α-keto-β-diimine nickel-catalyzed olefin polymerization: Effect of ortho-aryl substituents and preparation of stereoblock copolymers

Article abstract of DOI:10.1002/pola.28631

A series of α-keto-β-diimine nickel complexes (Ar-N = C(CH₃)-C(O)-C(CH₃)=N-Ar)NiBr₂; Ar = 2,6-R-C₆H₃-, R = Me, Et, iPr, and Ar = 2,4,6-Me₃-C₆H₃-) was prepared. All corresponding ligands are unstable even under an inert atmosphere and in a freezer. Stable copper complex intermediates of ligand synthesis and ethyl substituted nickel complex were isolated and characterized by X-ray. All nickel complexes were used for the polymerization of ethene, propylene, and hex-1-ene to investigate their livingness and the extent of chain-walking. Low-temperature propene polymerization with less bulky ortho-substituents was less isospecific than the one with isopropyl derivative. Propene stereoblock copolymers were prepared by iPr derivative combining the polymerization at low temperature to prepare isotactic polypropylene (PP) block and at a higher temperature, supporting chain-walking, to obtain amorphous regioirregular PP block. Alternatively, a copolymerization of propene with ethene was used for the preparation of amorphous block.

Full text of DOI:10.1002/pola.28631

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a-Keto-b-Diimine Nickel-Catalyzed Olefin Polymerization: Effect of
Ortho-Aryl Substituents and Preparation of Stereoblock Copolymers
1
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Anatolij Sokolohorskyj, Ondrej Zeleznık, Ivana Cısarova, Johannes Lenz, Albena Lederer,
1
Jan Merna
1
ꢁ
Department of Polymers, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic
2Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, 128 40 Prague 2, Czech
Republic
3
€
Leibniz Institute of Polymer Research, Hohe Strasse 6, D-1069 Dresden, Germany and Technische Universitat Dresden, Dresden,
01062, Germany
Correspondence to: J. Merna (E-mail: merna@vscht.cz)
Received 8 March 2017; accepted 8 April 2017; published online 00 Month 2017
DOI: 10.1002/pola.28631
ABSTRACT: A series of a-keto-b-diimine nickel complexes (Ar-
N 5 C(CH3)-C(O)-C(CH3)5N-Ar)NiBr2; Ar 5 2,6-R-C6H3-, R 5 Me,
Et, iPr, and Ar 5 2,4,6-Me3-C6H3-) was prepared. All correspond-
ing ligands are unstable even under an inert atmosphere and
in a freezer. Stable copper complex intermediates of ligand
synthesis and ethyl substituted nickel complex were isolated
and characterized by X-ray. All nickel complexes were used for
the polymerization of ethene, propylene, and hex-1-ene to
investigate their livingness and the extent of chain-walking.
Low-temperature propene polymerization with less bulky
ortho-substituents was less isospecific than the one with iso-
propyl derivative. Propene stereoblock copolymers were
prepared by iPr derivative combining the polymerization at low
temperature to prepare isotactic polypropylene (PP) block and
at a higher temperature, supporting chain-walking, to obtain
amorphous regioirregular PP block. Alternatively, a copolymer-
ization of propene with ethene was used for the preparation of
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amorphous block. 2017 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: a-keto-b-diimine nickel; block copolymers; catalysis;
isotactic poly(propylene); living polymerization; living olefin
polymerization; polyolefin; stereoblock copolymers
approximately 100 times less active than the corresponding
INTRODUCTION The research interest in late transition metal
catalysts lies in their low oxophilicity and therefore higher
tolerance to polar functional groups.1,2 Some of them also
undergo “chain-walking” reactions in which the growing cen-
ter migrates along the growing polymer chain through a
series of b-hydride elimination and reinsertion steps. Typical
examples of such catalysts are diimine complexes of nickel
and palladium.3 Chain-walking enables the formation of vari-
ous types of polyethylenes ranging from almost linear to
hyperbranched ones.1,4 On the contrary, higher a-olefins are
straightened after their insertion and form more linear poly-
mers.5,6 a-Diimine nickel and palladium complexes also
belong among of few catalytic systems suitable for living ole-
fin polymerization allowing the preparation of block olefin
copolymers including stereoblock copolymers.7–14 Soon after
the discovery of a-diimine complexes, their six-membered
analogs based on b-diimine ligands were introduced.15
b-Diimine nickel complexes of these basic ligands were
a-diimine
counterpart
and
afforded
less-branched
polyethylene.
Much more active nickel complexes with b-diimine ligand
substituted by electron withdrawing group connected to
ligand backbone were developed later by Bazan.16–20 Activity
comparable with that of known a-diimine complexes was
achieved with a-keto-b-diimine complex 5d (Scheme 1).
Complexation of organoaluminium cocatalyst to the carbonyl
group of ligand, leading to decrease of electron density at
nickel metal center, was described as the reason for the high
activity. a-Keto-b-diimine nickel complex 5d was also
reported to initiate living polymerization of propene, hex-1-
ene, and even of ethene, which is the case of only a-diimine
palladium complexes8 or nickel a-diimine complexes with
highly rigid ligands.21,22 Trimethylaluminium-free methylalu-
minoxane (MAO) had to be used to suppress transfer and
Dedicated to Prof. Jan Roda on the occasion of his 70th birthday.
