New mononuclear and polynuclear perfluoroarylmetatate cocatalysts for stereospecific olefin polymerization

Article abstract of DOI:10.1021/om0341698

Mononuclear and polynuclear perfluoroarylborate, -aluminate, and -gallate cocatalysts for metal-locene-mediated olefin polymerization have been synthesized as trityl fluoride adducts of known perfluoroarylmetalloid cocatalysts. Propylene polymerization ex

Full text of DOI:10.1021/om0341698

932
Organometallics 2004, 23, 932-935
New Mon on u clea r a n d P olyn u clea r
P er flu or oa r ylm eta la te Coca ta lysts for Ster eosp ecific
Olefin P olym er iza tion
Ming-Chou Chen, J ohn A. S. Roberts, and Tobin J . Marks*
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113
Received September 15, 2003
Summary: Mononuclear and polynuclear perfluoroaryl-
borate, -aluminate, and -gallate cocatalysts for metal-
locene-mediated olefin polymerization have been synthe-
sized as trityl fluoride adducts of known perfluoroaryl-
metalloid cocatalysts. Propylene polymerization experi-
ments using these species as cocatalysts with the Cs-sym-
metric precatalyst [Me2C(Cp)(fluorenyl)]ZrMe2 reveal a
marked counteranion dependence of polymerization ac-
tivity, product polymer syndiotacticity, and relative [m]
and [mm] stereoerror abundance, with the polynuclear
perfluoroaryl cocatalysts uniformly giving enhanced pro-
duct polymer stereoregularity vs the neutral analogues.
Recently, several sterically encumbered perfluoroaryl
group 13 cocatalysts, tris(2,2′,2′′-perfluorobiphenyl)bor-
ane (PBB; 4),4a tris(â-perfluoronaphthyl)borane (B(C10-
F7)3; PNB),4b and trityl tris(2,2′,2′′-perfluorobiphenyl)-
fluoroaluminate (Ph3C+PBA-; 8)8a have been synthe-
sized and their cocatalytic polymerization characteris-
tics studied. Bulky PBB and PNB generate cationic
Metallocenium ion pair complexes (A; eq 1) produced
in the reaction of metallocenes with various activators
(cocatalysts) have been recognized as the active species
in single-site olefin polymerization, and clear evidence
shows that interionic structures and interactions have
a profound influence on catalyst activity, lifetime, stabil-
ity, chain-transfer characteristics, and stereoregula-
tion.1,2 Well-known cocatalysts include tris(perfluoro-
species with significantly higher activities for olefin
polymerization and copolymerization than does B(C6F5)3.
Via M-F-Al coordination to the metal center, PBA-
affords more tightly bound and stereochemically im-
mobile ion pairs which exhibit decreased enchainment
and chain transfer rates, but more importantly, de-
pressed syndiospecificity-degrading, rrmr stereoerror-
inducing site epimerization rates in ion pairs generated
from Cs-symmetric [Me2C(Cp)(fluorenyl)]ZrMe2 (1; eq
2).9 These observations suggest that it would be of great
phenyl)borane (B(C6F5)3; 3)3 and related perfluoro-
arylboranes,4 ammonium or trityl salts of B(C6F5)4- (5),5
and related perfluoroarylborates,6 alanes (6),7 and alu-
minates.8
* Corresponding author.
(1) For recent reviews, see: (a) Gibson, V. C.; Spitzmesser, S. K.
Chem. Rev. 2003, 103(1), 283-315. (b) Pe´deutour, J .-N.; Radhakrish-
nan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid Commun. 2001,
22, 1095-1123. (c) Chen, Y.-X.; Marks, T. J . Chem. Rev. 2000, 100(4),
1391-1434. (d) Chem. Rev. 2000, 100, 1167-1682. (e) Top. Catal. 1999,
7, 1-208. (f) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew.
Chem., Int. Ed. 1999, 38, 428-447.
interest to synthesize new cocatalysts with substantially
modified steric and charge-dispersing properties and to
assess their influence on the polymerization process.
Here we report the synthesis, molecular structures,
(2) For recent cocatalyst studies, see: (a) Busico, V.; Cipullo, R.;
Cutillo, F.; Vacatello, M.; Castelli, V. V. Macromolecules 2003, 36,
4258-4261. (b) Mohammed, M.; Nele, M.; Al-Humydi, A.; Xin, S.;
Stapleton, R. A.; Collins, S. J . Am. Chem. Soc. 2003, 125, 7930-7941.
