3.3. Activation Energies and Stereoselectivities
From
Figure 3, the most stable
iBu growing chain conformation is 1,2-
re, A. Thus, the 1,2-
re propylene insertion on the front side is the preferred pathway for the study of the reactivity and stereoselectivity of five salicylate donors. Considering the 1,2-
si insertion, the conformation 1,2-
si, A is the most favorable route, but it has less stable energy than the 1,2-
re, A conformation by 3.9 kcal/mol. For the π-complex, we observed that the position of isopropyl substituents on the donors is closer to the methyl moiety of PP in the
re enantioface than in the
si enantioface. Thus, the π-complex of the
re enantioface is probably stabilized by the dispersion-type interaction between the methyl moiety and the substituents.
According to Cossee and Arlman [
33,
34,
35] and Brookhart and Green [
36], the insertion step involves the π-complex formation between the active catalyst and the olefin, the formation of the four-centered transition state, and the generation of the insertion product. Thus, the π-complex formation energy (ΔE
π) and the intrinsic activation energy (E
a) (the energy difference between the transition state structure (TS) and the π-complex) can be used to assess the reactivity of the ZN catalysts. Since the flexible cluster model is employed in this work, the π-complexation energy is then included the relaxation of the surface apart from the coordination energy. The stereoselectivity of polypropylene can be evaluated from the relative barrier (Rel.), which is the difference between the TS energy of 1,2-
si (E
TS(
si)) and 1,2-
re (E
TS(
re)) insertions. Additionally, it is also useful to estimate the apparent activation energy (E
a(app)) which is ∆E
π + E
a and equivalent to the transition state energy relative to the dissociation channel of the catalyzed PP polymerization. Since the chelate mode was the preferred adsorption mode for salicylate donors, this mode was then selected for the construction of the active MgCl
2 (110) surface.
Table 4 lists π-complex formation energy, intrinsic activation energy, apparent activation energy, and relative barrier for the insertion step of the ZN-catalyzed PP polymerization with five salicylate donors.
The π-complex formation energies (ΔE
π) of the five salicylate donors are in the range of −58–−64 kcal/mol. Generally, the 1,2-
re insertion complex gives lower ΔE
π than its 1,2-
si counterpart, suggesting the more stable π-complex of the 1,2-
re insertion mode. Structures of the π-complex of (a) 1,2-
re and (b) 1,2-
si insertions of ZN-catalyzed PP polymerization with the salicylate (SID-4) donor are shown in
Figure 5. From
Figure 5, we observed that the position of isopropyl substituents on the donors is closer to the methyl moiety of PP in the
re enantioface π-complex than in the
si enantioface. Thus, the
re enantioface π-complex is probably stabilized by the dispersion-type interaction between the methyl moiety and the substituents. The π-complex of 1,2-
re insertion for the case of SID-3 provides the strongest ΔE
π (−63.5 kcal/mol), while that of 1,2-
si for SID-2 gives the weakest interaction (−58.0 kcal/mol). There seems to be no direct relationship between activity and ΔE
π. Comparing with DIBP (−37.4 kcal/mol for 1,2-
si and −41.7 kcal/mol for 1,2-
re insertions [
23]), π-complex formation energies of all primary(1,2) insertion modes for five salicylates are larger. However, the π-complexation energy of the ZN without the electron donor is only −30 (
si face) and −33 (
re face) kcal/mol [
17], which suggests that the electron donor stabilizes the π-complex.
Intrinsic activation energies (E
a) of the five salicylate donors for both insertion modes are between 3.9 to 9.0 kcal/mol, and the 1,2-
re insertion has a higher activation barrier than its corresponding 1,2-
si. This similar trend was also found for the 1,2-
re /1,2-
si insertion with DIBP (4.7/4.5 kcal/mol) and dibenzoyl sulfide (6.5/4.4 kcal/mol) [
23]. When we considered the relationship between the intrinsic activation energies of 1,2-
re and 1,2-
si insertions and the log of experimental PP activities of five salicylate donors, we found R
2 = 0.98 and 0.97, respectively. Thus, the ln(activity) is directly related to the intrinsic activation barrier of the insertion step. (For the first-order kinetics, lnk = −E
a/RT). The lowering of the activation barrier will enhance the activity of the ZN catalyst. There seems to be a relation between E
a(app) and the ln(activity). We obtained R
2 of 0.99 and 0.64 for the E
a(app) of the 1,2-
re and -
si insertions, respectively. The activity is related to the TS energy of the 1,2-
re insertion. Thus, the donor which can better stabilize TS energy of 1,2-
re insertion will provide a higher activity for the catalyst. Interestingly, while ΔE
π does not have any relation to the activity, the E
a and E
a(app) do. We also found a correlation between E
a/E
a(app) and E
ads to be around 0.94. This suggests that factors which provide high adsorption energy will be the same for E
a/E
a(app).
