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Title

Coadsorption Model for First‑Principle
Description of Roles of Donors in Heterogeneous
Ziegler‑Natta Propylene Polymerization

Author(s)Taniike, Toshiaki; Terano, Minoru

Citation

Journal of Catalysis, 293: 39‑50

Issue Date

2012‑07‑04

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/11452

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Toshiaki
Taniike, Minoru Terano, Journal of Catalysis,
293, 2012, 39‑50,
http://dx.doi.org/10.1016/j.jcat.2012.06.001

Description

Japan Advanced Institute of Science and Technology

Coadsorption Model for First-Principle Description of Roles of Donors
in Heterogeneous Ziegler-Natta Propylene Polymerization

Toshiaki Taniike* and Minoru Terano

School of Materials Science, Japan Advance Institute of Science and Technology, 1-1 Asahidai,
Nomi, Ishikawa 923-1292, Japan*CORRESPONDING AUTHOR
Tel: +81-761-51-1622; Fax: +81-761-51-1625; E-mail: taniike@jaist.ac.jp1

ABSTRACT: Systematic periodic density functional calculations were conducted to clarify themechanism for donors to exert steric and electronics influences on propylene polymerization
using heterogeneous Ziegler-Natta catalysts. It was concluded that TiCl4 preferentially adsorbs
as mononuclear species on the MgCl2 (110) surface and the coadsorption of donors with it isenergetically viable. The coadsorption of donors on the (110) surface reinforces the electron
density of the Ti mononuclear species, and sterically transfers the underlying C2 symmetry toconvert the originally aspecific mononuclear species into isospecific one. The nearest
coadsorption of ethylbenzoate (EB) not only sterically induces the isospecificity of the Timononuclear species but also electrostatically improves the regiospecificity in propylene
insertion. In addition, EB prevents sterically-demanding chain transfer to propylene, increasing
the molecular weight of the produced polypropylene. Thus theoretically derived “coadsorption
model” is highly consistent with a variety of experimentally known facts and believed to be
useful for the ab-initio prediction of new donor structures.

KEYWORDS: Ziegler-Natta propylene polymerization; density functional theory; donor;
mechanism; coadsorption; active site2

1. Introduction
Since the discovery of Ziegler-Natta (ZN) olefin polymerization catalysts, tremendous
researches and developments have continuously progressed their mechanistic understandings aswell as commercial applications in the huge polyolefin field. Especially, developments ofhomogeneous metallocene and post-metallocene catalysts have opened new chemistry for
precise olefin polymerization. For example, early or late transition metals combined with avariety of tailored ligands have enabled stereo- and regio-selective living polymerization ofvarious olefins, incorporation of branches as well as bulky and polar groups into polyolefinbackbone, stereo/chemo block/gradient copolymerization, and so on [1-11]. Aside from these
extensive progresses in homogeneous catalysis, traditional heterogeneous ZN catalysts still keep
the top priority in the industrial olefin polymerization, especially accounting for more than 99%
of about 50 million tons/year of isotactic polypropylene (iPP) over the world.
The key technology of heterogeneous ZN catalysts for the iPP production is at theconcomitant usage of MgCl2 support and organic electron donors [12]. MgCl2 enhances both theconcentration of active Ti species and their activity [13,14], while donors bring about dramatic
improvement of the stereospecificity of the catalysts as well as significant alternation of othercatalytic properties such as the productivity, hydrogen response, comonomer incorporation
efficiency, and molecular weight distribution (MWD) of produced iPP [15,16]. Continuous
developments over three decades have invented four families of donor systems, that is, the first
benzoate/benzoate, most widely used phthalate/alkoxysilane, 1,3-diether/(alkoxysilane) featured
with high hydrogen response, and succinate/alkoxysilane for broader MWD. Thus, one can saythat catalytic performances of heterogeneous ZN catalysts can be altered based on the3

combination of different internal and external donors with MgCl2, in contrast to themetal-ligands combination for homogeneous catalysts. However, the largest difference between
the heterogeneous and homogeneous catalysts is attributed to the absence of a clear answer for
how the combination of specific donors with MgCl2 controls the catalytic properties of active Ti
species.
Activated MgCl2 as catalyst support usually has δ-form, characterized with small dimensionsof Cl-Mg-Cl tri-layers disorderly stacked along the [001] direction [17,18]. Its most stable (001)basal plane is coordinatively saturated [19] and inactive to the adsorption of TiCl4 [20,21] (i.e.
catalytically irrelevant). Catalytically relevant surfaces are low-index planes that expose
unsaturated Mg2+ ions. Especially, the (110) and (100) lateral planes have been long believed as
representative ones [17,18] (we employ the crystallographic notation of (100) rather than (104)[19] in order to stand on the same ground with historical discussion). While the dominant
exposure of the (100) lateral planes is estimated for equilibrium crystallographic morphology of
MgCl2 [19], activated MgCl2 can also have the (110) lateral plane, presumably as a result of
non-equilibration or equilibration shifted in the presence of adsorbates such as TiCl4 and donors.
Mori et al. observed with transmission electron microscope that lateral surfaces of mechanically
activated MgCl2 are dominantly composed by the (110) and (100) planes [22], while Andoni et
al. reported preferential growth of MgCl2 crystal along the [110] direction in the presence of
1,3-diether [23]. Recent density functional theory (DFT) calculations exactly pointed out shifted
equilibrium, where the (110) termination prevailed the (100) termination in the presence of ether
adsorbates [24].
Structures of surface TiCl4 species and active sites have been discussed based on the MgCl24

(110) and (100) surfaces. TiCl4 epitaxially adsorbs on these MgCl2 surfaces due to similar ionicradii between Ti4+ and Mg2+, bearing TiCl4 mononuclear species on the (110) surface and Ti2Cl8
dinuclear species on the (100) surface [25]. Active sites for propylene polymerization are
regarded as monoalkylated trivalent Ti species [15,26-28], resulting from extraction and alkyl
substitution of terminal Cl atoms with alkylaluminum activator. In analogous to the first andsecond generation TiCl3 catalysts [29,30], the mononuclear species on the (110) surface and the
dinuclear species on the (100) surface were respectively assigned as aspecific and isospecificactive sites [25]. Although the nuclearity of surface TiCl4 species has not been experimentallyconcluded yet [31-33], most of DFT studies have agreed on the energetic preference of the TiCl4mononuclear species on the (110) surface [34-39].
On the mechanism of stereospecific improvements by the addition of donors, the firstmicroscopic proposal was made by Busico, Corradini and their coworkers [25]. They proposedthat four-fold coordinated Mg2+ ions on the (110) surface are preferentially poisoned by
selective adsorption of donors, which prevents the formation of the aspecific mononuclear
species on the (110) surface and increases the fraction of the isospecific dinuclear species on the(100) surface. The proposal implicitly assumes that isospecific active sites are the same
dinuclear species irrespectively of donor structures, and the degree of the stereospecificimprovement depends on the selectivity of the donor adsorption on the (110) surface.
Afterwards, it became clear that polymer microstructures reflecting active site structures are
sensitive to donor structures [40-45], suggesting that donor molecules are directly involved in
the active sites. Since the coordination of donor molecules to TiCl4 is spectroscopically
excluded [46], researchers now agree that donor molecules adsorb on MgCl2 surfaces and5interact with nearby Ti species in a non-bonded manner to improve the stereospecificity of the
Ti species. Busico et al. postulated a three-site model as a versatile active site image in
MgCl2-supported ZN catalysts [47], which was later modified by Terano et al. [48]. The
three-site model describes the stereospecificity of supported Ti species irrespectively of theirlocation ((110) or (100)) and nuclearity (mononuclear or dinuclear) but according to the absenceor presence of bulky ligands, L1,2, (Cl or donor) at the neighboring undercoordinated metal sites(Mg, Ti or Al). Since the Ti dinuclear species on the (100) surface does not have any
coordinative unsaturation at the neighboring metal sites, alternative idea becomes increasingly
predominant that donors coadsorb with the Ti mononuclear species on the (110) surface toconvert it into an isospecific active site [49-52]. Recent cluster DFT calculations firstly provedthat the coadsorption of two succinate molecules at the L1 and L2 positions can convert the
aspecific Ti mononuclear species into isospecific one on the MgCl2 (110) surface [49].
Based on these backgrounds, it is plausible that the coadsorption of donors modifies thecatalytic performances of the Ti mononuclear species on the MgCl2 (110) surface. However, theidea is still evidenced by few aspects such as stronger adsorption of TiCl4 on the (110) surface[34-38] and the presence of coordination vacancies at L1,2 on the same surface. Many otheraspects remain unclarified such as energetic feasibility of the coadsorption of donors, electronicand steric interactions upon the coadsorption, and their influences not only on thestereospecificity but also on other catalytic properties like regiospecificity and molecularweight.
The aim of the present study is to offer a convincing conclusion on the mechanism for donors
to modify the polymerization performance of ZN catalysts by means of systematic periodic DFT

