From THF to Furan: Activity tuning and mechanistic insight via sidearm donor replacement in group IV amine bis(phenolate) polymerization catalysts

Article abstract of DOI:10.1021/om030276q

A family of olefin polymerization catalysts based on group IV metal complexes of amine bis(phenolate) ligands were studied. Weakening the sidearm donor of the ligands was found to affect the activity of the resultant polymerization catalysts. An increase in termination rate for both zirconium and titanium complexes led to either a decrease in the molecular weight of the resultant product, or a loss of living polymerization character.

Full text of DOI:10.1021/om030276q

Organometallics 2003, 22, 3013-3015
3013
F r om THF to F u r a n : Activity Tu n in g a n d Mech a n istic
In sigh t via Sid ea r m Don or Rep la cem en t in Gr ou p IV
Am in e Bis(p h en ola te) P olym er iza tion Ca ta lysts
Stanislav Groysman, Israel Goldberg, and Moshe Kol*
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Tel Aviv 69978, Israel
Elisheva Genizi and Zeev Goldschmidt*
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
Received April 15, 2003
Sch em e 1
Summary: Weakening the sidearm donor of amine bis-
(phenolate) titanium and zirconium polymerization cata-
lysts causes a 10-fold enhancement in activity of the
former and no difference in activity of the latter, while
increasing the termination to propagation ratios in both.
Group IV cyclopentadienyl-free complexes are attract-
ing an ever-increasing amount of attention as catalysts
for R-olefin polymerization,1 since they are highly
versatile, may exhibit novel polymerization modes, and
lead to previously inaccessible polymers.2 Yet, in con-
trast to the extensively studied metallocenes,3 a thor-
ough structure-activity understanding that may lead
to control of catalyst activity and well-defined polymers
is lacking. Recently, we introduced a family of olefin
polymerization catalysts based on group IV metal
complexes of amine bis(phenolate) ligands4 (Scheme 1).
These complexes possess a “rigid” [ONO] core, thereby
allowing accurate structure-activity relationship study
by varying several peripheral parameters. The most
critical parameter was found to be the sidearm donor,
as its absence led to fast catalyst deactivation in the
zirconium series and to oligomerization in the titanium
series.
Ligands bearing a strong sidearm donor led to some
remarkable reactivities. Especially when this donor was
a THF group, the corresponding zirconium dibenzyl
complex exhibited a very high activity in polymerization
of 1-hexene, and the corresponding titanium dibenzyl
complex led to a slow but living polymerization of
1-hexene at room temperature for an exceptionally long
time of 6 days.4f Herein we introduce an amine bis-
(phenolate) ligand featuring a weak sidearm donor, i.e.,
furan, and study the activity of its resulting catalysts
in polymerization of 1-hexene. The different electronic
properties combined with similar steric bulks of THF
and furan shed light on the intricate parameters
controlling the activity of these catalysts (Figure 1).
The ligand precursor was prepared by condensing 2,4-
di-tert-butylphenol, formaldehyde, and 2-(aminomethyl)-
furan and reacted cleanly with the corresponding tet-
rabenzylmetal precursors to yield the dibenzyl zirconium
and titanium complexes 1a ,b, respectively, in high
yields. The 1H NMR and 13C NMR spectra indicated
that both complexes are Cs symmetrical. Pale yellow
crystals of 1a suitable for an X-ray structure determi-
nation were obtained upon recrystallization from tolu-
ene.5 The crystal structure was consistent with the NMR
data and confirmed the binding of the furan oxygen to
the zirconium (Figure 2). The structure reveals a
slightly distorted octahedral geometry, with the phe-
nolate oxygens being mutually trans (157.86°) and the
benzyl groups mutually cis (108.30°). As expected for
(1) (a) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 428. (b) Gibson, V. C.; Spitzmesser, S. K. Chem.
Rev. 2003, 103, 283. (c) Coates, G. W. J . Chem. Soc., Dalton Trans.
2002, 467. (d) Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem.,
Int. Ed. 2002, 41, 2236.
