Synthesis and characterization of ethylene-1-hexene copolymers using homogeneous Ziegler-Natta catalysts

Summary This study investigated the copolymerization of ethylene with 1-hexene using the homogeneous Et[Ind]₂ZrCl₂ and [Ind]₂ZrCl₂ catalysts. The Et[Ind]₂ZrCl₂ catalyst gave a higher catalytic activity than the [Ind]₂ZrCl₂ and also showed a better incorporation of 1-hexene for the same comonomer concentration in the feed. Thermal analysis (DSC) and viscosity measurements showed that an increase of the 1-hexene incorporated in the copolymer results in a decrease of the melting point, crystallinity and molecular weight of the polymer formed. The reactivity ratios for ethylene and 1-hexene confirmed the more successful incorporation of the comonomer for the polymerization catalyzed by Et[Ind]₂ZrCl₂.     

Copolymerization of ethylene and 1-octene by homogeneous and different supported metallocenic catalysts

In this work, the performance of the homogeneous catalyst system based on Et(Flu)₂ZrCl₂/MAO was evaluated on the copolymerization of ethylene and 1-octene. Characteristics of some of the produced polymers were also investigated. A study was performed to compare this system with that of Cp₂ZrCl₂/MAO. The influence of different support materials for the Cp₂ZrCl₂ was also evaluated, using silica, MgCl₂, and the zeolite sodic mordenite NaM. An increase in activity was observed in relation to the comonomer addition for the two homogeneous catalysts. The copolymers produced by the Et(Flu)₂ZrCl₂/MAO system showed higher molecular weight and narrower molecular weight distribution. We verified that the catalyst supported on SiO₂ was the most active one, although the copolymers produced with the catalyst supported on NaM showed higher molecular weight and lower molecular weight distribution. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 724–730, 2001     

Comonomer sequence distribution in ethylene/1-hexene copolymers

Summary The¹³C NMR spectral assignments for ethylene/1-hexene copolymers are reexamined. Revised assignments are obtained for the 34–36 ppm region. A computational scheme is devised to extract information on composition and sequence distribution from the spectral intensities. A computerized analytical approach that fits the observed intensities to statistical models is also available.     

Temperature-induced structural changes in some random ethylene/1-hexene copolymers

Powder diffraction measurements on five unoriented samples of poly(ethylene-co-hexene) were performed in the temperature range 23-200 °C using synchrotron X-radiation in combination with an area detector. Melting and recrystallization was found to improve the crystallinity of the samples, resulting in a denser packing along [010] or b for the most crystalline samples. A high content of cohexene (branching) reduces both the melting temperature T_m and the crystallinity. There appears to be a slight increase in T_m with increasing molecular weight M_n of the sample. The coefficient of thermal expansion (CTE) along [100], α_a, was found to be always positive, in the range (26-34) × 10⁻⁵ K⁻¹ up to melting, with the larger values for the most crystalline samples. The CTE in the chain direction, i.e. α_c along [001], is negative, ranging from (−0.6 to −8.0) × 10⁻⁵ K⁻¹. The thermal response in the [010] direction is more complex, in most cases being significantly different in the heating and cooling sequences. The unit cells expand nearly linearly in the temperature range from RT to about 20 °C below T_m. Increasing T towards T_m brings about an enhanced asymmetry in the C-C-H valency angles and a small rotation of the polymer chains with a concomitant expansion of the interchain contacts lying approximately in the bc-plane. Melting and recrystallization induce a shortening of these contacts and both the atomic and the molecular movements involved in the structural changes are reversed during cooling.     

Investigation of ethylene/1-octene copolymerization models by carbon-13 NMR

Statistical reaction models have been used to fit C-NMR spectra for ethylene/1-octene copolymers and to describe the polymerization reaction probabilities. Ten models ranging in complexity from a one-site Bernoulli probability to multiple site second-order Markov systems were studied. Model parameters were determined by fitting the experimental integrations of replicated spectra using a maximum likelihood method. The best fit to the experimental NMR spectra was obtained with a two-site model, one site producing mainly high-density polymer following a Bernoulli probability model, while the second site allows more incorporation of octene following a chain-end controlled probability described by first-order Markov statistics. © 1994 John Wiley & Sons, Inc.     

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.     

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.     

Mechanical properties of ethylene/1-hexene copolymers with tailored short chain branching distributions

Recently, we have investigated a metallocene catalyst system that can produce polyethylene and ethylene/α-olefin copolymers with tailored molecular weight and short chain branching distributions (SCBD). Ethylene/α-olefin copolymers produced with this system have narrow molecular weight distributions as expected from metallocene catalysts. However, these copolymers are quite unique in that their SCBDs are broad and sometimes bimodal, similar to Ziegler-Natta LLDPE.To examine the effect of these broad SCBDs on the polymer properties, a series of poly(ethylene-co-1-hexene) resins with very distinct, and in some cases bimodal crystalline distributions, were synthesized. The attractive feature of the resins in this study is that their molecular weight distributions are similar but each possesses a different SCBD, thus effectively minimizing the effect of molecular weight on the properties investigated.It was found that the tensile properties of a copolymer could be controlled by the ratio of the crystalline species present in the sample. In this study, a balance of stiffness and toughness was exhibited by a copolymer containing a large proportion of crystalline material and a small fraction of material of lower crystallinity.     

