Copolymerization of ethylene and 1-octadecene with the Cp2ZrCl2/MAO and Cp2HfCl2/MAO catalyst systems

Summary The comparison of the copolymers obtained with the Cp₂ZrCl₂/MAO and Cp₂HfCl₂/MAO catalyst systems showed that the catalyst having hafnocene was much more reactive towards 1-octadecene than zirconocene. The comonomer concentration had to be three times higher in the zirconocene copolymerization than in the hafnocene copolymerization when the level of 6 mol-% was reached. Although the hafnocene catalyst was more reactive towards 1-octadecene, the molecular weights were higher than in the copolymers obtained with the zirconocene catalyst.The total activity of the zirconocene was 10 times higher than with the hafnocene catalyst. With the zirconocene catalyst the activity towards ethylene was constantly increasing by increasing the comonomer concentration but stayed nearly constant with the hafnocene catalyst. It seemed that there is no rate enhancement effect upon comonomer addition with the hafnocene catalyst.     

Thermal characterization of ethylene polymers prepared with metallocene catalysts

A new fractionation technique, segregation fractionation by differential scanning calorimetry (DSC), was applied to both lab-scale and commercial metallocene copolymers produced with a variety of comonomers. Comparisons were made to other fractionation methods, such as temperature rising elution fractionation (TREF) and crystalline analysis fractionation (CRYSTAF). The new method represents an alternative tool for the relative qualitative analysis of chemical composition distribution (CCD) and the technique was found useful for the characterization of comonomer unit distribution. Differences in the catalyst system and in the thermal treatment were reflected in the melting pattern of the copolymers, obtained after fractionation. The new method was used to study CCD broadening in lab scale slurry polymerizations, and it was found that the effect of polymerization time and comonomer feed system on the CCD can be followed by segregation fractionation measurements. Dynamic mechanical analysis (DMA) was also used to evaluate the heterogeneity of the polyethylene structure. Studies of β-transitions showed differences in the tan δ-density correlations, which were influenced by the comonomer type. In most cases the intensity of the β-transition increased with branching (decreasing density), but for ethylene-1-octadecene copolymers the situation was opposite at lower densities. This may be attributed to the crystallization of C₁₆H₃₃ branches.     

The effect of the ethylene pressure on its reaction with 1-hexene, 1-octene and 4-methyl-1-pentene

Summary Copolymers of ethylene and 1-hexexe, 1-octene and 4-methyl-1-pentene were obtained using Et[In]₂ZrCl₂/MAO catalyst at various pressures. The increase of 1-hexene and 1-octene concentration in the feed increases catalyst activity(g/nZr.h.bar) and productivity(g/nZr.h). For 4-methyl-1-pentene the activity is independent on comonomer concentration. Increasing the ethylene pressure the productivity of the copolymerization increases and the activity shows a weak decay. Characterization of the copolymer shows that at higher pressure the cristallinity of the copolymers is higher due to lower comonomer incorporation. There is a good linear correlation of cristallinity with comonomer concentration in the feed for 1-hexene and 1-octene at a fixed pressure, but not for 4-methyl-1-pentene.     

Syndiotactic polypropylene and its copolymers with alpha-olefins. Effect of composition and length of comonomer

A set of copolymers of propene and alpha olefins (1-hexene, 1-octene and 1-octadecene) and the corresponding homopolymer (sPP) have been synthesized using a syndiotactic metallocene catalyst. The effect of either the incorporation or length of these conomomeric units on the structure and final properties exhibited has been analyzed. As expected, there is a considerable decrease in crystallinity with the increase of comonomer content. Thus, a completely amorphous copolymer is obtained if the molar fraction is high enough. The structural variations drastically influence the viscoelastic and mechanical behaviors of these copolymers. Three or four relaxation processes can be observed depending on composition and length of comonomer. At temperatures above the process associated with the glass transition (β relaxation), a deep drop (in one or two steps) in E′ and a shoulder in E″, overlapped with that β mechanism, are observed. The existence of a relaxation involving crystalline regions is postulated because of its variation with crystal characteristics. Moreover, a relaxation related to internal motions within the comonomeric units is seen in the range of very low temperatures. This process, primarily ascribed to movement of methylenic segments within the comonomer, is strongly depending on composition and length of the pendant aliphatic chains on incorporated units. On the other hand, a decrease in stiffness and microhardness as well as the brittle–ductile transition are observed by simply varying composition when deformation takes place at room temperature.     

