kinetics, polymer molecular weights, and microstructure in zirconocene-catalyzed 1-hexene...

Kinetics, Polymer Molecular Weights, and Microstructurein Zirconocene-Catalyzed 1-Hexene Polymerization

XIA ZHAO,1 GEORGE ODIAN,1 ALBERT ROSSI2

1 College of Staten Island, City University of New York, 2800 Victory Boulevard, Staten Island, New York 10314

2 Exxon Chemicals Company, Linden, New Jersey 07036

Received 18 December 1999; accepted 4 August 2000

ABSTRACT: 1-Hexene was polymerized by rac-(dimethylsilyl)bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride catalyst and methylaluminoxane cocatalyst over the tem-perature range 0–100 °C. The polymerization rate, polymer molecular weight, andpolymer microstructure (stereospecificity and regiospecificity) were studied as a func-tion of the temperature and the concentrations of monomer, catalyst, and cocatalyst.© 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3802–3811, 2000Keywords: zirconocene; 1-hexene polymerization

INTRODUCTION

Soluble metallocene catalysts are of increasingtechnological importance because of their poten-tial for finer control of molecular weight, molecu-lar weight distribution, and polymer micro-structure. In this article, we report the use ofrac-(dimethylsilyl)bis(4,5,6,7-tetrahydro-1-inde-nyl)zirconium dichloride catalyst (I), abbreviatedas rac-(CH3)2Si(H4Ind)2ZrCl2, and methylalumi-noxane (MAO) cocatalyst, [Al(CH3)O]n, to poly-merize 1-hexene. More specifically, we studiedthe kinetics of the polymerization and termina-tion processes and the stereospecificity and regio-specificity of the polymer as a function of thetemperature and the concentrations of monomer,catalyst, and cocatalyst.

EXPERIMENTAL

Materials

Rac-dimethylsilyl(H4Ind)2ZrCl2, hereafter calledzirconocene and abbreviated Zr, was synthesizedaccording to the literature procedure.1 MAO wasobtained as a 10 wt % solution in toluene fromAlbemarle and was used without further purifi-cation. 1-Hexene (bp 5 63–64 °C) was obtainedfrom J. T. Baker and was dried over a mixture ofAl2O3 and 4-Å molecular sieves. Anhydrous tolu-ene was obtained from Aldrich and further driedover 4-Å molecular sieves.

Correspondence to: G. Odian (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 3802–3811 (2000)© 2000 John Wiley & Sons, Inc.

3802

Polymerization

All manipulations, except the oven drying of theglassware, were carried out in a Labconco glovebox under a nitrogen atmosphere. The glove boxcontained a purification system for the removal ofwater and oxygen from the internal atmosphere.The glassware was oven-dried above 115 °C for atleast 4 h and then was kept in the glove box for atleast 1 h before use. The glassware was subse-quently rinsed with a 0.05 wt % solution of MAOin toluene to remove any residual moisture left onits inner surfaces.

We prepared a solution of the zirconocene cat-alyst by adding 10 mL of 10 wt % MAO to 90 mLof toluene, allowing the mixture to stand for 15 m,and then adding 100 mg of the zirconocene cata-lyst. Each polymerization was carried out in apressure tube with a threaded stopper that sealedagainst a Teflon O-ring (Ace Glass 8648-23). Bulk1-hexene and solutions of 1-hexene in toluenewere polymerized. The monomer solution (5 mL)was added to the pressure tube and was followedfirst by 10 wt % MAO and then by the zirconocenesolution. Polymerizations were carried out in thetemperature range 0–100 °C with various concen-trations of monomer (0.80–8.0 M) and zircono-cene (1.0–10 mM) at MAO/Zr ratios of 10–1500.After polymerization for a specified time period,each reaction mixture was quenched by treat-ment with a 10% aqueous NaOH solution at roomtemperature, which was followed by two wash-ings with water. Polymer was isolated from theresulting organic layer through the evaporation ofunreacted monomer and solvent with a Speed Vacat 0.125 torr and 65 °C for more than 6 h. Themonomer conversion was obtained from the dryweight of the polymer.

