Isomeric effect of the Et(H4Ind)2Zr(CH3)2 catalyst on the copolymerization of ethylene and styrene:...

Isomeric Effect of the Et(H4Ind)2Zr(CH3)2 Catalyston the Copolymerization of Ethylene and Styrene: AComputational Study

SONIA MARTINEZ, JAVIER RAMOS, VICTOR L. CRUZ, JAVIER MARTINEZ-SALAZAR

Departamento de Fısica Macromolecular, Instituto de Estructura de la Materia, Consejo Superior de InvestigacionesCientıficas, Serrano 113 Bis, 28006 Madrid, Spain

Received 23 March 2006; accepted 26 May 2006DOI: 10.1002/pola.21569Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A density functional theory (B3LYP) computational study of the ethylene–styrene copolymerization process using meso-Et(H4Ind)2Zr(CH3)2 as the catalyst is pre-sented. The monomer insertion barriers in meso species are evaluated and comparedwith previously obtained barriers in rac diastereoisomers. Differences related to ethyl-ene homopolymerization and ethylene–styrene copolymerization activities as well asstyrene incorporation into the copolymer are found between the meso and rac diaster-eoisomers. Nevertheless, a migratory insertion mechanism seems to hold for both dia-stereoisomeric species. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:

4752–4761, 2006

Keywords: computer modeling; copolymerization; density functional theory (DFT)methods; ethylene–styrene copolymerization; metallocene catalysts; quantum chemis-try; rac–meso diastereoisomers

INTRODUCTION

Ziegler–Natta heterogeneous catalysts are com-monly used in the polymer industry for ethylenecopolymerization with a-olefins. However, theyare inefficient for ethylene–styrene copolymer-izations as they show low activity and poor incor-poration of styrene with values lower than 1%.1,2

Furthermore, as these catalysts present differentactive centers, the styrene content in the copolymerchanges among the different copolymer chains. Inrecent years, the use of several homogeneous cata-lysts based on titanium or zirconium and activatedby methylaluminoxane has permitted the satis-factory synthesis of ethylene–styrene copolymers.Depending on the catalyst structure and polymer-

ization conditions (the ethylene/styrene concen-tration ratio in the feed, temperature, etc.), abroad variety of materials can be produced. Forexample, a low amount of styrene in the polymerbackbone gives rise to crystalline, thermoplasticmaterials. With an increase in the styrene con-tent, a reduction in the crystallinity is obtained. Afurther increase in styrene yields fully amorphousproducts that exhibit elastomeric properties. Whenthe styrene content is sufficiently high to move theglass transition up to ambient temperatures, rigidpolymers are obtained.3–8

Despite the great number of experimental stud-ies concerning ethylene–styrene copolymerizationusing homogeneous catalysts,3–5 our previous com-putational studies9–13 constitute pioneering workin trying to explain theoretically some experimen-tal results obtained for this process. Some of thesestudies are related to the rac-Et(H4Ind)2Zr(CH3)2catalyst, which produces copolymers with low sty-rene concentrations, around 1 mol %.10,11 How-

Correspondence to: V. L. Cruz (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 4752–4761 (2006)VVC 2006 Wiley Periodicals, Inc.

4752

ever, as far as we know, the only experimentalwork available in the literature concerning theEt(H4Ind)2Zr(CH3)2 catalyst for ethylene–styrenecopolymerization reports a high styrene concentra-tion, about 20 mol %, when the styrene concentra-tion in the feed is 80 mol %.14 Unfortunately, thereis no mention of which diastereoisomer is used (racor meso) in that patent.

The synthesis of the zirconocene complex cata-lyst Et(H4Ind)2ZrCl2 leads mainly to the rac dia-stereoisomer.15–17 Nevertheless, the amount ofthe meso form that is produced is also significant.The rac isomer can be obtained in its pure stateby the stirring of the mixture with hot tolueneand then cooling to �20 8C. On the other hand, iftoluene solutions of bridge zirconocenes areexposed to daylight, a conversion of the rac dia-stereoisomer into the meso diastereoisomer takesplace.18 This conversion can also be acceleratedby irradiation with UV light until it reaches itsphotostationary state.19 The final rac/meso pro-portion and interconversion rate depend on thestructures of the different metallocenes.

