Ethylene and propylene polymerization by a series of highly electrophilic, chiral monocyclopentadienyltitanium catalysts

Article abstract of DOI:10.1021/om970522w

The compounds Cp*TiMe₂C₆F₅, Cp*TiMe₂OC₆F₅, and Cp*TiMe₂Cl (Cp* = η⁵-C₅Me₅) react with the borane B(C₆F₅)₃ to form the thermally unstable, chiral complexes Cp*TiMe(C₆F₅)-(μ-Me)B(C₆F ₅)₃, Cp*TiMe(OC₆F₅)(μ-Me)B(C₆F ₅)₃, and Cp*TiMeCl(μ-Me)B(C₆F₅)₃, respectively, which are similar to the known Cp*TiMe₂(μ-Me)B(C₆F₅)₃. All four μ-Me compounds behave as sources of the highly electrophilic species [Cp*TiMeE]⁺ (E = Me, Cl, C₆F₅, OC₆F₅) when treated with the borane, the last three being chiral, and all four systems exhibit catalytic activities for the polymerization of ethylene to high-molecular-weight polyethylene. Despite the chirality at titanium of three of the compounds, polymerization of propylene by all of them results in the formation of atactic, elastomeric polypropylene. NMR analyses of the propylene polymers formed show that initiation involves 1,2-insertion into a Ti-Me bond, and while propagation involves primarily head-to-tail 1,2-insertions, an unusually high (by metallocene standards) proportion of the insertions also involves 2,1-misinsertions but essentially no 1,3-enchainment. The major olefinic end groups are vinylidene (CH₂=CMe-), resulting from β-hydrogen transfer following a 1,2-insertion, and vinyl (CH₂=CH-), resulting from β-hydrogen transfer from the methyl group following a 2,1-insertion or, more likely, β-methyl transfer following a 1,2-insertion. Small amounts of internal olefins are also formed via β-hydrogen transfer following a 2,1-insertion. An EPR study of the Cp*TiMe₃/ B(C₆F₅)₃ system in toluene indicates that <0.01% of the titanium is occasionally present during polymerization as a complex of titanium(III), suggesting that a contribution to catalysis by titanium(III) species is unlikely.

Full text of DOI:10.1021/om970522w

1502
Organometallics 1998, 17, 1502-1510
Eth ylen e a n d P r op ylen e P olym er iza tion by a Ser ies of
High ly Electr op h ilic, Ch ir a l
Mon ocyclop en ta d ien yltita n iu m Ca ta lysts
Sean W. Ewart, Mark J . Sarsfield, Dusan J eremic, Tracey L. Tremblay,
Edan F. Williams, and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Received J une 23, 1997
The compounds Cp*TiMe2C6F5, Cp*TiMe2OC6F5, and Cp*TiMe2Cl (Cp* ) η5-C5Me5) react
with the borane B(C6F5)3 to form the thermally unstable, chiral complexes Cp*TiMe(C6F5)-
(µ-Me)B(C6F5)3, Cp*TiMe(OC6F5)(µ-Me)B(C6F5)3, and Cp*TiMeCl(µ-Me)B(C6F5)3, respectively,
which are similar to the known Cp*TiMe2(µ-Me)B(C6F5)3. All four µ-Me compounds behave
as sources of the highly electrophilic species [Cp*TiMeE]+ (E ) Me, Cl, C6F5, OC6F5) when
treated with the borane, the last three being chiral, and all four systems exhibit catalytic
activities for the polymerization of ethylene to high-molecular-weight polyethylene. Despite
the chirality at titanium of three of the compounds, polymerization of propylene by all of
them results in the formation of atactic, elastomeric polypropylene. NMR analyses of the
propylene polymers formed show that initiation involves 1,2-insertion into a Ti-Me bond,
and while propagation involves primarily head-to-tail 1,2-insertions, an unusually high (by
metallocene standards) proportion of the insertions also involves 2,1-misinsertions but
essentially no 1,3-enchainment. The major olefinic end groups are vinylidene (CH2dCMe-
), resulting from â-hydrogen transfer following a 1,2-insertion, and vinyl (CH2dCH-),
resulting from â-hydrogen transfer from the methyl group following a 2,1-insertion or, more
likely, â-methyl transfer following a 1,2-insertion. Small amounts of internal olefins are
also formed via â-hydrogen transfer following a 2,1-insertion. An EPR study of the Cp*TiMe3/
B(C6F5)3 system in toluene indicates that <0.01% of the titanium is occasionally present
during polymerization as a complex of titanium(III), suggesting that a contribution to
catalysis by titanium(III) species is unlikely.
Recent research into the formation of highly stereo-
of cationic complex as the active species in Cp′2ZrCl2/
regular polypropylene (PP) has highlighted group 4
metallocene catalysts of the type Cp′2ZrCl2/MAO (Cp′
) η5-functionalized cyclopentadienyl, MAO ) methyla-
luminoxane) as potentially viable alternatives to exist-
ing commercial catalysts.1,2 The active species are
generally accepted as being 14-electron cations of the
type [Cp′2MMe]+, which can be tailored to produce iso-
or syndiotactic PP via appropriate functionalization of
the cyclopentadienyl rings.1 The crystal structures of
non-MAO complexes Cp′2ZrMe(µ-Me)B(C6F5)3 have also
been reported;3a-c these dissociate the borate anion
[BMe(C6F5)3]- to generate the catalytically active spe-
cies [Cp′2ZrMe]+,3 thus confirming the role of this type
MAO systems.
A growing interest in the related half-sandwich
complexes of the type Cp′ML3 has led our group and
others to the formation of rather similar cationic species
by the general method of alkyl ligand abstraction using
4
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S0276-7333(97)00522-0 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 03/15/1998

Monocyclopentadienyltitanium Catalysts
Organometallics, Vol. 17, No. 8, 1998 1503
F igu r e 1. Proposed structure of Cp*TiMeE(µ-Me)B(C6F5)3
(A-D).
F igu r e 2. Structure of metallocenes of the type (η5-5,6-
X2C9H5)2ZrCl2/MAO (E).
the strongly Lewis acidic borane B(C6F5)3 and formation
of alkyl-bridged complexes of the type Cp*TiR2(µ-
R)B(C6F5)3 (Cp* ) η5-C5Me5) (eq 1).4j,r,u,v,5
molecular weights.8c However, the relative rates of
initiation, propagation, termination, and chain transfer
are all affected unpredictably by ligand substitution,
and the reasons for the observed deactivation were not
assessed in detail,8c although it has been shown else-
where that enhanced Lewis acidity of the metal can
result in stronger binding of the counteranion.3 The
latter phenomenon could result in both inhibition of
monomer coordination and more facile termination of
polymer growth via olefin ligand displacement. Utiliza-
tion of a counteranion which, for steric or electronic
reasons, could not coordinate strongly would obviate the
latter modes of deactivation, and thus the effects of
increased metal ion Lewis acid strength on catalytic
activity arguably remain open for discussion.
