- Email: [email protected]
18. Reaction Mechanisms in Metallocene-Catalyzed Olefin Polymerization
193
18. Reaction Mechanisms in Metallocene-Catalyzed Olefin Polymerization
H. BRINTZINGER,
S. BECK, M. LECLERC, U. STEHLING and W. ROLL
Fakultat fur Chemie, Universitat Konstanz, 0-78434 Konstanz, Germany ABSTRACT
1. Studies by 'H NMR on equilibria between contact ion pairs such as Cp2ZrCH3d+...H3C-B(C6F5)3d-and binuclear alkyl zirconocene cations of the type (Cp,ZrCH,),b
- CH,)
+
lead to the conclusion that these binuclear species must
generally be considered as participants in all homogeneous Ziegler-Natta systems.
2. Different polypropene chain lengths, which are obtained from cis- and trans1D - propene with the catalyst en(thind)2ZrC12/MA0,show that exchange of a-H with
a-D atoms affect the rate of chain growth by a large kinetic isotope effect; this supports the notion that an a-agostic interaction facilitates the olefin insertion step. 3. A strong increase in polymer chain lengths, which is caused by the presence of amethyl groups in ansa-zirconocene catalysts, is shown, by the effects of propene pressure on ,M ,
to be due to the suppression of the otherwise predominant direct
I3 - H-transfer to a coordinated olefin molecule by these a-substituents. INTRODUCTION Open questions with regard to the mechanisms of metallocene-catalyzed olefin polymerizations concern the equilbria which lead to catalyst activation and deactivation, the factors which control the rate and stereoselectivity of the olefin insertion step, and the mechanisms of chain termination. Some recent studies related to these questions are reported here. EXPERIMENTAL
I . Solutions of B(c~F,),
' 1 and of CP~Z~(CH,)~ in
C,D,
(10 - 40 mM) were
combined in various proportions under extreme exclusion of humidity (flamed glassware, glovebox techniques) and their 'H NMR spectra measured at room temperature on a Bruker AC 250 MHz spectrometer.
194 H . Brintzinger, S. Beck. M. Leclerc, U. Stehling and W. Roll
2. Cis- and trans-a-deuterated propene were prepared by lithiation o f cis- and trans-chlorpropene, respectively, and subsequent cleavage with D20. The samples thus obtained were purified by repeated distillation from dry MAO. Polymerizations were conducted at 5OoC with en(thir~d)~ZrCI,/MAOin toluene ([Zrl =
M, Al:Zr
= 1200:l 1 at 1 bar. The molecular weights of the polymer products were determined
from their 13C NMR spectra, run at 13OOC in CD ,C , ,I
by the ratio of n-propyl end-
group and methyl side-chain signals at 14.3 and 20.0-21.8 ppm, respectively. 3. Polymerizations were conducted with MAO-activated Me2Si(benzind)2ZrC12and Me,Si(2-Me-ben~ind)~ZrCI,([Zrl = 1.25
M, A1:Zr = 15800:1, T,
= 5OoC), at
propene pressures between 1 and 7 bar. The molecular weights of the polymer products were determined by GPC (BASF AG, Dept. ZKP). RESULTS
1. Binuclear Cations in Metallocene-Based Zlegler-Natta Catalysts. Indications for the occurrence of binuclear cations of the type (Cp2ZrCH3),@CH3)
+
have been
reported in several instances.'-4 In the 'H-NMR spectra of reaction systems containing B(C6F5), and an excess of Cp2Zr(CH3), in C&6, we observe at room temperature t w o distinct species of this kind. Based on the chemical shifts and the relative intensities of each of their signal sets, both o f these species are undoubtedly ion pairs of composition (Cp2ZrCH3)2@-CH3)+ H&-B(C&),-;
since one of them becomes more
prominent on dilution at the expense of the other, we assign the former t o a solventseparated and the latter t o an associated ion pair consisting of a binuclear cation and a methyl borate anion (Figure 1). For the reaction described by equation 1, we determine an equilibrium constant K, and H,C-B(C6F,),-
= 1.O
f 0.2; this indicates that Cp2Zr(CH3),
+.
