Coordination polymerization high density polyethylene

Solution polymerization is of limited commercial utihty in free-radical polymerization but finds ready applications when the end use of the polymer requires a solution, as in certain adhesives and coating processes [i.e., poly(viityl acetate) to be converted to poly(viityl alcohol) and some acryhc ester finishes]. Solution polymerization is used widely in ionic and coordination polymerization. High-density polyethylene, poly butadiene, and butyl rubber are produced this way. Table 10.2 shows the diversity of polymers produced by solution polymerization, while Figure 10.2 is the flow diagram for the solution polymerization of vinyl acetate. 

We make polyethylene resins using two basic types of chain growth reaction free radical polymerization and coordination catalysis. We use free radical polymerization to make low density polyethylene, ethylene-vinyl ester copolymers, and the ethylene-acrylic acid copolymer precursors for ethylene ionomers. We employ coordination catalysts to make high density polyethylene, linear low density polyethylene, and very low density polyethylene. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

Ethylene Polymers. Depending on the polymerization conditions, three major types of polyethylene are manufactured low-density polyethylene (LDPE) by free-radical polymerization, linear low-density polyethylene (LLDPE) by copolymerization of ethylene with terminal olefins, and high-density polyethylene (HDPE) by coordination polymerization. The processes yield polymers with different characteristics (molecular weight, molecular weight distribution, melt index, strength, crystallinity, density, processability). 

Strong interest in late transition metal olefin polymerization catalysts resulted in the development of new five-coordinate Fe and Co systems (69) that afford highly linear, crystalline, high-density polyethylene.587-589 A new class of single-component, neutral Ni catalysts based on salicylaldimine ligands (70) was reported to be active in the polymerization of ethylene 590,591... 

Several commercial processes are used to produce high-density polyethylene. All employ more moderate pressures and most also use lower temperatures than the low-density polyethylene processes. The Ziegler-developed process uses the mildest conditions, 200-400 kPa (2 atm) and 50-75°C, to polymerize a solution of ethylene in a hydrocarbon solvent using a titanium tetrachloride/aluminum alkyl-based coordination catalyst. After quenching the polymerized mixture with a simple alcohol, the catalyst residues may be removed by extraction with dilute hydrochloric acid or may be rendered inert by a proprietary additive. The product is almost insoluble in the hydrocarbon solvent, so is recovered by centrifuging and drying. The final product is extruded into uniform pellets and cooled for shipping to fabricators. 

The three major classes of polyethylene are described by the acronyms HOPE. LDPE. and LLDPE. High-density polyethylene (HOPE) is a linear, semicrystalline ethylene homopolymer Tm 135 °C) prepared by Ziegler—Natta and chromium-based coordination polymerization technology. Linear low-density polyethylene (LLDPE) is a random copolymer of ethylene and a-olefins (e.g.. 1-butene. 1-hexene, or... 

The period following the Second World War saw the emergenee, with an accelerated speed, of new polymerization methods in 1953-1954, polymerization catalysis by coordination was developed by K. Ziegler and G. Natta (Nobel Prize, 1963), which led to for high-density polyethylene (PEHD) and polypropylene (PP). Anionic polymerization and the concept of living polymerization proposed by M. Szwarc in 1956 led to the design of blocks copolymers and the first macromolecular architectures. We then saw the emergence of catalysis by metallocene in 1980 by W. Kaminski. Radical polymerization controlled by M. Sawamoto and K. Matyjaszewski in 1994 combined the benefits of radical and ionic polymerization without the drawbacks of the former. 

Bonduel et al prepared PE-coated MWCNTs by in situ coordination polymerization, using bis(pentamethyl-ri 5-cyclopentadienyl)zirconium(IV) dichloride (Cp2 ZrCl2) as a typical polymerization catalyst from methylalu-minoxane (MAO)-functionalized nanotubes at 50 °C, 2.7 bar for 1 h. The grafted PE amount was 72 wt%. The PE-grafted CNTs could be well dispersed in high-density polyethylene (HDPE) matrix by melt blending. 

Even with the same monomer, the properties of a polymer can vary significantly depending on how it is prepared. Free-radical polymerization of ethylene gives low-density polyethylene coordination polymerization gives high-density polyethylene. The properties are... 

The coordinative (stereospecific) polymerization mechanism differs from the previous ones. It caused a real revolution in the polymer world when developed in the 1950s by the scientists Ziegler (in Germany) and Natta (in Italy)—both Nobel Prize Laureates in 1963. The principle here is the use of specific catalysts that orient the mers in the chain into a highly ordered configuration. By this mechanism, ethylene forms a linear (branchless) chain, so-called high density polyethylene (specific mass 0.95 to 0.96) which was developed by Ziegler. 

With respect to titanium catalysts for polymerization of ethylene or propylene, Ziegler synthesized the first high-density polyethylene and Natta prepared isotactic polypropylene by means of coordination catalysts about 50 years ago. The preparation methods of catalysts have been studied extensively. These TiCL3 catalysts have very high activity for homopolymerization of ethylene and propylene, whereas, they exhibit low activity for random copolymerization of ethylene with propylene when compared to vanadium catalysts. Refer to Ziegler-Nata Catalyst, Vanadium Catalysts, and EP Terpolymer, (Source Elastomer Technology Handbook, N. P. Cheremisinoff - editor, CRC Press, Boca Raton, Florida, 1993). 

High-density polyethylene (HDPE) is made with Ziegler-Natta (Z-N) catalyst systems. It has a totally different structure from that obtained by radical polymerization in having a much lower degree of branching (0.5-3 vs. 15-30 side chains per 500 monomer units). Chain transfer to polymer is not possible in coordination polymerization. 

The polymerization processes used in the synthesis of both isotactic polypropylene (i-PP) and high density polyethylene (HOPE) involve the use of transition-metal catalysts called Ziegler-Natta catalysts, which utilize a coordination type mechanism reaction. 

Around 1950, several groups independently discovered ionic catalysts that were able to produce linear polymers from ethylene, and this led to the development of a commercial process to make high-density polyethylene (HDPE). Compared to LDPE, linear polyethylene has a higher crystallinity in the solid phase and is thus harder and stronger. Such catalysts have come to be called complex coordination catalysts (CCC). Since initiation occurs only at active catalyst sites, the polymerization is of the step-reaction type. And because each particle contains multiple reactive sites having different reactivities, these catalysts yield polymers with rather broad molecular weight distributions. 

Chien, J. C. W., Coordination Polymerization , Academic Press, New York, 1975. A memorial to Ziegler who discovered the catalyst system that made it possible to polymerize ethylene at low pressure to linear, high density polyethylene. 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

If the monomer and polymer are not mutually soluble, the bulk reaction mixture will be heterogeneous. The high pressure free radical process for the manufacture of low density polyethylene is an example of such reactions. This polyethylene is branched because of self-branching processes illustrated in reaction (6-89). Branches longer than methyls cannot fit into the polyethylene crystal lattice, and the solid polymer is therefore less crystalline and rigid than higher density (0.935-0.96 g cm ) species that are made by coordination polymerization (Section 9.5). 

The high-density polymer is virtually linear and can be obtained from ethylene using a coordination type catalyst (alkyl aluminum and TiCU) or by polymerization on a metal oxide catalyst. The structure of both polymers is usually indicated by the structure (-CH2CH2-)n, although the branching in the low density polymer implies the presence of -CH< groups. The same type of bonds may be present in crosslinked polyethylene. 

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