Rate of Ethylene Polymerization with the Catalyst ... - ACS Publications

Chapter 6

Rate of Ethylene Polymerization with the Catalyst System (η -RC H ) ZrCl -Methylaluminoxane 5

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Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: June 22, 1992 | doi: 10.1021/bk-1992-0496.ch006

Effects of Cyclopentadienyl Ring Substituents Peter J. Τ. Tait, Brian L. Booth, and Moses O. Jejelowo Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, United Kingdom

Ethylene was polymerized, at 60°C, using the homogeneous catalyst system (η -RC H ) ZrCl -methylaluminoxane (where R = H, Me, n-Pr, i-Pr and t-Bu). A modest increase in the maximum rate of ethylene polymerization according to the order t-Bu < Η < i-Pr < n-Pr < Me was observed. However active centre studies carried out using a CO radio-tagging method demonstrated that active centre concentrations, C*, remained more or less constant for all the catalysts systems, leading to the conclusion that there is some variation in the propagation rate constant, k , for the different catalyst systems investigated. Possible explanations are given in terms of the opposing influences of the electronic and the steric effects of the ring substituents. 5

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Soluble zirconium complexes with aluminoxane co-catalysts have proved to be highly active and v e r s a t i l e homogeneous catalysts f o r the polymerization of ethylene and propylene(1-3). Recent work by Kaminsky(4) and Ewen(5) have shown that substituents i n the η cyclopentadienyl rings of ( n - C H ) Z r C l a f f e c t both the rate of polymerization of ethylene and the molecular weight of the polymer. Kaminsky(4) has proposed that i n the case of ( n - C M e ) Z r C l and methylaluminoxane, the broad molecular weight d i s t r i b u t i o n can be explained by supposing two d i f f e r e n t active centres C and C which d i f f e r not only i n t h e i r reaction rates, but also i n that d i f f e r e n t prereaction times are required f o r t h e i r formation. Ewen(5) has found that the number average molecular weights, M , decrease l i n e a r l y with polymerization rates i n the systems ( n - R C H ) Z r C l (where R = H, Me and Et) and ( n - C M e ) Z r C l , and has stated that the observed 5

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0097-6156/92/0496-0078$06.00/0 © 1992 American Chemical Society

In Catalysis in Polymer Synthesis; Vandenberg, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Cyclopentadienyl Ring Substitueras

comparative rates of polymerization i n these systems can be explained by a balance between the s t e r i c and the e l e c t r o n i c e f f e c t s of the substituents on the cyclopentadienyl rings. Previous work i n t h i s Department(6) has also shown that using ( n - R C H ) T i C l {where R = H, Me, ( C H 0 ) S i ( C H ) } differences i n the r e l a t i v e rates of hydrogénation of o l e f i n s were observed with v a r i a t i o n i n the nature of R. In an attempt to investigate the e f f e c t s of substituents on the cyclopentadienyl rings i n more d e t a i l , a systematic study of the polymerization of ethylene using the complexes ( n - R C H ) Z r C l (where R = H, Me, i - P r , n-Pr and t-Bu) and methylaluminoxane has been c a r r i e d out, and the results of t h i s investigation are now reported. s

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Results and Discussion. Polymerizations of ethylene were c a r r i e d out at 60 °C and 1 atm pressure using the catalyst system (η -RC H ) ZrCl -methylaluminoxane, and t y p i c a l rate-time p r o f i l e s are shown i n Figures 1, 2 and 3, where R i s Me, n-Pr and t-Bu, respectively. Except f o r the system where R = t-Bu, the maximum rate was achieved within 20 s of adding the l a s t component of the polymerization mixture, and t h i s rate was then maintained f o r the duration of the polymerization. Thus, the prereaction times were extremely short, and no s i g n i f i c a n t differences i n these times were observed when using the d i f f e r e n t ( n - R C H ) Z r C l complexes. When R = t-Bu, however, the maximum rate was achieved only after 8 min and the rate then decreased t o a steady low l e v e l . From a comparison, however, of Figures 1, 2 and 3 i t i s apparent that there i s a very s i g n i f i c a n t difference i n the nature of the rate-time plots when R i s t-Bu. A schematic representation of the v a r i a t i o n of the polymerization rate with v a r i a t i o n i n the R group i s also shown i n Figure 4. V a r i a t i o n of the cyclopentadienyl substitutent l e d only to a s l i g h t v a r i a t i o n i n the maximum rate of ethylene polymerization. Whilst the v a r i a t i o n was not large i t was nevertheless r e a l and increase i n catalyst a c t i v i t y followed the order t-Bu < H < i - P r < n-Pr < Me. Comparative values of the polymerization rates are l i s t e d i n Table I. 5

