Polypropylene polymerisation mechanism.

Although there is still disagreement about the exact nature of the mechanism of propylene polymerisation, there is agreement about the exact nature of the mechanism of propylene polymerisation, there is agreement about the...

  1. The local polymerisation rate is given by the equation:

    in which, in addition to the propagation constant Kp, C is the active site concentration, and M is the monomer concentration.

  2. The active sites are located on the faces and edges of the titanium trichloride crystals.
  3. The polymer chain is initiated by the insertion of the olefin into a carbon-metal bond. The two postulated mechanisms for polymerisation are monometallic and bimetallic.

a) Monometallic mechanism (Polypropylene polymerisation mechanism).

Figure 3.5: Monometallic Mechanism for Polymerisation of Propylene

In this mechanism, schematized in Figure 3.5, the active site consists of a pentacoordinate Ti atom, with a vacant position at the edge of the crystal. The polymer grows according to the following mechanism (the square in the schematic represents the vacant position):

The active site is considered to possess sufficient chiral activity to generate a mostly stereoregular polymer, thanks to the scheme of alternating insertion site (vacancy site) as monomer molecules are inserted.

Another theory, still within the monometallic scheme, suggests that the olefin is inserted directly onto the carbon-titanium bond by cis-addition. This mechanism requires very high polarisation of the carbon-metal bond, and metals with very small atomic radii and high localised positive charge. In this theory, the driving force necessary to promote isotactic propagation would be provided by a steric interaction between the substituent at the α-position of the olefin and the bonds involving the metal atom. However, there is experimental evidence for the fact that stereospecificity depends on the alkyl metal group used as a co-catalyst.
For example, Natta found that isotacticity decreases with the following order:

Al(C2H5)2I > Al(C2H5)2Br > Al(C2H5)2Cl > Al(C2H5)3

In general, proponents of the monometallic theory argue that the differential impact of these forms of co-catalyst is due to the different modes of interaction of each of them on the metal either by adsorption of them on carbon atoms adjacent to the active sites, or to perturbations introduced into the crystal structure by their presence.