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2017 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of a-keto-b-diimine nickel complexes.
termination reactions. Interestingly, achiral a-keto-b-diimine
complex 5d was capable of enantiomorphic site control to
yield moderately isotactic polypropylene at low temperatures
([mmmm] 5 0.85, Tm 5 134 8C).18,20
over CaH2 and cryo-distilled under nitrogen. Hex-1-ene
(99%, Aldrich) was dried over sodium/potassium alloy and
distilled under nitrogen. Methylaluminoxane (MAO) (10 wt
% solution in toluene, Aldrich) was used as received.
b-Diimine ligands were synthesized and purified according
to reported procedures.23,24 Complex 5d was prepared
according to literature.16 Catalysts were dosed as a solid
directly to the polymerization vessel or in the form of
dichloromethane solutions that were stored at 5 8C. Elemen-
tal analyses (C, H, N) were performed on an Elementar Vario
EL III Analyzer. ATR-IR spectra were recorded on an FTIR
spectrometer Nicolet 6700 using diamond crystal.
Despite the promising catalytic behavior of 5d, it is the only
one a-keto-b-diimine nickel complex published in open litera-
ture.16,18–20 Derivative combining isopropyl and methyl group
in ortho-aryl positions of the ligand was reported in the pat-
ent; however, no polymerization data were provided.17
Herein, we report on a series of three novel a-keto-b-diimine
nickel complexes 5a–5c (Scheme 1) and their catalytic
behavior in ethene, propene, and hex-1-ene polymerization.
The variation of ortho-aryl substituents was the main focus
in the design of the complexes as these positions are crucial
for analogous a-diimine catalysts suitable for living polymer-
izations.7,12 Preparation of propene block copolymers using
complex 5d is reported for the first time.
Ligand Synthesis
Oxidation of b-diimine ligands was carried out analogously
to the procedure reported by Yokota.25 Copper intermediates
were obtained as microcrystalline powders and 3a,b were
characterized by powder X-ray diffraction.26,27 Decomposi-
tion of complexes 3a-d led to the formation of a-keto-b-dii-
mine ligands in the form of yellow oil or solid.
EXPERIMENTAL
2,4-Bis(2,6-dimethylphenylimino)pentan-3-one
(4a).
Cu
Materials
intermediate 3a (0.12 g, 0.15 mmol) was dissolved in 15 mL
chloroform. An aqueous solution of ammonium hydroxide
(6%, 40 mL) was added, and the mixture was allowed to stir
for 30 min. The organic layer was separated, and the treat-
ment was repeated four times until the aqueous layer
stopped turning blue. The organic layer was washed three
All manipulations with air-sensitive compounds were done
using standard Schlenk techniques. Nitrogen (SIAD,
99.999%), ethene (Linde, 99.9%), and propene (SIAD,
99.5%) were purified by passing through a column packed
with Cu-catalyst and molecular sieves to remove traces of
oxygen and water. Dichloromethane (p.a., Penta) was dried
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times with water (40 mL) and dried over anhydrous MgSO4.
Evaporation of the solvent afforded 4a as a yellow solid in
79% yield.
diamond) 1699, (C 5 O), 1636 cm21 (C 5 N). Anal. calcd. for
C23H28N2ONiBr2: C, 48.72, H, 4.98, N, 4.98. Found: C, 48.54,
H, 5.20, N, 4.85.
1H NMR (500 MHz, C6D6, d): 6.93 (4H, b, Hm), 6.90 (2H, b,
Hp), 1.99 (12H, s, o-CH3), 1.72 (6H, s, a-Me). 3C NMR (500
MHz, C6D6, d): 193.42 (ketone carbon), 168.13 (imine car-
bon), 128.06, 125.14, 124.21, 123.90 (aromatic carbons),
18.00 (o-CH3), 16.24 (a-Me). IR (ATR, diamond) 1703,
(C 5 O), 1668 cm21 (C 5 N).
Complex 5c
Using the same procedure as for 5a, 4c (0.30 g, 0.78 mmol)
were reacted with (DME)NiBr2 (0.22 g, 0.71 mmol) resulting
in red-brownish powder of 5c in 54% yield. Recrystallization
from CH2Cl2 solution layered by hexane gave single mono-
crystals suitable for X-ray analysis.
1H NMR (500 MHz, CDCl3, d): 27.1 (s, 4H, CH2CH3), 23.9 (s,
4H, CH2CH3), 22.0 (s, 4H, Hm), 2.6 (s, 12H, CH2CH3), 2.0–0.1
(m, 2H), 216.3 (s, 2H, Hp), 227.8 (s, 6H, a-Me). IR (ATR, dia-
mond) 1697 (C 5 O), 1646 cm21 (C 5 N). Anal. calcd. for
C25H32N2ONiBr2: C, 50.46, H, 5.42, N, 4.71; found: C, 50.40,
H, 5.60, N, 4.62.