Because of the symmetry of this type of ansa-metallocene catalyst, the rac form (C2 symmetry)gives isotactic homopolymers, whereas the mesoform (Cs symmetry) produces atactic homopoly-mers. Also, rac-Et(H4Ind)2ZrCl2 is more active forpropylene homopolymerization than its meso dia-stereoisomer.20 In the polymerization of higher a-olefins such as 1-pentene, 1-hexene, and 1-octene,the meso isomers of different metallocenes withsubstituted indenyl ligands are much more activethan the rac forms of these compounds.19

Finally, the possibility of a retention mecha-nism during the polymerization process has beenproposed for catalytic systems based on the meso-Et(H4Ind) ligand.

19,21 This mechanism should be-come favorable for higher a-olefins. In the reten-tion mechanism, the growing chain might alwaysoccupy the same coordination position in succes-sive coordination steps. This means that the poly-mer chain could move back to the previous coordi-nation position after monomer insertion, which isthe opposite of the generally accepted chain migra-tory insertion mechanism proposed by Cossee.22

Our interest in this study is to explore the activ-ity and ability of comonomer insertion of the meso-Et(H4Ind)2Zr(CH3)2 catalyst and the amount ofstyrene in copolymers synthesized with this cata-lyst and to determine whether the retention me-chanism is preserved for the ethylene/styrenecopolymerization. The computed values are com-pared with those obtained for the rac diaster-eoisomer.11

COMPUTATIONAL METHODS

Geometry, energy, and vibrational frequency cal-culations have been performed with the B3LYPhybrid density functional theory model,23,24 whichhas been shown to be quite reliable for both geom-etry and energy calculations when applied toorganometallic systems.25,26 All calculations wereperformed with the Gaussian98 package.27

The LANL2DZ basis set was employed for allatoms. This basis set makes use of a Dunning–Huzinaga full double f on first-row atoms andthe Los Alamos National Laboratory 2 electroncore pseudopotential for the innermost electronsplus a Dunning–Huzinaga full double f for theouter atomic shells on the Zr atom.28 In a previ-ous study, it has been shown that adding polar-ization functions to the basis set has a smalleffect on the energy profiles calculated for the in-sertion reactions.13

Transition-state geometries were obtained bythe synchronous transit-guided quasi-Newton29

method to locate an estimated saddle point in thepath from the reactant to the product. Subse-quently, we completely optimized this saddlepoint by following the negative eigenvector. Fre-quency calculations were performed to check thenature of the stationary points found. All thetransition states were checked over one imaginaryfrequency visualizing then the correspondingeigenvector.

Complexation energies were calculated as thedifference between the p-complex structure andthe separated species. Insertion barriers werecalculated as the difference between the transi-tion state and the most stable structures (sepa-rated species or p complex). Reaction energieswere calculated as the difference between themost stable products and the separated species.All the results are discussed in terms of Gibbsfree energies.

Thermodynamic data were calculated at 308.15 Kand 3 bar with standard statistical thermody-namic methods implemented in the Gaussian98package.27 To calculate the Gibbs free energy pro-files, thermal and entropic contributions weretaken into account.

For these calculations, the role assigned to thecocatalyst was to create the active site by alkyl-ating the catalyst precursor and removing one ofthe methyl groups. The cationic zirconium spe-cies with a vacancy in the coordination spherewas then considered as the starting point for thepolymerization reaction with the monomers.

COPOLYMERIZATION OF ETHYLENE AND STYRENE 4753

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

RESULTS AND DISCUSSION

Cationic Species meso-Et(H4Ind)2Zr(CH3)+

The meso-Et(H4Ind)2Zr(CH3)2 species is a cata-lyst precursor for the polymerization. For theinsertion of the monomer to take place, the cati-onic species meso-Et(H4Ind)2Zr(CH3)

þ, having avacant position, needs to be created. Because ofthe symmetry of meso species, the two vacantpositions available are not equivalent. This is, in-deed, a key difference between this species and itsrac diastereoisomer, as can be seen in Scheme 1.The two vacant positions differ from each other intheir relative position to the aromatic ligands. Inone case, the vacant site can be located betweenthe aromatic ligands. In this case, the vacant siteis inward, and it is expected to be sterically morehindered than the vacant site in the rac diaster-eoisomer. These species are denoted meso1. In theother case, the vacant site is located on the oppo-site side to the aromatic ligands. These specieshaving the vacant position outward and at thesame time less hindered than in the case of therac species are denoted meso2.

Taking into account the notation explained pre-viously, we find that there are two different cationicmeso-Et(H4Ind)2Zr(CH3)

þ catalytic species thatcan start the polymerization process. These speciesare denoted cation meso1 (cm1) and cation meso2(cm2), which present the vacant position inwardand outward, respectively. The optimization of thecm2 structure leads to the cm1 structure becauseof the less steric interactions in the latter species.

