A C2-symmetric, living α-diimine Ni(II) catalyst: Regioblock copolymers from propylene

Article abstract of DOI:10.1021/ja0540021

An isomerically pure, C₂-symmetric Ni(II) complex was synthesized and found to be living for propylene polymerization. The regioselectivity of the catalyst varied greatly with reaction temperature, with regioregular, highly isotactic polypropyl

Full text of DOI:10.1021/ja0540021

Published on Web 09/15/2005
A C2-Symmetric, Living r-Diimine Ni(II) Catalyst: Regioblock Copolymers
from Propylene
Anna E. Cherian, Jeffrey M. Rose, Emil B. Lobkovsky, and Geoffrey W. Coates*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853-1301
Received June 16, 2005; E-mail: gc39@cornell.edu
Scheme 1. Synthesis of rac-1 and Polymerization of Propylene
The development of new methods for the synthesis of polyolefins
of defined molecular weight, stereochemistry, and comonomer
composition continues to be an important frontier in the field of
polymer synthesis. Early transition metal catalysts have been
developed that give precise control over polymer stereochemistry1
and are, in some cases, living;2 however, they are generally inferior
at polymerizing functional alkenes. In contrast, late transition metal
catalysts are more functional group tolerant and can also show living
behavior, but they generally produce amorphous, atactic polymers.3
Given the promise of Ni(II) and Pd(II) R-diimine catalysts originally
reported by Brookhart for living and functional alkene polymeri-
zation,3,4 we began to develop chiral ligands in an attempt to
promote isotacticity in these systems.5 We recognized the challenge
in this approach, as these catalysts generally make amorphous
polymers due to both poor regioselectivity and chain-end isomer-
ization during polymerization.6 In fact, at low temperatures, the
growing chain end can induce syndiotactic enchainment in the case
of propylene.7 We have reported a new class of chiral anilines and
their incorporation in R-diimine Ni(II) catalysts that exhibit
isoselectivity in trans-2-butene polymerization, but no regio- or
stereocontrol in propylene polymerization.8 Herein we report a C2-
symmetric Ni(II) catalyst that exhibits both living behavior and
isospecificity in the polymerization of propylene.
above -40 °C. We believe the active centers are stable to chain
termination below this temperature as well, but Mw/Mn values
increase due to polymer precipitation. To demonstrate the living
behavior of rac-1 at 0 °C, we carried out propylene polymerizations
over a 6 h period (Figure 1). Molecular weights increased linearly
with polymer yield, and Mw/Mn values were consistently narrow
for all polymerizations.
The microstructure of the PP changes significantly with Trxn. At
-78 °C, the polymer is both regioregular and highly isotactic.
Analysis of the methyl region of the 13C NMR spectra of the iso-
PPs made below -40 °C shows a predominant mmmm peak, with
errors which are inconsistent with typical stereoerrors (mmmr or
mmrr), but rather appear to be regioerrors due to 3,1-insertions1a
(Figure 2). With Ni(II) catalysts, this error occurs by a 2,1-insertion
with subsequent chain-walking, yielding -(CH2)3- linkages (3,1-
insertions) in the polymer backbone. While these are minor errors
when Trxn is very low, the percentage of 3,1-insertions increases
with increasing Trxn up to 56.2% at Trxn ) 22 °C. In fact, this
polymer with methyl branches more closely resembles poly-
(ethylene-co-propylene) than polypropylene, with Tg ) -59.9 °C.
At Trxn ) -78 °C, Tg and Tm values are consistent with iso-PP.
This dramatic change in polymer structure provides a unique
Condensation of 2 equivalents of rac-4-methyl-2-(1-(2,4,6-
trimethylphenyl)ethyl)aniline with acenaphthenequinone yields one
isomerically pure C2-symmetric R-diimine ligand in 78% isolated
yield. Interestingly, the reversibility of imine formation and the
lower solubility of the racemic ligand isomer give a nonstatistical
4
stereochemical mixture. Metalation with (dimethoxyethane)NiBr2
followed by crystallization gave complex rac-1 in 79% yield
(Scheme 1). The molecular structure of rac-1 was determined by
single-crystal X-ray diffraction (Scheme 1); the complex exhibits
crystallographic C2-symmetry, with the mesityl groups stacked
above and below the acenaphthene backbone. This conformation
appears to be the sole solution structure of the complex based on
1H NMR spectroscopy. Although rac-1 shows very low activity
and stereoselectivity for trans-2-butene polymerization in the
presence of methylaluminoxane (MAO), it is active for the
polymerization of both ethylene and propylene.
The regiochemistry of polypropylene (PP) produced by rac-1/
MAO shows an interesting temperature dependence (Scheme 1).