2,4-Bis(2,4,6-trimethylphenylimino)pentan-3-one (4b). Using
the same procedure as for 4a, 3b (0.22 g, 0.27 mmol) was
decomposed to give 4b as a yellow solid in 79% yield.
1H NMR (500 MHz, C6D6, d): 6.75 (4H, b, Hm), 2.15 (6H, s, p-
CH3), 2.01 (12H, s, o-CH3), 1.75 (6H, s, a-Me). 13C NMR (500
MHz, C6D6, d): 193.64 (ketone carbon), 168.36 (imine car-
bon), 145.30, 133.07, 129.13, 128.35, 124.98 (aromatic car-
bons), 20.78 (p-CH3), 17.97 (o-CH3), 16.25 (a-Me). IR (ATR,
diamond) 1703, (C 5 O), 1666 cm21 (C 5 N).
Complex 5d
1H NMR (500 MHz, CDCl3, d): 21.4 (s, 4H, Hm), 10.7 (br, 2H,
CHCH3), 3.3 (s, 24H, CHCH3), 2.0–0.1 (m, 2H), 215.0 (s, 2H,
Hp), 225.5 (s, 6H, a-Me). IR (KBr) 1701, (C 5 O), 1632 cm21
(C 5 N). Anal. Calcd. for C29H40N2ONiBr2: C, 53.49, H, 6.19,
N, 4.30; found: C, 53.40, H, 6.20, N, 4.30.
2,4-Bis(2,6-diethylphenylimino)pentan-3-one (4c). Using the
same procedure as for 4a, 3c (0.24 g, 0.27 mmol) was
decomposed to give 4c as a yellow oil in 90% yield.
Hex-1-Ene Polymerization
1H NMR (500 MHz, C6D6, d): 7.04–6.67 (6H, m, HAr), 2.39
(8H, m, CH2CH3), 1.75 (6H, s, a-Me), 1.06 (12H, t, CH2CH3).
13C NMR (500 MHz, C6D6, d): 193.45 (ketone carbon),
168.11 (imine carbon), 145.45, 131.20, 126.59, 124.69 (aro-
matic carbons), 24.77 (CH2CH3), 16.60 (a-Me), 14.30
(CH2CH3). IR (ATR, diamond) 1702, (C 5 O), 1670 cm21
(C 5 N).
Polymerizations were carried out under dry nitrogen in mag-
netically stirred 15 mL glass ampoules. The ampoules with
toluene, hex-1-ene, and MAO were placed in a bath kept at a
desired temperature and tempered for 15 min. The polymer-
ization was initiated by addition of fresh catalyst solution.
After the allotted reaction time, the polymerization was ter-
minated by pouring the reaction mixture to a large excess of
ethanol acidified with HCl, washed with ethanol, and dried
24 h at 50 8C under vacuum.
Synthesis of Nickel Complexes
Nickel complexes were synthesized analogously to the proce-
dure reported for 5d16 by the reaction of the corresponding
ligand and (1,2-dimethoxyethane)nickel(II) bromide ((DME)-
NiBr2) in CH2Cl2.
Ethene Polymerization
A 100-mL glass reactor with a magnetic stirring bar was
evacuated and placed under an ethylene atmosphere. The
solvent and MAO were added to nitrogen flux, and reactor
was then allowed to stir under desired pressure of ethene
for 15 min at the prescribed polymerization temperature.
Polymerization was started by the injection of the catalyst
solution against nitrogen flux. The pressure of ethylene was
kept constant during the reaction. After the allotted polymer-
ization time, the reaction was terminated by pouring the
polymerization mixture to a large excess of ethanol acidified
with HCl. Polyethylene was separated by filtration, washed
with ethanol, and dried under vacuum overnight at 50 8C.
Complex 5a
4a (0.25 g, 0.78 mmol) and (DME)NiBr2 (0.22 g, 0.71 mmol)
were reacted in CH2Cl2 at room temperature for 2.5 h, evap-
orated, extracted by diethylether, dissolved in CH2Cl2, and fil-
tered. After evaporation, 5a was obtained as red-brownish
powder in 83% yield.
1H NMR (500 MHz, CDCl3, d): 26.7 (s, 12H, o-Me), 22.2 (s,
4H, Hm), 216.6 (s, 2H, Hp), 228.8 (s, 6H, a-Me). IR (ATR,
diamond) 1700, (C 5 O), 1642 cm21 (C 5 N). Anal. calcd. for
C21H24N2ONiBr2: C, 46.80, H, 4.49, N, 5.20; found: C, 46.53,
H, 4.83, N, 5.02.
Propene Polymerization
Polymerizations of propene were carried out in toluene in a
magnetically stirred jacketed 100-mL glass reactor. The MAO
solution was injected against nitrogen flux and evaporated to
dryness. The solvent was added against propene flux and
reactor was then cooled in dry ice/ethanol bath, and a speci-
fied volume of propylene was condensed into the reactor.
The reactor was cooled to the appropriate reaction tempera-
ture by Julabo F70 cryostat, and the polymerization was
Complex 5b
Using the same procedure as for 5a, 4b (0.27 g, 0.78 mmol)
were reacted with (DME)NiBr2 (0.22 g, 0.71 mmol) resulting
in red-brownish powder of 5b in 90% yield.