Here we describe the production of high-molecular-
weight polyethylene (PE) and high-molecular-weight
regio- and stereoirregular PP using the catalytic sys-
tems Cp*TiMeE(µ-Me)B(C6F5)3, in which E varies in
electron-withdrawing capability (E ) Me (A), Cl (B),
OC6F5 (C), C6F5 (D)). The syntheses7a and solution
dynamics5c,7 of compounds A, C, and D have been
previously reported, and while similar studies of B have
been thwarted by as yet unidentified side reactions
during the reaction of Cp*TiMe2Cl with B(C6F5)3, the
similarities in the catalytic properties of B to those of
A, C, and D justify its inclusion. The catalysts studied
here are of interest not only because they offer a
potential test of the effects of electron-withdrawing
groups on polymerization processes but also because
catalysts B-D are the first chiral, monocyclopentadi-
enyl systems to be studied in this way. Stereoregular
propylene polymerization by metallocene systems gen-
erally requires significant steric crowding at the cata-
lytic site, ensured by appropriate elaboration of the
cyclopentadienyl rings,1,2 and it is important to ascertain
the effects on propylene polymerization of a chiral
environment right at the active site, the metal ion.
Cp*TiR3 + B(C6F5)3 h Cp*TiR2(µ-R)B(C6F5)3 (1)
We have proposed that the methyl-bridged bonding
mode in Cp*TiMe2(µ-Me)B(C6F5)3 (A, Figure 1, E ) Me;
CD2Cl2, -50 °C)5 is similar to that found in Cp′2ZrMe-
(µ-Me)B(C6F5)3 in solution and in the solid state3a-c and
have demonstrated the versatility of A as an active
polymerization initiator for a number of olefins.6 In
addition, polymerization of ethylene and propylene has
been achieved using CpTiCl3 and Cp*TiCl3 activated
with large excesses of MAO, and Pellecchia has reported
the polymerization of ethylene using the related MAO-
free catalysts Cp*MR2(µ-R)B(C6F5)3 (M ) Zr, R ) CH2-
Ph; M ) Ti, R ) CH2Ph, Me), although propylene was
polymerized only with the benzyl complexes.4j
During the initiation step for Ziegler-Natta catalysis,
insertion of the monomer into an alkyl-metal bond is
required. The active species in half-sandwich com-
pounds [Cp*MR2]+ contains a redundant R ligand
during the polymerization, and thus replacement of one
of the R groups with other coordinated anions provides
scope for altering the steric and electronic properties of
the metal, and possibly for altering the catalyst activity.
Since cationic complexes generally form more active
polymerization catalysts than do neutral species,1 we
have hypothesized that substitution of one of the methyl
groups of [Cp*TiMe2]+ with a more electron withdraw-
ing ligand could result in the formation of more active
initiators.7a This expectation is contrary to current
conventional wisdom for metallocene systems,8 since, for
instance, incorporation of electron-withdrawing groups
on the Cp′ rings of metallocenes of the type (η5-5,6-
X2C9H5)2ZrCl2/MAO (Figure 2, E) has been found to
result in both decreased activity and reduced polymer
(5) Gillis, D. J .; Tudoret, M.-J .; Baird, M. C. J . Am. Chem. Soc. 1993,
115, 2543. (b) Gillis, D. J .; Quyoum, R.; Tudoret, M.-J .; Wang, Q.;
J eremic, D.; Roszak, A. W.; Baird, M. C. Organometallics 1996, 15,
3600. (c) Wang, Q.; Quyoum, R.; Gillis, D. J .; J eremic, D.; Tudoret,
M.-J .; Baird, M. C. J . Organomet. Chem. 1997, 527, 7.
Resu lts a n d Discu ssion
(6) Quyoum, R.; Wang, Q.; Tudoret, M.-J .; Gillis, D. J .; Baird, M. C.
J . Am. Chem. Soc. 1994, 116, 6435. (b) Barsan, F.; Baird, M. C. J .
Chem. Soc., Chem. Commun. 1995, 1065. (c) J eremic, D.; Wang, Q.;
Quyoum, R.; Baird, M. C. J . Organomet. Chem. 1995, 497, 143. (d)
Wang, Q.; Baird, M. C. Macromolecules 1995, 28, 8021. (e) Wang, Q.;
Quyoum, R.; Gillis, D. J .; Tudoret, M.-J .; J eremic, D.; Hunter, B. K.;
Baird, M. C. Organometallics 1996, 15, 693.
(7) Sarsfield, M. J .; Ewart, S. W.; Tremblay, T. L.; Roszak, A. W.;
Baird, M. C. J . Chem. Soc., Dalton Trans. 1997, 3097. (b) Tremblay,
T. L.; Ewart, S. W.; Sarsfield, M. J .; Baird, M. C. J . Chem. Soc., Chem.
Commun. 1997, 831. (c) Ewart, S. W.; Sarsfield, M. J .; Baird, M. C.
Unpublished results.
P olym er iza tion P r ocesses. Abstraction of a methyl
ligand from Cp*TiMe2E using 1 equiv of B(C6F5)3 in
methylene chloride provides a general route to a number
of active olefin coordination catalysts of the type
Cp*TiMeE(µ-Me)B(C6F5)3 (A-D) (Figure 1). For typical
polymerization runs, toluene or methylene chloride
solutions of the catalyst precursors Cp*TiMe2E were
saturated with monomer before addition of B(C6F5)3,
which resulted in rapid formation of polymer in reac-
tions completed within 5-20 min. The results of ethyl-
ene and propylene polymerization runs initiated at -78,
(8) Mogstad, A.-L.; Waymouth, R. M. Macromolecules 1992, 25, 2282.
(b) Piccolrovazzi, N.; Pino, P.; Consiglio, G.; Sironi, A.; Moret, M.
Organometallics 1990, 9, 3098. (c) Lee, I.-M.; Gauthier, W. J .; Ball, J .
M.; Iyengar, B.; Collins, S. Organometallics 1992, 11, 2115.

1504 Organometallics, Vol. 17, No. 8, 1998
Ewart et al.
Ta ble 1. P olym er iza tion of Eth ylen e (25 °C)
sample
catalyst
precursor
A* (kg of PE/
yield (g) ((mol of Ti) h atm)
run no.a
1
2
3
4
Cp*TiMe3 (A)
0.23
0.16
0.08
0.22
46
32
16
44
Cp*TiMe2Cl (B)
Cp*TiMe2C6F5 (D)
Cp*TiMe2OC6F5 (C)
a
Run time 5 min.