A computational study of iron-based Gibson-Brookhart catalysts for the copolymerisation of ethylene and 1-hexene

A combined QM/MM study of the ethylene/1-hexene copolymerisation with bisiminepyridine iron(II) is presented. It has been found experimentally that these catalysts do not copolymerise ethylene with 1-hexene. Based on the mechanism of propagation and termination processes proposed by Deng et al., we have performed calculations for the ethylene and 1-hexene monomers in order to give a suitable explanation to the experimental findings. The propagation reaction is divided into two fundamental steps: the backside monomer capture and the backside monomer insertion. The energy barriers for these steps are, respectively, 2.11 and 0.59 kcal/mol for the ethylene monomer and 6.62 and 5.43 kcal/mol for 1-hexene. Additionally, the backside π-complex formation for 1-hexene is an endothermic process by 0.72 kcal/mol. Therefore, the ethylene propagation reaction is very favourable as compared to the 1-hexene propagation. In the same way, the termination reaction is also divided into two elementary steps: the frontside monomer capture and the β-hydrogen transfer steps. The associated energy barriers for these two processes are, respectively, 5.83 and 6.55 kcal/mol for the ethylene monomer and 6.03 and 8.38 kcal/mol for 1-hexene. So, the differences between the rate-limiting step of the propagation and termination energy barriers are 4.44 kcal/mol for the ethylene and 1.76 kcal/mol for the 1-hexene monomer. These facts are in good agreement with concurrent experimental results. Furthermore, the role of the bulky ancillary ligands has been analysed.     

Carbon-13 nuclear magnetic resonance study of ethylene-1-octene and ethylene-4-methyl-1-pentene copolymers

Detailed assignments of ¹³C n.m.r. signals of ethylene-1-octene and ethylene-4-methyl-1-pentene copolymers, members of the linear low-density polyethylene (LLDPE) family, are presented. The equations relating signal intensities to the monomer sequences are given. Using these equations, the characterization of these copolymers by the triad monomer sequences is possible. From the analysis by the triad sequence, it is suggested that these 1-olefins have a tendency to be present isolated in the copolymer chain.     

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.     

Solubility parameters of ethene/1-octene random copolymers

Random copolymers of ethene with 1-octene were investigated by means of pressure-volume-temperature (PVT) measurements. Solubility parameters were calculated from the thermal expansion coefficients and isothermal compressibilities. While expansion coefficients are virtually independent of 1-octene content, compressibilities decrease with increasing 1-octene incorporation. The solubility parameters varied between 17.3 and 15.2 MPa^1/2 when 1-octene incorporation was varied between 2 and 100 wt%. In the framework of the solubility parameter concept copolymers containing less than 95 wt% 1-octene were shown to be immiscible with isotactic poly(propene). Received: 30 October 1997/Revised version: 18 December 1997/Accepted: 18 December 1997     

Tensile behaviour of homogeneous ethylene copolymers

The tensile behaviour of homogeneous ethylene–butene copolymer and heterogeneous ethylene–butene copolymer has been compared in uniaxial tension. The homogeneous copolymer exhibits a more marked double yield behaviour. Composition distribution and heat treatment greatly influence the double yield behaviour of ethylene copolymer. A more homogeneous composition distribution has the same effect as rising draw temperature and decreasing draw rate on the double yield behaviour of ethylene copolymers. It is found that the occurrence of the double yield phenomenon is distinguished by the difference between two yield strains. Metallocene‐based ethylene copolymer shows better tensile break properties than conventional ethylene copolymer. © 2002 Society of Chemical Industry     

Does the length of the short chain branch affect the mechanical properties of linear low density polyethylenes? An investigation based on films of copolymers of ethylene/1-butene, ethylene/1-hexene and ethylene/1-octene synthesized by a single site metallocene catalyst

Three nearly identical linear low density polyethylene resins based on copolymers of ethylene with 1-butene (B), 1-hexene (H) and 1-octene (O) were utilized to investigate the effect of short chain branch length on the mechanical properties of blown and compression molded (quenched and slow cooled) films. The content of short chain comononer in the three copolymers was ca. 2.5-2.9 mol% that corresponded to a density of 0.917-0.918 g/cm³. Within a given series, the tensile properties of these films do not show any significant difference at slow deformation rates (up to 510 mm/min), even though the DSC and TREF profiles of ‘H’ and ‘O’ differed slightly in comparison to ‘B’. However, at higher deformation rates (ca. 1 m/s), the breaking strength of these films was found to increase with increasing short chain branch length. In addition, the Spencer impact and Elmendorf tear strength of the blown films were also observed to increase with increasing short chain branch length. Further, dart impact strength and high-speed puncture resistance (5.1 m/s) of 1-octene and 1-hexene based samples was also observed to be higher than that based on 1-butene. The blown films displayed low and comparable levels of equivalent in-plane birefringence and crystalline orientation by wide angle X-ray scattering. This confirms that the differences in mechanical properties in the blown film series are not attributable to differences in molecular orientation. The deformation behavior of both the compression molded and blown films were also investigated in a well-defined controlled regime by analyzing their essential work of fracture. It was found that the essential work of fracture of films based on 1-hexene and 1-octene was higher than that of films based on 1-butene. While the origin of these differences in mechanical properties with increasing short chain branch length is not fully understood, the present investigation confirms this effect to be pronounced at high deformation rates for both the blown and compression molded quenched films.     

