCM) results in higher CTS energy barriers. This increase in the predicted CTS energy barriers is larger for butanol and MEK than for p-xylene. The higher rate constants of chain transfer to n-butanol reactions compared to those of chain transfer to MEK and p-xylene reactions suggest the higher CTS reactivity of n-butanol.

Kinetic Features of Hexene-1 Polymerization over Supported Titanium-Magnesium Catalyst

The data on the effect of cocatalyst composition and polymerization conditions on activity and MWD of polyhexene (PH) produced over a high-activity titanium-magnesium catalyst are obtained. The cocatalyst has an extremely strong effect on the MWD of polyhexene. When triisobutylaluminum is used as a cocatalyst, PH with high Mw and narrow MWD is produced. The use of TEA decreases the Mw of PH and broadens the MWD. It is found the ratio between the rate constant of chain transfer to TEA (ktrAl) and the propagation rate constant (kp) is two orders of magnitude higher than the ktrAl/kp value calculated for polymerization of ethylene and propylene. We attributed this effect to formation of polyhexene in the solution, as opposed to polymerization of ethylene and propylene in slurry polymerization. The addition of hydrogen at polymerization of hexene-1 significantly increases activity and stereospecificity.

Comparison of Selected Zirconium and Hafnium Amine Bis(phenolate) Catalysts for 1-Hexene Polymerization

The kinetics of 1-hexene polymerization using a family of three zirconium and hafnium amine bis-phenolate catalysts, M[t-Bu-ONXO]Bn2 (where M = Zr (a) or Hf (b), and X = THF (1), pyridine (2), or NMe2 (3)), have been investigated to uncover the mechanistic effect of varying the metal center M. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction-specific rate constants involved in a given polymerization process. This study builds upon the mechanism of polymerization for 1a–3a, which has been previously reported by applying the same methodology to the hafnium containing analogues, 1b–3b. It has been observed that each elementary step-specific rate constant that involves the insertion of a monomer is reduced by an order of magnitude. As previously reported for catalysts 1a–3a, a quantitative structure–activity relationship was uncovered between the logarithm of the monomer-independent chain transfer rate constants and the Hf–X bond distance for catalysts 1b–3b. However, this dependence on the pendant ligand is 2.7 times weaker for the Hf-containing analogues versus those containing Zr. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure–activity relationships.

Kinetic regularities of catalytic ethylene polymerization on single- and multi-site cobalt and vanadium bis(imino)pyridine complexes

The number of active centers Cp in the homogeneous complexes LCoCl2 and LVCl3 (L = 2,6-(2,6-R2C6H3N=CMe)2C5H3N; R = Me, Et, tBu) and the propagation rate constants kp have been determined by the radioactive 14CO quenching of ethylene polymerization on these complexes in the presence of the methylaluminoxane (MAO) activator. For the systems studied, a significant portion of the initial complex (up to 70%) transforms into polymerization-active centers. The catalysts based on the cobalt complexes are single-site, and the constant kp in these systems is independent of the volume of substituent R in the ligand, being (2.4−3.5) × 103 L mol−1 s−1 at 35°C. The much larger molecular weight of the polymer formed on the complex with the tert-butyl substituent in the aryl rings of the ligand compared to the product formed on the complex with the methyl substituent is due to the substantial (∼11-fold) decrease in the rate constant of chain transfer to the monomer. At the early stages of the reaction (before 5 min), the vanadium complexes contain active centers of one type only, for which kp = 2.6 × 103 L mol−1 s−1 at 35°C. An increase in the polymerization time to 20 min results in the appearance, in the vanadium systems, of new, substantially less reactive centers on which high-molecular-weight polyethylene forms. The number of active centers Cp in the 2,5-tBu2LCoCl2 and 2,6-Et2LVCl3 systems with the MAO activator increases as the polymerization temperature is raised from 25 to 60°C. The activation energies of the chain propagation reaction (Ep) have been calculated. The value of Ep for complex 2,5-tBu2LCoCl2 is 4.5 kcal/mol. It is assumed that the so-called “dormant” centers form in ethylene polymerization on the 2,6-Et2LVCl3 complex, and their proportion increases with a decrease in the polymerization temperature. Probably, the anomalously high value Ep = 14.2 kcal/mol for the vanadium system is explained by the formation of these “dormant” centers.

