Characterization of ethylene/propylene copolymers by means of a GPC-4D technique

Copolymer composition and comonomer distribution are important magnitudes in polymer material that have a big effect on different kind of properties and consequently there are several ways to study.In this work several ethylene/propylene copolymers synthesized with two different metallocene catalysts and a Ziegler–Natta catalyst and covering a wide composition range were studied. Characterization was carried out by nuclear magnetic resonance (¹³C NMR) and by gel permeation chromatography with 4 detectors (GPC-4D): refractive index, viscosity, multi-angle light scattering and infrared detectors.Different behaviour in the comonomer distribution along the molecular weight was obtained for metallocene and for ZN copolymers as expected due to the differences between these catalytic systems. Nevertheless, Ziegler–Natta copolymers present more homogeneous comonomer distribution due to the synthesis method. Study of conformation of chains in solution was improved by defining the scaling law of R_g against the number of repeat units because it avoids the effect of the repetitive unit size. Both metallocene copolymer sets show similar dependence of q value with the copolymer composition, however Ziegler–Natta copolymers show different behaviour with q values independent on copolymer composition. This different behaviour has been related with the effects of the heterogeneity of the ethylene distribution and of the molecular weight of the samples.     

Studies of structural composition distribution heterogeneity in ethylene/1-hexene copolymers using thermal fractionation technique (SSA): Effect of catalyst structure

Investigations into the compositional heterogeneity of ethylene/1-hexene copolymers obtained with various zirconocene/MAO catalysts, either homogeneous or supported on inorganic carriers such as a complex of magnesium chloride with tetrahydrofuran or methyl alcohol, were conducted. The dependence between metallocene structure, as well as catalyst immobilization, and the compositional heterogeneity of the related products was investigated. It was found that the heterogeneity of copolymers is determined by the metallocene catalyst structure. The amount of peaks on the DSC thermograms of copolymers and their division increase with the increase of bulkiness of the ligand in the catalytic system. The immobilization of the investigated catalysts on the magnesium carrier leads to an increase of the copolymer's compositional heterogeneity. However, the modification of the MgCl₂ carrier by tetrahydrofuran or methyl alcohol seems to not have any influence on the copolymers’ CCD.     

Copolymerization of ethylene and 1-hexene with supported metallocene catalysts: Effect of support treatment

The effect of different catalyst support treatments in the 1-hexene/ethylene copolymerization with supported metallocene catalysts was investigated through the analysis of molecular weight and chemical composition distributions by means of high temperature gel permeation chromatography (GPC) and crystallization analysis fractionation (CRYSTAF). The molecular weight distributions of all copolymers are narrow, indicating a uniform catalyst site. However, with respect to chemical composition distribution, some supported catalysts show broad and sometimes bimodal distributions, which indicates the presence of two or more active site types.     

Study of the compositional heterogeneity of ethylene–hexene-1 copolymers by thermal fractionation technique by means of differential scanning calorimetry

The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl₄/MgCl₂ (TMC) and a vanadium–magnesium catalyst VCl₄/MgCl₂ (VMC), a supported zirconocene catalyst Me₂Si(Ind)₂ZrCl₂/SiO₂ (MAO), and a chromium-oxide catalyst CrO₃/SiO₂. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions.     

Dynamic DSC,SAXS and WAXS on homogeneous ethylene-propylene and ethylene-octene copolymers with high comonomer contents

Ethylene-propylene (EP) and ethylene-octene (EO) copolymers polymerized with the aid of homogeneous vanadium and metallocene catalysts were compared by DSC and time-resolved simultaneous SAXS-WAXS-DSC at scanning rates of 10 and 20°C min^?1 using synchrotron radiation. An EP copolymer with a density of 896 kg m^?3 (about 89 mol % ethylene) after compression moulding gave orthorhombic WAXS reflections. The crystallinity as a function of temperature [w ^c (T)] calculated from these reflections using the two-phase model was in good agreement withw ^c (T) calculated fromc _p measurements using DSC. Thec _p measurements also enabled calculation of the baselinec _p and the excessc _p. The SAXS measurements revealed a strong change in the long period in cooling and in heating. The SAXS invariant as a function of temperature showed a maximum in both cooling and heating, which could be explained from the opposing influences of the crystallinity and the electron density difference between the two phases. Two EO copolymers with densities of about 871 kg m^?3 (about 87 mol% ethylene) no longer showed any clear WAXS reflections, although DSC and SAXS measurements showed that these copolymers did crystallize. The similarity between the results led to the conclusion that the copolymers, though based on different catalyst systems — vanadium and metallocene — did not have strongly different sets of propagation probabilities of chain growth during polymerization. On the basis of a Monte Carlo simulation model of crystallization and morphology, based on detailed knowledge of the microchain structure, the difference between WAXS on the one hand and DSC and SAXS on the other could be explained as being due to loosely packed crystallized ethylene sequences in clusters. These do cause the density and the electron density of the cluster to increase (which is measurable by SAXS) and the enthalpy to decrease (which is measurable by DSC) but the clusters are too small and/or too imperfect to give constructive interference in the case of WAXS. Of an EP copolymer with an even lower ethylene content (about 69 mol %), the crystallization and melting processes could still be readily measured by DSC and SAXS, which proves that these techniques are eminently suitable for investigating the crystallization and melting behaviour of the copolymers studied.     

