Coordination polymerization

Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes.^[1][2]

Types of coordination polymerization of alkenes

Heterogeneous Ziegler–Natta polymerization

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.^[3]

Homogeneous Ziegler–Natta polymerization

In some applications heterogeneous Ziegler–Natta polymerization has been superseded by homogeneous catalysts such as the Kaminsky catalyst discovered in the 1970s.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.^[4] Examples of monomers that can be incorporated are methyl vinyl ketones,^[5] methyl acrylate,^[6] and acrylonitrile.^[7]

Illustrative metallocene-based coordination catalysts

Kaminsky catalysts are based on metallocenes of group 4 metals (Ti, Zr, Hf) activated with methylaluminoxane (MAO).^[8][9] Polymerizations catalysed by metallocenes occur via the Cossee–Arlman mechanism. The active site is usually anionic but cationic coordination polymerization also exists.

Simplified mechanism for Zr-catalyzed ethene polymerization

Specialty monomers

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.^[10]

Butadiene polymerization

The annual production of polybutadiene is 2.1 million tons (2000). The process employs a neodymium-based homogeneous catalyst.^[11]

Principles

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).

Coordination polymerization of other substrates

Coordination polymerization can also be applied to non-alkene substrates. Dehydrogenative coupling of silanes, dihydro- and trihydrosilanes, to polysilanes has been investigated, although the technology has not been commercialized. The process entails coordination and often oxidative addition of Si-H centers to metal complexes.^[12][13]

Lactides also polymerize in the presence of Lewis acidic catalysts to give polylactide:^[14][15]

See also

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

References

1. Polymer science and technology (2000) Robert Oboigbaotor Ebewele
2. Kent and Riegel's handbook of industrial chemistry and biotechnology, Volume 1 2007 Emil Raymond Riegel, James Albert Kent
3. James C.W. Chien, ed. (1975). Coordination Polymerization A Memorial to Karl Ziegler. Academic Press. ISBN 978-0-12-172450-4.
4. Nakamura, Akifumi; Ito, Shingo; Nozaki, Kyoko (2009). "Coordination−Insertion Copolymerization of Fundamental Polar Monomers". Chemical Reviews. 109 (11): 5215–44. doi:10.1021/cr900079r. PMID 19807133.
5. Johnson, Lynda K.; Mecking, Stefan; Brookhart, Maurice (1996). "Copolymerization of Ethylene and Propylene with Functionalized Vinyl Monomers by Palladium(II) Catalysts". Journal of the American Chemical Society. 118: 267–268. doi:10.1021/ja953247i.
6. Drent, Eite; Van Dijk, Rudmer; Van Ginkel, Roel; Van Oort, Bart; Pugh, Robert. I. (2002). "Palladium catalysed copolymerisation of ethene with alkylacrylates: polar comonomer built into the linear polymer chainElectronic supplementary information (ESI) available: NMR data for entries 1, 9, 10, 12 and size exclusion chromatographic data for entries 1, 3, 8, 12". Chemical Communications (7): 744–745. doi:10.1039/b111252j. PMID 12119702.
7. Kochi, Takuya; Noda, Shusuke; Yoshimura, Kenji; Nozaki, Kyoko (2007). "Formation of Linear Copolymers of Ethylene and Acrylonitrile Catalyzed by Phosphine Sulfonate Palladium Complexes". Journal of the American Chemical Society. 129 (29): 8948–9. doi:10.1021/ja0725504. PMID 17595086.
8. Walter Kaminsky (1998). "Highly Active Metallocene Catalysts For Olefin Polymerization". Journal of the Chemical Society, Dalton Transactions (9): 1413–1418. doi:10.1039/A800056E.
9. Klosin, J.; Fontaine, P. P.; Figueroa, R. (2015). "Development of Group Iv Molecular Catalysts for High Temperature Ethylene-Α-Olefin Copolymerization Reactions". Accounts of Chemical Research. 48 (7): 2004–2016. doi:10.1021/acs.accounts.5b00065. PMID 26151395.
10. Eugene Y.-X. Chen (2009). "Coordination Polymerization of Polar Vinyl Monomers by Single-Site Metal Catalysts". Chem. Rev. 109 (11): 5157–5214. doi:10.1021/cr9000258. PMID 19739636.
11. Friebe, Lars; Nuyken, Oskar; Obrecht, Werner (2006). "Neodymium-Based Ziegler/Natta Catalysts and their Application in Diene Polymerization". Advances in Polymer Science. 204: 1–154. doi:10.1007/12_094. ISBN 978-3-540-34809-2.
12. Aitken, C.; Harrod, J. F.; Gill, U. S. (1987). "Structural studies of oligosilanes produced by catalytic dehydrogenative coupling of primary organosilanes". Can. J. Chem. 65 (8): 1804–1809. doi:10.1139/v87-303.
13. Tilley, T. Don (1993). "The Coordination Polymerization of Silanes to Polysilanes by a "σ-bond Metathesis" Mechanism. Implications for Linear Chain Growth". Accounts of Chemical Research. 26: 22–9. doi:10.1021/ar00025a004.
14. R. Auras; L.-T. Lim; S. E. M. Selke; H. Tsuji (2010). Poly(lactic acid): Synthesis, Structures, Properties, Processing, and Applications. Wiley. ISBN 978-0-470-29366-9.
15. Odile Dechy-Cabaret; Blanca Martin-Vaca; Didier Bourissou (2004). "Controlled Ring-Opening Polymerization of Lactide and Glycolide". Chem. Rev. 104 (12): 6147–6176. doi:10.1021/cr040002s. PMID 15584698.