are equally strong Lewis bases toward the cation CpzZrCH3
Even in the presence o f excess Lewis-acid activator A, binuclear cations could be present in amounts comparable t o the contact-ion pair CP~Z~CH,~+.-H,C-Ab-,
if
excess A is capable of efficiently complexing the anion H3C - A - according t o eq. 2: 2 Cp2ZrCH,d+-H3C -Ab-
+ (Cp,ZrCH,),(p
- CH,)
+
f A-H,C
-A -
(2)
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
6+5.4
6+5.6
6+0.3
6-0.1
6-o*1
=
195
6+5.7
1.020.2
0 + H,C-B(C,F,), (6+1.3, separated)
=
0.520.1 mM"
0 H3C-B(C6F5)3
(&+lo, associated)
(c.f. Li+ H$-B(C6F,),-
6+0.85)
Figure 1. Equilibria between contact ion pairs, excess dimethyl zirconocene and alternative binuclear zirconocene cations, with 'H NMR shift values. If binuclear cations do not contribute to chain growth, as indicated by a recent study,
4,
but still allow chain termination to occur, their presence might explain the
shortening of chain fengths associated with elevated zirconocene concentration^.^) 2. The Olefin Insertion Step. In previous studies, we have observed stereokinetic
isotope effects 61 for the hydro-oligomerization of cis- and trans-1D-1-hexene by Cp2ZrC12/MA0and en(thind),ZrCI2/MAO;') based Ziegler-Natta catalysts
these and related studies on scandocene-
support the notion that an agostic interaction of an
u - H atom of the migrating polymer chain with the metal center facilitates the olefin insertion step, as proposed by Rooney, Green and B r o ~ k h a r t . ~ ~ ' ~ )
196 H . Brintzinger, S. Beck, M. Leclerc, U . Stehling and W. Roll
We have now studied the polymerization of cis- and trans- 1D-propene with en(thind)2ZrCI/MA0, and find that the mean chain length obtained with the trans isomer, PN(trans) = 128, is about 2.8 times larger than that obtained with the cis isomer PN(cis) = 45. This indicates that the olefin insertion step is favored by a large isotope effect (k,/k,
= 2.8) when an a-H atom, rather than an a-D atom, is placed in
the agostic bridging position, as it is to be expected from consecutive insertion reactions of trans- and cis- 1D-propene, respectively (Figure 2). These results provide experimental support for recent theoretical studies on the course of the olefin insertion step in cationic metallocene catalysts.l1-l3'
0-HT
\
DHC=CMeR'
cis-1D-propene:
kD PN = 45
kD
0-HT
\ DHC=CMeR'
Figure 2. Reaction schemes for consecutive insertions of trans-1D-propen (top), which place an a-H atom in the agostic bridging position, and of cis-1D-propene, which place an a-D atom in this position.
3. Chain Termination Mechanisms. Previous metallocene-based polymerization catalysts have given much shorter polymer chain lengths than classical heterogeneous catalysts; recently however, polymers with molecular weights of several hundred thousands have become available by use of ansa-zirconocenecatalysts with a-methyl s u b ~ t i t u e n t s . ' ~ ~In' ~studies ) on the effect of propene pressure on the polymer
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
197
molecular weights, we find the molecular weight of polypropene obained with (CH3I2Si-bridgedbis(indeny1) and bis(bedndenyl1complexes to dependent very liitle on propene concentration (Table 1). This indicatesthat the dominant chain termination process is R-H transfer to a coordinated olefin molecule, in accord with previous evidence from studies on the end-groupdistribution in ethene-propenecopolymers. Catalyst
L
IIMAO
Benzlnd
1
0.31
29 800
88
IIMAO
Benzlnd
2
0.66
35 100
88
IIMAO
Benzlnd
3
1.02
38 600
88
IIMAO
Benzlnd
7
2.43
39600
90
IIIMAO
2-MeBenzlnd
1
0.31
80500
92
IIIMAO
2-MeBenzlnd
2
0.66
137 100
92
IIIMAO
2-MeBenzlnd
3
1.02
182 200
93
II/MAO
2-MeBenzlnd
5
1.72
247 700
93
plbar
c(C3H6)
M ,
% mmmm
Table 1. Effect of propene pressure on the molecular weight of polypropene obtained with MAO-activatedMe2Si(benzindI2ZrCl2(I,top) and Me2Si(2-Me-benzind)2ZrC12(11, bottom). T, 5OOC; [Zrl 1.25*10-6mol/L; [AIl:[Zrl 15 800. With o-methyl substituted ansa-zirconocenesas catalysts, however, the molecular weight of polypropene shows a strong increase with propene pressure (Table 11, in accord with expectations for a chain termination by R-H transfer to the metal center. From a plot of PN-' versus c(C3H6)-' (Figure 31, we determine that both types of catalysts have almost identical rate constants for R-H transfer to the metal (kTM), whereas the rate constant for R-H transfer to olefin (kTo) is about ten times smaller for the complex with a-methyl substituents. These substituents thus appear to interfere with the transition state for R-H transfer to a coordinated olefin (Figure 41, which appears to be sterically rather demanding, as indicated by a relatively large lateral extension angle of more than 180". 'I
198 H. Brintzinger, S. Beck, M. Leclerc, U. Stehling and W. Roll
2.50
2.00
a
1.50
\ 0
z
0
1.00
0.50
0.00
0.50
1.00 1
/
1.50 c(C,H,)
2.00
2.50
3.00
3.50
[Vmoll
Figure 3. Plot of PN-' vs. c ( C ~ H , ) - ~for Me2Si(benzind)2ZrC12(I, top) and
Me2Si(2-Me-benzind)2ZrC12(11, bottom). PN-' = c(C,H,)-~ *(kTM/kp) k,,/k,
+ kTo/kp gives
as the slope and kTo/k, as the abscissa intercept of each graph.