(2) For an example of living and highly active ethylene polymeri-
zation see: (a) Mitani, M.; Mohri, J .; Yoshida, Y.; Ishii, S.; Tsuru, K.;
Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S.; Matsugi,
T.; Kashiwa, N.; Fujita, T. J . Am. Chem. Soc. 2002, 124, 3327. For an
example of living and syndiotactic polymerization of propylene see: (b)
Tian, J .; Hustad, P. D.; Coates, G. W. J . Am. Chem. Soc. 2001, 123,
5134. For examples of living polymerization of high R-olefins see: (c)
Scollard, J . D.; McConville, D. H. J . Am. Chem. Soc. 1996, 118, 10008.
(d) Baumann, R.; Davis, W. M.; Schrock, R. R. J . Am. Chem. Soc. 1997,
119, 3830. (e) J eon, Y. M.; Park, S. J .; Heo, J .; Kim, K. Organometallics
1998, 17, 7, 3161. (f) Mehrkhodavandi, P.; Schrock, R. R. J . Am. Chem.
Soc. 2001, 123, 10746. For an example of living and isotactic polym-
erization of 1-hexene see: (g) Tshuva, E. Y.; Goldberg, I.; Kol, M. J .
Am. Chem. Soc. 2000, 122, 10706.
(3) (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev.
2000, 100, 1253. (b) Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt, R.;
Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34,
1143.
(4) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Weitman, H.; Gold-
schmidt, Z. Chem. Commun. 2000, 379. (b) Tshuva, E. Y.; Goldberg,
I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Organometallics 2001, 20,
3017. (c) Tshuva, E. Y.; Groysman, S.; Goldberg, I.; Kol, M.; Gold-
schmidt, Z. Organometallics 2002, 21, 662. (d) Tshuva, E. Y.; Goldberg,
I.; Kol, M.; Goldschmidt, Z. Inorg. Chem. Commun. 2000, 3, 611. (e)
Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Chem. Commun.
2001, 2120. (f) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.;
Goldschmidt, Z. Inorg. Chim. Acta 2003, 345, 137.
10.1021/om030276q CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/24/2003

3014 Organometallics, Vol. 22, No. 15, 2003
Communications
F igu r e 3. “Semi-living” polymerization of neat 1-hexene
by 1b/B(C6F5)3.
F igu r e 1. The two ligand precursors described in this
work. 1a and 2a denote the corresponding zirconium
dibenzyl complexes; 1b and 2b denote the corresponding
titanium dibenzyl complexes.
F igu r e 4. Mw dependence on time (min) in a mixed-
catalyst (1b and 2b) 1-hexene polymerization experiment.
Each vertical pair of points represents a single aliquot
taken at a given time.
binding resulting from a more electron-deficient metal
center. On the basis of the low steric demands of the
furan group, and on previous crystal structures, we
propose that the furan oxygen is bound to the titanium
center in 1b and that its binding is weak.
The polymerization studies were performed in neat
1-hexene by activating the precatalysts 1a and 1b by
addition of approximately equimolar amounts of B(C6F5)3.
The catalytic system 1a /B(C6F5)3 was found to be almost
as active in 1-hexene polymerization as 2a /B(C6F5)3:4f
that is, weakening the donor on the sidearm electroni-
cally does not appear to affect the polymerization rate
significantly for zirconium. However, the molecular
weight of the poly(1-hexene) obtained was reduced
substantially: Mw ) 15 000-25 000 for 1a /B(C6F5)3,
relative to Mw ) 80 000-120 000 produced by 2a /
B(C6F5)3. This difference may stem from an increased
termination-reactivation pathway in 1a /B(C6F5)3 as a
result of the less tightly bound sidearm donor.
Surprisingly, the sidearm donor replacement had a
much more pronounced effect for the titanium com-
plexes. Following addition of B(C6F5)3, the titanium
precatalyst 1b was found to be at least 10-fold more
active than the corresponding titanium complex bearing
THF as a sidearm donor (2b),4f with the activity
corresponding to ca. 200 g mmolcat-1 h-1. In fact, this is
the most active amine bis(phenolate) titanium catalyst
reported so far. In contrast to its zirconium analogue,
1b/B(C6F5)3 produced very high molecular weight poly-
(1-hexene) (a polymer having Mw ) 550 000 was ob-
tained after polymerization of neat 1-hexene at room
temperature for 4 h). Although this polymerization
F igu r e 2. ORTEP view of 1a (50% probability ellipsoids).