Melting behavior of polyethylene/α‐olefin copolymers: Narrow composition distribution copolymers and fractions from Ziegler–Natta and single‐site catalyst products

The melting temperature and heat of fusion were measured for an extensive series of compositionally uniform copolymers of ethylene with butene‐1, hexene‐1, and octene‐1. Fractions and whole polymers that exhibited minimal interchain compositional heterogeneity were from commercial copolymers made with either Ziegler–Natta (ZN) or single‐site metallocene catalysts. The present results do not support recent claims that ZN and corresponding metallocene catalyst copolymers melt at significantly different temperatures, nor the implication that comonomer incorporation is “blocky” in ZN copolymers. In five of the six comonomer/catalyst systems the dependencies of the melting temperature on comonomer type and amount were scarcely distinguishable. This common behavior is the same as that for a model random copolymer, so we conclude that most ethylene/α‐olefin copolymers have random distributions of ethylene sequences. The exception in the present study is a metallocene ethylene/butene‐1 copolymer that melts at lower temperatures and apparently has perceptibly alternating sequence distributions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3416–3427, 2004     

Comonomer Distribution in Polyethylenes Analysed by DSC After Thermal Fractionation

Ethylene copolymers exhibit a broad range of comonomer distributions. Thermal fractionation was performed on different grades of copolymers in a differential scanning calorimeter (DSC). Subsequent melting scans of fractionated polyethylenes provided a series of endothermic peaks each corresponding to a particular branch density. The DSC melting peak temperature and the area under each fraction were used to determine the branch density for each melting peak in the thermal fractionated polyethylenes. High-density polyethylene (HDPE) showed no branches whereas linear low-density polyethylenes (LLDPE) exhibited a broad range of comonomer distributions. The distributions depended on the catalyst and comonomer type and whether the polymerisation was performed in the liquid or gas phase. The DSC curves contrast the very broad range of branching in Ziegler—Natta polymers, particularly those formed in the liquid phase, with those formed by single-site catalysts. The metallocene or single-site catalysed polymers showed, as expected, a narrower distribution of branching, but broader than sometimes described. The ultra low-density polyethylenes (ULDPE) can be regarded as partially melted at room temperature thus fractionation of ULDPE should continue to sub-ambient temperatures. The thermal fractionation is shown to be useful for determining the crystallisation behaviour of polyethylene blends.This revised version was published online in November 2005 with corrections to the Cover Date.     

Isothermal crystallization of propylene-1-decene copolymers

The isothermal crystallization and subsequent melting behavior of one propylene homopolymer and three propylene-1-decene copolymers with different comonomer contents prepared by metallocene catalyst were studied using differential scanning calorimetry (DSC). It is found that the Avrami exponent of the propylene copolymers decreases gradually with the increase of comonomer content, from 3.0 for the propylene homopolymer to 1.4 for the copolymer with 7.83 mol% 1-decene units. Higher comonomer content also weakens the dependence of crystallization rate constant and crystallization halftime on temperature. Double melting peaks, which correspond to α and γ crystal phases, respectively, are observed for all copolymers under isothermal crystallization. The result shows that higher crystallization temperature is favorable to the segregation of α and γ crystal phases, resulting in higher proportion of γ crystal phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural Development During Thermal Fractionation of Polyethylenes

In this study, the stepwise isothermal crystallization or thermal fractionation of Ziegler—Natta and metallocene based polyethylenes (ZN-PE and m-PE) with two kinds of branch lengths (ethyl and hexyl) and branch compositions were studied using simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The crystal long period and the invariant were determined by SAXS, and the variations of crystal unit cell parameters and the degree of crystallinity were determined by WAXD. The arithmetic mean length (Ln), the weightedmean length (Lw) and the broadness index (Lw/Ln) of the studied polyethylenes were previously determined by DSC. Results from these studies were interpreted using the model of branch exclusion, which affects the ability of the chain-reentry into the crystal phase. Multiple SAXS peaks and step-change in crystallinity change (WAXD) were seen during heating, which corresponded well with the crystal thickness distribution induced by stepwise crystallization. The effects of the heterogeneity of the 1-olefin branch length and the distribution on the crystal long period and the invariant as well as the degree of crystallinity were discussed.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal and structural investigation of random ethylene/1-hexene copolymers with high 1-hexene content

