The presented scheme introduces us to a more detailed view of how the electrons are arranged around Zr.
The zirconium cation increases its stability since the electrons of 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 is still electron-hungry; when a vinyl monomer is added, such as propylene, for example, with its double bond it contributes the electrons to share
Contribution of electrons from the vinyl bond to the catalytic complex
A more detailed analysis of this complex will be useful to understand the mechanism of polymerization. For this study, it is useful to review the electronic structure of olefin and zirconium. Let us start with the former: in a molecule with a vinylic carbon-carbon group, it is formed by an s-bond and a p-bond:
Orbitals, vinyl
From the figure it follows that the p-bond consists of two p-type orbitals: one is the bonding orbital (blue in the figure) while the other is the anti-bonding orbital (red). The first orbital has 2 lobes between the carbon atoms, while the second has 4 lobes opening away from the carbon atoms. Normally, the electrons are located in the p-bonding orbital, since the anti-bonding orbital is of very high energy and under usual circumstances remains empty.
As for Zirconium, it has 5 d orbitals, although in the figure above only 2 of them are indicated. The figure below shows the lobes of an empty d orbital (green) and a full d orbital (pink).
Orbitals d, Zr.
Actually the 2 d orbitals have a more complex spatial distribution (the schemes of the 5 4d orbitals are added below) as shown in the figure:
In what follows, the simplified form shown above (two-dimensional figure) is used instead of the 3D representations to highlight only the concepts of interest. The empty d-orbital is the electron-hungry one, and will try to take the pair from the vinyl bond of the olefin, which, because of its high electron density, is in a position to share that pair. Thus, the p-bonding orbital of olefin and the empty d orbital of zirconium pair up and share the electron pair.
Vinyl bond electron pairing
Once these orbitals are together, a close proximity of the full Zr orbital to the empty p-antibonding orbital of the molecule with the vinyl group results as a consequence. This causes a second pair of electrons to be shared between these orbitals as well.
Secondary electron interaction
This second shared electron pair makes the Zr-olefin complex stronger and sets the stage for the polymerization process as seen below. If the simplified representation is used again, the complex between Zr and propylene will have the representation in this figure:
This complex, although stabilized in terms of no separation of Zr and propylene, allows electron mobility and re-arrangement of the complex into a new form.
Since the presence on Zr of the electrons shared with the vinyl bond is evident, the electrons of the electron pair of the covalent bond between Zr and the methyl carbon concentrate on the latter and and are displaced to form a bond between the methyl carbon and a carbon in the vinyl bond of propylene..
This complex, although stabilized in terms of no separation of Zr and propylene, allows electron mobility and re-arrangement of the complex into a new form.
Since the presence on Zr of electrons shared with the vinyl bond becomes evident, the electrons of the electron pair of the covalent bond between Zr and the methyl carbon concentrate on the latter and move to form a bond between the methyl carbon and a carbon in the vinyl bond of propylene.
Meanwhile, the electron pair that formed the Zr-propylene complex shifts and forms a covalent bond between Zr and one of the propylene carbons.
Sequence addition
The process proceeds through a transition with 4 intermediate states. The zirconium cation in the last stage appears as it started in the first stage, with still a missing ligand and with a new agostic association, although now with a hydrogen from a C-H bond of the propylene.
If a new propylene molecule approaches, the process repeats. The propylene coordinates with the Zr and then the displacements of the electronic pairs occur, comparing the above 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 vinylic double bond of the approaching monomer which upon opening produces the coordination in the insertion.
Addition of a new monomer molecule to the polymer chain.