Determination of the catalytic active species in the polymerization of propylene by titanium benzamidinate complexes

Article abstract of DOI:10.1021/om0602198

The catalytic behavior of the monomeric titanium bis(benzamidinate) [η-C₆H₅-C(NSiMe₃)₂] ₂TiCl₂ (1), the dimeric titanium mono(benzamidinate) {[η-C₆H₅-C(NSiMe₃

Full text of DOI:10.1021/om0602198

2656
Organometallics 2006, 25, 2656-2666
Determination of the Catalytic Active Species in the Polymerization
of Propylene by Titanium Benzamidinate Complexes
Victoria Volkis,† Anatolii Lisovskii,† Boris Tumanskii,† Michael Shuster,‡ and
Moris S. Eisen*,†
Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of
Technology, Haifa, 32000, Israel, and Carmel Olefins Ltd., P.O.B. 1468, Haifa 31014, Israel
ReceiVed March 8, 2006
The catalytic behavior of the monomeric titanium bis(benzamidinate) [η-C6H5-C(NSiMe3)2]2TiCl2 (1),
the dimeric titanium mono(benzamidinate) {[η-C6H5-C(NSiMe3)2]TiCl3]}2 (2), and the monomeric titanium
mono(benzamidinate) complex η-C6H5-C(NSiMe3)2]TiCl3]‚THF (3) activated by methylalumoxane (MAO)
has been compared in the polymerization of ethylene and propylene. Despite structural and symmetrical
differences, the activities of all precatalysts were found to be alike, indicating that rearrangements toward
similar active species are operative during the polymerization regardless of the starting materials. To
shed some light on the mechanistic pathways for the formation of such active species from the different
titanium benzamidinate complexes and on the role of the aluminum cocatalyst in the activation process,
corresponding aluminum benzamidinate dichloro and dimethyl complexes were synthesized and compared
to the titanium complexes. The formation of the different active sites was monitored using NMR and
ESR spectroscopy, trapping experiments with [60]fullerene, and MALDI-TOF mass spectroscopy. The
results obtained for the different benzamidinate titanium complexes and proposed mechanistic pathways
for their activation reactions with MAO and their fate after the addition of the olefins are presented and
discussed in this publication. In addition, viscoelastic and rheological mechanical properties of the polymers
are disclosed.
as promising alternatives to the cyclopentadienyl complexes.4
The benzamidinate complexes are normally obtained as a
mixture of racemic C2-symmetry cis-octahedral structures (A),
which usually contain two ligands in a chelating coordination
mode (M ) group 4 early transition metals, X ) halides, amines,
or alkyl groups).
Introduction
The flourishing application of “well-defined” single-site group
3 and 4 metallocene catalysts for the polymerization of
R-olefins,1 together with sophisticated investigations on the role
of the ligand composition and structure in the stabilization of
the active centers,2 has facilitated the search of the nonmetal-
locene systems as alternative catalysts for this polymerization
process.3 Among the various organometallic compounds syn-
thesized during the past decade, the complexes of early transition
metals with the bidentate N,N′-bis(trimethylsilyl) benzamidinate
ligations [η-RC(NR′)2]- (where R ) C6H5 or substituted
phenyls, R′ ) alkyl, aryl, or SiMe3) have been widely described
* To whom correspondence should be addressed. E-mail: chmoris@
techunix.technion.ac.il.
† Technion-Israel Institute of Technology.
‡ Carmel Olefins Ltd.
Compared to the cyclopentadienyl ligands, the benzamidinate
ligations strongly differ from them with unique electronic
(1) (a) Kaminsky, W. In AdVances in Catalysis; Gates, B. C., Kno¨zinger,
H., Eds.; Academic Press: San Diego, 2002; Vol. 46, p 89. (b) Mu¨lhaupt,
R. In Polypropylene: An A-Z Reference; Karger-Kocsis, J., Ed.; Kluwer
Academic Publisher: Dordrecht, 1999; p 454. (c) Brintzinger, H.-H.; Fisher,
D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1143, and references therein. (d) Alt, H. G.; Ko¨ppl, A.
Chem. ReV. 2000, 100, 1205, and references therein. (e) Kaminsky, W. J.
Chem. Soc., Dalton Trans. 1998, 1413. (f) Kojoh, S.-I.; Kashiwa, N. Chem.
Rec. 2004, 3, 342. (g) Kojoh, S.; Fujita, T.; Kashiwa, N. Recent Res. DeV.
Polym. Sci. 2001, 5, 43. (h) Lin, S.; Waymouth, R. M. Acc. Chem. Res.
2002, 35, 765.
(2) (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b)
Hajela, S.; Bercaw, J. E. Organometallics 1994, 13, 1147. (c) Brown, S.
J.; Gao, X.; Harrison, D. G.; Koch, L.; Spence, R. E. H.; Yap, G. P. A.
Organometallics 1998, 17, 5445. (d) Catalyst Design for Tailor-Made
Polyolefins; Soga, K., Terrano, M., Eds.; Elsevier: Tokyo, 1994. (e)
Corradini, P. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 391. (f) Busico,
V.; Cipullo, R.; Kretschmer, W.; Talarico, G.; Vacatello, M.; Van Axel
Castelli, V. Macromol. Symp. 2002, 189, 127. (g) Alt, H. G. In Synthetic
Methods of Organometallic and Inorganic Chemistry; Herrmann, Ed.;
Thieme: Stutgart, 2002; Vol. 10, p 167.
(3) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(b) Gibson, V. C.; Marshall, E. L. Comput. Coord. Chem. 2004, 9, 1. (c)
Lamberti, M.; Mazzeo, M.; Pappalardo, D.; Zambelli, A.; Pellecchia, C.
Macromol. Symp. 2004, 213, 235. (d) Makio, H.; Kashiwa, N.; Fujita, T.
AdV. Synth. Catal. 2002, 344, 477. (e) Busico, V.; Cipullo, R. Prog. Polyn.
Sci. 2001, 26, 443.
(4) (a) Dehicke, K.; Ergezinger, C.; Hartmann, E.; Zinn, A., Ho¨sler, K.
J. Organomet. Chem. 1988, 352, C1. (b) Sheldrick, G. M. Chem. Ber. 1988,
121, 1403. (c) Edelmann, F. T. Coord. Chem. ReV. 1994, 137, 403, and
references therein. (d) Jayarante, K. C.; Sita, L. R. J. Am. Chem. Soc. 2001,
123, 10754. (e) Hagadorn, J. R.; Arnold, J. Organometallics 1998, 17, 1355.
(f) Hagadorn, J. R.; Arnold, J. J. Chem. Soc., Dalton Trans. 1997, 3087.
(g) Hagadorn, J. R.; Arnold, J. Organometallics 1994, 13, 4670. (h)
Hagadorn, J. R.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 893. (i) Littke,
A.; Sleiman, N.; Bensimon, C.; Richeson, D. S.; Yap, G. P. A.; Brown, S.
Organometallics 1998, 17, 446. (j) Gomez, R.; Duchateau, R.; Chernega,
A. N.; Teuben, J. N.; Edelmann, F. T.; Green, M. L. H. J. Organomet.
Chem. 1995, 491, 153.