Key atoms are labeled. Selected bond distances (Å) and
angles (deg): Zr(1)-O(2) ) 2.000(2), Zr(1)-O(3) ) 1.978(2),
Zr(1)-O(4) ) 2.684(2), Zr(1)-N(5) ) 2.434(2), Zr(1)-C(6)
) 2.273(3), Zr(1)-C(7) ) 2.299(3); O(3)-Zr(1)-O(4) )
85.60(6), O(2)-Zr(1)-O(4) ) 80.11(6), C(8)-C(6)-Zr(1) )
98.6(2), C(14)-C(7)-Zr(1) ) 119.5(2).
the weak furan donor, the Zr-O bond distance was
found to be exceptionally long (2.68 Å). This bond is
longer than all other M-O bonds, and most M-N bonds
reported thus far, for metal-sidearm donor bonds in
group IV metal complexes of this ligand family.4 In
contrast, the Zr-O bond distance for the complex
bearing a THF sidearm donor (2a ) was found to be very
short (2.37 Å).4f Significantly, except for the sidearm
donor-to-metal bond distances, the bindings of these two
[ONOO]-type ligands to the zirconium are essentially
identical. A narrower Zr-C-C angle for the top benzyl
group in 1a relative to 2a may indicate a partial η2
(5) Crystal data for 1a ‚2C7H8, C63H79NO3Zr, Mr ) 989.49, mono-
clinic, space group P21/n; a ) 16.2970(2) Å; b ) 15.7960(2) Å; c )
22.0160(3) Å; â ) 101.3020(8)°; V ) 5557.62(12) Å3; Z ) 4, D ) 1.183
g/cm3; F ) 0.242 mm-1; 9396 independent reflections collected; 7537
unique reflections with I > 2σ(I); R1(I > 2σ(I)) ) 0.0448; wR2(I > 2σ-
(I)) ) 0.1161; R1(all data) ) 0.0620; wR2(all data) ) 0.1273. Data were
collected on a Kappa CCD diffractometer using Mo KR radiation (λ )
0.710 73 Å) at 110 K. For full details concerning crystallographic
analysis see the Supporting Information.

Communications
Organometallics, Vol. 22, No. 15, 2003 3015
F igu r e 5. GPC traces of three consecutive aliquots of the mixed-catalyst polymerization experiment (see Figure 3 and
Supporting Information), featuring the well-resolved peaks of the low-molecular-weight and high-molecular-weight polymers.
Blue line (polymerization time 36 min): Mw(RT ) 8.76 min) ) 12 900, PDI ) 1.20; Mw(RT ) 7.41 min) ) 126 000, PDI )
1.17. Red line (polymerization time 52 min): Mw(RT ) 8.55 min) ) 16 900, PDI ) 1.12; Mw(RT ) 7.19 min) ) 175 700,
PDI ) 1.13. Black line (polymerization time 74 min): Mw(RT ) 8.42 min) ) 21 800, PDI ) 1.12; Mw(RT ) 7.00 min) )
223 900, PDI ) 1.16.
cannot be defined as truly living (a slow broadening of
PDI was observed, and the final polymer sample had a
PDI of 1.37), very narrow polydispersities (1.05-1.09)
were recorded for the first 2 h of polymerization, after
which the molecular weight of the polymer was higher
than 200 000 (Figure 3). Evidently, reducing the strength
of the sidearm donor in the titanium series resulted in
a significant increase in propagation rate but concur-
rently encouraged termination to some extent, thereby
bringing about a reduction of the living polymerization
character.
The availability of two structurally related catalysts,
both of which undergo almost no termination and
feature a 10-fold difference in activity (i.e. 1b and 2b),
enables the evaluation of possible polymeryl transfer
between two propagating species, by GPC monitoring
of the progress of a “mixed-catalyst” polymerization
experiment. Recently, polymeryl transfer has been
investigated by several groups, since, if feasible, it could
be employed in the formation of block copolymers.6-8
The polymerization was performed in neat 1-hexene,
using a ca. 7:1 ratio of 2b to 1b, respectively, and
B(C6F5)3 as an activator. During the whole experiment,
two resolved peaks could be detected in the GPC trace:
the low-molecular-weight peak was attributed to the
action of 2b/B(C6F5)3 and the high-molecular-weight
peak to the action of 1b/B(C6F5)3. Significantly, both
catalysts were found to react at their “native” polym-
erization rates, as evident from the molecular weights.