Critical insights into ethylene-1-alkene copolymers and long-chain branched polyethylene microstructure on thermo-rheological and sealing behavior of multicomponent systems

The microstructure-property relationships in multicomponent ethylene-1-alkene copolymers with different branching in the microstructures are demonstrated. The metallocene catalyzed linear low density polyethylene (mLLDPE), was miscible with both autoclave grade low density polyethylene (LDPE-A) and/or tubular grade low density polyethylene (LDPE-T). For these multicomponent systems, the rheological response was distinctly differentiating and sensitive to the microstructure of LDPE, at higher shear regimes. The thicker lamellae of LDPE-T and/or LDPE-A might co-crystallize if there is a high density polyethylene-like fraction present in the mLLDPE. Even though the macro parameters like density and melt index (MI) of the investigated multicomponent systems are comparable, the subtle differences in the microstructure manifested by type and distribution of comonomer and/or branching affected the sealing performance. Both high comonomer content and comonomer distribution in the mLLDPE matrix affording a higher fraction of material melting below 120°C were found to be critical for the heat sealing. The fraction of material melting at lower temperatures, attributed to the tertiary branches present in the hyper-branched microstructure of LDPE-A, participate in the sealing process, and lower the sealing temperature. It was evident that mLLDPE with asymmetric distribution of lamellae is more sensitive to the microstructure of the LDPE used.     

Synthesis and properties coming from the copolymerization of propene with α-olefins using different metallocene catalysts

Copolymerization of propene with two α-olefins (1-hexene and 1-octadecene) using iso- and syndioselective metallocene catalysts (EtInd₂ZrCl₂, Et(2-MeInd)₂ZrCl₂, Me₂SiInd₂ZrCl₂, Ph₂CFluCpZrCl₂ and Me₂CFluCpZrCl₂) activated with methylaluminoxane (MAO) is reported. The so-called comonomer effect was seen in the catalytic activity of the Me₂SiInd₂ZrCl₂/MAO system. Incorporation of syndiotactic copolymers was greater than that of isotactic copolymers. The molecular weight of the isotactic copolymers was not affected significantly by the presence of the comonomer, but the molecular weights of the copolymers obtained with the syndioselective catalysts decreased with increasing comonomer concentration in the medium. Tensile properties were studied. Syndiotactic copolymers with incorporation of the order of 6 mol% of 1-octadecene presented elastomeric properties.     

Effect of crystallization conditions on the morphological structure of ethylene-1-butene copolymers obtained by different ziegler-natta catalyst systems

Four ethylene- 1 -butene copolymers of about the same comonomer content but obtained with different supported Ziegler-Natta catalyst systems have been studied. The effects of the catalyst and the crystallization conditions on the morphological structure have been analyzed. These two factors'clearly affect the melting endotherms and the most probable crystallite thickness of the copolymers, although no important differences were found in the crystalline contents. The catalyst system influences the melting pattern due to changes in the chemical composition distribution, i.e., variations in the comonomer content between chains of different molecular weight.     

Comparison of propylene/ethylene copolymers prepared with different catalysts

This study compared a series of experimental propylene/ethylene copolymers synthesized by a transition metal‐based, postmetallocene catalyst (xP/E) with homogeneous propylene/ethylene copolymers synthesized by conventional metallocene catalysts (mP/E). The properties varied from thermoplastic to elastomeric over the broad composition range examined. Copolymers with up to 30 mol % ethylene were characterized by thermal analysis, density, atomic force microscopy, and stress–strain behavior. The xP/Es exhibited noticeably lower crystallinity than mP/Es for the same comonomer content. Correspondingly, an xP/E exhibited a lower melting point, lower glass transition temperature, lower modulus, and lower yield stress than an mP/E of the same comonomer content. The difference was magnified as the comonomer content increased. Homogeneous mP/Es exhibited space‐filling spherulites with sharp boundaries and uniform lamellar texture. Increasing comonomer content served to decrease spherulite size until spherulitic entities were no longer discernable. In contrast, axialites, rather than spherulites, described the irregular morphological entities observed in xP/Es. The lamellar texture was heterogeneous in terms of lamellar density and organization. At higher comonomer content, embryonic axialites were dispersed among individual randomly arrayed lamellae. These features were characteristic of a copolymer with heterogeneous chain composition. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1651–1658, 2006     

Synthesis of linear low density polyethylene with a nano-sized silica supported Cp2ZrCl2/MAO catalyst