10.1021/om0602198 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/12/2006

Polymerization of Propylene by Ti Complexes
Organometallics, Vol. 25, No. 10, 2006 2657
properties.4i,j As a four-electron donor, the anionic moiety
[RC(NSiMe3)2]- polarizes the M-N bonds, promoting greater
electrophility of the metal center, as compared to the six
electrons of the cyclopentadienyl ligands. The possibility to
simply modify the steric and electronic properties of the
benzamidinate-based ligations, through changes in either the
organic substituents at the nitrogen atom and/or different
functional groups at the aromatic ring, makes these ligands very
attractive for the synthesis of various organometallic complexes
as potential catalytic precursors for polymerization and other
R-olefin transformations.5
electron-donor solvent or additional nucleophilic ligands, such
as PPh3.5b,6e Thus, in tetrahydrofuran at room temperature, the
dimer produces a red crystalline solid of monomeric complex
η-C6H5-C(NSiMe3)2]TiCl3]‚THF (3), in which one molecule of
THF is coordinated to the titanium center (eq 5).
The catalytic properties of several group 4 bis(benzamidinate)
complexes in the polymerization of R-olefins have been
previously investigated.4j,5b,6 When activated with methylalu-
moxane (MAO) or other cocatalysts, these complexes demon-
strated high catalytic activity for the polymerization of ethylene,
propylene, and styrene.5b,7 It was shown that the stereoregularity
of the obtained polymers depends on the composition of the
early transition metal benzamidinate complexes and the condi-
tions under which the polymerization reactions were performed.
Thus, for example, we have found that for some benzamidinate
zirconium complexes the change of the monomer concentration
during the process allowed the formation of various types of
polypropylene (atactic, isotactic, or elastomeric).7
Among a great variety of benzamidinato-transition metal
compounds, the monomeric titanium bis(benzamidinate) [η-C6H5-
C(NSiMe3)2]2TiCl2 (1)6 and the dimeric titanium mono(benz-
amidinate) {[η-C6H5-C(NSiMe3)2]TiCl3]}2 (2)5b,6e,8 complexes
have been synthesized and studied as catalytic precursors for
the polymerization of R-olefins. The reaction of benzonitrile
with lithium bis(trimethylsilyl)amide produces the bidentate
lithium salt [η2-C6H5-C(NSiMe3)2Li] (eq 1) as either a mono-
meric, dimeric, or oligomeric complex depending on the solvent
(TMEDA, ether, or hexane) used.9 The consecutive reaction of
2 equiv of the latter ligands with titanium tetrachloride in
toluene, at room temperature, yields a brown-red solution, from
which 60-70% of pure complex 1 was isolated (eq 2)4a-c,7
Thus, complexes 1 and 2, having a similar chemical nature,
differ by the space symmetry and number of ligations surround-
ing the metal center and, as a consequence, are expected to
exhibit different reactivities and stereoselectivity in the polym-
erization of R-olefins. Hence, the mono(benzamidinate) titanium
complexes 2 and 3 activated by MAO were found to be active
for the polymerization of styrene, whereas the bis(benzamidi-
nato) titanium complex 1 was completely inactive in this process
under similar reaction conditions.5b,6e For ethylene, the polym-
erizations have been performed at low and high pressure with
the zirconium bis(benzamidinate) dialkyl complexes.10 The
corresponding mono(benzamidinate) zirconium trialkyl complex
has shown lower activities as compared to the bis(benzamidi-
nate) complex.6f,10 It is notable that theoretical calculation has
predicted a higher activity in the polymerization of ethylene
for the mono-benzamidinate derivative.11
Furthermore, regarding titanium, none of the mono(benz-
amidinate) complexes 2 and 3 were found to be active for the
polymerization of propylene at a monomer pressure of 1 atm.5b,6e
This result is similar to our findings, in which the bis(benz-
amidinate) titanium dichloride complex (1) activated with MAO
also does not polymerize propylene at atmospheric pressure.7
However, when complex 1 was used in the polymerization of
(6) (a) Duchateau, R.; Van Wee, C. T.; Meetsma, A.; Van Duijnen, P.
T.; Teuben, J. H. Organometallics 1996, 15, 2279. (b) Averbuj, C.; Tish,
E.; Eisen, M. S. J. Am. Chem. Soc. 1998, 120, 8640. (c) Walter, D.; Fisher,
R.; Friedrich, M.; Gebhardt, P.; Go¨rls, H. Chem. Ber. 1996, 129, 1389. (d)
Gomez, R.; Green, M. L. H.; Haggitt, J. L. J. Chem. Soc., Dalton Trans.
1996, 939. (e) Flores, J. C.; Chien, J. C. W.; Rausch, M. D. Organometallics
1995, 14, 2106. (f) Herscovics-Korine, D.; Eisen, M. S. J. Organomet. Chem.
1995, 503, 307. (g) Gomez, R.; Green, M. L. H.; Haggitt, J. L. J. Chem.
Soc., Chem. Commun. 1994, 2607.
(7) (a) Volkis, V.; Shmulinson, M.; Averbuj, C.; Lisovskii, A.; Edelmann,
F. T.; Eisen, M. S. Organometallics 1998, 17, 3155. (b) Volkis, V.;
Nelkenbaum, E.; Lisovskii, A.; Hasson, G.; Semiat, R.; Kapon, M.;
Botoshansky, M.; Eishen, Y.; Eisen, M. S. J. Am. Chem. Soc. 2003, 125,
2179. (c) Lisovskii, A.; Volkis, V.; Eisen, M. S. Proc. 9th Int. Symp. Varna,
Bulgaria, 2000, p 301. (d) Yang, P.; Baird, M. C. Organometallics 2005,
24, 6013.
(8) (a) Boere, R. T.; Oakley, R. T.; Reed, R. W. J. Organomet. Chem.
1987, 331, 161. (b) Fenske, D.; Hartman, E.; Dehnicke, K. Z. Naturforsch.
1988, 43B, 1611.
(9) Lisovskii, A.; Botoshansky, M.; Eisen, M S. J. Chem. Soc., Dalton
Trans. 2001, 11, 1692.
(10) Walter, D.; Fischer, R.; Go¨rls, H.; Koch, J.; Scheweder, B. J. J.
Organomet. Chem. 1996, 508, 13.
(11) Ramos, J.; Cruz, V.; Munoz-Escalona, A.; Martinez-Salazar, J.
Polymer 2001, 42, 7275.
When the lithium complex reacts with Me3SiCl in toluene,
the neutral silylated benzamidinate ligand is formed7a (eq 3).
Reaction of TiCl4 with stoichiometric amounts of this neu-
tral ligand in dichloromethane produces 92% of complex 27b
(eq 4).
Interestingly, the dimeric complex 2 can be easily converted
into the corresponding monomeric complex by using an
(5) (a) Richter, J.; Edelmann, F. T.; Noltemeyer, M.; Schmidt, H.-G.;
Shmulinson, M.; Eisen, M. S. J. Mol. Catal. A: Chem. 1998, 130, 149. (b)
Flores, J. C.; Chien, J. C. W.; Rausch, M. D. Organometallics 1995, 14,
1827. (c) Mercando, L. A.; Handwerker, B. M.; MacMillan, H. J.; Geoffroy,
G. L.; Rheingold, A. L.; Owens-Waltermire, B. E. Organometallics 1993,
12, 1559. (d) Yamamoto, Y.; Akiba, K. Heterocycles 1979, 13, 297. (e)
Piskov, V. B.; Kasperovich, V. P. Zhu. Organich. Khim. 1978, 14, 820.

