Polyethylene coordination polymerization

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 presence of long chain branches in low density polyethylene (LDPE) accounts for the difference in properties e.g. higher melt strength, greater toughness for the same average molecular weight) between LDPE and linear low density polyethylene (LLDPE, made by coordination polymerization). 

Transition metal catalysis plays a key role in the polyolefin industry. The discovery by Ziegler and Natta of the coordination polymerization of ethylene, propylene, and other non-polar a-olefins using titanium-based catalysts, revolutionized the industry. These catalysts, along with titanium- and zirconium-based metallocene systems and aluminum cocatalysts, are still the workhorse in the manufacture of commodity polyolefin materials such as polyethylene and polypropylene [3-6],... 

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. 

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). 

Polyolefins. Ultra-High-Molecular-Weight Polyethylene.TIus is easily made by conventional low-pressure coordination polymerization. Hercules Hi-fax 1900 has a molecular weight of 2.5-5.0 million. Its outstanding properties are low coefficient of friction (0.11) abrasion-resistance superior to nylon, polyurethane, and steel unbreakable in the Izod notched impact test and high resistance to most inorganic and many organic chemicals. 

Terminally brominated PE as PE macroinitiator can be produced by other methods. It has been reported that vinyl terminated PE produced by a bis(phenoxy-imine)metal complex and MAO catalyst system (Mn = 1800, Mw/Mn = 1.70) was converted to terminally 2-bromoisobutyrate PE through the addition reaction of 2-bromoisobutyric acid to the vinyl chain end. Polyethylene-Wodc-poly( -bulyl acrylate) (PE-fo-PnBA) from terminally brominated PE by ATRP procedure has also been produced [68]. It was reported that degenerative transfer coordination polymerization with an iron complex can be used to prepare terminally brominated PE as a macroinitiator [69]. A Zn-terminated PE prepared using an iron complex and diethylzinc,... 

The crystallinity of polyethylene prepared by coordination polymerization can be decreased in a controlled fashion by the addition of a small amount of another alkene, such as 1-pentene. Show the structure of the resulting polymer and explain why its crystallinity is reduced. 

High-molecular-weight polymers of polyethylene oxide of MW > 100,000 are prepared by a coordination polymerization with alkali earth metal compounds. These catalysts activate the epoxide ring by forming complexes with the oxygen in the oxide with the alkali earth metal ... 

Poly(e-caprolactone) (PCL) is synthesized by anionic, cationic or coordination polymerization of e-caprolactone. Degradable block copolymers with polyethylene glycol, diglycolide, substituted caprolactones and /-valerolactone can also be synthesized. Like the lactide polymers, PCL and its copolymers degrade both in vitro and in vivo by bulk hydrolysis, with the degradation rate affected by the size and shape of the device and additives. 

Coordination polymerizations are often accompanied by isomerization. By means of the ternary catalytic system VC14, (acac)3Fe, and Et3Al, propene can yield crystalline polyethylene and the amorphous ethylene—propene copolymer. Many more such cases have, of course, been observed. Probably of greatest importance are those where a non-polymerizing 2-alkene is is-omerized to 1-alkene prior to propagation [355]. 

Very highly branched polymers, like polyethylene made by free-radical, high-pressure processes, will have Mw/Mn ratios of 20 and more. Most polymers made by free-radical or coordination polymerization of vinyl monomers have ratios of from 2 to about 10. The M /M ratios of condensation polymers like nylons and thermoplastic polyesters tend to be about 2, and this is generally about the narrowest distribution found in commercial thermoplastics. 

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). 

Refers to linear low-density polyethylene, also produced by coordination polymerization at low pressures. It is not a homopolymer of ethylene see text for details. 

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... 

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. 

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. 

In a recent study, Yu et al. combined the palladium-diimine-catalyzed metallocene polymerization and ATRP to synthesize polyethylene-h-polystyrene and polyethylene-h-poly(butyl acrylate) [165]. A relatively new coordination olefin polymerization method - degenerative transfer coordination polymerization - was recently combined with ATRP to prepare block and graft copolymers with linear polyethylene segments [166-169]. 

To better understand the fundamental and practical differences between step-growth polymerization and chain-growth polymerization (see Table 1.4), consider the industrial chain-growth polymerization of ethylene (by either coordination polymerization or high-pressure free-radical polymerization) to produce polyethylene. 

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