Molecular Mechanics Study of Zirconocenium Catalyzed lsospecific Polymerization of Propylene

ZHENCTIAN YU and JAMES C. W. CHIEN'

Department of Polymer Science and Engineering, Department of Chemistry, Materials Research Laboratories, University of Massachusetts, Amherst, Massachusetts 01003

SYNOPSIS

Stereocontrol energy ( AE') is investigated as a measure of enantioselectivity of Q ~ Q -

zirconocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using T complex ( *C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis ( l-q5-indenyl) - ( 1 ) and rac- ethylenebis ( 1-v5-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the *-complexes or the transition states. The stereoselectivity is greater if there is a-agostic interaction; it is lowered in the cases of p and y agostic interactions. The 13C- NMR steric pentad distribution indicates the poly(propy1ene) to be much less stereoregular than that predicted by AE'. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. 0 1995 John Wiley & Sons, Inc. Keywords: isospecific polymerization stereocontrol energy agostic interaction steric insertion probability

I NTRODUCTI 0 N

Linear poly (ethylene) is produced by chromium catalysts of the Philips Petroleum Co. and by Zie- gler-Natta TiC13 catalysts, but only the latter can be used to manufacture highly isotactic poly- (propylene) ( i-PP) . iPP is characterized by high melting transition ( T,) , high homosteric pentad distribution ( [ m4] ) , and solvent resistance (insol- uble in refluxing n-heptane) .l The technology has evolved from simple TiC13 to very sophisticated supported systems.2 Corradini and coworkers3 had performed rigid model molecular mechanics calcu- lations of these catalyst systems and concluded that those coordinately unsaturated chiral Ti (111) ions situated at the edges, steps, and relief in the lateral face of TiC13 are stereoselective sites for propylene polymerizations.

Homogeneous ansa-metallocene complexes of group IV elements have been shown to produce is-

* To whom all correspondence should be addressed. Journal of Polymer Science: Part A Polymer Chemistry, Vol. 33, 125-135 (1995) 0 1995 John Wiley & Sons, Inc. CCC 0sS7-624X/95/010125-11

o t a c t i ~ , ~ , ~ syndiotactic, and alternating crystalline- amorphous stereoblock7 polymers of propylene. Rigid model molecular mechanics calculations found rac-ethylene ( 1-q5-indenyl) zirconium dichloride ( 1 ) to be isospecific' and isopropylidene ( 1-q5-cyclo- pentadienyl) ( l-q5-fluorenyl) hafnium dichloride to be syndiospecific in propylene polymerization, in agreement with observation.

Ab initio molecular orbital (MO) calculations have been reported for the system Cl,Ti+( CH3)2 + C3H6 + CH3Ti+C1(C4H9)loa and ClaZr+CH3 + C2H4 + Cl2Zr+C3H7.lla Optimized structures of the a-complexes ( aC ) and transition states (TS ) were obtained and the potential energy profile for the electronic rearrangements that occur along the reaction coordinate were reported. Morokuma and coworkers lo and RappC. and coworkers l1 combined molecular orbital with molecular mechanics calculations l2 (MM) to calculate the steric energies (Eo) of the ?rC and of the TS. The Eo values are smaller for isotactic enchainment than for the others such as syndiotactic and secondary insertions, as expected. This difference ( AEO) , which is the mea- sure of the stereocontrol, is quite large. This suggests that these catalysts produce i-PP of high steric reg-

126

126 YU AND CHIEN

ularity at all polymerization temperatures (T,) due to AEo B RT,.

These theoretical results could be used to predict the microstructure of i-PP and to compare it with observation. This was not attempted by these in- vestigators. The purpose of this work is to assess the quantitative merits of the MO/MM method. This was done by first calculating the stereocontrol energy of a catalyst, relating it to the steric insertion probability (PO), then calculating the theoretical steric sequence distribution, and finally comparing it with the steric microstructures observed by 13C- NMR in PP produced by the catalyst. We also em- ployed the MO/MM method to calculate regiose- lectivity and to estimate the magnitude and effects of agostic interactions.

Table I. Zirconocenium Models"

Fixed Geometrical Parameters for

Bond Distance/ Transition Angle x-Complex State

Zr-C, Cg,-C2 (olefin) Zr-Olefin (centroid) Zr-C1 (olefin) Zr-C1 (olefin) C,-C2 (olefin) C,-Zr-C1 Zr-C,-C2 C2-C1-Zr ca-cz-cl C2-Zr-CI

2.28 8, 1.33 8, 2.85 A 2.97 8, 2.90 8,

76.9" 73.7'

26.2'

2.38 8, 1.42 8,

2.36 A 2.4 A

2.15 8, 81.3' 72.9' 87.3"

118.5'

a Reference lob.

COMPUTATIONAL METHODS

The catalysts precursor investigated here are ruc-ethylenebis ( 1-7 5-indenyl ) dichlorozirconium ( 1 ) and rac-ethylenebis ( l-v5-4,5,7,8-tetrahydroin- denyl) dichlorozirconium (2 ) . The corresponding 4d0 fourteen-electron metallocenium ions are the catalytic species. They can be formed by reacting the metallocene precursors either with methylalu- minoxane (MAO) or with a cation-forming agent ( CFA) .5 The difference between various cocatalysts is largely due to the strength of the ion-coupling between the metallocenium species and the counter- anions.13 The former is denoted in this article by a positive charge on 1 (or 2) and the model polymer chain P (2-methylpentyl) with a monomer M, 1+( M ) P or 2+( M ) P, or written as Et ( Ind)2Zr+P or Et ( IndH4)2Zr+P.

The program used to perform the molecular me- chanics calculation was Allinger's MM2.14 The steric potential energy is a sum of the stretching energy due to bond length deformation, the bending energy arising from bond angle deformations and out-of- plane bending, the torsional energy associated with rotation about bonds, and nonbonded interaction energy. This augmented MM2 force field provides a good approximation for transition metal com- p o u n d ~ . ~ ~

The initial model geometry is based on the known molecular structure of (R, R ) Et ( IndH4)2ZrC12 .16 To prove the model's reliability, we have made arbitrary change of the Zr-centroid distance or the ring - CH2 - CHz - ring torsional angle followed by the MM2 optimization. In each case the altered structure returned to the X-ray diffraction structure.