2658 Organometallics, Vol. 25, No. 10, 2006
Volkis and Eisen
plex was crystallized from toluene-hexane (4:1) solution at -40
°C. The resulting brown crystals were filtered and dried under
vacuum to obtain 2.11 g (58%). Anal. Calc for C13H23N2Si2Cl2Al:
C, 43.33; H, 6.37; N, 7.76; Cl, 19.67. Found: C, 42.65; H, 6.54;
propylene at higher pressures (liquid propylene ∼10 atm), an
elastomeric polypropylene was produced.7
Our main goal was to investigate the influence of the
compositional and structural features of the mono- and bis-
(benzamidinato) titanium complexes on their activity for the
polymerization of ethylene and propylene at high pressure and
on the properties of the polymers produced. In addition, to shed
some light on the mechanistic pathways for the formation of
the active species from the different titanium benzamidinate
complexes and on the role of the aluminum cocatalyst in the
activation process, corresponding aluminum benzamidinate
dichloro and dimethyl complexes were synthesized. The catalytic
activity of these aluminum complexes in the polymerization of
propylene and the properties of the achieved polymers were
compared with those obtained by the titanium complexes. In
addition the formation of the different active sites was monitored
using NMR and ESR (C60 trapping experiments) spectroscopy
and MALDI-TOF mass spectroscopy. The results obtained
regarding the active complexes and their fate in their reactions
with MAO and olefins are presented and discussed in this
publication. In addition, viscoelastic and rheological properties
of the polymers are disclosed.
1
N, 7.38; Cl, 19.58. H NMR (C7D8, 500 MHz): δH 0.08 (s, 18H,
TMS), 7.11 (br, 5H, Ph). 13C NMR (C7D8, 125 MHz): δ 0.07
(TMS); 123, 127, 131 (Ph). 29Si NMR (C7D8, 99.3 MHz): δ 3.84
ppm.
Synthesis of Benzamidinate Aluminum Dimethyl Complex
9
7. Benzamidine PhC(NTMS)N(TMS)2 (3.36 g, 0.01 mol) was
suspended in 150 mL of dry hexane under inert conditions and
cooled to -78 °C. Then 0.72 g (0.01 mol) of trimethyl aluminum
(as 10% solution in hexane, Aldrich) was added dropwise. The
reaction mixture was allowed to warm to room temperature and
stirred for 3 h. The resulting solution was concentrated by vacuum,
removing part of the hexane until precipitation commenced. The
white powder was filtered and dried under vacuum to obtain 1.9 g
(59%). Anal. Calc for C15H29N2Si2Al: C, 56.25; H, 9.06; N, 8.75.
1
Found: C, 56.82; H, 9.22; N, 8.71. H NMR (C7D8, 500 MHz):
δH 0.11 (s, 18H, TMS), 6.9 (br, 5H, Ph), 0.99 (s, 6H, AlMe2). 13
C
NMR (C7D8, 125 MHz): δ 0.05 (TMS); 7.13 (Al-Me); 122, 128,
132 (Ph). 29Si NMR (C7D8, 99.3 MHz): δ 3.83 ppm.
Propylene Polymerization Experiments. The polymerization
of propylene was studied using complexes 1-5 as catalytic
precursors. The complexes were activated by methylalumoxane at
different molar Al:Ti ratios. The polymerization was performed in
a 100 mL stainless steel reactor equipped with a magnetic stirrer.
The reactor was charged with a certain amount of complex and
MAO inside a glovebox and connected to a high-vacuum line. After
introducing 5-6 mL of the solvent (CH2Cl2 or toluene) under an
argon stream, the reactor was cooled to 123 K and vacuum
evacuated. Then 30-40 mL of liquid propylene was vacuum
transferred into the reactor. The temperature was then raised in a
thermostated bath and the stirring began. The pressure in the reactor
was measured with a manometer. After stirring for a definite period
of time, the reaction was quenched by exhausting the unreacted
C3H6 in a well-ventilated hood, followed by the introduction of a
30-40 mL mixture of acetylacetone and water (1:20) to decompose
all the MAO, inducing the formation of Al(acac)3. The polymer
was filtered, washed with methanol and acetone, and dried under
vacuum at 50 °C.
Experimental Section
General Procedures. All manipulations with air-sensitive
materials were performed with the exclusion of oxygen and moisture
in Schlenk-type glassware and a high-vacuum (10-6 Torr) line. For
storage of materials, a nitrogen-filled Vacuum Atmospheres glove-
box with a medium-capacity recirculator (1-2 ppm O2) was used.
The gases (argon and nitrogen) were purified by passage through
a MnO oxygen-removal column and a Davison 4 Å molecular sieve
column. Analytically pure solvents were distilled under N2 from
Na/K-benzophenone (tetrahydrofuran), Na/K alloy (hexane), Na
(toluene), or P2O5 (dichloromethane). All solvents for vacuum line
manipulations were stored in a vacuum over Na/K alloy. Benzoni-
trile and TMEDA were freshly distilled under argon and degassed.
C60, AlMe3, and AlCl3 (Aldrich) was used as received. Methyl-
alumoxane (Witco) was prepared from a 30% suspension in toluene
by vacuum evaporation of the solvent at 25 °C/10-5 Torr.
Complexes [η-C6H5-C(NSiMe3)2]2TiCl2 (1), {[η-C6H5-C(NSiMe3)2]-
TiCl3}2 (2), and [η-C6H5-C(NSiMe3)2]TiCl3‚THF (3) were prepared
as described in the literature.4a-c,5b,6e
The NMR measurements of the benzamidinate complexes were
conducted on Teflon J. Young valve-sealed NMR tubes after
vacuum transfer of the solvent in a high-vacuum line and recorded
on Bruker Avance 300 or 500 MHz spectrometers. MALDI-TOF
LD+ and LD- experiments were performed on a Waters MALDI
mass spectrometer. ESR spectra were recorded on a Bruker EMX-
10/12 X-band (ν ) 9.4 GHz) digital ESR spectrometer equipped
with a Bruker N2-temperature controller. All spectra were recorded
at microwave power 10, 1 mW, 100 kHz magnetic field modulation
of 1.0-0.5 G amplitude. Digital field resolution was 2048 points
per spectrum, allowing all hyperfine splittings to be measured
directly with accuracy better than 0.2 G. Spectra processing and
simulation were performed with Bruker WIN-EPR and SimFonia
software.
Ethylene Polymerization. The polymerization of ethylene was
studied using complexes 1 and 2 as catalytic precursors. The
complexes were activated by methylalumoxane at a Ti:Al molar
ratio of 1:1000. The polymerization was performed in a 100 mL
stainless steel reactor equipped with a magnetic stirrer. The reactor
was charged with a certain amount of complex and MAO inside a
glovebox and connected to a high-vacuum line. After introducing
5-6 mL of toluene under an argon stream, the reactor was filled
with ethylene at a pressure of 20 atm. The pressure was kept
constant during the polymerization. After stirring for a definite
period of time, the reaction was quenched by exhausting the
unreacted ethylene in a well-ventilated hood, followed by the
introduction of a 30-40 mL mixture of acetylacetone and water
(1:20) to decompose all the MAO. The polymer was filtered,
washed with acetone, and dried under vacuum at 50 °C.
