Initiation of Polymerization - American Chemical Society

32 Homogeneous Lanthanide Complexes as Polymerization and Oligomerization Catalysts: Mechanistic Studies Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

P. L . WATSON and T. HERSKOVITZ Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, DE 19898

The trivalent lanthanide complexes M(η5-C5Me5) 2 CH 3 •L (M = Yb, Lu; L = diethyl ether, tetrahydrofuran or trimethylaluminum) and divalent M(η5-C5Me5)2•L complexes (M = Yb, Eu, Sm; L = diethyl ether or tetrahydrofuran) are catalysts for polymerization of ethylene (hexane, 30-100°C, 60 psig; Mv product ~105) and oligomerization of propene. The initial insertion reactions of propene with Lu(η 5 -C 5 Me 5 ) 2 CH 3 •L, studied by NMR, give well characterized Lu(η 5 -c 5 Me 5 ) 2 -isobutyl and -2,4-dimethylpentyl complexes. Prior dissociation of ligand L is necessary. Constants for the dissociative pre-equilibria are determined from the rates of the propene insertion reactions and ligand exchange rates. Rates of ethylene polymerization in the presence of various ligands L are modified qualitatively as for the propene insertion reactions viz. L = no ligand > diethyl ether > THF > Al (CH3)3 (slowest rate). Ethylene propagation is faster than propene propagation by ~104. Much e f f o r t has been devoted during the l a s t 30 years toward understanding the mechanisms o p e r a t i v e i n the c o o r d i n a t i o n c a t a l y s i s o f ethylene and α-olefin p o l y m e r i z a t i o n u s i n g Z i e g l e r Natta systems (metal h a l i d e and aluminum a l k y l , sometimes with Lewis base m o d i f i e r s ) . Aspects o f the complex heterogeneous r e a c t i o n s have been e l u c i d a t e d (1-5) but the intimate mechanistic d e t a i l - f o r example the r o l e o f i n h i b i t o r s and promoters, k i n e t i c s and thermodynamics o f chain growth, modes o f chain t r a n s f e r and t e r m i n a t i o n - comes p r i m a r i l y from s t u d i e s o f homogeneous c a t a l y s t s (_5~Z) Ethylene p o l y m e r i z a t i o n c a t a l y z e d by the w e l l - c h a r a c t e r i z e d homogeneous lanthanide complexes [ M ( C H R ) R ' ] studied,

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0097-6156/83/0212-0459$06.25/0 © 1983 American Chemical Society In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Downloaded by UNIV OF CINCINNATI on November 14, 2014 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch032

460

INITIATION OF

POLYMERIZATION

i n a d d i t i o n to some model r e a c t i o n s with s u b s t i t u t e d o l e f i n s . ( 8 / £ ) Mechanistic d e t a i l s of the a c t u a l o l e f i n i n s e r t i o n r e a c t i o n s were obscured i n these systems by the s t a b i l i t y of the a l k y l - b r i d g e d dimeric s t r u c t u r e s and by the slow r a t e of α-olefin i n s e r t i o n r e l a t i v e t o f u r t h e r i n s e r t i o n o f a second o l e f i n and/or β-hydrogen elimination. S t e r e o s p e c i f i c p o l y m e r i z a t i o n o f 1 , 3 - d i e n e s ( 1 0 - 1 8 ) (to buta diene) and isoprene homo- and copolymers), d i m e r i z a t i o n of propene ( 1 9 ) and r e c e n t l y s t e r e o s p e c i f i c p o l y m e r i z a t i o n of acetylene ( 2 0 ) to high c i s - c o n t e n t p o l y a c e t y l e n e have a l l been reported u s i n g lanthanide c a t a l y s t s . Sen ( 2 1 ) has reported the p r e p a r a t i o n o f c a t i o n i c europium systems (which perhaps f u n c t i o n as c a t i o n i c i n i t i a t o r s ) f o r p o l y m e r i z a t i o n of norbornadiene and 1 , 3 - c y c l o h e x a diene. Described i n t h i s paper i s a model system - one i n which w e l l c h a r a c t e r i z e d lanthanide complexes e x h i b i t high c a t a l y s t a c t i v i t i e s f o r ethylene p o l y m e r i z a t i o n but where the corresponding oligomer i z a t i o n of propene i s s u f f i c i e n t l y slowed so t h a t stepwise i n s e r t i o n of the o l e f i n can be s t u d i e d q u a n t i t a t i v e l y and a l l important intermediates observed or i s o l a t e d . Emphasized i n t h i s paper i s the e f f e c t of added Lewis a c i d s and bases on the r a t e of o l e f i n i n s e r t i o n s , and comparison between ethylene and propene r e a c t i o n s . The c a t a l y s t s , of general s t r u c t u r e M(n -Cp*)2^3·L (M = Yb, Lu; Cp* = C ( C H ) ; L = C H L i , ether, THF or A l ( C H ) ) , polymerize ethylene a t r a t e s comparable t o Z r ( b e n z y l ) / A l 0 . Thus they are l e s s a c t i v e by two orders of magnitude than the zirconium-aluminoxane systems reported r e c e n t l y by Kaminsky and Sinn, ( 2 2 ) but much ι more a c t i v e than most other reported homogeneous and heterogeneous ethylene p o l y m e r i z a t i o n c a t a l y s t s . (23_, 2 4 ) 5