The zirconocenium ion was formed by removing two C1 ligands and replacing them with P.

Morokuma and coworkers have calculated the TC and TS geometries of HzSi (Cp),Zr+( C2H4) CH3 for the ethylene insertion process. We have selected the certain geometrical parameters of these ub initio MO calculations (Table I ) to be fixed. The C, and Cz atoms ( sp2 type) of the propylene molecule are maintained in the same plane with the Zr atom and the C, atom of P, two single bonds (Zr-C, and C1- C,), and two weak bonds ( Zr-C1 and Zr-C2) are de- fined in the x-complex. One single bond between the C1 and C2 atoms (sp3 type) and three weak bonds are fixed for the corresponding atoms in the TS. The remainder of the molecular structure was op- timized using the Block-Diagonal Newton Raphson method with the above described MM2 augmented force field. The same tactics were employed to cal- culate the effects of agostic interactions, the relevant Zr - H - C bond distances were taken to be those found by the ub initio MO calculation. All the cal- culations were performed by the Cache modeling software system (2.7 version, 1991 ) .

The calculation of values Eo by the MM force field by itself could be questionable because of un- certain steric, electronic, and quantum mechanical contributions. However, the present treatment deals with the comparison of species having identical at- oms and bonds and differing only in the steric ar- rangements. The stereocontrol is the difference be- tween Eo of the reaction intermediates having the re and si face of propylene. In the case of agostic interactions, we obtain Eo values for different ge- ometries of the 2-methylpentyl group. Our experi-

ISOSPECIFIC POLYMERIZATION OF PROPYLENE 127

ence indicates a reliability of 1 kcal/mol for the AEo values.

RESULTS AND DISCUSSION

Stereoselectivity

Steric energies for the PC and TS were calculated for l + ( M ) P and Z'(M)P. The results are sum- marized in Table 11. The first two rows are for the primary (1, 2 ) insertion into P, which has the pre- vious monomer unit also inserted in the primary mode with its @-carbon in the S configuration. The steric energy is smallest for the complexion and in- sertion of the re propylene face with the 0-carbon in the S configuration, i.e., the meso enchainment. The primary insertion of a si propylene face intro- duces an R configuration monomer in the racemic enchainment with steric energy Eo ( r ) . The differ- ence between these two steric energies for the PC or the TS is the isotactic stereocontrol energy for the two reaction intermediates,

In our calculations, Eo( m ) is taken to be zero as the reference. A large positive A E o ( r - m ) value is com- mensurate with isoselectivity.'s The results in Table I1 show that both PC and TS are enantioselective

Table 11. Relative Steric Energies for Propylene Insertion

and that the former has larger AEo( r - m ) values than the latter. The 2 + ( M ) P system is more iso- specific than 1+( M ) P, as observed experimentally. There will be a small number of the r -insertion er- rors (steric misinsertion) , the occurrence of which is both determined by the Boltzmann factor and other causes to be suggested in the conclusion.

Regioselectivity

In the third and fourth rows of Table I1 (cases 3 and 4) the new monomer is inserted in the secondary ('41) mode for the re and sipropylene, respectively. It is interesting to note that the relative PC steric energies is only about 2 kcal/mol greater than the reference case 1. If this were the only determining factor, then the catalyst would produce poly- (propylene) containing a large number of head-to- head and tail-to-tail regioirregular units. However, the TS steric energy for the 2, 1 insertion is very large, about 6 kcal/mol greater than the reference value. This energy difference between the secondary ( s ) and primary ( p ) at TS, AE$s(s - p ) , is the re- giocontrol energy in the transition state. This result indicates that the regioselectivity of insertion is governed by the TS.

Following the secondary insertion, a subsequent insertion by either the 1,2 or 2, l mode becomes very unfavorable. The relative steric energy for the PC increases to 8-10 kcal/mol, it increases to 12-14 kcal/mol for the TS (cases 5 to 8 in Table 11). In

AE" (kcal/mol)'

Mode of Insertion (R, R)Et(Ind)* Zr+ P (R, R)Et(IndH&Zr+ P Polymer

Regio Stereo Config.d Case ?rC TS Case ?rC TS

Pa P"

Pb Pb Sb

Sb

sa SB

re' si re si re si re si

ss 1 RS 2

3 4 5 6 7 8

0.0 3.5 2.0 2.1 7.8 7.0 9.5 8.7

0.0 2.9 5.8 6.2

13.1 12.7 11.8 11.2

1' 2' 3' 4' 5' 6' 7' 8'

0.0 4.2 2.4 2.3 8.4 8.1

10.1 9.1

0.0 3.4 6.2 6.6

13.8 13.1 12.6 11.8

a The previous monomer was inserted by the primary mode ( p = primary; s = secondary).

' Preferred prochiral face of propylene. The previous monomer was inserted by the secondary mode.

Configuration on the left is for the last inserted monomer; configuration on the right is for the next to the last inserted propylene. AEo = Ej - El, where Ej is the steric energy of the j-th case (El is reference as zero), and is equal to AEo(r - rn) for the cases

1, 2 and l', 2', or is equal to AE&(s - p ) for the cases 1, 3 and l', 3'.

