Coordination polymerization

Coordination polymerisation is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center.^[1][2] The development of this polymerization technique started in the 1950s with heterogeneous Ziegler-Natta catalysts based on titanium tetrachloride and an aluminium co-catalyst such as methylaluminoxane. 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).

Polymerizations catalysed by metallocenes occur via the Cossee-Arlman mechanism. The active site is usually anionic but cationic coordination polymerization also exists.

In many applications Ziegler-Natta polymerization is succeeded by metallocene catalysis polymerization. This method is based on homogeneous metallocene 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 ethylene and propylene. The development of coordination polymerization that enables copolymerization with polar monomers is more recent.^[3] Examples of monomers that can be incorporated are methyl vinyl ketones^[4] methyl acrylate^[5] and acrylonitrile.^[6]

See also

• Cossee-Arlman mechanism
• Ziegler–Natta catalyst
• Polymerization
• Coordination bond

References