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Within the context of lanthanide chemistry i t i s i n t e r e s t i n g to note r e p o r t s by Chinese ( 1 0 , 1 5 ) and Russian ( 1 2 ) s c i e n t i s t s that a c t i v i t i e s o f lanthanide-based Z i e g l e r - N a t t a c a t a l y s t s f o r p o l y m e r i z a t i o n of 1 , 3 - d i e n e s show much higher a c t i v i t y f o r e a r l y metals than f o r the end-row metals Er-Lu. The p o t e n t i a l f o r higher a c t i v i t y analogs of the complexes d e s c r i b e d here i s thus evident . Experimental A l l s y n t h e t i c manipulations were c a r r i e d out i n a Vacuum Atmospheres HE-63 Drybox under a very slow continuous purge of dry N using p r e d r i e d glassware. Solvents (THF, ether, toluene, pentane) were d r i e d by d i s t i l l a t i o n from Na/benzophenone under N2. Ethylene and propene were purchased from Matheson (Research grade, 9 9 . 9 8 ° / m i n and 9 9 . 7 % min r e s p e c t i v e l y ) . NMR measurements were made on a N i c o l e t 3 6 0 MHz s p e c t r o meter. Both 1 H and 1 3 c chemical s h i f t s were referenced to TMS by c o r r e c t i n g the s h i f t s r e l a t i v e to r e s i d u a l deuterated solvent peaks. Paramagnetic m a t e r i a l s were r e f e r e n c e d (in Hz) to the h i g h e s t f i e l d s o l v e n t peak (+ = down f i e l d ) . Samples f o r NMR 2

In Initiation of Polymerization; Bailey, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

32.