Fractionation of Polymers. The polymer samples were fraction-
ated by extraction with refluxing hexane. A 250 mL round-bottom
flask was charged with the solvent (100 mL) and attached to a
Soxhlet extractor. The polymer (∼1 g) was packed into a cellulose
thimble placed into the extractor. After 72 h of extraction the solvent
was evaporated under vacuum from both hexane-soluble and
hexane-insoluble fractions. The results are expressed as a percentage
of the total amount of polymer.
Synthesis of Benzamidinate Aluminum Dichloride Complex
6. Li benzamidinate [PhC(NTMS)2]Li6f,7a (2.7 g, 0.01 mol) was
dissolved in 250 mL of dry toluene under inert conditions and
cooled to -78 °C. Then 1.33 g (0.01 mol) of aluminum trichloride
was dissolved in 50 mL of dry toluene in a dropping funnel and
added dropwise to the lithium benzamidinate ligand. The reaction
mixture was allowed to warm to room temperature and stirred for
24 h. The resulting solution was filtered through a fritted glass,
and the solution was concentrated by vacuum to produce a solid
product. The resulting monochloroaluminum benzamidinate com-
Polymerization of Propylene under ESR Monitoring. A Teflon
J. Young valve-sealed NMR tube was charged in the presence or
absence of fullerene (2 mg/mL solution in toluene) with the

Polymerization of Propylene by Ti Complexes
Organometallics, Vol. 25, No. 10, 2006 2659
Table 1. Data for the Polymerization of Propylene Using Precatalysts 1, 2, and 3 Activated by Methylalumoxane
process conditionsa
Al:Ti ratio
properties of polypropylene
entry
cat.b
solvent
yield of polymer, g
Complex 1
activity,c A × 10-5
Mw
mwdd
mmmm %
1
2
3
4
16.0
16.0
17.0
16.0
500
1000
2000
1000
CH2Cl2
CH2Cl2
CH2Cl2
toluene
0.20
1.93
4.28
3.23
0.043
0.40
0.84
0.69
51 000
47 000
29 000
59 000
2.83
3.13
5.81
3.70
12.8
14.9
15.7
10.9
Complex 2
Complex 3
5
6
7
8
23.9
25.1
25.1
25.1
500
1000
2000
1000
CH2Cl2
CH2Cl2
CH2Cl2
toluene
1.69
3.10
5.31
4.73
0.24
0.38
0.71
0.63
80 000
51 000
38 000
58 000
3.20
3.76
6.33
3.97
34.1
20.6
18.4
11.9
9
10
20.4
20.4
1000
1000
CH2Cl2
toluene
2.30
3.37
0.38
0.55
55 000
62 000
3.77
3.26
14.7
9.9
a 23°C, 10.2 atm, 6 mL of solvent, 40 mL of C3H6, 180 min. b mol Ti × 10-6
.
.
c g PP × mol Ti-1 × h-1 d Molecular weight distribution.
at 20 atm exhibiting unexpectedly equal activities (5.8 × 103 g
corresponding complex, MAO, and a deuterated solvent inside a
drybox. The NMR tube was connected to a high-vacuum line,
frozen, pumped, thawed, and filled with propylene to 1 atm. The
consumption of the propylene were monitored via a pressure gauge
and refilled after reaching 700 mmHg. The ESR spectrum was
measured after 1, 2, 6, and 12 refills of propylene.
PE/(mol cat × h × atm) for complex 1 and 5.6 × 103
g
PE/(mol cat × h × atm) for complex 2). The similar activities
spurred us to evaluate the relationship between the number of
ancillary ligations in similar metal complexes, their symmetry,
and their catalytic reactivity. To estimate these effects, a
comparison study in the polymerization of propylene was
performed using complexes 1-3, and the results are shown in
Table 1.
As well as complex 1, complexes 2 and 3 were found to be
active in the polymerization of propylene only at high pressure.
The polymerization activities for all these complexes in toluene
are somewhat larger than those obtained in dichloromethane
(compare entries 2 and 4, 6 and 8, 9 and 10). In CH2Cl2, at the
Al:Ti molar ratio of 500, the activity of 1 is smaller than that
of complex 2 (entries 1 and 5), whereas at Al:Ti ) 1000 the
activities of all precatalysts in this solvent were found to be
alike (entries 2, 6, and 9). In toluene, the polymerization activi-
ties of the complexes also differed insignificantly. These results
induce us to consider that possible similar actiVe species are
operative in the polymerization process regardless of the starting
materials.
For all the complexes, the elastomeric polypropylene pro-
duced in toluene has somewhat higher molecular weight than
the polymers formed in dichloromethane. In addition, the
polymers were characterized by a wide polydispersity. Raising
the MAO concentration in the catalytic mixture decreases the
molecular weights of the polymers and increases their molecular
weight distribution. This result normally indicates that the
aluminum transfer mechanism is operative, inducing the lower
molecular weights fractions. However, according to the 13C
NMR analysis of all the obtained polymers, the double-bond
chain-end signals were found to be characteristic for a â-hy-
drogen elimination process.13 Interestingly, for corresponding
zirconium benzamidinate complexes we have shown that the
chain termination pathway involves â-methyl elimination.7a,b
The formation of lower molecular weight polymers is therefore
a result of the formation of different species with probably
different activity, inducing higher molecular weight distributions.
Despite the different structural features of the used precata-
lysts, the molecular weights of the polymers created under the
same conditions were unexpectedly found to be alike (compare
entries 2, 6, and 9 for CH2Cl2, or 4, 8, and 10 for toluene). As
regards the stereoregularity of the monomer insertion, augmen-
Characterization of the Polymers. NMR spectra of the
polymers were conducted in deuterated tetrachlorethane at 85 °C
and recorded on a Bruker Avance 500 MHz spectrometer. The
preheating of our polymer samples at 85 °C dissolved in 0.4 mL
of tetrachloroethane-d2 (4 h) yielded clear homogeneous solutions.
13C NMR spectra of random polymer samples were measured at
85 and 120 °C, exhibiting reproducible pentad data.12 Chemical
1
shifts for H and 13C NMR were referenced to internal solvent
resonances and reported relative to SiMe4.
Molecular weights of polymers were determined by the GPC
method on a Waters-Alliance 2000 instrument using 1,2,4-trichlo-
robenzene as a mobile phase at 150 °C and referenced to polystyrene
standards.
Melting crystallization behavior of the polymers was examined
using a Perkin-Elmer DSC-7 differential scanning calorimeter. Three
runs (heating-cooling-heating) at a rate of 10 deg/min in the range
30-190 °C were performed for each sample of polymer. The second
heating exothermal peak temperature was taken as a melting point.
Prior to studying their properties, the polymers were first
devolatalized by processing on Collin roll mills and then compres-
sion molded at 180 °C using a Collin heated plate hydraulic press.
Viscoelastic properties of the compression-molded samples were
tested in both molten and solid state using an ARES dynamic
mechanical thermal analyzer (Rheometric Scientific). The solid
materials were tested in a torsion deformation mode using rec-
tangular samples cut from the compression-molded plates. Dynamic
temperature ramp experiments at 1 Hz oscillation frequency were
conducted in the temperature range -50 to +100 °C at a heating
rate of 5 deg/min. The applied deformation was about 0.2%.