128 YU AND CHIEN

other words, the polymerization is essentially stalled following the secondary insertion as pointed out by Pino." We can write the reverse regiocoordinations 2,l-TC as,

(3) Zr+P + C3Hs e Zr 'P

t (Me)CH = CH2

and the reverse regio insertion 2,l-TS as,

Zr +P S Zr+CH(Me)CH,P .t (4)

(Me)CH = CHI

Due to the unfavorable energetics for the contin- uation of the monomer insertion, 1,2- or 2,l-rC

Zr+CH(Me)CH,P + C3H, e (C,H,)Zr+CH(Me)CH2P ( 5 )

the @-hydride transfer takes place,

Zr+CH(Me)CH2P -c Zr'H + (Me)CH=CHP ( 6 )

This internal olefin (Me)CH=CHP is very un- reactive toward Ziegler-Natta catalysts and ceases to grow any further. The @-hydride elimination from the @-methyl group is less facile. But in the absence of other alternative processes, it is observed to occur

ZrfCH(Me)CH2CH2P -, Zr+H + CH2=CHCH2CH2P ( 7 )

The reinsertion of the resulting olefin into the Zr'H bond forms the tetramethylene sequence, z1 which had been referred to as 1,3 misinsertion,

Zr+H + CH2=CHCH2CH2P -+ Zr+(CH,),P (8 )

The data in Table I1 may suggest that the sec- ondary insertion preceded by a primary insertion (cases 3, 4 and 3', 4') is nonenantioselective, and that there is a slight isoselectivity for the primary insertion (in favor of s i ) preceded by a secondary insertion (cases 5,6 and 5', 6'). They are in contrast with experimental results,zza,h where the si (or r e ) insertion is actually favored for 3 and 4 (or 5' and 6'). Therefore, it indicates that the MM2 calcu- lated steric control energy has reliability of about 1 kcal/mol.

A comparison of the cases 5,6 (or 5', 6') with the cases 7, 8 (or 7', 8') indicates that, following the secondary insertion, the TC state favors a subse- quent primary insertion, whereas the reverse is true in the TS, which slightly favors the secondary in- sertion. The experimental resultzza showed that only the primary insertion occurs in this case, which is

rather consistent with our results for rC. In fact, Morokuma and coworkers also reported that the TC and TS may give different regioselectivities.loh

The relative steric energy for every case in Table I1 is greater for catalyst 2+ than 1'. Therefore, the catalyst derived from precursor 2 is predicted to be slightly more regio- and stereo-selective than those belonging to 1, in agreement with experimental ob- servations.

The PP produced by 1 or by 2 activated with MA0 have 13C-NMR resonances attributable to head-to-head and tail-to-tail microstructures. The polymer formed by 2 at -15OC contains 0.51% of the 2 , 1 inserted monomer, or one for every 150-200 primary insertion^.'^ The relative steric energies for the two insertion modes (cases 1' and 2' in Table 11) are 0.0 and 6.2 kcal/mol, respectively; AE$s(s - p ) is 6.2 kcal/mol. The effective regiocontrol en- ergy (vide infru) must be applied, i.e., AE$s( s - p ) = 2.7 4 0.1 (kcal/mol) can be calculated so that the frequencies of the secondary insertion may give one in every 194 primary insertions according to the fol- lowing equation:

Ago& Interactions

Brintzinger and coworkersz4 and Piers and BercawZ5 had raised the question of the effects of agostic in- teractions in Ziegler-Natta catalysis. In general, if the agostic interaction stabilized the intermediate along the reaction coordinate, then this process is rendered more favorable than other competing pathways. For instance, if there is a significant @- hydrogen agostic interaction, then the P-hydride elimination is facilitated to lower the polymer mo- lecular weight.

Morokuma and coworkers lob reported no strong a-agostic interactions in either ( MezCp)2Zr+CH3 or HzSi ( Cp),Zr+( C2H4) CH3. However, the transition state of the latter has one C - H bond of the methyl group stretched by 0.03 A consistent with a slight a C - H agostic interaction. Two equilibrium struc- tures were found by the MO/MM calculation for HzSi (Cp),Zr+( C3H7). One structure has a y C - H agostic interaction, the C, - H bond is stretched by 0.03 A, another has a @ C-H agostic interaction with the C, - H bond stretched by 0.06 A.

We have investigated the effects of various kinds of agostic interactions on the stereoselectivity of the zirconocenium catalyst. Table I11 contains the steric

ISOSPECIFIC POLYMERIZATION OF PROPYLENE 129

Table 111. Transition State Geometry and Steric Energy with Agostic Interaction for (R, R) It (M)P

TS Geometry Steric Energy (kcal/mol)

Agostic Distances AE" Interaction Type Structure' Fixed' re si ( r - m)

None 3 65.2 68.1 2.9 a 3 Ccu "2 H2A Zr 71.6 74.2 3.6

4b Cp "2 H *L2 Zr 77.6 79.1 1.5 4a CY "2 H2& Zr 84.4 85.1 0.7 Y

Db

a Reference lob. Experimental Zr - H = 2.16 8, for /3-agostic interaction (reference 40). In A.

energies computed for the meso and racemic en- chainments of propylene in the TS of 1 + ( M ) P with a, @, and y agostic interactions. The results are very interesting and important. The stereocontrol energy without any agostic interactions is 2.9 kcal/mol. The a-agostic interaction raised AE$s(r - m ) to 3.6 kcal/mol, i.e., isoselectivity is enhanced by this in- teraction. However, isoselectivity is reduced in the case of C,-H agostic interaction; the value of AE$s(r - m ) is lowered to 1.5 kcal/mol. In other words, the @-agostic interaction not only facilitates the @-hydride transfer leading to a lower polymer molecular weight, but it also makes the polymeriza- tion less stereospecific. Even more surprising is the result that the transition-state structure with the C, - H agostic interaction has AE$, ( r - m ) of only 0.7 kcal/mol. This value of AEo is too small to be meaningful. Our results are consistent with another independent theoretical study (Extended Hukel calculation) by Janiak, 26 where he concluded that the a-agostic interaction becomes increasingly im- portant around and beyond the TS for the olefin insertion, and the @-agostic interaction leads to a stable ground state for the C,-H -+ Zr contact.

Steric Insertion Probability

Since the number of atoms, electrons, and bonds are identical for the meso and racemic monomer inter- actions, we can relate AEo(r - m ) at TC or TS di- rectly to the steric insertion probability for the meso enchainment,

where E o ( m ) and E o ( r ) are the steric energies for the meso and racemic insertions of the re and si face propylene, respectively, appropriate for the aC or TS structures.

