Coordination polymerization

Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes.

Coordination polymerization started in the 1950s with heterogeneous Ziegler–Natta catalysts based on titanium tetrachloride and organoaluminium co catalysts. The mixing of TiCl4 with trialkylaluminium complexes produces Ti(III) containing solids that catalyze the polymerization of ethene and propene. The nature of the catalytic center has been of intense interest but remains uncertain. Many additives and variations have been reported for the original recipes.

In some applications heterogeneous Ziegler–Natta polymerization has been superseded by homogeneous catalysts such as the Kaminsky catalyst discovered in the 1970s. The 1990s brought forward a new range of post metallocene catalysts. Typical monomers are nonpolar ethene and propene. The development of coordination polymerization that enables copolymerization with polar monomers is more recent. Examples of monomers that can be incorporated are methyl vinyl ketones, methyl acrylate, and acrylonitrile. Kaminsky catalysts are based on metallocenes of group 4 metals (Ti, Zr, Hf) activated with methylaluminoxane (MAO). Polymerizations catalysed by metallocenes occur via the Cossee–Arlman mechanism. The active site is usually anionic but cationic coordination polymerization also exists.

Many alkenes do not polymerize in the presence of Ziegler–Natta or Kaminsky catalysts. This problem applies to polar olefins such as vinyl chloride, vinyl ethers, and acrylate esters.

The annual production of polybutadiene is 2.1 million tons (2000). The process employs a neodymium based homogeneous catalyst.

Coordination polymerization has a great impact on the physical properties of vinyl polymers such as polyethylene and polypropylene compared to the same polymers prepared by other techniques such as free radical polymerization. The polymers tend to be linear and not branched and have much higher molar mass. Coordination type polymers are also stereoregular and can be isotactic or syndiotactic instead of just atactic. This tacticity introduces crystallinity in otherwise amorphous polymers. From these differences in polymerization type the distinction originates between low density polyethylene (LDPE), high density polyethylene (HDPE) or even ultra high molecular weight polyethylene (UHMWPE).