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Understanding Catalyst Generations.

Ziegler-Natta catalyst generations for PE and PP are differentiated by activity and selectivity to control the isotacticity of monomers on the polymer chain, which determines polymer properties. Generations 1, 2, and 3 were used for both PE and PP. Gen 1 and 2, introduced in the 1950s and 1970s respectively, aren't used any more. They were precipitated solids of titanium tetrachloride without supports, activated by aluminum trichloride with diethyl aluminum chloride. Gen 2 was higher activity than Gen 1.

Gen 3, introduced in the 1980s and still in use, is also based on titanium chloride. It's the first with magnesium chloride support and internal electron donors--monoesters like ethyl benzoate and methyl-p-toluate--which greatly increase activity. Internal electron donors also control isotacticity in PP, or the orientation of propylene monomer "thumbs" on a polymer chain.

Generations 4 and 5 apply primarily to stereo specificity for PP. Gen 4 is an offshoot of Gen 3 introduced in 1990, using diesters like diisobutyl phthalate as internal donors and silanes like dicyclopentyl-dimethoxy silane (DCPDMS) as external donors for super high activity and high selectivity for isotactic PP. Gen 4 is still by far the most widely used catalyst for PP, accounting for 90 percent of production globally, Clariant says.

Gen 5 is based on the same support as Gen 4 but with "non-phthalate" internal donors like diethers or succinates and optionally used without external donors. Diether and succinate internal donors were patented in the 1990s, but not commercialized until the early 2000s after pressure from the European Union's Reach program (Registration, Evaluation, Authorization, and Restriction of Chemicals). Reach was adopted in 2006 because of concerns that phthalates are endocrine disruptors.

After that, the generations get fuzzy. Dow introduced a non-phthalate catalyst with a catechol dibenzoate internal donor in 2011, called Consista, with so much higher activity and isotacticity that Dow called it 6th generation. Grace, which bought the Consista catalyst business in 201 3, also markets it as "Generation 6." Competitor Clariant calls it Generation 5.5 since it's one among a number of high-activity non-phthalate catalysts.

Instead Clariant calls metallocene and single-site catalysts for high performance PE and PP Generation 6. Single-site catalysts aren't Ziegler-Natta chemistry, but after almost 30 years, they are undeniably the next important group of catalysts, particularly for PE. They started with metallocene catalysts with two cyclopentadiene rings, introduced in the late 1980s, and expanded into post-metallocene single-site catalysts in the 1 990s and early 2000s.

Post-metallocenes, also called "molecular" catalysts," are part-metallocene or non-metallocene like one cyclopentadiene ring and something else or no cyclopentadiene at all. If a metallocene catalyst molecule looks like a bee with stubby wings, post-metallocene catalyst molecules look like a butterfly.

The first metallocene catalysts made liquid EPDM for synthetic rubber in the 1980s. The first metallocenes for PEwere in 1991 from ExxonMobil and Dow followed by Mitsui Chemicals Inc. and Mitsubishi Chemical Corp. in Tokyo and a host of other companies. The first metallocenes for PP came in the mid-1990s from Hoechst AG in Germany, now part of LyondellBasell; ExxonMobil; Fina Inc., Dallas, Texas, now part of Total S.A. in France (for syndiotactic PP with monomer "thumbs" alternating on the polymer chain vs. isotactic PP, with thumbs on one side like a comb); Mitsui Chemicals; and Chisso polypropylene, now part of Mitsubishi Chemical.

Metallocene-made plastics were more difficult to process and more expensive, so they didn't catch on at first. Polymerizing with metallocene catalysts was also tricky for resin producers because any variation in impurities in the feedstock during polymerization could cause a loss of reaction. Post-metallocene catalysts, on the other hand, are lower cost and more efficient over a range of feedstock impurities. Metallocene and post-metallocene catalysts are both used with MAO cocatalyst or other boron-based activators like borates.

Both metallocenes and post-metallocenes make homopolymers and copolymers with homogeneous comonomer distribution and narrow MWD with a wide range of comonomer content and comonomer sizes, i.e. C4-C10 or higher. They can also make complex stereo regular heterophasic copolymers with ethylene, propylene, and a third monomer like butene into bimodal and trimodal polymers with broader MWD. Single-site and Ziegler-Natta catalysts aren't compatible, but can be used in sequence.

Single-site catalysts still account for very little global polyolefin production, primarily LLDPE. "Bottom line: sixth-generation is not yet an accepted term and not well-defined. Including all metallocenes in it is not correct," says Kenneth Sinclair, principal of polymer consultant STA+Research in Lucey, Wash. "I think it will eventually be defined to include the latest advanced single-site catalyst systems as they become more widely used."
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Publication:Plastics Engineering
Date:May 1, 2018
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