Chain-Initiation Step into themeso-Et(H4Ind)2Zr(CH3)

+ Species

Ethylene, primary or 1,2-styrene, and secondaryor 2,1-styrene insertions into cm1 and cm2 spe-cies have been studied. In what follows, the mainresults are emphasized.

Ethylene Insertion into themeso-Et(H4Ind)2Zr(CH3)

+ Species

Geometric parameters for the p-complex andtransition-state structures corresponding to eth-ylene insertion into the cm1 and cm2 species arepresented in Table 1. In both p complexes, the eth-

Scheme 1. Cationic structures for (a) rac-Et(H4Ind)2Zr(CH3)þ and (b) meso-

Et(H4Ind)2Zr(CH3)þ species.

4754 MARTINEZ ET AL.


ylene monomer is parallel to the Zr��CME bond.Transition-state structures present the character-istic a-agostic interaction of these species.

Complexation energies and Gibbs free energybarriers for ethylene insertion into the cm1 andcm2 species are presented in Table 2. Correspond-ing values for ethylene insertion into the rac-Et(H4Ind)2Zr(CH3)

þ species, cation rac (cr), obtainedin a previous study11 are also given. Ethylene com-plexation into cm2 species (�1.9 kcal/mol) is morefavorable than that into cm1 species (2.5 kcal/mol).Ethylene complexation into cr species is intermedi-ate (0.0 kcal/mol). However, the ethylene insertionbarrier is higher for cm2 species (11.9 kcal/mol)than for cm1 species (9.8 kcal/mol). Both barriersare higher than that corresponding to the cr spe-cies (8.8 kcal/mol).

The most stable products for ethylene insertioninto cm1 and cm2 species present a b-agostic in-teraction and are �10.3 and �10.1 kcal/mol morestable than the separated species. According tothe notation explained previously, the productcorresponding to an ethylene insertion into cm1 isnamed product-E meso2 and presents the vacantsite outward because of the alternating mecha-nism. In the same way, the product correspondingto an ethylene insertion into cm2 is named prod-uct-E meso1 and presents the vacant site inward.Both possibilities are depicted in Scheme 2.

From Table 2, it can be concluded that ethylenecomplexation into cm2 species is more favorablethan that into cm1 species. However, the insertionenergy barrier for cm2 species is higher than thatfor cm1 species. On the other hand, product-Emeso2is slightly more stable than product-E meso1.

The population ratio between the cm1 p com-plex and cm2 p complex, nMeso2/nMeso1, can be cal-culated with Boltzmann statistics:

nMeso2=nMeso1 ¼ exp ��GcomplexationMeso2

RT

� ��

exp ��GcomplexationMesol

RT

� �ð1Þ

where nMeso2 and nMeso1 stand for the populationsof the cm2 p complex and cm1 p complex, respec-tively; DGcomplexationMeso1 and DGcomplexationMeso2

are the complexation energies for the cm1 andcm2 species, respectively; R is the gas constant;and T is the absolute temperature. The calculatedvalue for the ratio nMeso2/nMeso1 for the ethylenemonomer is 1207. Furthermore, based on thispopulation ratio and on the Gibbs free energyinsertion barriers, the probability ratio of ethyl-ene insertion into cm2 and cm1, PMeso2/PMeso1,can be calculated according to the following equa-tion:

PMeso2=PMeso1 ¼ nMeso2

nMeso1exp ��G#

insertionMeso2

RT

!,

exp ��G#insertionMeso1

RT

!ð2Þ

where DG#insertionMeso1 and DG#

insertionMeso2 arethe Gibbs free energy insertion barriers forthe cm1 and cm2 species, respectively. Fromthese calculations, it can be concluded that about97% of ethylene insertions must occur in cm2species with the vacant site in the outward posi-tion.

Table 1. Geometric Parameters for p-Complex and Transition-State Structures Formed after Ethylene Insertioninto the meso-Et(H4Ind)2Zr(CH3)

þ Speciesa

Zr��C1 Zr��C2 Zr��CMe C1��C2 C2��CMe Zr��Ha

cm1 p complex 2.759 3.013 2.261 1.360 3.139 —Transition state 2.373 2.680 2.339 1.425 2.200 2.233


a Distances are given in angstrom. CME is the carbon atom bonded to zirconium, and C1 and C2 are ethylene carbon atoms.