1H NMR (500 MHz, CDCl3, d): 31.2 (s, 6H, p-Me), 26.8 (s,
12H, o-Me), 21.9 (s, 4H, Hm), 229.4 (s, 6H, a-Me). IR (ATR,
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initiated by the injection of the catalyst solution against
nitrogen flux. After the desired time, the reaction was termi-
nated by pouring the reaction mixture to a large excess of
the ethanol acidified with HCl, and the polymer was washed
with ethanol and dried 24 h at 50 8C under vacuum.
X-Ray
Crystallographic data for 5c was collected on Nonius Kap-
paCCD diffractometer equipped with Bruker APEX-II CCD
detector by monochromatized MoKa radiation (k 5 0.71073
Å) at the temperature of 150(2) K. The structure was solved
by direct methods (SHELXS)29 and refined by full matrix
least squares based on F2 (SHELXL97).29 The hydrogen
atoms on carbon were calculated into idealized positions and
were refined as fixed (riding model) with assigned tempera-
ture factors
Propene Block Copolymers
The first block of a copolymer was prepared in an analogous
way to propene homopolymerization. After a desired time
for the first block formation, a sample of the reaction mix-
ture for SEC and DSC analysis was taken by a cannula and a
second block was synthesized in two different ways. For the
second block with regioirregular PP structure, the tempera-
ture of the reaction was increased to 0 8C. For the second
block with the structure of random ethene/propene copoly-
mer, the reactor was repeatedly (after each 15–30 min)
shortly fed with ethene (1.5 bar).
Hiso(H) 5 1.2 Ueq (pivot atom) or 1.5 Ueq for methyl moiety.
One of the methyl moiety is disordered into two positions
with occupational factors 0.6:0.4.
To improve the resolution of the complex, PLATON30
/
SQUEEZE procedure was used to correct the data for the
presence of the disordered solvent. Two potential solvent
cavities with the volume of 136 Å3 were found at special
positions of inversion. Sixty-nine electrons worth of scatter-
ing were located in the void, highest peak corresponds to
Polymer Characterization
electron density 5.34 e Á Å23
.
Molar masses of polyethylenes and polypropylenes were
determined by high-temperature PL 220 chromatograph
equipped with refractometric and viscometric detector. Sepa-
ration was performed on two Olexis mixed columns (Poly-
mer Laboratories, 13 lm) at 150 8C in 1,2,4-
trichlorobenzene at an elution rate of 1 mL Á min21. Apparent
molar masses were calculated using narrow PS standards
calibration. Molar masses of poly(hex-1-enes) were deter-
mined using Waters Breeze chromatographic system
equipped with RI detector operating at 880 nm and multian-
gle laser light scattering (MALLS) miniDawn TREOS from
Wyatt operating at 658 nm. Separations were performed
with two columns (Polymer Laboratories Mixed C) at 35 8C
in THF at an elution rate of 1 mL Á min21. The dn/dc value
0.078 6 0.002 mL Á g21 for poly(hex-1-ene) in THF at 35 8C
was used.12 Molar masses of PP block copolymers were
determined using SEC-RI chromatograph against PS calibra-
tion (580–1,230,000 g/mol). Separation was performed on
one Polymer Laboratories Mixed C at 35 8C in chloroform at
Crystal data for 5c: C25H32Br2N2NiO, Mr 5 595.05, mono-
clinic, P 21/n (No 14), a 5 9.8413 (3) Å, b 5 12.2506 (4) Å,
c 5 22.6665 (8) Å, b 5 96.974 (1)8; Z 5 4, Dx 5 1.457
mg Á m23, dark red crystal of dimensions 0.40 3 0.32 3
0.18 mm, the numerical absorption correction was applied
(l 5 3.68 mm21) Tmin 5 0.34 Tmax 5 0.55; 20,957 diffraction
collected (hmax 5 27.5˚), 6228 independent (Rint 5 0.023) and
5136 observed (I > 2r(I)). The refinement converged (D/
r
max50.001) to R 5 0.033 for observed reflections and
wR(F2) 5 0.069, GOF 5 1.10 for 297 parameters and all 6228
reflections. The final difference map displayed no peaks of
chemical significance (Dqmax 5 0.44, Dqmin 5 20.46 e Á Å23).
Crystallographic data for structural analysis have been
deposited with the Cambridge Crystallographic Data Centre,
CCDC no. 1522889 Copies of this information may be
obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EY, UK (e-mail: deposit@ccdc.cam.ac.
uk or www: http://www.ccdc.cam.ac.uk).
an elution rate of 1 mL Á min21
.
1H NMR spectra of ligands and polymers were measured in
CDCl3 at 25 8C on Bruker Avance DRX 500 spectrometer.
