Strong ion pairing effects on single-site olefin polymerization: Mechanistic insights in syndiospecific propylene enchainment [4]

Full text of DOI:10.1021/ja011558p

J. Am. Chem. Soc. 2001, 123, 11803-11804
11803
Scheme 1
Strong Ion Pairing Effects on Single-Site Olefin
Polymerization: Mechanistic Insights in
Syndiospecific Propylene Enchainment
Ming-Chou Chen and Tobin J. Marks*
Department of Chemistry
Northwestern UniVersity
EVanston, Illinois 60208-3113
ReceiVed June 26, 2001
Although there is considerable circumstantial evidence that ion
pairing has significant consequences for single-site polymerization
catalyst1 activity, lifetime, stability, chain-transfer characteristics,
and stereoregulation,2 actual mechanistic structure-function con-
nections have remained ill-defined. In principle, the accepted
pathway for syndiospecific propylene enchainment by Cs-sym-
metric catalysts should be a particularly sensitive probe of
cocatalyst/counteranion2,3 effects since olefin enchainment neces-
sarily occurs in concert with “chain-swinging” (eq 1, R )
polypropylene fragment).4 It is known that rates of similar
reorganization processes are sensitive to, and one metric of, ion
pairing strength in model metallocenium systems (R ) H, alkyl
group),3,5 and thought that analogous “back-skipping” processes
without concomitant enchainment are a major source of polypro-
pylene stereoerrors (Scheme 1).4 Herein we communicate the first
systematic study of counteranion effects on propylene enchain-
ment stereochemistry by the archetypical Cs-symmetric precatalyst
[Me2C(Cp)(fluorenyl)]ZrMe2 (1),6 using a fairly broad array of
structurally/coordinatively diverse counteranions3 as a function
of temperature, propylene pressure, and solvent polarity. It will
be seen that effects can be large and are to a significant degree
understandable in terms of established trends in ion pairing
strength and dynamics.7
under 1.0 atm propylene pressure in toluene from -10° to + 60
°C using conditions minimizing mass transfer and exotherm
effects;3e,f,5 product isolation and characterization utilized standard
techniques.5b,8,9 Several trends are evident in the data (Table 1,
Figure 1). Product polydispersities are consistent with well-defined
single-site processes and are rather temperature-, anion-insensitive.
Polymerization rates are highly anion-sensitive, with the most
strongly (PBA-)5b and weakly (MeB(2-C6F5C6F4)3-, B(C6F5)4-)2a,b,5d
coordinating anions generally affording the lowest and highest
polymerization rates, respectively. Not surprisingly,1,4 product
molecular weights fall with rising reaction temperature, although
the superiority of strongly coordinating PBA- might not, a priori,
be predicted. Most interesting, however, is the pattern in polypro-
pylene stereoerrors ([m], [mm]) as a function of anion and
temperature (Figure 1C), and which are concentration-invariant
over a 32-fold range in a control experiment with 6.8 It can be
seen that the PBA- catalyst exhibits far higher syndiotacticity,
with far lower [m] and somewhat lower [mm] stereoerrors. As
temperature is increased, all systems exhibit a precedented erosion
in syndiotacticity,10 however that of the PBA- catalyst is least,
with the principle factor being greater temperature insensitivity
of the [m] stereoerrors versus that of the other anions. Interest-
Under rigorously anhydrous/anaerobic conditions, 1 was acti-
vated with the perfluoroaryl borane, borate, and fluoroaluminate
reagents shown in eq 2. Polymerizations were first carried out
(1) For recent reviews, see: (a) Gladysz, J. A. Ed. Chem. ReV. 2000, 100,
1167-1682. (b) Marks, T. J.; Stevens, J. C., Eds. Top.Catal. 1999, 7, 1-208.
(c) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed.
1999, 38, 428-447. (d) Jordan, R. F.; Ed. J. Mol. Catal. 1998, 128, 1-337.
(2) (a) Chen, Y.-X.; Marks, T. J. in ref 1a, pp 1391-1434, and references
therein. (b) Luo, L.; Marks, T. J. in ref 1b, pp 97-106.
(3) For recent cocatalyst studies, see: (a) Chen, Y.-X.; Kruper, W. J.; Roof
G.; Wilson, D. R. J. Am. Chem. Soc. 2001, 123, 745-746. (b) Zhou, J.;
Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.; Bochmann, M.
J. Am. Chem. Soc. 2001, 123, 223-237. (c) Chase, P. A.; Piers, W. E.; Patrick,
B. O. J. Am. Chem. Soc. 2000, 122, 12911-12912. (d) LaPointe, R. E.; Roof,
G. R.; Abboud, K. A.; Klosin, J. J. Am. Chem. Soc. 2000, 122, 9560-9561.
(e) Sun, Y. M.; Metz, M. V.; Stern, C. L.; Marks, T. J. Organometallics 2000,
19, 1625-1627. (f) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Nickias, P.
N.; Marks, T. J. Angew. Chem., Int. Ed. 2000, 39, 1312-1316.