WATSON AND HERSKOViTZ

Homogeneous

Lanthanide

461

Complexes

were approximately 0.05 M i n lanthanide complex. O l e f i n was condensed i n t o the sample on the vacuum l i n e and then the sample was sealed under vacuum. Elemental analyses were done by Franz Pascher, M i c r o a n a l y t i s c h e s Lab., Bonn, West Germany. Ethylene p o l y m e r i z a t i o n s were c a r r i e d out i n a g l a s s 500cc batch r e a c t o r with high speed (1000 rpm) s t i r r i n g and a thermocouple f o r i n t e r n a l temperature sensing. The s t a i n l e s s s t e e l head a l s o had an i n l e t f o r gases and septum p o r t f o r c a t a l y s t i n j e c t i o n and sampling. Absence of a l l poisons was ensured i n each p o l y m e r i z a t i o n by e i t h e r : a) t i t r a t i o n with s a c r i f i c i a l c a t a l y s t under 10 p s i g C H u n t i l polyethylene formation ensued; or b) a d d i t i o n o f 10~ moles of tetraneophylzirconium. T y p i c a l runs i n v o l v e d p r e e q u i l i b r a t i o n of s o l v e n t (cyclohexane, 150 mL) with C H (60 p s i g ) i n the r e a c t o r , followed by i n j e c t i o n o f c a t a l y s t (10~3 t o 10~ mol). Reaction times were u s u a l l y 60-120 sec and were terminated with e t h a n o l / a c e t i c a c i d quench. Average p o l y m e r i z a t i o n r a t e s were c a l c u l a t e d from the t o t a l polymer y i e l d obtained during the r e a c t i o n . Ethylene uptake p r o f i l e s were always monitored using a mass flowmeter (Brooks 5810) on the ethylene feed l i n e , and recorded f o r a n a l y s i s . I t should be noted t h a t the a b b r e v i a t i o n Cp* = n 5 - c ( C H ) i s used i n the experimental s e c t i o n below. 2

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Li[MCp* (CH^) ] (THF) ^ 3 , Zb (M = Lu) , 36 (M = Yb) . (25) To a 0°C s o l u t i o n of L i [ M C p * C l ] (THF) 3 (26) was added 2 eq. CH3ÏX. A f t e r 30 min the s o l u t i o n was evaporated to dryness. Ether e x t r a c t s of the s o l i d residues were f i l t e r e d and the f i l t r a t e cooled, y i e l d i n g m i c r o c r y s t a l l i n e white (2b) or yellow (£g) product. For 2A: -^H NMR (glyme-d ) δ-1.77 (s, 2CH , LU-CH3) , 1.86 (m, 12H, THF), 2.28 (s, 30H, r i n g CH3, overlapping THF) and 3.68 (m, 12H, THF). Anal. C a l c d . f o r [ C H L i L u O , i . e . 1 THF): C, 56.11; H, 8.33; L i , 1.25; Lu, 31.44. Found: C, 56.59; H, 8.40; L i , 1.17; Lu, 31.20. Less than 0.15% C l found. For 2B: Anal. C a l c d . f o r [ C 3 H L i O Y b ] : C, 58.6; H, 8.60; L i , 0.90; Yb, 24.83. Found: C, 58.53; H, 8.83; L i , 1.00; Yb, 24.82. [N.B. Samples f o r elemental a n a l y s i s were s t o r e d under vacuum f o r s e v e r a l weeks r e s u l t i n g i n some slow l o s s o f THF]. 9

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Li[MCp* (CH^) 3, 3A (M = Lu), 3B (M = Yb). Solid L i [ M C p * ( C H ) ] ( T H F ) 3 was heated under vacuum a t 75° f o r 1-2 ! H NMR i n glyme-d^ô showed no THF or ether. 2

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days.

MCp* Al(CH^) , £b (M = Lu), (M = Yb). To a s t i r r e d s l u r r y of Li[MCp* (CH3) ] ( s o l v e n t - f r e e ) i n toluene was added 2 eq. A l ( 0 ^ 3 ) 3 . A f t e r most of the s o l i d s were taken i n t o s o l u t i o n the mixture was f i l t e r e d . The product was c r y s t a l l i z e d (-40°C) from the concentrated f i l t r a t e (M = Lu, white; M = Yb, p u r p l e ) . For 4A: Ε NMR (toluene-dg) 62.06 (s, 30H, Cp*), -0.11 (s, 6H, b r i d g i n g CH3) and -0.21 (s, 6H, t e r m i n a l CH3)· A n a l . C a l c d . f o r [C H AlLu]: C, 54.13; H, 7.95; A l , 5.07; Lu, 32.86. 9

4

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λ

2 4

4 2


462

INITIATION OF POLYMERIZATION

Found: C, 54.16; H, 7.94; A l , 4.86; Lu, 32.70. For 4B: H NMR (toluene-dg) overlapping peaks a t +92 (Wi ^30 Hz) and +62 Hz (est. Wi ^40 Hz), i n r a t i o 10:4. Anal. Calcd. f o r [ C 4 H A l Y b ] : C, 54.32; H, 7.98; A l , 5.09; Yb, 32.61. Found: C, 53.90; H, 7.95; A l , 4.88; Yb, 32.50. 2

