Copolymerization Of 1-hexene Research Articles

Effect of Fluoride‐Modification on the Phillips Cr/SiO2 Catalyst for Ethylene Polymerization

AbstractIn this work, a fluoride‐modified Phillips catalyst was investigated by using combined experimental and computational methods. The addition of fluoride to the Phillips Cr/SiO2 catalyst can increase the activity of the catalyst calcined at a low temperature and the molecular weight of the polyethylene product. DFT calculations were performed to show that the difference of the Gibbs free energy barriers between chain transfer and chain propagation increased after the introduction of the fluoride onto the silica surface, which is in accordance with the increased molecular weight of the polyethylene produced by the fluoride‐modified Phillips Cr/F‐SiO2 catalyst. The computational results also reflect the increase of the activity of the ethylene/1‐hexene copolymerization after the introduction of fluoride, although the modification has little effect on the regioselectivity of the produced ethylene/1‐hexene copolymer. Moreover, copolymers produced by the fluoride‐modified Phillips catalyst with more short‐chain branches (SCBs) in the high‐molecular‐weight fraction and a significant enhancement of the environmental stress crack resistance can be explained from the results of the DFT calculations.

SANS Evidence of Liquid–Liquid Phase Separation Leading to Inversion of Crystallization Rate of Broadly Distributed Random Ethylene Copolymers

Aiming to understand the inversion of crystallization kinetics observed by DSC, detailed SANS investigations of the melt structure of a broadly distributed ethylene–1-hexene copolymer have been undertaken in a wide range of temperatures that were reached either by heating the solid or cooling from the homogeneous melt state. In both cases, the observed SANS signal transitions from a scattering cross section consistent with a homogeneous melt state (high temperature range) to an intensity that in the low Q region displays the characteristics of the Porod region for particles dispersed in a homogeneous matrix (low melt temperature range). The latter structure is consistent with demixing of the highly branched molecules and corroborates the postulated liquid–liquid phase separation (LLPS) as an explanation for the peculiar crystallization kinetics observed by DSC. The solution temperature is found at 160 °C by heating the solid and at 150 °C when cooling from the one-phase melt, thus denoting the effect of c...

Dynamic mechanical analysis of ethylene/1‐hexene copolymers: The effect of the catalyst type on the short‐chain branching distribution and properties of the amorphous and crystalline phases

ABSTRACTDynamic mechanical analysis was used to study ethylene/1‐hexene copolymers with different compositions, molecular weight distributions, and profiles of short‐chain branching (SCB) versus molecular weight. These copolymers were produced over a highly active supported titanium–magnesium catalyst (TMC), a highly active supported vanadium–magnesium catalysts (VMC), and a supported zirconocene catalyst. A higher fraction of the crystalline phase in the copolymers prepared with VMC was shown to result in higher elastic modulus values. β relaxation was found to be sensitive to the SCB distribution versus the molecular weight. The copolymers prepared with the zirconocene catalyst and VMC were characterized by more uniform SCB distributions and higher temperatures of β relaxation compared to the copolymers prepared with TMC. The mobility of the polymer chains at room temperature in the amorphous phase obtained by the spin‐probe method rose with increasing branch content in the copolymers and was not sensitive to different SCB distribution profiles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44638.

Titanium compounds as catalysts of higher alpha-olefin-based super-high-molecular polymers synthesis

The synthesis of polymers of 10 million or more molecular weight is a difficult task even in a chemical lab. Higher α-olefin-based polymer agents of such kind have found a narrow but quite important niche, the reduction of drag in the turbulent flow of hydrocarbon fluids such as oil and oil-products. In its turn, searching for a catalytic system capable to produce molecules of such a high length and to synthesize polymers of a low molecular-mass distribution is part of a global task of obtaining a high-quality product. In this paper we had observed a number of industrial catalysts with respect to their suitability for higher poly-α- olefins synthesis. A number samples representing copolymers of 1-hexene with 1-decene obtained on a previous generation catalyst, a microsphere titanium chloride catalytic agent had been compared to samples synthesized using a titanium-magnesium catalyst both in solution and in a polymer medium.