NMR Analysis

1H NMR and 13C NMR spectra of poly(1-hexene)samples were obtained on Varian 500-MHz and600-MHz spectrometers. The conditions for quan-titative 1H NMR were as follows: 10% (w/v) sam-ple in CDCl3, 40 °C, 30° pulse angle, 2-s totaldelay between pulses, 500 scans, and tetrameth-ylsilane as an internal reference. The conditionsfor quantitative 13C NMR were the following: 10%(w/v) sample in CDCl3, 15 mg of Cr(AcAc)3, in-verse gated decoupling, 40 °C, 90° pulse angle, 3-stotal delay between pulses, 10,000 scans, andCDCl3 as an internal reference.

Polymer Molecular Weight

Molecular weights were obtained from 1H NMRthrough a comparison of the signal areas for thesingle-bond and double-bond regions. The molec-ular weight calculation was based on the assump-tion of one double bond per polymer molecule, anassumption verified in an earlier work.2 Somemolecular weight determinations were also madewith size exclusion chromatography (SEC) at40 °C with a Waters 150C instrument, tetrahy-drofuran as the mobile phase, a set of four ultra-styragel columns (106, 105, 104, and 103 Å) as thestationary phase, and calibration with polysty-rene standards.

RESULTS AND DISCUSSION

Polymerization Rate (Rp)

Polymerizations were carried out at 0, 50, 80, and100 °C with various concentrations of monomer(0.80–8.0 M) and zirconocene (1.0–100 mM) atvarious MAO/Zr ratios (10–1200). A typical set ofdata is shown in Figure 1, with the monomerconsumption plotted against the reaction time.The slope of each plot yielded Rp. Figure 2 showsa log–log plot of Rp versus the monomer concen-tration, whose slope yielded the kinetic order ofdependence of Rp on the monomer concentration.Corresponding data were obtained for the depen-dence of Rp on zirconocene. Table I lists the ordersof dependence of Rp on zirconocene and monomerat different temperatures. Rp was first-order inzirconocene at all temperatures. The order of de-pendence on monomer was first-order at 0 °C. Thedependence was higher than first-order at thehigher temperatures but only slightly above theprecision of the data.

The experimental results were compatible witha reaction mechanism involving initiation by re-action between zirconocene and MAO to form theactive catalyst species C*, which subsequentlyadded monomer to yield the initial propagatingcenter M*:

Zr 1 MAO -|0K1

C* (1)

C* 1 M -|0K2

M* (2)

ZIRCONOCENE-CATALYZED 1-HEXENE POLYMERIZATION 3803

Propagation proceeded by successive additions ofmonomer to the propagating center:

M* 1 MO¡kp

M* (3)

When reaction 2 was irreversible and fast (i.e.,[M*] 5 [C*]), Rp was

Rp 5 K1kp@Zr#@MAO#@M# (4)

which shows the first-order dependencies of Rp onzirconocene and monomer.3 If reaction 2 was re-versible and slow (i.e., [M*] Þ [C*]), Rp was

Rp 5 K1K2kp@Zr#@MAO#@M#2 (5)

A minor contribution of eq 2 to the overall pro-cess was indicated by the observed slightlyhigher-than-first-order dependence of Rp onmonomer at 50 °C. A higher-than-first-order de-

pendence of Rp on monomer has been reportedin other olefin polymerizations catalyzed bymetallocenes.4,5

A study of the effect of the MAO concentrationon Rp was inconclusive. Polymerization occurredonly when the [MAO]/[Zr] ratio exceeded about150. Presumably, this corresponded to the mini-mum MAO concentration required to activate thezirconocene catalyst through alkylation. Rp in-creased at higher [MAO]/[Zr] ratios but leveled offwhen the [MAO]/[Zr] ratio exceeded about 600.Thus, the effective range of the MAO concentra-tions over which Rp varied was small, whichgreatly limited the precision and accuracy of thedata. Within these limitations, Rp was approxi-mately first-order in the MAO concentration.

Rp increased with increasing temperature. Aplot of ln Rp versus 1/T was linear, and the slopeyielded 59 kJ/mol as the activation energy for Rp.This activation energy was in the range observedfor metallocene-catalyzed polymerizations ofa-olefins.6

Figure 1. Monomer consumption versus reaction time at different initial monomerconcentrations. [MAO] 5 62 mM; [Zr] 5 52 mM; T 5 100 °C.