42

MCp* CH 'ether, 5& (M = L u ) , 5B, (M = Yb).(27) M C p * A l ( C H ) was d i s s o l v e d i n d i e t h y l ether. On c o o l i n g the s o l u t i o n t o -30°C c r y s t a l s o f the product formed (5A white; 5B orange). For 5&: ! H NMR (toluene-d ) 6-0.05 (s, 3Îi7 Lu-CH^), 1.09 ( t , 6H, e t h e r ) , 2.19 (s, 30H, Cp*), 3.49 (q, 4H, e t h e r ) ; ( c y c l o h e x a n e - d ) : -0.95 (Lu-CH ) , 1.20 ( e t h e r ) , 1.95 (Cp*), 3.56 ( e t h e r ) . 3-H NMR (-90° 0.04 M L u C p * C H - e t h e r and 0.4 M ether i n t o l u e n e - d ) : 6-0.55 (s, Lu-Me), 0.41 ( t , CH coord, e t h e r ) , 0.60 ( t , CH coord, ether), I. 17 ( t , CH f r e e e t h e r ) , 2.04 (s, Cp*), 2.79 (q, CH coord, e t h e r ) , 2.86 (q, coord, e t h e r ) , and 3.16 (q, CH f r e e e t h e r ) . Anal. C a l c d . f o r [ C H O L u ] : C, 56.17; H, 8.11, 0, 2.99; Lu, 32.73. Found: C, 56.04; H, 8.11, 0, 2.90; Lu, 33.20. For 5β: ! H NMR (toluene-d ) J = +9 Hz, Wi = 90 Hz. A n a l . C a l c d . f o r [ C H O Y b ] : C, 56.37; H, 8.14; 0, 3.00; Yb, 32.49. Found: C, 56.05; H, 8.13; O, 2.90; Yb, 32.35. 9

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[MCp* CH ] , 2h (M = L u ) . To a s t i r r e d s o l u t i o n o f MCp* CH *ether i n toluene was added 1-3 eq. N E t . Solvent was removed under vacuum. The r e s u l t i n g r e s i d u e s were washed with n-pentane, c o l l e c t e d by f i l t r a t i o n and d r i e d . 1H NMR ( c y c l o h e x a n e - d , 0.05 M, 25°C): 6-1.10 (s, 3H, Lu-CH ) and 2.01 (s, 30H, Cp*); ( t o l u e n e - d , 0.05 M, 25°C): 6-0.5 (s, 3H, Lu-CH ) and 2.13 (s, 30H, Cp*). Anal. Calcd. f o r [ C H L u ] : C, 54.77; H, 7.22; Lu, 38.00. Found: C, 54.44; H, 7.19; Lu, 37.90. 2

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MCp* CH -THF, (M = L u ) , 6B (M = Yb). MCp* CH .THF (M = Yb, Lu) was prepared by d i s s o l v i n g MCp* CH *ether i n THF. Evaporation of s o l v e n t followed by r e c r y s t a l l i z a t i o n from n-pentane (-40°) gave the white (Lu) or orange (Yb) product. For 6A: H NMR (glyme-d ) 6-1.18 (s, 1CH , Lu-CH ), 1.80 (m, 4H,~THF overlapping Cp*), 1.89 (s, 10CH , Cp*) and 3.64 (m, 4H, THF) ( t o l u e n e - d ) : 6-0.79 (s, 1CH , Lu-CH ), 1.21 (m, 4H, THF), 1.93 (s, 10CH , Cp*) and 3.28 (m, 4H, THF). ^ NMR (-90°, 0.04 M LuCp* CH «THF and 0.02 M THF i n t o l u e n e - d ) : 6-0.32 (s, Lu-CH ), 1.13 (unres., 3 and 3 ' CH o f coord. THF), 1.57 (m, f r e e THF), 2.27 (s, Cp*), 3.04 (unres., α CH of coord. THF), 3.38 (unres., o/ CH o f coord. THF), 3.81 (m, f r e e THF). Anal. C a l c d . f o r [ C H L u O ] : C, 56.38; H, 7.76; Lu, 32,86. Found: C, 56.51; H, 7.81; Lu, 32.75. Anal. Calcd. f o r [ C H O Y b ] : C, 56.58; H, 7.79; Yb, 32.61. Found: C, 56.61; H, 7.83; Yb, 32.75. 2

