Once the above process has been completed, a new monomer molecule (propylene) has been incorporated into the chain, in the isotactic configuration.
If the sequence is repeated with the next molecule, it will follow the same steps as its predecessor and end up positioned in the same way, with the methyl group on the same side of the backbone.
The steps of the insertion process for this new molecule are similar to those of its predecessors and follow a rigorous sequence: Coordination with the Zr+ will occur on the same side from which the previous molecule coordinated to the last one added (the direction of approach from the right or left of the Zr+ alternates with each monomer added, as seen in the figures of the previous chapter. The orientation of the methyl group of the monomer approaching from the left or right of the Zr+ is determined by the location of the aromatic rings of the indenyl groups: taking the front view in the figure below, if the monomer approaches from the right of the Zr+, the methyl group should face up (to avoid the indenyl group below), and if it approaches from the left, the methyl will face down (the indenyl is now on top).
Since the insertion of the vinylic double bond is rigorously alternated between the right and left of the Zr+, the fact that one monomer molecule has the methyl oriented upward-facing outward and the next one downward-facing outward is the reason for the isotacticity of the polymer. As the polymer chain rotates around its long axis to accommodate the opening of each vinyl bond that is incorporated, the right-left alternation of the double bond opening is exactly offset by the up-down alternation of the position of the methyls in the monomer approach and causes them all to be on the same side of the polymer chain (isotacticity).