F igu r e 3. Representation for isotactic PP containing
stereoerrors caused by (a) chain-end control and (b) enan-
tiomorphic site control.
0, and 25 °C are reported in Tables 1-4, although it
should be noted that the polymerizations initiated at
25 °C were not thermostated effectively and quickly
became quite hot. For PE, catalytic activities (16-46
kg of PE/((mol of Ti h atm)) were somewhat lower than
for CpTiCl3/MAO (62 kg PE/((mol of Ti h atm)) (Table
1), although, of course, conditions were not identical.4s
The PE materials formed were insoluble in 1,2,4-
trichlorobenzene (145 °C), and molecular weights are
therefore conservatively estimated to be >300 000. The
PE formed is generally linear, as indicated by single
species (E ) C6F5, OC6F5) do not, presumably for steric
reasons.7c Solvent effects on the propylene polymeriza-
tions are discussed below.
Ster eor egu la r ity. The relative stereochemistries of
consecutive propylene units in the PP polymer chain are
best described as sequences of isotactic meso (m) dyads
and/or syndiotactic racemic (r) dyads.9 Polymer micro-
structure analysis by 13C{1H} NMR provides valuable
information on the mechanism of monomer insertion,10
the methylene, methine, and methyl resonances being
observed at δ 45-47, 28-29 and 19-23, respectively.
Integration of the nine observable methyl pentad reso-
nances provides information on the relative distribution
of the dyads, with m:r ratios being 1:0, 0:1, and 1:1 for
ideal cases of iso-, syndio-, and atactic polymers, re-
spectively.9,10 For polymerizations operating under
stereocontrol, isolated stereoerrors provide information
on whether the regulating mechanism is chain-end
control (regulated by the last inserted monomer) or
enantiomorphic site control (regulated by the ligand
arrangement at the metal).10b,c In the former, a stereo-
error is propagated until the next defect corrects the
error (Figure 3a), while in the latter the error is
corrected immediately (Figure 3b). Statistical evidence
for the chain-end control mechanism results from
analyses of the stereoerrors of isolated r dyads exhibit-
ing a characteristic mmmr:mmrm pentad ratio of 1:1.
In contrast, the evidence for enantiomorphic site control
results from analyses of the stereoerrors of isolated rr
triads, with the population of the mmmr:mmrm:mrrm
pentads in a 2:2:1 ratio.10
The four catalyst systems (A-D) produce very similar
atactic PP products in toluene at a given polymerization
temperature, and representative 13C{1H} NMR spectra
(50.38 MHz) of PP samples produced in reactions
initiated at 25, 0, and -78 °C are respectively shown
in parts a-c of Figure 4. In all three cases, the spectra
exhibit prominent methylene, methine, and methyl
resonances, as anticipated. Interestingly, however,
while the NMR spectrum of the polymer prepared at
-78 °C exhibits no other resonances, spectra of the
samples prepared at 0 and 25 °C, especially the latter,
exhibit multiple resonances in the regions δ 12-16 and
30-40. The methyl and methylene resonances of the
higher temperature materials are also relatively com-
plex, and it is clear that the polymerization processes
1
resonances at δ 1.39 in the H NMR spectra of soluble
fractions (1,1,2,2-C2D2Cl4 at 135 °C).
In a previous paper, we reported that Cp*TiMe2(µ-
Me)B(C6F5)3 (A) exhibits low activity for polymerization
of propylene in toluene,6a but we have since found that
propylene polymerization is in fact possible with all of
the catalysts A-D under rigorously dry conditions. The
reasons for the earlier failures are not clear but were
probably a result of insufficiently anhydrous conditions.
Results of the propylene polymerizations are completely
reproducible and are reported in Tables 2 and 4.
Polymerization runs at -78 °C in toluene resulted in
the formation of PP with increased Mw (3 × 105-2 ×
106) when the more electron-withdrawing ligands were
used (D ≈ C > B > A). These results are in contrast to
what has been reported for some metallocene systems,
where an increase in the electron-withdrawing ability
of ligand substituents was found to reduce both catalytic
activity and the molecular weights of the polymers
formed.8 For example, compounds of the type (η5-5,6-
X2C9H5)2ZrCl2/MAO (Figure 2, E) were examined for the
polymerization of ethylene,8c with the result that smaller
amounts of lower molecular weight PE were obtained
when X was more electron-withdrawing. Unfortunately,
no assessment could be made between the relative rates
of polymerization and chain transfer, nor were the
relative affinities of the cationic metal centers for
monomer as opposed to counteranion assessed.
Previous low-temperature NMR evidence has shown
that A and C but not D dissociate into separate ion pairs
to some extent in CD2Cl2, as in eq 2.7 The counterion
Cp*TiMeE(µ-Me)B(C6F5)3 h
[Cp*TiMeE(solvent)]+[BMe(C6F5)3]- (2)
must be displaced by an incoming monomer for polym-
erization to occur, and thus the relative polymerization
activities in toluene (A > C > D; Table 2) generally
correlate with the ease of the borate displacement of eq
2. The trend is certainly significant for the polymeriza-
tions at -78 °C, where unimodal, nearly monodisperse
polymers were reproducibly obtained. On the other
hand, these conclusions are clouded by the fact that the
complex cation [Cp*TiMe2]+ forms the arene complex
[Cp*TiMe2(η6-toluene)]+ in toluene5b while [Cp*TiMeE]+
(9) Tonelli, A. E. NMR Spectroscopy and Polymer Microstructure;
VCH: New York, 1989. (b) Busico, V.; Cipullo, R.; Monaco, G.;
Vacatello, M.; Segre, A. L. Macromolecules 1997, 30, 6251.
(10) Ewen, J . J . Am. Chem. Soc. 1984, 106, 6355. (b) Doi, Y.;
Asakuru, T. A. Makromol. Chem. 1975, 176, 507. (c) Sheldon, R. A.;
Fueno, T.; Tsunstsuga, T.; Kurukawa, J . J . Polym. Sci., Part B 1965,
3, 23. (d) Xu, J .; Feng, L.; Yang, S. Macromolecules 1997, 30, 2539.

Monocyclopentadienyltitanium Catalysts
Organometallics, Vol. 17, No. 8, 1998 1505
A (kg of PE/
Ta ble 2. P olym er iza tion of P r op ylen e (in Tolu en e)
catalyst
precursor
run no.d
temp (°C)
10-5Mw
Mw/Mn
% m dyadsb
% 2, 1
yield (g)
((mol of Ti) h atm)
4
5
6
7
8
Cp*TiMe3
Cp*TiMe2Cl
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
Cp*TiMe3
-78
-78
-78
-78
0
0
0
0
25
25
25
25
3.4
5.6
23.0
20.0
3.0
c
4.0
2.8
2.6
1.6
2.7
1.7
1.3
1.9
1.7
1.7
5.3
c
3.7
3.3
5.2
5.4
3.9
4.9
54.7
57.7
67.1
69.8
49.7
50.0
44.3
44.1
48.5
49.8
42.4
43.8
<1
<1
nd
nd
4
6
5
11
12
0.86
0.86
0.43
0.65
4.20
2.36
3.09
3.06
2.40
1.56
1.45
1.84
43
43
22
33
214
119
156
155
122
79
9
Cp*TiMe2Cl
10
11
12
13
14
15
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
Cp*TiMe3
Cp*TiMe2Cl
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
12
12
74
94
a
b
1
d
Calibrated using polystyrene standards. m ) mm +
/
mr. c Bimodel. Run time 5 min.