3804 ZHAO, ODIAN, AND ROSSI

Chain Transfer

Chain transfer stops the propagation of individ-ual chains with the formation of vinylene, trisub-stituted, and vinylidene double-bond end groups.The transfer mechanisms are b-hydride transfersfrom normal and reverse propagating centers, be-fore and after rearrangement.2,7 1H NMR wasused to identify vinylene, trisubstituted, and vi-

nylidene double-bond end groups in poly(1-hex-ene) by their signals at 5.35, 5.16, and 4.70 ppm,respectively. Figure 3 shows the variation of eachdouble-bond end group with temperature. Onlyabout 10% of the double bonds were trisubsti-tuted double bonds over the temperature rangestudied. The vinylidene and vinylene contentswere quite temperature-sensitive, with vinyli-dene end groups predominant at higher temper-atures and vinylene predominant at lower tem-peratures.

Plots of the different double-bond end groupsversus time yielded the transfer rates (Rtr). Thedependence of Rtr for vinylene, trisubstituted, andvinylidene double-bond formations on zirconoceneand monomer concentrations were obtained fromappropriate log–log plots of Rtr versus concentra-tion. Table II lists the orders of dependence of Rtr

on the zirconocene and monomer concentrationsat different temperatures. The order of depen-dence of Rtr on the monomer concentration variedfrom about one-quarter to one and one-half, de-

Figure 2. Dependence of Rp on the initial monomer concentration at different tem-peratures. [MAO] 5 62 mM; [Zr] 5 52 mM.

Table I. Effect of the Zirconocene and MonomerConcentrations on Rp

T (°C)

Order ofDependence

[Zr] [M]

0 0.94 0.9650 1.1 1.380 0.95 1.4

100 1.0 1.3


pending on the type of double bond and tempera-ture. The order of dependence of Rtr on the zir-conocene concentration varied from first-order to

slightly above second-order, depending on thetype of double bond and temperature.

The results on the monomer dependence of Rtrwere consistent with a combination of unimolecu-lar (eq 6a) and bimolecular (eq 7a) transfers. Thetwo transfers differed in whether monomer wasinvolved in a b-hydride transfer (expulsion of zir-conocene) from the propagating center. The orderof dependence on the monomer concentration alsodepended on whether reaction 2 was irreversibleand fast ([M*] 5 [C*]), that is, eqs 6b and 7bapplied, or was reversible and slow ([M*] Þ [C*]),that is, eqs 6c and 7c applied. At one extreme,there was zero-order dependence of Rtr on themonomer concentration if transfer occurred via eq6a, with reaction 2 being irreversible and fast. Atthe other extreme, there was second-order depen-dence of Rtr on the monomer concentration iftransfer occurred via eq 7a, with reaction 2 beingreversible and slow:

M*O¡ktr1

Polymer 1 Zr (6a)

Figure 3. Dependence of the double-bond end groups on the temperature. [M] 5 8.0M; [MAO] 5 62 mM; [Zr] 5 52 mM; conversions 5 50–60%.

Table II. Effect of the Zirconocene and MonomerConcentrations on Rtr

T (°C) End Group

Order ofDependence

[Zr] [M]

0 CH2AC, 2.4 0.28OCHAC, 1.5 0.75OCHACHO 1.2 1.0





Rtr1 5 K1ktr1@Zr#@MAO# if @M*# 5 @C*# (6b)

Rtr1 5 K1K2ktr1@Zr#@MAO#@M#

if @M*# Þ @C*# (6c)

M* 1 MO¡ktr2

Polymer 1 M* (7a)

Rtr2 5 K1ktr2@Zr#@MAO#@M# if @M*# 5 @C*# (7b)

Rtr2 5 K1K2ktr2@Zr#@MAO#@M#2

if @M*# Þ @C*# (7c)

The greater than first-order dependence of Rtr onthe zirconocene concentration for vinylidene andvinylene end groups at 50 and 0 °C indicated thatzirconocene was involved in the b-hydride trans-fer process. Table II shows that the higher-than-

first-order dependencies on the zirconoceneconcentration were coupled with the lower-than-first-order dependencies on the monomer concen-tration. Apparently, zirconocene aided the expul-sion of zirconocene (i.e., b-hydride transfer) fromthe propagating center when monomer did nothave that role, and the effect was temperature-dependent. Zirconocene was more effective thanmonomer in the b-hydride transfer process atlower temperatures; monomer was more effectiveat higher temperatures.