6calculations. For this purpose, we have performed a reliable level and amount of DFT
calculations and to compare the obtained results with a number of previous experimental results.
In the former part, results are presented and discussed for the independent adsorption of TiCl4
and donors as well as their coadsorption and resultant interactions on the MgCl2 (110) and (100)surfaces. The obtained conclusions support the coadsorption of donors with Ti mononuclear
species on the (110) surface from viewpoints of the energetic stability and consistency withexperimental facts. In the latter part, the influences of the coadsorption of ethylbenzoate (EB) as
the simplest industrial donor were systematically examined on the polymerization performanceof the Ti mononuclear species. We found that the coadsorption of EB not only converts the
aspecific Ti mononuclear species into isospecific one, but also accounts for experimentally
known enhancements in the regiospecificity and polymer molecular weight [44,45,55,56]. Thus,
the present article proposes that the coadsorption of donors on the (110) surface is theoreticallyand experimentally highly consistent, and enables the first comprehensive molecular-leveldescription of the role of donors.

2. Numerical
All the DFT calculations were performed in a spin unrestricted manner using the DMol3
package [57]. The Perdew-Burke-Ernzerhof (PBE) functional [58] was chosen, since PBE gave
the best agreement with the experimental activation energies of olefin insertion among several
functionals of the generalized gradient approximation (GGA) type. However, it is notable that
other functionals such as PW91 [59], BP [59,60], and BLYP [60,61] provided similar
conclusions to PBE in a qualitative meaning. A double numerical polarized (DNP) basis set [57]7was employed together with effective core potentials [62,63]. DMol3 instructs that thenumerical basis sets are designed to minimize the basis set superposition error. The convergencecriteria for normal geometry optimization were set to 0.01255 kcal/mol in energy and 2.510
kcal/mol·Å in force.
As in our previous studies [34,51,52], the most stable -crystalline phase was assumed for the
MgCl2 bulk, and its (110) and (100) lateral surfaces were employed as catalytically
representative cuts. Recently, Busico et al. reported the instability of the bulk-terminated (100)surface compared to the (104) surface, based on dispersion-corrected DFT calculations [19]. On
the other hand, Boero et al. showed that the bulk-terminated (100) surface reconstructs in a waythat terminal Cl atoms migrate to coordinate tri-coordinated Mg atoms [38]. The resultant (100)surface exposed only penta-coordinated Mg atoms on the surface, similarly to the (104) surface[38]. The (100) surface employed in this study corresponds to the reconstructed one with similar
stability to the (104) surface. The (110) and (100) surfaces were expressed by the slab modelwith 1.5 nm of a vacuum layer. A p(2x2) unit cell of 6 atomic layer thickness (composed of 24
MgCl2 units) was used for the (110) surface, and a p(3x1) unit cell of 14 atomic layer thickness(composed of 36 MgCl2 units) for the (100) surface. A p(4x1) unit cell with the same thickness
was employed only for TiCl4 dinuclear species on the (100) surface. 2 x 1 x 1 k-point mesh wasapplied to the p(3x1) (100) unit cell, while the gamma point was used for the other unit cells.
Both the thickness of the unit cells and the density of the k-points were determined in terms ofthe converged adsorption energies of TiCl4 and TiCl3 mononuclear species on the MgCl2
surfaces [34,51]. Also, the widths of the unit cells were decided not to have direct stericinteractions between adsorbates in neighboring cells. It is notable that small clusters composed8

of several to 10 MgCl2 units could not correctly describe electronic interaction with Ti species,
suggesting the necessity of a large unit cell for reliably expressing the nature of the MgCl2
surfaces. The MgCl2 slabs were fully relaxed except the bottom two atomic layers fixed in theirbulk positions during geometry optimization and transition state search.
It has been widely accepted that active sites of ZN catalysts for propylene polymerization aremonoalkylated trivalent Ti species [15,26-28]. Though tetravalent species also has a highactivity for propylene polymerization as in group IV metallocene catalysts, TiCl4 which isusually situated in an octahedral symmetry on MgCl2 [31-33,64] might be difficult to offer avacant site for monomer coordination. Here isobutyl (iBu) or methyl (Me) was employed as thealkyl group, i.e. as the growing chain. iBu is the minimal unit to describe the growing chain in asequence of the 1,2 insertion of propylene. Based on 13C-NMR chain end analyses using
13

C-enriced alkylaluminum, it was clarified that the stereoselectivity of the propylene insertion

into iBu-Ti was as high as those observed in the growing chain [40,41]. On the contrary, the first
propylene insertion into shorter alkyl-Ti was less stereoselective [40,41]. In this way, the size ofthe alkyl group is important to convey the site chirality to inserting propylene, and the smallest
Me (i.e. the least effects from the site chirality) was useful for us to investigate electronic
influences of donors.
Generally, alkyl-Ti and olefin can undergo two kinds of reactions in polymerization: insertion
for the chain growth, which follows the Cossee-Arlman mechanism [65] assisted with agostic
interaction [66], and chain transfer for the termination of the chain growth, where monomer
extracts β-hydrogen of a growing chain to renew the growing chain. Both of the reactions go
through  complexation between the Ti center and olefin. In this study, transition states of the9insertion and chain transfer to propylene were evaluated for all possible pathways which differ
from each other in terms of the configurations of the growing chain and monomer. For example,
four insertion pathways, namely, 1,2-re, 1,2-si, 2,1-re, and 2,1-si, exist for the propylene
insertion into Me-Ti. The same number of pathways exists for the propylene insertion into
iBu-Ti for each configuration of iBu. Transition states for the propylene insertion wereapproximated at the energy maxima along a reaction coordinate, the C2-Cα distance for the 1,2insertion and the C1-Cα distance for the 2,1 insertion. The reaction coordinate was scanned over2.0-2.6 Å with a step size of 0.05 or 0.1 Å. As the potential energy profiles of olefin insertionare flat along the reaction coordinate [49,67], such the linear scan was believed to be accurateenough for the identification of the transition states. The energy maxima were mostly obtainedin the range of 2.1-2.3 Å of the reaction coordinate [49,67-69]. As for the transition states of thechain transfer to propylene, the reaction coordinates were assumed as the C2-Hβ distance for the
1,2 orientation and the C1-Hβ distance for the 2,1 orientation. First, approximate transition states
were similarly decided at the energy maxima along the reaction coordinates, where the potential
energy profiles were relatively steep. Then, accurate transition states were searched in a way tosatisfy the energy maximum for the normal mode corresponding to the reaction coordinates andthe energy minima for the other normal modes, starting from numerically derived Hessian
matrices of the approximate transition states. The higher convergence criteria (0.01255 kcal/mol
in energy and 0.6275 kcal/mol·Å) were employed in the geometry optimization and transition
state search relating to the resting states,  complexation, insertion and chain transfer, in orderto improve the accuracy of energetic values.
In this study, activation energies of the insertion and chain transfer were given as apparent

10

values, which are defined as the energetic difference between resting states as the starting pointof a catalytic cycle and transition states. Apparent activation energies are normally equal tothose experimentally obtained from the Arrhenius plots for heterogeneous catalysis, as long as
the adsorption of reagents onto active sites is far from being saturated, which is the case for ZN
olefin polymerization under usual conditions [14,70]. We have neglected the contributions ofentropy and zero-point vibration in the reactions, in cost of the relatively large unit cells.
However, in separate calculations using MgCl2 clusters with tractable sizes, these contributions
were mostly constant among different pathways for the insertion and chain transfer: about 16-17
kcal/mol at 350 K for the entropy and about –1 kcal/mol for the zero-point vibration. Similarinsensitivities were previously reported by Ziegler and his coworkers [70,71]. Consequently, theenergies of arbitrary two pathways were believed to be nicely compared even without thosecontributions, for example, in estimating the stereo- and regio-selectivities of propylene
insertion, the most probable molecular weight, and so on.
In the end of this section, we would like to mention the computational accuracy. Judging fromexperimentally known selectivities for the propylene insertion with ZN catalysts, energies
around a few kcal/mol become important. Although a few kcal/mol in an absolute energy isbeyond the accuracy of the employed method, it is expected that differential energies which
determine selectivities are more accurate due to error cancellation between two absoluteenergies. With a similar reason, it is believed that influences of donors on the propylene
polymerization properties are safely discussed in terms of relative energies. Moreover, bycalculating activation energies among many different pathways, safe tendency must be derived
for the influences of donors on the polymerization properties. Such a comprehensive DFT

11survey with a sufficiently large MgCl2 model has been scarcely implemented for ZN catalysts,
and this is one of the purposes of the present DFT calculations.

