Vinyl-addition polymerization of norbornene catalyzed by (pyrazol-1-ylmethyl)pyridine divalent iron, cobalt and nickel complexes

Article abstract of DOI:10.1016/j.poly.2011.08.021

Complexes of 2-((3,5-dimethyl)-1H-pyrazol-1-ylmethyl)pyridine (L1), 2-((3,5-ditert-butyl-1H-pyrazol-1-yl)methyl)pyridine (L2), 2-((3,5-diphenyl)-1H- pyrazol-1-yl)methyl)pyridine (L3), 2-((3,5-bis(trifluoromethyl)-1H-pyrazol-1- ylmethyl)pyridine (L4) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine (L5) with cobalt(II), iron(II) and nickel(II), Ni(L1)Cl₂ (1), Co(L1)Cl₂ (2), Fe(L1)Cl₂ (3), Ni(L2)Cl₂ (4), Ni(L3)Cl₂ (5), Co(L3)Cl₂ (6), Fe(L3)Cl₂ (7), Ni(L4)Cl₂ (8) and Ni(L5)Cl₂ (9), were used as catalyst precursors to produce vinyl-addition type norbornene polymers. Both the identity of the metal center and nature of ligand affected the polymerization behaviour of the resultant catalysts. Nickel catalysts were generally more active than the corresponding iron and cobalt analogues. The polynorbornene produced have high molecular weights (0.5-2.1 × 10⁶ g/mol) and narrow molecular weight distributions. Analyses of polymer microstructure using NMR and IR spectroscopy confirmed the polymers produced to be vinyl-addition polynorbornene.

Full text of DOI:10.1016/j.poly.2011.08.021

Polyhedron 30 (2011) 2878–2883
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Vinyl-addition polymerization of norbornene catalyzed by
(pyrazol-1-ylmethyl)pyridine divalent iron, cobalt and nickel complexes
Letitia L. Benade a, Stephen O. Ojwach a,1, Collins Obuah a, Ilia A. Guzei a,b, James Darkwa a,
⇑
a Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
b Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Complexes of 2-((3,5-dimethyl)-1H-pyrazol-1-ylmethyl)pyridine (L1), 2-((3,5-ditert-butyl-1H-pyrazol-
1-yl)methyl)pyridine (L2), 2-((3,5-diphenyl)-1H-pyrazol-1-yl)methyl)pyridine (L3), 2-((3,5-bis(trifluoro-
methyl)-1H-pyrazol-1-ylmethyl)pyridine (L4) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)
pyridine (L5) with cobalt(II), iron(II) and nickel(II), Ni(L1)Cl2 (1), Co(L1)Cl2 (2), Fe(L1)Cl2 (3), Ni(L2)Cl2
(4), Ni(L3)Cl2 (5), Co(L3)Cl2 (6), Fe(L3)Cl2 (7), Ni(L4)Cl2 (8) and Ni(L5)Cl2 (9), were used as catalyst pre-
cursors to produce vinyl-addition type norbornene polymers. Both the identity of the metal center and
nature of ligand affected the polymerization behaviour of the resultant catalysts. Nickel catalysts were
generally more active than the corresponding iron and cobalt analogues. The polynorbornene produced
have high molecular weights (0.5–2.1 Â 106 g/mol) and narrow molecular weight distributions. Analyses
of polymer microstructure using NMR and IR spectroscopy confirmed the polymers produced to be vinyl-
addition polynorbornene.
Received 14 July 2011
Accepted 17 August 2011
Available online 7 September 2011
Keywords:
Iron
Cobalt
Nickel catalysts
Structures
Norbornene polymerization
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
complexes bearing phosphoraniminato ligands exhibit high cata-
lytic activities for the polymerization of norbornene to produce
Polynorbornene is a high performance speciality polymer pre-
pared from bicycle-[2.2.1]hept-2-ene, commonly known as nor-
bornene [1]. Norbornene polymerization can be achieved via
three pathways, namely; ring opening metathesis polymerization
(ROMP), cationic or radical polymerization and vinyl or addition
polymerization [2]. Polymers derived from free radical process
have undesirable properties such as low molecular weights and
irregular polymer microstructure. As such, these polymers are
not suitable for applications that require high tensile strength.