The transfer reaction rates increased with theMAO concentration within the narrow concentra-tion range described for Rp. The transfer reactionrates also increased with temperature over thetemperature range 0–100 °C. The transfer reac-tion rates increased more steeply with tempera-ture than did Rp. This is usually the situation forolefin polymerizations because transfer activationenergies are greater than propagation activationenergies.

Figure 4. Dependence of the polymer molecular weight on the temperature. [M] 5 8.0M; [MAO] 5 62 mM; [Zr] 5 52 mM; conversions 5 40–45%.


Polymer Molecular Weight

M# n values from 1H NMR and SEC were generallywithin 10–30% of each other; the SEC valueswere higher than the 1H NMR values. The poly-mer molecular weight distribution was narrow.M# w/M# n from SEC was generally only slightlygreater than 2, indicative of the single-site natureof the zirconocene catalyst. For example, M# w/M# nfor a polymerization at 50 °C was 2.2 at a 10.4%conversion and increased slowly with conversion,reaching 2.9 at an 86.2% conversion.

The SEC M# values were used to study theeffects of the reaction parameters on the polymermolecular weight. Figure 4 shows the effect oftemperature on the polymer molecular weight.The polymer molecular weight decreased with in-creasing temperature. This was the expected re-sult because the polymer molecular weight de-pended on Rp/Rtr and the transfer reaction ratesincreased more sharply with temperature thandid Rp. The polymer molecular weight increased

modestly with increasing monomer concentration(Fig. 5). This occurred because Rp showed aslightly higher order dependence on monomerthan did the transfer reactions rates (Tables I andII). Similar results were reported by Resconi etal.8 in the polymerization of propene with rac-ethylenebis(Ind)2ZrCl2 and MAO.

The polymer molecular weight decreasedmodestly with increasing zirconocene concen-tration at 0 and 50 °C in line with the higherorder dependence of Rtr on the zirconocene con-centration compared to the dependence of Rp onthe zirconocene concentration (Tables I and II).There was a negligible effect of the zirconoceneconcentration on the polymer molecular weightat 80 and 100 °C because Rp and Rtr had thesame dependence on the zirconocene concentra-tion.

There was a negligible effect of the MAO con-centration on the polymer molecular weight overthe temperature range 0–100 °C because Rp and

Figure 5. Dependence of the polymer molecular weight on the monomer concentra-tion at different temperatures. [M] 5 8.0 M; [Zr] 5 52 mM; [MAO] 5 62 mM; conversions5 40–60%.


Rtr both had the same (very slight) dependence onthe MAO concentration.

Polymer Microstructure

The stereospecificity and regiospecificity of the1-hexene polymerization were determined by 13CNMR. The polymer was highly isotactic under arange of polymerization conditions. Figure 6

shows the 13C NMR spectrum in the saturatedcarbon region for a mostly isotactic polymer (syn-thesized at 80 °C, [M] 5 8.0 M, [Zr] 5 52 mM,[MAO] 5 62 mM.). The signal for C3 (the firstmethylene carbon of the butyl branch attached tothe methine carbon) was used to quantify theisotacticity at the pentad level with the assign-ments of Asakura et al.9 The stereochemical as-signments are shown in the expanded spectrum

Figure 6. 13C NMR spectrum of an isotactic poly(1-hexene). [M] 5 8.0 M; [Zr] 5 52mM; [MAO] 5 62 mM; T 5 80 °C.


in the top portion of Figure 6. Table III shows theeffects of the temperature and the monomer, zir-conocene, and MAO concentrations on the poly-mer isotacticity.

Isotactity decreased sharply with increasingtemperature, for example, from 97% isotactic pen-tads at 0 °C to 47% at 100 °C ( [M] 5 8.0 M, [Zr]5 52 mM, [MAO] 5 62 mM). Busico and Cipullo10

observed similar effects in polymerizations of pro-pene with zirconocene catalysts. Isotacticity in-creased with increasing monomer concentration,with the effect of the monomer leveling off aboveapproximately [M] 5 4 M. Isotacticity decreasedwith increasing zirconocene and MAO concentra-tions. These results were rationalized by the cat-alyst site control mechanism. Isotactic placementoccurred through the tight coordination of thepropagating center, catalyst counterion, andmonomer. Increased temperature loosened the co-ordination and allowed some syndiotactic place-ment. After an isotactic addition of a monomermolecule to the propagating center, that isotacticplacement was not locked in until the next mono-mer molecule was added; that is, there was thepossibility of reversal of addition. Higher mono-mer concentrations decreased the time availablefor such reversibility, and the extent of isotactic-ity was increased. Higher zirconocene and MAOconcentrations at a constant monomer concentra-tion resulted in increased concentrations of prop-agating centers. The amount of monomer avail-

able per propagating center decreased, there wasmore time available for the isomerization of thelast added monomer molecule, and isotacticitydecreased.

