Ethylene with a-Olefins

Many copolymers of ethylene with a-olefins are prepared commercially. Thus ethylene is copolymerized with butene-1, where a comonomer is included to lower the regularity and the density of the polymer. Many copolymers are prepared with transition metal oxide catalysts on support. The comonomer is usually present in approximately 5% quantities. This is sufficient to lower the crystallinity and to markedly improve the impact strength and resistance to environmental stress cracking. Copolymers of ethylene with hexene-1, where the hexene-1 content is less than 5%, are also produced for the same reason. 

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Heterogeneous Ziegler-Natta catalysts composed of titanium trichloride and alkylaluminum have been used to prepare block copolymers of ethylene with a-olefins 44-46), even though there is no known example of such a catalyst meeting the requirement for a living polymerization. The produced block copolymers have broad molecular weight distributions (Mw/Mn = 4 20) and are present in small concentrations... 

Copolymers of ethylene with a-olefins, such as the short-chain branched LLDPE (linear low-density polyethylene) impact materials or the EPD (ethylene-propylene-diene copolymer) rubbers represent major percentages of the total polyolefin production, due to their desirable mechanical properties. Solid-state MgCl2-supported Ziegler-Natta catalysts however, have unfavourable reactivity... 

Some products are sligthly modified by copolymerization of ethylene with a-olefins > InTHN at 135 C ... 

Medium Density High Density Polyethylene MDPE (or MDHDPE) is produced by copolymerization of ethylene with a-olefins using Ziegler-Natta, supported chromium or single site catalysts. MDPE cannot be produced by free radical polymerization. MDPE has a linear structure similar to LLDPE, but comonomer content is lower. Density is typically 0.93-0.94 g/cm. MDPE is used in geomembrane and pipe applications. 

As mentioned in Chapter 1, ethylene is always the more reactive olefin in systems used to produce copolymers involving a-olefins (LLDPE and VLDPE). An important process consideration for copolymerizations is the reactivity ratio. This ratio may be used to estimate proportions needed in reactor feeds that will achieve the target resin. However, fine tuning is often required to obtain the density or comonomer content desired. Reactivity ratios were discussed previously (Chapter 2) in the context of free radical polymerization of ethylene with polar comonomers. Reactivity ratios are also important in systems that employ transition metal catalysts for copolymerization of ethylene with a-olefins to produce LLDPE. Discussions of derivations and an extensive listing of reactivity ratios for ethylene and the commonly used a-olefins are provided by Krentsel, et al. (1). 

It is also possible to copolymerize ethylene with a-olefins such as propene, 1-butene, 1-pentene, 1-hexene, and 1-octene, forming linear low-density polyethylene (LLDPE). The product of copolymerization parameters V2 obtained by using ethylenebis(l-indenyl)zirconium dichloride (11) indicates random incorporation of the comonomer [38]. 

G.D. Bukatov, L.G. Yechevskaya, V.A. Zakharov, Copolymerization of ethylene with a-olefins by highly active supported catalysts of various composition, in W. Kaminsky, H. Sinn (Eds.), Transition Met. Organomet. Catal. Olefin Polym., Proc. Int. Symp., Springer, Berlin, Germany, 1988, pp. 101-108, Meeting Date 1987. 

This review covers families of catalysts based largely on chelating nitrogen-based ligands that are active for the homopolymerization of ethylene and the copolymerization of ethylene with a-olefins and polar comonomers. Late metal catalysts not active for ethylene polymerization are not included, but when a system is active for ethylene polymerizations, its activity for polymerizations beyond ethylene are included. Catalyst syntheses, structures, activities, and chain-growth mechanisms and the influence of these factors on the structures of the resulting polymers are discussed. 

Using metallocene catalysts it has proved possible to tailor the microstructure of the polymers by fine-tuning of the ligands. Besides polyethylene, it is possible to co-polymerize ethylene with a-olefins such as propylene, but-l-ene, pent-l-ene, hex-l-ene, and oct-l-ene, in order to produce LLDPE. In addition, many kinds of co-polymers and elastomers, and new structures of polypropylenes, polymers and co-polymers of cyclic olefins can be obtained. Furthermore, catalysts with chiral centers can be beneficial in stereospecific polymerization to build the desired isotactic products. 