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Results and D i s c u s s i o n Synthesis and C h a r a c t e r i z a t i o n of Lutetium- and YtterbiumMethyl Complexes. The s y n t h e t i c s t r a t e g y o u t l i n e d i n Scheme 1


32.


Homogeneous

Lanthanide

Complexes

463

allows the p r e p a r a t i o n of a f a m i l y of methyl complexes having the general s t r u c t u r e MCp* CH 'L (M = Lu, Yb; Cp* = n - C ( C H ) ; L = Lewis bases: ether, THF, CH3L1; and Lewis a c i d s : AlMe , MCp* CH ). (25, 26, 27) S t a t i c s t r u c t u r e s of s e v e r a l complexes - [YbCp* (CH ) ] L i ( T H F ) ( e t h e r ) , YbCp* CH ·THF and YbCp* CH -ether - have been confirmed by X-ray c r y s t a l l o g r a p h y . Ligand arrangements are represented adequately by the drawings i n Scheme 1. A l l these s t r u c t u r e s c o n t a i n a b a s i c sandwich arrangement of metal atom between r| -C5(CH )5 r i n g s (mutually staggered) s i m i l a r t o s t r u c t u r e s reported f o r r e l a t e d halogen complexes.(26) The methyl group and l i g a n d L are coordinated i n the plane between the n 5 - C ( C H ) r i n g s . An ORTEP drawing of YbCp* CH*ether i s provided i n the abstract,(28) and of YbCp* CH «THF i n F i g u r e 1. V a r i a b l e temperature 1H NMR provides information about the l a b i l i t y of l i g a n d s L i n LuCp* CH *L complexes (Eq. 1). For a l l l i g a n d s L considered here Eq. 1 l i e s s t r o n g l y t o the left. D i s s o c i a t i v e rather than a s s o c i a t i v e exchange i s confirmed 5

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LuCp* CH^-L Ζ J

^

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•

LuCpSCH-, Z

+

L

(1)

3

by the independence of 1H NMR coalescence temperatures on concentration of excess L. Exchange of coordinated ether or THF (L) i n 5A o r 6A with uncoordinated L i s extremely r a p i d a t 25°C i n toluene or cyclohexane. At -90°C, l i m i t i n g s p e c t r a show sharp separate resonances f o r coordinated L and added f r e e L. Intramolecular exchange - presumably r o t a t i o n a l - of the chemically e q u i v a l e n t a, s i t e s and 3, 3^ s i t e s o f coordinated L i s f a s t on the NMR time s c a l e above -90°C f o r ether and above -65°C f o r THF. Coalescence measurements show t h a t i n t e r m o l e c u l a r l i g a n d exchanges then become r a p i d a t higher temperatures. Dis s o c i a t i v e l i g a n d exchange r a t e s at 15°C (Eq. 1 forward, extrapo l a t e d from data i n Figure 2) are 4.5xl0 s " f o r ether and 1.2xl0 s " f o r THF i n toluene-dg. Data shown i n Figure 2 i n d i c a t e t h a t these l i g a n d d i s s o c i a t i o n processes are 2-3 times f a s t e r i n cyclohexane than i n toluene. The s t r u c t u r e of M C p * A l ( C H ) (4, see Scheme 1) i s i n f e r r e d from the NMR spectrum, and by analogy with the s t r u c t u r e s of YbCp* AlCl (26) and Y ( C H ) A 1 ( C H ) . ( 2 9 ) Exchange w i t h A l ( C D ) occurs w i t h i n minutes a t I5°C. B r i d g e - t e r m i n a l methyl group s i t e exchange (extrapolated from coalescence temperatures i n F i g u r e 2) a t 15°C i s ^0.3 s " . I f t h i s exchange r e s u l t s from d i s s o c i a t i o n of A l ( C H ) , then 0.3 s " i s the r a t e of Eq. 1 forward. However, i f the exchange i s i n t e r m o l e c u l a r then 0.3 s " i s an upper l i m i t f o r the d i s s o c i a t i v e process. Since the resonance f o r f r e e , added A l ( C H ) (at δ-0.3, superimposed on the l o w e r - f i e l d h a l f of the coalescenced b r i d g e - t e r m i n a l resonance) i s a l s o broadened i t i s l i k e l y that i n t e r m o l e c u l a r exchange i s 5

