Ethylene a olefin

Densities and crystallinities of ethylene—a-olefin copolymers mosdy depend on their composition. The classification ia Table 1 is commonly used (ASTM D1248-48). VLDPE resias are usually further subdivided iato PE plastomers of low crystallinity, 10—20%, with densities ia the range of 0.915—0.900 g/cm, and completely amorphous PE elastomers with densities as low as 0.860 g/cm. ... 

Commercial production of PE resias with densities of 0.925 and 0.935 g/cm was started ia 1968 ia the United States by Phillips Petroleum Co. Over time, these resias, particularly LLDPE, became large volume commodity products. Their combiaed worldwide productioa ia 1994 reached 13 X 10 metric t/yr, accouatiag for some 30% market share of all PE resias ia the year 2000, LLDPE productioa is expected to iacrease by 50%. A aew type of LLDPE, compositioaaHy uniform ethylene—a-olefin copolymers produced with metallocene catalysts, was first introduced by Exxon Chemical Company in 1990. The initial production volume was 13,500 t/yr but its growth has been rapid indeed, in 1995 its combiaed production by several companies exceeded 800,000 tons. 

Most Kaminsky catalysts contain only one type of active center. They produce ethylene—a-olefin copolymers with uniform compositional distributions and quite narrow MWDs which, at their limit, can be characterized by M.Jratios of about 2.0 and MFR of about 15. These features of the catalysts determine their first appHcations in the specialty resin area, to be used in the synthesis of either uniformly branched VLDPE resins or completely amorphous PE plastomers. Kaminsky catalysts have been gradually replacing Ziegler catalysts in the manufacture of certain commodity LLDPE products. They also faciUtate the copolymerization of ethylene with cycHc dienes such as cyclopentene and norhornene (33,34). These copolymers are compositionaHy uniform and can be used as LLDPE resins with special properties. Ethylene—norhornene copolymers are resistant to chemicals and heat, have high glass transitions, and very high transparency which makes them suitable for polymer optical fibers (34). 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

The ability to control the polymer from the design of the catalyst, coupled with high catalytic efficiency has led to an explosion of commercial and academic interest in these catalysts. Exxon started up a 30 million lb/5rr ethylene copol3rmer demonstration plant in 1991 using a bis-cyclopentadienyl zirconium catalyst of structure 1. The Dow Chemical Company (Dow) began operating a 125 million Ib/yr ethylene/l-octene copolymer plant in 1993 and has since expanded production capacity to 375 million Ib/yr. This paper will focus on the structure / property relationships of the catalysts used by Dow to produce single-site ethylene a-olefin copolymers. 

These are some key advantages that the metallocene catalysts have over conventional Ziegler-Natta catalysts and hence it is highly probable that inter-and intra-chain heterogeneity expected in ethylene-a-olefins copolymers can be controlled through the use of the metallocene system. 

Ethylene-alkyne metathesis, 26 955-956 Ethylene-a-olefin copolymers,... 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Figure 9. Relative scission efficiencies of short-chain branches in the 7 radiolysis of ethylene-a-olefin copolymers. Branch types are indicated.

Table IV. Yields of Isobutene and 1-Butene from Irradiation of Ethylene-a-Olefin Copolymers at 150°C...

The selective orientation may indicate a steric hindrance from the branch. Internal olefins like 2-butene tend to be poisons. They adsorb strongly but do not copolymerize to any significant extent. 2-Methylpropene is not very reactive either. In the absence of ethylene, a-olefins can be polymerized over Cr/silica but their reactivity is much lower than that of ethylene. Sometimes adding a-olefins to the reactor will improve activity, not because they are more reactive monomers, but because they are better reducing agents. 

Densities and crystallinities of ethylene-a-olefin copolymers mostly depend on their composition. The classification in Table 4 is commonly used (ASTM D1248-48). 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

Michelotti, M., Altomare, A. and Ciardelli, F., Ethylene/a-Olefins Cooligomerization versus Copolymerization by Zirconocene Catalysts , Polymer, 37, 5011-5016 (1996). 

Also, a series of well-defined polyolefins, including perfectly branched polyethylene and ethylene/a-olefin copolymers, have been synthesised via acyclic diene metathesis polycondensation [scheme (28)] [47] these well-defined polyolefins have been designed to model the crystallisation of polyethylene and olefin copolymers. 

Copolymerization Parameters for Ethylene/a-Olefin Copolymerization with Various Metallocene/MAO Catalysts 1... 

Monocyclopentadienyl metallocene catalysts, (VII), not requiring methylalu-moxane as a co-catalyst were prepared by Canich [6] and used to prepare ethylene/a-olefin copolymers. 

Maleic anhydride grafted ethylene-a-olefin copolymer... 

Ethylene-styrene interpolymers exhibit a novel balance of properties that are uniquely different from polyethylenes and polystyrenes. In contrast to other ethylene-a-olefin copolymers, ESI display a broad range of material response ranging from semicrystalline, through elastomeric to amorphous. The styrenic functionality and unique molecular architecture of ESI are postulated to be the basis of the versatile material attributes such as processability (shear thinning, melt strength and thermal stability), viscoelastic properties, low-temperature toughness and broad compatibility with other polymers, fillers and low molecular weight materials. 

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