On the other hand, polynorbornene, produced via the vinyl-addi-
tion process [3] exhibits superior properties such as good mechan-
ical strength, high chemical resistance, optical transparency, low
birefringence, high glass transition temperatures, low dielectric
constant and large refractive indices [4]. Vinyl-addition polymeri-
zation is usually catalyzed by transition metal complexes [5].
Several complexes of iron, cobalt and nickel have been reported
as catalysts for vinyl-addition polymerization of norbornene and
strained cyclic olefins in general [5,6]. Among these complexes, cat-
alysts derived from nickel appear to be the most active in the vinyl-
addition polymerization of norbornene. For example, nickel(II)
high molecular weight polymers [6b]. In another related work,
vinyl-addition polymerization of norbornene catalyzed by neutral
salicylaldiminato nickel(II) complexes was reported to give very
high activities of up to 2.86 Â 106 g polymer/mol Ni h [6c]. Indeed
a number of nitrogen-donor nickel complexes have been found to
be very good catalysts for the vinyl polymerization of norbornene
[7].
To date, there are few reports in literature that employ pyrazole
or pyrazolyl-based metal complexes as catalysts for norbornene
polymerization. For instance, nickel and palladium complexes of
pyridine–pyrazolyl ligands display moderate activities of 100 kg
polymer/mol Ni h [8]. Over the past 10 years, we have extensively
demonstrated the versatility of late transition metal pyrazole and
pyrazolyl metal complexes as olefin oligomerization and polymer-
ization catalysts [9] and showed that the advantage of using pyraz-
olyl ligands as nitrogen donor lies in the weaker r-donor ability of
pyrazoles compared to pyridines, imines or other commonly used
nitrogen donors. Thus pyrazolyl metal complexes produce electro-
philic metal centers that promote coordination of substrates for
subsequent catalytic transformations. This therefore makes pyraz-
olyl metal complexes attractive for catalysis that involves coordi-
nation such as vinyl-addition polymerization of norbornene. In
the current work, we have investigated pyrazolyl complexes of
iron(II), cobalt(II) and nickel(II) as catalysts for the polymerization
of norbornene and herein report our results.
⇑
Corresponding author. Tel.: +27 (11) 559 2838; fax: +27 (11) 559 2819.
E-mail address: jdarkwa@uj.ac.za (J. Darkwa).
Present address: Department of Chemistry, Maseno University, Private Bag,
1
Maseno 40105, Kenya.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.08.021

L.L. Benade et al. / Polyhedron 30 (2011) 2878–2883
2879
3.53; N, 9.83%. ESI-MS (m/z): 439 {[M]+(35Cl), 100%}, 441
{[M]+(37Cl), 64.2%}. IR (ATR, cmÀ1): 1078 (C–C), 1588 (C@N).
2. Experimental
2.1. Materials and methods
2.2.5. [{2-((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine}FeCl2] (7)
Complex 7 was prepared from FeCl2 (0.05 g, 0.35 mmol) and L3
(0.11 g, 0.35 mmol) and isolated as a yellow solid. Yield = 0.10 g
(66%). Anal. Calc. for C21H17Cl2N3Fe: C, 57.57; H, 3.91; N, 9.59.
Found: C, 57.21; H, 4.25; N, 9.91%. ESI-MS (m/z): 436 {[(M+(35Cl),
100%}, 438 {[(M+(37Cl), 63.0%}. IR (ATR, cmÀ1): 1084 (C–C), 1593
(C@N).