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INITIATION OF

POLYMERIZATION


464


Homogeneous Lanthanide

Complexes





MCH-d -

tol-d

MCH-d

ether

THF

THF

THF

A1(CH ) 3 t o l - d g

Al(CH )

5A

6A

6A

6A

4A

4A

8

8

14

3 MCH-d..14

tol-d

14

b

àv

38.3

32.9^

42.4

e

52.75^

e

X

170.1

146.2

188.4

234.4

120.3

c

-1 8 0MHz k (s ) 27.08

1

1

94.5

-10

-5.5

-5 9

-65

172.3

46.4

19.1

148.0

190.8

765.5

206.3

84.8

657.5

847.7

448.0

1054.8

237.4^ 100.1

545.6

e

122.8

Τ (°C) Δν 360MHz k ( s " )

Coalescence measurements' for ligand exchange.

120

107

-10

7

11

-63

-48

-55

S 0.3

~1.2xl0

~4.5xl0

1

1 4

3

5

Τ (°C) k ^ C s " )

a. Measurements were made on s o l u t i o n s 0.5M i n complex and 0.5M i n added l i g a n d L. Changes o f t h e c o o r d i n a t e d - l i g a n d l i n e shapes w i t h t e m p e r a t u r e were i n d e p e n d e n t o f c o n c e n t r a t i o n o f added L. b. Δν = ν(free) - ν(coord). c. C a l c u l a t e d f o r two s i t e model, k = /2πΔν a t c o a l e s c e n c e . d. By e x t r a p o l a t i o n . e . P r o t o n s α t o oxygen. ^. P r o t o n s β t o oxygen, g. Toluene-dg and m e t h y l c y c l o h e x a n e - d .

3

3

tol-dg

ether

5A

tol-dgS

ether

5A

Solvent

Ligand

Complex

Figure 2.


32.

WATSON

AND

Homogeneous Lanthanide Complexes

HERSKOViTZ

467

o c c u r r i n g . Again the exchange r a t e i s independent of the amount of added l i g a n d , A l (0113)3. The dimer [MCp* CH3]2* §r can a l s o be viewed as a Lewis a c i d adduct. Monomer and dimer are i n r a p i d e q u i l i b r i u m down t o -60°C, g i v i n g r i s e to a s i n g l e LU-CH3 resonance i n the H NMR spectrum of The l i m i t i n g spectrum o f §A at -90°C shows a 1:1:2 p a t t e r n f o r the Cp* peaks and a 1:1 s e t f o r the LU-CH3 groups. The s t r u c t u r e shown i n Scheme 1 i s proposed f o r t h i s dimer. Presumably s t e r i c bulk c o n s t r a i n s the two LuCp* fragments t o be mutually orthogonal and prevents b r i d g i n g o f both methyl groups. The methyl complexes are thus w e l l - c h a r a c t e r i z e d with r e s p e c t to s t r u c t u r e and l a b i l i t y o f l i g a n d L. Comparatively, l a b i l i t y o f L decreases i n the s e r i e s MCp* CH3 êther>THF>AlMe . As w i l l be seen, t h i s i s a l s o approximately the o r d e r i n g o f e q u i l i b r i u m constants Κ f o r Eq. 1. 2