Random ethylene/1-hexene copolymers with the 1-hexene content in the range from 2 to 28 mol% were produced with a novel post-metallocene catalyst and analyzed by three techniques, FTIR, ¹³C NMR, and DSC. The 1-hexene content and the sequence distribution in the copolymers were determined by means of FTIR-M and ¹³C NMR. The crystallization behavior of the copolymers was studied by DSC under dynamic and isothermal conditions; the Avrami model was used to analyze the crystallization kinetics. It was found that both the 1-hexene content and the crystallization temperature affect the relative crystallinity. The bulk crystallization rate decreases with the 1-hexene content and reduces exponentially with an increase of T _c. The melting behavior of isothermally crystallized samples was also investigated and it was found that the melting temperatures of the copolymers under equilibrium conditions were related to the composition.     

Heterogeneity of active species in metallocene catalysts

From a detailed analysis of poly[ethylene-co-(1-hexene)] obtained with typical metallocene catalysts, it was found that metallocene catalysts give two kinds of copolymers which differ in crystallinity. It may be thus plausible to assume that there are two kinds of active species in metallocene catalysts.     

COMONOMER SEQUENCE DISTRIBUTION IN METALLOCENE-BASED ETHYLENE/1-BUTENE (S-34) AND ETHYLENE/1-HEXENE (S-43) COPOLYMERS

Metallocene based polyethylenes were prepared by SINOPEC's "metallocene adduct" technologyin a gas phase fluidized bed model reactor. The ~(13)C-NMR spectra of ethylene/1 -butene (S-34) and ethylene/1-hexene(S-43) copolymers were studied in a manner analogous to that established by Hsieh and Cheng. Thecomonomer sequence distributions of copolymer samples were obtained. The results show that thesemetallocene based copolymers contain a small amount of butene and hexene, and the EE and EEE sequencesare dominant.     

Inter and intra-molecular branching distribution of tailored LLDPEs inferred by melting and crystallization behavior of narrow fractions

Here we report the melting and isothermal crystallization behavior of two sets of fractions obtained from a film-grade metallocene catalyzed ethylene-1-hexene resin with enhanced mechanical properties. One set of fractions was obtained by molecular weight fractionation, the second set was obtained fractionating by content of 1-hexene. The melting behavior, crystallization kinetics and supermolecular morphology of the fractions are analyzed in reference to the behavior of model systems with uniform inter-chain branching content and a random intra-chain distribution. While melting and crystallization kinetics of molecular weight fractions conforms to the bivariate (molecular weight-comonomer content) distribution of the original copolymer, the behavior of 1-hexene compositional fractions indicate a blockier branching distribution in the highly branched high molar mass fractions. Major differences with model random copolymers are also observed in the supermolecular morphology of the latter fractions.     

Homopolymerization and copolymerization of vinyl chloride over supported metalorganic catalysts

The homopolymerization of vinyl chloride and its copolymerization with ethylene over dibutyl ether–modified SiO₂-supported Ziegler–Natta catalysts based on titanium and vanadium chlorides have been studied. The supported metal complexes are sufficiently active in the polymerization of vinyl chloride. Their activity depends on the catalyst composition and conditions of formation of the catalyst on the surface of the support. The chain structure of the resulting polyvinyl chloride (PVC) has been studied by NMR spectroscopy. The thermal properties of the synthesized PVC have been investigated by differential scanning calorimetry. The PVC obtained possesses enhanced thermal stability owing to the specific features of its chain structure. Vinyl chloride polymerization over the supported metalorganic catalyst proceeds mainly via a free-radical mechanism. Process conditions have been found for conducting the copolymerization of vinyl chloride with ethylene over supported metal complexes resulting in the formation of true statistical copolymers, which is confirmed by IR and NMR spectroscopy.     

茂金属聚乙烯的支化非均匀性与结晶形态

以美国Exxon公司生产的茂金属短支链聚乙烯 (SCBPE)为研究对象 ,用DSC热分级方法研究了不同支链含量聚乙烯的支化非均匀性 ,并用TEM表征茂金属聚乙烯在不同条件下熔体结晶的形态 .SCBPE经逐步结晶分级后 ,其DSC熔融曲线出现多峰 ,表明茂金属SCBPE仍具有很大的非均匀性 .支链含量愈高 ,熔点愈低 ,非均匀性增加 .SCBPE的晶体形态随平均支链含量的增加片晶厚度减小 ,片晶尺寸分布增加 ;含量较低时 ,SCBPE在不同温度下均生成球晶结构 .随结晶温度的降低支链含量较多的分子也能参与结晶 ,故生成的片晶厚度减小 .从相分离的熔体中结晶 ,大尺度的片晶集聚体间的分离和小尺度的片晶之间相分离同时存在 .     