All reactions were carried out under inert atmosphere using
standard Schlenk techniques. Solvents were dried over appropriate
drying reagents prior to use. Bicycle-[2.2.1]hept-2-ene (norborn-
ene) and methylaluminoxane (MAO) were purchased from Sigma
Aldrich and used without further purification. The ligands 2-(3,5-
dimethylpyrazolylmethyl)pyridine (L1), 2-((3,5-ditert-butyl-1
H-pyrazol-1-yl)methyl)pyridine (L2), 2-((3,5-diphenyl-1H-pyra-
2.2.6. [{2-((3,5-bis(trifluoromethyl)-1H-pyrazol-1-
zol-1-yl)methyl)pyridine
(L3),
2-(3,5-bis(trifluoromethyl)-1
yl)methyl)pyridine}NiCl2] (8)
H-pyrazol-1-yl)methyl)pyridine (L4) and 2,6 bis-(3,5-dimethyl-
1H-pyrazol-1-yl)methyl)pyridine (L5) [10] and complexes
Ni(L1)Cl2 (1), Ni(L2)Cl2 (4) and Ni(L5)Cl2 (9) were synthesized
according to literature procedures [10,11]. NMR spectra were re-
corded on a Varian Gemini 3000 instrument. IR spectra were re-
corded using a Bruker Tensor 27 FTIR using an Attenuated Total
Reflector (ATR) attachment. Microanalyses were performed on a
Vario Elementar III microcube CHNS analyser. High resolution
mass spectra for the Schiff base ligands were recorded on a Waters
API Q-TOF Ultima. Differential scanning calorimetry (DSC) analysis
was conducted on a Mettler Toledo DSC822. Molecular weight and
molecular weight distribution of polymers were determined by
size exclusion chromatography (SEC) on a WGE Q1000 Gel Perme-
ation Chromatograph.
Complex 8 was prepared from [NiCl2 (0.05 g, 0.40 mmol) and L4
(0.09 g, 0.40 mmol) as a green solid. Yield = 0.06 g (68%). Anal. Calc.
for C11H7Cl2F6N3Ni: C, 31.10; H, 1.66; N, 9.89. Found: C, 30.94; H,
1.36; N, 9.73%. ESI-MS (m/z): 422 {[(M+(35Cl), 99.7%}, 424
{[(M+(37Cl), 97.1%}. IR (ATR, cmÀ1): 1156 (C–C), 1607 (C@N).
2.3. Polymerization of norbornene
Polymerization reactions were carried out in a Schlenk tube
equipped with a magnetic stirrer bar. In a typical procedure, com-
plex 1 (6 mg, 20 lmol) was placed in a dry Schlenk tube, evacuated
and degassed toluene (30 mL) was added. The Schlenk tube was
flushed with nitrogen and norbornene monomer (5 mL) in toluene
(10 mL) added via a cannula. Polymerization was initiated by the
addition of 10 mL of methylaluminoxane (MAO) as co-catalyst.
After 1 h of reaction time, the polymer formed was precipitated
by addition of methanol to the reaction mixture. The polymer
was isolated by filtration and dried under vacuum.
2.2. Synthesis of iron and cobalt complexes
2.2.1. [{2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)pyridine}CoCl2] (2)
To a suspension of CoCl2 (0.10 g, 0.75 mmol) in dichlorometh-
ane (20 mL) was added a solution of L1 (0.14 g, 0.75 mmol) in
dichloromethane (20 mL). The solution was allowed to stir for
24 h and the complex precipitated by slow addition of hexane to
afford compound 2 as a blue solid. Recrystallization from CH2Cl2/
hexane mixture at À4 °C gave single crystals suitable for X-ray
analyses. Yield = 0.17 g (69%). Anal. Calc. for C11H13Cl2N3Co: C,
41.67; H, 4.13; N, 13.25. Found: C, 41.41; H, 4.54; N, 13.60%. ESI-
MS (m/z): 316 {[M]+(35Cl), 100%}, 318 {[M]+(37Cl), 64.1%}. IR (ATR,
cmÀ1): 1083 (C–C), 1590 (C@N).
2.4. Single crystal X-ray crystallography
A typical experiment is described here for 2. Suitable crystals
were selected under oil, under ambient conditions and attached
to the tip of a MiTeGen MicroMountÓ. The crystal was mounted
in a stream of cold nitrogen at 100(2) K and centered in the X-
ray beam by using a video camera. The crystal evaluation and data
collection were performed on a Bruker CCD-1000 diffractometer
with MoK
crystal distance of 4.9 cm. The initial cell constants were obtained
from three series of scans at different starting angles. Each series
consisted of 20 frames collected at intervals of 0.3° in a 6° range
about with the exposure time of 10 s per frame. A total of 67
a (k = 0.71073 Å) radiation and the diffractometer to
Compounds 3, 6, 7 and 8 were prepared using the procedure de-
scribed for 2.
x
x
2.2.2. [{2-((3,5-dimethyl-1H-pyrazole-1-yl)methyl)pyridine}FeCl2] (3)
Complex 3 was prepared from FeCl2 (0.10 g, 0.75 mmol) and L1
(0.14 g, 0.75 mmol) and isolated as a brown solid. Yield = 0.18 g
(77%). Anal. Calc. for C11H13Cl2N3Fe: C, 42.08; H, 4.17; N, 13.38.
