New phenoxyketimine titanium complexes: Combining isotacticity and living behavior in propylene polymerization

Article abstract of DOI:10.1021/ja044268s

A series of titanium bis(phenoxyketimine) olefin polymerization catalysts were synthesized and screened for propylene polymerization. The phenoxyketimine ligands contain pentafluorophenyl N-aryl groups and ortho-phenol substituents of varying size. Cataly

Full text of DOI:10.1021/ja044268s

Published on Web 11/26/2004
New Phenoxyketimine Titanium Complexes: Combining Isotacticity and
Living Behavior in Propylene Polymerization
Andrew F. Mason and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853-1301
Received September 21, 2004; E-mail: gc39@cornell.edu
Scheme 1. Olefin Polymerization Using Phenoxyketimine
Catalysts
Research focusing on homogeneous olefin polymerization ca-
talysis has proliferated in recent years. In addition to metallocene-
type catalysts, transition metal active sites are now supported by
ancillary ligand frameworks incorporating a wide variety of
structural motifs.1 Homogeneous catalyst systems offer both the
option to use ligand geometry for precise control of polymer
stereochemistry and the potential to use living catalysts to synthesize
block copolymers. Many catalysts can generate highly isotactic
polypropylene (iPP),2 and there are a number of living catalysts
for propylene polymerization.3 Despite 50 years of intense research
since Natta’s synthesis of iPP,4 living isospecific propylene
polymerization catalysts remain unreported. Such catalysts could
be used to synthesize iPP block copolymers that have valuable
applications as compatibilizers and thermoplastic elastomers. Sita5
and Kol6 have developed catalysts for the living, highly isospecific
polymerization of 1-hexene. Busico has reported that Kol-type
catalysts are controlled for propylene polymerization and have been
used to generate iPP-block-PE (PE ) polyethylene) copolymers.7
Bis(phenoxyimine)titanium catalysts were originally reported for
the polymerization of ethylene,8a but later studies showed that they
had promise for syndiospecific propylene polymerization behavior.8b-e
If the phenoxyimine (PHI) ligands contain ortho-fluorinated N-aryl
groups and bulky ortho-phenol substituents (R1, see Scheme 1),
then the resulting catalysts are active for living, highly syndiospe-
cific propylene polymerization despite being C2-symmetric both
in solution and in the solid state.8d,e Ligand isomerization between
monomer insertions to give enantiomeric sites is believed to cause
this unusual stereocontrol, which is dictated by the chain end of
the polymer.8c,9 As the size of R1 is reduced for this class of aldimine
(R3 ) H) PHI catalysts, syndiotacticity decreases.8e Fujita and co-
workers have been able to generate iPP using zirconium and
hafnium PHI complexes with alkylaluminum/borate activators.10
The PHI ligands react with the aluminum co-catalyst under these
conditions; activation of the same complexes with methylalumi-
noxane (MAO) yields atactic, regioirregular polypropylene. We
have previously reported a series of titanium phenoxyketimine
) H) complex 6 can be compared to complex 4. When activated
with MAO, 1-6 are active catalysts for propylene polymerization
(Table 1). Catalysts 2-4 produce PPs that are substantially isotactic.
All catalysts except for 1 form PP with narrow molecular weight
distributions indicative of living behavior (Mw/Mn e 1.17). As with
aldimine catalysts, activity increases as the size of R1 decreases.
The stereoerrors present in the PP 13C NMR spectra (mmmr, mmrr,
and mrrm pentads, 2:2:1 ratio; Figure 1) are consistent with an
enantiomorphic site-control enchainment mechanism. This contrasts
previously reported syndiospecific PHI catalysts, which govern
polymer stereochemistry by chain-end control.8c Polypropylenes
produced by catalysts 2-4 are not completely regioregular: the
NMR spectra contain small peaks arising from head-to-head and
tail-to-tail mis-insertions (<5%). There are no olefin peaks in the
NMR spectra, suggesting that â-H and â-alkyl elimination reactions
do not occur during the polymerization.
Of the catalysts tested, 4 produces PP with the highest isotacticity
and was used to further investigate catalyst behavior. To demon-
strate the living behavior of 4, we carried out propylene polymer-
izations from 1 to 10 h at 0 °C (Figure 2). The molecular weight
increased linearly with polymer yield, and Mw/Mn values were
consistently narrow for all polymerizations. Another requirement
for living catalysts is the ability to synthesize monodisperse block
