Extremely narrow-dispersed high molecular weight polyethylene from living polymerization at elevated temperatures with o-F substituted Ti enolatoimines

Article abstract of DOI:10.1021/ja805304p

With a new titanium imineenolato complex, ethylene is polymerized in an unprecedented living fashion affording a polymer with a high molecular weight (M_n > 105 g mol^-1) and an extremely narrow distribution (M_w/M_n 1.01) at the same time; the living character is also retained even at an elevated temperature of 75 °C, and blockcopolymers are accessible. Copyright

Full text of DOI:10.1021/ja805304p

Published on Web 09/12/2008
Extremely Narrow-Dispersed High Molecular Weight Polyethylene from Living
Polymerization at Elevated Temperatures with o-F Substituted Ti
Enolatoimines
Sze-Man Yu and Stefan Mecking*
UniVersity of Konstanz, Chair of Chemical Materials Science, Department of Chemistry, UniVersita¨tsstr. 10,
D-78457 Konstanz, Germany
Received July 9, 2008; E-mail: stefan.mecking@uni-konstanz.de
Living polymerization techniques are an essential tool for the
synthesis of polymers with controlled architectures, such as
monodisperse polymers or blockcopolymers.1 Polymerization in a
living fashion requires the essential absence of chain termination
and irreversible transfer. In olefin polymerization, chain transfer
(occurring by ꢀ-hydride transfer or transfer to cocatalyst) and
termination can be suppressed by polymerization at low tempera-
tures.2 This, however, results in low catalyst activities and limited
molecular weights attainable especially for semicrystalline polymers.
Therefore, considerable effort has been directed at the development
of new catalysts. Albeit a significant number of catalyst systems
with living polymerization characteristics at ambient or elevated
temperature have been reported,3-10 the synthesis of linear
Figure 1. ORTEP plot of the molecular structure of 3a. Ellipsoids are
shown with 50% probability. The H-atoms are not shown for clarity.
polyethylene (PE) with a high molecular weight and truly narrow
distribution remains a challenge.
Phenoxyimine Ti complexes with o-fluorine substitution of the
N-aryl moiety, discovered by Fujita et al., are highly active versatile
catalysts for, e.g., the synthesis of narrowly distributed (ultra high
molecular weight) PE and of block copolymers.3 The o-F substit-
uents have been suggested to suppress chain transfer by interaction
with the ꢀ-hydrogen atoms of the growing PE chain.3,11 At 25 °C
linear PE with Mw/Mn as low as 1.05 was obtained, when a short
reaction time (3 min) and diluted ethylene (0.04 atm) were
employed for this very reactive system. The PE formed with this
Scheme 1. Synthesis of Complexes 3a and 3b
narrow distribution has a molecular weight of several 104
mol-1 3b,c,f
Living characteristics can be retained for longer reaction
g
Activation of 3a-c with an MAO cocatalyst affords ethylene
polymerization catalysts (Table 1). Astonishingly, 3a affords high
molecular weight linear PE with an extremely narrow molecular
weight distribution of Mw/Mn 1.01 at room temperature (entry 2;
cf. Supporting Information (SI) for GPC trace and details of
molecular weight determination). The polymer molecular weight
determined agrees within experimental error with the theoretical
value calculated from the amount of polymer obtained and catalyst
precursor employed. This shows that the catalyst precursor is rapidly
converted to a single type of active species, chains are initiated
simultaneously, and essentially no chain transfer or termination
occurs. The catalyst is highly stable during ethylene polymerization.
After 1 h of living polymerization, narrowly distributed ultrahigh
molecular weight polyethylene is obtained (entry 7).15 By contrast,
a comparatively much broader polydispersity (Mw/Mn 1.45) is
observed with the nonfluorinated 3c (entry 1). The even broader
molecular weight distribution obtained with 3b (R ) CH3, entry
3) supports the fact that the living behavior of 3a is not due to a
steric effect of the o-fluorine substituents. Interestingly, relatively
high molecular weights are obtained with 3b and 3c which exceed
the calculated molecular weights for the hypothetical case that the
polymerization was living (vide infra). Considering also the broad
molecular weight distributions, this suggests that by contrast to the
o-fluorinated 3a the catalyst precursors 3b and 3c are inefficiently
converted to the active species and that actually, by comparison to
.
times also at 50 °C, as evidenced by a linear increase of molecular
weight with time to afford PE with, e.g., Mw/Mn 1.19, Mn 1.4 ×
105 g mol-1 after 15 min of polymerization.3a For complexes with
other N,O-chelating ligands, however, no living polymerization was
observed to result from o-F substitution,12,13 and living polymer-
ization occurs also for nonfluorinated N,O-chelated Ti complexes.3e,4
This prompted us to study the effect of o-F substitution in a different