Found: C, 41.72; H, 4.04; N, 13.80%. ESI-MS (m/z): 312
{[M]+(35Cl), 100%}, 314 {[M]+(37Cl), 62.5%}. IR (ATR, cmÀ1): 1086
(C–C), 1592 (C@N).
reflections were obtained. The reflections were successfully in-
dexed by an automated indexing routine built in the SMART pro-
gram. The final cell constants were calculated from a set of 6406
strong reflections from the actual data collection. The data were
collected by using the full sphere data collection routine to survey
the reciprocal space to the extent of a full sphere to a resolution of
0.71 Å. A total of 9431 data were harvested by collecting four sets
of frames with 0.3° scans in
x and one set with 0.45° scans in u
2.2.3. [2-((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine}NiCl2] (5)
Complex 5 was prepared from NiCl2 (0.05 g, 0.42 mmol) and L3
(0.13 g, 0.42 mmol) as a green solid. Yield = 0.13 g (72%). Anal. Calc.
for C21H17Cl2N3Ni: C, 57.20; H, 3.89; N, 9.53. Found: C, 57.33; H,
3.52; N, 9.73%. ESI-MS (m/z): 439 {[M]+(35Cl), 100%}, 441
{[M]+(37Cl), 97.6%}. IR (ATR, cmÀ1): 1093 (C–C), 1594 (C@N).
with an exposure time 20 s per frame. These highly redundant
datasets were corrected for Lorentz and polarization effects. The
absorption correction was based on fitting a function to the empir-
ical transmission surface as sampled by multiple equivalent mea-
surements [12].
The systematic absences in the diffraction data were consistent
ꢀ
for the space groups P1 and P1. The E-statistics strongly suggested
ꢀ
2.2.4. [{2-((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine}CoCl2] (6)
Complex 6 was prepared from CoCl2 (0.06 g, 0.48 mmol) and L3
(0.15 g, 0.48 mmol) as a blue solid. Single crystals suitable for X-ray
analyses were grown by slow diffusion of hexane into a dichloro-
methane solution of 6. Yield: 0.13 g (59%). Anal. Calc. for
the centrosymmetric space group P1 that yielded chemically rea-
sonable and computationally stable results of refinement [13] A
successful solution by the direct methods provided most non-
hydrogen atoms from the E-map. The remaining non-hydrogen
atoms were located in an alternating series of least-squares cycles
and difference Fourier maps. All non-hydrogen atoms were refined
C21H17Cl2N3Co: C, 57.17; H, 3.88; N, 9.52. Found: C, 57.00; H,

2880
L.L. Benade et al. / Polyhedron 30 (2011) 2878–2883
from single crystal X-ray crystallographic analyses confirmed these
structures.
Crystals of complexes 2 and 6 suitable for single crystals X-ray
MCl2
N
analyses were grown by slow diffusion of hexane into a CH2Cl2
solution of the corresponding complex at À4 °C. The data collection
and refinement parameters are given in Table 1 while selected
bond lengths and angles of 2 and 6 are listed in Table 2. Molecular
structures of 2 and 6 are shown in Figs. 1 and 2, respectively. In the
solid state structures of 2 and 6, the cobalt center adopts a
distorted tetrahedral geometry, in which the coordination sphere
around the metal centers consist of either the bidentate ligand
L1 or L3 and two chloride atoms. The Co–N and Co–Cl distances
in 2 and 6 fall in the usual ranges [14]. The coordination environ-
ment of the Co center is 6 is substantially more distorted than that
in 2. Two parameters are very indicative. There first one is the
dihedral angle between the planes defined by atoms Co1, N1, N3
and Co1, Cl1, Cl2. In 6, this angle is 98.37(13)°, whereas in 2 it is
closer to 90° at 86.23(5)°. The second parameter is the angle be-
tween the midpoint between atoms N1 and N3, Co1 and the mid-
point between Cl1 and Cl2. This angle is 180° in an ideal
tetrahedron, but in 2 it is 178.9° and a dramatic 169.9° in 6. Ligand
L3 occupies 41.8% of the Co coordination sphere in 6 whereas L1
shields only 38.6% of the metal center in 2. The closer position of
the N atoms to the Co center in 6 results in a slightly wider N–
Co–N angle in 6. Thus, the apparent bulk of the L3 ligand in 6
and the mutual molecular arrangement in the lattice cause the dis-
tortion. Interestingly, the two N atoms in 6 approach the metal
center somewhat closer than in 2 despite the fact that ligand L3
is substantially more sterically demanding than L1.
