Polypropylene polymerization mechanism.
Although there is still disagreement about the exact nature of the mechanism of propylene polymerization, there is agreement about the exact nature of the mechanism of propylene polymerization, there is agreement about the...
- The local rate of polymerization is given by the equation:
in which, in addition to the propagation constant Kp, C is the concentration of active sites, and M is the monomer concentration.
- The active sites are located on faces and edges of the titanium trichloride crystals.
- The polymer chain is initiated by the insertion of the olefin into a carbon-metal bond. The two postulated mechanisms for polymerization are monometallic and bimetallic.
a) Monometallic mechanism (polypropylene polymerization mechanism).
Figure 3.5: Monometallic Mechanism for Propylene Polymerization
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 alternation scheme of the insertion site (vacant site) as the 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 polarization of the carbon-metal bond, and metals with very small atomic radii and high localized 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 employed as cocatalyst.
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 occurrence of these forms of cocatalyst 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.