Kinetics of ethylene polymerization reactions with chromium oxide catalysts

Principal kinetic data are presented for ethylene homopolymerization and ethylene/1‐hexene copolymerization reactions with two types of chromium oxide catalyst. The reaction rate of the homopolymerization reaction is first order with respect to ethylene concentration (both for gas‐phase and slurry reactions); its effective activation energy is 10.2 kcal/mol (42.8 kJ/mol). The r₁ value for ethylene/1‐hexene copolymerization reactions with the catalysts is ～30, which places these catalysts in terms of efficiency of α‐olefin copolymerization with ethylene between metallocene catalysts (r₁ ～ 20) and Ti‐based Ziegler‐Natta catalysts (r₁ in the 80–120 range). GPC, DSC, and Crystaf data for ethylene/1‐hexene copolymers of different compositions produced with the catalysts show that the reaction products have broad molecular weight and compositional distributions. A combination of kinetic data and structural data for the copolymers provided detailed information about the frequency of chain transfer reactions for several types of active centers present in the catalysts, their copolymerization efficiency, and stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5315–5329, 2008     

Study of the compositional heterogeneity of ethylene-1-hexene copolymers via thermal fractionation with the use of differential scanning calorimetry

The compositional heterogeneity of ethylene-1-hexene copolymers synthesized with various types of supported catalysts, namely, the titanium-magnesium catalyst TiCl₄/MgCl₂ and the zirconocene catalyst SiO₂(MAO)/Me₂Si(Ind)₂ZrCl₂, is studied via the method of successive self-nucleation-annealing (SSA) with the use of differential scanning calorimetry. On the basis of the data on the temperatures of individual peaks on SSA curves, the thickness of lamellas composed of macromolecules with a certain degree of short-chain branching is estimated. The copolymer synthesized with the zirconocene catalyst has a narrower range of fusion and does not contain large lamellas composed of molecules with a low degree of short-chain branching. With the use of the broadness index, it is shown that the copolymer synthesized with the zirconocene catalyst has a more uniform distribution of the comonomer than does the copolymer synthesized with the titanium-magnesium catalyst. For the copolymers synthesized with the titanium-magnesium catalyst, the compositional heterogeneity increases with an increase in the content of 1-hexene.     

Targeted synthesis of homo-and copolymers of propylene in liquid propylene initiated by highly efficient homogenous metallocene catalysts

Studies devoted to the homo-and copolymerization of propylene with ethylene and higher olefins (1-butene, 1-hexene, 4-methyl-1-pentene, 1-octene) in liquid propylene under the action of homogeneous metallocene catalysts of various types are surveyed in brief. The main kinetic features of the processes and the properties of the polymers are discussed. The optimal conditions for the highly efficient syntheses of isotactic, syndiotactic, hemiisotactic, and stereoblock PPs are described. It is shown that the combined cocatalyst—polymethylaluminoxane coupled with (i-Bu)₃Al—shows promise for the processes under consideration. Depending on the type of catalyst used, the copolymerization of propylene with ethylene yields copolymers with a block, random, or close to alternating distribution of comonomer units in a polymer chain. The copolymerization of propylene with higher olefins in the monomer bulk initiated by highly active sterically hindered isospecific catalytic systems shows an ideal character, and the reactivity ratios are r ₁ ≈ r ₂ ≈ 1; that is, the composition of the copolymer is equal to the composition of the monomer mixture at all comonomer ratios. It is demonstrated that the synthesis of homo-and copolymers of propylene in the monomer bulk in the presence of modern homogeneous catalysts is promising for highly efficient production of both traditional and new polymer materials with a unique combination of mechanical and thermal properties.     

Melting and crystallization of tetrafluoroethylene-ethylene copolymers

The melting and crystallization of copolymers of tetrafluoroethylene with ethylene, synthesized in bulk and in suspension by semi-flow method, were studied by DSC. X-ray diffractions and infrared spectra of the copolymers were measured and new crystalline reflections different from those of the homopolymers were observed. The melting temperature of the copolymers synthesized in bulk depends strongly on the composition and exhibits several maxima. A certain small decrease in the melting temperature within the range of the alternating composition is observed. For alternating copolymers synthesized in suspension, the peaks are monomodal indicating a higher structural and chemical homogeneity of the copolymer. The nonisothermal crystallization kinetics in the temperature interval from 260 to 255°C of the alternating copolymer prepared in suspension can be described by a modified Avrami equation. The mechanism of nucleation and nuclei growth during the nonisothermal crystallization of the tetrafluoroethylene-ethylene copolymer is close to that of polyethylene.