## 2 Results and Discussion
### Hydrogen Reactivates the Reaction  
Figure 2 lists the energies of the reaction coordinates transition states (TSs)/intermediates (IMs) involved in the hydrogen reaction process, and Figure 3 lists the main transition states and configurations of the intermediates for the DVB on to the zirconocene catalyst. The DVB unit in [Zr]-DVB-PP\({}_{1}\) is more spatially crowded than the propylene unit; therefore, further insertion of propylene into the zirconium-carbon bond of [Zr]-DVB-PP\({}_{1}\) to inactivate the catalytic system cannot occur easily. However, because hydrogen is smaller in size, it can be inserted into the zirconium-carbon bond of the [Zr]-DVB-PP\({}_{1}\) to form a coordinated DVB-PP\({}_{1}\) and into the catalytically active Zr-H site on the zirconocene. Figure 2: The relative free energy of the reaction pathway involving hydrogen is given in kcal/mol, and the relative energy of BVPE is given in brackets; the relative enthalpy is also given in parentheses for reference (in kcal/mol). H\({}_{2}\) first coordinates to the zirconium center across the transition state (TS1) to obtain the dihydro complex IM1 (Figures 2 and 3). The bond length of Zr-C\({}_{1}\) (2.423 A) is longer than that of [Zr]-DVB-PP\({}_{1}\) (2.355 A), and the H\({}_{1}\)-H\({}_{2}\) bond is slightly lengthened from 0.743 A (in free H\({}_{2}\)) to 0.744 A in the configuration of the transition state TS1, owing to the insertion of H\({}_{2}\); the bond lengths of Zr-H\({}_{1}\) and Zr-H\({}_{2}\) are 3.314 A and 3.311 A, respectively. Relative to the reactant ([Zr]-DVB-PP\({}_{1}\) and H\({}_{2}\)), the barrier for the transition state TS1 is 12.7 kcal/mol. Subsequently, the intermediate IM1 is formed; exothermic, 1.3 kcal/mol, compared with the transition state TS1. In the IM1 configuration, the Zr-C\({}_{1}\) (2.507 A) and H\({}_{1}\)-H\({}_{2}\) (0.783 A) bonds are longer than those in the transition state, TS1; the Zr-H\({}_{1}\) (2.103 A) and Zr-H\({}_{2}\) (2.081 A) bonds are shorter than those in the transition state, TS1. Then, the intermediate, IM1, evolves to the four-member transition state, TS2, across an energy barrier of 2.8 kcal/mol, and the Zr-C\({}_{1}\) bond (2.601 A) and H\({}_{1}\)-H\({}_{2}\) bond (0.930 A) become longer. Figure 3: Optimized structures of key transition states (TSs) and intermediates (IMs) in the DVB reaction pathway shown in Figure 2. The key length is in angstroms. After the transition state, TS2, the breaking of the H-H bond leads to the intermediate, IM2, which is exothermic, 18.8 kcal/mol. Compared with the TS2 configuration, the Zr-C\({}_{1}\) bond (4.411 A) of IM2 is much longer, and the Zr-H\({}_{1}\) bond (1.818 A) of IM2 is shortened by 0.112 A. The intermediate IM2 is then decomposed into two parts: DVB-PP\({}_{1}\) and [Zr]-H; exothermic, 2.4 kcal/mol. The Zr-C\({}_{1}\) bond is broken. Due to DVB-PP\({}_{1}\) being separate from [Zr]-H in product, the distances between Zr and the center of the two side-C5 rings in [Zr]-H are 2.191 and 2.181 A, respectively, which are shorter than those in intermediate IM2 (2.251 and 2.283 A). The interaction between the central metal Zr and the two side-C\({}_{5}\) ring ligands becomes stronger, so the Zr-H\({}_{1}\) bond (1.828 A) of [Zr]-H is slightly longer than that of IM2 by 0.010 A. During the entire hydrogen introduction reaction, the Zr-C\({}_{1}\) and H\({}_{1}\)-H\({}_{2}\) bonds gradually become longer until they break, and the Zr-H\({}_{1}\) and C\({}_{1}\)-H\({}_{2}\) bonds gradually become shorter until they form stable bonds. The Wiberg bond indices (WBI) and natural charges (NBO) for some key bonds and atoms of the reaction pathway involving hydrogen are shown in Table 1. As the reaction proceeds, the WBI values of the Zr-C\({}_{1}\) bond (from 0.609 to 0.003) and those of the H\({}_{1}\)-H\({}_{2}\) bond (from 1.000 to 0.000) gradually become smaller from the reactant ([Zr]-DVB-PP\({}_{1}\) and H\({}_{2}\)) to the intermediate IM2, indicating the break of the Zr-C\({}_{1}\) and the H\({}_{1}\)-H\({}_{2}\) bond.