2
Ta ble 3. En d Gr ou p An a lysis Resu lts for P P
tion of monomer into titanium-hydrogen bonds. The
alternative 2,1 (secondary), insertion, resulting in the
methine (CH) carbon being bound to the metal, results
in regioerrors when 1,2-incorporation is the dominant
process and can be detected as head-to-head or tail-to-
tail enchainments (Figure 5). In general, 2,1-defects
may be corrected immediately to give isolated errors
(Figure 5b), or a number of 2,1-misinsertions may occur
prior to reversion to the major 1,2-mechanism (Figure
5a).
P r od u ced a t 70 °C in Tolu en e
amt of
amt of
amt of
sample
run no.
catalyst
precursor
vinyl vinylidene 2-butenyl
10-4Mw
(%)a
(%)a
(%)a
16
17
18
19
Cp*TiMe3
Cp*TiMe2Cl
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
4.1
2.8
3.8
3.7
40
39
40
32
53
53
52
59
7
8
8
9
From 1H NMR spectra.
a
at higher temperatures are rather more complicated
than might be anticipated (see below).
As mentioned above, the 13C{1H} NMR spectra of PP
prepared at 0 °C and above exhibit resonances in the
regions δ 38.2-42.4, 32.2-34.7, and 12.4-15.6 (Figure
4b,c). As has been shown elsewhere,13,14a these groups
of resonances correspond to the methylene, methine,
and methyl resonances of 2,1-enchainments, respec-
tively, and it is clear that the samples prepared at 0 °C
and above are highly regioirregular. Indeed, the 13C-
{1H} spectra of the elastomeric polymers produced here
resemble those of the elastomeric PP prepared using
soluble vanadium catalysts of the type VCl4/AlEt2X (X
) halogen).13a Integration of the total methine region
of the NMR spectrum of PP formed at 25 °C indicates
that >10% of the monomer units are oriented in a tail-
to-tail manner, stereoirregularities in the neighboring
units causing broadening of the resonances.13b,c Par-
ticularly characteristic is the strong methine resonance
at δ 30.4, attributed to the methine carbons of tail-to-
tail units which are followed by blocks of 2,1-
insertions.13b,c Since a head-to-head unit must occur
before primary 1,2-insertions are reestablished, 10% is
therefore a lower limit for the number of 2,1-insertions
in these polymers. There were, however, no resonances
at δ 37.2, 30.8, and 27.5, which would be indicative of
1,3-enchainments.14a,b
The methyl pentad resonances are clearly resolved in
each spectrum and, for the sample prepared at -78 °C,
the pentad distributions deviate from ideally atactic
(70% m dyads). An increase in the intensity of the
mmmm pentad (δ 21.7) is clearly apparent (Figure 4c),
indicating the presence of isotactic segments caused by
an only moderately effective stereoregulating mecha-
nism. Although the cationic catalysts formed from C
and D are chiral, careful inspection of the methyl mmmr
(δ 21.8) and mrmm (δ 20.7) pentads reveals that this
slight stereoregulation is caused by chain-end control
(mmmr:mrmm ≈ 1:1).10,11 Interestingly, however, as
the polymerization temperature is raised (Figure 4a,b),
the PP products show slight tendencies toward syndio-
tacticity (50-58% r dyads). Similar changes from iso-
to syndiotacticity with increasing temperature have
been attributed in other systems to the thermodynamic
aspects of the transition state during the 1,2-insertion
of an incoming monomer unit4g,w,12 and are probably not
a result of ligand influence at the metal center. Further
analyses of the pentad structures, as reported else-
where,10d also showed the absence of stereoblocks;
the atactic PP prepared is in all cases completely
random.
Such a high degree of regioerror incorporation is
unprecedented with metallocene catalysts, which nor-
mally produce PP containing ∼1% of isolated 2,1-
errors,14a,b although somewhat similar behavior has
been observed for supported CpTiCl3 and Cp*TiCl3
systems.14c,d Indeed, incorporation of 2,1-misinserted
units reduces the activity of some metallocene catalysts
Regior egu la r ity. Most Ziegler-Natta propylene
polymerizations proceed via 1,2 (primary)-incorporation
of monomer into the polymer-metal bond such that the
methylene carbon (CH2) of an inserting propylene
becomes bound to the metal; the resulting polymer
contains an isobutyl end group, although subsequent
â-hydrogen elimination chain transfer processes (see
below) result in most of the eventually isolated polymer
containing n-propyl end groups arising from 1,2-inser-
(13) Doi, Y. Macromolecules 1979, 12, 248. (b) Asakura, T. A.;
Nishiyama, Y.; Doi, Y. Macromolecules 1987, 20, 616. (c) Grassi, A.;
Zambelli, A.; Resconi, L.; Albizzati, E.; Mazzocchi, R. Macromolecules
1988, 21, 617.
(11) Note that the mmrm pentad needs to be corrected because of
overlap with the rrmr pentad. Since the rrmr signal is equal in
intensity to the mrrr signal, then mmrm ) (mmrm + rrmr) - mrrr.
(12) Erker, G.; Fritze, C. Angew. Chem., Int. Ed. Engl. 1992, 31,
199.
(14) Cheng, H. N.; Ewen, J . A. Makromol. Chem. 1989, 190, 1931.
(b) Guerra, G.; Longo, P.; Corradini, P.; Resconi, L. J . Am. Chem. Soc.
1997, 119, 4394. (c) Park, J . R.; Shiono, T.; Soga, K. Macromolecules
1992, 25, 521. (d) Uozumi, T.; Toneri, T.; Soga, K.; Shiono, T. Macromol.
Rapid Commun. 1997, 18, 9.