Figure 4. Model of the reaction complex for 13-H transfer to a coordinated olefin; (I-
methyl groups (shaded) interfere with the formation of this reaction complex.
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
199
ACKNOWLEDGEMENTS Financial support of this work by the VW Foundation and BMFT is gratefully acknowledged. REFERENCES 1.
X.Yang, C.L.Stern and T.J.Marks, Angew. Chem., Int. Ed. Engl. JQ4,1406 (19911, Organometallics N, 840 (1991).
2.
G.G.Hlatky and H.W.Turner, quoted in ref. 4.
3.
M.Bochmann and S.J.Lancaster, J. Organomet. Chem.,
a, C1 (1992);
M.Bochmann, results reported a t symposium "40 Years Ziegler-Natta Catalysts", Freiburg (1993).
14,91 (1993).
4.
N.Herfert and G.Fink, Makroml. Chem. Rapid Commun.,
5.
W.Kaminsky, M.Miri, H.Sinn and R.Woldt, Makromol. Chem. Rapid Commun.
4, 41 7 (1983); W.Kaminsky, K.Kulper and SNiedoba, Makromol. Chem. Macromol. Symp.
3,377 (1986); W.Kaminsky, A.Bark
and R-Steiger, J. Mol.
Catal., 74, 109 (1992). 6.
L.Clawson, J.Soto, S.L. Buchwald, M.L.Steigerwald and R.H.Grubbs, J. Am. Chem. SOC.,1pz, 3377 (1985).
7.
H.Krauledat and H.H.Brintzinger, Angew. Chem., Int. Ed. Engl.,
B,1412
(1990); M.Leclerc and H.H. Brintzinger, in preparation. 8.
W.E.Piers and J.E. Bercaw, J. Am. Chem. SOC., 1 1 2 , 9 4 0 6 (1990); D.W.Cotter and J.E.Bercaw, J. Organomet. Chem. 417, C1 (19911.
9.
D.T.Laverty and J.J.Rooney, J. Chem. SOC., Faraday Trans. L9, 869 (1983).
10.
M.Brookhart and M.L.H.Green, J. Organomet. Chem.,
m, 395
M.Brookhart, M.L.H.Green and L.Wong, Prog. Inorg. Chem.,
(1983);
X,1 (1986).
11.
M.Prosenc, C.Janiak and H.H. Brintzinger, Organometallics, 11,4036 (1992).
12.
H.Kawamura-Kuribayashi, N.Koga and K.Morokuma, J. Am. Chem. SOC., 114 8687 (1992).
n,432 (1994).
13.
T.K.Woo, L.Fan and T.Ziegler, Organometalllics
14.
WSpaleck, M.Antberg, J.Rohrmann, A.Winter, B-Bachmann, P.Kiprof, J.Behm and W.A.Herrmann, Angew. Chem. Int. Ed. Engl.,
a,1347
(1992);
J.Rohrmann, V.Dolle, A.Winter and F.Kuber, Eur. Pat. Appl. 921 20287.5 (1991 1.
200
15.
H . Brintzinger, S . Beck, M. Leclerc, U . Stehling and W. Roll
E.Karl, W.RtiII, H.H.Brintzinger, B.Rieger and U-Stehling, Eur. Pat. Appl. 92108888.6 ( 1991 1.
m, 428 (1989).
16.
T.Tsutsui, A.Mizuno and N-Kashiwa, Polymer,
17.
P.Burger, K.Hortmann and H.H.Brintzinger, Makromol. Chem., Macromol. Symp., 66, 127 (1993).