N
N
Cl
N
M
R
CH2Cl2
R
N
N
Cl
R
R
R = Me, M = Ni, (1); R = Me, M = Co, (2); R = Me, M = Fe (3); R = tBu, M = Ni, (4);
R = Ph, M = Ni, (5); R = Ph, M = Co, (6); R = Ph, M = Fe, (7); R = CF3, M = Ni, (8).
Scheme 1. Synthesis of 2-(pyrazolylmethyl)pyridine iron, cobalt and nickel com-
plexes 1–8.
with anisotropic displacement coefficients. All hydrogen atoms
were included in the structure factor calculation at idealized posi-
tions and were allowed to ride on the neighboring atoms with
relative isotropic displacement coefficients.
The crystal proved to be a non-merohedral twin with the minor
component contribution of approximately 15%. The refinements
based on data from both twin components were worse than the
one based on the reflections from the major component; thus the
results of based on the major component are presented herein.
The final least-squares refinement of 156 parameters against
3581 data resulted in residuals R (based on F2 for I P 2
r) and wR
(based on F2 for all data) of 0.0385 and 0.1080, respectively. The fi-
nal difference Fourier map was featureless. The molecular diagram
is drawn with 50% probability ellipsoids.
3.2. Polymerization of norbornene
Complexes 1–9 were investigated for their ability to catalyze
the polymerization of norbornene using MAO as the co-catalyst.
Table 3 contains the optimization results for the norbornene poly-
merization using catalyst 1, while Table 4 shows all the polymeri-
zation data for catalysts 1–9.
3. Results and discussion
3.1. Synthesis and characterization of metal complexes
The general procedure for the preparation of complexes 1–8 is
shown in Scheme 1. Whereas the nickel complexes 1, 4 and 9 are
known [11], the rest of the complexes are new.
First, catalyst 1 was used to investigate the effect of co-catalyst
to catalyst ratio by varying the Al/Ni ratio from 250:1 to 1500:1.
The optimum ratio was found to be 1000:1 (Table 3, entries 1–4).
It was clear that lower ratios produced fewer active sites to initiate
the polymerization reaction than at 1000:1, leading to reduced
activity. Similar observations have been reported for the [(3,5-di-
Due to the paramagnetic nature of the isolated iron, cobalt and
nickel complexes, 1H NMR spectra of these complexes were broad
and showed large contact shifts; hence NMR spectroscopy was not
useful in their characterization. A combination of micro-analyses,
mass spectrometry and single crystal X-ray crystallography for 2
and 6 were thus used to elucidate their structures. Indeed molecu-
lar ions from the mass spectra were consistent with the molecular
weights of monometallic compounds. For instance, the molecular
ions of 2 and 3 were observed at M+ = 318 and 314, respectively
correspond to the molecular formulae of these compounds pro-
posed in Schemes 1 and 2. The solid state structures of 2 and 6
methyl-pyrazol-1-yl)-phenyl-methylene]-phenyl-amine
nickel
complex by Wang and co-workers, where a decrease in Al/Ni ratio
from 800:1 to 400:1 resulted in a decrease in the activity from
6.64 Â 105 to 2.21 Â 105 g/mol [15]. Increasing reaction time for
catalyst 1 from 60 to 90 min resulted in significant increase in per-
centage conversion from 90% to 99% (Table 3, entries 3 and 6). This
indicates the stability of catalyst 1 in the norbornene polymeriza-
tion reaction. It is also well known that monomer concentration
NiCl2
N
N
N
N
N
Ni
N
CH2Cl2
N
N
N
N
Cl Cl
(9)
Scheme 2. Synthesis of 2,6-bis(3,5-dimethylpyrazolylmethyl)pyridine nickel complex 9.