(5) (a) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358-
10370. (b) Chen, X.-Y.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J. J. Am.
Chem. Soc. 1998, 120, 6287-6305. (c) Deck, P. A.; Beswick, C. L.; Marks,
T. J. J. Am. Chem. Soc. 1998, 120, 1772-1784. (d) Jia, L.; Yang, X.; Stern,
C. L.; Marks, T. J. Organometallics 1997, 16, 842-857.
(6) Razavi, A.; Thewalt, U. J. Organomet. Chem. 1993, 445, 111-114.
(7) Presented in part at the 221st ACS National Meeting, San Diego, CA,
April 1-5, 2001, Abstract INORG 65.
(8) See Supporting Information for full experimental details.
(9) NMR assay: see ref 4 and references therein.
(4) (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. in ref 1a, pp 1253-
1345. (b) Coates, G. W. in ref 1a, pp 1223-1252. (c) Veghini, D.; Henling,
L. M.; Burkhardt, T. J.; Bercaw, J. E. J. Am. Chem. Soc. 1999, 121, 564-
573. (d) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem.
Soc. 1988, 110, 6255-6256.
(10) For 1/MAO, syndiotacticity falls with increasing temperature,10a while
for C1-symmetric catalysts, isotacticity sometimes increases with increasing
temperature:10b (a) Kleinschmidt. R.; Reffke, M.; Fink, G. Macromol. Rapid
Commun. 1999, 20, 284-288. (b) Grisi, F.; Longo, P.; Zambelli, A.; Ewen,
J. A. J. Mol. Catal. A: Chem. 1999, 140, 225-233.
10.1021/ja011558p CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/02/2001

11804 J. Am. Chem. Soc., Vol. 123, No. 47, 2001
Communications to the Editor
Table 1. Propylene Polymerization Results with 1 + Indicated
Cocatalystsa
actb rrrrc rmmr
mmrr
rrmr
Mw
d
cocat Tp
entry (cat) (°C) (×104) (%) (%) (%) ([mm]) (%) ([m]) (×103) Mw/Mn
1
2
3
4
5
6
7
8
9
2 (6) -10
3.1 90.0 1.1
2.2
2.2
1.9
1.5
3.1
3.1
3.0
2.6
6.8
5.3
5.3
3.9
5.1
4.6
4.0
3.9
4.1
4.0
4.2
4.4
2.2
0.6
0.6
0.0
10.6
4.1
79.8
201
229
290
79
101
112
147
1.75
1.83
1.95
1.86
1.81
1.85
1.95
1.85
2.38
1.82
1.82
1.95
1.89
1.81
1.68
1.76
1.76
2.24
1.92
1.78
3 (7) -10
58
93.3 1.1
94.0 1.0
4 (8) -10 920
5 (9) -10
1.8 96.5 0.7
2 (6)
3 (7)
4 (8)
5 (9)
2 (6)
25
25 350
25 890
25
60
60 100
44
69.4 1.5
83.6 1.5
83.8 1.4
91.0 1.3
30.0 3.5
48.3 2.5
47.0 2.6
4.2
1.3
20
25
21.1
16.7
17.2
8.6
14.7
7.9
7.4
4.3
18.0
17.6
17.3
17.8
11.9
10 3 (7)
11 4 (8)
12 5 (9)
13 2 (6)
14 3 (7)
15 4 (8)
16 5 (9)
17 2 (6)
18 3 (7)
19 4 (8)
20 5 (9)
53.1
55.8
66.5
33.9
58.6
63.2
70.8
97.6
93.3
60
80
60
2.5 71.0 1.9
60e
60e
60e
60e
18
30
37
53.8 2.5
71.3 2.0
73.6 2.0
5.8 81.0 1.8
25f 266
49.5 2.0
49.8 2.0
50.3 1.9
49.5 2.0
25f
25f 453
25f
96
86
104
127
a Under 1.0 atm of propylene in 50 mL of toluene with precise
polymerization temperature control (exotherm < 3 °C). See Supporting
Information for full experimental details. b Units: g polymer/(mol cat.×
atm × h). c Pentad analysis by 13C NMR. d GPC relative to polystyrene
standards. e Under 5.0 atm of propylene. f Under 1.0 atm of propylene
in 50 mL of 1,3-dichlorobenzene.
Figure 1. (A) Polymerization activity, (B) polypropylene molecular
weight, and (C) pentad distribution (%) data for polypropylenes produced
by 1 + indicated cocatalysts under 1.0 atm of propylene from -10° to
60 °C. (D) Pentad distribution (%) data for polypropylenes produced by
1+ indicated cocatalysts as a function of propylene pressure at 60 °C.
Lines connecting data points are drawn as a guide to the eye.
ingly, [mm] stereoerrors are far less temperature-sensitive, with
the PBA- catalyst again slightly superior. In contrast, the
-
MeB(C6F5)3 catalyst exhibits the lowest syndiotacticity with
greatest increase of [m] and [mm] with rising polymerization
temperature.