(c) Abramo, G. P.; Li, L.; Marks, T. J . J . Am. Chem. Soc. 2002, 124,
13966-13967. (d) Li, L.; Metz, M. V.; Li, H.; Chen, M.-C.; Marks, T. J .
J . Am. Chem. Soc. 2002, 124, 12725-12741. (e) Metz, M. V.; Schwartz,
D. J .; Stern, C. L.; Marks, T. J .; Nickias, P. N. Organometallics 2002,
21, 4159-4168. (f) Metz, M. V.; Sun, Y. M.; Stern, C. L.; Marks, T. J .
Organometallics 2002, 21, 3691-3702. (g) Wilmes, G. M.; Polse, J . L.;
Waymouth, R. M. Macromolecules 2002, 35, 6766-6772. (h) Lancaster,
S. J .; Rodriguez, A.; Lara-Sanchez, A.; Hannant, M. D.; Walker, D. A.;
Hughes, D. H.; Bochmann, M. Organometallics 2002, 21, 451-453. (i)
Rodriguez, G.; Brant, P. Organometallics 2001, 20, 2417-2420. (j)
Kaul, F. A. R.; Puchta, G. T.; Schneider, H.; Grosche, M.; Mihalios,
D.; Herrmann, W. A. J . Organomet. Chem. 2001, 621, 177-183. (k)
Chen, Y.-X.; Kruper, W. J .; Roof, G.; Wilson, D. R. J . Am. Chem. Soc.
2001, 123, 745-746. (l) Zhou, J .; Lancaster, S. J .; Walker, D. A.; Beck,
S.; Thornton-Pett, M.; Bochmann, M. J . Am. Chem. Soc. 2001, 123,
223-237. (m) Kehr, G.; Roesmann, R.; Frohlich, R.; Holst, C.; Erker,
G. Eur. J . Inorg. Chem. 2001, 535-538. (n) Mager, M.; Becke, S.;
Windisch, H.; Denninger, U. Angew. Chem., Int. Ed. 2001, 40, 1898-
1902.
(3) (a) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015-10031. (b)Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem.
Soc. 1991, 113, 3623-3625.
(4) (a) Li, L.; Stern, C. L.; Marks, T. J . Organometallics 2000, 19,
3332-3337. (b) Li, L.; Marks, T. J . Organometallics 1998, 17, 3996-
4003. (c) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J . J . Am. Chem.
Soc. 1996, 118, 12451-12452. (d) Also see refs 2c-e. (e) For a recent
chelating borane review, see: Piers, W. E.; Irvine, G. J .; Williams, V.
C. Eur. J . Inorg. Chem. 2000, 2131-2142.
(5) (a) Chien, J . C. W.; Tsai, W.-M.; Rausch, M. D. J . Am. Chem.
Soc. 1991, 113, 8570-8571. (b) Yang, X.; Stern, C. L.; Marks, T. J .
Organometallics 1991, 10, 840-842. (c) Ewen, J . A.; Elder, M. J . Eur.
Pat. Appl. 426637, 1991; Chem. Abstr. 1991, 115, 136987c, 136988d.
(6) For related fluorinated tetraarylborates, see: (a) References 2h-
j,l. (b) J ia, L.; Yang, X.; Stern, C. L.; Marks, T. J . Organometallics 1997,
16, 842-857. (c) J ia, L.; Yang, X.; Ishihara, A.; Marks, T. J . Organo-
metallics 1995, 14, 3135-3137.
(7) (a) Reference 2f. (b) Bochmann, M.; Sarsfield, M. J . Organome-
tallics 1998, 17, 5908-5912. (c) Biagini, P.; Lugli, G.; Abis, L.;
Andreussi, P. U. S. Patent 5,602,269, 1997.
10.1021/om0341698 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/04/2004

Communications
Organometallics, Vol. 23, No. 5, 2004 933
Sch em e 1
reactivity, and single-site polymerization catalytic char-
acteristics of a new series of mononuclear and poly-
nuclear perfluoroarylborates, -aluminates, and -gallates.