A nano-sized silica supported Cp₂ZrCl₂/MAO catalyst was used to catalyze the copolymerization of ethylene/1-hexene and ethylene/1-octene to produce linear low-density polyethylene (LLDPE) in a batch reactor. Under identical reaction conditions, the nano-sized catalyst exhibited significantly higher polymerization activity, and produced copolymer with greater molecular weight and smaller polydispersity index than a corresponding micro-sized catalyst, which was ascribed to the much lower internal diffusion resistance of the nano-sized catalyst. Copolymer density decreased with the increase of polymerization temperature, probably due to the decrease of reactivity ratio r ₁ and ethylene solubility with increasing temperature. Polymerization activity of the nano-sized catalyst increased rapidly with increasing comonomer concentration. Ethylene/1-octene exhibited higher polymerization activity and had a stronger comonomer effect than ethylene/1-hexene.     

Copolymerization of ethylene/α‐olefins using bis(2‐phenylindenyl)zirconium dichloride metallocene catalyst: structural study of comonomer distribution

Catalysts have a major role in the polymerization of olefins and exert their influence in three ways: (1) polymerization behaviour, including polymerization activity and kinetics; (2) polymer particle morphology, including bulk density, particle size, particle size distribution and particle shape; and (3) polymer microstructure, including molecular weight regulation, chemical composition distribution and short‐ and long‐chain branching. By tailoring the catalyst structure, such as the creation of a bridge or introducing a substituent on the ligand, metallocene catalysts can play a major role in the achievement of desirable properties. Kinetic profiles of the metallocene catalyst used in this study showed decay‐type behaviour for copolymerization of ethylene/α‐olefins. It was observed that increasing the comonomer ratio in the feedstock affected physical properties such as reducing the melting temperature, crystallinity, density and molecular weight of the copolymers. It was also observed that the heterogeneity of the chemical composition distribution and the physical properties were enhanced as the comonomer molecular weight was increased. In particular, 2‐phenyl substitution on the indenyl ring reduced somewhat the melting point of the copolymers. In addition, the copolymer produced using bis(2‐phenylindenyl)zirconium dichloride (bis(2‐PhInd)ZrCl₂) catalyst exhibited a narrower distribution of lamellae (0.3–0.9 nm) than the polymer produced using bisindenylzirconium dichloride catalyst (0.5–3.6 nm). The results obtained indicate that the bis(2‐PhInd)ZrCl₂ catalyst showed a good comonomer incorporation ability. The heterogeneity of the chemical composition distribution and the physical properties were influenced by the type of comonomer and type of substituent in the catalyst. Copyright © 2010 Society of Chemical Industry     

DSC analysis of ethylene/1-butene copolymers obtained with different ziegler–natta catalysts

The crystallization and melting behaviour of two sets of ethylene/1-butene copolymers have been analysed by DSC. The samples, with comonomer content in the range from 0 to 21.5 mol%, were obtained by industrial processes using both Mg/Ti-based catalyst systems. The composition dependences of melting and crystallization temperatures were found to be strictly affected by the catalyst type. Moreover, logarithmic plots of the melting and crystallization enthalpy as a function of the ethylene content (mol%) in the copolymers fitted linear relationships whose slopes have been related to the critical sequence length of crystallizable ethylene units, depending on the catalytic system. These results are compared with those reported in the literature for ethylene/1-butene copolymers synthesized by other catalysts and are accounted for by a different distribution of the comonomer units in the macromolecules of the two sets of samples.     

Nonisothermal melting and crystallization studies of homogeneous ethylene/α-olefin random copolymers

A study of nonequilibrium melting, nonisothermal, and isothermal crystallization behavior of ethylene/1-octene (EO) random copolymers, produced using metallocene catalysts has carried out. As branch (or defect) content increases, the nonisothermal and isothermal crystallization rates, melting temperatures, and heats of fusion decrease. There is also a branch length effect on melting temperature depression, the melting temperature depression of EO random copolymers with hexyl branches were significantly larger than those of ethylene/1-butene (EB) and ethylene/1-propene (EP) copolymers having ethyl and methyl branches, respectively. The melting temperatures of homogeneous random copolymers have been found to be always lower than those of fractions of heterogeneous copolymers, having approximately the same branch content and molecular weight. Hence, defect distribution in copolymer systems is at least as important a parameter as the defect content. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1893–1905, 1998     

Ethylene/1-olefin copolymerization behaviour of vanadium and titanium complexes bearing salen-type ligand

Ethylene/1-olefin copolymerization using vanadium and titanium complexes bearing tetradentate [O,N,N,O]-type ligand and EtAlCl₂ or MAO as a cocatalyst is carried out. In the presence of the vanadium complex activated with EtAlCl₂ is observed (a) negative “comonomer effect”, (b) high comonomer incorporation and narrow chemical composition distribution (CCD), (c) unexpected copolymer microstructure, and (d) increased molecular weight of copolymers when compared with the homopolymer. In contrast, titanium catalyst gives copolymers with lower 1-olefin content and broad CCD. Supported complexes show higher activity, lower 1-olefins incorporation and give copolymers with ultra high molecular weights.     