3. Results and discussion
3.1. States of TiCl4 and donors on MgCl2
3.1.1. Energetic aspect
When one infers the mechanism for donors to improve the catalyst stereospecificity, the statesof TiCl4 and donors on catalyst surfaces must be considered. TiCl4 can adsorb on the MgCl2(110) surface only as the mononuclear species (Fig. 1a). The adsorption energy calculated by us
was –20 kcal/mol (Table 1). On the other hand, TiCl4 accepts both the mononuclear anddinuclear forms on the (100) surface (Figs. 1b and 1c). Their adsorption energies (per Ti) wererespectively –13 and –11 kcal/mol, both of which were much smaller than that on the (110)
surface (Table 1). Many of previous calculations reported similar trends for the adsorptionenergies of TiCl4 on the MgCl2 (110) and (100) surfaces [34-38], even though energetic
magnitudes were more or less dependent on exchange-correlation functionals. Thus, it seems
clear that TiCl4 adsorbs more strongly on the (110) surface. It is noticeable that the most recent
calculations by D’Amore et al. based on the-state-of-the-art exchange correlation functionals
concluded only the (110) surface can bind TiCl4 [39].
Ester-type donors show similar adsorption energies on the (110) and (100) surfaces (Fig. 2
and Table 1). Monofunctional benzoate adsorbs with a monodentate structure on both of thesurfaces, giving ca. –30 kcal/mol of the adsorption energies. Bifunctional phthalate andsuccinate can adsorb either as a bidentate structure or as bridging structures on the (110) surface,12

while only a (intra-layer) bridging structure is allowed on the (100) surface. The adsorptionenergies of these diesters are about –30-40 kcal/mol, irrespectively of the adsorption mode andthe surface type. On the contrary, 1,3-diether adsorbs only with a bidentate structure on the(110) surface due to its short oxygen-oxygen distance, giving –31 kcal/mol of the adsorptionenergies, while the adsorption energy on the (100) surface was less than half. The results inTable 1 are completely in line with recent DFT calculations by Correa et al. [49].
The surface-selective adsorption of 1,3-diethers was previously employed to support the
hypothesis of the formation of the isospecific dinuclear species on (100) through the selective
poisoning of (110) [72]. However, recent experimental results [23] reported preferential growthof one kind of lateral surfaces in the presence of 1,3-diether. The obtained lateral surfaces wereassigned by them as the (110) surface, since 1,3-diether selectively adsorbs and stabilizes the(110) surface so as to induce its selective exposure. Thus, it appears problematic to assume theco-exposure of the (110) and (100) surfaces in certain cases. Instead, it is necessary to consider
the role of internal donors in catalyst preparation to understand the states of TiCl4 and donors on
MgCl2 support. In general, preparation of ZN catalysts accompanies the fabrication of activated
MgCl2 support in the presence of TiCl4 and/or internal donor at an elevated temperature.
Activated MgCl2 support is featured not only with great structural disorder along the [001]direction [17] but also with small crystalline dimensions [18], which plausibly result from thestabilization of lateral surfaces by the adsorption of TiCl4 and internal donor. Consequently, it isexpected that the adsorption behaviors of TiCl4 and internal donors strongly affect the structureof MgCl2 support. For example, in the presence of an adsorbate which prefers the (110) surface(such as 1,3-diether), the (110) surface is supposed to be preferentially stabilized and exposed.

13

On the contrary, in the presence of a non-selective adsorbate (ester-type donors), both the (110)and (100) surfaces are supposed to be similarly stabilized and exposed. The experimental reportby Andoni et al. [23] supports the above discussion: in the presence of non-selective ester-type
donors, the morphology of MgCl2 became rectangular, indicating the co-presence of the (110)and (100) lateral cuts, while only hexagonal morphologies were obtained in the presence of
selective 1,3-diether, corresponding to the preferential exposure of (110). When both the (110)and (100) surfaces are exposed for ester-type donors, TiCl4 is supposed to preferentially replace
an internal donor on the (110) surface, due to the preferential adsorption of TiCl4 on (110). On
the other hand, TiCl4 necessarily attaches to the (110) surface when 1,3-diether is employed as
an internal donor.
Provided that an internal donor induces the exposure of both/either the (110) and/or (100)surfaces, it is important to consider the states and interactions of TiCl4 and donors on these two
surfaces. Recently, we have reported that there is no significant attraction or repulsion between
TiCl4 and EB on the two MgCl2 surfaces [51,52], and that they tend to coadsorb close to one
another at a high surface coverage, which is the case for real catalysts containing as many as one
TiCl4 and donor molecules per 10-20 MgCl2 units [15]. Considering the result that TiCl4 adsorbs
more strongly on the (110) surface as the mononuclear species, coadsorption of mononuclear
TiCl4 with donors on the MgCl2 (110) surface is concluded as energetically the most plausible
scenario.

3.1.2. Electronic aspect
Electronic interactions among MgCl2, TiCl4 and donors are totally different between the (110)14and (100) surfaces. In our previous DFT calculations [51,52], it was found that the MgCl2 (110)
surface donates much more electron density into TiCl4 than the (100) surface. Moreover,coadsorption of EB further reinforced the electron density of TiCl4 on the (110) surface, whileelectron supplied from donors to MgCl2 support was hardly transferred into TiCl4 on the (100)surface. Soga and Shiono examined the electron density of Ti species supported on various
halides with X-ray photoelectron spectroscopy (XPS) and concluded that one of the advantagesof MgCl2 as a support is attributed to its high electron donation ability [53]. In addition, Zhao et
al. clarified with XPS that internal donors further enhance the electron density of Ti centers [54].
In these points of view, TiCl4 coadsorbed with donors on the (110) surface fulfills theexperimentally obtained electronic aspects, and consequently regarded to be catalytically more
important species than that on the (100) surface.

3.1.3. Steric aspect
Isoselective propylene polymerization is generally catalyzed by a C2 (or C1) symmetric activesite. In typical metallocene catalysts, a C2 symmetric space is generated around a metal centerby the coordination of bridged cyclopentadienyl ligands. In ZN catalysts, the epitaxial
adsorption of TiCl4 induces a C2 symmetry around Ti due to the structure of MgCl2 surfaces,
which is the case for the mononuclear species on the (110) surface and the dinuclear species onthe (100) surface. In spite of the adequate symmetry, the Ti mononuclear species is aspecificowing to the lack of the stereo-regulating ligands at L1,2 [47]. On the contrary, the dinuclear
species is classified as an isospecific site due to the presence of Cl ligands at L1,2 [47]. As was
mentioned in the introduction, polymer microstructure analyses led to a conclusion that donors

15

interact with nearby Ti species and form new isospecific active sites, whose performances are
sensitive to the molecular structures of donors [40-45]. On the other hand, it is known that
agglomeration of Ti species progresses in the course of the reduction of TiCl4 into Ti3+ and Ti2+
by reacting with alkylaluminum. An electron spin resonance (ESR) study by Sergeev et al.
revealed that the population of ESR-active Ti3+ mononuclear species was greatly enhanced by
the addition of donors [73]. We have recently found that isolated Ti mononuclear species ismuch less isospecific than agglomerated Ti species in the absence of donors, but the isolated Ti
species is more efficiently converted into highly isospecific sites upon the addition of donors[43]. These suggest that donors not only prevent the agglomeration of Ti species but also
increase the isospecificity of the Ti mononuclear species. At this point, we have reached ahypothesis that the coadsorption of donors generally occurs with the TiCl4 mononuclear specieson the (110) surface.
Based on these backgrounds, we describe here how donors improve the isospecificity of theTi mononuclear species through their random adsorption [51,52] on the (110) surface, using an
illustration with quadrants introduced by Correa et al. [49]. Originally, the C2 symmetry of theTi mononuclear species on the (110) surface arises from the fact that the Oh axes of the Ti center
projected on the surface obliquely cross with MgCl2 tri-layers, as shown in Fig. 3a.
Consequently, the reaction spaces around the Ti center illustrated by white and gray quadrants in
Fig. 3a (right) are necessarily non-equivalent, which gives a two-fold rotation axis at the Ti
center without mirror planes (i.e. C2 symmetric). However, the Ti mononuclear species is still
aspecific due to the absence of steric hindrance that transfers the underlying symmetry to thegrowing chain and monomer. In other words, the growing chain and monomer cannot