18. Reaction Mechanisms in Metallocene-Catalyzed Olefin Polymerization
H. BRINTZINGER,
S. BECK, M. LECLERC, U. STEHLING and W. ROLL
Fakultat fur Chemie, Universitat Konstanz, 0-78434 Konstanz, Germany ABSTRACT
1. Studies by 'H NMR on equilibria between contact ion pairs such as Cp2ZrCH3d+...H3C-B(C6F5)3d-and binuclear alkyl zirconocene cations of the type (Cp,ZrCH,),b
- CH,)
+
lead to the conclusion that these binuclear species must
generally be considered as participants in all homogeneous Ziegler-Natta systems.
2. Different polypropene chain lengths, which are obtained from cis- and trans1D - propene with the catalyst en(thind)2ZrC12/MA0,show that exchange of a-H with
a-D atoms affect the rate of chain growth by a large kinetic isotope effect; this supports the notion that an a-agostic interaction facilitates the olefin insertion step. 3. A strong increase in polymer chain lengths, which is caused by the presence of amethyl groups in ansa-zirconocene catalysts, is shown, by the effects of propene pressure on ,M ,
to be due to the suppression of the otherwise predominant direct
I3 - H-transfer to a coordinated olefin molecule by these a-substituents. INTRODUCTION Open questions with regard to the mechanisms of metallocene-catalyzed olefin polymerizations concern the equilbria which lead to catalyst activation and deactivation, the factors which control the rate and stereoselectivity of the olefin insertion step, and the mechanisms of chain termination. Some recent studies related to these questions are reported here. EXPERIMENTAL
I . Solutions of B(c~F,),
' 1 and of CP~Z~(CH,)~ in
C,D,
(10 - 40 mM) were
combined in various proportions under extreme exclusion of humidity (flamed glassware, glovebox techniques) and their 'H NMR spectra measured at room temperature on a Bruker AC 250 MHz spectrometer.
194 H . Brintzinger, S. Beck. M. Leclerc, U. Stehling and W. Roll
2. Cis- and trans-a-deuterated propene were prepared by lithiation o f cis- and trans-chlorpropene, respectively, and subsequent cleavage with D20. The samples thus obtained were purified by repeated distillation from dry MAO. Polymerizations were conducted at 5OoC with en(thir~d)~ZrCI,/MAOin toluene ([Zrl =
M, Al:Zr
= 1200:l 1 at 1 bar. The molecular weights of the polymer products were determined
from their 13C NMR spectra, run at 13OOC in CD ,C , ,I
by the ratio of n-propyl end-
group and methyl side-chain signals at 14.3 and 20.0-21.8 ppm, respectively. 3. Polymerizations were conducted with MAO-activated Me2Si(benzind)2ZrC12and Me,Si(2-Me-ben~ind)~ZrCI,([Zrl = 1.25
M, A1:Zr = 15800:1, T,
= 5OoC), at
propene pressures between 1 and 7 bar. The molecular weights of the polymer products were determined by GPC (BASF AG, Dept. ZKP). RESULTS
1. Binuclear Cations in Metallocene-Based Zlegler-Natta Catalysts. Indications for the occurrence of binuclear cations of the type (Cp2ZrCH3),@CH3)
+
have been
reported in several instances.'-4 In the 'H-NMR spectra of reaction systems containing B(C6F5), and an excess of Cp2Zr(CH3), in C&6, we observe at room temperature t w o distinct species of this kind. Based on the chemical shifts and the relative intensities of each of their signal sets, both o f these species are undoubtedly ion pairs of composition (Cp2ZrCH3)2@-CH3)+ H&-B(C&),-;
since one of them becomes more
prominent on dilution at the expense of the other, we assign the former t o a solventseparated and the latter t o an associated ion pair consisting of a binuclear cation and a methyl borate anion (Figure 1). For the reaction described by equation 1, we determine an equilibrium constant K, and H,C-B(C6F,),-
= 1.O
f 0.2; this indicates that Cp2Zr(CH3),
+.