type of complex to shed further light on this issue.
Ti(IV) enolatoimine complexes [(N∧O)2TiCl2] substituted in the
2,6-position of the N-aryl moiety were prepared in good yields
(70%) by reaction of the corresponding ketoenamines with
[Ti(NMe2)2Cl2] and chlorination of the resulting mixed amido-
chloro intermediate with BCl3 (Scheme 1; reaction of the Li salts
of the ketoenamines with TiCl4 did not afford the desired
compounds). The unsubstituted analogue 3c (R ) H), which is
known to polymerize ethylene in a nonliving fashion, was prepared
for comparison as previously reported.14
In the solid state, 3a possesses a C2-symmetric, distorted
octahedral geometry (Figure 1). The two oxygen atoms are oriented
trans to one another (O-Ti-O angle 164°), while the two nitrogen
atoms (N-Ti-N 92°) and the two chlorine atoms are positioned
in a cis fashion. The latter is a prerequisite for polymerization
catalysis.
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10.1021/ja805304p CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Results of Ethylene Polymerization with 3a-ca
b
T
[°C]
time
[min]
TOF
[mol(C2H4) mol(Ti)-1 h-1
polymer yield
[g]
Tm
[°C]
Mn (calcd)
Mw/Mnc
entry
complex
Mnc [104 g mol-1
]
]
[104 g mol-1
]
1
2
3
4
5
6
7
3c
3a
3b
3a
3a
3a
3a
25
25
25
0
50
75
25
15
15
15
15
15
15
60
21 400
37 100
14 300
30 000
42 900
42 900
36 100
0.15
0.26
0.10
0.21
0.30
0.30
1.01
138
139
139
140
139
139
137
26
27
19
19
32
26
99
(15)
26
(10)
21
30
30
1.45
1.01
2.62
1.13
1.03
1.15
1.17
100
a Reaction conditions: 1 atm of ethylene pressure, 1 µmol of catalyst, toluene 250 mL, 2000 equiv of MAO (molar ratio). b Measured by DSC.
c Determined by GPC referenced to linear PE (cf. SI for details).
substituents also affect the activation reaction, which is effective only
for 3a but not for analogues lacking o-fluorine substitution.
Acknowledgment. Financial support by the DFG (Me 1388/
3-3) is gratefully acknowledged. S.M. is indebted to the Fonds der
Chemischen Industrie. We thank Lars Bolk (Konstanz) and Dieter
Lilge (Basell, Frankfurt) for GPC analysis and Hans Brintzinger
and Inigo Go¨ttker-Schnetmann for fruitful discussions.
Supporting Information Available: Preparative procedures and
characterization data, X-ray data for 3a. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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Figure 2. Plots of Mn and Mw/Mn versus time for ethylene polymerization
at 75 °C with 3a (reaction conditions as in entry 6).
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the o-fluorinated analogue, the intrinsic rate of chain growth is
higher in the active species, which are prone to chain transfer.
The effect of polymerization temperature was studied (entries
2, 4, 5, and 6). The extremely narrow molecular weight distribution
is retained at an elevated temperature of 50 °C. PE with Mw/Mn
1.03 and Mn 3.2 × 105 g mol-1 is obtained after 15 min of
polymerization (entry 5). Remarkably, molecular weight control is
retained even at 75 °C in polymerization with 3a (entry 6).
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25, 50, and 75 °C (Figure 2 and SI). A linear increase of Mn with
polymerization time and narrow molecular weight distributions were
observed for all runs, thus corroborating the highly controlled nature
of the polymerization.
By comparison to the well studied highly active phenoxyimine
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copolymers, with a hard PE block and a soft atactic polypropylene
block. After complete ethylene consumption at 25 °C, addition of
propylene afforded a block copolymer with apparent Mn 1.9 × 105
g mol-1, Mw/Mn 1.12, PE content 60 wt %. Notably, the living
character is also retained at 50 °C (Mn 2.4 × 105 g mol-1, Mw/Mn
1.12, PE content 40 wt %. For details, cf. SI).
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In conclusion, the conveniently accessible novel enolatoimine
Ti complex 3a upon MAO activation polymerizes ethylene in a
living fashion with unprecedented molecular weight control and
temperature stability. This provides viable access to high molecular
weight PE with an extremely narrow molecular weight distribution
(Mw/Mn 1.01) and corresponding block copolymers. In addition to
phenoxyimine Ti complexes, this is another example of a system in
which o-F substitution appears beneficial for living polymerization of
ethylene as well as propylene. Our studies indicate that these
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(15) DSC (SI) shows typical behaviour of UHMWPE. In the first heating trace
a narrow melt transition (144 °C) and high crystallinity (76%) are observed
for the nascent polymer. In the second heating cycle, crystallinity is much
lower (49%) due to entanglement hindering recrystallization from the melt.
JA805304P
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J. AM. CHEM. SOC. VOL. 130, NO. 40, 2008 13205