Mao Catalysts

K.J. Smith, E.C. Sanford (Editors), Progress in Catalysis 0 1992 Elsevier Science Publishers B.V. All rights reserved.

169

A KINETIC STUDY OF THE POLYMERIZATION OF PROPYLENE WITH Et(Ind),ZrCI,/MAO CATALYSTS J. Huang and G. L. Rempel Department of Chemical Engineering, University of Waterloo Waterloo, Ontario, N2L 3G 1, Canada

Abstract The kinetics of polymerization of propylene with Et(Ind)2ZrC12/MA0 catalysts was investigated by monitoring gaseous propylene consumption in a computer-controlled constant pressure reactor system. The effects of [Zr], [Al]/[Zr], temperature and stirring speed on the polymerization were statistically studied. It was found that the polymerization rate reached a maximum at the beginning of the reaction, and gradually decreased with reaction time. The catalyst activity was found to strongly depend on the ratio of [Al]/[Zr]. The existence of two types of active centers with different activity and stability is proposed in an attempt to explain the experimental results.

INTRODUCTION

Since the early 1980's, there has been increasing interest in Ziegler-Natta polymerization of a-olefins with a new family of homogeneous catalysts based on Group 4B transition metallocenes and alumoxane oligomers. The catalysts not only show extremely high activity for a-olefin polymerization, but also have the ability to control the stereochemical structure of the resultant polyolefins. The catalytic system consisting of racemic ethylene bis(indeny1)zirconium dichloride and methyl alumoxane, Et(Ind),ZrCI,/MAO, is one of the most important catalysts of this type, and it is believed to be the first homogeneous system capable of polymerizing isotactic polypropylene. Polymerization of propylene with Et(Ind),ZrCI,/MAO has been extensively studied in recent years. Most investigations have concentrated on the unique feature of steric structures of produced polypropylene. In contrast, there are very few publications involving a kinetic study of the system. Recently, Chien and et al"] reported that the overall activation energy for propylene polymerization with Et(Ind),ZrCI,/MAO is 10.6 kcal/moI at polymerization temperatures from -55°C to 80°C. Using tritium radiolabeling, they showed that not all Zr atoms are catalytically active. The existence of two types of active species with different

stereoselectivity, activity and tendency towards chain transfer in the catalytic system was suggested. This paper reports on a kinetic study of the same polymerization system via a different approach. In order to obtain reliable experimental results, statistical analysis is employed in this work.

EXPERIMENTAL 1. Catalyst Preparation The catalyst component Et(Ind),ZrCI, was synthesized according to the methods of Brintzingerl'l and C~llins'~'. The cocatalyst M A 0 was synthesized as follows: 20 g of Al,(SO,),. 18H,O was suspended in 130 ml of toluene under N2 atmosphere. 31.5 ml of trimethylaluminum was added dropwise to the suspension at 0°C over a period of 1 hr. Then the reaction mixture was allowed to warm up to 40°C and the reaction continued for 48 hrs. The solid residue was removed by filtration. The filtrate was evaporated under vacuum at 40°C. The MA0 product was obtained as white crystals in 33% yield. The molecular weight of M A 0 was determined cryoscopically in solvent benzene. (MW = 1300-1500, that is, the number of repeat units n=22-26). 2. Propylene Polymerization Polymerization reactions were carried out in a computer-controlled constant pressure reactor ~ystem.'~,~] Polymerization was performed at a selected temperature and propylene pressure. Solvent toluene was refluxed over Ndbenzophenone and freshly distilled before use. Propylene (C.P. grade) was purified by passing through a column of 4A molecular sieves and an oxygen trap. Polymerization was initiated by dropping a small glass bucket containing given amounts of Et(Ind),ZrCl, into solvent toluene which contained a given amount of MA0 and was saturated with propylene. The propylene consumption was recorded using the computer controlled reactor system with a sampling period of seconds. Polymerization was stopped by adding a few ml of acidic methanol into the reaction mixture. Polymer product was precipitated in methanol, collected by filtration, and dried in a vacuum oven at 50°C.

3. Polymer Analyses The polypropylene (PP) products were fractionalized into three Extmction fractions, i.e., C,-soluble, C,-insoluble/C,-soluble and C,-insoluble, by extraction with n-

hexane and n-heptane successively. About 2 g of fine powder of polymer is extracted in a Soxhlet extractor for 12 hrs by 180 ml of boiling n-hexane or n-heptane. The insoluble polymer fraction is vacuum dried while the soluble fraction is recovered by evaporating off the solvent from the extract. Differential Scanning Calorimeter (DSC) The melting temperature (TJ of PP samples was measured using a Perkin-Elmer System IV differential scanning calorimeter. The weight of sample was about 5 mg and a heating rate of 20"C/min was employed. T, was

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obtained from the second scan after complete melting and cooling of the samples. I d m r e d Spectroscopy A Nicolet 520 FTIR spectrometer was used to record IR spectra. Samples were prepared by compressing a mixture of PP with KBr at room temperature. The PP was not subjected to heat treatment. The sample was sandwiched between NaCl discs for IR examination.The baseline was drawn as recommended by Burfield and