Table 2. Complexation Energy (DGcomplexation),Insertion Energy Barrier (DG#

insertion), and ReactionEnergy (DGreaction) Values for Ethylene Insertioninto the rac-Et(H4Ind)2Zr(CH3)

þ, cr, andmeso-Et(H4Ind)2Zr(CH3)

þ Species for cm1 and cm2a

DGcomplexationb DG#

insertionb DGreaction

b

cr cm1 cm2 cr cm1 cm2 cr cm1 cm2

0.0 2.5 �1.9 8.8 9.8 11.9 �10.3 �10.3 �10.1

a The structures for cr, cm1, and cm2 are shown in Scheme 1.b Defined in the Computational Methods section.



Primary or 1,2-Styrene Insertion into themeso-Et(H4Ind)2Zr(CH3)

+ Species

Some of the most relevant geometric parametersfor p-complex and transition-state structures cor-responding to 1,2-styrene insertion into cm1 andcm2 species are presented in Table 3. By compar-ing Tables 1 and 3, we can conclude that both 1,2-styrene p complexes are more asymmetric thanethylene p complexes. Transition-state structuresexhibit the characteristic a-agostic interaction ofthese species.

Complexation energies and Gibbs free energybarriers for ethylene insertion into cm1 and cm2species are collected in Table 4. Values for 1,2-sty-rene insertion into the rac-Et(H4Ind)2Zr(CH3)

þ

species11 are also given. 1,2-Styrene complexationinto species with the vacant site in the outwardposition, cm2, is more favorable than the corre-sponding complexation into species with thevacant site in the inward position, cm1 (�2.7 and�1.5 kcal/mol, respectively). Nonetheless, 1,2-sty-rene complexation into cr species is the mostfavorable process, presenting a Gibbs free com-plexation energy of �3.9 kcal/mol. In all cases,

styrene complexation is more favorable than theethylene one because of the higher nucleophiliccharacter of the styrene monomer.

The Gibbs free energy barrier for 1,2-styreneinsertion into cm2 (18.2 kcal/mol) is slightly higherthan the corresponding barrier into cm1 (17.9 kcal/mol). Both energy barriers are higher than the 1,2-styrene insertion barrier into cr (14.5 kcal/mol). Onthe other hand, 1,2-styrene insertion barriers arehigher than ethylene insertion barriers because ofthe bulkier size of the styrene monomer.

Direct 1,2-styrene insertion products into cm1and cm2 species present a c-agostic interaction.The calculated Zr��Hc distances are 2.336 and2.387 A, respectively. These products evolve intomore stable products with a b-agostic interaction,yielding Zr��Hb distances of 2.166 and 2.260 A,respectively. However, the most stable productsfound for both species present a p-benzyl interac-tion between the phenyl ring and the metal atom.The same interaction was found for 1,2-styreneinsertion into cr. These most stable products arenamed product-1,2S meso2 and product-1,2S meso1and correspond to 1,2-styrene insertion into cm1and cm2, respectively. The Gibbs free reaction ener-

Scheme 2. Cationic structures for meso-Et(H4Ind)2Zr(CH3)þ species (cm1 and cm2)

and ethylene insertion products in these species (product meso2 and product meso1).



gies for cm1 and cm2 are �5.1 and �2.6 kcal/mol,respectively.

nMeso2/nMeso1 can be calculated again withBoltzmann statistics according to eq 1. The calcu-lated value is 7. Furthermore, the probability ra-tio of 1,2-styrene insertion into cm2 and cm1,PMeso2/PMeso1, can also be calculated with Gibbsfree energy barriers and eq 2. It can be concludedthat 80% of 1,2-styrene insertions should takeplace in cm2 species with the vacant site in theoutward position.

Secondary or 2,1-Styrene Insertion into themeso-Et(H4Ind)2Zr(CH3)

+ Species

Geometric parameters for 2,1-styrene p complexand transition-state structures for both cm1 andcm2 species are presented in Table 5. By compar-ing the Zr��C1 and Zr��C2 distances presentedin Tables 1 and 5, we can see that both 2,1-sty-rene p complexes are more asymmetric than eth-ylene p complexes. This asymmetry is higher forcm1 with the vacant site in the inward position.On the other hand, Zr��C1 and Zr��C2 distancesin 2,1-styrene p complexes are higher than thosein 1,2-styrene p complexes. This could be due tothe more steric hindrance for 2,1-styrene inser-tion, where the carbon atom with the phenyl sub-

stituent approaches the metal atom, and ishigher for cm1 species with the vacant site in theinward position. Moreover, the transition-statestructure for cm1 species could not be obtained,most likely because of this high steric hindrance.On the other hand, in the 2,1-styrene transition-state structure for cm2 styrene insertion, the mon-omer is more symmetrically located and presentsthe characteristic a-agostic interaction of these spe-cies as well.