1506 Organometallics, Vol. 17, No. 8, 1998
Ta ble 4. P olym er iza tion of P r op ylen e in CH2Cl2
Ewart et al.
sample
catalyst
precursor
A (kg of PP/
((mol of Ti) h atm)
c
b
run no.a
temp (°C)
10-5Mw
Mw/Mn
yield (g)
20
21
22
23
24
25
26
27
Cp*TiMe3
Cp*TiMe2Cl
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
Cp*TiMe3
Cp*TiMe2Cl
Cp*TiMe2C6F5
Cp*TiMe2OC6F5
25
25
25
3.0
3.2
3.5
10.8
9.9
5.9
7.9
2.0
1.7
1.8
1.9
0.86
0.60
0.51
1.06
0.52
0.31
0.04
0.04
172
120
102
212
26
16
2
2
25
2.5
-78
-78
-78
-78
11.5c
21.3c
11.1c
10.2c
a
b
Polymerization time 5 min. All GPC curves are multimodel. c Values from GPC curve with highest Mw.
F igu r e 5. Block (a) and isolated (b) 2,1-insertions. Aster-
isks denote denotes the methine signals observed at δ 30.4
in Figure 4a,b.
immediately after a 2,1-insertion.16,17 Considering the
fact that the molecular weights and activities reported
here are relatively high for PP formed at 25 °C (Mw
≈
3 × 105, A ≈(2-4) × 105), the restrictions on 1,2-
insertions after a 2,1-regioerror must be considerably
lower for these half-sandwich complexes than in met-
allocene systems. It seems likely that the relatively
high propensity toward 2,1-insertion observed here
arises from the less crowded nature of the half-sandwich
complexes relative to metallocenes, thus permitting
sterically more demanding secondary insertions to occur
more readily.
The fact that the 13C NMR spectra of PP samples
prepared at -78 °C exhibit no resonances in the regions
δ 38.2-42.4, 32.2-34.7, and 12.4-15.6 (Figure 4c)
implies that 2,1-misinsertions do not occur at this
temperature. The reasons for this behavior are not
clear, but it seems likely that steric factors result in a
sufficiently high energy of activation for 2,1-insertion
that this process occurs at a relatively low rate at -78
°C. The systems described here thus differ from the
above-mentioned vanadium systems, which also give
regioirregular PP, but at all temperatures studied.13b
High-molecular-weight atactic PP is currently of
interest because of its elastomeric properties,18 and
different types of elastomeric PP (elPP) have been
discovered recently. For instance, the C2v symmetric
catalyst Me2Si(9-Flu)2ZrX2 (of type E) with MAO pro-
F igu r e 4. 13C{1H} NMR spectra of polypropylene made
using D in toluene at initiation temperatures of (a) 25 °C,
(b) 0 °C, and (c) -78 °C.
considerably, since insertion of propylene after a 2,1-
insertion occurs at a rate 100 times slower than that
after a 1,2-insertion,15 resulting in lower activities in
these systems. In addition, it has been shown that 2,1-
insertion is often followed by â-H elimination to give
2-butenyl groups (see below);16,17 since 2,1-regioerrors
within polymer chains are rarely found, it has thus
been inferred that chain termination normally occurs
(15) Busico, V.; Cipullo, R.; Corradini, P. Makromol. Chem. Rapid
Commun. 1992, 13, 15. (b) Busico, V.; Cipullo, R.; Corradini, P.
Makromol. Chem. Rapid Commun. 1992, 13, 21. (c) Busico, V.; Cipullo,
R.; Corradini, P. Makromol. Chem. Rapid Commun. 1993, 14, 97.
(16) J u¨ngling, S.; Mu¨lhaupt, R.; Stehling, U.; Brintzinger, H.-H.;
Fischer, D.; Langhauser, F. J . Polym. Sci., Part A 1995, 33, 1305. (b)
Tsutsui, T.; Kashiwa, N.; Mizuno, A. Makromol. Chem. Rapid Com-
mun. 1990, 11, 565.
(17) Resconi, L.; Piemontesi, F.; Camurati, I.; Balboni, D.; Sironi,
A.; Moret, M.; Rychlicki, H.; Zeigler, R. Organometallics 1996, 15, 5046.

Monocyclopentadienyltitanium Catalysts
Organometallics, Vol. 17, No. 8, 1998 1507
{1H} spectra discussed above, and analyses of the
olefinic end groups were now possible, although the
molecular weights were still much higher than those of
PP samples, for which accurate end group analyses have
been achieved elsewhere utilizing 13C{1H} NMR spec-
troscopy.
a
b
The olefinic region of the 1H NMR spectrum is shown
in Figure 7, where there are observed quite complex
resonances attributable to vinyl (δ 5.78, m; δ 4.99, m)
and vinylidene (δ 4.74, s; δ 4.67, s) end groups in
approximately equal amounts, as well as weaker inter-
nal olefin resonances at ∼δ 5.35. The resonances at ∼δ
5.35, attributed to 2-butenyl end groups17a,18 arising
from â-hydride elimination after a 2,1-insertion, are
weak, suggesting that the â-hydrogen elimination pro-
cess of Figure 6d does not occur to a significant extent.
Proportions of the three types of olefinic functionalities,
c
d
1
based on integrations of the H chemical shift data, are
F igu r e 6. Chain termination mechanisms involving a 1,2-
insertion followed by (a) â-H elimination from C-2, (b) â-Me
elimination from C-2, and 2,1-insertion followed by (c) â-H
elimination from C-1 (Me) and (d) â-H elimination from
C-3 (two modes).
listed in Table 3, and as can be seen, catalysts A-D all
produce very similar materials. While the vinylidene
resonances clearly provide firm evidence for the ex-
pected â-hydrogen elimination process of Figure 6a, it
is interesting to note the presence of relatively strong
resonances attributable to vinyl end groups. The latter
may arise either via the â-methyl elimination process
of Figure 6b or, following 2,1 insertion, via â-hydride
elimination from the methyl group (Figure 6c). The
formation of low-molecular-weight polypropylene using
highly substituted metallocenes has been attributed to
facile â-methyl elimination,19c,21 but no â-methyl elimi-
nation was evident using the less encumbered mixed
cyclopentadienyl system Cp*CpZrCl2/MAO, possibly for
steric reasons.10 No such steric argument can be
applied to the mono-Cp* systems investigated here,
however, since the number of vinyl end groups present
in PP samples prepared using both Cp*TiMe2Cl/
B(C6F5)3 and the more sterically demanding
Cp*TiMe2C6F5/B(C6F5)3 are the same (∼40%) within
experimental error. Thus, it is of interest to ascertain
whether â-methyl elimination is a factor in these
systems.
The 100.6 MHz13C{1H} NMR spectrum in the region
δ 10-50 is shown in Figure 8, where the effects of
greater chemical shift dispersion are obvious. Although
only pentad resonances of the higher molecular weight
materials could be resolved at 50.38 MHz, even at 120
°C, in this case hexad resolution of the methyl region
was easily achieved in spectra run at room temperature.