copolymers via sequential monomer addition. To demonstrate this
ability, we synthesized an iPP-block-E/P (E/P ) ethylene/propylene
copolymer) diblock copolymer using 4. Narrow molecular weight
distributions (both Mw/Mn ) 1.10) were measured for both the initial
iPP block (Mn ) 28.1 kg/mol) and the final diblock product (Mn )
62.0 kg/mol).
The effect of temperature on polymerization behavior was also
investigated (Table 1). Activity and molecular weight both increased
with increasing temperature, and molecular weight distributions
remained narrow. At low temperatures, isotacticity increased but
catalyst activity was greatly reduced. Preliminary R,ω-diene cy-
clopolymerization experiments with 4/MAO support a 2,1-insertion
mechanism, as with other PHI catalysts.12
Catalysts 1-5 display polymerization behavior significantly
different than that of previously reported PHI catalyst systems. The
addition of a seemingly remote R3 substituent shifts polymer
stereochemistry from slightly syndiotactic to isotactic. We propose
that the R3 group is bulky enough to prevent catalyst racemization,
t
(R3 * H) catalysts with bulky ortho substituents (R1 ) Bu) for
the living polymerization of ethylene.11 In this communication, we
report a living titanium catalyst that produces crystalline, predomi-
nantly isotactic PP.
t
Although ketimine catalysts with R1 ) Bu (1) are active for
living ethylene polymerization, they are sparingly active and
generally not well behaved for propylene polymerization. We
reasoned that smaller ortho substituents might reduce steric conges-
tion around the active metal center and result in active catalysts
for the formation of stereoregular polypropylene. Complexes 1-6
contain ligands with N-pentafluorophenyl groups and were syn-
thesized using previously reported procedures.8d,11 Ketimine com-
plexes 1-5 have R3 ) Ph and have ortho-phenol substituents that
t
decrease in size from R1 ) Bu to R1 ) H. Phenoxyaldimine (R3
9
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J. AM. CHEM. SOC. 2004, 126, 16326-16327
10.1021/ja044268s CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Propylene Polymerization Data for Catalysts 1-6/Methylaluminoxanea
Tacticitye
1
c
d
catalyst
methodb
Trxn
(°
C)
yield (g)
TOF (h-
)
M
n (g/mol)d
Mw/Mn
[m4]
[r4]
Rf
Tg
(
°
C)g
Tm (°
C)g
1h
2
3
A
A
A
A
A
A
B
B
B
B
0
0.16
0.13
0.40
1.11
1.22
2.05
0.44
0.17
0.32
1.23
9
35
bimodal
2710
broad
1.12
1.17
1.11
1.12
1.13
1.15
1.16
1.13
1.10
0.07
0.46
0.45
0.53
0.08
<0.01
0.61
0.54
0.48
0.27
0.26
<0.01
<0.01
<0.01
0.13
NAk
0.85
0.85
0.89
NAk
NAk
0.91
0.89
0.85
0.77
-8.6
-18.4
-14.1
-12.5
-5.5
NDl
NDl
NDl
69.5
NDl
NDl
96.4
70.1
NDl
NDl
0
0
0
0
0
105
293
323
1690
41
138
267
967
7290
4
27940
35440
123100
33700
13580
16760
59370
5
6i
4j
4
0.22
-2.9
-20
<0.01
<0.01
<0.01
0.02
-12.9
-13.8
-12.0
-9.2
0
4
4
20
50
a General conditions: catalyst in toluene (5 mL) was added to a propylene-saturated PMAO-IP solution (100 mL of toluene; [Al]/[Ti] ) 150) for 3.0 h.
b Method A: 0.03 mmol of catalyst, reactor pressure maintained at 30 psi during polymerization. Method B: 0.01 mmol of catalyst, closed reactor to
maintain a constant initial [propylene] at different temperatures. c Turnover frequency (TOF): mol propylene/(mol Ti‚h). d Determined using gel permeation
chromatography in 1,2,4-C6H3Cl3 at 140 °C versus polyethylene standards. e Determined by integration of the methyl region of the 13C NMR spectrum.
f Enantiofacial selectivity parameter, calculated from the 13C NMR spectrum using the equation [m4] ) R5 + (1 - R)5. g Determined using differential
h
j
scanning calorimetry (2nd heating). trxn ) 15.0 h. i 0.01 mmol 6. trxn ) 24.0 h. k Not applicable; data does not fit site control statistics. l None detected.
mers and continue to investigate the influence of ligand substituent
effects on polymer tacticity.
Acknowledgment. G.W.C. gratefully acknowledges support
from the Packard and Sloan Foundations, as well as support from
Mitsubishi Chemicals. The authors thank Dr. S. Reinartz for
synthesizing complex 1. This research made use of the Cornell
Center for Materials Research Shared Experimental Facilities
supported through the NSF MRSEC program (DMR-0079992).
Supporting Information Available: Catalyst synthesis and char-
acterization, X-ray data for 4, ethylene polymerization data, and block
copolymer characterization. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 1. 13C NMR (1,1,2,2-C2D2Cl4, 125 MHz, 100 °C) of isotactic PP
formed by 4/MAO at 0 °C. Unmarked shifts are due to regioinversions.
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