The rheological properties of the molten polymers were tested
at 180 °C using a parallel plate (25 mm) setup in a dynamic
frequency sweep mode. The oscillation frequency varied from 0.05
to 15 Hz with the deformation amplitude of 2%. The complex
viscosity data were approximated using the Carreau model. Shore
A hardness of the elastomeric compression-molded samples was
measured using a digital instrument according to the ASTM D2246.
Results and Discussion
In our search for catalysts for the polymerization of olefins,
we have studied and compared the activity and selectivity of
complexes 1 and 2 activated by MAO in the polymerization of
ethylene. We have found that these complexes are active in the
polymerization of ethylene, producing HDPE (mp ) 130 °C)
(12) Yoder, J. C.; Bercaw, J. E. J Am. Chem. Soc. 2002, 124, 9.
(13) Resconi, L.; Piemontesi, F.; Franciscono, G., Abis, L.; Fiorani, T.
Organometallics 1992, 114, 1025.

2660 Organometallics, Vol. 25, No. 10, 2006
Volkis and Eisen
Table 2. Data for the Fractionation of the Polymers PP-1
and PP-2, Obtained in Entries 4 and 8 (Table 1): Properties
of the Hexane-Soluble (PP-HS) and Hexane-Insoluble
(PP-HI) Polymeric Fractions
sample
wt %
Mw
mwd
mmmm, %
PP1 (whole polymer)
PP1-HS
PP1-HI
PP2 (whole polymer)
PP2-HS
PP2-HI
100
98.2
1.8
100
96.7
3.3
59 000
55 000
238 000
58 000
56 000
216 000
3.1
3.2
3.3
3.8
3.1
3.6
10.9
10.0
74.4
11.9
10.0
74.5
tation of the MAO concentration results in similar isotacticities
for the polymers created by complex 1 in CH2Cl2 and in a slight
decrease in stereoregularity for the polymers produced by
complex 2. The high polydispersity of the obtained polypro-
pylenes, which is presumably a consequence of the presence
of various active species, producing polymers with a wide
spectrum of molecular weights and different tacticities, encour-
aged us to perform fractionations of the whole polymers using
refluxing hexane, and the results are given in Table 2.
The data of Table 2 show that the main part (97-98%) of
polypropylene is a hexane-soluble (HS) fraction. The molecular
weight of the HS fraction for PP-2 (polymer obtained by
complex 2) is equal to that of the polymer PP-1 (polymer
obtained by complex 1). For both polymers, the isotacticities
of the hexane-soluble and, accordingly, hexane-insoluble frac-
tions were found to be also equal.
The results of the fractionation of the polypropylenes PP-1
and PP-2 confirm the compositional heterogeneity of the
polymers, which were found to be a mixture mostly consisting
of an elastomeric fraction, with properties similar to elastomers
described by Resconi14 (hexane-soluble fraction), and a high
molecular weight polymeric fraction with larger isotacticity
(hexane-insoluble fraction). It is important to point out that the
tacticity of the hexane-soluble fraction was found to be similar
to that observed for Resconi’s elastomeric polymers;12 however,
the MW was noticeably lower. One possible reason for the
elastomeric behavior of such low tacticity and low MW
polymers is the presence of comparatively large amounts of ethyl
and butyl fragments, formed as a result of misinsertions, as
found for R-diimine late transition metal complexes in the
polymerization of propylene.15
Figure 1. 13C NMR spectra of different fractions of polypropylenes
PP-1 and PP-2: (a) PP-1, hexane-soluble fraction; (b) PP-1, hexane-
insoluble fraction; (c) PP-2, hexane-soluble fraction; (d) PP-2,
hexane-insoluble fraction. All polymers were obtained with the same
equimolar amount of the corresponding catalysts, 6 mL of toluene,
Ti:Al ) 1:1000, RT, 4 h, 10.2 atm propene.
Table 3. Pentad and Triad Distribution for Hexane-Soluble
Fractions of PP-1 and PP-2
pentad analysis
PP-1, %
PP-2, %
mmmm
mmmr
rmmr
10.2
9.9
5.2
10.0
8.5
4.8
mmrr
mmrm+rmrr
mrmr
11.5
20.5
10.3
8.1
11.2
21.9
10.1
9.8
rrrr
rrrm
mrrm
12.4
10.9
13.1
10.6
triad analysis
PP-1, %
PP-2, %
mm
mr
rr
25.3
43.3
31.4
22.3
43.2
34.5
aluminum benzamidinate dichloride (6) and dimethyl (7)
complexes were synthesized by the reaction of the lithium
benzamidinate ligand [η-C6H5-C(NSiMe3)2Li] with AlCl3 or the
reaction of the neutral benzamidinate ligand [C6H5-C(dNSiMe3)-
(NSiMe3)2] with AlMe3,16 correspondingly (Scheme 1).
We have investigated the features of activation of complexes
1 and 2 with methylalumoxane followed by 29Si NMR,
discriminating various species formed at different steps of the
activation (Figure 2). The spectra obtained for those complexes
were compared to the spectra of the aluminum benzamidinate
complexes 6 and 7.
In Figure 2, the spectrum a exhibits the clean signal of the
pure complex 1. This signal has no matches with the signals
obtained in spectrum b of complex 1 activated with MAO. It is
important to point out that the central signals in spectrum b
match the signal obtained in both spectrum c and d, corre-
sponding to the aluminum dichloro benzamidinate (6) and
dimethyl aluminum benzamidinate (7) complexes, respectively.
Similarly, spectrum e of the pure complex 2 exhibits only one
single signal, but after the addition of MAO (f) the same three
signals as obtained with complex 1 (spectrum b) are observed
(the intensities are somewhat different due to the amount of
MAO used and the formation of Ti(III) species, vide infra).
Hence, on the basis of our previous results for cationic bis-
(benzamidinate) complexes,7b,17 we can substantiate that the
small signal at the lowest chemical shift in spectrum b belongs
to the bis(benzamidinate) titanium methyl cationic complex,
whereas the large signal at the highest magnetic field corre-
sponds to the mono(benzamidinate) alkyl cationic complex.15
The 13C NMR spectra of the different polymeric fractions
are presented in Figure 1. As can be concluded from Figure 1,
the NMR characterization of different polymer fractions indi-
cates that the hexane-soluble fractions of samples PP-1 and
PP-2, as well as the hexane-insoluble fractions, are almost
identical as regards the activity, tacticity, and stereoerrors.
Moreover, the comparison of the pentad and triad distribution
for the soluble fractions of PP-1 and PP-2 (Table 3) shows also
absolute similarity of these samples.
To explain these unexpected similarities, it is conceivable
that during the polymerization process the different complexes
undergo rearrangements, forming equal number of species,
similar in their structure, which are active for the polymerization
reaction. To corroborate this hypothesis, one of such active
species was postulated to be the corresponding aluminum
benzamidinate complex that can be formed as a result of the
interaction of the studied titanium benzamidinates with methyl-
alumoxane. To confirm experimentally this assumption, the
(14) Resconi L.; Jones, R. L.; Rheigold, A. L.; Yap, G. P. A.
Organometallics 1996, 15, 998.