Novel SiO2‐Supported Chromium Oxide(Cr)/Vanadium Oxide(V) Bimetallic Catalysts for Production of Bimodal Polyethylene

Novel SiO2‐supported Cr–V bimetallic oxides catalysts, which were capable of one‐pot synthesizing polyethylene with bimodal molecular weight distribution, had been successfully developed. These catalysts were prepared with a simple introduction of vanadium oxide onto the traditional Phillips CrOx/SiO2 catalyst. The polymerization activity over the Cr–V bimetallic catalysts was significantly improved compared with the V‐free Phillips CrOx/SiO2 catalyst. Moreover, the bimodal ethylene/1‐hexene copolymer prepared by the novel catalyst showed a better short chain branch distribution with more 1‐hexene comonomer inserted into the PE chains with high molecular weight, which would be favorable for the long term mechanical properties of the bimodal PE as potential high grade pipe materials.

The ablative behavior of high linear chains and its role in the thermal stability of polyethylene: a combined P‐TREF−TGA study

AbstractEthylene copolymers contain short chain branches (SCBs) which have great influence on their properties. An ethylene/1‐hexene copolymer distinguished in terms of the number of butyl SCBs was precisely separated based on differences in crystallizability using the preparative temperature‐rising elution fractionation (P‐TREF) method and was then studied via TGA and DSC. A comparison between the results obtained and the literature suggested a short chain branch distribution (SCBD) functionality for a included in the general linear form Tm (°C) = −a(SCBD) × (SCB) + b. P‐TREF−TGA results showed that the highly linear chains acted as ablative layers which could increase the thermal stability and durability of polyethylene in the absence of any mineral additive. Furthermore, the P‐TREF−TGA data displayed an interesting interrelationship between temperature at maximum rate of degradation (Tmax) and the number of butyl SCBs over all the heating rates (10, 25, 50 and 100 °C min‐1). The role of the number of butyl SCBs in thermal degradation was exhausted by higher heating rates, whereas the ablation capability was enhanced. Kinetic studies demonstrated that the activation energy dropped on increase in butyl branch content within the backbone. © 2015 Society of Chemical Industry

Enhanced rate of formation of trigonal phase in blends of homogeneous isotactic propylene-1-hexene copolymers

Blends of a miscible pair of propylene-1–hexene (PH) copolymers with 11 and 21 mol% of 1-hexene have been studied in reference to their polymorphic behavior, lamellar structure and crystallization kinetics using in situ WAXD, DSC and AFM. As pure components, PH21 crystallizes in a trigonal packing in the whole range of undercooling while PH11 develops monoclinic crystallites at low undercooling or the mesomorphic form at high undercooling. In the blends, the level of overall crystallinity obtained from WAXS increases from 17 to 25% and scales directly with increasing content of PH21. Due to differences in crystallization kinetics, two separate populations of crystallites develop in the blends, each of a different polymorph. As expected, the content of trigonal phase decreases with increasing PH11, however, the rate of formation of trigonal phase in the whole range of undercooling increases with addition of PH11, which as a pure component does not form trigonal phase. This unexpected enhanced kinetics of formation of trigonal phase with blending is attributed to the increasing composition of 1-hexene in the melt during evolution of the monoclinic phase in the first stage of isothermal crystallization of the blends.