It will be seen that most are excellent cocatalysts,
affording high catalytic activity and improved enchain-
ment stereocontrol for syndiotactic propylene polymer-
ization.
Reaction of trityl fluoride (Ph3CF; 2)10 with B(C6F5)3
(3) and B(2-C6F5C6F4)3 (4)4a yields the corresponding
trityl fluorometalate salts tris(perfluorophenyl)fluorobo-
rate (9)11 and trityl tris(2,2′,2′′-nonafluorobiphenyl)-
fluoroborate (10; Scheme 1).12 19F NMR spectra of both
fluoroborates exhibit characteristic broad, upfield B-F
resonances at δ -186.99 (9) and -185.00 ppm (10).13
The structure of anion 10 was confirmed by X-ray
diffraction14 and features an unassociated trityl cation
and a sterically congested chiral, essentially C3-sym-
metric borate anion (Scheme 1). Depending on the
reagent molar ratio, either of three trityl tris(perfluo-
rophenyl)fluoroaluminates [(Ph3C+)x{Fx[Al(C6F5)3]y}x-
;
x ) 1, y ) 1, 11; x ) 1, y ) 2, 12; x ) 2, y ) 3, 13) can
be isolated in good yield from reaction of 2 with
Al(C6F5)3 (6; Scheme 1). Trityl tris(2,2′,2′′-nonafluoro-
biphenyl)fluoroaluminate (Ph3C+[FAl(2-C6F5C6F4)3]-;
8)8a also undergoes reaction with 6 to afford the trityl
fluoro-bridged mixed aluminate (Ph3C+[(C6F5)3AlFAl-
(2-C6F5C6F4)3]-; 14). These fluoroaluminates were char-
acterized by standard techniques15 and also by single-
crystal diffraction (13, 14; Scheme 1). The structure of
13 features two unassociated trityl cations and a steri-
cally congested pseudo-C3-symmetric trinuclear anion,16
where the three Al centers are connected by two µ-F
atoms in a nearly linear Al-F-Al-F-Al configura-
tion.17 The crystal structure of 14 features an unasso-
ciated trityl cation and a sterically congested fluoro-
bridged Al anion, with the two Al centers bridged by a
single F atom, forming a nearly linear Al-F-Al con-
figuration.18 Trityl fluorobis[tris(perfluorophenyl)gallate]
(Ph3C+F[Ga(C6F5)3]2-; 15) is derived from a “one pot”
reaction of 2 with Ga(C6F5)3 (7; Scheme 1), in turn
generated in situ from reaction of B(C6F5)3 + Ga(CH3)3
(eq 3).19 The structure of 15 was confirmed by X-ray
(8) (a) Chen, Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J . J .
Am. Chem. Soc. 1998, 120, 6287-6305. (b) Chen, Y.-X.; Stern, C. L.;
Marks, T. J . J . Am. Chem. Soc. 1997, 119, 2582-2583. (c) Elder, M.
J .; Ewen, J . A. Eur. Pat. Appl. EP 573,403, 1993; Chem. Abstr. 1994,
121, 0207d.
(9) (a) Chen, M.-C.; Marks, T. J . J . Am. Chem. Soc. 2001, 123,
11803-11804. (b) Chen, M.-C.; Roberts, J . A. S.; Marks, T. J . J . Am.
Chem. Soc., in press.
Ph3CF
B(C F )
Ph C+{F[Ga(C F ) ] }-
GaMe3 +
8
(3)
6
5 3
3
6
5 3 2
2
3
15 (67%)
(10) Oishi, M.; Yamamoto, H. Bull. Chem. Soc. J pn. 2001, 74(8),
1445-1454.
diffraction20 as the first linear fluoro-bridged organogal-
lium complex.21
(11) A similar fluoroborate (Li+[FB(C6F5)3]-) has been claimed
previously. See: Klemann, L. P.; Newman, G. H.; Stogryn, E. L. U. S.
Patent 4,139,681, 1979.
The activation reactivities and olefin polymerization22
cocatalytic characteristics (Table 1; Figure 1) of salts
9-15 were investigated with respect to the Cs-sym-
(12) See the Supporting Information for full experimental details.