Experiments at increased propylene pressures reveal general
increases in syndiotacticities (Figure 1D) and generally uniform
but modest increases in product molecular weights8 (Table 1,
entries 13-16), arguing that chain-transfer to monomer4,11 is a
significant but probably not the only termination pathway.
Interestingly, increases in [rrrr] and declines in [m] with increased
propylene pressure are smallest for the PBA- catalyst. The
stereochemical consequences of increasing [propylene] are usually
ascribed to increased enchainment rates versus those of competing,
tacticity-degrading site epimerization (Scheme 1B).4,11a Finally,
polymerizations were carried in 1,3-dichlorobenzene (ꢀ ) 5.04)
(Table 1), with the net result being compression in the dispersion
of polymerization rates and collapse of [rrrr], [m], and [mm] %
to experimentally indistinguishable values of 50, 17.5, and 4%,
respectively, for all cocatalysts.8
These results suggest a mechanistic picture in which anion-
specific ion pairing effects modulate not only the rate of
enchainment and chain transfer, but more importantly, the relatiVe
rates of enchainment versus [m]-enhancing/syndiotacticity-
degrading site epimerization (Scheme 1B).12 More tightly bound,
stereochemically immobile anions should depress epimerization
rates,5 with computational studies arguing that propagation in
nonpolar media involves concerted anion displacement and
monomer enchainment rather than highly endergonic unimolecular
ion pair separation.13 To the extent that rates of dynamic NMR-
quantifiable unimolecular reorganization processes (detected by
averaging of magnetically diastereotopic sites)5 in [Me2C(Cp)-
(fluorenyl)]ZrMe+X- ion pairs mirror barriers in eq 1, we find
that the PBA- ion pair indeed has by far the highest barrier (∆Gq
g 25 kcal/mol vs 19.5 (2) kcal/mol for MeB(C6F5)3-).14 Other
NMR and crystallographic data support the strong coordinative
properties of PBA-.5b In contrast to these results, misinsertion or
chain epimerization4c processes producing [mm] stereoerrors
(Scheme 1 C and D) would not appear to be directly influenced
by decoupling from chain-swinging, and indeed it is found that
the low level of [mm] is far less anion-dependent although,
interestingly, these results show that the ion pairing does affect
enantiofacial discrimination and chain epimerization as well
(Figure 1C).15 Last, spectroscopic,5 theoretical,13 and polymeri-
zation studies16 argue that polar solvents significantly weaken ion
pairing in other single-site systems, and in accord with a picture
that ion pairing modulates syndiospecific enchainment, we find
here that differential anion effects on propagation rates diminish8
and those on stereoerrors completely Vanish in a more polar
solvent.
Acknowledgment. Financial support by DOE (DE-FG02-86ER1351)
is gratefully acknowledged. M.-C. C. thanks Dow Chemical for a
postdoctoral fellowship, Dr. L. Li for helpful discussions, and Dr. P.
Nickias of Dow for GPC measurements.
Supporting Information Available: Details describing syntheses of
6-9; polymerization experiments and polymer characterization; tables
of pressure- and solvent-dependent polymerization data (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA011558P
(13) (a) Lanza, G.; Fragala`, I. L.; Marks, T. J. J. Am. Chem. Soc. 2000,
122, 12764-12777. (b) Lanza, G.; Fragala`, I. L. in ref 1b, pp 45-60. (c)
Lanza, G.; Fragala`, I. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 8257-
8258. (d) Chan, M. S. W.; Ziegler, T. Organometallics 2000, 19, 5182-5189,
and references therein.
(14) (a) Line broadening is found to be independent of concentration over
an 8-fold range for 6. (b) For 7 and 8, extensive thermal decomposition occurs
at > 80 °C. (c) In o-xylene-d10.
(15) The weak pressure dependence of %[mm] argues for a misinsertion
origin.
(11) (a) For Cs catalysts, lower propylene concentrations correlate with
lower product molecular weights and tacticities (mostly [m]-type stereoerrors).4c
(b) In contrast, declining isotacticity with increasing monomer concentration
is observed in C1 catalysts: Kukral, J.; Lehmus, P.; Feifel, T.; Troll, C.; Rieger,
B. Organometallics 2000, 19, 3767-3775. (c) For another recent monomer
concentration study, see: Lin, S.; Tagge, C. D.; Waymouth, R. M.; Nele, M.;
Collins, S.; Pinto, J. C. J. Am. Chem. Soc. 2000, 122, 11275-11285.
(12) For general kinetic models, see: (a) Bravakis, A. M.; Bailey, M. P.;
Pigeon, M.; Collins, S. Macromolecules 1998, 31, 1000-1009. (b) Gauthier,
W. J.; Corrigan, J. F.; Taylor, N. J.; Collins, S. Macromolecules 1995, 28,
3771-3778.
(16) (a) Herfert, N.; Fink, G. Makromol. Chem. 1992, 193, 773-778. (b)
Coevoet, D.; Cramail, H.; Deffieux, A. Makromol. Chem. Phys. 1999, 200,
1208-1214.