Non-isothermal crystallization kinetics of LLDPE prepared by in situ polymerization in the presence of nano titania

In this research work, a zirconocene/MAO complex was used as a catalyst for the copolymerization of 1-hexene and ethylene in the presence of nano titania doped with 1 % of manganese (TiO2/Mn), which was used as a drop in filler. It was investigated from the 13C NMR analysis that 1-hexene incorporation increases with the addition of nano filler. The degree of crystallinity (DOC), catalytic activity and molecular weight of the nanocomposites were studied by a differential scanning calorimeter (DSC), yield analysis and gel permeation chromatography (GPC), respectively. It was found that DOC, catalytic activity, molecular weight and molecular weight distribution were strongly influenced by the addition of nano filler due to the increase of 1-hexene incorporation. As a result, an increase in catalytic activity and a decrease in DOC were observed due to the addition of nano filler. The non-isothermal crystallization kinetics of the produced copolymer was studied using a model proposed by Ozawa and Mo et al. It was observed that the crystal growth rate is slowed by the nano filler. The activation energy (E A) was determined by the Kissinger method, and it was found that E A is increased incrementally with the loadings of the nano filler, confirming a slower crystallization process.

Effects of supported metallocene catalyst active center multiplicity on antioxidant-stabilized ethylene homo- and copolymers

A silica-supported bis(n-butylcyclopentadienyl) zirconium dichloride [( n BuCp)2ZrCl2] catalyst was synthesized. This was used to prepare an ethylene homopolymer and an ethylene–1-hexene copolymer. The active center multiplicity of this catalyst was modeled by deconvoluting the copolymer molecular mass distribution and chemical composition distribution. Five different active site types were predicted, which matched the successive self-nucleation and annealing temperature peaks. The thermo-oxidative melt stability, with and without Irganox 1010 and Irgafos 168, of the above polyethylenes was investigated using nonisothermal differential scanning calorimetric (DSC) experiments at 150 °C. This is a temperature that ensures complete melting of the samples and avoids the diffusivity of oxygen to interfere into polyethylene crystallinity and its thermo-oxidative melt degradation. The oxidation parameters such as onset oxidation temperature, induction period, protection factor, and S-factor were determined by combining theoretical modeling with the DSC experiments. Subsequently, these findings were discussed considering catalyst active center multiplicity and polymer microstructure, particularly average ethylene sequence length. Several insightful results, which have not been reported earlier in the literature, were obtained. The antioxidant effect, for each polymer, varied as (Irganox + Irgafos) ≈ Irganox > Irgafos > Neat polymer. The as-synthesized homopolymer turned out to be almost twice as stable as the corresponding copolymer. The antioxidant(s) in the copolymer showed higher antioxidant effectiveness (AEX) than those in the homopolymer. Irganox exhibited more AEX than Irgafos. To the best of our knowledge, such findings have not been reported earlier in the literature. However, mixed with Irganox or Irgafos, their melt oxidation stability was comparable. The homopolymer, as per the calculated S-factor, showed Irganox–Irgafos synergistic effect five times that of the copolymer. This illustrates how the transition in backbone structure, from exceedingly high to low ethylene sequence length, influences antioxidant synergistic performance. Finally, this study shows a DSC-aided approach that can elucidate the effect of polyethylene structural backbone on its thermo-oxidative melt degradation as well as antioxidant synergism in a facile fashion.

Tandem Action of SNS-Cr and CGC-Ti in Preparation of Ethylene-1-Hexene Copolymers from Ethylene Feedstock

Ethylene–1-hexene copolymer materials, ranging from semicrystalline linear low-density poly­ethylene (LLDPE) to completely amorphous polyethylenes (PEs), are prepared from ethylene alone in a single reactor by the tandem polymerization of bis(2-dodecylsulfanyl-ethyl)amine–CrCl3 (SNS–Cr) and (N-tert-butylamido) (tetramethylcyclopentadienyl)–titanium dichloride (CGC–Ti) at 75 °C and under atmospheric pressure. The polymerization activities are on the order of 105–106 g (mol Ti)−1 h−1. 1-Hexene incorporation in the resulting copolymers can be adjusted by varying the Cr–Ti molar ratio and/or applying a short period of pre-trimerization. Copolymers with high 1-hexene incorporation up to 15 mol% are obtained. Few vinyl and vinylene chain ends are detected by 13C NMR, suggesting that 1-hexene does not act as a chain-transfer agent. Narrow molecular-weight distributions with polydispersities from 1.9 to 2.6 are obtained, characteristic of a single-active-site nature of the catalyst system.