Catalytic performance of ZnO nanoparticle in formation of LLDPE/ZnO nanocomposites

ZnO nanoparticle was used for preparing supported catalyst, which was applied in copolymerization of ethylene and 1-octene to obtain LLDPE/ZnO nanocomposite. There were two different impregnation methods (in situ and ex situ) in preparing the nano-ZnO supported catalyst. The investigation to compare both methods was conducted by employing various 1-octene initial concentrations in copolymerization. It was found that a heterogeneous catalytic system comprised a supported catalyst, prepared by in situ impregnation, provided higher catalytic activities and 1-octene incorporations compared to those of ex situ impregnation under similar condition perhaps due to closer similarity to a homogeneous system. For the ex situ impregnation, it was found that when zirconocene was directly impregnated onto the support, the catalytic activity decreased. This was due to zirconocene close vicinity to the supports and even deep into the support structure proved by XPS and TGA measurements. Therefore, it was more inaccessible to monomer attack and reducing the catalytic activity. The separate study on each catalytic system relating to the comonomer effect was also conducted by applying initial comonomer concentrations varied between 0 and 18?mmol. The increase in catalytic activity with increasing comonomer concentration can be considered as a positive comonomer effect, and the opposite was true for a negative comonomer effect. It was found that both positive and negative comonomer effects occurred in in situ impregnation and ex situ impregnation systems with Zn/(Al?+?Zr) support, whereas only positive comonomer effect was found in an ex situ impregnation system with Zn/Al support. This suggested that the comonomer effect was varied according to the nature of each system. The polymer properties, such as relative crystallinity and thermal properties were also investigated and found to alter with 1-octene concentration.     

Polymerization and characterization of ethylene/propylene and ethylene/1-octene copolymers produced with bridged Zr- and Hf-based metallocenes

Ethylene/propylene (E/P) and ethylene/1-octene (E/O) copolymers were polymerized with two bridged metallocene catalyst systems, Et(Ind)₂ZrCl₂/MAO and Et(Ind)₂HfCl₂/MAO, respectively. The copolymers produced and some commercial reference copolymers were characterized by DSC, SEC, DMA and ¹³C NMR. The Hf-catalysed E/P polymerizations showed much lower activities than the corresponding Zr-catalysed polymerizations but gave polymers with high molar mass. The Hf-based copolymers also showed two melting peaks which may be indicative of several active sites of the catalyst. A comparison of E/P copolymers, containing about 20 mol-% propylene and produced with Zr, Hf and homogeneous V-catalysts, respectively, indicated that the Hf and V-catalysts gave material more similar to each other. The E/O copolymers produced with Zr-catalysts gave very low molar masses and the reactivity ratios, calculated from the NMR data, indicated that the Hf-catalyst has a slightly higher reactivity for 1-octene and the Zr-catalyst some better reactivity for ethylene. Segregation fractionation studies by DSC indicated that a lower 1-octene feed gives more heterogeneous copolymers and the DMA measurements reveal the existence of a linear correlation between the 1-octene content and the intensity of the tan δ_max peak.     

Effects of shearing and comonomer content on the crystallization behavior of poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate)

Poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate) (PBSEMS) random copolymers were prepared with different comonomer compositions. The effects of shearing and comonomer content on the crystallization behavior of these copolymers were investigated at 80 °C. The thermal and morphological properties of the resulting samples were also discussed. The copolymers showed a longer induction time and a slower crystallization rate with increasing comonomer content. The promoting effect of shear on the overall crystallization behavior was more notable for those copolymers containing more 2‐ethyl‐2‐methyl succinic acid (EMSA) units. The melting temperature of ‘as‐prepared’ poly(butylene succinate) (PBS) was ca. 115 °C, while that of the copolymers varied from 112 to 102 °C. Higher comonomer contents in the copolymers gave rise to lower melting temperatures and broader melting peaks. In addition, the isothermally crystallized samples showed multiple melting endothermic behavior, the extent of which depended on the comonomer content. The copolymers showed different wide‐angle X‐ray diffraction (WAXD) patterns from that of neat PBS, depending on the comonomer content and shear applied during crystallization. With increasing comonomer content, the copolymers crystallized without shearing, showing the shifting of a diffraction peak to a higher angle, while those crystallized under shear did not show any peak shift. Copyright © 2004 Society of Chemical Industry     