Of interest here, analysis of the spectrum revealed weak
resonances at δ ∼22.5, ∼23.5, and ∼25.2, which are
reasonably attributed to the methyl and methine reso-
nances of isobutyl end groups. These resonances were
clearly much more intense than those of the olefinic end
groups, which were not observed at all, and thus it
would seem that â-methyl elimination processes may
well occur to a significant extent. However, the unusu-
ally high complexity of the vinyl and vinylidene 1H
resonances suggest that the corresponding 13C reso-
nances would be complex multiplets and that individual
signals could be indistinguishable from baseline noise.
duces atactic elPP with molecular weights of >200 00018d
and random incorporation of isotactic blocks.18b,c In
contrast, the elastomeric PP produced by the Coates-
Waymouth C2v/C2 symmetric “oscillating” catalyst con-
tains isotactic blocks.18a In all of these cases, the degree
of regioerror incorporation is at the most 1%.
Ch a in Tr a n sfer Mech a n ism s. Identification of the
chain transfer processes involved in the termination of
polymer chain growth is an important aspect in the
control of molecular weights and polydispersities and
may in principle be effected by NMR analysis of the
unsaturated chain end groups. For polypropylene, the
main chain transfer mechanisms during group 4 met-
allocene/MAO polymerizations involve â-hydrogen trans-
fer to the metal or to a coordinated monomer unit,
resulting in a vinylidene end group (Figure 6a),18 chain
transfer to the Al cocatalyst, resulting in saturated end
groups following hydrolysis,19 and â-methyl elimination,
giving vinyl end groups (Figure 6b).20 Following 2,1-
insertions, â-hydride transfer may result in the forma-
tion of both vinyl (Figure 6c) and 2-butenyl end groups
(Figure 6d).17,18
Unfortunately, the molecular weights of the PP
samples discussed above were too high and the materi-
als were too insoluble for the weak end group resonances
to be identifiable by NMR spectroscopy. Samples of PP
of relatively low molecular weight (Mw ≈ 4 × 104, Mw/
Mn ≈ 5) and hence also of greater solubility were
therefore prepared at 70 °C, utilizing all four catalysts
A-D. 1H and 13C{1H} NMR spectra were obtained at
400.14 (Figure 7) and 100.6 (Figure 8) MHz, respec-
tively, higher frequencies than was the case for the 13C-
(18) Hauptman, E.; Waymouth, R. M.; Ziller, J . W. J . Am. Chem.
Soc. 1995, 117, 11586. (b) Gauthier, W. J .; Corrigan, J .; Taylor, N. J .;
Collins, S. Macromolecules 1995, 28, 3771. (c) Gauthier, W. J .; Collins,
S. Macromolecules 1995, 28, 3779. (d) Resconi, L.; J ones, R. L.;
Rheingold, A. L.; Yap, G. P. A. Organometallics 1996, 15, 998.
(19) Tsutsui, T.; Mizuno, A.; Kashiwa, N. Polymer 1989, 30, 428.
(b) Kaminsky, W.; Ahlers, A.; Moller-Lindenhof, N. Angew. Chem., Int.
Ed. Engl. 1989, 28, 1216. (c) Resconi, L.; Piemontesi, F.; Franciscono,
G.; Abis, L.; Fioriani, T. J . Am. Chem. Soc. 1992, 114, 1025.
(21) Eshuis, J . J . W.; Tan, Y. Y.; Teuben, J . H.; Renkema, J . J . Mol.
Catal. 1990, 62, 277. (b) Eshuis, J . J . W.; Tan, Y. Y.; Meetsma, A.;
Teuben, J . H. Organometallics 1992, 11, 362. (c) Mise, T.; Kageyama,
A.; Miya, S.; Yamazaki, H. Chem. Lett. 1991, 1525.
(20) Chien, J .; Wang, B. J . Polym. Sci., Polym. Chem. 1990, 28, 15.
(b) Resconi, L.; Bossi, S.; Abis, L. Macromolecules 1990, 23, 4489.

1508 Organometallics, Vol. 17, No. 8, 1998
Ewart et al.
F igu r e 7. 1H NMR spectrum (400 MHz) in the olefinic region of low-molecular-weight PP prepared at 70 °C.
F igu r e 8. 13C{1H} NMR spectrum (100.6 MHz) of a low-molecular-weight PP prepared at 70 °C.
Thus, the question concerning â-methyl elimination
processes perhaps remains moot.
termination induced by hydrolysis of Ti-CH2CHMe∼P
linkages.
As pointed out above, predominant â-hydrogen elimi-
nation would result in PP containing mostly n-propyl
end groups. Although a sharp singlet at δ 14.2 is
reasonably attributable to the n-propyl methyl reso-
nance, the 13C resonances of this group are largely
obscured by the main backbone methylene resonance
and the above-mentioned, broad resonances which
result from regioirregularities. Significant â-methyl
elimination would, however, result in chain transfer
processes involving 1,2-insertion of monomer into newly
formed Ti-Me bonds to give isobutyl end groups, as are
obtained in the initiation step (see above), with 13C{1H}
resonances at ∼δ 23 (Me) and ∼δ 25.5 (methine). If the
initiation step were the only process leading to isobutyl
end group formation, relatively few polymer chains
should contain isobutyl groups and the isobutyl 13C{1H}
resonances should be much weaker than those of the
olefinic end groups, which occur in all polymer chains.
If, however, all chain transfer processes involved â-m-
ethyl elimination, then the isobutyl and olefinic groups
should be present in comparable amounts, although
some of the former would also result from chain
It must be made clear that the conclusions reached
for PP prepared at 70 °C do not necessarily apply to
materials prepared at lower temperatures. Although
we can reasonably presume the identity of catalysts of
the type [Cp*TiMeE]+ at -78 °C, which produce PP
exhibiting 1H NMR spectra that contain just methylene,
methine, and methyl resonances (Figure 4c), the po-
lymerization chemistry at 0 °C and above is clearly
much more complicated, as attested to by Figure 4a,b
and the broadened, albeit unimodal, molecular weight
distributions (Table 2). Unfortunately, the very high
molecular weights and low solubilities of the polymeric
materials obtained at the lower temperatures prevented
their thorough characterization by NMR spectroscopy.
Solven t Effects on th e P olym er iza tion P r ocess.
There is compelling evidence that counterion interac-
tions can play a vital role in the activity of metallocene
catalysts, quite significant increases in activities of
metallocene systems being observed when, for instance,
[MeB(C6F5)3]- is replaced by the more weakly coordi-

Monocyclopentadienyltitanium Catalysts
Organometallics, Vol. 17, No. 8, 1998 1509
nating anion [B(C6F5)4]-.22 Catalysts incorporating
para-substituted fluoroarylborate counteranions exhibit
intermediate activities,3d and it is now clear that
displacement of counterion by olefin from the inner
coordination sphere of the metal cation can be non-
trivial.
these polymerizations may be more complex than was
first anticipated.