Inferring Comonomer Content Using Crystaf: Uncertainty Analysis

We estimate the comonomer content in random copolymers, through the use of semi‐empirical models. We extend the model of Anantawaraskul et al. by expanding the number of model parameters from 4 to 9. Using available data on well‐characterized ethylene/1‐hexene copolymers, we randomly select a subset to train the model, and regress model parameters. We test the ability of the parametrized model to infer comonomer content on the rest of the data. We quantify the predictive ability by exploring the effect of the quantity and quality of the training data. The accuracy and precision of the inference improve as the amount of training data increases, and as datasets span the domain more evenly.

A Novel SiO2‐Supported Cr–V Bimetallic Catalyst Making Polyethylene and Ethylene/1‐Hexene Copolymers with Bimodal Molecular Weight Distribution

A novel SiO2‐supported Cr–V bimetallic catalyst for ethylene and ethylene/α‐olefin polymerization is investigated. This catalyst is prepared using the residual surface hydroxyl groups in a SiO2‐supported vanadium catalyst to support bis(triphenysilyl) chromate (BC) in order to get the merits from both the vanadium‐based catalyst and the S‐2 catalyst. By investigation of the polymerization behavior and the microstructures of their polymers, several vital factors, such as the addition amount of vanadium species and the dosage of 1‐hexene, on the ethylene/1‐hexene copolymerization are systematically investigated. The results show that the activity of each bimetallic catalyst is higher than that of the S‐2 catalyst, and the product from the bimetallic catalyst shows a higher molecular weight (MW) with a bimodal molecular weight distribution (MWD). The copolymers obtained using the bimetallic catalyst also show a better short‐chain branch distribution (SCBD) than that using the S‐2 catalyst through a temperature rising elution fractionation (TREF) cross successive self‐nucleation and annealing (SSA) characterization. image

Synthesis and characterization of phosphine-(thio)phenolate-based half-zirconocenes and their application in ethylene (co-)polymerization

A series of novel half-zirconocenes containing phosphine-(thio)phenolate chelating ligands of the type Cp'Zr(thf)Cl2[X-2-R(1)-4-R(2)-6-(PPh2)C6H2] (Cp' = C5Me5, 2a: X = O, R(1) = Ph, R(2) = H; 2b: X = O, R(1) = (t)Bu, R(2) = H; 2c: X = O, R(1) = R(2) = (t)Bu; 2d: X = O, R(1) = SiMe3, R(2) = H; 2e: X = S, R(1) = SiMe3, R(2) = H; Cp' = C5H5, 3b: X = O, R(1) = (t)Bu, R(2) = H; 3c: X = O, R(1) = R(2) = (t)Bu; 3d: X = O, R(1) = SiMe3, R(2) = H; 3e: X = S, R(1) = SiMe3, R(2) = H) have been synthesized in high yields (74-85%). These complexes were identified by (1)H, (13)C and (31)P NMR as well as elemental analyses. Structural analysis for 2a-b and 2d revealed that these complexes adopt six-coordinate, distorted octahedral geometry around the zirconium center. These novel half-zirconocenes possessed high catalytic performance for ethylene polymerization at high temperature in the presence of MMAO. The phosphine-phenolate-based Cp* complexes produced high molecular weight polymers (M(W) > 400,000), while the Cp analogues displayed much higher activities at high temperature. Complex 3c with MMAO showed a maximum ethylene polymerization activity of 17,580 kg mol(Zr)(-1) h(-1) at 75 °C. In addition, the Cp based complexes 3b-e could promote ethylene and 1-hexene copolymerization with high activities.

