Stereospecific Catalytic Polymerization of α-Olefins

α-olefins are often polymerized in the presence of stereospecific catalysts such as metallocenes or Ziegler-Natta catalysts.¹ These catalysts are able to restict the addition of monomer molecules to a specific regular orientation (tacticity). In the case of isotactic orientation, all alkyl groups are positioned at the same side of the molecule with respect to the polymer backbone, and in the case of syndiotactic orientation the position of the alkyl groups alters. 

The coordination polymerization of olefins has many advantages over conventional polymerization. For example, it allows for excellent control of the molecular weight (MW), tacticity, and comonomer incorporation and results in narrow MW distribution. Usually, only very small quantities of catalyst is required.

The most important α-olefin is propylene; it is the second most widely used commercial polymer today and almost exclusively produced via stereospecific polymerization. Compared to conventional polypropylene, it shows very little creep at room temperature under load even if of comparatively low molecular weight. It is also more crystalline due to the regular structure and has a higher heat deflection temperature and thus a higher service temperature.

The mechanism of α-olefin polymerization using stereoselective catalysts includes initiation, propagation, transfer and termination reactions:

Initiation: The polymerization reaction starts when the olefin monomer coordinates to the transitional metal atom bearing a positive charge followed by insertion.

Propagation: The insertion of monomer molecules into the ionic Me-C bond at each active center results in the formation of long polymer chains:

Cp₂Me⁺-R' + n CH₂=CHR → Cp₂Me⁺-(CH₂-CHR)_n-R'

The instant polymerization rate is

R_p = k_p · [Cat] · [M]

where [Cat] is the concentration of active propagation centers, [M] is the concentration of monomers and k_p is the rate constant which is usually very high for most transition metal catalysts.

Elimination: Polymerization stops when the polymer chain is disengaged from the active center. The most common termination reaction is the β-hydride elimination:

Cp₂Me⁺-(CH₂--CHR)_n-R' + CH₂=CHR → Cp₂Me⁺-H + CH₂=CHR-Polymer

Depending on the charge distribution, the metal hydride can often initiate a new polymerization reaction (β-hydride transfer reaction). In fact, chain termination reactions are usually very unlikely events in Ziegler-Natta and metallocene catalytic polymerization resulting in too high molecular weights. To control the molecular weight, quenching agents are often added. For example, the catalyst can be deactivated by contact with water or alcohol.

The stereoselctivity in α-olefin polymerization depends on many factors. The most important factor is the stability of the chemisorbed catalytic complex. Its stability is affected by the ion radius of the metal atoms linked to the metalorganic compound, irregularities of the catalyst, and surface energy of the crystall lattice. Another important factor is the polymerization temperature. In general, the degree of tacticity decreases with increasing temperature.

Regulation of the average molecular weight is of great importance in the polymerization of olefins because the mechanical and viscoelastic properties depend on the molecular weight. For example, an iso- or syndiotactic polypropylene of high molecular weight is difficult to process in the melt state due to its high viscosity whereas a high molecular weight is a lesser problem for atactic polypropylene. In fact, a high molecular weight is often required to achieve similar high tensile strength and low creep properties.

Compounds with a partial ionic character such as metal-alkyls are often very effective in lowering the molecular weight of polyolefins. These compounds transfer negative groups to the catalytic complex, i.e. they act as alkyl transfer agents. Other compouds having a partial ionic character such as alkyl halides are also effective in chain growth stopping and/or molecular weight reduction. The molecular weight can also be reduced by carrying out the polymerization in the presence of hydrogen. In this case, the decrease in MW depends on the hydrogen partial pressure in the reaction vessel. However, in the case of Ziegler-Natta catalysts, hydrogen can also effect the steric purity.² The reduction of molecular weight is caused by proton saturation of the growing chain ends (elimination) and by hydride ions replacing alkyl groups attached to the catalyst (Ziegler-Natta catalysts). These hydrid ions react with propylene to form alkyl groups which can reinitiate chain growth.

¹Karl Ziegler was awarded the Nobel price in chemistry (1963) for the discovery of the first titanium-based catalysts, and Guilo Natta for using them to prepare stereoregular polymers from polypropylene.

²Giulio Natta, J. of Poly. Sci., Vol. 34, 531-549 (1959)