are equally strong Lewis bases toward the cation CpzZrCH3
Even in the presence o f excess Lewis-acid activator A, binuclear cations could be present in amounts comparable t o the contact-ion pair CP~Z~CH,~+.-H,C-Ab-,
if
excess A is capable of efficiently complexing the anion H3C - A - according t o eq. 2: 2 Cp2ZrCH,d+-H3C -Ab-
+ (Cp,ZrCH,),(p
- CH,)
+
f A-H,C
-A -
(2)
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
6+5.4
6+5.6
6+0.3
6-0.1
6-o*1
=
195
6+5.7
1.020.2
0 + H,C-B(C,F,), (6+1.3, separated)
=
0.520.1 mM"
0 H3C-B(C6F5)3
(&+lo, associated)
(c.f. Li+ H$-B(C6F,),-
6+0.85)
Figure 1. Equilibria between contact ion pairs, excess dimethyl zirconocene and alternative binuclear zirconocene cations, with 'H NMR shift values. If binuclear cations do not contribute to chain growth, as indicated by a recent study,
4,
but still allow chain termination to occur, their presence might explain the
shortening of chain fengths associated with elevated zirconocene concentration^.^) 2. The Olefin Insertion Step. In previous studies, we have observed stereokinetic
isotope effects 61 for the hydro-oligomerization of cis- and trans-1D-1-hexene by Cp2ZrC12/MA0and en(thind),ZrCI2/MAO;') based Ziegler-Natta catalysts
these and related studies on scandocene-
support the notion that an agostic interaction of an
u - H atom of the migrating polymer chain with the metal center facilitates the olefin insertion step, as proposed by Rooney, Green and B r o ~ k h a r t . ~ ~ ' ~ )
196 H . Brintzinger, S. Beck, M. Leclerc, U . Stehling and W. Roll
We have now studied the polymerization of cis- and trans- 1D-propene with en(thind)2ZrCI/MA0, and find that the mean chain length obtained with the trans isomer, PN(trans) = 128, is about 2.8 times larger than that obtained with the cis isomer PN(cis) = 45. This indicates that the olefin insertion step is favored by a large isotope effect (k,/k,
= 2.8) when an a-H atom, rather than an a-D atom, is placed in
the agostic bridging position, as it is to be expected from consecutive insertion reactions of trans- and cis- 1D-propene, respectively (Figure 2). These results provide experimental support for recent theoretical studies on the course of the olefin insertion step in cationic metallocene catalysts.l1-l3'
0-HT
\
DHC=CMeR'
cis-1D-propene:
kD PN = 45
kD
0-HT
\ DHC=CMeR'
Figure 2. Reaction schemes for consecutive insertions of trans-1D-propen (top), which place an a-H atom in the agostic bridging position, and of cis-1D-propene, which place an a-D atom in this position.
3. Chain Termination Mechanisms. Previous metallocene-based polymerization catalysts have given much shorter polymer chain lengths than classical heterogeneous catalysts; recently however, polymers with molecular weights of several hundred thousands have become available by use of ansa-zirconocenecatalysts with a-methyl s u b ~ t i t u e n t s . ' ~ ~In' ~studies ) on the effect of propene pressure on the polymer
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
197
molecular weights, we find the molecular weight of polypropene obained with (CH3I2Si-bridgedbis(indeny1) and bis(bedndenyl1complexes to dependent very liitle on propene concentration (Table 1). This indicatesthat the dominant chain termination process is R-H transfer to a coordinated olefin molecule, in accord with previous evidence from studies on the end-groupdistribution in ethene-propenecopolymers. Catalyst
L
IIMAO
Benzlnd
1
0.31
29 800
88
IIMAO
Benzlnd
2
0.66
35 100
88
IIMAO
Benzlnd
3
1.02
38 600
88
IIMAO
Benzlnd
7
2.43
39600
90
IIIMAO
2-MeBenzlnd
1
0.31
80500
92
IIIMAO
2-MeBenzlnd
2
0.66
137 100
92
IIIMAO
2-MeBenzlnd
3
1.02
182 200
93
II/MAO
2-MeBenzlnd
5
1.72
247 700
93
plbar
c(C3H6)
M ,
% mmmm
Table 1. Effect of propene pressure on the molecular weight of polypropene obtained with MAO-activatedMe2Si(benzindI2ZrCl2(I,top) and Me2Si(2-Me-benzind)2ZrC12(11, bottom). T, 5OOC; [Zrl 1.25*10-6mol/L; [AIl:[Zrl 15 800. With o-methyl substituted ansa-zirconocenesas catalysts, however, the molecular weight of polypropene shows a strong increase with propene pressure (Table 11, in accord with expectations for a chain termination by R-H transfer to the metal center. From a plot of PN-' versus c(C3H6)-' (Figure 31, we determine that both types of catalysts have almost identical rate constants for R-H transfer to the metal (kTM), whereas the rate constant for R-H transfer to olefin (kTo) is about ten times smaller for the complex with a-methyl substituents. These substituents thus appear to interfere with the transition state for R-H transfer to a coordinated olefin (Figure 41, which appears to be sterically rather demanding, as indicated by a relatively large lateral extension angle of more than 180". 'I
198 H. Brintzinger, S. Beck, M. Leclerc, U. Stehling and W. Roll
2.50
2.00
a
1.50
\ 0
z
0
1.00
0.50
0.00
0.50
1.00 1
/
1.50 c(C,H,)
2.00
2.50
3.00
3.50
[Vmoll
Figure 3. Plot of PN-' vs. c ( C ~ H , ) - ~for Me2Si(benzind)2ZrC12(I, top) and
Me2Si(2-Me-benzind)2ZrC12(11, bottom). PN-' = c(C,H,)-~ *(kTM/kp) k,,/k,
+ kTo/kp gives
as the slope and kTo/k, as the abscissa intercept of each graph.