(15) McCord, E. F.; McLain, S. J.; Nelson, L. T. J.; Arthur, S. D.;
Coughlin, E. B.; Ittel, S. D.; Johnson, L. K.; Tempel, D.; Killian, C. M.;
Brookhart, M. Macromolecules 2001, 34, 362.
(16) For a similar synthesis see: Barker, J.; Aris, D. R.; Blacker, N. C.;
Errington, W.; Phillips, P. R.; Wallbridge, M. G. H. J. Organomet. Chem.
1999, 586, 138.

Polymerization of Propylene by Ti Complexes
Organometallics, Vol. 25, No. 10, 2006 2661
Scheme 1. Synthesis of Aluminum Benzamidinate Complexes
Scheme 2. Plausible Rearrangement of Complex 1
The formation of the aluminum complex involves the
complex 2), all signals including the signals from the aluminum
benzamidinate complexes disappeared from the spectra. This
behavior was found to be a result of the formation of Ti(III)
species, which were indicated using ESR analysis (vide infra).
The activities of complexes 6 and 7 in the high-pressure
polymerization of propylene were found to be an order of
metathesis of the benzamidinate ligand from the titanium
complexes to the aluminum center at MAO. After adding a large
concentration of MAO (>1:20 for complex 1 and >1:2 for
magnitude less reactive (∼0.5 × 104 g PP × mol Al-1 × h-1
)
than for the corresponding titanium precatalysts 1 and 2. The
molecular weight, polydispersity, and tacticity of the elastomers
produced by the aluminum benzamidinate complexes were also
found to be close to the hexane-soluble (HS) polymeric fractions
obtained by the titanium complexes. At the same time, the low
activity of complexes 6 and 7 activated by MAO allowed us to
conclude that the formation of the hexane-soluble fraction (more
than 90% of the whole polymers produced by the catalytic
systems 1/MAO and 2/MAO) that may be produced by species
obtained by the partial migration of the benzamidinate ligand
from Ti to Al is insignificant. Hence, the formation of the
polydisperse hexane-soluble elastomeric polypropylenes mainly
depends on the presence of the other two active intermediates
(unless NMR-silent but catalytically active species are obtained).
The additional two signals on the 29Si NMR spectra (Figure
2b,f) are representative of two different complexes, each of
Figure 2. 29Si NMR monitoring of the formation of new species
from complexes 1 and 2 in toluene-d8: (a) pure complex 1, (b)
complex 1 + MAO at a 1:20 respective ratio, (c) pure complex 6,
(d) pure complex 7, (e) pure complex 2, (f) complex 2 + MAO at
a respective 1:1 ratio.18
(17) Volkis, V. Benzamidinate Complexes as Active Catalysts for the
Polymerization of R-Olefins and Silanes. Ph.D. Thesis. Technion-Israeli
Institute of Technology, 2000.

2662 Organometallics, Vol. 25, No. 10, 2006
Scheme 3. Plausible Mechanism for the Disproportionation of Complex 2
Volkis and Eisen
Scheme 4. Proposed Ti(III) Intermediates in the Activation of Complexes 1 and 2 with MAO
which is probably responsible for the formation of different
fractions. Assuming that a bis(benzamidinate) titanium methyl
complex, with C2 symmetry, is responsible for the small isotactic
hexane-insoluble fraction, a second monomeric mono(benz-
amidinate) titanium cationic alkyl complex must be responsible
for the formation of the elastomeric hexane-soluble polymer.19
Hence, it is reasonable that complex 2 rearranges the same as
complex 1, forming the same active species. Plausible mech-
anisms for the rearrangements for complexes 1 and 2 are
described in Schemes 2 and 3.
The rearrangement mechanisms can be explained by elemen-
tary reactions well known for group 4 metal complexes.20 In
Scheme 2, the rearrangement of complex 1 starts with the
formation of the corresponding dimethyl titanium bis(benz-
amidinate) complex by the reaction with MAO. The methide
abstraction will form the cationic complex 5, or if a metathesis
is operative, one benzamidinate ligand will be transferred to
the aluminum center, as observed in the 29Si NMR. The
corresponding methide abstraction from the benzamidinate
titanium trimethyl with MAO will yield the corresponding
cationic complex 4. In Scheme 3, the dimeric complex 2 reacts
with MAO toward the monomeric complex in a fashion similar
to that in the formation of complex 3. Upon its reaction with
MAO, this complex can induce either methide abstraction,
forming complex 4, or the metathesis reaction, eliminating
TiMe4 and producing the aluminum benzamidinate moiety. In
addition, a ligand redistribution of the benzamidinate titanium
trimethyl will produce also complex 1 and TiMe45b,21 (a second
possibility regards the ligand redistribution of the cationic
titanium benzamidinate dialkyl complex, although if operative,
it should be much slower).5b,6e,22 The abstraction of a methide
ligand from complex 1 with MAO will form complex 5.
As mentioned above, in the reaction of either complex 1 or
2 with MAO some Ti(III) intermediates were found and
characterized by ESR. Figure 3 illustrates the ESR of the
intermediates formed, when MAO was added to the toluene
solution of complex 1 at Ti:Al ratio 1:1000. The ESR spectra
of the paramagnetic intermediates were measured at room
temperature and at low temperatures as frozen glasses. The ESR
(18) The benzamidinate aluminum complexes (6 and 7) are unaffected
by the presence of an excess of MAO. Moreover, the reaction of the neutral
benzamidinate ligand with an excess of MAO exhibits the same 29Si NMR
signal, at 3.83 ppm, as obtained for complexes 6 and 7.
(19) Low-temperature polymerization of monobenzamidinate titanium
dialkyl cationic complexes has been shown to produce stereoregular
syndiotactic polypropylene (rr ) ∼50%); see refs 19a,b.
(20) Bresler, L. S.; Ivanova, V. P.; Khachaturov, A. S. Zh. Strukt. Khim.
1976, 17, 257.
(21) Wu, Y.-D.; Peng, Z.-H.; Chan, K. W. K.; Liu, X.; Tuinman, A. A.;
Xue, Z. Organometallics 1999, 18, 2081.
(22) (a) Liguori, D.; Centore, R.; Tuzi, A.; Grisis, F.; Sessa, I.; Zambelli,
A. Macromolecules 2003, 26, 5451. (b) Liguori, D.; Grisis, F.; Sessa, I.;
Zambelli, A. Macromol. Chem. Phys. 2003, 204, 164.