Furthermore, the two catalysts retain their original
living character in the presence of each other. Espe-
cially, 2b/B(C6F5)3 remained living throughout the
experiment, as supported by the linear increase of
molecular weight vs time and very narrow polydisper-
sities. 1b/B(C6F5)3 showed living behavior in the first
2-3 h only, after which the polymerization in this
system decayed (Figures 4 and 5). All this supports the
existence of two independent propagating species that
do not communicate with each other under these condi-
tions. We thus propose that an alkyl-bridged dinuclear
species is not a viable intermediate in the polymeriza-
tion pathway of the catalysts of this family.
In conclusion, weakening the sidearm donor of amine
bis(phenolate) ligands affected the activity of the result-
ing polymerization catalysts. An apparent increase in
termination rate for both zirconium and titanium
complexes led to a decrease in molecular weights of the
resulting poly(1-hexene) (Zr) or to loss of living polym-
erization character (Ti) and may be associated with the
ease of detaching the sidearm donor from the metal. The
similarity in polymerization rates for zirconium in
contrast to the unexpected 10-fold increase in polym-
erization rate for titanium may reflect the latter’s higher
response to steric effects, as the smaller and remote
furan in comparison to THF leads to a more open site.
A polymeryl exchange between two propagating tita-
nium species does not seem to occur; thus, the possibility
of alkyl-bridged dinuclear species in this system is
unlikely. We are investigating further structure-activ-
ity relationships in this family.
(6) For examples of polymeryl transfer and stereoblock polypropy-
lene formation in the presence of alkyl aluminum species see: (a)
Chien, J . C. W.; Iwamoto, Y.; Rausch, M. D.; Wedler, W.; Winter, H.
H. Macromolecules 1997, 30, 3447. (b) Chien, J . C. W.; Iwamoto, Y.;
Rausch, M. D. J . Polym. Sci., Part A 1999, 37, 2439. (c) Lieber, S.;
Brintzinger, H.-H. Macromolecules 2000, 33, 9192. (d) Przybyla, C.;
Fink, G. Acta Polym. 1999, 50, 77. (e) Kukral, J .; Lehmus, P.; Klinga,
M.; Leskela¨, M.; Rieger, B. Eur. J . Inorg. Chem. 2002, 1349.
(7) For methyl-methyl and methyl-polymeryl group transfer in
methyl-bridged dimeric zirconium cations, see: J ayaratne, K. C.; Sita,
L. R. J . Am. Chem. Soc. 2001, 123, 10754.
(8) For methyl-bridged dimeric zirconium complexes and their
activity in R-olefin polymerization, see: (a) Bochmann, M.; Lancaster,
S. Angew. Chem., Int. Ed. Engl. 1994, 33, 1634. (b) Chen, Y. X.; Stern,
C. L.; Yang, S.; Marks, T. J . J . Am. Chem. Soc. 1996, 118, 12451. (c)
J ia, L.; Stern, C. L.; Marks, T. J . Organometallics 1997, 16, 842. (d)
Vollmerhaus, R.; Rahim, M.; Tomaszewski, R.; Xin, S.; Taylor, N. J .;
Collins, S. Organometallics 2000, 19, 2161. (e) Mehrkhodavandi, P.;
Bonitatebus, P. J ., J r.; Schrock, R. R. J . Am. Chem. Soc. 2000, 122,
7841. (f) Keaton, R. J .; J ayaratne, K. C.; Fettinger, J . C.; Sita, L. R. J .
Am. Chem. Soc. 2000, 122, 12909.
Ack n ow led gm en t. This research was supported by
the Ministry of Science, Culture and Sports and was
also supported in part by the Israel Science Foundation,
founded by the Israel Academy of Sciences and Hu-
manities.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving synthetic details and spectroscopic data for 1a ,b,
polymerization procedures, and crystallographic data for 1a .
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM030276Q