Number of branches per 1000 total carbon atoms (N) was
determined according to literature.12,20 13C NMR spectra of
polypropylenes were measured in C2D2Cl4 at 115 8C using
INVGATE pulse sequence, 10-s relaxation delay, and 5000
scans to ensure quantitative spectra. Spectra were assigned
as described in literature.20
RESULTS AND DISCUSSION
Catalysts Synthesis
The synthesis of catalyst was carried out according to the
Scheme 1. a-Keto-b-diimine ligand synthesis is based on the
procedure developed by Yokota et al.25 and consists of com-
plexation of b-diimine ligands 1a–1d to copper complexes
2a–2d which are then oxidized by molecular oxygen to give
copper complexes 3a–3d. These complexes are then decom-
posed by NH4OH, yielding the desired a-keto-b-diimine
ligands 4a–4d. Whereas the starting b-diimine ligands and
copper complexes 2a–2d and 3a–3d are stable on air, the
a-keto-b-diimine ligands 4a–4d are unstable even if they are
stored in the dark under an inert atmosphere at 220 8C.
Except ligand 4d, isolatable in a crystalline state, the other
ligands, isolated as oils or amorphous solids, has to be
DSC measurements were performed on a TA Instrument
module Q100 at a rate of 10 8C Á min21 for both heating and
cooling. Melting temperatures and enthalpies of fusion were
obtained from the second heating run. Enthalpies of the
fusion 207 and 297 J Á g21 for hypothetical 100% crystalline
isotactic polypropylene and polyethylene, respectively, were
used to calculate the degree of their crystallinity.28
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TABLE 1 Ethene Polymerization Initiated by 5a–5d/MAO in
Toluene
TOFa
(h21
Mn
Tm
b
c
Run
Cat.
)
(kgÁmol21
)
Ðb
(8C)
ac (%)
Nd
1
5a
5b
5c
5c
5d
1,910
2,400
1,640
3,990
2,100
219
301
302
333
377
1.49
1.77
1.28
1.35
1.41
119
123
96
25
26
15
21
24
22
23
36
30
30
2
3
4e
114
114
5
[Ni] 5 0.4 mM, Al/Ni 5 200, T 5 0 8C, toluene 20 mL, p(ethene) 5 1.5 bar,
t
p 5 40 min.
a
Turnover frequency: mol ethene/(molNi Á tp).
b
Molar mass and dispersity determined by SEC in 1,2,4-trichloroben-
FIGURE 1 ORTEP drawing of 5c. Thermal ellipsoids are set at
the 30% probability level. The second position of disordered
methyl moiety is omitted for clarity. The selected bond distan-
zene at 150 8C versus PS standards.
c
Melting temperature and crystallinity determined by DSC from the
second heating run.
Number of branches per 1000 carbon atoms determined by 1H NMR.
˚
d
ces (A) and angles (8): Br1-Ni1 2.3383 (4), Br2-Ni1 2.3521 (4),
e
p(ethene) 5 5.5 bar.
Ni1-N2 1.991 (2), Ni1-N1 1.995 (2), O1-C2 1.206 (3), N1-C1 1.279
(3), N2-C3 1.278 (3); N2-Ni1-N1 94.15 (8), N2-Ni1-Br1 108.92 (6),
N1-Ni1-Br1 112.78 (6), N2-Ni1-Br2 103.29 (6), N1-Ni1-Br2 102.83
(6), Br1-Ni1-Br2 129.075 (18). [Color figure can be viewed at
wileyonlinelibrary.com]
and 1.5 bar ethene, PE with slightly broadened molar mass
-
distribution (D 5 1.41) was obtained, which could be
ascribed to increased extent of the transfer reactions at a
higher temperature and eventually also a different batch of
MAO cocatalyst. This indicates the limited window of reac-
tion temperatures at which the living ethene polymerization
can be achieved with these catalysts. At the same conditions,
complexes 5a and 5b bearing methyl substituents in ortho-
aryl positions yielded PE with broader molar mass distribu-
tion, indicating an increased proportion of transfer reactions
compared to propagation. This shows the importance of
bulkiness of ortho-aryl substituents in preventing the forma-
tion of transfer reaction transition state, the structure of
which could resemble the one known for relative a-diimine
complexes, that is, pentacoordinated bis-olefin hydrido com-
plex1 (Fig. 2). Larger substituents could destabilize such a
transition state by blocking axial positions above and below
the ligand–metal plane and suppress the role of transfer. The
complexed to metal in the order of hours to avoid their
decomposition. Ligand 4d can be stored in the fridge for
weeks or easily purified by recrystallization from hot hexane.
This decomposition of a-keto-b-diimine ligands is highly
accelerated in solution, and only traces of acidity in dichloro-
methane or chloroform lead to their decomposition to a
complex mixture of products in the order of minutes. Thus,
a-keto-b-diimine ligands were complexed with (DME)NiBr2
in dichloromethane immediately after their synthesis, result-
ing in more stable a-keto-b-diimine nickel(II) bromide com-
plexes 5a–5d. These complexes were isolated in 54–90%
yields in the form of red-brownish microcrystalline powders.