L.L. Benade et al. / Polyhedron 30 (2011) 2878–2883
2881
Table 1
Crystal and structure refinement data for complexes 2 and 6.
Parameters
2
6
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
11H13Cl2CoN3
C21H17Cl2CoN3
443.22
100(2)
1.54178
triclinic
P212121
7.6585(3)
15.3491(6)
16.3800(6)
90
90
90
1925.49(13)
4
1.529
9.618
908
317.07
100(2)
0.71073
triclinic
ꢀ
p1
8.136(2)
8.722(2)
10.206(3)
85.603(4)
79.773(4)
64.061(4)
641.0(3)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Dcalc (mg/m3)
1.643
1.735
322
Absorption coefficient (mmÀ1
F(000)
)
Theta range for collection (°)
Index ranges
2.60–30.06
À10 6 h 6 11
À12 6 k 6 12
0 6 l 6 14
3581
5.40–64.40
À8 6 h 6 8
À14 6 k 6 17
À19 6 l 6 19
29911
Fig. 1. Molecular structure of complex 2. All hydrogen atoms have been omitted for
Reflections collected
clarity.
Independent reflections
Completeness to theta = 25.0° 99.8%
3581 [Rint = 0.0483]
3086 [Rint = 0.0470]
92.7%
Maximum and minimum
0.8457 and 0.5157
0.3678 and 0.1251
transmission
Refinement method
full-matrix least-
squares F2
full-matrix least-
squares F2
Data/restraints/parameters
3581/0/156
0.991
3086/0/245
1.034
Goodness-of-Fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0385,
wR2 = 0.1044
R1 = 0.0463,
wR2 = 0.1080
1.346 and À0.489
R1 = 0.0491,
wR2 = 0.1201
R1 = 0.0535,
wR2 = 0.1239
1.12 and À0.66
Largest difference peak and
hole (e ÅÀ3
)
Table 2
Selected bond lengths (Å) and angles (°) for complexes 2 and 6.
2
6
Bond length (Å)
Co(1)–N(1)
Co(1)–N(3)
Co(1)–Cl(1)
Co(1)–Cl(2)
2.0503(19)
2.0609(19)
2.2236(8)
2.2250(9)
Co(1)–N(1)
Co(1)–N(3)
Co(1)–Cl(1)
Co(1)–Cl(2)
2.023(4)
2.048(4)
2.2457(12)
2.2331(11)
Bond angles (°)
N(1)–Co(1)–N(3)
N(1)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
N(1)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
Cl(1)–Co(1)–Cl(2)
90.97(8)
N(1)–Co(1)–N(3)
N(1)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
N(1)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
Cl(1)–Co(1)–Cl(2)
93.29(14)
123.04(10)
103.89(10)
106.60(10)
116.12(5)
116.12(5)
113.45(6)
108.55(6)
110.00(6)
114.93(5)
116.30(3)
Fig. 2. Molecular structure of complex 6. All hydrogen atoms have been omitted for
clarity.
affect the rates of olefin polymerization reactions [15]. Doubling
the norbornene/Ni ratio from 2500:1 to 5000:1 resulted in an in-
crease in polymer yield, but showed a drastic drop in percentage
conversion from 90% to 60% (Table 3, entries 3 and 5). We attribute
this decrease in percentage conversion to reduced number of ac-
tive sites of the catalyst that is accessible to the monomer. A sim-
ilar observation was also made by Myagmarsuren and co-workers
[16] where they reported a slight increase in polymer yield but a
large decrease in turn-over number with increase in monomer
concentration.
from 500:1 to 2500:1 was followed by a slight decrease in molec-
ular weight from 10.5 Â 105 to 9.6 Â 10 g/mol. This trend contrast
the observations made for salicylaldimnato nickel(II) catalysts
where an increase in Al/Ni from 500:1 to 2000:1 resulted in an in-
crease in polymer molecular weight from 5.73 Â 105 to 8.40 Â
105 g/mol [7b]. We, however, observed that there was no signifi-
cant influence of monomer concentration on polymer molecular
weight at norbornene/Ni ratios of 2500:1 and 5000:1. This is also
consistent with vinyl-addition polymerization process where chain
termination occurs via b-hydride elimination but not transfer of
polymer chain to the monomer [17].