Complexation energies and Gibbs free energybarriers for ethylene insertion into cm1 and cm2species are collected in Table 6. Values for 2,1-styrene insertion into the rac-Et(H4Ind)2Zr(CH3)

þ

species11 are also given. 2,1-Styrene complexationinto cm2 species, with the vacant site in the out-ward position, is more favorable than complexa-tion into cm1 species. The difference between theircomplexation energies is now 7.4 kcal/mol. Thisvalue is higher than the corresponding energydifferences for ethylene (4.4 kcal/mol) and for1,2-styrene (1.2 kcal/mol). nMeso2/nMeso1 can becalculated for the 2,1-styrene monomer with eq 1.The high value found for the nMeso2/nMeso1 ratio,173,000, indicates that the number of 2,1-styrenep complexes corresponding to cm2 species must benotably higher than the number of p complexescorresponding to cm1 species. For these reasons,it can be concluded that 2,1-styrene insertionsshould likely occur in the outward position, whichcorresponds to cm2 species.

The Gibbs free energy barrier for 2,1-styreneinsertion into cm2 species is 22.3 kcal/mol. Thisvalue is 5.2 kcal/mol higher than the correspond-ing value for rac species. The most stable productsfound for 2,1-styrene insertion into cm1 and cm2species present a b-agostic interaction and arenamed here product-21S meso2 and product-21Smeso1, respectively. The Zr��Hb distances in theseproducts are 2.182 and 2.229 A, respectively. Theyare both more stable than the separated species by�2.6 and�3.7 kcal/mol, respectively.





DGcomplexationb DG#


b


�3.9 �1.5 �2.7 14.5 17.9 18.2 �4.6 �5.1 �2.6

a The structures for cr, cm1, and cm2 are shown in Scheme 1.b Defined in the Computational Methods section.

Table 3. Geometric Parameters for p-Complex and Transition-State Structures Formed after 1,2-StyreneInsertion into the meso-Et(H4Ind)2Zr(CH3)

þ Speciesa




a Distances are given in angstroms. CME is the carbon atom bonded to zirconium, C1 is the styrene carbon atom without thephenyl substituent, and C2 is the styrene carbon atom with the phenyl substituent.



Chain-Propagation Step

The results explained in the Chain-InitiationStep into meso-Et(H4Ind)2Zr(CH3)

þ Species sec-tion indicate that there are three different spe-cies that could contribute to the polymer chaingrowth. These species are represented as meso-Et(H4Ind)2ZrCH2CH2CH3

þ, meso-Et(H4Ind)2ZrCH2

CH(Ph)CH3þ, and meso-Et(H4Ind)2ZrCH(Ph)CH2

CH3þ, corresponding to first ethylene, 1,2-styrene,

and 2,1-styrene insertions into themeso-Et(H4Ind)2Zr(CH3)

þ species, respectively. Nevertheless, fromeach of them, two different possibilities of propaga-tion can be obtained, depending on the location ofthe vacant position with respect to the aromaticligands. Species with inward and outward vacantpositions are noted here as meso1 and meso2,respectively.

Monomer Insertion into the meso-Et(H4Ind)2ZrCH2CH2CH3

+ Species

The results discussed in the Ethylene Insertioninto the meso-Et(H4Ind)2Zr(CH3)

þ Species sectionshow that the majority of the products, after a firstethylene insertion into themeso-Et(H4Ind)2Zr(CH3)

þ

species, correspond to monomer insertions intocm2. The resulting species is named product-E

meso1 and exhibits the vacant site in an inwardposition. Nonetheless, product-E meso2 species,which presents the vacant site in an outwardposition, is slightly more stable than the former(�10.3 and �10.1 kcal/mol, respectively).