用13C-NMR表征高乙烯含量乙烯-丙烯无规共聚物的结构与性能

采用淤浆聚合法,使用3种聚丙烯催化剂制备了高乙烯含量乙烯-丙烯无规共聚物,并采用核磁共振碳谱对共聚物结构和性能进行了表征。结果表明:在相同聚合条件下,使用二醇酯化合物作为内给电子体的ND催化剂同已商业化的两种Ziegler-Natta催化剂CAT1和CAT2聚合得到的乙烯-丙烯无规共聚物中的乙烯摩尔分数接近,为75%~78%;使用ND催化剂聚合得到的乙烯-丙烯无规共聚物链段中乙烯-丙烯单元序列分布更均匀;使用ND催化剂聚合得到的乙烯-丙烯无规共聚物中橡胶相相对含量高于采用CAT1和CAT2催化剂。     

High resolution solution and solid state NMR characterization of ethylene/1-butene and ethylene/1-hexene copolymers fractionated by preparative temperature rising elution fractionation

Preparative TREF was used to fractionate 2 commercial LLDPE polymers. These polymers had similar MFI values, density and comonomer content, but differed in comonomer type, 1-butene vs 1-hexene. High resolution solution NMR and solid state NMR was used to characterize the copolymer fractions. Distinct differences in chemical composition distribution could be observed from solution NMR results, and these correlated well with solid state analyses. Conclusions regarding the molecular make-up and crystallization phenomena are made.     

Synthesis of 1-hexene/1,7-octadiene copolymers using coordination polymerization and postfunctionalization with triethoxysilane

Homo- and copolymerization of 1-hexene (H) and 1,7-octadiene (O) were done using two different catalysts 1,4-bis(2,6-diisopropylphenyl)acenaphthenediiminedibromo nickel (II) and rac-ethylenebis(indenyl)zirconium dichloride [rac-Et(Ind)₂ZrCl₂]. The metallocene catalyst showed higher activity than the nickel α-diimine catalyst in homo- and copolymerization. The ¹H NMR studies confirmed the formation of copolymers containing 8–47% of 1,7-octadiene. In the copolymerization of hexene and diene, as the amount of incorporated diene in the copolymers increased, their T _g increased. TGA results showed that thermal stability of the polymer increases with the increase of 1-hexene incorporation in the polymer chain. Finally 1-hexene/1,7-octadiene copolymers were functionalized by triethoxysilane in the presence of hexachloroplatinic acid. The ¹H NMR spectrum of the functionalized samples showed that the double bonds in the copolymer structure were completely eliminated. The DSC analysis showed higher T _gs for the functionalized copolymer. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48934.     

1H-NMR/13C-NMR studies of branched structures in PVC obtained at atmospheric pressure

Summary The ¹H-NMR-spectra of raw poly (vinyl chloride) obtained at atmospheric pressure (U-PVC) have revealed the presence of high concentrations of branches. The content of labile chlorine was determined by reaction with phenole in order to estimate the branch points with tertiary chlorine. The branch length of reductively dehalogenated U-PVC by¹³C-NMR analysis have provided evidence for both short chain branches including chloromethyl groups and 2.4-dichloro-n-butyl groups and long chain branching. For a number of U-polymers the total amount of branching ranges from 7.5 to 13.5/1000 C. The ¹³C-NMR measurements point to a ratio of methyl/butyl branches of 1 1 and short chains/long chains of 6 1 .     

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     

Comonomer enhancement effect of 1-hexene in ethylene copolymerization catalyzed over MgCl2/THF/TiCl4 catalysts

Homo- and copolymerization of ethylene were performed by using a catalyst system composed of TiCl₄/THF/MgCl₂ complex activated with AlEt₃ at 70°C and 3 atm. To investigate the effect of the compositional difference of the catalyst on the rates of homo- and copolymerization and on the reactivity in ethylene–hexene copolymerization, a series of six catalysts with different compositions (Mg/Ti = 0.4–16.5) were prepared by coprecipitation. The catalytic activity in ethylene polymerization increased sharply with the Mg/Ti ratio from 21 (Mg/Ti = 0.4) to 1477 kg PE/g-Ti h (Mg/Ti = 16.5). The activity in copolymerization with 1-hexene also increased with Mg/Ti ratio. The values of r₁ were 120, regardless of Mg/Ti ratios within the experimental error range. Enhancement of the polymerization rate by the addition of 1-hexene in the reaction medium was observed only for the catalysts of low Mg/Ti ratio. This unusual effect of 1-hexene on the polymerization rate was explained by chemical and physical processes that occurred during polymerization. © 1993 John Wiley & Sons, Inc.