Although the complexes are paramagnetic, their structure
could be characterized by 1H NMR spectroscopy which gave
broad but well-resolved signals of all protons. Further, ligand
complexation to nickel was confirmed by shifting of C 5 N
FTIR signals in ligands (1664–1670 cm21) by ꢀ30 cm21 in
the corresponding complexes (1632–1646 cm21). In the case
of 5c, a single crystal suitable for X-ray analysis was
obtained by crystallization from dichloromethane solution
layered by n-hexane. The solid-state structure of 5c is very
similar to structure 5d16 having a distorted tetrahedral
geometry and boat-shaped ligand structure (Fig. 1).
Olefins Homopolymerization
Ethene Polymerization
Prepared a-keto-b-diimine complexes 5a–5d were initially
tested in ethene polymerization after activation by 200 equiv.
of MAO at 0 8C in toluene (Table 1). Complex 5d, previously
studied by Bazan, catalyzed ethene polymerization leading to
polyethylene (PE) with dispersity below 1.10 at 210 8C and
3.5 bar ethene pressure.18 At our conditions, that is, at 0 8C
FIGURE 2 Suggested structure of transfer reaction transition
state.
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TABLE 2 Propene Polymerization Initiated by 5a–5d/MAO in Toluene
ncat
TOFa
Mn
b
Run
Cat
(lmol)
T (8C)
tp (h)
(h21
)
(kgÁmol21
)
Ðb
NPMc
Tg (8C)
Tm (8C)
ad (%)
Ne
d
d
6f
5a
5a
5b
5b
5c
5c
5c
5d
5d
5d
5d
5d
10
20
10
20
10
20
20
10
40
10
20
20
210
230
210
230
210
230
250
210
210
230
230
250
2
108
6
35
2.03
1.54
1.35
1.52
1.40
1.18
1.69
1.16
1.18
1.10
1.86
1.59
0.15
0.04
0.04
0.04
0.03
0.11
0.63
0.06
0.15
0.35
0.34
0.14
242
239
245
240
246
237
235
221
217
215
220
220
–
–
250
270
233
254
238
271
288
290
284
nd
7
8f
18
2
108
103
124
123
211
11
–
–
52
7
–
–
9
18
2
–
–
10f
11
12
13f
14
15
16
17
38
13
7
–
–
42
25
2
–
–
68
66
65
94
70
98
1
2
3
8
8
17
124
324
136
54
4
188
179
34
2
2
18
8
121
4
295
308
d
Al/Ni 5 200, m(propene) 5 15 g, toluene 40 mL.
Glass transition temperature, melting temperature, and crystallinity
a
Turnover frequency: mol propylene/(molNi Á tp).
determined by DSC from the second heating run.
e
b
Molar mass and dispersity determined by SEC in 1,2,4-trichloroben-
Number of branches per 1000 carbon atoms determined by 1H NMR.
f
zene at 150 8C.
p(propene) 5 1.5 bar.
c
Number of polymer molecules per one Ni center NPM 5 polymer
mass/(molNi Á Mn).
increase of ortho-aryl substituents to ethyl groups seems to
be sufficient to prevent transfer as PE prepared with 5c dis-
played even narrower molar mass distribution than the bulk-
iest complex 5d (Table 1, run 3 vs. run 5).
Another important parameter to suppress chain-walking is
the reaction temperature which was maintained between
210 and 250 8C (Table 2). Moderate isoselectivity
([mmmm]50.43 2 0.86) in propene polymerization was pre-
viously observed by Bazan for 5d18,20 between 210 and
260 8C, yielding PPs with melting temperature 59–134 8C.
The superior suppression of transfer reactions by ethyl
ortho-aryl groups was previously observed also in the case
of a-diimine nickel complexes.12 The increase of ethene pres-
sure with 5c (Table 1, run 3 vs. run 4) leads to the broaden-
ing of molar mass distribution which is in accordance with
previous observations for complex 5d.18 Similarly, for
a-diimine complexes, the size of substituents influences
branching of PE as a consequence of altered extent of chain-
walking with respect to propagation.3 More linear PEs are
obtained by catalysts 5a and 5b, bearing smaller ortho-aryl
substituents than 5c and 5d. The key step of chain-walking,
that is, b-H elimination, can only occur in Ni-alkyl complexes.
Lower steric hindrance of ligand leads to easier coordination
of monomer to Ni center forming an olefin-alkyl complex,
thus blocking a vacant position necessary for b-H elimination
and decreasing the extent of chain-walking.3 The increase of
PE branching, expressed as a number of branches per 1000
total carbon atoms, correlates well with the decrease in their
melting temperatures (Table 1).
At first, all a-keto-b-diimine complexes 5a–5d activated by
dried MAO were compared in propene polymerization at
1.5 bar at 210 8C (propene does not condense) in toluene
(Table 2, runs 6, 8, 10, 13). Turnover frequencies calculated
from the amount of PP obtained after 2 h are not in clear
correlation with the bulkiness of ligand ortho-aryl substitu-
ents 5d ꢀ 5a > 5b ꢀ 5c.