Moreover, we found the highest occupied molecular orbital (HOMO) energies of SID-1 to SID-5 donors (−0.249, −0.242, −0.247, −0.238 and −0.235 a.u., respectively) to be also linearly related to ln(PP activity) with R2 = 0.94. The donor with higher HOMO provides higher activity for the ZN catalyst. More interestingly, with the least square fit for Ea of 1,2-re and 1,2-si insertions and HOMO we obtained R2 of 0.94 and 0.87, respectively. Thus, HOMO data is better correlated with Ea of 1,2-re insertion. The relation between HOMO and Ea could be well explained by the Frontier Molecular Orbitals (FMO). This information suggests that the more potent salicylate donors should have high HOMO energy (less negative value). However, we should restrict our observation to within the same class of compounds.
Figure 6 illustrates the transition state structures of two enantiofaces of the primary (1,2) insertion modes of the ZN-catalyzed PP polymerization with SID-4. From the Figure, it appears that TS structures are similar to the corresponding π-complex structures, where the methyl moiety of PP is positioned furthest from the donor in the primary (1,2)-
si, and in the primary (1,2)-
re they are in the close vicinity. If the E
a for the insertion step is controlled solely by the steric interaction between the methyl moiety of PP and the substituents of the donor, as suggested by Cavallo et al. [
39], the TS of the 1,2-
re insertion should be less stable (higher) than that of the 1,2-
si. From the values of E
a(app) in
Table 4, the energy of the TS of the 1,2-
re insertion is lower than that of 1,2-
si for all electron donors. In other words, the TS of the 1,2-
re with salicylate donor is more stable than the TS of the 1,2-
si. Thus, the close encounter of the methyl moiety of PP and substituents on the donor provides a favorable interaction to the TS of 1,2-
re insertion. Thus, the dispersion-type interaction should be another effect. To prove our hypothesis, the π-complex formation energy (ΔE
π), the intrinsic activation energy (E
a), the relative barrier (Rel.), and the apparent activation energy (E
a(app)) of the five salicylate donors (SID-1–5) were calculated using the B3LYP method. These values are given in
Table S3 in supporting information. The values in
Table S3 differ from those in
Table 4. This suggests the importance of dispersion interaction. However, without dispersion (values in
Table S3) the (1,2)-
re remains the preferred insertion mode for salicylate donors (noticing from E
a(app), and Rel.), in exception of SID-1. Thus, we still believed that the stereoselectivity of the ZN catalyst is controlled by the steric interaction between the electron donor and the methyl moiety of propylene, in agreement with Corradini [
52], Cavallo [
39], and the Taniike groups [
26]. However, the (1,2)-
re insertion gains extra stability from the dispersion interaction. Unlike when the B3LYP-D3 method was employed for the calculations, no relation between computed values of ΔE
π or E
a or Rel. or E
a(app)) and the activities of the five salicylate donors was observed when the calculations were performed using B3LYP (see
Table S4). This implies the significance of the dispersion-type interaction for this system. Therefore, we can conclude that there exists a dispersion-type interaction between the methyl moiety of PP and the substituents of the donors, which helps to stabilize the TS of the 1,2-
re insertion. The higher activation energy of the 1,2-
re insertion mainly comes from the stronger π-complex formation of the
re enantioface.
The polymerization can be determined by relative barriers (Rel.). Theoretically, higher Rel. would relate to the higher stereoselectivity. The relative barrier of ZN-catalyzed PP polymerization with the five salicylate donors of SID-1 to SID-5 are given in
Table 4. Experimentally, the stereoselectivity can be indicated by the percent isotactic sequence length (%mm) and isotacticity index (%I.I). These values for the five salicylate donors are listed in
Table 1. The positive value of the relative barrier indicates that the TS with a
re-coordinated propylene face is more stable than the corresponding
si-enantioface. From the result, the SID-4 system gave the highest value for the relative barrier (3.9 kcal/mol) and hence the highest stereoselectivity (%mm = 91.0 and %I.I = 98.6). The SID-1 system has the lowest relative barrier (1.1 kcal/mol) and the lowest selectivity (%mm = 85.5 and %I.I = 96.3). Compared with the industrial diisobutyl phthalate donor, we have previously reported that the transition state for the primary (1,2)-
re is 4.0 kcal/mol below that for primary (1,2)-
si. [
23]. The %mm and %I.I of PP prepared by the diisobutyl phthalate internal donor are 91.0% and 91.7%, respectively [
16]. Thus, the relative barrier of the ZN-catalyzed PP polymerization correlates well with the experimental stereospecificity data [
16]. Also, the %selectivity, which should be closely related to both %mm and %I.I., can be estimated using the Curtin-Hammett principle (see the details in
Table S6 of Supporting Information). The linear relationship between the %selectivity for the five salicylate donors and %mm and %I.I was observed with R
2 of 0.74 and 0.55, respectively. Since the substituents on the electron donor play roles in the stability of the TS of the 1,2-
re insertion as mentioned earlier, types of substituents on the donor can be used to assess the selectivity of the ZN catalyst. The SID-1 donor which has the lowest Rel. (1.0 kcal/mol) contains an H atom as the substituent for R
1 and R
2 and phenyl for R
3. The SID-4 donor with
iPr on R
1 and R
2 while having the same substituent on R
3 as the SID-1 gives the largest Rel. (3.9 kcal/mol). Similarly, with
tBu on R
2 and R
3, the SID-5 also has large Rel. (3.6 kcal/mol). Thus, the substituents on R
1 and R
2 enhance the stability of the TS for 1,2-
re insertion towards that for 1,2-
si. The
iPr substituent provides a more favorable interaction than
tBu. Possibly the
tBu is too bulky. The R
3 substituent plays a smaller role than those on R
1 and R
2, since it gives a small difference for the relative barriers of SID-2 (3.5 kcal/mol) and SID-3 (3.2 kcal/mol). It should be noted that suggestions for improving stereoselectivity by salicylate electron donors are the same as those for improving activity. Therefore, the salicylate donor that provides high activity will also provide high stereoselectivity.
