The diagram presented here provides a more detailed view of how electrons are arranged around Zr.
The zirconium cation becomes more stable because the electrons in the carbon-hydrogen covalent bond of the methyl group are shared with Zr in what is known as an α-agostic association. Even so, Zr remains electron-deficient; when a vinyl monomer is added—such as propylene, for example—its double bond provides the electrons to be shared.
Electron contribution from the vinyl bond to the catalytic complex
A more detailed analysis of this complex will be helpful in understanding the mechanism of polymerization. For this study, it is useful to examine the electronic structure of the olefin and zirconium. Let’s start with the former: in a molecule with a vinyl carbon-carbon group, this group consists of an s bond and a p bond. The latter can be illustrated in the figure:
Orbital, vinyl
It follows from the figure that the p bond consists of two p-type orbitals: one is the bonding orbital (shown in blue in the figure), while the other is the antibonding orbital (red). The first orbital has two lobes between the carbon atoms, while the second has four lobes extending outward from the carbon atoms. Normally, the electrons are located in the bonding p orbital, since the antibonding orbital has very high energy and, under typical conditions, remains empty.
As for zirconium, it has 5 d orbitals, although only 2 of them are shown in the figure above. The figure below shows the lobes of an empty d orbital (green) and a filled d orbital (pink).
d orbitals, Zr.
In fact, the two d orbitals have a more complex spatial distribution (diagrams of the five 4d orbitals are provided below), as shown in the figure:
In what follows, we use the simplified form shown earlier (a two-dimensional figure) instead of 3D representations to highlight only the concepts of interest. The empty d orbital is electron-hungry and will attempt to take the electron pair from the vinyl bond of the olefin, which, due to its high electron density, is capable of sharing that pair. Thus, the bonding p orbital of the olefin and the empty d orbital of zirconium pair up and share the electron pair.
Electron pairing of the vinyl bond
Once these orbitals are combined, the result is that the fully occupied orbital of Zr comes very close to the empty p-antibonding orbital of the molecule containing the vinyl group. This also causes a second pair of electrons to be shared between these orbitals.
Secondary electron interaction
This second shared electron pair strengthens the Zr-olefin complex and lays the groundwork for the polymerization process, as shown below. Using the simplified representation again, the complex between Zr and propylene is depicted in this figure:
Although this complex is stable in the sense that Zr and propylene do not separate, it allows for electron mobility and the rearrangement of the complex into a new form.
Since the presence of electrons shared with the vinyl bond on Zr is evident, the electrons of the electron pair in the covalent bond between Zr and the methyl carbon are concentrated on the latter and shift to form a bond between the methyl carbon and a carbon in the vinyl bond of propylene.
Although this complex is stable in the sense that Zr and propylene do not separate, it allows for electron mobility and the rearrangement of the complex into a new form.
Since the presence of electrons shared with the vinyl bond on Zr is evident, the electrons of the electron pair in the covalent bond between Zr and the methyl carbon are concentrated on the latter and shift to form a bond between the methyl carbon and a carbon in the vinyl bond of propylene.
Meanwhile, the electron pair that made up the Zr-propylene complex moves and forms a covalent bond between the Zr and one of the carbon atoms in the propylene.
Addition Sequence
The process occurs through a transition involving four intermediate states. The zirconium cation in the final stage appears as it did at the beginning of the process, still missing one ligand and with a new agostic association, although now it includes a hydrogen atom from a C-H bond in propylene.
If a new propylene molecule approaches, the process repeats. The propylene coordinates with the Zr, and then the electron pair shifts occur. Comparing the previous figures, the orientation of the coordination bond between the Zr+ and the growing polymer chain alternates between the left and right sides of the metal cation as an inevitable consequence of the position of the vinyl double bond in the approaching monomer, which, upon opening, results in coordination upon insertion.
Additionof a newmonomer moleculeto the polymer chain.