This indicates that ligands steric hindrance is not the only
factor influencing the catalyst activity. Similarly in the case
of a-diimine ligands, polymerization rate could be increased
by the decrease of insertion barrier caused by the destabili-
zation of the ground state of monomer insertion by bulkier
ligands.11,12,31,32 The narrowest molar mass distribution was
obtained for 5d for which living behavior was proved in pre-
vious studies.18 The decrease of ortho-ligand bulkiness
increased molar mass dispersity indicating an increase of
transfer reactions. While complexes 5b and 5c afforded PP
with dispersity bellow 1.40, which could still be considered
as a controlled polymerization process, complex 5a produced
a polymer with dispersity around 2, characteristic for single-
site catalyst but deviating substantially from the controlled
character. Comparison of the molar mass values with theo-
retical ones allows the quantification of a number of polymer
molecules per one nickel center (Table 2, column NPM). In
most of the cases, NPM values are very low (albeit calculated
using only apparent molar mass values from polystyrene
Propene Polymerization
Our main focus in developing new a-keto-b-diimine com-
plexes was propene polymerization to prepare isotactic poly-
propylene (PP) in a living manner. We hypothesized that
complexes with smaller ortho-aryl substituents could display
decreased chain-walking extent and could lead to PPs with
higher regioregularity, the first assumption for the possibility
of good stereoregularity.
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calibration), showing the efficiency of MAO as a cocatalyst is
rather low.
To suppress side reactions and achieve a better controllabil-
ity of propene polymerization, further experiments were per-
formed at 230 to 250 8C. At these conditions, propene was
condensed into the reactor to increase its concentration and
partially compensate for lowered propagation rate. Catalysts
activities are, however, much lower below 230 8C; thus, the
polymerization time was prolonged to 8–42 h.
The decrease in molar mass dispersity with the decrease in
reaction temperature to 230 8C was observed for complexes
5a, 5c, and 5d (Table 2, run 7) while it led to broadening of
molar mass distribution for complex 5b. For complex 5c, dis-
persity 1.18 (run 11) was achieved demonstrating the well-
controlled character of propene polymerization. Interestingly,
the most active complex 5d, which yielded PP with very nar-
row molar mass distribution after 2 h, produced a polymer
with rather broad molar mass distribution after prolongation
of the reaction time to 18 h (run 15 vs. run 16). Similarly,
further decrease in temperature 250 8C led to PPs with sub-
stantially increased dispersity and only low molar mass
(Table 2, runs 12 and 17). The increase in polymer molar
mass dispersity at decreased temperature could indicate that
the reaction mixture is less homogeneous due to the limited
access of monomer to polymer coil. In the case of 5c and 5d
that both afford partially crystalline PPs, this inhomogeneity
could also result from the separation of formed polymer
from the reaction mixture. The regular increase of dispersity
with the decrease in reaction temperature was previously
also observed for 5d.20
Our highest interest lied in the ability of new a-keto-b-
diimine complexes to catalyze isospecific propene polymeri-
zation. Whereas the reference complex 5d produced isotactic
PP capable of crystallization already at 210 8C, the less
bulky complex 5c produced amorphous polymer up to
230 8C and only slightly crystalline PP was formed by 5c at
250 8C. The least bulky complexes 5a and 5b formed only
amorphous PPs at 230 8C with very low polymerization
FIGURE 3 1H NMR (upper) and 13C NMR (lower) spectra (500
MHz, 115 8C, 1,1,2,2-C2D2Cl4) of polypropylenes obtained by
catalysts 5a (run 7) and 5d (run 16) at 230 8C.
This shows that the mechanism of chain-walking during
higher a-olefin polymerization may be different for a-keto-b-
diimine complexes than for a-diimine derivatives. This might
be connected with the different geometry of a-keto-b-diimine
complexes having six-membered ligand–metal ring compared
to five-membered ligand–metal ring in a-diimine complexes.
The decrease in reaction temperature caused the decrease of
chain-walking for all catalysts as indicated by an increase of
branching. The reaction temperature thus seems to be the
only tool which allows the preparation of regular PPs. Com-
parison of 13C NMR spectra of PPs prepared by 5a and 5d
(Fig. 3) shows much more regular structure of PP prepared
by bulkier complex 5d. Due to regioselectivity and isospeci-
ficity of 5d, the most abundant methyl signal of PP (run 16)
corresponds to mmmm pentad (44%). In contrast, PP pre-
pared by 5a (run 7) shows plenty of signals connected with
1,3-insertions of propene leading to oligoethylene sequences
and only very low proportion of mmmm pentads (ꢀ5%).
1
activities. Prepared PPs were characterized by H NMR spec-
troscopy and selected samples also by 13C NMR spectroscopy
(Fig. 3) to determine their branching and deduce the infor-
mation about the extent of chain-walking. In the case of a-
diimine complexes, smaller ortho-aryl substituents lead to
the decrease of chain-walking1,3 as
a consequence of
improved access of monomer to the growing center and the
transformation of alkyl–metal complexes (the species which
can undergo chain-walking) to alkyl–olefin–metal complex.
In contrast to our expectations that a-keto-b-diimine com-
plexes could behave similarly as their a-diimine analogs, the
decrease of ortho substituents bulkiness led to the increase
of chain-walking in propene polymerization. This is indicated
by the decrease in a number of branches in prepared PPs
with the decrease of ligand bulkiness. Thus, the most regular
PP was formed by the bulkiest complex 5d, followed by 5c
and 5a,b.