Effects of Pendant Ligand Binding Affinity on Chain Transfer for 1-Hexene Polymerization Catalyzed by Single-Site Zirconium Amine Bis-Phenolate Complexes

The kinetics of 1-hexene polymerization using a family of five zirconium amine bis-phenolate catalysts, Zr[tBu-ON(X)O]Bn2 (where X = THF (1), pyridine (2), NMe2 (3), furan (4), and SMe (5)), has been investigated to uncover the mechanistic effect of varying the pendant ligand X. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction specific rate constants involved in a given polymerization process. The mechanism of polymerization for 1-5 was similar and the necessary elementary reaction steps included initiation, normal propagation, misinsertion, recovery from misinsertion, and chain transfer. The latter reaction, chain transfer, featured monomer independent β-H elimination in 1-3 and monomer dependent β-H transfer in 4 and 5. Of all the rate constants, those for chain transfer showed the most variation, spanning 2 orders of magnitude (ca. (0.1-10) × 10(-3) s(-1) for vinylidene and (0.5-87) × 10(-4) s(-1) for vinylene). A quantitative structure-activity relationship was uncovered between the logarithm of the chain transfer rate constants and the Zr-X bond distance for catalysts 1-3. However, this trend is broken once the Zr-X bond distance elongates further, as is the case for catalysts 4 and 5, which operate primarily through a different mechanistic pathway. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure-activity relationships.

Copolymerization of ethylene with α‐olefins over supported titanium–magnesium catalysts. II. Comonomer as a chain transfer agent

AbstractThe data on the effect of comonomer (propylene and 1‐hexene) on molecular weight (Mw), molecular weight distribution (MWD), and content of terminal double bonds were obtained for ethylene/α‐olefin copolymers produced over a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence of hydrogen. The experimental data on the effect of comonomer concentration on Mw of polymers were used to calculate the ratios between the effective rate constants of chain transfer with monomer and the propagation rate constant. It was shown that the effective rate constant of chain transfer with monomers increases in the row of monomers: ethylene < 1‐hexene < propylene. Meanwhile, the data on the effect of copolymers on content of terminal double bonds of various types demonstrate that different reactions of chain transfer with comonomer may simultaneously occur during copolymerization. It results in simultaneous formation of terminal vinylidene and trans‐vinylene bonds. Therefore, the calculated rate constants of chain transfer with comonomer are complex values, which include the rate constants of chain transfer with comonomer occurring via different mechanisms. The data on MWD, short chain branching (SCB) and terminal double bonds content of different types were obtained by molecular weight fractionation of copolymers followed by the analysis of narrow fractions. The analysis of the data on MWDs of SCB and terminal double bonds shows that active sites of the TMC are considerably heterogeneous with respect to the rates of different chain transfer reactions with monomers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Ethylene polymerization over supported titanium‐magnesium catalysts: Effect of polymerization parameters on the molecular weight distribution of polyethylene

AbstractThe data on the effects of polymerization duration, cocatalyst, and monomer concentrations upon ethylene polymerization in the absence of hydrogen, and the effect of an additional chain transfer agent (hydrogen) on the molecular weight (MW), molecular weight distribution (MWD), and content of vinyl terminal groups for polyethylene (PE) produced over the supported titanium‐magnesium catalyst (TMC) are obtained. The effects of these parameters on nonuniformity of active sites for different chain transfer reactions are analyzed by deconvolution of the experimental MWD curves into Flory components. It has been shown that the polymer MW grows, the MWD becomes narrower and the content of vinyl terminal groups in PE increases with increasing polymerization duration. It is assumed to occur due to the reduction of the rate of chain transfer with AlEt3 with increasing polymerization duration. The polydispersity of PE is found to rise with increasing AlEt3 concentration and decreasing monomer concentration due to the emergence of additional low molecular weight Flory components. The ratios of the individual rate constants of chain transfer with AlEt3, monomer and hydrogen to the propagation rate constant have been calculated. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Titanium-magnesium catalysts for propylene polymerization: The effect of donors

