Coordination polymerization HDPE

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). 

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. 

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). 

Complex 6.37 has been shown to be an active catalyst for the manufacture of HDPE. Note that 6.37 is a 15-electron complex with chromium in the formal oxidation state of 3 +. The mechanism of polymerization involves generation of coordinative unsaturation through the dissociation of a THF molecule from 6.37. The evidence for an oxidation state of 3+ in the commercial catalyst comes from the fact that complex 6.38 is active for polymerization. However, complex 6.39, identical to 6.38 in every respect except the oxidation state of the metal ion, is inactive. Note that the oxidation state of chromium in 6.39 is 2+. 

Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

Loop reactors combine the thermal characteristics of the tubular reactors with the residence time distribution of the CSTRs. HDPE and i-PP are produced in loop reactors using coordination catalysts by means of slurry polymerization [22]. HDPE uses isobutane as continuous phase (Chevron-PhilKps process) and i-PP uses the monomer as continuous phase (Spheripol process). 

Contrary to the polymerizations mentioned above, homopolymerization by coordination catalysis ( Ziegler, Phillips, etc. catalysts) leads to polymers that are almost perfectly linear and thus highly crystaUizable. They exhibit a very high density (HDPE) since their high degree of crystallinity ( 70%) confers upon them a volumlc mass that can reach 0.97 g cm . Moreover, Phillips HDPEs carry an unsaturation at the chain end which results from a spontaneous transfer 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. 

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