Polymerization of Propylene by Ti Complexes
Organometallics, Vol. 25, No. 10, 2006 2663
Figure 3. ESR characterization of complex 1 activated by MAO
in toluene: (a) at room temperature (for determination of g-fac-
tor spectrum recorded with 2,2,6,6-tetramethylpiperidine-N-oxyl
(TEMPO), g ) 2.0059, the three lines of TEMPO marked by stars);
(b) at 150 K without TEMPO; (c) at 170 K with TEMPO.
spectrum at 290 K (Figure 3a) illustrates two single isotopic
signals: a minor signal with an upfield shifted g-factor
(compared to organic radicals) at giso ) 1.982 (Ti-1a) (line width
) 26.0 G) and a major signal, at giso ) 1.959 (Ti-2) (line width
) 8.3 G). The signal of the latter species (Ti-2) in frozen glass
is characterized by a three-axis anisotropy of the g-factor,23
which results in three ESR signals (g1 ) 1.985, g2 ) 1.969,
and g3 ) 1.927), as shown in Figure 3b (Figure 3c shows the
calibration at low temperature with TEMPO ) 2,2,6,6-tetra-
methylpiperidine-N-oxyl). The observed strong anisotropy of
the g-factor in frozen glasses and the low value of the
giso-factor in solution are typically exhibited by paramagnetic
Ti(III) complexes.24 The broad resonance of the minor signal
(Ti-1a) was found to have a weak anisotropy of the g-factor in
frozen glass. This widening of the signal is normally a result
of spin exchange processes within agglomeration of various
titanium species, which probably contain aluminum from MAO
(Scheme 4b). Intriguingly, a similar signal of aggregates has
been observed by Baird et al. for Cp*TiCl3 activated by MAO.25
On the basis of the steric equivalence between cyclopentadienyl
ligands and benzamidinates, it seems plausible that the structure
of the agglomeration species involves only one benzamidinate
ancillary ligand per metal center, as observed for the corre-
sponding metallocenes.22 Regarding the signal observed for the
titanium species with the strong anisotropy, we can relate this
signal to the trigonal bipyramidal intermediate Ti-2, with two
benzamidinate ligands (Scheme 4a), having the three expected
symmetry axes.21a
Figure 4. ESR and MALDI-TOF monitoring of the mixture of
complex 1 with fullerene C60 activated by MAO: (a) ESR spectrum
at 290 K in toluene, (b) MALDI-TOF spectrum, (c) ESR spectrum
of the reaction mixture under visible light (λ > 600 nm).
agent),23b the intensity of the Ti-1a signal on the ESR was found
to be higher (Figure 4a) compared with the same signal without
the preaddition of C60. This result indicates that the addition of
C60 induces the formation (breaks or avoids the aggregates) of
an additional intermediate maintaining its chemical nature due
to the similar exhibited g-factor and the temperature anisotropy.
The irradiation of this reaction mixture in the cavity of the ESR
spectrometer by visible light (λ > 600 nm) gives rise only to
reversible dissociation of the dimer of the fullerenyl radicals
MeC60-C60Me h 2 •C60Me, and the corresponding signal was
also observed (Figure 4c).26 MALDI-TOF analysis shows one
methyl radical trapped on one fullerene molecule (Figure 4b).
The formation of the methyl radical is via the reduction of the
dimethyl Ti(IV) complex with MAO, producing the Ti(III) and
the methyl radical.
The ESR analysis of the active species, formed from complex
1 activated by MAO, without fullerene, at different steps of
the polymerization reaction with propylene (performed in the
NMR tube) shows that the intensity of the Ti-2 species decreases
When the activation of complex 1 with MAO was performed
in the presence of fullerene C60 (used as a radical-trapping
(23) Bolton, J. R.; Wertz, J. E. Electron Spin Resonance: Elemental
Theory and Practical Applications; Chapman and Hall: New York, 1986.
(24) (a) Lukens, W. W.; Smith, M. R., III; Andersen, R. J. Am. Chem.
Soc. 1996, 118, 1719, and references therein. (b) Bonoldi, L.; Abis, L.;
Fiocca, L.; Fusco, R.; Longo, L.; Simone, F.; Srea, S. J. Mol. Catal. A:
Chem. 2004, 219, 47. (c) Ye, Z.; Wang, W.-J.; Zhu, S. J. Appl. Polym. Sci.
2003, 89, 2474. (d) Pattiasina, J. W.; Heeres, H. J.; van Bolhuis, F.;
Meetsma, A.; Teuben, J. H.; Spek, A. L. Organometallics 1987, 6, 1004.
(25) Williams, E. F.; Murray, M. C.; Baird V. C. Macromolecules 2000,
33, 261, and references therein.
(26) (a) Krusic, P. J.; Wasserman, E.; Keizer, P. N.; Morton, J. R.;
Preston, K. F. Science 1991, 254, 1184. (b) Tumanskii, B. L.; Kalina, O.
G. Radical Reactions of Fullerenes and Their DeriVatiVes; Kluwer Academic
Publishers: Dordrecht: The Netherlands, 2001. (c) Taylor, R. Lecture Notes
on Fullerene Chemistry: A Handbook for Chemists; Imperial College
Press: London, 1999.

2664 Organometallics, Vol. 25, No. 10, 2006
Volkis and Eisen
Figure 5. ESR monitoring of the active species formed from
complex 1 during propylene polymerization reaction in a ESR
tube: (a) before polymerization, (b) after two back-fillings, (c) after
six back-fillings (lines of TEMPO marked by stars).
Figure 7. ESR and MALDI-TOF monitoring of the mixture of
complex 2 with fullerene C60 activated by MAO: (a) ESR spectrum
at 290 K in toluene, (b) MALDI-TOF spectrum, (c) ESR spectrum
of reaction mixture under visible light (λ > 600 nm).
Figure 6. ESR monitoring of complex 2 activated by MAO in
toluene: (a) at room temperature (lines of TEMPO marked by stars),
(b) at 150 K.
very rapidly during the polymerization (Figure 5). The signal
of Ti-1a remains unchangeable, indicating that this kind of
species is not active in the polymerization process. After six
fillings of the ESR tube (see Experimental Section), the Ti-2
species completely disappeared from the spectrum, forming the
polymer and regenerating Ti(IV) species.27
When the same reaction was carried out in the presence of
C60 as a function of the addition of propylene, the same trend
was obtained. The signal of Ti-2 totally disappeared but the
signal of Ti-1a, which was obtained with relative high intensity
(Figure 4a), was reduced to the same intensity as observed
without fullerene. This result indicates that some of the
nonagglomerate species (proposed to be Ti-1) observed in the
signal of Ti-1a are oxidized to Ti(IV).24
Similar ESR studies were also performed for complex 2. In
contrast to complex 1, and to corroborates the formation of the
presumably mono(benzamidinate) species (Ti-1 and/or Ti-1a),
the activation of complex 2 with MAO in toluene induces the
Figure 8. ESR monitoring of the active species formed from
complex 2 during propylene polymerization reaction: (a) before
polymerization, (b) after two back-fillings.
formation of a large Ti-1 and Ti-1a ESR signal having
temperature behavior similar to that of the same signal obtained
in the activation of complex 1 (Figure 6). The signal of Ti-2 is
absent from this ESR spectrum, indicating that no dispropor-
tionation is obtained from the cationic complexes. The presence
of the fullerene C60 did not affect the activation of complex 2
(compare ESR in Figures 6 and 7). In addition, the irradiation
of the ESR tube with visible light allows, as shown before, the
production of the C60-Me radicals, as observed in the ESR
(Figure 7c). MALDI-TOF characterization of such samples also
showed the methyl radicals trapped on the fullerene (Figure 7b).
(27) For other Ti(III) octahedral allylic complexes that are oxidized to
Ti(IV) by propene during the polymerization, see: Ray, B.; Neyroud, T.
G.; Kapon, M.; Eichen Y.; Eisen, M. S. Organometallics 2001, 20, 3044.