The analogous racemic insertion probability is

, - E 0 ( r ) / R T 0

P ( r ) = - E o ( r ) / R T + e - E o ( m ) / R T e

1 - - 1 + , - A E o ( m - r ) / R T '

To calculate the steric sequence distribution for comparison with the 13C-NMR data, one needs to adopt a stereochemical control model. The two sim- plest ( limiting) stereochemical control models for a-olefin polymerizations are the enantiomorphic-site model ( E ) in which the chain propagation is deter- mined by the chirality of the catalyst,27 and the chain-end control model (B) in which the asym- metry of the last inserted monomer unit controls the microstructure of the polymer.28 In the B model, P , is the conditional probability for the isotactic placement corresponding to the [ m ] diad abundance, [ r ] = Pr = ( 1 - P , ) . In this case po(m) = P,. In the E model, the conditional probability for the is- otactic placement is Pd. The steric &ad distributions are [ m ] = P ; + ( 1 - P : ) , [ r ] = 2Pd(1 - Pd). p o ( m ) here is identified with Pd. The expressions for the longer sequence distributions have been given by Cheng.29 There have been numerous attempts of the statistical analysis of NMR spectra, most of which were aimed to fit observed steric distributions with the stereochemical control models using the conditional probabilities as arbitrary variables.

A large value of the stereocontrol energy corre- sponds to high stereoselectivity. A catalyst with AEo( r - m ) = 3-4 kcal/mol would produce i-PP of nearly perfect stereoregularity (i.e., [ m 4 ] > 0.99).

130 YU AND CHIEN

Furthermore, this steric purity should be retained over the experimental range of Tp (from -55 to 80°C) according to eq. (10) when AEo(r - m ) %- RT,. Such is indeed the case for the heterogeneous TiC13 catalysts, but it is not true for the zirconocene catalysts.

The AEo( r - m ) value for 2+( M ) P is 4.2 and 3.4 kcal/mol for the 7rC and TS, respectively. i-PP pro- duced by catalyst 2/MAO at -15OC is not uniform in stereoregularity. The polymer can be separated into two fractions, one extractable with refluxing n - heptane and the insoluble fraction. The experimen- tal steric sequence distributions are given in columns 2 and 5 of Table IV. The steric purities are obviously much lower than the steric sequence lengths pre- dicted by eq. (10) for the MO/MM2 calculated AEo( r - m ) , A smaller effectivestereocontrol energy, AE'( r - m ) can bring the observed and calculated stereoregularity into agreement. AE'( r - m ) values were found by lowering AEo(r - m ) by 0.01 kcall mol, and calculating the new p o ( m ) values and the steric pentad distribution both for the E-model and B-model. This distribution was compared with the observed distribution from the 13C-NMR spectra, and the mean deviation (MD) was calculated. The above process was repeated until the MD was less than a prescribed level ( < 5% ) .

The 13C-NMR spectra of the n-heptane insoluble fraction are in good agreement with the microstruc- ture calculated with the E-model for an effective stereocontrol energy of 2.88 kcal/mol using the ef- fective steric insertion probability,

1 p ' (m) = 1 + e -AE' (r -m) /RT.

The steric sequence distributions can be obtained directly from the p ' ( m ) . For instance, the homo- steric meso pentad abundance is2'

[ m 4 ] ~ = ~ ' ( m ) ~ + (1 - ~ ' ( m ) ) ~ (13)

[m4]B = ~ ' ( m ) ~ . (14)

The MD is 0.04% for the E-model; it is a about five times larger for the B-model. The difference between the MDs for the two models is

AMD = MD(B) - MD(E) . (15)

The AMD is 0.16 in favor of the enantiomorphic- site stereochemical control model in case of n -hep- tane insoluble fraction.

The result for n-heptane soluble fraction was ac- counted for by a AE'( r - m ) = 2.65 kcal/mol and

Table IV. Et(IndH4)2ZrC12/MA0 (T, = 258 K)

Theoretical and Observed Steric Sequence Distributions of i-PP Produced by

Fraction AE'(r - m) (kcal/mol)

n-HeptanelSoluble 2.65

n-Heptane/Insoluble 2.88

P' (m) ut %

0.994 42

0.996 58

Steric sequence Ob" E b B" Ob E B

[mmmm] [ mmmr] [rmmr] [ mmrr] [mrmm]

4- [ rmrr] [ rmrm] [rrrr] [rrrm] [ mrrm] MD (%) AMD (%)

0.972 0.009 0 0.009

0.004

0 0 0 0.004

0.972 0.011 0 0.011 0

0 0 0 0 0.006 0.11

0.978 0.011 0 0 0.011

0 0 0 0 0 0.31

0.20

0.982 0.007 0 0.006

0.002

0 0 0 0.003

0.982 0.007 0 0.007

0 0 0 0 0.004 0.04

0.986 0.007 0 0 0.007

0 0 0 0 0 0.20

0.16

Reference 4d. Calculated for enantiomorphic site stereochemical control model. ' Calculated for chain end stereochemical model.

ISOSPECIFIC POLYMERIZATION OF PROPYLENE 13 1

p'(m) = 0.994 as shown in columns 1-4 of Table IV. The lower stereoregularity of this fraction ac- counts for a higher solubility of the polymer fraction.

Variation of Steric Structure of Poly ( propylene)

Poly ( propylene ) formed by ama-zirconocene cat- alyst at very low Tp is mostly insoluble in refluxing n-heptane, has a high T, and a high homosteric m- pentad p o p ~ l a t i o n . ~ ~ , ~ ~ The low- T, polymerization is best achieved with the 'bare' zirconocenium ion generated in situ3' by reactions of the precursor with an organic cation such as triphenylcarberium ion and with an inert and noncoordinative anion such as tetrakis (pentaflurophenyl) borate in the presence of a trialkylaluminum compound as the alkylating and scavenging agent. The effective stereocontrol energy that fits experimental pentad distributions is only slightly smaller than the AEo(r - m ) cal- culated by MO /MM2, as illustrated in Table IV.