1


2

r

2

3

Reaction o f Methyl Complexes with Propene. Products. Reaction o f [MCp* CH3] / §, with propene i n i t i a l l y y i e l d s i s o b u t y l complexes MCp* CH CH(CH ) , 9, as shown i n Scheme 2. Secondary, slower r e a c t i o n o f propene with 9 g i v e s the 2,4-dimethylpentyl species J.Q, and subsequently higher oligomers. Confirmation o f the s t r u c t u r e s comes from s e v e r a l sources. GC-MS of hydrolyzed samples o f 9 and 10 show isobutane and 2,4-dimethylpentane ( r e s p e c t i v e l y ) as the only C^ or C7 isomers formed, confirming the r e g i o s p e c i f i c i t y o f the i n s e r t i o n s . -^H NMR spectra of 9A and 10A (and o f H s p e c i f i c a l l y - l a b e l l e d samples o f 9A and 10A) are f u l l y assigned (30) and c o n s i s t e n t with proposed s t r u c t u r e s . A d d i t i o n a l l y , 9A i s a l s o formed from the r e a c t i o n of LuCp* H,(31) 11A,.with isobutene ( r e a c t i o n 5, Scheme 2 ) , and 10A i s formed by a d d i t i o n o f e i t h e r 2,4-dimethylpentene t o LuCp* H or by a d d i t i o n o f 4-methylpentene t o LuCp* CH3. Polypropene i s not formed i n these systems. A n a l y s i s (by h y d r o l y s i s then GC) of mixtures o f propene (60 p s i ) and 8A (0.1 M i n cyclohexane) r e v e a l s that oligomers a t l e a s t up to C 4 are formed. However the product d i s t r i b u t i o n above C^Q becomes p r o g r e s s i v e l y more complex, i n d i c a t i n g chain t r a n s f e r and chain t e r m i n a t i o n processes which compete w i t h propagation. Both § and XQ are t h e r m a l l y unstable with h a l f - l i v e s at 25°C o f ^3 h r . The thermal decomposition of 9A i n cyclohexane i s under study as a model f o r c h a i n t e r m i n a t i o n f o r these systems. Both 3-hydrogen e l i m i n a t i o n ( g i v i n g 11A and isobutene) and β-alkyl e l i m i n a t i o n (giving 8A and propene) are important k i n e t i c a l l y a c c e s s i b l e processes, both f o r 8A and f o r the longer chain LuCp* (CH CHCH ) CH complexes. (31) K i n e t i c s . K i n e t i c a n a l y s i s of the r e a c t i o n o f the dimer 8A with propene confirms t h a t d i s s o c i a t i o n t o the a c t i v e monomer 7A i s necessary and t h a t the rate-determining step i s i n s e r t i o n of «propene i n t o the Lu-CH bond o f 7A (reactions 1 and 2 i n Scheme 2). At 15°C Κ = k i / k . ! = 4.1x10-3 M'ând k = 1.22x10"! M S " 2

2

2

2

3

2

2

2

2

2

2

2

2

3

n

3

3

_1

2


1

INITIATION OF


468

POLYMERIZATION

Scheme 2.


32.


469

Homogeneous Lanthanide Complexes

i n cyclohexane. Previous k i n e t i c s t u d i e s (27) o f the r e a c t i o n of LuCp*2CH3-ether, 5, with propene a l s o showed the presence o f a d i s s o c i a t i v e p r e e q u i l i b r i u m g i v i n g 7A and ether followed by a slower o l e f i n i n s e r t i o n . The r a t e expression d e r i v e d (Eq. 2) i s c o n s i s t e n t with Scheme 2. Knowing k e x a c t l y , values f o r 2