Effects of supported (nBuCp)2ZrCl2 catalyst active center multiplicity on crystallization kinetics of ethylene homo- and copolymers

Two different supported zirconocene, that is, bis(n-butylcyclopentadienyl) zirconium dichloride (nBuCp)2ZrCl2, catalysts were synthesized. Each catalyst was used to prepare one ethylene homopolymer and one ethylene–1-hexene copolymer. Catalyst active center multiplicity and polymer crystallization kinetics were modeled. Five separate active center types were predicted, which matched the successive self-nucleation and annealing (SSA) peak temperatures. The predicted crystallinity well matched the differential scanning calorimetric (DSC) values for a single Avrami–Erofeev index, which ranged between 2 and 3 for the polymers experimented. The estimated apparent crystallization activation energy Ea did not vary with cooling rates, relative crystallinity α, and crystallization time or temperature. Therefore, the concept of variable/instantaneous activation energy was not found to hold. Ea linearly increased with the weight average lamellar thickness Lwav DSC-GT; and for each homopolymer, it exceeded that of the corresponding copolymer. Higher Ea, hence slower crystallization, was identified as a pre-requisite to attain higher crystallinity. Crystallization parameters were correlated to polymer backbone parameters, which are influenced by catalyst active center multiplicity.

Influence of copolymer fraction composition in ultrahigh molecular weight polyethylene blends with ethylene/1‐hexene copolymers on material physical and tensile properties

AbstractThe reactor blends (RBs) with bimodal molecular weight distribution on the base of ultrahigh molecular weight polyethylene (UHMWPE) and low molecular weight random ethylene/1‐hexene copolymers (CEH) were synthesized by two‐step processes including ethylene polymerization followed by ethylene/1‐hexene copolymerization over rac‐(CH3)2Si(Ind)2ZrCl2/methylaluminoxane catalyst. The four series of blends differed in a composition of copolymer fraction that was varied in a wide range (from 3.0 to 37.0 mol % of 1‐hexene). The differential scanning calorimetric study shows the double melting behavior of the net semicrystalline CEHs, which can be attributed to intramolecular heterogeneity in chain branch distribution. The introduction of CEHs leads to the modification of nascent RB crystalline and amorphous phases. Physical and tensile properties as well as melting indexes of the materials depend not only on the percentage of copolymer fraction that varied from 6.9 to 35.8 wt % but also on its composition. The increase of copolymer fraction with high content of 1‐hexene (≥11.0 mol %) in the blends leads to the change of the character of stress–strain curves; the materials behave as elastomers. Controlled regulation of copolymer fraction characteristics in the synthesis yields RBs combining the enough high strength, good plastic properties with enhanced melting indexes as compared with the net UHMWPE. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40151.

Silica-supported (n BuCp)2 ZrCl2 : effect of catalyst active center distribution on ethylene-1-hexene copolymerization

Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts—silica/MAO/(nBuCp)2ZrCl2 (Catalyst 1) and silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2 (Catalyst 2), where MAO is methylaluminoxane—were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene–1-hexene Copolymer 1 and Copolymer 2, respectively. Fouling-free copolymerization, catalyst kinetic stability and production of free-flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self-nucleation and annealing experiments, as well as by an extended X-ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO-pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co-monomer effect—both by 1-hexene—were common. Each copolymer demonstrated vinyl, vinylidene and trans-vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended. © 2013 Society of Chemical Industry

Effects of Supported (nBuCp)2ZrCl2Catalyst Active-Center Distribution on Ethylene–1-Hexene Copolymer Backbone Heterogeneity and Thermal Behaviors

Two catalysts, denoted as catalyst 1 [silica/MAO/(nBuCp)2ZrCl2] and catalyst 2 [silica/nBuSnCl3/MAO/(nBuCp)2ZrCl2] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene–1-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive self-nucleation and annealing (SSA)], and 13C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. 13C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter- and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, nE MPDSC-GT and nE MPNMR-Flory, as well as the weight-average lamellar thicknesses, Lwav DSC–GT and Lwav SSA DSC, were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo- and copolymers increased linearly as a function of LMPDSC-GT. This indicates that the homo- and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene−α-olefin linear low-density polyethylenes (LLDPEs).