(13) Similarly, the F-Al resonance of Ph3C+PBA- (8) appears at δ
-175.60 ppm.8a
(14) For 10 (50% probability thermal ellipsoids in Scheme 1),
selected bond lengths (Å) and bond angles (deg) are as follows: B1-
F1 ) 1.437(6), B1-C1 ) 1.649(7), B1-C13 ) 1.659(6); F1-B1-C1 )
105.3(4), F1-B1-C13 ) 106.8(3), C1-B1-C13 ) 114.4(4), C1-B1-
C25 ) 112.2(3).
(15) The 19F NMR spectra of complexes 11-14 exhibit broad singlets
at δ -168.61, -167.30, -166.49, and -169.38 ppm, respectively,
indicative of an Al-F bond.

934 Organometallics, Vol. 23, No. 5, 2004
Communications
Ta ble 1. Com p a r ison of P r op ylen e P olym er iza tion Resu lts w ith 1 + th e In d ica ted Coca ta lysts a t 25 °C
u n d er 1.0 a tm of P r op ylen ea
e
g
cocat. (R ) C6F5;
R′ ) C12F9)
amt
time
amt of PP
(g)
∆Tc
activityd
Tm
rmmrf
mmrr
rrmr
rrrr
Mw
Mw/
expt no.
(µmol)
(min)
(°C)
(×106)
(°C)
(%)
(%)
(%)
(%)
(×103)
79
81
101
94
112
74
138
147
144
147
121
77
Mn
1b
2
BR3 (3)
20
10
10
20
4.8
20
10
1.6
2.5
20
2.6
20
10
40
60
5
12
1.25
45
4
2
3
75
5
40
3
5.9
1.0
1.0
3.0
1.0
3.0
1.0
2.0
2.0
1.0
0.5
1.0
0.5
3.0
0.44
0.087
3.5
0.35
8.9
0.056
2.0
14
7.9
0.20
4.4
0.098
2.33
101.4
104.5
130.3
137.6
130.7
139.5
142.1
143.7
143.5
145.7
145.8
138.0
140.5
1.5
1.6
1.5
1.7
1.4
1.7
1.6
1.5
1.4
1.3
1.5
1.6
1.5
3.1
3.1
3.1
3.2
3.0
2.8
2.6
2.5
2.5
2.6
2.4
2.7
2.5
10.6
10.3
4.2
2.8
4.2
3.4
2.4
2.3
2.2
1.3
1.4
3.4
2.5
69.5
70.2
83.5
86.3
84.0
85.5
88.7
88.9
89.3
91.0
90.8
86.2
88.6
1.81
1.99
1.85
2.11
1.95
2.2
1.95
2.08
1.98
1.85
1.91
2.85
1.93
Tr+FBR3- (9)
BR′3 (4)
0.87
2.92
1.39
0.89
0.84
1.32
0.79
0.99
5.0
3b
4
Tr+FBR′3- (10)
5b
6
Tr+BR′4- (5)
AlR3 (6)
7
8
9
Tr+ FAlR3- (11)
Tr+F(AlR3)2- (12)
Tr+F2(AlR3)32- (13)
Tr+FAlR′3- (8)
Tr+(AlR3FAlR′3)- (14)
GaR3 (7)
10b
11
12
13
0.94
1.30
1.17
Tr+F(GaR3)2- (15)
129
a
b
In 50 mL of toluene with precise polymerization temperature control (exotherm <3 °C). See ref 11b. c Internal temperature variation
((). In units of g of polymer/((mol of cat.) atm h). e Second scan by DSC. f Pentad analysis by 13C NMR ((0.3%). GPC relative to
polystyrene standards.
d
g
F igu r e 1. Activity (A), syndiotacticity (rrrr%) (B), and stereoerror (rrmr%) data (C) for polypropylenes produced by
metallocene 1 + the indicated cocatalysts at 25 °C under 1.0 atm of propylene.