LLDPE synthesis via SiO2–Ga-supported zirconocene/MMAO catalyst

In this present study, the linear low-density polyethylene (LLDPE) was synthesized via ethylene/1-octene copolymerization with the zirconocene/MMAO catalyst by in situ impregnation of different silica (SiO₂) supports. The SiO₂ supports used were small-pored (SP) and large-pored (LP) sizes with and without Ga modification. It was found that the SP-SiO₂ support exhibited higher polymerization activity (～1.5 times) than that obtained from the LP-SiO₂ one. This can be attributed to the lower amount of MMAO being present inside the SP-SiO₂ support resulting in higher content of MMAO at the external surface. The higher activity in ethylene/1-octene copolymerization was also found with the supported catalyst having Ga modification onto both SP-and LP-SiO₂ supports_. The results demonstrated that the introduction of Ga may improve ability of supports to immobilize metallocene catalyst. Based on ¹³C NMR measurement, it indicated that all synthesized polymers were typical LLDPE having random distribution of comonomer.     

On the melting behavior of isothermally crystallized 1‐octene linear low‐density polyethylene copolymers

The melting behavior of two 1‐octene linear low‐density polyethylene (LLDPE) copolymers is investigated. One made using Dow′s INSITE constrained geometry catalyst technology (LLDPE‐A) and the other using titanium‐based Ziegler–Natta catalysts (LLDPE‐B). Both have similar comonomer content as well as melt flow index. Differential scanning calorimetry (DSC) was used throughout the work. Isothermal crystallizations in the DSC for several times were carried out at various temperatures between 90 and 100°C for LLDPE‐A and between 105 and 112.5°C for LLDPE‐B. As a result of the isothermal crystallizations for both copolymers, multiple melting peaks are found in the DSC traces on subsequent heating. The melting behavior was also examined as a function of heating rate (1, 2.5, 5, 10, and 20°C/min). The multiple melting behavior indicates that they are inhomogeneous. In addition, a melting–recrystallization process was shown to be responsible for the appearance of one of the melting peaks in LLDPE‐B. A lowering in heating rate from the crystallization temperature favors the occurrence of melting–recrystallization during the dynamic experiment. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2022–2028, 2001     

Isothermal crystallization of metallocene‐based propylene/α‐olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene‐1 copolymers and two propylene/octene‐1 copolymers prepared with metallocene catalyst were investigated. It is found that γ‐modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (T_c) the crystallization rate constant changes more rapidly with T_c and the crystallization half‐time substantially increases. Double melting peaks were also observed at high T_c, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005     

Diffusivity of n-hexane in poly(ethylene-stat-octene)s assessed by molecular dynamics simulation

Molecular dynamics simulations have been used to study diffusion of n-hexane in wholly amorphous poly(ethylene-stat-octene)s with comonomer contents ranging from 0 to 11.5 mol%. The branches in the polymer increased the specific volume by affecting the packing of the chains in the rubbery state in accordance with experimental data. The diffusion of n-hexane at penetrant concentrations between 0.5 and 9.1 wt% was simulated within time-scales between 0.1 and 0.2 μs. The penetrant diffusivity unexpectedly decreased with increasing comonomer content. The penetrant molecule motion statistics showed that systems with high comonomer content showed a greater tendency for short distance motion (over a sampling period of 3 ps) whereas the systems with lower comonomer content showed penetrant motion over longer distances. It seems that the branches retarded local chain mobility of the polymer thereby trapping the penetrant molecules. All systems studied showed a minimum in penetrant diffusivity at ca. 1 wt% n-hexane and a marked increase in diffusivity at higher penetrant concentrations. The volumetric data for the different polymer-penetrant systems were consonant with additional volumes of the different components. Comparison between simulated diffusivities (for a wholly amorphous polymer) and experimentally obtained diffusivity data for semicrystalline polymers showed that constraining effect of the crystals were substantial for the highly crystalline systems and that it gradually decreased with decreasing crystallinity.