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Table I Polymerization System ( η - R C H ) ZrCl /Methylaluminoxane/Ethylene : E f f e c t of Cyclopentadienyl Ring Substituent on Polymerization Rate 5

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Rate /Kg PE

H Me n-Pr i-Pr t-Bu

(mmol Z r ) "

1

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9.1 10.1 9.7 9.6 7.6* 3

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[Zr] = 0.024 mmol dm" ; Al/Zr = 1000; Toluene =0.30 dm" ; Τ = 60 °C [C H ] = 0.0353 mol dm" *Rate i s the maximum l e v e l attainable. 3

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80

CATALYSIS IN POLYMER SYNTHESIS

20

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60

Ti me /min

Figure 1. Plot of rate of polymerization of ethylene versus time using the catalyst system (n -MeC H ) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. [Zr] = 0.024 mmol dm" ; toluene = 0.30 dm Al/Zr : Δ 500; • 700; Ο 1000. 5

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Ti me /min

Figure 2. Plot of rate of polymerization of ethylene versus time using the catalyst system (n -n-PrC H ) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. [Zr] = 0.024 mmol dm" ; toluene =0.30 dm Al/Zr : Δ 500; • 700; Ο 1000. s

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Cyclopentadienyl Ring Substituents

TAIT ET AL.

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%


JZ

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/min

Figure 3. Plot of rate of polymerization of ethylene versus time using the catalyst system (t-BuC H*) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. [Zr] = 0.024 mmol dm" ; toluene = 0.30 dm Al/Zr : Δ 500; • 700; ο 1000. s

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Figure 4. Schematic representation of e f f e c t of substituent R on the rate of polymerization of ethylene using the catalyst system (n -R-C H ) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. s

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The use of CO-radio-tagging as a method for the determination of active centre concentration has the great advantage that i t allows determination of C* values as a function of polymerization time. Typical p l o t s of C* as a function of polymerization time are shown in Figures 5 and 6. In a l l systems studied the value of C* remained constant throughout the duration of the polymerization. The use of the equation (8) R = k C* [M] where [M] i s the monomer concentration, kept constant throughout the duration of these polymerization experiments, allows the propagation rate c o e f f i c i e n t , k , to be calculated at various times within a given polymerization. Typical values of k as a function of polymerization time are shown i n Figures 7 and 8. The behaviour of the polymerization system where R=t-Bu i s again very d i f f e r e n t from any of the other polymerization systems which were investigated i n t h i s study. It has been suggested by Kaminsky(4) that, perhaps, i n these reactions there are two types of active centres. The f i r s t type, C *, i s produced rapidly but inserts monomer u n i t s quite slowly, while the second, C ~, requires a longer prereaction formation time, but once formed inserts monomer units at a faster rate than those of the C ~ type. If the nature of the substituent on the cyclopentadienyl ring can a f f e c t the r a t i o of the active centres, C ~ , then an active centre study should be able to detect t h i s phenomenon. We have shown previously(7) for the polymerization of ethylene using the homogeneous catalyst system (n -C H ) ZrCl -methylaluminoxane that using a C0 radio-tagging method for active centre determination, a l l zirconium atoms are involved i n active centres. In order to obtain accurate values of C* i t i s , however, e s s e n t i a l to use a short contact time. Using t h i s same method, active centre studies were carried out on the representative systems where R = H, Me and t-Bu. It can be seen from the r e s u l t s given i n Table II that, within the l i m i t s of experimental error, the values of C* are unaffected by the nature of R. However, these results do not provide any information on the types of centres which may be present. As can be seen from an examination of Table I I , the observed small changes i n the r e l a t i v e rates of polymerization with the type of R substituent are due to a similar v a r i a t i o n of the propagation rate constant. I t can also be seen from Table II that the values of kp decrease i n the order Me > H > t-Bu, i . e . , the same order as was found for the r e l a t i v e rates of polymerization. This lends support to the proposal f i r s t made by Ewen(5) that the main influences on k values i n these systems, are the electronic and the s t e r i c effects of the R substituents. Thus, the inductive and hyperconjugative effects of the a l k y l substituents w i l l increase the electron density of the n -cyclopentadienyl ligand leading to an increased electron density at the zirconium atom which i n turn leads to an increased rate of polymerization. This increase i n polymerization rate may be due to a lowering of the s t a b i l i t y of the n -alkene-zirconium bond resulting i n a more weakly coordinated monomer, f a c i l i t a t i n g insertion into the growing polymer chain. A l t e r n a t i v e l y , or, i n


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TAIT ET AL.