The chain transfer reaction with hydrogen at propylene polymerization over Ti-Mg catalysts (TMCs) of composition TiCl4/D1/MgCl2-AlEt3/D2 is studied in a wide hydrogen concentration range. A two-step mechanism of this reaction is suggested. This mechanism accounts for the fractional order of the reaction with respect to hydrogen concentration. Constants of chain transfer reaction with hydrogen are determined for TMC with different donors: 1,3-diether or dibutyl phthalate as D1 and tetraethoxysilane or dicyclopentyldimethoxysilane as D2. In propylene polymerization over the TMCs, the length of the polymer chain is mainly determined by the ratio of the propylene and hydrogen concentrations because the propagation and chain transfer rate constants are comparable. The rate constant of chain transfer with hydrogen at ethylene polymerization is significantly (more than one order of magnitude) less, and higher hydrogen concentrations are required for attaining the same degree of polymerization. The results of this study might be helpful in simulation of industrial polymerization processes and in control of the polymer molar mass.

Role of Chain Transfer Agents in Free Radical Polymerization Kinetics

This study deals with modeling the propagation and the chain transfer reactions in the free radical polymerization of ethylene, methyl methacrylate (MMA), and acrylamide (AM). The chain transfer agents modeled in the free radical polymerization of ethylene are the experimentally widely used species such as ethylene, methane, ethane, propane, trimethylamine, dimethylamine, chloroform, and carbon tetrachloride. The role of 4-X-thiophenols as chain transfer agents in the polymerization of MMA and AM has been investigated. Geometry optimizations have been carried out with the B3LYP/6-31+G(d) methodology. Reaction rate constants are calculated via the standard transition-state theory with the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d), MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d), and M05-2X/6-311+G(3df,2p)//B3LYP/6-31+G(d) methodologies, which reproduce qualitatively the experimental trends for the chain transfer rate constants. The usage of simple continuum models with the MPWB1K/6-311+G(3df,2p)//B3LYP/6-31+G(d) meth...

Factors Affecting the Apparent Chain Transfer Rate Constants of Chromium Metalloradicals: Mechanistic Implications

When a high concentration of (C5Ph5)Cr(CO)3• (1) is present during the initiator-induced oligomerization of MMA, UV−vis spectra show that the concentration of the metalloradical decreases sharply at the beginning of the reaction (an “induction period”) and remains constant subsequently (a “stable period”). With dimethyl 2,2‘-azobis(isobutyrate) (AIBMe) as the initiator the decrease is due to the transfer of H• from the methyl isobutyryl radical and the chain-carrying radicals to 1, forming the corresponding hydride 2. When the catalysis of chain transfer during a polymerization reaction is studied by the Mayo method, it is assumed that all the added 1 remains active. In truth the chain transfer constant (CS) and chain transfer rate constant (ktr) from the Mayo method are only “apparent” values; the true [1] during the polymerization is lower, and the true values of CS and ktr for this Cr catalyst are larger than those obtained from a Mayo plot. Simulations based on a simplified kinetic model reproduce our observations satisfactorily.

Thiophenols as chain transfer agents in the polymerization of vinyl monomers

The behaviour of a series of 4-substituted thiophenols as chain transfers in the photopolymerization of the acrylamide and methyl methacrylate was studied. The addition of concentrations lower than 1 mM of thiophenols to the polymerization of acrylamide in aqueous solutions reduces slightly the polymerization rate but decreases markedly the polymer molecular weight. Chain transfer constants were determined by a modified Mayo equation that consider the variation of the polymerization rate with the chain transfer agent. The addition of thiophenols to the polymerization of methyl methacrylate in organic media reduced considerably the polymer molecular weight without variations in the polymerization rate. The chain transfer rate constants for both monomers are well correlated with the electron donor-acceptor ability of the 4-substitutents, increasing with electron donating groups. An amino substituent in the 4-position increases 10 times the chain transfer constant with respect to the unsubstituted compound. No correlations were found with reported data on stability of the thiyl radicals or the addition rate to the monomer. The electron-donating effect of 4-substituents on the chain transfer constant is related to polar structures in the transition state, and suggests that the main controlling chain transfer step by thiophenols is the H-abstraction from the S–H.

