The Cossee–Arlman mechanism in
polymer chemistry is the main pathway for the formation of C–C bonds in the
polymerization of
alkenes. The mechanism features an intermediate
coordination complex that contains both the growing polymer chain and the monomer (alkene). These
ligands combine within the
coordination sphere of the metal to form a polymer chain that is elongated by two carbons.[1]
The details of this mechanism can be used to explain the
stereoregularity of the
polymerisation of
alkenes using
Ziegler–Natta or
metallocenecatalysts. Stereoregularity is relevant for unsymmetrical alkenes such as
propylene. The
coordination sphere of the
metalligands sterically influences which end of the propylene attaches to the growing polymer chain and the relative stereochemistry of the methyl groups on the polymer. The stereoregularity is influenced by the ligands. For the
metallocene catalysts, the cyclopentadienyl ligands (or their surrogates) fulfill this role. For heterogeneous catalysts, the stereoregularity is determined by the surface structure around the active site on the catalyst particle, and can be influenced by additives such as
succinates or
phthalates, which tend to block specific sites, while leaving others (with different stereoreactivity) to catalyse the polymerization.
References
^Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010.
ISBN1-891389-53-X
Publications of historic interest
Allegra G. (1971). "Discussion on the Mechanism of Polymerization of a- Olefins with Ziegler–Natta Catalysts". Die Makromolekulare Chemie. 145 (3668): 235–246.
doi:
10.1002/macp.1971.021450119.
Cossee, P. J. (1964). "Ziegler–Natta catalysis I. Mechanism of polymerization of α-olefins with Ziegler–Natta catalysts". J. Catal. 3 (1): 80–89.
doi:
10.1016/0021-9517(64)90095-8.
Arlman, E. J. (1964). "Ziegler–Natta catalysis II. Surface structure of layer-lattice transition metal chlorides". J. Catal. 3 (1): 89–99.
doi:
10.1016/0021-9517(64)90096-X.
Arlman, E. J.; Cossee, P. (1964). "Ziegler–Natta catalysis III. Stereospecific polymerization of propene with the catalyst system TiCl3---AlEt3". J. Catal. 3 (1): 99–104.
doi:
10.1016/0021-9517(64)90097-1.