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40 Polymerization Time /min

Figure 5. V a r i a t i o n of active centre concentration with duration of polymerization of ethylene using the catalyst system (n -MeC H ) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. [Zr] = 0.024 mmol dm" ; Al/Zr = 1000; toluene 0.30 dm ; ethylene = 0.0353 mol dm" . S p e c i f i c a c t i v i t y of *CO = 1.57 χ 1 0 dpm m o l ; contact time = 4 min; mole r a t i o C 0 : Zr = 23:1. s

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Figure 6. V a r i a t i o n of active centre concentration with duration of polymerization of ethylene using the catalyst system (n -t-BuC H ) ZrCl /methylaluminoxane at 60 °C and 1 atm pressure. [Zr] = 0.024 mmol dm ; Al/Zr = 1000; toluene =0.30 dm ; ethylene = 0.0353 mol dm" . S p e c i f i c a c t i v i t y of C 0 = 1.57 χ 1 0 dpm mol" ; contact time = 4 min; mole r a t i o CO : Zr = 23:1. s

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Figure 7. V a r i a t i o n of the propagation rate c o e f f i c i e n t with duration of polymerization of ethylene using the catalyst system (n -MeC H ) ZrCl methylaluminoxane. Experimental d e t a i l s as f o r Figure 5. s

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Figure 8. V a r i a t i o n of the propagation rate c o e f f i c i e n t with duration of polymerization of ethylene using the catalyst system (n -t-BuC H ) ZrCl /methylaluminoxane. Experimental d e t a i l s as f o r Figure 6. s

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addition, a higher electron density at the zirconium atom may weaken the zirconium-carbon σ-bond to the growing polymer chain which could also f a c i l i t a t e insertion of the monomer, and hence lead to increased rate of polymerization. TABLE II Polymerization System (RC H*) ZrCl /Methylaluminoxane/Ethylene: V a r i a t i o n of Active Centre Concentration and Propagation Rate Constant with Polymerization Time Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: June 22, 1992 | doi: 10.1021/bk-1992-0496.ch006

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R^(add) /mol C H (mol Z r ) "

R

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/min

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/mol

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C* (mol Z r ) "

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/dm

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mol" s "

x

H

10 24 60

89 89 89

1.07 0.93 0.94

23.6 27.2 26.9

Me

10 24 60

99 99 99

0.95 0.97 0.98

29.4 28.8 28.5

4 20 40 60

79 25 6.9 6.8

1.02 0.98 0.98 0.99

22.0 7.2 2.0 2.0

Bu

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Contact time = 4 min; S p e c i f i c a c t i v i t y of C 0 = 1.57 χ 1 0 dpm mol" ; R (add) i s the rate of polymerization at the time of C 0 addition; t = polymerization time; Other conditions as i n Table I. 1

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Opposing these e f f e c t s an increase i n the s t e r i c bulk of the a l k y l substituent may hinder the approach of the incoming monomer and decrease the rate of polymerization. Thus, s t e r i c crowding i s expected to increase steadily from R = Me, to R = i - P r . The observed rates of polymerization may then r e f l e c t a combination of the s t e r i c and the e l e c t r o n i c e f f e c t s and may then reach a maximum when R = Me. When R = t-Bu the s t e r i c factor now outweighs the e l e c t r o n i c factor and the r e l a t i v e rate i s less than when R = H (see Figure 3). This explanation agrees well with those advanced by Ewen(5). The same explanation i s presumably true f o r the catalyst system ( n C Me ) ZrCl -methylaluminoxane studied by Kaminsky(4), where the rate of polymerization was noted to be some orders of magnitude less than the unsubstituted cyclopentadienyl complex. There i s an interesting difference between the results obtained with ( n - R C H ) Z r C l (R = H, Me) on the one hand, and those f o r s

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R = t-Bu on the other. In the former case, both C~ and k remain constant throughout the duration of the polymerization. Hence, the rate of polymerization remains steady over h i s period. In contrast, when R = t-Bu, although the value of C~ remains constant over the polymerization period the value of k decreases (see Figure 8). I t can, therefore, be concluded that i t i s t h i s decrease i n the value of k which i s responsible f o r the d i f f e r e n t k i n e t i c rate-time p r o f i l e observed f o r the n - ( t - B u C H * ) Z r C l catalyst. In the model as proposed by Kaminsky(9) f o r the active centre, the zirconium atom has a s t e r i c a l l y crowded environment. Thus, access of the monomer to the t r a n s i t i o n metal centre i s expected t o be sensitive to the size of the ligands attached to the t r a n s i t i o n metal. The presence of a t-Bu group could be expected to give r i s e to more severe s t e r i c crowding, evidently so much so that the k decreases with the extent of polymer formation. S t e r i c considerations may also explain the decrease i n a c t i v i t y which we, and others, have observed when ethylaluminoxane i s used i n place of methylaluminoxane. p