Microstructure and properties of branched polyethylene: Application of a three‐phase structural model

AbstractA combined study of microstructure and mechanical properties for a series of single‐site metallocene ethylene/1‐hexene copolymers is presented. The analysis of the results focuses on the effect of branching, which in turn modulates crystallinity and density, on the values of the elastic modulus obtained in tensile‐deformation experiments. An extensive literature review is done in order to compare our own results with previous studies. The typical variation found in the elastic modulus is discussed in terms of the existence of a rigid amorphous phase. This rigid phase controls the macroscopic mechanical behavior of the materials, giving rise to an increase of the elastic modulus as the crystallinity does. Mechanical coupling models for heterogeneous systems are applied in order to describe the experimental results of the elastic modulus as a function of the variation of three different phase fractions, i.e., crystalline, rigid amorphous (or interfacial), and mobile amorphous. The phase fraction values obtained from the analysis of the mechanical properties are in qualitative agreement to those found experimentally in our group by Raman infrared spectroscopy and from the literature by positron annihilation lifetime spectroscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Study of the compositional heterogeneity of ethylene/1–hexene copolymers produced over supported catalysts of different composition

AbstractSeparation into narrow MWD fractions (liquid–liquid fractionation) and preparative TREF (temperature rising elution fractionation) with subsequent analysis of fractions by GPC, FTIR, and 13C NMR spectroscopy were used to study the comonomer distribution of ethylene/1–hexene copolymers produced over highly active supported titanium‐ and vanadium‐magnesium catalysts (TMC and VMC) and a supported zirconocene catalyst. These catalysts produce PE with different MWD: Mw/Mn values vary from 2.9 for zirconocene catalyst, 4.0 for TMC, and 15 for VMC. 1‐Hexene increases polydispersity to 25 for copolymer produced over VMC and hardly affects MWD of the copolymer produced over TMC and zirconocene catalysts. The most broad short chain branching distribution (SCBD) was found for ethylene/1–hexene copolymers produced over TMC. VMC and supported zirconocene catalyst produce copolymers with uniform profile of SCB content vs. molecular weight in spite of great differences in Mw/Mn values (22 and 2.5 respectively). TREF data showed that majority of copolymer produced over supported zirconocene catalyst was eluted at 70–90°C (about 85 wt %). In the case of VMC copolymer's fractions were eluted in the broad temperature interval (40–100°C). Accordingly, TREF data indicate a more homogeneous SCBD in copolymer, produced over supported zirconocene catalyst. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Copolymerization of ethylene with α‐olefins over supported titanium–magnesium catalysts. I. Effect of polymerization duration on comonomer content and the molecular weight distribution of copolymers

AbstractThe data on the effect of polymerization duration on molecular weight (MW), molecular weight distribution (MWD), and content of α‐olefin were obtained for ethylene/1‐hexene copolymers produced on a supported titanium–magnesium catalyst (TMC) upon polymerization in the absence and presence of hydrogen. An increase in MW, narrowing of the MWD, and a decrease in 1‐hexene content in the copolymer takes place with increasing polymerization duration. It was shown by molecular weight fractionation of copolymers obtained at different polymerization duration followed by analysis of narrow fractions to determine the butyl branching content that the decrease comonomer content in copolymers with increasing polymerization duration is accounted for by the decrease in the share of low molecular weight fractions with an increased butyl branch content. These data, in combination with the data concerning the narrowing of the MWD of copolymers and the decrease in activity with polymerization duration allow arriving at conclusion that active sites producing low molecular weight polymer with an increased butyl branching content is predominantly deactivated with polymerization. Active sites producing high molecular weight polymer has a low ability to copolymerize ethylene with α‐olefins; however, these sites show higher stability during copolymerization. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012