It is a universally exhibited behavior for all Cz symmetric isoselective zirconocene catalysts3' that as Tp increases, i-PP comes progressively more sol- uble in low-boiling solvents 4d and has lower T, ,32,33

The latter polymers have low molecular weights and stereoregularity. In fact, Natta34 utilized these physical properties to isolate the highly isotactic polypropylene, which is insoluble in refluxing n- heptane. The 13C-NMR sequences of i-PP fractions separated by solvent extraction are given in Table V.

The theoretical stereocontrol energy ( AEo ( r - m) in Table 11) is 3.5 kcal/mol for the TC and 2.9 kcal/mol for the TS. The effective stereocontrol en- ergy AE' ( r - m) is only 1.77 kcal/mol for the most stereoregular material insoluble in n -heptane ( last 4 columns in Table V) . The values of AE'( r - m) decrease with the decreasing boiling point of the solvent; they are 1.27, 1.06, and 0.58 kcal/mol for the n-hexane, n-pentane, and diethyl ether frac- tions, respectively (Table V) . The [ m4] populations are 0.92, 0.77, 0.66, and 0.31 for the fractions of polymer obtained at Tp = -55"C, which are insoluble in n-heptane and soluble in n-hexane, n-pentane, and diethyl ether, respectively.

Table V also compares the observed steric se- quence distributions for fractions of i-PP obtained at 0°C and -55°C with the calculated results using E model. It is encouraging that the same AE'( r - m) value fits both series of samples. This is consistent with the fact that each fraction is formed at different Tp by catalytic species with a given effective ster- eocontrol energy. The homosteric m-pentad popu-

lation of the fractions extracted by the same solvent at two different Tp are related by

The above results may be explained by the exis- tence of several types of catalytic species, Zrf , which differ in stereoselectivity. The subscript j corre- sponds to A, E, P, c6, c 7 , and c 7 i , i.e., the solvent ranking, which can dissolve the polymer produced by the Zrf species.

The zirconocene catalysts are widely referred to as single-site catalysts. This implies a unique mo- lecular structure, which is fixed in a crystalline state because the site is associated with heterogeneous catalysts. If this were true, then the catalyst should produce a single type of macromolecules indepen- dently of Tp, albeit having a most probable molecular weight distribution but uniform microstructure, re- flected by I3C-NMR steric distributions, T,, and solubility. Polymerization by the zirconocene cata- lysts approaches these expectations for a single-site catalyst a t very low Tp. However, deviation from the single-site highly stereospecific behavior occurs for all Cz symmetric catalysts and it differs only in the extent of the decrease of stereoselectivity with Tp and other experimental conditions. This is to be expected for metal complexes in solution. There should be a large number of thermally accessible isomeric states in equilibrium. One can relate these states to the stereoregularity of the polymer they produce,

Zr&, e Zrg G Zr& e Zr& e Zr: e Zr: (17) In reality, there is probably a continuous distribution of isomeric states due to the combinations of all the vibrations. For instance, the Zrf cation may occur in states involving a, p, or y agostic interactions (vide supra). The diethyl ether soluble polymers, which have steric distributions corresponding to AE' ( r - m) of 0.58 kcal/mol, may evolve from the transition state with the y-agostic interaction. The transition state for the ,B-agostic interaction was calculated to be 1.5 kcal/mol. This lies between the AE'( r - m) values of 1.27 and 1.77 kcal/mol for the c6 and c7i insoluble fractions, respectively (Ta- ble V).

CONCLUSION

Heterogeneous Ziegler-Natta catalysts are highly stereospecific and regiospecific over a broad range

CI w

N

Table

V.

by E

t (In

d),Z

rCl,/

MA

O (

AE'

: kc

al/m

ol)

The

oret

ical

and

Obs

erve

d St

eric

Seq

uenc

e D

istr

ibut

ions

for

the

Var

ious

Fra

ctio

ns o

f P

P P

rodu

ced n-

Hep

tane

l In

solu

ble

Eth

er

n-P

enta

ne

n-H

exan

e Fr

actio

n T

p (K

) 21

8 27

3 21

8 27

3 21

8 27

3 21

8 27

3 A

E' (

r - m

) 0.

58

1.06

1.

27

1.77

p' (m

) 0.

792

0.74

4 0.

920

0.87

6 0.

949

0.91

2 0.

983

0.96

3 w

t%

0.9

4.3

0.4

2.4

3.1

2.9

86.2

74

.4

~ ~~

~~

~ ~~

Sequ

ence

O

b"'

Eb

Ob

E

Ob

E

Ob

E

Ob

E

Ob

E

Ob

E

Ob

E

[mm

mm

] [m

mm

r]

[rm

mr]

[ m

mrr

] [m

rmm

] +

[rm

rr]

[rm

rm]

[ rrr

r]

[ rrr

m]

[ mrr

m]

MD

(W

)

0.31

0.

15

0.06

3 0.

14

0.31

3 0.

167

0.02

7 0.

167

0.05

4

0.26

0.

16

0.05

0.

15

0.23

0 0.

163

0.03

6 0.

163

0.07

2

0.66

0.

13

0.02

0.

11

0.66

0 0.

114

0.00

5 0.

114

0.01

1

0.38

0.

175

0.05

8 0.

13

0.51

6 0.

147

0.01

2 0.

147

0.02

4

0.77

0.

064

0.02

7 0.

062

0.77

1 0.

082

0.00

2 0.

082

0.00

5

0.51

0.

17

0.05

2 0.

13

0.63

2 0.

122

0.00

6 0.

122

0.01

3

0.92

0.

023

0.00

8 0.

023

0.92

0 0.

031

0 0.03

1 0

0.89

0.

036

0.01

8 0.

029

0.82

9 0.

063

0.00

1 0.

063

0.00

3 0.

08

0.13

0.

03

0.06

7 0.

032

0.05

2 0.

004

0.00

8 0.

054

0.05

4 0.

027

0.05

4 0.

083

2.83

0.07

2 0.

072

0.03

6 0.

082

0.08

2 2.

58

0.01

1 0.

011

0.00

5 0.

011

0.05

7 1.

06

0.02

4 0.