Na tu r e of th e Active Ca ta lytic Sp ecies. The
compounds Cp*TiMeE(µ-Me)B(C6F5)3 (E ) Me, C6F5,
OC6F5) are all precursors of the cationic species
[Cp*TiMeE]+ which, on the basis of a very extensive
literature,1,3-6 are reasonably believed to be the initia-
tors of polymer chain growth. However, organotita-
nium(IV) compounds are susceptible to reduction, and
there is now extensive evidence for the formation of
titanium(III) species, detected utilizing EPR spectros-
copy, in many olefin polymerization systems involving
monocyclopentadienyltitanium compounds. Thus, not
surprisingly, treatment of many monocyclopentadi-
enyltitanium(IV) compounds with alkylaluminum com-
pounds results in at least partial reduction to EPR-
active species.4k,24 However, treatment of compounds
of the type Cp*TiR3 with B(C6F5)3, expected to give only
[Cp*TiR2]+, has also been reported to yield EPR-active
titanium(III) species, some of which are found to behave
as olefin polymerization catalysts.4u,x In addition, con-
strained-geometry compounds of the type (η5-C5H4SiR2-
NR′)TiMe (R, R′ ) alkyl, aryl), in which η5 coordination
of the η5-C5H4SiR2NR′ ligand is reinforced by simulta-
neous coordination of the amide nitrogen atom to the
titanium(III), are also good catalysts for the polymeri-
zation of ethylene and 1-octene,25 and we have therefore
utilized EPR spectroscopy to assess the possibility that
titanium(III) species are involved in the catalytic system
discussed here.
Our EPR investigation has involved treatment of
Cp*TiMe3 with B(C6F5)3 in the solvents chlorobenzene
and toluene, in the absence and in the presence of
propylene under normal polymerization conditions. In
contrast to results reported elsewhere,4u we find that
the EPR spectrum of a reaction mixture containing
equimolar amounts of Cp*TiMe3 and B(C6F5)3 in chlo-
robenzene exhibited only a very weak doublet at g )
1.994 (A ) 8.4 G). The magnitude of the hyperfine
splitting implies that the resonance may reasonably be
attributed to the type of titanium(III) hydride reported
from the reaction of CpTi(OBu)3 with MAO,4k although
on the basis of the integrated intensity, the titanium-
(III) species amounted to much less than 0.01% of the
total titanium in solution. Repetition of this experiment
in a toluene solution saturated with propylene resulted
in the formation of copious amounts of polymer, as
described above, and the EPR spectrum now exhibited
either no titanium(III) resonance or a weak singlet at g
) 1.981. Although the absence of hyperfine coupling
to a hydride ligand may well be a result of olefin
insertion,4k it seems unlikely that major proportions of
the polymers formed by the catalyst systems described
here result from titanium(III) catalysis since, in at least
one case, an active polymerizing mixture was EPR
silent. However, in view of the somewhat broad mo-
lecular weight distributions in some cases, we cannot
rule out the possibility that some of the polymers formed
are generated by titanium(III).
There is, however, conflicting evidence concerning the
effects of solvent polarity on catalyst activity.23 Al-
though a solvent of higher dielectric constant (CH2Cl2,
ꢀ ) 9.08) is expected to promote separation of ion pairs
(eq 2), shifting the equilibrium to the right to a greater
extent than in toluene (ꢀ ) 2.38) and thus increasing
the catalyst activity, Chien et al. have reported negli-
gible solvent effects on the activities of the rac-Et-
(ind)2ZrCl2/MAO system23c and Oliva et al. have noted
a 100-fold increase in the activity of Cp2TiCl2 when
activated with AlMe3/AlMe2F23a on going from toluene
to CH2Cl2, while Fink et al. have found a 15-fold
increase in activity of a related zirconocene/MAO
system.23b
We have therefore repeated the propylene polymer-
izations in CH2Cl2 under the same conditions used for
polymerizations in toluene, in part because we have
spectroscopically characterized the catalysts A-D at
-50 °C in CD2Cl2, and polymerization in the same
solvent would provide strong evidence that the cationic
species [Cp*TiMeE]+ do indeed bind and activate mono-
mers during polymerization. Indeed, for both solvents
at -78 °C, we find that the amount of polymer produced
generally decreases as E becomes more electron-
withdrawing, i.e. as the strength of the Ti+-MeB-
-
(C6F5)3 interaction increases.7 However, the amount
of polymer obtained in toluene is actually higher than
that obtained in CH2Cl2, in which ion-pair dissociation
is presumably more facile. While we have previously
shown that [CpTiMe2]+ coordinates toluene in an η6-
fashion,5a,b possibly contributing to disruption of the
interaction between the metal center and the counterion
and thus resulting in a greater activity than one would
otherwise expect, toluene coordination could also be
expected to result in lowered polymerization activity
because of competition for coordination sites by solvent.
Another factor, difficult to assess, is the probability that
those systems exhibiting broad molecular weight dis-
tributions involve multiple catalytic sites, possibly of
varying lifetimes.
Possible evidence for mechanistic similarities in the
two solvents, however, is that the NMR spectra of the
PP (13C{1H} NMR) formed at 25 °C in CH2Cl2 are very
similar to those of the PP produced in toluene. How-
ever, while polymerizations are complete within 5 min
in CH2Cl2 under the same conditions which require 20
min in toluene, the activities of the catalysts A-D in
CH2Cl2 are actually less than in toluene after correcting
for the increased solubility of propylene in CH2Cl2
compared to toluene. Therefore, there does not appear
to be an increase in the activities of the catalysts A-D
in the more polar solvent, and the role of toluene in
(24) Yim, J .-H.; Chu, K.-J .; Choi, K.-W.; Ihm, S.-K. Eur. Polym. J .
1996, 32, 1381. (b) Xu, G.; Lin, S. Macromolecules 1997, 30, 685. (c)
Grassi, A.; Lamberti, C.; Zambelli, A.; Mingozzi, I. Macromolecules
1997, 30, 1884. (d) Xu, J .; Zhao, J .; Fan, Z.; Feng, L. Macromol. Rapid
Commun. 1997, 18, 875.
(25) Rosen, R. K.; Nickias, P. N.; Devore, D. D.; Stevens, J . C.;
Timmers, F. J . U.S. Patent 5374696, 1994.
(22) Chien, J . C. W.; Song, W.; Rausch, M. D. J . Polym. Sci., Polym.
Chem. 1994, 32, 2387.
(23) Longo, P.; Oliva, L.; Grassi, A.; Pellecchia, C. Makromol. Chem.
1989, 190, 2357. (b) Herfert, N.; Fink, G. Makromol. Chem. 1992, 193,
773. (c) Vizzini, J . C.; Chien, J . C. W.; Babu, G. N.; Newmark, R. A. J .
Polym. Sci., Polym. Chem. 1994, 32, 2049.