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Conclusions. The very high rates of ethylene polymerization which are obtained at 60 °C and 1 atmosphere pressure, using the homogeneous catalyst system (η -RC H ) ZrCl -methylaluminoxane a r i s e from the quantitative p a r t i c i p a t i o n of the zirconium atoms i n active centre formation, and the high values of propagation rate constants. The v a r i a t i o n i n the rate of ethylene polymerization with v a r i a t i o n i n the nature of the R group on the cyclopentadienyl unit can be explained from the opposing e f f e c t s of the e l e c t r o n i c and the s t e r i c e f f e c t s a t t r i b u t a b l e t o these substituent groups. The values of C" remain constant with polymerization time as do the derived values of k except when R = t-Bu. In t h i s case, the catalyst system shows a k i n e t i c rate-time p r o f i l e which r e f l e c t s a decrease i n k with polymerization time, C* remaining constant throughout the duration of polymerization. 5

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Experimental. Bis-(η -cyclopentadieny1) or ( η alkylcyclopentadienyl)-zirconium (IV) dichloride(12-14) and methylaluminoxane(2,10,15) were prepared according t o l i t e r a t u r e procedures. Radio-labelled carbon monoxide was supplied by ICI p i c , Petrochemicals and P l a s t i c s D i v i s i o n , Billingham, U.K. Ethylene (CP Grade, A i r Products) was dried by passage over 13X and 4A molecular sieves, and solvents were dried using standard procedures(16).

Polymerization and determination of active centre concentration. Details of the polymerization are as detailed previously(11). The procedure f o r active centre concentration determination has been described i n another p u b l i c a t i o n 7), a contact time of 4 min was used for t h i s work.

References. 1. H. Sinn, W. Kaminsky, Adv. Organomet. Chem., 18, 99 (1980). 2. W. Kaminsky, M. Miri, H. Sinn, R. Woldt, Makromol. Chem., Rapid Commun., 4, 417 (1983). 3. J. Herwig, W. Kaminsky, Polymer Bull., 9, 464 (1983).


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W. Kaminsky, Κ. Kulper, S. Niedoba, Makromol. Chem., Macromol. Symp., 3, 377 (1986). J.A. Ewen in "Catalytic Polymerization of Olefins", Ed. T. Keii and Κ. Soga, Elsevier, Amsterdam, p.271, 1986. B.L. Booth, G.C. Ofunne, P.J.T. Tait, J. Organomet. Chem., 315, 143 (1986). P.J.T. Tait, B.L. Booth, M.O. Jejelowo, Makromol. Chem., Rapid Commun., 9, 393 (1988). P.J.T. Tait in "Transition-Metal Catalyzed Polymerizations: Alkenes and Dienes", Ed. R.P. Quirk, Harwood Acad. Publ., New York, Vol. A, p.115, 1983. W. Kaminsky in "Transition-Metal Catalyzed Polymerizations: Alkenes and Dienes", Ed. R.P. Quirk, Harwood Acad., New York, Vol. A, p.225, 1983. J.A. Ewen, J. Am. Chem. Soc., 106, 6355 (1984). M. Abu-Eid, S. Davies and P.J.T. Tait, Polym. Prepr., Am. Chem. Soc., 24, 114 (1983). P.C. Wailes, R.S.P. Coutts, H. Weigold in "Organometallic Chemistry of Titanium, Zirconium and Hafnium", Academic Press, New York, 1974. M.F. Lappert, C.J. Pickett, R.I. Riley, P.I.W. Yarrow, J. Chem. Soc., Dalton Trans., 805 (1981). E. Samuel, Bull. Soc. Chim., France 3548 (1966). G.A. Razuvaev, Yu.A. Sangalov, Yu.Ya. Nel'Kenbaum, K.S. Minsker, Bull. Acad. Sci. USSR, Div. Chem. Sci., Engl. Trans., 11, 2434 (1975). A.I. Vogel, "Textbook of Practical Organic Chemistry including Qualitative Organic Analysis", 4th Edn., p.264, Longmans, London - New York, 1978.

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Rate of Ethylene Polymerization with the Catalyst ... - ACS Publications

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