012

0.02

4 0.

073

0.08

2 3.

85

0.00

5 0.

005

0.00

2 0.

005

0.00

5 1.

24

0.01

3 0.

013

0.00

6 0.

013

0.01

3 3.

23

0.00

1 0.

001

0 0.00

1 0.

016

0.42

0.00

3 0.

003

0.00

1 0.

003

0.03

2 3.

50

0.04

0.

07

0.06

3 0.

071

0.05

2 0.

052

0.07

8 0.

078

0.01

0.

005

0.01

0.

029

0.04

2 0.

025

0.06

7 0.

059

0.00

3 0.

007

0.00

3 0.

034

0.01

7 0.

013

0.00

9 0.

009

0.00

1 0.

007

0.00

1 0.

012

0.00

7 0.

07

0.06

3 0.

071

a.b Fo

otno

tes

are

the

sam

e as

in T

able

IV

.

ISOSPECIFIC POLYMERIZATION OF PROPYLENE 133

of polymerization conditions, For instance, the commercially supported TiC13 catalyst produces highly isotactic PP ( T,,, > 165°C) of high molecular weight over a broad temperature range by either slurry, liquid monomer, or a gas phase process. If the homogeneous Ziegler-Natta catalysts were to be useful in the industrial production of isotactic poly- a-olefins, they also need to possess these attributes.

The ethylene-bridged indenyl systems, 1 and 2, as well as other C2 symmetric ansa-zirconocene pre- cursors, suffer significant loss of stereospecificity with the increase of Tp, resulting in the depression of T Z and in the homosteric pentad popula- tion.32,33,3536 Th e presence of poly (propylene) of low steric purity, i.e., acetone and diethyl ether soluble polymer molecules, decreases the onset of melting to as low as 60°C.4d In other words, the material exhibits a very low deflection temperature and a poor creep behavior.

The loss of isospecificity with Tp is at least partly attributed to the perturbation in the molecular structure of the catalytic species. These changes af- fect molecular weights as well. For instance, the M,/ M,, value for PP produced by 1 /MA0 above 0°C is much larger than the most probable value. The GPC curves of PP obtained with 2/MAO are distinctly bimodal. If a polymerization reaction is quenched with tritiated methanol and PP separated into a number of fractions, we that the catalytic species responsible for the polymerization actually differ greatly in the rate constants for the propa- gation and the P-hydride elimination as well as in their stereospecificity. Furthermore, the distribution of these catalytic species can be significantly altered by changing the dielectric constant medium.37 One of the possible reasons for the loss of stereoselectiv- ity with increasing Tp was proposed to be different transition states involving various agostic interac- tions.

We have identified at least three other processes that cause a loss of stereospecificity and have verified them e~perimentally.~' The first is the introduction of steric inversion through the exchange of two chains of opposite stereochemical configurations (RR - - - RR) and ( SS - * SS ) belonging to the two antipodes of the catalyst ( + ) and ( - ) . The normal facial insertions are

( + ) / R R R - - .RR (18)

and

The reverse facial insertions are far less probable as shown in this article. However, the exchange pro- cess,

(+)/SS. .SS + ( - ) /RR. - .RR (20)

which is highly probable, effectively introduces a steric inversion. This exchange can also involve alu- minum alkyl if it is present.

The second is isomerization of the chiral rucemic zirconocene to an achiral meso deri~ative.~' The conversion may occur via ring slippage,

ruc- 1,2-ethylene ( 1 -q 5-indenyl )2Zr + P

ruc- 1 ,%ethylene ( 1-7 5-indenyl ) ( 1 -f '-indeny1 ) -

Zr+P 5 meso-l,Z-ethylene( l-q5-indenyl)-

(1-q'-indenyl)Zr+P meso-1,2-ethylene-

( l -q ' - inden~l)~Zr+P (21)

or the severance of one indenyl-Zr bond:

ruc-1,Z-ethylene ( l-q5-indenyl)2Zr+P

ruc- 1 ,%ethylene ( 1-7 5-indenyl ) ( 1 -indeny1 ) - Zr'P 5 meso-1,2-ethylene-

( l-q5-indenyl)2Zr+P (22)

The meso complex has a lower catalytic activity and is without stereoselectivity.

Migration of Zr+ along the polymer chain had been observed in the zirconocenium catalysis of 1- hexene p~lymerization.~' Finally, the l+(M)P (or 2+( M)P) may undergo hydride elimination, followed by the reinsertion of the olefin in the 1'-H or 2+-H without stereoselectivity.

At end of this article, we briefly compare our MO/ MM2 calculation with Corradini's nonbonded (NB) calculation (rigid model). The steric control energies we obtained are smaller than those calculated by Corradini et al. For instance, AE:c(r - m) is 3.5 kcal/mol (or 4.2 kcal/mol) in this work and is 7.2 kcal/mol (or 8.2 kcal/mol) in NB calculation in cases of 1+( M ) P (or 2+( M ) P) . This could be orig- inated from the different geometries between our

134 YU AND CHIEN

K / T S and their 7r-complex and two kinds of force fields (NB vs. MM2) as well. Both calculations, however, indicate that steric interaction causes the enantioface selection of the olefin for coordination and that 2' is more isospecific selective than 1+.

This work was supported by the Materials Research Lab- oratories of the National Science Foundation at the Uni- versity of Massachusetts.

REFERENCES AND N O T E S

1.

2.

3.

4.

5.

6.