16

distinguish the two kinds of the quadrants without appropriate steric hindrance.
When one considers coadsorption of donors with the Ti mononuclear species, ester-type
donors can be energetically non-selectively placed at sites SA-SE’ in Fig. 3a [51,52]. However,steric impacts of coadsorbed donors depend on the coadsorption sites. Donors placed at the sites
SA and SA’ are sterically the most important due to the shortest carbonyl-Ti distance, while stericinfluences from the other sites are regarded as secondary otherwise negligible. This situation is
illustrated in Fig. 3b. Among ester molecules non-selectively distributed at SA-SE’, molecules
placed at SA and SA’ occupy the gray quadrants and force the growing chain and monomer toorient into the white quadrants. On the other hand, diether-type donors can coadsorb at the sites
SB-SE’ except SA and SA’ through the bidentate coordination again in a non-selective manner.
The sterically most important locations are SC and SC’, while the coadsorption at SE and SE’ may
have some level of influences. Thus, diester molecules occupy the white quadrants to orient thegrowing chain and monomer into the gray quadrants (Fig. 3c). On the contrary, the axes of theTi mononuclear species projected on the (100) surface are parallel or perpendicular to the
MgCl2 layers, and therefore coadsorption of donors cannot induce a C2 symmetry on the (100)surface.
In this way, the coadsorption of donors with the Ti mononuclear species on the (110) surface
is not only energetically and electronically advantageous, but also compatible with theenhancement of the isospecificity. Coadsorbed donors sterically transfer the C2 symmetry of theTi mononuclear species on the (110) surface to the growing chain and monomer so as to convert
an originally aspecific site into an isospecific one, which is automatically achieved as a result of
non-site-selective coadsorption of donors.

17

3.2. Influences of donors on the resting state
We have postulated the importance of the coadsorption of donors with the Ti mononuclear
species on the (110) surface in 3.1. Of course, it must not be the sole surface state of ZN
catalysts, exhibiting a variety of surface heterogeneity [43]. For example, the dominance of
agglomerated Ti species in the absence of donors suggests the location of Ti species on the(100) surface, whose fraction is largely reduced but still remains in the presence of donors [73].
Thus, we do not exclude the presence of the other Ti species (e.g. the dinuclear species on the(100) surface), but regard the coadsorption on the (110) surface as the most plausible state in thepresence of donors. Hereafter, taking EB as the simplest example of industrial donors, we
describe the electric and steric influences of coadsorption on the activity, stereospecificity, andregiospecificity of propylene polymerization. For this purpose, one EB molecule was placed at
the sterically most important site, SA (Fig. 3a), and incoming propylene was placed in the
opposite side of EB, based on the fact that steric influences of EB are mainly imposed on themuch bulkier growing chain [74]. The second EB molecule would be required at the site SA’ forthe second insertion, which was not considered in this study.
First, influences of EB were examined for the resting state (corresponding to the starting
point of the catalytic polymerization cycle). Fig. 4 illustrates four possible conformations of theresting state in the presence of EB. Similar four conformations also exist in the absence of EB(not shown). All the conformations possess agostic interactions, characterized by elongated C-H
distances compared with a typical C-H distance of 1.10 Å. The conformations in Figs. 4a and 4b
possess weak -agostic interactions with the corresponding C-H distances elongated into18

1.11-1.12 Å. They differ in the orientation of C of the growing chain (right and left), and are
precursors for the -agostic assisted insertion. The conformations in Figs. 4c and 4d possess
strong -agostic interaction with the corresponding C-H distances elongated into 1.14-1.15 Å.
They differ in the orientation of C (back and front), and are precursors for the -agostic
assisted insertion and the chain transfer to monomer. Conformations with -agostic interactions
as a direct product of the -agostic assisted insertion [75,76] were not considered due to thelimitation of the iBu growing chain. For all the conformations, the symmetry is trigonal
bipyramidal (TBP) rather than Oh.
Table 2 summarizes the relative stability of each conformation with respect to the most stable
-agostic conformation in the absence of EB, where each conformation is denoted according to
the type of the agostic interaction and to the orientation of C. The relative stabilities of theconformations in the presence of EB were derived by {E(TiCl2iBu/EB/MgCl2) –E(TiCl2iBu/MgCl2)} – {E(TiCl3/EB/MgCl2) – E(TiCl3/MgCl2)}. Since the stability of TiCl3 issimilar with and without EB (corresponding to a marginal coadsorption energy [51,52]), theobtained values mostly comprise of the energetic difference of TiCl2iBu with and without EB. In
the absence of EB, the most stable conformations are those with the -agostic interaction, whilethe -agostic conformations are about 4 kcal/mol less stable. The orientation of C did not affectthe stability for both the - and -agostic conformations. The nearest coadsorption of EB
destabilized the - and -agostic conformations by ca. 5 kcal/mol and by ca. 3 kcal/mol,
respectively. Since the degrees of the destabilizations were rather insensitive to the orientationof C for both the - and -agostic conformations, the influence of EB on the resting state wasconcluded as mostly electrostatic. This was also supported by the fact that the least bulky Me

19

was destabilized by EB similarly to the -agostic conformations. The coadsorption of EB
enriches the electron density of Ti species on the (110) surface. As a result, alkyl groups withpoor electron affinity are destabilized and the agostic interactions are weakened. The
destabilization was more obvious for the -agostic conformations with stronger agostic
interactions.
The values in the parentheses in Table 2 show the equilibrium population of eachconformation, calculated based on the Arrhenius law at 350 K (a typical polymerization
temperature). Since the transformation of the -agostic conformations into the -agostic
conformations required only 5-6 kcal/mol of the activation energies (much lower than the freeenergy of the insertion), mutual transformation among different conformations was regarded to
be fast enough to achieve the equilibrium population. Both in the absence and presence of EB,
the -agostic conformations dominate. However, EB increased the populations of the -agostic
conformations by a factor of 10, suggesting that EB may restrict the -agostic assisted insertionand the chain transfer to monomer compared with the -agostic assisted insertion.

3.3. Influences of donors on the π complex
Second, influences of EB on the  complexation of propylene were examined. Fig. 5 shows
the structures of the  complexes which were examined in this study. Propylene preferentiallycoordinates to Ti3+ with its double bond aligned perpendicularly to the Ti-C bond [77]. complexes with the double bond aligned parallel to the Ti-C bond also formed saddle points,
but they were always 1-3 kcal/mol less stable than those with the perpendicular alignment andnot shown here. For each conformation of the resting state, four orientations of propylene were20

calculated, where the methyl group of propylene was placed at either of R1-4. The position of themethyl group at C was also varied (at either RA or RB) for the -agostic conformations. In thepresence of EB, propylene was attached to the opposite side of EB, while the growing chain was
inclined to the side of EB.
For the simplest instruction, the adsorption of ethylene, propylene and fluoroethylene(C2H3F) to TiCl2Me was first studied in the absence of EB, and their adsorption energies (∆Ead)are summarized in Table 3. ∆Ead for ethylene was calculated to be –9.45 kcal/mol, in roughagreement with the previous DFT calculation (–7.3 kcal/mol) by Cavallo et al. [68] in spite ofdifferences in the functional and the model size. The ∆Ead values of propylene andfluoroethylene were insensitive to the positions (R1-4) of the methyl and fluorine substituents,and on average –8.62 and –6.37 kcal/mol, respectively. Thus, the presence of the substituents
weakened the binding of olefins, which was more obvious for electron withdrawing fluorine
than for the electron donating methyl group. This fact suggests that the  electron donation to
Ti3+ is important for the adsorption of olefins to Ti3+. The slightly weaker binding of propylene
than ethylene is likely attributed to the steric reason. The ∆Ead values for the same three kinds ofolefins in the presence of EB are shown in Table 4. The coadsorption of EB weakened thebinding of ethylene, propylene and fluoroethylene by 1.5, 2.4, and 1.0 kcal/mol on average,
respectively, which resulted from the competitive electron donations from EB and olefin into
TiCl2Me.
The replacement of Ti-Me with Ti-iBu largely weakened the binding of propylene. In theabsence of EB, the average ∆Ead value was –2.83 kcal/mol for Ti-iBu as compared with –8.62
kcal/mol for Ti-Me (Table 3). Both the stabilization of the resting state through the -agostic