3.4. Comparing with Other Internal Electron Donors
The isobutyl 2-benzyloxy-3,5-isopropyl benzoate or salicylate donor (SID-4) was selected to represent five salicylate donors, since this compound gives the highest activity [
16]. Apparent activation energies (E
a(app)) of primary (1,2)-
re insertion in kcal/mol, relative barrier (Rel.) in kcal/mol, activity in kg-PP/gCat for di-
n-butyl-2-cyclopentyl malonate, dibenzoyl sulfide, diisobutyl phthalate donors and in kg-PP/gTi for diisobutyl phthalate and salicylate donors, and %isotacticity in %mmmm (pentrad) for di-
n-butyl-2-cyclopentyl malonate, dibenzoyl sulfide, diisobutyl phthalate donors, and in %mm (triad) for diisobutyl phthalate and salicylate donors are listed in
Table 5. In our previous work, we have shown that E
a(app) can be used on par with the experimental activity [
23]. Thus, it was used for the comparison of the activity of the ZN catalyst with various electron donors, whereas Rel. was utilized for comparing %isotacticity.
Among all internal electron donors in
Table 5, the ZN catalyst with salicylate donor (SID-4) gives the lowest (highest negative) E
a(app) and hence, the best activity. This is followed by those with dibenzoyl sulfide, di-
n-butyl-2-cyclopentyl malonate, and diisobutyl phthalate donors, which have a higher E
a(app) (less negative) and, thus, less activity. The apparent activation energy of the ZN catalyst without a donor is the least negative and, therefore, the catalyst shows the poorest activity. In the order of experimental activity, the list of donors is sulfide > malonate, phthalate. This order agrees with the calculated apparent activation energies. Unfortunately, the salicylate donor (SID-4) does not share the same unit for activity. Thus, it cannot be directly compared with other internal donors. By comparing E
a(app), the ZN catalyst with the salicylate donor probably yields similar activity to that of sulfide donor. Using data of malonate, sulfide, and phthalate donors, we obtained a linear equation between ln(activity) and E
a(app) with R
2 of 0.98. From the equation, we estimated the activity of the ZN catalyst with salicylate donor to be 40.4 kg-PP/gCat.
Excluding data of the malonate donor in
Table 5, it can be seen that the %Isotacticity is well predicted by the relative barrier (Rel.) [
16]. The ZN with sulfide donor gives Rel. of 11.9 kcal/mol and has a %mmmm of 91.7, while that with phthalate donor gives Rel. of 4.0 kcal/mol and have %mmmm of 88.7 [
23]. The salicylate donor which has comparable Rel. to the phthalate donor provides the polypropylene product with similar %isotacticity. Interestingly, the malonate donor has a negative relative barrier, implying that the TS for the 1,2-
si insertion is more stable. Moreover, the ZN catalyst with the malonate donor yields the polypropylene product with better %isotacticity (%mmmm = 97.5) than that with the sulfide donor [
13]. Thus, the interaction involved in the insertion step of the ZN-catalyzed PP polymerization with the malonate donor must be different from others.
The relative barrier can be decomposed to contributions from ΔE
π and E
a. Since the transition state energy relative to the dissociation channel (E
TS) is E
a(app) and Rel. = E
TS(
si) − E
TS(
re). Therefore,
where ∆∆E
π = ∆E
π(
si) − ∆E
π(
re), and ∆E
a = E
a(
si) − E
a(
re). The decomposition of the relative barrier of malonate, sulfide, phthalate, and salicylate (SID-4) donors is given in
Table 6.
The positive sign of the value means that the value for the
re face of the propylene monomer is lower than that of the
si face and vice versa. From
Table 6, the contribution of ΔE
π to the relative barrier favors the
re face insertion, while that of E
a favors the
si face insertion for all donors. Thus, the nature of interactions that controlled the stereoselectivity is the same for all donors. However, for salicylate, sulfide, and phthalate donors, the magnitude of the ΔE
π contribution is larger than that of the E
a contribution. Thus, the selectivity is dictated by the contribution of ΔE
π and
re face insertion is preferred. Whereas it is contrary to the malonate donor. Therefore, the selectivity is controlled by the E
a contribution and the
si face insertion is preferred.