-diZ&'L] dt

=

kik [7A-L][propene] k^lL]

the p r e e q u i l i b r i u m constants Κ = k / k _ can be d e r i v e d from the observed r a t e s o f r e a c t i o n o f the complexes with propene under p s e u d o - f i r s t order c o n d i t i o n s . These are shown i n F i g u r e 3 f o r v a r i o u s l i g a n d s L. I n h i b i t i o n o f propene i n s e r t i o n by ether o r monomer 7A i s m i l d but k i n e t i c a l l y important a t the c o n c e n t r a t i o n s used f o r NMR experiments (^0.05 Μ ) , slowing the r a t e s by a f a c t o r o f 20-30. I n h i b i t i o n i s very pronounced w i t h the stronger l i g a n d s Al(CH )3 - T h i s k i n e t i c i n h i b i t i o n i s due t o the thermo dynamic s t a b i l i t y o f t h e adducts (see Figure 4 ) . In f a c t , complexation o f 7L by both A l f C H ^ ^ and THF slows the propene i n s e r t i o n r e a c t i o n s u f f i c i e n t l y t h a t only very low steady-state concentrations o f the i n i t i a l L u - i s o b u t y l product are observed. Observed r a t e s f o r r e a c t i o n o f propene with LuCp* CH *L (L = LuCp*2CH , ether, THF, AlMe ) are a l l dependent on propene c o n c e n t r a t i o n . The h i g h e s t energy t r a n s i t i o n s t a t e i s t h e r e f o r e always t h a t f o r the o l e f i n i n s e r t i o n , not f o r l i g a n d d i s s o c i a t i o n . I n h i b i t i o n o f the r e a c t i o n o f i s o b u t y l complex 9A with propene does not occur with added ether, i n d i c a t i n g t h a t formation o f 9A-ether i s not s i g n i f i c a n t . However, THF does i n h i b i t t h i s r e a c t i o n and a value o f Κ = k4/k_4 (Scheme 2) = 'VLxlO M " " i s obtained. The r e l a t i v e a b i l i t y o f 7A and 9A t o coordinate THF i s probably l a r g e l y s t e r i c i n o r i g i n ? F i n a l l y , the r a t e o f propene i n s e r t i o n i n t o the Lu-C bond o f i s o b u t y l complex 9A, V L . l x l O " ^ M~1 s"^ i s a good estimate o f the r a t e o f propagation d u r i n g f u r t h e r o l i g o m e r i z a t i o n o f propene. 1


(2)

?

a

n

d T

H

1

F

3

2

3

3

3

-2

1

Polymerization o f Ethylene by Methyl-Lutetium and -Ytterbium Complexes. Products. Ethylene (60 p s i , 30-100°C) i s polymerized r a p i d l y by 10"" t o Ι Ο " M cyclohexane s o l u t i o n s o f the methyl complexes 3, 4, 5, 6, and 8 (Table). Low Mw oligomers were not formed ( a n a l y s i s by GC) i n experiments l a o r 3a, Table. A t 160°C both the Lu and Yb e t h e r a t e s produced very l i t t l e polyethylene CVL0~ o f the Table y i e l d s a t 40-100°) r e f l e c t i n g thermal decomposition o f the a c t i v e s p e c i e s . Inherent v i s c o s i t i e s of the polymer (Table) show t h a t M ^ 1 0 and t h a t p o l y m e r i z a t i o n a t higher temperatures reduces the molecular weight (Figure 5 ) . I n f r a r e d s p e c t r a l a n a l y s i s o f the polyethylenes obtained i n the Table showed no H(R)C=CR groups t h a t would r e s u l t from α-olefin formation then i n c o r p o r a t i o n i n t o growing polymer then chain t e r m i n a t i o n v i a 4

6

3

5

v

2



e l

LuCp*CH

8A 3

3

3

k

3

2

1.2xl0~

-

3 . U X 1 0 "

1.7xl0~

2.3xl(T

observed

5

6

5

4

( s

1 )

2

2

b

2

l.OxlO"

4.1xl0~

2.4xl0"

1.4xl0~

2

3

5

4

3

(M)

1.9xl0"

K

1

1

1

-

Initiation of Polymerization - American Chemical Society

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