Polyethylene is made with three families of catalyst: Ziegler-Natta, Phillips chromium, and metallocene or other single-site catalysts. Between them they polymerise ethylene into all HDPE and LLDPE. LDPE is the exception: it is made at high pressure by free-radical initiators, with no catalyst at all.
On a polyethylene grade‘s datasheet, the catalyst family tells a buyer how broad the resin’s molecular weight distribution is and, in LLDPE, how evenly the comonomer is spread along the chains.
Ziegler-Natta Catalysts
A Ziegler-Natta catalyst for polyethylene is titanium chloride on a magnesium chloride support, activated by an aluminium-alkyl cocatalyst such as triethylaluminium.
Ziegler-Natta polyethylene has a broad molecular weight distribution. It counts as narrow only beside chromium-made polyethylene, which is broader still; single-site polyethylene is narrower than both.
Phillips Chromium Catalysts
A Phillips catalyst is chromium oxide on a silica or silica-alumina support. Phillips chromium catalysts make about half of the world’s HDPE.
Phillips polyethylene has a very broad molecular weight distribution, and its chains include long-chain branches. Phillips HDPE has an Mw/Mn above 15 to 20.
Metallocene and Single-Site Catalysts
A metallocene catalyst is a single organometallic molecule: a Group 4 metal (titanium, zirconium or hafnium) held between two cyclopentadienyl rings and activated with methylaluminoxane (MAO).
“Single-site” means every active centre in the catalyst is the same. The class also covers other well-defined catalysts that are not metallocenes.
Single-site polyethylene has a narrow molecular weight distribution with a single peak. Metallocene LLDPE is traded under the label mLLDPE.
Which Catalyst Is Used for HDPE, LLDPE and LDPE
Of the three polyethylene types, HDPE is made with Phillips chromium and Ziegler-Natta catalysts and LLDPE mostly with Ziegler-Natta catalysts, both by catalytic polymerisation at comparatively low temperature and pressure. LDPE is made by free-radical polymerisation at high pressure, with no catalyst. Ziegler-Natta catalyst residue stays in the polyethylene: since the high-activity magnesium-supported catalysts of the early 1970s, it is not removed.
HDPE
HDPE is made with both Phillips chromium and Ziegler-Natta catalysts. Chromium HDPE suits blow-moulded containers, fuel tanks and pipes.
Bimodal HDPE, with two molecular weight peaks, is made by running reactors in series or with two catalysts in a single reactor. “Bimodal” on a datasheet describes the molecular weight distribution those processes produce; it does not name a separate catalyst family.
LLDPE
Most LLDPE is made with supported Ziegler-Natta catalysts, and a smaller part with metallocene catalysts. Chromium catalysts make LLDPE only in modified form.
Ziegler-Natta and metallocene LLDPE differ in how the comonomer is spread. Ziegler-Natta LLDPE is a blend of chains with uneven comonomer content; in metallocene LLDPE the comonomer is spread more evenly.
LDPE
LDPE is made without a catalyst: its polymerisation is started by free-radical initiators, oxygen among them, fed into a high-pressure reactor. An initiator is used up as it starts each chain, which is why it is not counted as a catalyst.