High density polyethylene, HDPE coordination polymerization

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). 

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. 

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. 

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. 

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. 

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