The development of bifunctional catalysts for specific catalytic sequences of reactions in which the product of the first reaction can serve as substrate for the second is of great importance. There are many examples of such reactions. They are, for instance, the monomer-isomerizing polymerization of heptene-2, heptene-3 and 4-methyl-2-pentene and the combination of propene disproportionation with oligomerization, etc. Bifunctional catalysts are most widely used for ethylene copolymerization with a-butene in situ in the production of so-called low-density linear polyethylene (LDLPE). All general methods for LDLPE production are based on incorporation into a PE backbone of short-chain branches, which can be made by catalytic copolymerization of ethylene with a-olefins C3-C10. A macromolecnlar ligand offers wide possibilities of joining the different types of active site in the same matrix (see also Section 12.5.2). 

Boisson C, Bo5 ron O, Macko T, Cossoul E, Baverel Laetitia, Martigny E (2013) Homogeneous copolymers of ethylene with a-olefins S3mthesized with metallocene catalysts and their use as standards for TREF calibration. In Proceedings 4th international conference on polyolefin characterization, Houston, October 2012. Macromolecular Symposia (in press)... 

Copolymers of Ethylene with a-Olefins and Ethylene with Carbon Monoxide... 

The two monomers of major interest, styrene and ethylene, are well known and details can be found on all aspects of their technology elsewhere. Poly(ethylene-co-styrene) is primarily produced via solution polymerization techniques using metallocene catalyst/co-catalyst systems, analogous to the production of copolymers of ethylene with a-olefin monomers. Solvents that can be employed include ethyl-benzene, toluene, cyclohexane, and mixed alkanes (such as ISO PAR E, available from Exxon). The thermodynamic properties of poly(ethylene-co-styrene), including solvent interactions and solubility parameter assessments, are important factors in relation to polymer manufacture and processing, and have been reported by Hamedi and co-workers (41). 

Minick, J. Moet, A. Hiltner, A. Baer, E. Chum, S. P. Crystallization of very low density copolymers of ethylene with a-olefins. J. Appl. Polym. Sci. 1995, 58, 1371-1384. 

Polymers obtained by copolymerization of ethylene with a-olefins display diverse physical and chemical properties. Applications of these materials are directed related with extent of comonomer incorporated into the final polymer. More important is their thermal behavior which is dependent of the comonomer content and distribution.Synthesis of precisely placed methyl-branched PE was the first attempt in tiiese modeling studies. Continuation of this research led to the development of ethyl-branched PE. These polymers are ideal models in comparison with those obtained by Ziegler-Natta or metallocene chemistry using ethylene/ 1-propylene and ethylene/1-butene, respectively. 

Copolymers of ethylene with a-olefins also known as LLDPE are an important topic due to their enhanced mechanical properties, structural simplicity, and industrial importance. Copolymers of ethylene with 1 -butene, 1-hexene, and 1-octene, have been obtained by copolymerization via Ziegler-Natta and metallocene chemistry.While many studies deal with modification of the catalyst and optimization of the reaction s conditions, thermal behavior of LLDPE is more important for understanding the morphology of these materials.Inspired by the success obtained modeling EP copolymers via ADMET polymerization, extension of this investigation led us to the synthesis of PE with only ethyl branches precisely placed along the main backbone, or precisely sequenced... 

While previous studies containing precisely placed methyl- and ethyl-branched PE copolymers via ADMET are perfectly sequenced models of EP and EB materials, copolymers of ethylene with a-olefins are obtained in a statistically random fashion using Ziegler-Natta or metallocene chemistry. Although these materials can be obtained totally randomized, imperfections over the branch identity are usually incorporated into the PE backbone due to tile chain nature of this chemistry. Randomly branched copolymers with only one kind of branch identity can be created using ADMET, copolymerization followed by exhaustive hydrogenation of a methyl-branched a,(o-diene with an unbranched a,to-diene yields statistically random EP copolymers. " Randomness in tiie final material is assured since copolymerization of both diene monomers is carried out, and no electronically or sterically major... 

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