The first monomer insertion into the meso-Et(H4Ind)2Zr(CH3)

þ species occurs mainly in theoutward position, which corresponds to cm2 spe-cies. It can be expected that second monomer in-sertion into the meso-Et(H4Ind)2ZrCH2CH2CH3

þ

species also occurs in the outward position, whichcorresponds to product-E meso2. However, prod-uct-E meso2 is more stable but less abundantthan product-E meso1. Monomer insertion intoproduct-E meso2 should lead to a chain-retentionmechanism in which the olefin insertion into themetal center always takes place from the same sideof the catalyst. This mechanism has been proposedby different authors for meso diastereoisomers,mainly for the insertion of bulky monomers.30,31

In the propagation step, the new monomer canapproach the metal center following a frontsideor backside path, depending on both the relativeposition of the monomer and the b-agostic inter-action.32 Frontside and backside complexationsand monomer insertions into product-E meso1and product-E meso2 have been evaluated. Thevalues obtained for these energies are pre-sented in Table 7. The p complexes are less stablethan their separated species (complexation energy¼ 7–11 kcal/mol). On the other hand, backsidecomplexations energies are higher than frontsideones. Moreover, the backside ethylene p complexin product-E meso2 could not be obtained. In thislast structure, the ethylene moved away from themetal atom up to distances higher than 6 A. Inaddition, frontside insertion yields lower Gibbsfree energy barriers than backside insertion.

It can be concluded that the most favorableprocess corresponds to a frontside ethylene inser-tion into product-E meso1. The Gibbs free energybarrier for this process is 12.4 kcal/mol. The ethyl-ene–metal distances (Zr��C1 and Zr��C2) in this

Table 5. Geometric Parameters for p-Complex and Transition-State Structures Formed after 2,1-StyreneInsertion into the meso-Et(H4Ind)2Zr(CH3)

þ Speciesa


cm1 p complex 2.649 3.495 2.266 1.375 2.266 —cm2 p complex 2.611 3.197 2.248 1.377 3.413 —

Transition state 2.640 2.452 2.333 1.437 2.094 2.206

a Distances are given in angstroms. CME is the carbon atom bonded to zirconium, C1 is the styrene carbon atom without thephenyl substituent, and C2 is the styrene carbon atom with the phenyl substituent.





DGcomplexationb DG#


b


�3.6 2.9 �4.5 17.1 —c 22.3 �3.6 �2.6 �3.7

a The structures for cr, cm1, and cm2 are shown in Scheme 1.b Defined in the Computational Methods section.c Not determined.



transition-state structure are 2.420 and 2.677 A,respectively. The short Ti��Ha distance in thisstructure, 2.170 A, indicates that this transition-state structure presents an a-agostic interaction.

The results presented in Table 7 show thatfrontside second ethylene insertion into product-Emeso1 is the most favorable process. Product-Emeso1 is the main product after a first ethylene in-sertion and presents the vacant site in an inwardposition. For this reason, in the ethylene insertion,the migration mechanism (flip-flop) is to be pre-ferred over the chain-retention mechanism.

Finally, ethylene insertion barriers into product-E meso1 and product-E meso2 are higher thanthose corresponding to rac species (12.4 and 14.0 kcal/mol vs 11.0 kcal/mol, respectively). These valuesindicate that meso species must be less active thanrac species for the ethylene homopolymerizationprocess.

Monomer Insertion into the meso-Et(H4Ind)2ZrCH2CH(Ph)CH3

+ Species

Experimentally, no styrene–styrene sequenceswere found in copolymers synthesized with a cat-alyst similar to the one studied in this work.33 Inthe same way, styrene–styrene insertion energiescalculated with a high-level method were veryhigh, suggesting a lack of styrene–styrene sequen-ces in the copolymer.11 For this reason, only ethyl-ene insertion was performed.

There are two different products after 1,2-sty-rene insertion into the meso-Et(H4Ind)2Zr(CH3)

þ

species. These products are named product-12Smeso1 and product-12S meso2 and correspond to1,2-styrene insertion into cm2 and cm1, respec-tively (see Scheme 2). As discussed earlier, prod-uct-12S meso1, which presents the vacant site inthe inward position, is the most abundant productafter the first 1,2-styrene insertion.

Frontside and backside ethylene insertions intoproduct-12S meso1 and product-12S meso2 werestudied. The p complex corresponding to a frontside ethylene insertion into product-12S meso1 isthe only p-complex structure obtained. In othercases, ethylene moved away from the metal to dis-tances near 6 A. This fact could be due to the stron-ger p-benzyl interaction found in product-12Smeso2 with respect to product-12S meso1. Thecomplexation energy and Gibbs free energy barrierfor ethylene insertion into product-12S meso1 are11.0 and 13.7 kcal/mol, respectively. From theseresults, it can be concluded that the migrationmechanism should be preferred over the retentionmechanism.