RESULTS AND DISCUSSIONS 1. Gas Consumption Curve and the Rate of Polymerization The cumulative propylene consumption curve for each run of polymerization can be generated from computer-recorded data. A typical propylene consumption curve is shown in Fig. 1. The computerized gas-uptake system can precisely monitor the polymerization process and provide sufficient data points, which enable calculation of the instantaneous polymerization rate from the derivatives of the gas consumption curve. The instantaneous polymerization rate as shown by the solid line in Fig.2 describes actual changes in catalyst activity, and is therefore preferable to the average polymerization rate as shown by the dotted line in Fig.2. Gas Uptake ( m m o l )

30 I

3000

Rate (kgPP/rnolZr.h)

[Zr]=lOPM [Al]/[Zr]=9000 T=30 'C 0

2

4

6

U

I0

0

I

Time (s) (Thousands)

Figure 1 An example of the propylene consumption curve.

Figure 2 average

2

3

4

5

6

7

8

9

10

Time (s) (Thousands)

The instantaneous R, and the curves.

Considering the lag time required by apparatus and experimental method, such as catalyst dissolution and dispersion, active center formation, and build up of pressure difference being detectable by sensor, we conclude that there is virtually no induction period at the beginning of the polymerization. The polymerization rate reaches its maximum immediately after the reaction is initiated, and then gradually decreases with reaction time. Since the rate of polymerization is changing, we adopt the average rate in the period from the 5th to 35th min. of reaction time as the initial rate of polymerization for further discussion.

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2. Experiments of 2'l Fractional Factorial Design and Statistical Analysis

Four variables, i.e., zirconocene concentration ([Zr]), the ratio of MA0 to zirconocene ([Al]/[Zr]), polymerization temperature (T,,) and stimng rate were selected for statistical analysis. Table 1 lists the two levels of the variables. Table 2 provides the 24-1 fractional factorial design. The matrix of "-" and signs represents the experimental means at high condition: "-" means the variable on the top of column at low level; level. Each experiment was duplicated and the average value was used for analysis. The experimental results, polymerization rate (RJ,melting temperature (Tdand stereoregularity (mmmm pentad content) of PP products are also listed in Table 2.

"+"

"+"

Table 1 The Selected Variables and Their two levels Variable

Symbol

Low Level 10 pM

Z R T S

[ZrI [A1l/[Zrl Temperature Stirring Rate

High Level 15 p M

go00

6000 30 "C

50 "C 125 /min

100 /min

Table 2 The Design of Experiments and Results RunNo. 1 2 3 4 5 6 7 8

Experiment Conditions

RQ

-

(kgPP/mol-Zr h)

+ + + +

+ + + -

t

+ + +

+

+ +

+ +

1420 2870 2250 3340 5920 6470 6410 7050

Tm

mmmm

130.2 131.2 131.3 131.6 117.3 115.8 115.3 117.2

0.90 0.91 0.91 0.92 0.86 0.88 0.82 0.88

("c)

The effects of individual variables and two-variable interactions on %, T, and the mmmm pentad content of PP products were calculated and are shown in Table 3. The results were also plotted on normal probability paper. According to statistics, the effects of non-significant factors are just a reflection of random noise. These effects should be normally distributed with mean zero and thus should form a straight line on normal probability paper. The factor whose effect does not fit such a straight line must be considered as "statistically significant".p]

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Table 3 The Effects of Variables on %, T,, and mmmm Pentad Content

Contrast

Effect

Contrast

35730 3730 2370 -270 15970 -1350 -230

932.5 592.5 -67.5 3992.5 -337.5 -57.5

989.9 1.7 0.9 2.7 -58.7 -0.9 -2.1

Total Z R ZXR T ZXT RXT S

mmmm

T,

RP

Factor

;[ 450

112.5

4.1

Effect 0.425 0.225 0.675 ..14.675 -0.225 -0.525 1.025

Contrast 7.06 0.08 -0.04 0.02 -0.22 0.04 -0.08 0.02

Effect 0.02 -0.01 0.005 -0.055 0.01 -0.02 0.005

Note: Calculation was done by assuming that all three-variable interactions and two-variable interactions involving S are non-significant. Effect = Contrast/4.

Prob;

30

RT

10

-20

-10

0

10

that factors Z and R are also significant. The other four factors appear to be statistically nonsignificant. The fact that Z is significant is unexpected, which is probably related to the existence of trace impurities in the whole reaction system. The impurities could reduce the number of active centers and/or change the ratio of [Al]/[Zr]. Since stirring rate is a nonsignificant factor, propylene mass transfer

cu

For the T, of PP (before extraction), only T is significant, that is, raising the polymerization temperature strongly reduces the stereospecificity of the active centers. The other Figure 3 A normal probability plot six factors are not significant to T,, those for effects on T,. effects fit a straight line on normal probability paper as shown in Fig.3. The mmmm pentad content of PP products was estimated from IR spectra using an equation given by Chien:'" Effect ( " C )

A998/A9,3= (0.864

k 0.004)[mmmm] + 0.10

where A,,/A,,, is the intensity ratio of peaks 998 cm-' to 973 cm-I. Although the estimation involves considerable error, the results for PP produced at 30°C fit the literature data well.['' Factor T was found to be significant to the mmmm content, however the R x T interaction

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has marginal significance. The other five factors appear to be non-significant.