21

interaction and the increased bulkiness around Ti mostly account for this reduction. The moststable species were  complexes with the -agostic conformation (back), giving the average∆Ead value of –5.40 kcal/mol. The  complexation with the -agostic conformation (front) was
less stable due to steric interference between H and propylene. The magnitudes of ∆Ead for the-agostic conformations were smaller than those for the -agostic conformation (back) by 3.2
kcal/mol on average. The energy difference of 3.2 kcal/mol roughly corresponds to the energetic
difference between the -agostic and -agostic conformations of the resting state (Table 2). The
contribution of the increased bulkiness was also observed in terms of the positional preferenceof the propylene methyl group. For example, when C was directed to the right side, the R4
position became disadvantageous due to the steric repulsion with C, while the R2 position was
relatively free from the repulsion, and so on.
The coadsorption of EB further reduced the magnitudes of average ∆Ead into –0.32 kcal/mol(Table 4). This destabilization partly originates from the prevention of the  electron donationfrom propylene due to the competitive electron donation from EB to TiCl2iBu. The steric
repulsion between iBu and EB also affected the ∆Ead values greatly, since the growing chain tilts
toward EB in the  complexation and sterically repulses with EB. The  complexation of
propylene with the -agostic conformations is sterically the most demanding to be least favoredin the presence of EB. Among the -agostic conformations, the right orientation of C (opposite
to EB) combined with the placements of the propylene methyl group at R1 or R2 led to the leaststeric interference, thus giving the most stable species. On the contrary, the left orientation of Ctended to be less favorable due to the steric repulsion between C and the carbonyl group of EB.
The ∆Ead values calculated for the propylene  complexation with Ti-iBu were always lower

22than the entropic losses around 14 kcal/mol, estimated using a cluster model at 350 K. This factassures that active sites are far from fully covered by propylene at typical polymerization
temperatures, thus validating the usage of the apparent activation energies in subsequent
insertion and chain transfer reactions [14,70]. Summing up the equilibrium populations
calculated for different structures of the  complexes at 350 K (Tables 3 and 4), the equilibrium
populations of the  complexes with specific enantiofaces of propylene or with specificconformations of the growing chain were given (Table 5). In the absence and even in thepresence of EB, the adsorption of propylene was not enantioselective (similar populations forthe re and si enantiofaces). This means that the stereoselectivity in propylene polymerization
results solely from the stereoselectivity at the transition states of insertion. As for theconformations of the growing chain, the  complexes with the -agostic conformation (back)were predominant in the absence of EB. However, the  complexes with the -agostic
conformation (right) became the main species in the presence of EB.

3.4. Influences of EB on the propylene insertion and chain transfer to propylene
As was mentioned in 3.1, the Ti mononuclear species on the (110) surface and its
coadsorption with donors are the most plausible scenarios, which can explain the experimentallyobserved electron donation picture and the amplification of C2 symmetry around the Ti species
by the addition of donors. Here, propylene polymerization performances of the Ti mononuclear
species on the (110) surface were evaluated in the absence and in the presence of coadsorbed
EB. For the validation of our scenarios, it is particularly important whether the Ti mononuclear
species coadsorbed with EB on the (110) surface can reproduce experimentally observed effects

23

of donors such as improvements of iso- & regio-specificities and enlargement of polymer
molecular weight [15,16,40-45,55,56].
Fig. 6 illustrates all the pathways examined for propylene insertion and chain transfer topropylene. The chain transfer to monomer is known as one of the major chain transfer pathways
in the absence of hydrogen [78,79]. In order to fully consider stereo and regio aspects of thesereactions, the position of the methyl group of propylene was varied at either of R1-4. The
placements of the methyl group at R1 and R2 correspond to the 1,2 orientation of propylene withthe re and si prochiral planes, while those at R3 and R4 correspond to the 2,1 orientation with thethe re and si prochiral planes, respectively. Propylene can react with Ti-iBu in different ways
according to the conformations of Ti-iBu. When Ti-iBu is situated in the -agostic
conformations, propylene can insert into the Ti-iBu bond with the aid of the -agostic
interaction, as shown in Figs. 6a and 6b. In addition to the orientation of C either in the right orleft side, the position of the methyl group at C was also varied either at RA or RB, where RA andRB are in the front and back sides against incoming propylene, respectively. In the case of the-agostic conformation (back), propylene can insert with the aid of the -agostic interaction(Fig. 6c). Differently from the other conformations, the -agostic conformation (front) cannot
be a precursor for the insertion. Instead, it can transfer -hydrogen to incoming propylene,
leading to the chain transfer to propylene (Fig. 6d). For the Ti-Me, only the -agostic assisted
insertion is feasible (Fig. 6e).
Table 6 corrects apparent activation energies (∆Eap) calculated for all the examined pathways
in the absence of EB. The 1,2 insertion of propylene into Ti-Me required 2.1 kcal/mol of ∆Eap,
regardless of the prochiral faces. The ∆Eap value for the ethylene insertion was –0.1 kcal/mol,24and that for the 1,2 insertion of fluoroethylene was 3.0 kcal/mol (on average). An inserting
olefin molecule gets polarized as a result of the -* mixing at the transition state: the carbon
atom of olefin that is bonding to C charges positively, while the carbon atom that is bonding to
Ti charges negatively [3,76,80]. Substituents of olefin electrostatically affect the stability of thepolarization at the transition state. An electron-withdrawing substituent at olefin further
withdraws electron density from positively charged C2 at the transition state of 1,2 insertion andthus destabilizes it, while an electron-donating substituent electrostatically stabilize thetransition state of 1,2 insertion. Accordingly, the highest ∆Eap value calculated for the 1,2insertion of fluoroethylene is simply explained with the electrostatic reason. On the other hand,
the higher ∆Eap value for the 1,2 insertion of propylene than for the ethylene insertion must be
attributed to steric congestion at the transition state, which exceeds the electrostatic contribution.
The ∆Eap values for the 2,1 insertion of propylene were 3.2-3.3 kcal/mol (again without
prochiral preference), 1.1 kcal/mol higher than those for the 1,2 insertion. Thus the Timononuclear species is moderately regioselective even in the absence of donors. Though highpropensity of MgCl2-supported ZN catalysts for the 1,2 insertion is well known [15,44,45], its
origin has not been fully understood yet. So far, two different explanations have beentheoretically made for cationic Ti(VI) homogeneous systems [76,81]. One is a steric origin thatthe methyl group of propylene repulses with ligands located on both sides of the Ti center in the
2,1 insertion. The other is an electronic origin that the electron-donating methyl group
destabilizes the transition state of the 2,1 insertion by further donating electron density into
negatively-charged C2. Judging from the fact that only quantum chemical calculations havesucceeded to reproduce clear preference to the 1,2 insertion in heterogeneous systems

25[44,49,70] (while molecular mechanics calculations failed [30]), the electronic origin must bevery important. In our additional calculations using chloroethylene having electron-withdrawing
Cl (whose size is similar to the methyl group for propylene), the 1,2 insertion was stilladvantageous but only by 0.6 kcal/mol over the 2,1 insertion. Thus, both the electrostatic andsteric contributions are believed to be important for the regiospecificity of ZN catalysts.
In the previous section, the replacement of Ti-Me by Ti-iBu largely reduced the magnitude of∆Ead for the  complexation of propylene, which was due to the stabilization of the resting statethrough the -agostic interaction and due to the increase of the bulkiness around Ti. The latter
factor also introduced energetic preferences toward specific conformations of  complexes inorder to minimize steric congestion. Similar consequences were obtained for the propylene
insertion. The replacement of Ti-Me by Ti-iBu gave rise to the increase of ∆Eap by 7.5 kcal/mol
on average, while the degree of the increase was dependent on the mutual conformations of thegrowing chain and propylene. Among -agostic assisted insertion pathways, the anti placementof the methyl group of propylene with respect to the C-C bond is favored for the 1,2 insertion,
while the syn placement is favored for the 2,1 insertion [29,49,77]. It was also found that the
methyl group at C of the growing chain was preferentially placed at RB (i.e. in the back side of
propylene) to reduce the steric congestion. As a consequence of these conformational
restrictions, two 1,2 insertion pathways had the similarly lowest ∆Eap values, ca. 7.4 kcal/mol,which is comparable with the experimentally determined activation enthalpies for the propylene
insertion (9-12 kcal/mol) [14,82,83], assuring the validity of the employed model and method inthis study. The two pathways are identical in terms of the anti placement of the propylene
methyl group and the location of the C methyl group at RB, while the orientation of the26