Reviews for Ziegler-Natta polymerization: (a) T. Keii, Kinetic of Ziegler-Natta Polymerization, Kodansha, Tokyo, 1972. ( b ) J. Boor, Jr., Ziegler-Natta Catalysis and Polymerization, Academic Press, New York, 1979. (c) Y. V. Kissin, Isospecific Polymerization of Olefins, Springer-Verlag, New York, 1985. J. C. W. Chien, Catalysis in Polymer Synthesis, E. J. Vandenberg, J. C. Salamone, Eds., ACS Symposium Series 496, Am. Chem. SOC., Washington, D.C., 1992,

(a ) P. Corradini, G. Guerra, R. Fusco, and V. Barone, Eur. Polym. J., 16, 835 (1980). ( b ) P. Corradini, V. Barone, and G. Guerra, Macromolecules, 1 5 , 1242 (1982). (c) P. Corradini, V. Barone, R. Fusco, G. Guerra, and V. Barone, Eur. Polym. J., 20, 1117 ( 1984). ( e ) V. Venditto, G. Guerra, P. Corradini, and R. Fusco, Eur. Polym. J., 2 7 , 4 5 (1991). Isospecific polymerization by metallocene/methylal- uminoxane: (a) J. A. Ewen, J. Am. Chem. SOC., 1 0 6 , 6355 (1984). ( b ) W. Kaminsky, K. Kulper, H. H. Brintzinger, and F. R. W. P. Wild, Angew. Chem. Znt. Ed. Engl., 24, 507 (1985). ( c ) B. Rieger, J. C. W. Chien, Polym. Bull., 2 1 , 159 (1989). ( d ) B. Rieger, X. Mu, D. T. Mallin, M. D. Rausch, and J. C. W. Chien, Macromolecules, 23,3559 (1990). ( e ) J. C. W. Chien, R. Sugimoto, J. Polym. Sci., Part A 2 9 , 459 (1991). ( f ) S. Miya, T. Mise, and H. Yamasaki, Chem. Lett. 1853 ( 1989). Isospecific polymerization by metallocene/cation forming agent: (a) X. Yang, C. L. Stern, and T. J. Marks, J. Am. Chem. SOC., 113, 3623 (1991). ( b ) J. A. Ewen, M. J. Elder, R. L. Jones, L. Haspeslagh, J. L. Atwood, S. G. Bott, and K. Robinson, Makromol. Chem. Macromol. Symp., 4 8 / 4 9 , 253 (1991). ( c ) J. C. W. Chien, W.-M. Tasi, and M. D. Rausch, J. Am. Chem. SOC., 113,8570 (1991). ( d ) W.-M. Tasi, M. D. Rausch, and J. C. W. Chien, Appl. Organomet. Chem., 7 , 71 (1993). ( e ) J. C. W. Chien, and W.-M. Tasi, Makromol. Chem. Macromol. Symp., 6 6 , 141 (1993). (a ) J. A. Ewen, R. L. Jones, and A. Razavi, J. Am. Chem. SOC., 110,6255 (1988). ( b ) J. A. Ewen, M. J. Elder, R. L. Jones, S. Curtis, H. N. Cheng, in Catalytic Olefin Polymerizations, T. Keii, K. Soga, Kodansha, Eds., Elsevier, Tokyo, 1990, p. 439.

pp. 27-55.

7. (A) D. T. Mallin, M. D. Rausch, Y.-G. Lin, S.-H.

8.

9.

10.

11.

12.

13.

14. 15.

16.

17.

18.

Dong, and J. C. W. Chien, J. Am. Chem. Soc., 1 1 2 , 2030 (1990). (b ) Y.-G. Lia, D. T. Mallin, J. C. W. Chien, and H. H. Winter, Macromolecules, 2 4 , 850 (1991). ( c ) G. H. Llinas, S.-H. Dong, D. T. Mallin, M. D. Rausch, Y.-G. Lin, H. H. Winter, and J. C. W. Chien, Macromolecules, 25,1242 (1992). ( d ) J. C. W. Chien, G. H. Llinas, M. D. Rausch, Y.-G. Lin, H. H. Winter, J. L. Atwood, and S. G. Bott, J. Am. Chem. SOC., 113, 8569 (1991). ( e ) J. Polym. Sci. Part A , 30,2601 (1992). ( a ) P. Corradini, G. Guerra, M. Vacatello, and V. Vil- lani, Gazz. Chim. Ztal. 1 1 8 , 1 7 3 (1988). (b ) L. Cavallo, G. Guerra, L. Oliva, M. Vacatello, and P. Corradini, Polym. Commun., 3 0 , 1 6 ( 1989). ( c ) V. Venditto, G. Guerra, P. Corradini, and R. Fusco, Polymer, 3 1 , 5 3 0 ( 1990). ( d ) L. Cavallo, P. Corradini, G. Guerra, and M. Vacatello, Polymer, 32, 1329 (1991). ( e ) P. Cor- radini and G. Guerra, Progr, Polym. Sci., 1 6 , 239 ( 1991 ) * L. Cavallo, G. Guerra, M. Vacatello, and P. Corradini, Macromolecules, 2 4 , 1784 ( 1991). H. Kawamura-Kuribayashi, N. Koga, and K. Morok- uma, J. Am. Chem. Soc., ( a ) 114, 2359 (1992). ( b ) 1 1 4 , 2359 (1992). ( b ) 114,8687 (1992). ( a ) L. A. Castonguay and A. K. RappB, J. Am. Chem. SOC., 1 1 4 , 5832 (1992). ( b ) J. R. Hart and A. K. RappB, J. Am. Chem. SOC., 1 1 5 , 6159 (1993). This approach was introduced and applied to organic reactions by: ( a ) D. C. Spellmeyer and K. N. Houk, J. Org. Chem., 5 2 , 9 5 9 (1987). ( b ) A. E. Dorigo and K. N. Houk, J. Org. Chem., 5 3 , 1650 (1988). The effect of anion on zirconcenium catalysis of pro- pylene polymerization was reported by J. C. w. Chien, W. Song, and M. D. Rausch, Macromolecules, 26,3239 ( 1993). The change in stereospecificity of zircono- cenium catalyst by medium dielectric constant was studied by J. C. Vizzini, J. C. W. Chien, G. N. Babu, and R. A. Newmark, unpublished results. N. L. Allinger, J. Am. Chem. SOC., 9 9 , 8 1 2 7 (1977). R. G. Brubaker and W. D. Johnson, Coord. Chem. Reu., 53 , l (1984). F. R. W. P. Wild, M. Wasincionek, G. Huttner, and H. H. Brintzinger, J. Organomet. Chem., 6 3 , 288 ( 1985). ( a ) J. W. Lauher and R. Hoffmann, J. Am. Chem. SOC., 98, 1729 (1976). ( b ) D. R. Armstrong, P. G. Perkins, and J. J. J. Steward, Chem. SOC. Dalton Trans., 1972. ( c ) G. Giunchi, E. Clementi, J. E. Ruiz- Vizcaya, and 0. Novaro, Chem. Phys. Lett., 4 9 , 8 (1977). ( d ) 0. Novaro, E. I., Blisten-Borojas, E. Cle- menti, G. Guinchi, and J. E. J. Ruiz-Vizcaya, Chem. Phys., 68,2337 (1978). ( e ) 0. Novaro, S. Chow, and P. Maynouat, J. Catal., 4 1 , 9 1 (1926). ( f ) N. Rosch and K. H. Johnson, J. Mol. Catal. 1 , 3 9 5 (1976). (g) A. C. BalazsandK. H. Johnson, J. Chem. Phys., 77, 3184 (1982). A puckered 4-membered ring for the a-complex had been suggested. However, the planar 4-membered ring