3. Temperature Influence As shown in the previous section, polymerization temperature is the most important factor influencing polymerization kinetics. Fig.4 shows that an increase of Tp from 30°C to 50°C increases the maximum R, about fourfold. This result is not consistent with previous literature report^.[^.'*'^^ It was reported that increase of T, did not increase 4 in terms of kgPP(mo1-Zr hr)-l due to the decrease of solubility of propylene in toluene. In regard to the variation of & with reaction time, it was found that & decreased faster at 50°C than it did at 30°C. It seems that catalyst is less stable at higher temperature and deactivated faster.

-

Rate (kgI'I'/rnolZr.h)

Rate (kgPP/rnolZr.h)

(Thousa~~ds)

3500 I

I

[Zl.]= I 0 jlhl

0" 0

1

, , 3 4 5 6 Tirnc ( s ) ('l'housands)

2

Figure 4 The effect of T, on R,.

7

0

0

1

2

3

4

5

6

7

0

g

10

Time ( s ) ('l'housands)

Figure 5 The effect of [Al]/[Zr] on R,.

4. Ratio of [AI]/[Zr] Fig.5 is a comparison of R, of two runs at the same conditions but different ratio of [Al]/[Zr]. Higher value of [AI]/[Zr] did cnhance the catalyst activity, but showed no apparent effect on catalyst deactivation at such conditions. Analysis of experimental results shows that the dependence of 4 on [Al]/[Zr] seems more significant at 30°C than at 50°C. 5. Active Centers In some polymerization experiments another type of polymerization rate curve was observed. After a few minutes of reaction, R, developed to its maximum value, and then I$ kept almost constant for several hours. No substantial rate decrease was observed as shown by curve C in Fig.6. In such cases, R, was quite low, about 1/4-1/3 of the expected value, This situation often occurred at 50°C. To explain the finding, we suggest that there are two types of active centers, namely, species I and 11. Species I is highly active but unstable, especially at elevated temperature; while species I1 is very stable but of relatively low activity. The catalytic activity of species I is about 3 to 4 times higher than species 11. It would appear that species I can be transformed into species I1 during the course of polymerization, but the transformation does not appear to be reversible. At higher T, and certain conditions, species I cannot be formed for unknown reasons, and the reaction system is dominated by the less active species 11. This assumption can explain the fact R, at higher

I75

Rate (kgPI'/molZr.h) (Thousrrrids) [Zr]= 1OpM

P p = l atrn

--0

0

I

2

3

'

'

'

'

4

5

G

7

-->.A 8

9

10

Time ( s ) ('l'housands)

Figure 6 R,, and deactivation rate due to different contents of species I and 11.

T, could be lower than R,, at lower T, previously reported by some researcher^[^*'^] and confirmed by our experiments. It appears that this is not due to the low solubility of propylene in toluene but rather than the formation of a low activity catalyst. The deactivation of the catalyst could be thought to result from this transformation. The ratio of species I and I1 in the system would determine the R,, and the deactivation rate. The higher the content of species I, the higher the %, and also the faster the catalyst deactivation. See Fig.6.

CONCLUSIONS

The processes of propylene polymerization with Et(Ind),ZrCI,/MAO catalyst were precisely monitored using a computerized gas uptake system. The instantaneous R,,curve derived from the experimental data provides an accurate description of the variation of catalyst activity during polymerization. There is almost no induction period at the beginning of polymerization. The catalyst activity is highest at the beginning of the reaction, and it decreases gradually with reaction titnc for most cases. Statistical analysis indicated that in respect to %, polymerization temperature is the most significant factor, followed by catalyst concentration and the ratio of cocatalyst to catalyst. Raising the reaction temperature increased the activity of the catalyst, but it also accelerated catalyst deactivation. The initial activity of the catalyst is dependent on the ratio of [Al]/[Zr]. For the T, of PP product, only polymerization temperature is statistically significant, and for the niniinni pentad content of PP, polymerization temperature is the most significant factor, which indicates that stereospecificity of this type of homogeneous catalyst is strongly influenced by thermal disturbance. In order to explain experimental results, the existence of two types of active centers with different activity and stability is proposed. The assumption can well explain two types of & curves, catalyst deactivation during polymerization, and the wide deviation of F$ values reported by different researchers. Further investigations are being carried out to confirm this assumption.

ACKNOWLEDGEMENT

Financial support of the research by the Natural Science and Engineering Research Council of Canada is gratefully acknowledged.

176

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