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TABLE 3 Hex-1-ene (C6) and 4-Methylpent-1-ene (4MP1) Poly-
merization Initiated by 5c/MAO in Toluene
b
T
tp
Mn
Run Monomer (8C)
(h) Conv.a (kgÁmol21
)
Ðb
Nc
18
19
20
21
C6
25
2
29
179
77
–
1.31 95
C6
210 18 13
1.12 133
4MP1
4MP1
25
2
0
0
–
–
–
–
210 18
–
Monomer/Ni 5 1000, [Ni] 5 1 mM, Al/Ni 5 200, total volume 10 mL.
a
Monomer conversion calculated from the mass of polymer obtained.
Molar mass and dispersity determined by SEC-MALLS in THF against
b
PS standards.
c
Number of branches per 1000 carbon atoms determined by 1H NMR.
Higher a-Olefin Polymerization
5d was also reported to initiate controlled polymerization of
hex-1-ene beside that of ethene and propene.18 Therefore, we
attempted hex-1-ene and 4-methylpent-1-ene polymerization
with the most promising new catalyst 5c, which displayed both
controlled and isospecific polymerization of propene (Table 3).
Hex-1-ene polymerization initiated by 5c/MAO yielded poly(hex-
1-ene) with relatively narrow molar mass distribution both at
210 and 25 8C, suggesting a controlled polymerization. Chain-
walking led to a significant polymer rearrangement (>40% of
1,6-insertions) at 25 8C as displayed by 95 br./1000 C compared
to poly(hex-1-ene) which would be obtained by regular 1,2-
insertions (167 br./1000 C). As in the case of propene, the chain-
walking extent decreased with the decrease in temperature giv-
ing a poly(hex-1-ene) with 20% of 1,6-inserted hexene units.
Polymerization of 4-methylpent-1-ene initiated by 5c/MAO did
not produce any polymer neither at 210 8C nor at 25 8C, which
could be ascribed to the high steric bulk of the monomer.
FIGURE 4 SEC-RI traces of first polypropylene block (dash) and
iPP-block-(ethene-co-propene) copolymer (solid), run 25.
test 5d for the preparation of stereoblock polymers based on
isotactic polypropylene (iPP) block connected to the rubbery
block. iPP block was prepared by 5d/MAO at 210 to 250 8C
followed by the growth of rubbery block by two different strat-
egies. The first strategy, adopted by Coates for a-diimine cata-
lysts,10,11 takes the advantage of a-keto-b-diimine Ni
complexes to undergo intensive chain-walking above 210 8C
to form amorphous regioirregular polypropylene (rirPP) block
(Table 4, runs 22–23). Thus, the only change in reaction tem-
perature can switch between the formation of semicrystalline
and an amorphous polymer. In our case, two block copolymers
iPP-block-rirPP were prepared by first polymerizing propene
at 230 8C for 1 h (Table 4, run 22) or at 250 8C for 16 h (run
23) followed by 30 min propene polymerization at 0 8C.
Preparation of Stereoblock Copolymers
Catalyst 5d was used for the homopolymerization of ethene,
propene, and hex-1-ene only.18 Due to its rare ability of living
and simultaneously isospecific polymerization, we decided to
The good control of polymerization is demonstrated by the
increase of molar mass after each reaction step while
TABLE 4 Preparation of Stereoblock Copolymers by 5d/MAO in Toluene
b
b
c
Run
22
Block
Mon.a
T (8C)
tp (h)
mp (g)
Tg (8C)
Tm (8C)
Mn (kgÁmol21
)
Ðc
1st
1st 12nd
1st
1st 12nd
1st
1st 12nd
1st
1st 12nd
1st
1st 12nd
C3
230
0
1
–
–
98
27
1.18
1.18
nd
C3
1.5
16
16.5
1
0.795
–
219 (251)
219
73
100
nd
nd
95
23
C3
250
0
109
C3
0.130
–
217
54 (90–120)
nd
24
C3
210
210
210
210
230
230
214
65
1.21
1.35
1.19
1.25
1.10
1.15
C3 1 C2
C3
2
0.961
–
224
67
155
74
25d
26
1
218
64
C3 1 C2
C3
2
1.245
–
217 (246)
215
64 (98)
94
215
34
2
C3 1 C2
2.5
0.172
218
92 (115)
41
c
[Ni] 5 0.5 mM, Al/Ni 5 200, toluene 20 mL.
Molar mass and dispersity determined by SEC in CHCl3 at 35 8C versus
a
Monomers: C3, propene (7.5 g); C2, ethene (1.5 bar).
Determined by DSC.
PS standards.
d
b
50 mL toluene. nd 5 not determined.
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ACKNOWLEDGMENTS
This work was supported in the frame of a joint project by
Czech Science Foundation (No. 15-15887J) and Deutsche
Forschungsgemeinschaft (No. LE 1424/7-1) and from specific
university research, the Ministry of Education, Youth and
Sports (MSMT No. 20/2016).
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