Monomer Insertion into the meso-Et(H4Ind)2ZrCH(Ph)CH2CH3

+ Species

Ethylene insertion into the meso-Et(H4Ind)2ZrCH(Ph)CH2CH3

þ species was studied. Similarly to theprevious case, the styrene insertion into this spe-cies was not performed because of the lack of sty-rene–styrene sequences in polymers synthesizedwith similar catalysts.33

Product-21S meso1 is the most abundant andalso most stable product after 2,1-styrene inser-tion into cm2 species. This product presents thevacant site in an outward position. The complex-ation and Gibbs free energy barriers for ethyleneinsertion into product-21S meso1 are 12.7 and17.6 kcal/mol, respectively. This insertion barrieris 5.2 kcal/mol higher than the ethylene insertionbarrier in product-E meso1 (12.4 kcal/mol). Thesevalues point toward a reduction of the catalystactivity due to the increase in the styrene con-tent in the copolymer. Nevertheless, this energydifference is higher for rac species (9.8 kcal/mol).This value indicates that the reduction in thecatalyst activity with the increase in the styrene

Table 7. Complexation Energy (DGcomplexation) and Insertion Energy Barrier (DG#insertion) Values for Ethylene

Insertion into the rac-Et(H4Ind)2ZrCH2CH2CH3þ (rac) and meso-Et(H4Ind)2ZrCH2CH2CH3

þ Species forProduct-E meso1 (meso1) and Product-E meso2 (meso2)a

DGcomplexationb DG#

insertionb

rac

meso1 meso2

rac

meso1 meso2

Frontside Backside Frontside Backside Frontside Backside Frontside Backside

7.5 9.1 10.9 7.9 —c 11.0 12.4 21.0 14.0 —c

a The structures for meso1 and meso2 are shown in Scheme 2.b Defined in the Computational Methods section.c Not determined.



content in the copolymer is lower for the mesospecies than for the rac species.

CONCLUSIONS

A computational study for ethylene–styrene copoly-merization using the meso-Et(H4Ind)2Zr(CH3)2 cat-alytic system has been performed. The catalyst ac-tivity and styrene insertion in the copolymer havebeen evaluated and compared with the corres-ponding values for the rac-Et(H4Ind)2Zr(CH3)2 cat-alytic system. Our results related to ethylene andstyrene insertions into meso species indicate thatthe migration mechanism is preferred over theretention mechanism for both monomers. On theother hand, the Gibbs free energy barriers fora second ethylene insertion into meso and racspecies are 12.4 and 11.0 kcal/mol, respectively.These values point out that the catalyst activityfor ethylene homopolymerization is lower formeso species than for rac species. Productsformed after a first 1,2-styrene insertion intomeso species can incorporate a new ethylenemonomer and continue the polymerization proc-ess. Corresponding products into rac species aredormant species and cannot continue the polym-erization process. These results seem to indicatethat the styrene content in the copolymer couldbe higher with the meso catalyst rather than therac catalyst. Finally, the Gibbs free energy barrierdifferences for ethylene insertion after first ethyl-ene and 2,1-styrene insertions into meso and racspecies (9.8 and 5.2 kcal/mol) indicate that cata-lyst activity decreases with increasing styrenecontent in the copolymer. Nevertheless, it isexpected that the reduction in the catalyst activ-ity will be lower for the meso species than for therac species.

Thanks are due to the Centro de Investigacion Cientıf-ica y Tecnologica (grant MAT2002-01242) for fundingthis investigation. S. Martınez was awarded a fellow-ship by the MEC. The authors also acknowledge theCentro de Supercomputacion de Galicia (Santiago deCompostela, Spain) and Centro de InvestigacionesEnergeticas, Medioambientales y Tecnologicas (Madrid,Spain) for the use of their computational resources.

REFERENCES AND NOTES

1. Soga, K.; Lee, D. Polym Bull 1988, 20, 237–241.2. Mani, R.; Burns, C. M. Macromolecules 1991, 24,

5476–5477.

3. Ren, J.; Hatfield, G. R. Macromolecules 1995, 28,2588–2589.

4. Oliva, L.; Caporaso, L.; Pellecchia, C.; Zambelli, A.Macromolecules 1995, 28, 4665–4667.

5. Arai, T.; Ohtsu, T.; Suzuki, S. Macromol RapidCommun 1998, 19, 327–331.

6. Thomann, Y.; Sernetz, F. G.; Thomann, R.; Kressler,J.; Mulhaupt, R. Macromol Chem Phys 1997, 198,739–748.

7. Pellecchia, C.; Oliva, L. Rubber Chem Technol1999, 72, 553–558.

8. Chen, H.; Guest, M. J.; Chum, S. P.; Hiltner, A.;Baer, E. Annu Tech Conf Proc 1998, 1808–1812.

9. Munoz-Escalona, A.; Cruz, V.; Mena, N.; Martınez, S.;Martınez-Salazar, J. Polymer 2002, 43, 7017–7026.

10. Martınez, S.; Cruz, V.; Munoz-Escalona, A.; Martınez-Salazar, J. Polymer 2003, 44, 295–306.

11. Exposito, M. T.; Martınez, S.; Ramos, J.; Cruz, V.;Lopez, M.; Munoz-Escalona, A.; Haider, N.; Martınez-Salazar, J. Polymer 2004, 45, 9029–9038.