metric precatalyst [Me2C(Cp)(fluorenyl)]ZrMe2 (1). As
monitored by in situ 1H and 19F NMR, reaction of 1
(methide abstraction) with 9-15 is rapid,12 and spec-
troscopically identified products are shown in Scheme
1. For reaction of 1 with cocatalyst 9, [Me2C(Flu)(Cp)-
ZrMe]+[MeB(C6F5)3]- (16) is observed as the major
product (∼30% yield),23 suggesting B-F/Zr-Me me-
tathesis. Not unexpectedly, lower polymerization activ-
ity (∼20%) but with similar polypropylene syndiotac-
ticity (∼70.2(3) rrrr%) results from propylene polymeri-
zation catalyzed by the 9-derived ion-pair complex,
compared to 1/B(C6F5)3 (3; Table 1, entries 1 and 2). For
reaction of 1 with cocatalyst 10, formation of a pair of
dinuclear {[Me2C(Flu)(Cp)ZrMe]2(µ-F)}+[FB(2-C6F5C6-
F4)3]- diastereomers (17) is observed in a 2:1 ratio by
NMR.24 In comparison to 1/PBB (4), lower polymeriza-
tion activity but increased polypropylene syndiotacticity
(86.3(3) vs 83.5(3) rrrr%) results from 1+10 (Table 1,
entries 3 and 4).
For reaction of 1 with cocatalysts 11-13, in situ NMR
assigns a mixture of [Me2C(Flu)(Cp)ZrMe]+[FAl(C6F5)3]-
(18) and a pair of diastereomeric µ-methyl complexes
{[Me2C(Flu)(Cp)ZrMe]2(µ-Me)}+{F[Al(C6F5)3]2}- (19) in
differing ratios for all three cocatalysts (Scheme 1).
Despite the complexity of these reactions, it is found
that the active species generated by in situ reactions
are effective agents for highly syndiospecific propylene
polymerization (∼89.0(3) rrrr%, Table 1).25 In compari-
son to the borate-derived catalyst 1+5 (84.0(3) rrrr%)
and alane-derived catalyst 1+6 (85.5(3) rrrr%), the new
bridged fluoroaluminate cocatalyst 12 provides a system
with the highest polymerization activity along with the
highest product stereoregularity. In the reaction of 1
with cocatalyst 14, a pair of diastereomeric µ-methyl
complexes {[Me2C(Flu)(Cp)ZrMe]2(µ-Me)}+[(C6F5)3AlFAl-
(2-C6F5C6F4)3]- (20; 3:2 ratio) is obtained (Scheme 1).
When cocatalysts 1-15 are compared in propylene
(16) For 13 (50% probability thermal ellipsoids in Scheme 1),
selected bond lengths (Å) and bond angles (deg) are as follows: Al1-
F1 ) 1.740(2), Al1-C1 ) 1.998(4), Al2-F1 ) 1.964(2), Al2-F2 ) 1.966-
(2), Al2-C19 ) 1.997(4), Al3-F2 ) 1.734(2), Al3-C37 ) 1.999(4); Al1-
F1-Al2 ) 168.66(14), F1-Al2-F2 ) 176.95(11), Al3-F2-Al2
)
173.53(15), F1-Al1-C1 ) 106.39(14), C1-Al1-C7 ) 110.36(16), F1-
Al2-C19 ) 89.63(13), F2-Al2-C19 ) 91.18(13), C19-Al2-C25 )
116.97(16), F2-Al3-C37 ) 104.84(14).
(17) (a) A similar M-F-M-F-M arrangement is seen in a Bi
system. For a recent review of metal fluorides, see: Roesky, H. W.;
Haiduc, I. J . Chem. Soc., Dalton Trans. 1999, 2249-2264. (b) For the
first structural study of a fluoro-bridged organoaluminum complex,
K+[(Et)3Al-F-Al(Et)3]-, with F-Al ) 1.80(6) Å, see: Natta, G.;
Allegra, G.; Perego, G.; Zambelli, A. J . Am. Chem. Soc. 1961, 83, 5033-
5033.
(18) For 14 (50% probability thermal ellipsoids in Scheme 1),
selected bond lengths (Å) and bond angles (deg) are as follows: Al1-
F1 ) 1.771(2), Al2-F1 ) 1.795(2), Al1-C1 ) 1.979(4), Al2-C31 )
1.999(4); Al1-F1-Al2 ) 176.93(15), F1-Al1-C1 ) 102.07(15), C1-
Al1-C7 ) 113.62(17), F1-Al2-C19 ) 100.24(14), C19-Al2-C31 )
116.62(17).
(19) Several products are observed in this reaction; however, pure
15 can be isolated in good yield by fractional crystallization. The 19F
NMR spectrum of 15 exhibits a characteristic broad, upfield resonance
at δ -210.11 ppm, indicative of Ga-F bond formation.