New Experimental Procedure To Determine the Recombination Rate Constants between Nitroxides and Macroradicals

A new experimental procedure for the determination of the recombination rate constant, kc, between a propagating macroradical and a nitroxide is proposed. It is based on a single pulse−pulsed lamp polymerization where the main chain growth breaking event between two consecutive pulses is the recombination of the macroradicals with the nitroxide. The recovered polymer is analyzed by size exclusion chromatography, and the parameters of the molar mass distribution are used to determine kc, in a similar way as that traditionally applied in the determination of the chain transfer rate constants, i.e., the Mayo method using the number- or weight-average degrees of polymerization and the full chain length distribution method. We named the technique RNR−PLP−SEC for radical nitroxide recombination−pulsed lamp polymerization−size exclusion chromatography. The particular polystyryl macroradical−SG1 nitroxide system was tested to validate it. To apply a consistency check, the experimental parameters have been varied according to the recommendations made by the IUPAC for the measurements of the propagation rate constants via PLP. The recombination rate constant kc was measured over a temperature range where no cleavage of the formed alkoxyamine might occur. At 40 °C kc = 2.6 × 105 L mol-1 s-1, and the value increased from 1.1 × 105 L mol-1 s-1 at 15 °C to 4.0 × 105 L mol-1 s-1 at 82 °C. The extrapolation at 120 °C led to 5.3 × 105 L mol-1 s-1, in good agreement with the values already reported in the literature.

Investigation of the Telomerization Kinetics of N-Isopropylacrylamide Using 3-Mercaptopropionic Hydrazide as Chain Transfer Agent

The telomerization (chain transfer polymerization) kinetics of N-isopropylacrylamide were investigated in various (hydro)organic solvents using 3-mercaptopropionic hydrazide as chain transfer agent (telogen). Except for the dioxane/water system telogen consumption rates were similar for all cases, while solvent effects could be observed for the monomer consumption rates. Chain transfer constants, as defined by the ratio of the rate constants for chain transfer and chain propagation (CT = ktr,/kp), were highest in DMF (10.3), a solvent unable to form hydrogen bonds or dipole−dipole interactions with the monomer, while a more promising value of CT = 1.7 was found for the 6:4 methanol/water mixture. The highest monomer consumption rates were observed for the 1:1 dioxane/water mixture. However, in this particular case the telogen consumption was also found to increase, as we propose due to a “hydrophobic effect” whereby polymer microaggregates serve to locally increase the concentration of telogen and/or mono...

Effect of alkylaluminum on ethylene polymerization catalyzed by 2,6-bis(imino)pyridyl complexes of Fe(II)

In the presence of tetraethylaluminoxane (TEAO), iron complexes were used to catalyze ethylene polymerizations with extremely high activities and generally produced polyethylene with a bimodal molecular weight distribution (MWD). This bimodal MWD of polyethylene was mainly derived from residual triethylaluminum in TEAO and was produced through a mechanism of chain transfer to aluminum. Ethylaluminoxane and tetraisobutylaluminoxane also were used to polymerize ethylene with high activities in the presence of iron complexes, and only polyethylene with a unimodal MWD was produced. The ratio of the rate constant of chain transfer to aluminum (k trA ) to the rate constant of chain propagation (k p ) was determined to be 0.12 for {[ArN=C(Me)] 2 C 5 H 3 N}FeCl 2 when Ar was 2,6-diisopropylphenyl (1) and 2.48 for {[ArN=C(Me)] 2 C 5 H 3 N}FeCl 2 when Ar was 2,6-dimethylphenyl (2); these values are far larger than those for metallocene-based catalysts. This explains why an iron complex usually produces polyethylene with a broader MWD than metallocene-based catalysts. Additionally, it can be concluded from the great difference between 1 and 2 with respect to k trA /k p that an iron complex with less congested aryl substituents is subjected to chain transfer to aluminum.