Polymerization of Propylene by Ti Complexes
Organometallics, Vol. 25, No. 10, 2006 2665
In parallel, we have followed the activation of the titanium
benzamidinate complexes in the presence of fullerene using also
29Si NMR. For example, when fullerene was added to complex
1 before the activation by MAO, two 29Si signals from
Ti-centered species were observed only after the addition of
propylene (Figure 9). The downfield signal is exactly at the
same chemical shift as obtained for complex 5, whereas the
second signal was obtained upfield shifted by ∼2.0 ppm, as
expected for complex 4 upon coordination with C60.
The thermal, mechanical, and rheological properties of the
polymers PP-1 and PP-2 were also investigated. The purified
nonfractionated polymers were found to be similar in their
processability. Polymer PP-1 or PP-2 produced by complex 1
or 2, respectively, activated with MAO was easily processed
on roll mills at 100-120 °C and compression molded at 100
°C.
DSC thermograms of the polymers are smooth, showing no
thermal effects on heating or cooling in the range 30-190 °C,
thus indicating a low-crystalline structure of the samples.
As may be seen from the DMTA curve for PP-1 (Figure 10a),
there is a monotonic gradual decrease of the storage modulus
(G′) with the temperature increase in the range -50 to -10
°C, followed by an abrupt drop indicative of the glass transition.
The single strong peak on tan δ versus temperature at about 0
°C (characteristic for the polypropylene glass transition), to-
gether with the 3 orders of magnitude decrease of G′′, is evi-
dence of the mainly amorphous structure of the tested polymer.
The rheological behavior of the polymers is represented by
shear viscosity versus shear rate dependencies at 180 °C
obtained from the oscillatory tests (Figure 10b). In the molten
state, the polymer demonstrates typical viscoelastic rheological
behavior. Both the storage (G′) and loss (G′′) modulus increase
with the oscillation frequency. In the studied frequency range,
G′, a measure of the elastic properties, rises with the frequency
faster than G′′, as a result of viscous properties. Thus, with the
frequency increase, the melts become more elastic. At low
oscillation frequency (low shear rate), the melts show liquidlike
behavior (G′′ > G′), typical for molten non-cross-linked
thermoplastics. For the sample PP-1 the G′/G′′ crossover
probably occurs at higher frequency, beyond the studied range.
The complex viscosity (η*) of the melts (Figure 10b), a
measure of their overall resistance to flow, was also found to
be similar for both polymers PP-1 and PP-2. η* changes with
Figure 9. 29Si NMR of the active species formed from complex 1
with C60 activated by MAO; effect of propylene: (A) before
polymerization, without fullerene, at Ti:Al ratio 1:20, (B) after the
beginning of polymerization, Ti:Al ratio 1:1000, in the presence
of fullerene.
It is important to point out that the intensity of the ESR signal
of Ti-1a/Ti-1 obtained in the activation of complex 2 with MAO
decreases after the addition of propylene, and a small signal
equal in intensity to the signal of Ti-1a found for complex 1
remains regardless the amount of propene added (Figure 8). This
result corroborates that in the activation of complex 2 the
proposed mono(benzamidinate) titanium trimethyl intermediate
is reduced to the corresponding mono(benzamidinate) Ti(III)
species (Ti-1 in Scheme 4b), which can react with MAO,
forming additional agglomerates (Ti-1a). With propylene, the
species Ti-1 is oxidized to Ti(IV), producing the expected
polymer, whereas the signal of the agglomerated inactive species
Ti-1a is unaffected.
Comparison of the polymerization results for complexes 1
and 2 under regular conditions in the presence of fullerene C60
as a radical-trapping agent (added after the activation with
MAO) shows no difference in activity. This allows us to
conclude that all observed Ti(III) intermediates have very low
polymerization activity if at all operative, or are produced and
transformed to corresponding Ti(IV) species when exposed to
propene. Interestingly, when fullerene C60 was added to com-
plex 1 before its activation with MAO, the resultant catalytic
system produced only 2% of the polypropylene, as compared
to the corresponding polymerization without fullerene. The
polymer was characterized as an isotactic solid (mmmm ) 75%
and broad mp, starts at 144, max at 151.2 °C; narrow
crystallization signal at 112 °C, ∆Hm ) 272 J/mol). It is
remarkable that the 2% is exactly the same amount of the
hexane-insoluble fraction of polypropylene that resulted under
regular conditions!! Hence, it seems plausible that C60 can
coordinate to the mono(benzamidinate) complex 4 (eq 6),
impeding the insertion of propene, whereas due to steric
hindrance in complex 5, containing two benzamidinate ligands,
this coordination is not operative and the stereoregular polymer
is obtained.
Figure 10. DTMA thermogram for the polypropylene PP-1 (a) and frequency dependences of the storage and loss moduli, and of the
complex viscosity for the blend of polymer PP-1 measured at 180 °C (b).

2666 Organometallics, Vol. 25, No. 10, 2006
Volkis and Eisen
the shear rate, demonstrating behavior typical for molten
polymers: Newtonian plateau in the range of low frequencies
and decrease of viscosity with increasing of shear rates (non-
Newtonian behavior).
identical as regards their properties. These unexpected similari-
ties are explained via likely rearrangements of the complexes,
forming an equal number of species, similar in their structure,
that are active for the polymerization reaction. Aluminum benz-
amidinate complexes were synthesized and are also obtained
in the reaction of the titanium complexes with MAO, but their
contribution to the total amount of polymer is very small.
Conclusion
The catalytic properties of the monomeric titanium bis-
(benzamidinate) [η-C6H5-C(NSiMe3)2]2TiCl2 (1), the dimeric
titanium mono(benzamidinate) {[η-C6H5-C(NSiMe3)2]TiCl3]}2
(2), and the monomeric titanium mono(benzamidinate) complex
η-C6H5-C(NSiMe3)2]TiCl3]‚THF (3) activated by MAO were
studied in the polymerization of ethylene and propylene. Being
similar by their chemical nature, but different by their space
symmetry and number of ligations surrounding the metal center,
complexes 1-3 were expected to exhibit diverse reactivities
and stereoselectivity in the polymerization of R-olefins. Unex-
pectedly, the catalytic behavior of all complexes was found to
be similar in the polymerization of ethylene and propylene.
Moreover, the difference in properties of the resultant polymers
produced by the different benzamidinate complexes (tacticities,
fractional composition, molecular weight, and viscoelastic and
rheological properties) was found to be insignificant. Fraction-
ation of the polymers shows the formation of a solid isotactic
hexane-insoluble minor fraction and an elastomeric hexane-
soluble major fraction. The hexane-soluble fractions of all
samples, as well as the hexane-insoluble fractions, were almost
The activation of complexes 1 and 2 by MAO was followed
by 29Si NMR, which discriminated between various species
(complexes 4 and 5) formed at different steps of the activation.
In addition to species 4 and 5, during the activation of either
complex 1 or 2 with MAO some Ti(III) intermediates were
found and characterized by ESR at room temperature and in
frozen glasses. Trapping experiments with C60, ESR studies,
MALDI-TOF spectroscopy, and NMR showed that Ti(III)
complexes are not active in the polymerization but rather are
oxidized by propene to Ti(IV), similar to other Ti(III) allylic
complexes.24
Acknowledgment. This research was supported by the
USA-Israel Binational Science Foundation under contract
2004075 and by the New York Metropolitan Research Fund
administered by the VPR office at the Technion-Israel Institute
of Technology.
OM0602198