At low temperature (-78 °C), the polymer is regioregular and
highly isotactic (iso-PP); at room temperature (22 °C), the polymer
is completely amorphous and regioirregular (rir-PP). We therefore
chose to study the effect of the polymerization temperature (Trxn
)
in more detail. As Trxn decreased from 22 to -78 °C, turnover
frequency (TOF) decreased from 966 to 1 h-1. Number average
molecular weight (Mn) data corresponded well to theoretical values
for a living polymerization. Further support for living polymeri-
zation behavior is the narrow Mw/Mn values (e1.11) at temperatures
Figure 1. Plot of PP Mn (O) and Mw/Mn (9) versus yield using rac-1/
MAO at 0 °C, determined by gel-permeation chromatography at 140 °C
(polyethylene standards).
9
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J. AM. CHEM. SOC. 2005, 127, 13770-13771
10.1021/ja0540021 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Propylene Polymerization Data for rac-1/Methylaluminoxanea
c
f
f
Trxn
C)
propylene
(g)
time
(h)
yield
(mg)
TOFb
Mn
Tg
C)
Tm
C)
1
c
entry
(
°
(h-
)
(g/mol)
Mw/Mn
[CH3]/[CH2]d
% 3,1e
(
°
(
°
1
2
3
4
5
6
22
0
-20
-40
-60
-78
5
5
5
5
15
15
1.0
2.0
6.0
24
48
96
690
550
445
280
110
73
966
385
104
16
3
57 100
48 200
31 500
19 400
9 000
1.11
1.08
1.09
1.10
1.34
1.37
0.21
0.31
0.48
0.67
0.80
1.00
56.2
43.1
26.8
13.9
7.6
-59.9
-54.4
-43.0
-27.0
-14.3
-0.5
NDg
NDg
NDg
68.8
129.7
137.3
1
5 700
0.0
a Polymerization conditions: toluene ) 25 mL, Ni ) 17 µmol, [Al]/[Ni] ) 270. b TOF (turnover frequency): mol propylene/(mol Ni‚h). c Determined
using gel-permeation chromatography in 1,2,4-C6H3Cl3 at 140 °C versus polyethylene standards. d Determined by 1H NMR. e Determined by the equation:
% 3,1-insertions ) [(1 - R)/(1 + 2R)] × 100, where R ) [CH3]/[CH2]. f Determined by differential scanning calorimetry (second heating). g None detected.
copolymers with multiple blocks of defined size and microstructure
from a single alkene. Future work will focus on the application of
this system for the synthesis of model iso-PP block copolymers,
as well as the development of systems with higher activity.
Acknowledgment. G.W.C. gratefully acknowledges support
from the Packard and Sloan Foundations, and Mitsubishi Chemicals.
This material is based upon work supported in part by the U.S.
Army Research Laboratory and the U.S. Army Research Office
under Grant No. DAAD19-02-1-0275, Macromolecular Architecture
for Performance (MAP) MURI. This research made use of the
Cornell Center for Materials Research Shared Experimental Facili-
ties supported through the NSF MRSEC program (DMR-0079992).
Supporting Information Available: Catalyst synthesis and char-
acterization, X-ray data for rac-1, propylene polymerization data, and
polymer characterization. This material is available free of charge via
Figure 2. 13C NMR spectrum (1,1,2,2-C2D2Cl4, 125 MHz, 135 °C) of iso-
PP formed by rac-1/MAO at -78 °C (Table 1, entry 6).
the Internet at http://pubs.acs.org.
opportunity for the synthesis of propylene homopolymers with
blocks of controlled sequence length and composition. Polypropy-
lene block copolymers are primarily made using four techniques.
First, catalysts with dynamic active sites can produce isotactic/
atactic stereoblock polypropylenes.9 Second, the sequential polym-
erization of propylene with other alkenes using living catalysts
produces isotactic and syndiotactic polypropylene block copoly-
mers.2 Third, chain exchange between catalysts of different ste-
reospecificities can be used for block copolymer synthesis.10 Fourth,
changing the solvent during a living propylene polymerization can
produce stereoblock polymers.11 We chose to investigate the simple
change of Trxn during chain growth as a straightforward technique
to give polymers with blocks that are well-defined in number, size,
and composition. Propylene was polymerized at -60 °C for 7 h to
give an iso-PP block with Mn ) 5 400 g/mol, Mw/Mn ) 1.24.
Warming to 0 °C for 1 h gave a diblock polymer (iso-PP-block-
rir-PP) with Mn ) 47 400 g/mol, Mw/Mn ) 1.12. Thermal analysis
of the diblock polymer by DSC showed characteristics of each of
the homopolymers, with Tg ) -44.5 °C and Tm ) 118.6 °C.
Polymerization at Trxn) -60, 0, then -60 °C gave a triblock
copolymer with elastomeric properties.
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In conclusion, we report a new chiral, living Ni(II) complex
which produces polypropylenes with microstructures ranging from
highly isotactic at low Trxn to regiorandom at high Trxn. By changing
Trxn during propylene polymerization, regioblock copolymers can
be synthesized. This represents a new strategy for the synthesis of
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