(20) For 15 (50% probability thermal ellipsoids in Scheme 1),
selected bond lengths (Å) and bond angles (deg) are as follows: F1-
Ga1 ) 1.896(2), F1-Ga2 ) 1.918(2), C1-Ga1 ) 1.991(4), C19-Ga2 )
1.992(4); Ga1-F1-Ga2 ) 173.37(12), F1-Ga1-C1 ) 103.02(13), C1-
Ga1-C7 ) 113.98(16), F1-Ga2-C19 ) 97.39(13), C19-Ga2-C25 )
119.52(16).
(21) For recent examples of organogallium-F complexes, see: (a)
Werner, B.; Kra¨uter, T.; Neumu¨ller, B. Organometallics 1996, 15,
3746-3751. (b) See ref 17a for examples of nonlinear fluoro-bridged
Ga complexes. (c) Kra¨uter, T.; Neumuller, B. Z. Anorg. Allg. Chem.
1995, 621, 597-606.
(23) The other product is an unidentified precipitate which exhibits
very broad peaks in the 1H NMR (CD2Cl2), and structural identification
is ambiguous.
(22) Polymerizations were carried out under 1.0 atm of propylene
pressure in toluene at 25 °C using conditions minimizing mass transfer
and exotherm effects; product isolation and characterization utilized
standard techniques.2d,9,12
(24) In the 19F NMR, the Zr-F-Zr and F-B signals of ion pair 17
appear at δ -77.97 and -184.47 (br), respectively. In addition, free
B(C12F9)3 is also observed in ∼50% yield.

Communications
Organometallics, Vol. 23, No. 5, 2004 935
polymerization, cocatalyst 14 achieves the highest syn-
dioselectivity (90.8(3)%), comparable to Ph3C+PBA- (8)
but with 20× greater catalytic activity (Table 1, entries
10 and 11; Figure 1). In the reaction of 1 with cocatalyst
15, rapid methide abstraction and formation of a
complex mixture, including unidentified dinuclear spe-
cies, is observed. In propylene polymerization, the
bridged fluoro-gallate 15 produces a polymer with high
product syndiotacticity (88.6(3)%), comparable to the
results for fluoroaluminates 11-13. Substantially higher
polymerization activity is exhibited by the fluoro-bridged
gallate 15, compared to Ga(C6F5)3.
In summary, several new classes of mono- and poly-
nuclear trityl fluorotris(perfluoroaryl)borates, -alumi-
nates, and -gallates have been synthesized and char-
acterized. These new cocatalysts demonstrate efficient
and rapid activation of Cs-symmetric group 4 complexes
to generate highly active single-site agents for propylene
polymerization.26 Polymerization activities (Figure 1A),
product molecular weights (Table 1), product syndio-
tacticities (Figure 1B), and %rrmr stereoerrors (Figure
1C) are markedly sensitive to the cocatalyst. These new
cocatalysts achieve higher stereoregulation by depress-
ing %rrmr stereoerrors. In addition, bridged fluoro
polynuclear cocatalysts effecting similar stereocontrol
exhibit significantly higher polymerization activities
than mononuclear cocatalysts.
Ack n ow led gm en t. Financial support by the DOE
(Grant No. DE-FG02-86ER1351) is gratefully acknowl-
edged. M.-C.C. thanks Dow Chemical for a postdoctoral
fellowship, Dr. M. Oishi for helpful discussions, and Dr.
P. Nickias of Dow for GPC measurements.
Su p p or tin g In for m a tion Ava ila ble: Text giving detailed
synthetic procedures and analytical data for 9-15 and com-
plete X-ray experimental details, tables of bond lengths,
angles, and positional parameters for 10 and 13-15, and text
giving a description of polymerization experiments. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(25) The highest catalytic activity is exhibited with the highest 19:
18 ratio (∼2:1) when cocatalyst 12 is used. The lowest polymerization
activity is observed at the lowest 19:18 ratio (∼1:1) when 11 is used.
Similar product syndiotacticities are observed for all three cocatalysts,
arguing that the same (or very similar) species serve as the active
catalyst(s) in all three polymerization systems. Since the activity
increases as 19 content is increased, this argues that 19 is the
precursor to the actual active species.
OM0341698
(26) Product polydispersities are consistent with well-defined single-
site processes (Table 1).