ISOSPECIFIC POLYMERIZATION OF PROPYLENE 135

is more reasonable. Furthermore, the energy difference due to torsional constraint is very small.

19. Castonguay and Rapp6 had performed molecular me- chanics calculations on the 'activated complex' state of the Et(IndH4)zZr'( M)CH3 They defined the 'ac- tivated complex' as the state that lies approximately half-way along the reaction pathway between the ground state and the transition state. The relative steric energy for insertion of the first re propylene molecule is a little less than 3 kcal/mol in favor of the isotactic enchainment. This energy increased to about 6 kcal/mol favoring the insertion of a second re propylene molecule to Et( IndH4)2Zr+( M ) CH2C- (CH3)2. These results are in substantial agreement with ours.

20. P. Pino, in Integration of Fundamental Polymer Sci- ence and Technology, Vol. 2, P. J. Lemotra, L. A. Ke- intjens, Eds., Elsevier Appl. Sci., New York, 1989, p. 3.

21. K. Soga, T. Shino, S. Takemura, and W. Kaminsky, Macromol. Chem., Rapid Commun., 8 , 3 0 5 (1987).

22. (a) A. Grassi, A. Zambelli, L. Resconi, E. Albizzati, and R. Mazzocchi, Macromolecules, 21, 617 ( 1988). ( b ) A. Mizuno, T. Tsutsui, and N. Kashiwa, Polymer, 33, 254 (1992).

23. T. Tsutsui, A. Mizuno, and N. Kashiwa, Makromol. Chem., 1 9 0 , 1 1 7 7 (1989) .

24. W. Roell, H. H. Brintzinger, B. Rieger, and R. Zolk, Angew. Chem. Int. Ed. Engl., 2 9 , 279 (1990).

25. W. E. Piers and J. E. Bercaw, J. Am. Chem. SOC., 1 1 2 , 9 4 0 6 (1990) .

26. C. Janiak, J. Organomet. Chem., 4 5 2 , 6 3 (1993). 27. ( a ) R. A. Sheldon, T. Fueno, T. Tsunstsugu, and J.

Furukawa, J. Polym. Sci. Part B, 3 , 23 (1965). ( b ) H. L. Frisch, C. L. Mallows, andF. A. Bovey, J. Am. Chem. Phys., 4 5 , 1 5 6 5 (1966).

28. F. A. Bovey and G. V. D. Tiers, J. Polym. Sci., 44, 173 (1960).

29. H. N. Cheng, in Modern Methods of Polymer Char-

acterization, H. G. Barth and J. W. Mays, Eds., John Wiley and Sons, New York, 1991, pp. 409-493.

30. (a ) S.-N. Zhu, X.-Z. Yang, and R. Chujo, Polym. J . (Tokyo), 1 5 , 8 5 9 (1983). ( b ) Y. Inoue, Y. Itabashi, R. Chujo, and Y. Doi, Polymer, 2 5 , 1640 (1984).

31. ( a ) J. C. W. Chien and W.-M. Tsai, Makromol. Chem. Macromol. Symp., 66, 141 (1993). ( b ) W.-M. Tasi, M. D. Rausch, and J. C. W. Chien, Appl. Organomet. Chem., 1, 71 (1993). ( c ) J. C. W. Chien and B.-P. Xu, Makromol. Chem., Rapid. Commun., 3 2 , 14, 109 (1993). (d ) W.-M. Tasi, J. Polym. Sci. Part A, Polym. Chem., 3 2 , 1 4 9 (1994). ( e ) J. C. W. Chien, W. Song, and M. D. Rausch, Macromolecules, 26,3239 ( 1993).

32. The I3C-NMR homosteric pentad sequence and T,,, of poly(propy1ene) is related by (I",,,)-' - (TO,)-' = - (R/AHf) - In[mmmm] where TO, = 148"Cand AHf = 2.7 kcal/mol for highly stereoregular i -PP .

33. P. J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, 1953, p. 568.

34. G. Natta, J. Polym. Sci., 1 9 5 9 , 34, 531. 35. F. Danusso and G. Gianotti, Eur. Polym. J., 4, 165

(1968) . 36. ( a ) G. Natta, SOC. Plast. Eng., 1 5 , 373 (1959). ( b )

R. L. Combs, D. F. Slonaker, F. B. Joyner, and H. W. Conver, J. Polym. Sci., Polym. Chem. Ed., 5 , 215 (1967).

37. J. C. Vizzini, J. C. W. Chien, G. N. Babu, and R. A. Newmark, J. Polym. Sci. Part A, Polym. Chem., to appear.

38. J. C. W. Chien, W. Song, Z. Yu, and M. D. Rausch, to appear.

39. G. N. Babu, R. Newmark, and J. C. W. Chien, Mac- romolecules, to appear.

40. R. F. Jordan, P. K. Bradley, N. C. Baenziger, and R. E. La Pointe, J. Am. Chem. SOC., 112,1281 (1990).

Received May 11, 1994 Accepted August 3, 1994