The regiospecificity of the polymer was deter-mined from a comparison of the 13C NMR signalareas for C2 of the normal (1,2-addition) and re-verse (2,1-addition) repeat units found at 32.6and 35.7 ppm, respectively. Table IV summarizesthe effects of the temperature and the monomer,zirconocene, and MAO concentrations on the poly-mer regiospecificity. In general, the polymer re-giospecificity was very high (1,2-addition), withvery modest variations due to experimental con-ditions. The directions of the changes in regio-specificity mirrored the changes in stereospecific-ity, for similar reasons. The extent of reverseaddition increased from 0.32% at 0 °C to 4.1% at100 °C. The effect of the monomer concentrationwas smaller, with the extent of reverse additionincreasing from 1.4% at 8.0 M to 3.0% at 0.8 M.Increasing concentrations of zirconocene andMAO yielded higher extents of reverse addition,although the effects were very small.

The authors gratefully acknowledge financial supportof this work by the Exxon Chemical Co.

REFERENCES AND NOTES

1. Welbourn, H. C. (Exxon). Eur. Pat. Appl. 89302675.7,Publ. No. 0 344 887 A2, March 17, 1989.

2. Rossi, A.; Odian, G.; Zhang, J. Macromolecules1995, 28, 1739.

Table III. Effects of the Polymerization Conditionson Isotacticity

T (°C)[M](M)

[Zr](mM)

[MAO](mM)

mmmm(%)

0 8.0 52 62 972.0 52 62 89

50 8.0 104 62 768.0 52 62 848.0 26 62 888.0 5.2 62 908.0 52 18 928.0 52 4.0 924.0 52 62 842.0 52 62 820.8 52 62 69

80 8.0 52 62 588.0 1.0 62 74

100 8.0 52 62 478.0 1.0 62 548.0 52 7.4 58

Table IV. Effects of the Polymerization Conditionson Regiospecificity

T (°C)[M](M)

[Zr](mM)

[MAO](mM)

2,1-RepeatUnits (%)

0 8.0 52 62 0.3250 8.0 99 62 1.6

8.0 52 62 1.48.0 26 62 1.08.0 5.2 62 0.948.0 52 16 1.18.0 52 3.7 0.854.0 52 62 1.42.0 52 62 1.70.8 52 62 3.0

80 8.0 52 62 3.9100 8.0 52 62 4.1


3. One referee suggested that our kinetic scheme wasoversimplified because we assumed only one type ofpropagating species. The referee indicated that asecond propagating species existed because of re-verse (2,1-addition). This was probably true, butthe amount of the second propagating species wasvery small in our system. The amount of reverseaddition was very small in our system, not exceed-ing 4.1%, as indicated in the Polymer Microstruc-ture section.

4. Jungling, S.; Mulhaupt, R.; Stehling, U.; Brintz-inger, H.-H.; Fischer, D.; Langhauser, F. MakromolChem Macromol Symp 1995, 97, 205.

5. Herfert, N.; Fink, G. Makromol Chem MacromolSymp 1993, 66, 157.

6. Chien, J. C. W.; Sugimoto, R. J Polym Sci Part A:Polym Chem 1991, 29, 459.

7. Folie, B.; Ruff, C. J. Polym Prep 1998, 39(1),201.

8. Resconi, L.; Fait, A.; Piemontesi, F.; Colonnesi, M.;Rychlicki, H.; Ziegler, R. Macromolecules 1995, 28,6667.

9. Asakura, T.; Demura, M.; Nishiyama, Y. Macro-molecules 1991, 24, 2334.

10. Busico, V.; Cipullo, R. Makromol Chem MacromolSymp 1995, 87, 277.


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