12. Martınez, S.; Exposito, M. T.; Ramos, J.; Cruz, V.;Martınez, M. C.; Munoz-Escalona, A.; Martınez-Salazar, J. J Polym Sci Part A: Polym Chem 2005,43, 711–735.

13. Ramos, J.; Munoz-Escalona, A.; Martınez, S.;Martınez-Salazar, J.; Cruz, V. J. Chem Phys 2005,122, 74901–74904.

14. Inoue, N.; Shiomura, T.; Kouno, M. (Mitsui ToatsuChemicals, Inc.). European Patent Application EP0572990A2, Dec 8, 1993.

15. Wild, F. R. W. P.; Wasiucionek, M.; Huttner, G.;Brintzinger, H. H. J Organomet Chem 1985, 288,63–67.

16. Collins, S.; Kuntz, B. A.; Taylor, N. J.; Ward, D. G.J Organomet Chem 1988, 342, 21–29.

17. Grossman, R. B.; Doyle, R. A.; Buchwald, S. L.Organometallics 1991, 10, 1501–1505.

18. Kaminsky, W.; Schauwienold, A. M.; Freidanck, F.J Mol Catal A 1996, 112, 37–42.

19. Vathauer, M.; Kaminsky, W. Macromolecules 2000,33, 1955–1959.

20. Collins, S.; Gauthier, W. J.; Holden, D. A.; Kuntz,B. A.; Taylor, N. J.; Ward, D. G. Organometallics1991, 10, 2061–2068.

21. Guerra, G.; Caballo, L.; Moscardi, G.; Vacatello, M.;Corradini, P. Macromolecules 1996, 29, 4834–4845.

22. Cossee, P. J Catal 1964, 3, 80–88.23. Becke, A. D. Phys Rev A 1988, 38, 3098–3100.24. Lee, C.; Yang, W.; Parr, R. G. Phys Rev B 1988, 37,

785–789.25. Musaev, D. G.; Morokuma, K. J Phys Chem 1996,

100, 6509–6517.26. Talarico, G.; Blok, A. N. J.; Woo, T. K.; Caballo, L.

Organometallics 2002, 21, 4939–4949.27. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria,

G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski,V. G.; Montgomery, J. A.; Stratmann, E.; Burant,J. C.; Dapprich, S.; Millan, J. M.; Daniels, A. D.;Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;



Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.;Pomelli, C.; Adamo, C.; Clifford, S.; Petersson,G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,D. K.; Rabuk, A. D.; Raghavachari, K.; Foresman,J. B.; Ciolowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;Martın, R. L.; Fox, D. J.; Keith, T. A.; Al-Laham, M. A.;Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challa-combe, M.; Gill, P. M. W.; Jonson, B. G.; Chen, W.;Wong, M. W.; Andres, J. L.; Head-Gordon, M.;Reploge, E. S.; Pople, J. A.Gaussian98 (RevisionA.1); Gaussian: Pittsburgh, PA, 1998.

28. Hay, P. J.; Wadt,W. R. J ChemPhys 1985, 82, 299–310.29. Peng, C. Y.; Schlegel, H. B. Isr J Chem 1993, 33,

449–454.30. Guerra, G.; Caballo, L.; Moscardi, G.; Vacatello, M.;

Corradini, P. Macromolecules 1996, 29, 4834–4845.

31. Vathauer, M.; Kaminsky, W. Macromolecules 2000,33, 1955–1959.

32. Lohrenz, J. C. W.; Woo, T. K.; Ziegler, T. J AmChem Soc 1995, 117, 12793–12800.

33. Oliva, L.; Izzo, L.; Longo, P. Macromol Rapid Com-mun 1996, 17, 745–748.



Isomeric effect of the Et(H4Ind)2Zr(CH3)2 catalyst on the copolymerization of ethylene and styrene:...

Documents

Transcript of Isomeric effect of the Et(H4Ind)2Zr(CH3)2 catalyst on the copolymerization of ethylene and styrene:...