Copolymers of ethylene with a-olefins

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... 

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)... 

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. 

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. 

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... 

Chain Structure. LLDPE resins are copolymers of ethylene and a-olefins with low a-olefin contents. Molecular chains of LLDPE contain units derived both from ethylene, —CH2—CH2—, and from the a-olefin, —CH2—CHR—, where R is C2H for ethylene—1-butene copolymers, for... 

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). 

Random copolymers of ethylene and a-olefins (l-aUcenes) can be obtained with Ziegler-Natta catalysts, the most important being those of ethylene and 1-butene (LLDPE) and of ethylene and propylene (EPM or EPR and EPDM). Some reactivity ratios are listed in Table 9.5. Tha ratios vary with the nature and physical state of the catalyst and in most instances, r r2 is close to unity. However, all these values show that ethylene is much more reactive than higher alkenes. Copolymers produced using Ziegler-Natta catalysts usually have a wide range of compositions. This may be due to the presence of different active sites in the catalyst giving rise... 

The motivation behind the molecular design of Nodax class PHA copolymers closely follows that of the well-known industrial polyolefin linear low density polyethylene (LLDPE). LLDPE is a random copolymer of ethylene with a small amount of a-olefin units, such as 1-butene or 1-hexene, which will result in the formation of the polymer chain structure with mcl alkyl side group branches. In a similar manner, one can envision the possibility of creating a polymer structure of LLDPE with a PHA backbone having short alkyl side chains, as depicted in Fig. 2. 

The synthesis of copolymers of ethylene and a-olefins is one of the important commercial advances made possible by the development of homogeneous olefin polymerization catalysts. The difference in reactivity between ethylene and a-olefins with many homogeneous catalysts is often one or two orders of magnitude. Thus, the controlled synthesis of copolymers comprised of these simple molecules has been challenging. 

A different approach of modeling ethylene-based copolymers is presented using acyclic diene metathesis (ADMEl) polymerization. Monomer s)mtiiesis dictates final polymer structure due to step-growth chemistry yielding only to olefin metathesis. While copolymerization of ethylene with a-olefins produces random distribution of alkyl branches along the PE backbone, polymerization of one kind of symmetric macromonomer produces PE with a perfectly known primary structure, as seen in Figure 1. 

Linear low-density polyethylene (LLDPE) is a copolymer of ethylene and a-olefins (generally 1-butene, 1-hexene, or 1-octene) with densities in the range 0.915-0.94 g cm . Products with even lower densities, down to 0.88 g cm, are sometimes called very low-density polyethylene (VLDPE) but are chemically identical to LLDPE. Copolymerization of ethylene with increasing amounts of a-olefins disrupts the order of the linear polyethylene chains by introducing short-chain branches. As a consequence, the density, crystallinity, and rigidity of LLDPE are lower than for HD PE. Linear low-density polyethylene is used predominantly in films, and shares the market with LDPE. 

This material, with a density down to 0.88 Mg m , is believed to be a copolymer of ethylene with a higher olefin. No pattern of applications seems to have been established. 

Polyethylene with density lower than 0.915 g/cm is generally regarded as a distinct type of polyethylene. Polymers with densities ranging between 0.890 and 0.915 g/cm are commercially available. These polymers are copolymers of ethylene and a-olefins and are made by processes similar to those described... 

Modeling of the melt viscosity of polyethylene and random copolymers of ethylene and a-olefins has been extensively dealt with in the past. Empirical viscosity models of the form of the Generalized Cross/Carreau models can and have been fitted to viscosity data for INSITE Technology Polymers. The shear viscosity data is usually measured at 190 °C in the molten regime from 0.1 - 100 rad/s. Of the several models available, the Cross model provides a good fit to the data with a minimum number of fitting parameters. The Cross model is of the form ... 

Ziegler-Natta catalysts were also designed to synthesize polyethylene (HDPE) and copolymers of ethylene with longer chain a-olefins (n-butene,... 

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