growing chain is opposite. This fact indicates that the Ti mononuclear species on the (110)
surface is aspecific (as has been conventionally believed) due to the inability to distinguish theorientation of the growing chain. As for the 2,1 insertion, the above-mentioned conformational
restrictions were rather small since the propylene methyl group does not largely interfere withthe growing chain. The energetically accessible pathways mostly had the C methyl group at RB,
whose average ∆Eap value, 8.9 kcal/mol, was 1.5 kcal/mol greater than the lowest 1,2 insertion
energies, i.e. moderately regioselective. A similar energetic difference (1.1-1.3 kcal/mol)
between the 1,2 and 2,1 insertion was previously reported in non-local DFT calculations [44,49].
The -agostic assisted 1,2 insertion exhibited similar ∆Eap values to the most preferable
-agostic assisted pathways, indicating that the 1,2 insertion of propylene can occur via bothtypes of the agostic assistances in the absence of EB. On the contrary, the -agostic assisted 2,1
insertion was disadvantageous compared with the -agostic assisted 2,1 insertion, plausibly
because the strong -agostic interaction increases the electron density of Ti and thus
electrostatically destabilizes the 2,1 insertion.
The -agostic conformation (front) leads to the chain transfer reaction to propylene. The
activation energies calculated for the four chain transfer pathways having different propylene
methyl placements are shown in Table 6. On the contrary to the insertion, the 1,2 placements ofthe propylene methyl group (at R1,2) were disadvantageous for the chain transfer. The chaintransfer energies with the 2,1 placement (at R3,4) were as low as the lowest 1,2 insertion energies,
indicative of a very low polymerization degree in the absence of EB.
Table 7 lists the ∆Eap values in the presence of an EB molecule coadsorbed at the nearest
Mg2+ site (SA). The 1,2 insertion of propylene into Ti-Me required on average 3.3 kcal/mol of27∆Eap, being 1.1 kcal/mol higher than in the absence of EB. Though the si prochiral orientation of
propylene (R2) was preferred over the re prochiral orientation (R1), the preference was as small
as ca. 0.5 kcal/mol to regard the steric influence of EB as secondary. Therefore, the increase ofthe ∆Eap values for Ti-Me was attributed mainly to the electrostatic destabilization of thetransition states, where the enhanced electron density at Ti restricted the  donation and theresultant activation of the double bond. Sacchi et al. [41] showed that the stereoselectivity of thefirst propylene insertion into Ti-Et was much lower than the average stereoselectivity in themain chain, while that into Ti-iBu was as high as the average stereoselectivity in the main chain.
Thus, the calculated small energy difference between the two prochiral planes of the insertion
into Ti-Me is consistent with their experimental observation that alkyl groups smaller than iBu
do not have sufficient repulsion with a bulky ligand, leading to lower stereoselectivity.
The average ∆Eap value for the 2,1 insertion into Ti-Me was 5.3 kcal/mol to be 2.0 kcal/mol
greater than that for the 1,2 insertion. Compared with the regiospecificity of 1.1 kcal/mol in theabsence of EB, the coadsorption of EB obviously increased the regiospecificity of the Timononuclear species. Similar ∆Eap values for the re and si prochiral orientations imply that theenhancement of the regiospecificity by EB is not caused by steric repulsion between the side Cl
ion (forwarded to propylene by EB) and the methyl group of propylene. Most plausibly, theadditional electron transfer from EB to Ti further enlarged electrostatic repulsion between the Ti
center and C2 carbon for the 2,1 insertion so as to destabilize it compared with the 1,2 insertion.
For Ti-iBu, comparison of the corresponding ∆Eap values in the absence and presence of EB(Tables 6 and 7) revealed that the coadsorption of EB basically increased the ∆Eap values. The
degree of the increase was dependent on the conformation of the growing chain as well as on

28

the positions of the methyl groups for Ti-iBu. First, the -agostic assisted insertion and the chaintransfer to propylene, both having the -agostic conformations as direct precursor, weresignificantly destabilized by the coadsorption of EB. The increases of the ∆Eap values were, on
average, 7.3 kcal/mol for the former and 4.8 kcal/mol for the latter. The destabilization of the-agostic assisted insertion pathways, having the -agostic conformations as direct precursor,
was less, but the way of the destabilization was not uniform. The average ∆Eap value became
1.57 kcal/mol higher for the pathways having the growing chain oriented toward the opposite(right) side of EB, and 2.12 kcal/mol for the growing chain oriented toward the EB (left) side, ascompared with the corresponding pathways in the absence of EB. On the other hand, theaverage ∆Eap value increased by 1.68 kcal/mol for the 1,2 insertion and 2.12 kcal/mol for the 2,1
insertion (among the -agostic assisted insertion). Thus, EB not only sterically biases theorientation of the growing chain, but also electrostatistically prevents the 2,1 regio-misinsertion.
The pathway having the lowest ∆Eap value possessed the positions of the methyl groups at R2(si) and at RB, and the orientation of the growing chain to the right side. Its ∆Eap value, 7.9
kcal/mol, was only ca. 0.5 kcal/mol higher than that of the corresponding pathway in theabsence of EB, as a result of minimal steric interference. On the other hand, the pathway having
the left-oriented growing chain as well as the placements of the methyl groups at R1 (re) and RB
was destabilized by ca. 2.8 kcal/mol, due to stronger steric repulsion between the growing chain
and EB. Consequently, the lowest ∆Eap value for the 1,2-re insertion became 2.3 kcal/mol higher
than that for the 1,2-si insertion, indicating that the nearest coadsorption of EB transforms the
aspecific Ti mononuclear species into isospecific one. Our previous calculations using a small
MgCl2 cluster [77] revealed significant influences of the rigidity of the Mg-carbonyl bond on29

the isospecificity, where fully relaxed EB never improved the isospecificity, while the fixationof the positions of the carbonyl group during propylene insertion resulted in high isospecificity.
Meanwhile, the present calculations using thick MgCl2 slab led to a different conclusion that afully relaxed EB molecule can enhance the isospecificity. This deviation arises from thedifference of the MgCl2 models, where a small MgCl2 cluster underestimated the rigidity of the
Mg-carbonyl bond. Thus, a sufficiently large MgCl2 model (such as the slab employed here) is
critically important for accurate description of the steric influence of donors in propylene
insertion.
The coadsorption of EB expanded the difference of the lowest ∆Eap values for the 1,2 and 2,1
insertion to 2.1 kcal/mol, as compared to 1.3 kcal/mol in the absence of EB. As long as we know,
this result corresponds to the first reproduction of the experimentally observed improvement of
the regiospecificity by the addition of donors. Previous DFT calculations reached a different
conclusion that the placement of succinate molecules at the nearest Mg2+ ions did not affect theregiospecificity [49]. This deviation is attributed to a rather small MgCl2 cluster in the previous
study [49], which cannot fully account for the electron re-distribution among MgCl2, Ti species
and donors. Meanwhile, the previous [49] and present DFT studies coincided in the fact that the
2,1 insertion prefers the prochiral plane (re) opposite to the plane (si) preferred by the 1,2insertion.
As a summary, EB coadsorbed at the nearest Mg2+ ion improved the isospecificity of theoriginally aspecific Ti mononuclear species through the steric repulsion with the growing chain,and the regiospecificity through the electron enrichment of the Ti species. Based on thesemechanisms, it is naturally expected that the degrees of the improvements of the specificities are30

dependent on methods and models employed. At least, a small MgCl2 model could reachconclusions which were inconsistent with the experimentally observed effects of donors.
Table 8 shows equilibrium probabilities of different reactions at 350 K, and those of theconformations of the growing chain involved in the reactions at the same temperature, whichwere calculated based on the data in Tables 6 and 7. As was already described, the entropic
contributions were assumed to be similar among the different reactions, and neglected in thecalculations. In the absence of EB, both the 1,2 and 2,1 insertion is not stereoselective without
capability to control the orientation of the growing chain. This is consistent with theconventional idea that the Ti mononuclear species on the MgCl2 (110) surface is aspecific,
lacking stereo-regulating ligands [25,47,49]. Though the 1,2 insertion is basically favored, theprobability of the regio-misinsertion among the insertion is estimated to be ca. 15% in theabsence of donors, which is much greater than the experimentally reported lowest
regiospecificity, 1.5% for the hexane-soluble fraction of polypropylene in the absence of donors[44]. This might be explained by two factors: first, completely atactic polypropylene is neverformed in actual heterogeneous ZN propylene polymerization, whose reason will be describedlater. Second, the regiospecificity is rather sensitive to the kind of DFT functionals [44]. The
probability of the chain transfer to monomer as high as 26% suggests inability of the chain
growth at the Ti mononuclear species on the (110) surface. This plausibly explains the reason
why completely atactic polypropylene is never obtained in heterogeneous ZN propylene
polymerization, and consistent with a known fact that less tactic polymer tends to be shorter andcontain a larger content of end unsaturation [78]. The four kinds of the growing chain
conformations are almost equally involved in the catalysis, although the resting state dominantly

