Branch distribution ethylene-hexene copolymers

Eig. 1. Melting curves (dsc) of two ethylene—1-hexene copolymers produced in a gas-phase process one with a uniform branching distribution (1-hexene content 2.5 mol %) and another with a nonuniform branching distribution (1-hexene content 2.8 mol %). 

Lower activation temperatures are usually preferred in commercial operations, despite the lower activity. The corresponding redistribution of sites results in improved polymer physical properties. An example is shown in Table 12, which is a list of the ESCR values, the resistance to chemical attack, of several ethylene-butene and ethylene-hexene copolymers. These copolymers were made at constant MI and density with Cr/ silica catalysts activated at various temperatures. The measured ESCR value decreases as the activation temperature is raised. This change results in part because the MW distribution is narrowed, but more so because the longest chains contain less branching.13... 

As noted in Section 6.9, when added to the reactor in trace amounts, some polar compounds can selectively inhibit some sites in preference to others. In addition to affecting the MW distribution, they sometimes have an influence on the branch content and distribution as well. An example of this behavior is shown in Figure 44. Ethylene-hexene copolymers were produced with a Cr/silica-titania catalyst activated at 850 °C. Methanol was added to the polymerization reactor in incrementally greater amounts to "titrate" the activity of the catalyst. As expected, the activity declined as methanol was added, reaching nearly zero at about three CH3OH molecules/Cr atom. The average polymer MW increased as methanol was added. The results imply that some sites, those more... 

The general structure of linear low density polyethylene is shown in Fig. 18.2 c). Linear low density resins are copolymers of ethylene and 1-alkenes principally 1-butene, 1-hexene, and 1-octene. Comonomer levels range from approximately 2 to 8 mole %. This family of polyethylene is widely known as LLDPE. Linear low density polyethylenes are polydisperse with regard to molecular weight and branch distribution. 

Chu K-J, Soares IBP, Penlidis A (2000) Variation of molecular weight distribution (MWD) and short chain branching distribution (SCBD) of ethylene/1-hexene copolymers produced with different in-situ supported metallocene catalysts. Macromol Chem Phys 201 340-348... 

In an experiment to determine if this type of catalyst may have properties required for industrial applications, Jolly et cd. carried out a polymerization at 80°C with catalyst 3. The results showed that this particular catalyst exhibited an activity of 50,750 Kg PE/mol Cr/hr with a constant polymerization rate for an extended period, thus indicating very beneficial polymerization properties for commercial applications. In addition, catalyst 6 was evaluated in ethylene/1-hexene copolymerization experiments. In neat 1-hexene an elastomeric material was isolated with an activity of 40,000 Kg PE/mol Cr/ hr under polymerization conditions described in Table 3.12, while in toluene containing 20 vol% 1-hexene, an ethylene/1-hexene copolymer was isolated with a DSC melting point of 112°C, suggesting a LLDPE material (i.e., density of ca. 0.91-0.92 g/cc) was produced with a homogeneous branching distribution, which is expected based on the polydispersity value of 1.58 for the same catalyst in an ethylene homopolymerization experiment. This copolymerization data also shows that this particular catalyst has necessary copolymerization characteristics for industrial applications. 

The xmiform branching distribution found in this new type of polyethylene, and the relatively high reactivity of these catalysts with higher 1 -olefins, has allowed the manufacture of ethylene/1-olefin copolymers containing very high levels of comonomer such as 1-butene and 1-hexene, i.e., 5-20 mol% comonomer. Consequently, the density range of the polyethylene... 

Figure 4.12 Density and 1-hexene content of ethylene/1-hexene copolymers with a heterogeneous branching distribution (—) and homogeneous (solid line) branching distribution [41].

Figure 4.14 DSC melting point data for ethylene/l-hexene copolymer with a hetergeneous branching distribution [41].

Figure 7.8 DSC melting curves of two ethylene/1-hexene copolymers. The homogeneous (uniform) branching distribution polymer contains 2.5 mol% 1-hexene and the heterogeneous (non-uniform) branching distribution polymer contains 2.8 mol%...

Amer and van Reenen [39] fractionated isotactic polypropylenes by TREE to get fractions with different molar masses but similar tacticities. The DSC results of the fractions indicated that the crystallization behaviour is strongly affected by the configuration (tacticity) and the molar mass of the PP. Soares et al. [40] proposed a new approach for identifying the number of active catalyst sites and the polymer chain microstructural parameters produced at each active site for ethylene/l-olefin copolymers synthesized with multiple-site catalysts. This method is based on the simultaneous deconvolution of bivariate MMD/CCD, which can be obtained by cross-fractionation techniques like SEC/TREE or TREE/SEC. The proposed approach was validated successfully with model ethylene/1-butene and ethylene/ 1-octene copolymers. Alamo and co-workers [41] studied the effects of molar mass and branching distribution on mechanical properties of ethylene/1-hexene copolymer film grade resins produced by a metallocene catalyst Molar mass fractions were obtained by solvent/non-solvent techniques while P-TREE was used for fractionation according to the 1-hexene content. 

Shan, C. L., Chu, K-J., Soares, J. B. R, Penlidis, A. Structure-property characteristics of ethylene/ 1-hexene copolymers with tailored short chain branching distributions. Soc. Plastics Engrs. ANTBC Proceedings (2000), pp. 1616-1619... 

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

Linear low-density polyethylene (LLDPE)440-442 is a copolymer of ethylene and a terminal alkene with improved physical properties as compared to LDPE. The practically most important copolymer is made with propylene, but 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene are also employed.440 LLDPE is characterized by linear chains without long-chain branches. Short-chain branches result from the terminal alkene comonomer. Copolymer content and distribution as well as branch length introduced permit to control the properties of the copolymer formed. Improvement of certain physical properties (toughness, tensile strength, melt index, elongation characteristics) directly connected to the type of terminal alkene used can be achieved with copolymerization.442... 

In 1975 Mitsui Petrochemicals introduced metallocene-made LLDPE Tafiner , with controlled comonomer placement, but rather low MW. In 1991, Dow Plastics produced developmental quantities of ethylene copolymers with up to 25 mole % of butene, hexene or octene, Affinity resins. The use of a metallocene catalyst with a single cyclopentadiene ring, resulted in a certain degree of randomization of the polymerization process. The catalyst produced PP with narrow molecular weight distribution, and a long chain branching, similar to LDPE. 

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