Figure 4. Model of the reaction complex for 13-H transfer to a coordinated olefin; (I-
methyl groups (shaded) interfere with the formation of this reaction complex.
18. Mechanisms of Metallocene-Catalyzed Olefin Polymerization
199
ACKNOWLEDGEMENTS Financial support of this work by the VW Foundation and BMFT is gratefully acknowledged. REFERENCES 1.
X.Yang, C.L.Stern and T.J.Marks, Angew. Chem., Int. Ed. Engl. JQ4,1406 (19911, Organometallics N, 840 (1991).
2.
G.G.Hlatky and H.W.Turner, quoted in ref. 4.
3.
M.Bochmann and S.J.Lancaster, J. Organomet. Chem.,
a, C1 (1992);
M.Bochmann, results reported a t symposium "40 Years Ziegler-Natta Catalysts", Freiburg (1993).
14,91 (1993).
4.
N.Herfert and G.Fink, Makroml. Chem. Rapid Commun.,
5.
W.Kaminsky, M.Miri, H.Sinn and R.Woldt, Makromol. Chem. Rapid Commun.
4, 41 7 (1983); W.Kaminsky, K.Kulper and SNiedoba, Makromol. Chem. Macromol. Symp.
3,377 (1986); W.Kaminsky, A.Bark
and R-Steiger, J. Mol.
Catal., 74, 109 (1992). 6.
L.Clawson, J.Soto, S.L. Buchwald, M.L.Steigerwald and R.H.Grubbs, J. Am. Chem. SOC.,1pz, 3377 (1985).
7.
H.Krauledat and H.H.Brintzinger, Angew. Chem., Int. Ed. Engl.,
B,1412
(1990); M.Leclerc and H.H. Brintzinger, in preparation. 8.
W.E.Piers and J.E. Bercaw, J. Am. Chem. SOC., 1 1 2 , 9 4 0 6 (1990); D.W.Cotter and J.E.Bercaw, J. Organomet. Chem. 417, C1 (19911.
9.
D.T.Laverty and J.J.Rooney, J. Chem. SOC., Faraday Trans. L9, 869 (1983).
10.
M.Brookhart and M.L.H.Green, J. Organomet. Chem.,
m, 395
M.Brookhart, M.L.H.Green and L.Wong, Prog. Inorg. Chem.,
(1983);
X,1 (1986).
11.
M.Prosenc, C.Janiak and H.H. Brintzinger, Organometallics, 11,4036 (1992).
12.
H.Kawamura-Kuribayashi, N.Koga and K.Morokuma, J. Am. Chem. SOC., 114 8687 (1992).
n,432 (1994).
13.
T.K.Woo, L.Fan and T.Ziegler, Organometalllics
14.
WSpaleck, M.Antberg, J.Rohrmann, A.Winter, B-Bachmann, P.Kiprof, J.Behm and W.A.Herrmann, Angew. Chem. Int. Ed. Engl.,
a,1347
(1992);
J.Rohrmann, V.Dolle, A.Winter and F.Kuber, Eur. Pat. Appl. 921 20287.5 (1991 1.
200
15.
H . Brintzinger, S . Beck, M. Leclerc, U . Stehling and W. Roll
E.Karl, W.RtiII, H.H.Brintzinger, B.Rieger and U-Stehling, Eur. Pat. Appl. 92108888.6 ( 1991 1.
m, 428 (1989).
16.
T.Tsutsui, A.Mizuno and N-Kashiwa, Polymer,
17.
P.Burger, K.Hortmann and H.H.Brintzinger, Makromol. Chem., Macromol. Symp., 66, 127 (1993).