1510 Organometallics, Vol. 17, No. 8, 1998
Ewart et al.
Atmospheres glovebox and dried, thoroughly deoxygenated
solvents. Polymer samples for high-temperature NMR studies
were prepared by dissolving ca. 200 mg of polymer in 2 mL of
o-dichlorobenzene in a 10 mm o.d. NMR tube. The 13C{1H}
spectra were referenced with respect to external TMS using
the residual carbons of internal diglyme (5 mm o.d. coaxial
tube placed within the sample) and were run at 120 °C on a
Bruker CXP-200 NMR spectrometer operating at 50.38 MHz;
5 000-10 000 transients were collected for each spectrum. For
To our surprise, we observe no evidence in toluene
for the species at g ) 1.979(3) and g ) 1.977(3) reported
elsewhere as arising from reaction of Cp*TiMe3 with
B(C6F5)3 in chlorobenzene.4u Further experiments will
hopefully shed light on this apparent anomaly.
Con clu sion s
The compound Cp*TiMe2(µ-Me)B(C6F5)3 and the chiral
compounds Cp*TiMe(C6F5)(µ-Me)B(C6F5)3, Cp*TiMe-
(OC6F5)(µ-Me)B(C6F5)3, and Cp*TiMeCl(µ-Me)B(C6F5)3
all polymerize ethylene to high-molecular-weight poly-
ethylene and propylene to atactic, elastomeric polypro-
pylene (PP). In all cases, the catalytically active species
are believed to be titanium(IV) complexes of the type
[Cp*TiMeE]+ (E ) Me, Cl, C6F5, OC6F5), but minor
contributions from related titanium(III) complexes can-
not be ruled out since very low concentrations of
titanium(III) species are sometimes detected via EPR
spectroscopy. Although the compounds Cp*TiMeE(µ-
Me)B(C6F5)3 (E ) C6F5, OC6F5), containing the more
electron-withdrawing ligands, were expected to be the
better catalysts, the anticipated results were only
partially achieved. While PP molecular weights are
higher with these catalysts, the activities are actually
lower because the greater Lewis acidity of the cationic
species [Cp*TiMeE]+ results in stronger coordination of
the [BMe(C6F5)3]- anion. Although comparisons of
activities with data for similar systems are difficult
because of differences in experimental procedures, the
compounds investigated here are definitely more active
than is CpTiCl3-MAO and probably comparable with
Cp*TiCl3-MAO and a series of MAO-activated [(2-
(dimethylamino)ethyl)cyclopentadienyl]titanium com-
pounds containing a substituted cyclopentadienyl group
incorporating a donor group in a side chain.4v
NMR analyses of the PP formed show that initiation
involves 1,2-insertion into a Ti-Me bond, while propa-
gation involves primarily head-to-tail 1,2-insertions. An
unusually high proportion of the propagation steps
involve 2,1-misinsertions but essentially no 1,3-enchain-
ment. The major olefinic end groups are vinylidene
(CH2dCMe-), resulting from â-hydrogen transfer fol-
lowing a 1,2-insertion, and vinyl (CH2dCH-), resulting
from â-hydrogen transfer from the methyl group fol-
lowing a 2,1-insertion or, more likely, â-methyl transfer
following a 1,2-insertion. Small amounts of internal
olefins are also formed via â-hydrogen transfer following
a 2,1-insertion. The indiscriminate nature of the pro-
pylene polymerization relative to catalysis by metal-
locenes appears to be a result of the relatively un-
crowded nature of the inner coordination sphere of the
mono-Cp* catalysts. The sterically rather demanding
transition state for 2,1-insertion can be achieved much
more readily than is the case for metallocene systems,
where sterically less demanding 1,2-insertion steps
predominate.
1
end group analyses of the PP prepared at 70 °C, H and 13C-
{1H} spectra were run at room temperature in CDCl3 on a
Bruker AM-400 spectrometer operating at 400.14 and 100.6
MHz, respectively. The NMR samples were prepared by
dissolving ca. 100 mg of polymer in a 5 mm NMR tube in 0.7
mL of CDCl3 and were referenced to external TMS using the
residual protons or carbons of the solvent. Assignments were
based on both the literature precedents cited and on model
compounds.26 EPR experiments involving equimolar amounts
of Cp*TiMe3 and B(C6F5)3 (0.01-0.06 mmol of each in 10 mL
solution) were run on a Bruker R-B70 EPR spectrometer at
∼9.7 GHz with the field centered at ∼3400 G; the magnetic
field was calibrated with DPPH (g ) 2.0037), and standard
solutions of TEMPO (1 × 10-3-1 × 10-5 M) were used for
quantitative estimates of the concentrations of titanium(III)
species present. Gel permeation chromatograms (GPC) of the
polymers were obtained at 145 °C in 1,2,4-trichlorobenzene
using a Waters Model 150-C GPC or in THF at room temper-
ature using a Waters Model 440 liquid chromatograph; data
were analyzed using polystyrene calibration curves. The
compounds Cp*TiMe3,27 Cp*TiMe2Cl,27 Cp*TiMe2OC6F5,7
7
28
Cp*TiMe2C6F5 and B(C6F5)3 were prepared according to
literature procedures.
P olym er iza tion of P r op ylen e a n d Eth ylen e. In
a
typical procedure, 0.06 mmol of catalyst precursor (Cp*TiMe2E,
E ) Me, Cl, C6F5, OC6F5) was dissolved in 5 mL of solvent
which was saturated with monomer. The solution was then
treated with B(C6F5)3 (0.06 mmol, 31 mg) in 5 mL of solvent.
Bubbling of monomer through the catalyst solution was
continued throughout the polymerization for a further 5-20
min before quenching with an excess of methanol. The
polymer samples were washed with methanol and dried under
reduced pressure for 16 h. In experiments performed at -78
°C, 2 mL of propylene was cannulated into the solution of
catalyst precursor, and the cocatalyst was added in 4 mL of
solvent.
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada (Research
and Strategic Grants to M.C.B. and Graduate Scholar-
ship to S.W.E.) and Alcan (Graduate Scholarship to
S.W.E.) for financial support. We also thanks Professor
J . K. S. Wan for allowing us time on the Bruker RB-70
EPR spectrometer.
OM970522W
(26) Pouchert, C. J .; Behnke, J . The Aldrich Library of 13C and 1H
FT NMR Spectra; 1st ed.; Aldrich: Milwaukee, WI, 1993; Vol. 1.
(27) Mena, M.; Royo, P.; Serrano, R.; Pellinghelli, M. A.; Tiripicchio,
A. Organometallics 1989, 8, 476.
Exp er im en ta l Section
All experiments were carried out under nitrogen (prepurified
grade) using standard Schlenk line techniques, a Vacuum
(28) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245.