31

exists as the  agostic conformations (Table 2).
The coadsorption of EB largely restricts the conformations of the growing chain which can be
involved in the catalysis with propylene. Most prominently, the coadsorption prevents the-agostic assisted pathways, which are sterically demanding. As a result, the probability of thechain transfer to monomer, the main cause to prevent the growth of completely atactic chain,
drops to 0.5%. This result is consistent with the fact that more isotactic polymer tends to belonger and contain a smaller end unsaturation fraction [78]. The second consequence brought
about by the coadsorption of EB is the restriction of the -agostic insertion into the left-oriented
growing chain, leading to a dramatic reduction of the probability for the 1,2-re insertion. The
stereoregularity estimated from the calculated probabilities corresponds to 82 mol% of mmmm,
rather lower than a typical experimental value (95 mol%) [15]. However, considering the
exponential sensitivity of mmmm, the estimation is regarded as quite reasonable. The probabilityof the 2,1 insertion moderately decreases from 15% to 6% by the coadsorption of EB. Though
the probability of 6% is still much higher than experimentally estimated regioselectivity [44,45],
it is more important that we could firstly reproduce the improvement of the regiospecificity inthe presence of donors. As was mentioned, one of the purposes of the present study is toqualitatively depict the steric and electronic influences of donors on propylene polymerization
by means of comprehensive DFT calculations. In this sense, the model based on the nearest
coadsorption of donors (as well as the employed methods) totally succeeded to reproduce
experimentally known improvements of stereo- and regio-specificities, and increase ofmolecular weight by the addition of donors [15,16,40-45,55,56], allowing us to propose aworking active site model, so called “coadsorption model.”32

4. Conclusions
A series of periodic DFT calculations have been conducted to conclude on the mechanism fordonors to influence polymerization performances of MgCl2-supported ZN catalysts. Systematicand exclusive calculations based on appropriate levels of methods and MgCl2 models have been
performed not only on the states and interactions of the catalytic components but also on theirinfluences on the polymerization performances, and the obtained results were validated in termsof various experimental aspects. Major findings are described as follows.
Considering that donors stabilize active MgCl2 surfaces to increase their fractions during
preparation [23,24], diether-type donors are expected to form (110)-rich MgCl2, whilemono- and di-ester-type donors can form MgCl2 exposing both the (110) and (100) surfaces.
In both of the cases, TiCl4 tends to adsorb on the (110) surface.

The mutual location of TiCl4 and donors on the (110) surface is energetically randomly
decided, and they tend to coadsorb close to each other at a high surface coverage, which isthe case for real catalysts.



The coadsorption of TiCl4 and donors on the (110) surface not only reproduces
experimentally observed enrichments of the Ti electron density in the presence of MgCl2
and donors [53,54], but also explains the amplification of C2 symmetry around the Ti
species as a result of random placements of donors.

In the absence of donors, the Ti mononuclear species on the (110) surface is totally
aspecific without capability to distinguish the orientation of the growing chain at thetransition state, while it is moderately regiospecific favoring the 1,2 insertion. The chain

33

transfer to propylene is as facile as the insertion, indicating no chain growth at thecompletely aspecific Ti mononuclear species.
The nearest coadsorption of EB i) converts the aspecific Ti mononuclear species into
isospecific one by sterically controlling the orientation of the growing chain at thetransition state, ii) improves the regiospecificity as a result of increased electrostatic
repulsion for the 2,1 insertion, iii) and dramatically prevents chain transfer to propylene,
thus enabling the chain growth at the Ti mononuclear species.
In this way, it was successfully proven that the coadsorption of donors with the Timononuclear species on the MgCl2 (110) surface is not only energetically plausible but also canreproduce a number of experimental facts such as the enhancements of iso- andregio-specificities and molecular weight by the addition of donors. The proposed “coadsorption
model” gives a comprehensive molecular-level description for the roles of donors as well as ascientific basis for the first-principle prediction of new donor structures.

Acknowledgement
This work was financially supported by Grants-in-Aid for Scientific Research from Japan
Society for the Promotion of Science, Japan Polypropylene Corporation, Samsung Total
Petrochemicals Co., Ltd., Sumitomo Chemical Co., Ltd., and Toho Titanium Co., Ltd.

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39

Figure captions
Fig. 1. Adsorption structures of TiCl4 on MgCl2 surfaces: a) mononuclear species on the (110)
surface, and b) mononuclear and c) dinuclear species on the (100) surface. Black: Mg, white: Cl,
purple: Ti.

Fig. 2. Adsorption structures of donors on MgCl2 surfaces (only relevant atoms are shown). a,b)
Benzoate adsorbed on the (110) and (100) surfaces in a monodentate fashion, c-e) phthalate
adsorbed on the (110) surface in bidentate, intra-bridging, and inter-bridging fashions, f)phthalate adsorbed on the (100) surface in an intra-bridging mode. Black: Mg, white: Cl (balls)& H (sticks), gray: C, red: O. The color schemes in Figs. 1 and 2 are used in subsequent Figs.

Fig. 3. Origins of a C2 symmetry and isospecificity of the Ti mononuclear species on the MgCl2(110) surface. a) Top view of Ti mononuclear active species on the (110) surface, where SA-E’
describe Mg2+ sites available for the coadsorption of donors. The projection of the reaction
space around the Ti species is schematically illustrated in the right side. Note that the two whiteand gray quadrants defined across the Oh axes of the Ti species are not equivalent in terms of theunderlying MgCl2 layers. b) Coadsorption of benzoate molecules at the sterically mostimportant SA,A’ sites to occupy the gray quadrants, and c) coadsorption of diester molecules atthe sterically most important SC,E,C’E’ sites to occupy the white quadrants.

Fig. 4. Conformations of the resting state at the Ti mononuclear species on the MgCl2 (110)
surface. a,b) -agostic conformations with right and left orientations of the growing chain (iBu),

40

and c,d) -agostic conformations with back and front orientations of the growing chain.
Hydrogen atoms relevant to the agostic interactions are yellow-colored. These conformations
were examined both in the absence and presence of EB.

Fig. 5. Structures of  complexes between propylene and the Ti mononuclear active species on
the MgCl2 (110) surface. a,b)  complexes for the -agostic conformations of the iBu growing
chain (right- and left-oriented), c,d)  complexes for the -agostic conformations of the iBu
growing chain (back- and front-oriented), and e)  complexes for the Me growing chain. f) The
position of the methyl group of propylene (yellow-colored) was varied at either of R1-4, and theposition of the methyl group at C of iBu (blue-colored) was varied at either RA or RB. These
structures were examined both in the absence and presence of EB.

Fig. 6. Pathways examined for propylene insertion and for chain transfer to propylene at the Timononuclear active species on the MgCl2 (110) surface. a,b) Insertion into Ti-iBu with the-agostic conformations (right- and left-oriented), c) insertion into Ti-iBu with the -agostic
conformation (back-oriented), d) chain transfer to propylene, and e) insertion into Ti-Me. f) The
positions of the methyl groups of propylene and at C of iBu were similarly varied. Thesepathways were examined both in the absence and presence of EB.

41

Table 1
Adsorption energies of TiCl4 and internal donors on MgCl2 surfaces.
MgCl2 surface Adsorbate

Structure∆Ead[kcal/mol](110)

Mononuclear–19.8

Benzoate

Monodentate–29.8

Phthalatea

Bidentateb

TiCl4
a

–38.8
b

Intra-layer bridge

–32.5

b

–33.7

Inter-layer bridgeab

Succinate

Bidentate–38.6bIntra-layer bridge

–36.9

b

–43.3

Inter-layer bridge
a

(100)

b

1,3-diether

Bidentate

–31.1

TiCl4

Mononuclear–13.0

Dinuclear–11.1

Benzoate

Monodentate–30.5

Phthalatea

Intra-layer bridgeb –42.4

Succinatea

Intra-layer bridgeb –45.0

a1,3-diethera Intra-layer bridgeb –7.5
aBenzoate:

ethylbenzoate(EB);phthalate:diethylphthalate;succinate:(2R,3R)-2,3-diisopropyldiethylsuccinate; 1,3-diether: 2,2-diisopropyl-1,3-dimethoxypropane.
b“Bidentate” is sometimes called as “chelate”. “Intra-layer bridge” and “inter-layer bridge”

correspond to bridging adsorption of donors within one MgCl2 layer and between two
neighboring MgCl2 layers, respectively. The latter is also called as “zip” [49].

42Table 2
Stability of different conformations of the resting state in the absence and presence of EB.
Conformationa

Relative stability [kcal/mol]b
Without EB

With EB

-agostic (right) 3.95 (0.19%)

7.58 (1.30%)

-agostic (left)

3.90 (0.20%)

7.17 (2.33%)

-agostic (back)

0.00 (55.6%)

5.62 (22.0%)

-agostic (front)

0.16 (44.0%)

4.77 (74.4%)

0.00

3.12

c

Me
a

The corresponding structures are shown in Fig. 4.

b

The relative stability of each conformation was calculated by setting the energy of the moststable β-agostic conformation (back) in the absence of EB as the zero-point energy. The stabilityof each conformation in the presence of EB was derived by {E(TiCl2iBu/EB/MgCl2) –E(TiCl2iBu/MgCl2)} – {E(TiCl3/EB/MgCl2) – E(TiCl3/MgCl2)}. The values in the parenthesis
are the equilibrium populations of the corresponding conformations at 350 K.
c

Me was used instead of iBu.

43

Table 3
 complexation energies of propylene in the absence of EBa.
Growing chain ∆Ead [kcal/mol]b
R1 (re)R2 (si)R3 (re)R4 (si)-agostic(right, RA)–2.24(0.22%)–3.52(1.43%)–1.01(0.04%)–1.86(0.13%)

-agostic(right, RB)–1.62(0.09%)–3.04(0.71%)–3.27(0.99%)–1.72(0.11%)

-agostic(left, RA)–0.73(0.03%)–3.40(1.21%)–3.63(1.67%)–2.76(0.48%)

-agostic(left, RB)–0.26(0.01%)–2.40(0.28%)–2.17(0.20%)–1.55(0.08%)

-agostic(back)

–5.77
–5.33
–5.27
–5.23
(36.4%) (19.2%) (17.7%) (16.8%)

-agostic(front)–2.66(0.41%)–2.36(0.27%)–2.73(0.45%)–3.36(1.13%)

Mec–8.14(11.5%)–8.89(33.6%)–8.87(32.9%)–8.59(21.9%)a

The corresponding structures are shown in Fig. 5.

b

The adsorption energy (∆Ead) of propylene was calculated based on E( complex) – {E(themost stable resting state) + E(gaseous propylene)}. The values in the parenthesis are theequilibrium populations of the corresponding adsorption states at 350 K.
c

Me was used instead of iBu. ∆Ead of ethylene was –9.45 kcal/mol and ∆Ead of fluoroethylene(on average) was –6.37 kcal/mol for Ti-Me.

44

Table 4
 complexation energies of propylene in the presence of EB.
Growing chain ∆Ead [kcal/mol]
R1 (re)R2 (si)R3 (re)R4 (si)-agostic(right, RA)

–1.38
–2.39
(25.4%) (5.96%)

-agostic(right, RB)–1.15(4.26%)

–0.88
–0.12
–2.23
(2.92%)(0.97%)(20.4%)

-agostic(left, RA)

0.02(0.79%)–0.62(2.00%)

–0.11
–1.81
(0.95%) (11.0%)

-agostic(left, RB)–0.31(1.27%)–1.48(6.91%)

–0.56
–0.77
(1.83%) (2.46%)

-agostic(back)–1.65(8.83%)

1.53(0.09%)

2.13–0.62(0.04%) (2.00%)

-agostic(front)0.55(0.37%)

1.71(0.07%)

0.76
0.64(0.27%) (0.33%)

Mea

–6.67
(33.8%)–5.93(11.6%)

–5.25
–6.95
(4.35%) (50.3%)

a

0.29
0.67
(0.54%) (0.31%)∆Ead of ethylene was –7.98 kcal/mol and ∆Ead of fluoroethylene (on average) was –5.41 for

Ti-Me.

45

Table 5
Equilibrium populations of the  complexes with specific enantiofaces of propylene or withspecific conformations of the growing chaina.
Conformation
R1,3 (re)Without EB With EB

b

58.2%

47.5%

41.8%

52.5%

-agostic (right)

3.72%

60.8%

-agostic (left)cR2,4 (si)

b
c

3.97%

27.2%

c

-agostic (back)

90.1%

11.0%

-agostic (front)c

2.26%

1.03%

a

Calculated based on the equilibrium populations of the  complexes at 350 K in Tables 3 and4.
bSummation over all the  complexes with specific enantiofaces of propylene (irrespectively of

the conformations of the growing chain).
cSummation over all the  complexes with specific conformations of the growing chain(irrespectively of the propylene enantiofaces).

46

Table 6
Apparent activation energies of propylene insertion and chain transfer to propylene in theabsence of EBa.
Growing chain∆Eap [kcal/mol]b
R1 (re)R2 (si)R3 (re)R4 (si)-agostic(right, RA)

15.4(0.00%)

9.20(1.34%)

10.8(0.13%)

9.45(0.93%)

-agostic(right, RB)

14.2(0.00%)

8.82
7.36(18.9%) (2.31%)

9.39(1.02%)

-agostic(left, RA)

9.71(0.64%)

11.3(0.06%)

10.8(0.13%)

14.5(0.00%)

-agostic(left, RB)

12.8
7.52
(15.0%) (0.01%)

8.66(2.91%)

8.90(2.06%)

-agostic (back)

9.21
7.91
7.37
(8.57%) (18.7%) (1.32%)

11.0(0.10%)

-agostic (front)c 8.42
(4.11%)

8.49(3.72%)

8.26
7.63(12.8%) (5.18%)

Med

2.07
(44.2%)

3.18(8.94%)

2.15(39.4%)

3.31(7.41%)a

The corresponding pathways are shown in Fig. 6.

b

The zero-point energy to calculate the apparent activation energies was set to the total energyof the most stable resting state and gaseous propylene [14,70]. The values in the parenthesis are
the probabilities of the corresponding pathways at 350 K.
c

Chain transfer to propylene.

d

Me was used instead of iBu. ∆Eap for ethylene insertion was calculated to be –0.07 kcal/mol,and ∆Eap for the 1,2 insertion of fluoroethylene was on average 2.95 kcal/mol.

47

Table 7
Apparent activation energies of the propylene insertion and chain transfer to propylene in thepresence of EB.
Growing chain∆Eap [kcal/mol]
R1 (re)R2 (si)R3 (re)R4 (si)-agostic(right, RA)

16.8
9.90
(0.00%) (4.26%)

12.6(0.09%)

11.5(0.41%)

-agostic(right, RB)

16.5
11.9
7.90
9.07
(0.00%) (76.0%) (14.1%) (0.24%)

-agostic(left, RA)

11.7
13.9
(0.31%) (0.01%)

13.5(0.02%)

15.3(0.00%)

-agostic(left, RB)

10.2
14.0
(2.89%) (0.01%)

11.6(0.37%)

11.0(0.86%)

-agostic (back)

13.8
14.4
(0.02%) (0.01%)

17.6(0.00%)

18.7(0.00%)

12.8
-agostic (front) 12.6
(0.09%) (0.07%)

11.9(0.25%)

14.7(0.00%)

Mea

5.31(2.37%)

5.26(2.55%)

a

3.51
3.03
(31.7%) (63.4%)

Me was used instead of iBu. ∆Eap for the ethylene insertion was 1.50 kcal/mol, and ∆Eap forthe 1,2 insertion of fluoroethylene was on average 4.29 kcal/mol.

48

Table 8
Equilibrium probabilities for the reactions and for the conformations of the growing chain
involved in the reactionsa.
Conformation

Without EB With EB

1,2-re insertionb

24.3%

3.59%

b

39.0%

89.6%

2,1-re insertionb

6.79%

4.71%

4.11%

1.69%

25.8%

0.46%

-agostic (right)

24.7%

94.5%

d

1,2-si insertion
2,1-si insertionbcchain transfer to propylened-agostic (left)

20.8%

4.99%

d

28.7%

0.03%

d

25.8%

0.46%

-agostic (back)-agostic (front)aCalculated based on the equilibrium probabilities of different pathways at 350 K in Tables 6

and 7.
b

Summation over all the 1,2 or 2,1 insertion pathways with the re or si prochiral plane.

cSummation over all the chain transfer pathways to propylene.

dSummation over the pathways with specific conformations of the growing chain.

49

Fig. 1.

Fig. 2.

50

Fig. 3.

Fig. 4.

51

Fig. 5.

Fig. 6.

52