Generally high molecular weight polymers of up to 10.5 Â
105 g/mol were obtained, typical of vinyl-addition type polynor-
bornene [1]. Changing the reaction conditions also affected the
polymer molecular weight. For example, an increase in Al/Ni ratio
The effect of catalyst structure on norbornene polymerization
activity was also investigated under the optimized conditions of

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L.L. Benade et al. / Polyhedron 30 (2011) 2878–2883
Table 3
Table 4
Influence of reaction parameters on the norbornene polymerization activity of
Effect of catalyst structure on polymerization of norbornene.a
catalyst 1.a
Yield (g)b
Yield (%)c
Mw  105d
Mw/Mn
d
Entry
Catalyst
Yield (g)b
Yield (%)c
Mw  105d
Mw/Mn
d
Entry
Al:Ni
[NBE/Ni]
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
4.50
1.50
3.05
3.50
3.70
1.35
2.50
3.02
3.21
90
31
61
70
74
27
50
61
64
5.13
6.05
6.47
5.33
nde
2.38
2.76
3.01
2.18
–
–
–
2.64
2.44
1
2
3
4
250
500
1000
1500
1000
1000
2500
2500
2500
2500
5000
2500
1.26
2.80
4.50
4.45
6.00
4.95
25
56
90
89
60
99
9.55
10.05
9.58
9.56
9.57
9.53
2.22
2.25
2.21
2.21
2.31
2.20
5
nde
6e
nde
5.17
21.01
a
Polymerization conditions: reaction time, t = 60 min; catalyst: 20 lmol; Al/
Ni = 1000; monomer concentration [NBE/Ni] = 2500; solvent: toluene (40 mL); ratio
of norbornene monomer (NBE) to the nickel catalyst.
a
Polymerization conditions: reaction time, t = 60 min; catalyst: 20 lmol; Al/
b
Mass of polymer obtained.
Ni = 1000; [NBE/Ni] = 2500; solvent: toluene (40 mL).
c
Mass of polymer/mass of monomer  100.
b
Mass of polymer obtained.
d
Determined by gel permeation chromatography.
c
Mass of polymer/mass of monomer  100.
e
Time of reaction, t = 90 min.
d
Determined by gel permeation chromatography.
e
Not determined.
N
Cl
N
Ni
N
Me
N
Cl
Cl
N
Ni
100
80
60
40
20
0
Ph
N
N
N
N
Cl
Me
N
M
N
Cl
Cl
N
Fe
N
Ni
Me
F3C
N
N
N
N
Cl
Cl
Ph
Cl
Cl
Me
CF3
N
Cl
N
Co
Me
N
Cl
Me
1
2
3
5
8
9
Catalyst
Fig. 3. Effect of catalyst structure on catalyst activity in norbornene polymerization.
Al/Ni of 1000, 1 h and norbornene/catalyst ratio of 2500:1 (Table
4). The nature of the metal was found to have a profound effect
on the catalytic activity. Under similar conditions, nickel catalysts
1 and 5, exhibited much higher activities than the corresponding
iron and cobalt complexes (2, 3, 6 and 7) (Fig. 3). For example,
while the nickel catalyst 1 gave 90% conversion, the corresponding
iron and cobalt catalysts, 2 and 3, gave conversions of 31% and 61%,
respectively (Table 4, entries 1–3). This is consistent with higher
catalytic activities generally observed for nickel in olefin transfor-
mation reactions compared to cobalt and iron analogues [18]. We
have also recently observed similar trend in ethylene oligomeriza-
tion where nickel complexes of L1 and L3 are highly active, while
the palladium, cobalt and iron show very low activity [9e,11].
Both the electronic and steric properties of the ligands influ-
enced the norbornene polymerization activity. For example, cata-
lyst 1 bearing electron-donating methyl groups on the pyrazolyl
motif showed greater activity (90%) than the corresponding cata-
lyst (8) containing electron-withdrawing CF3 groups (61%). It is
generally believed that electron-donating groups increased the
stability of the active species while electron-withdrawing groups
reduces the stability of the active species [7a]. Steric factors also
influenced the catalytic properties of the complexes used in this
study. While the iron catalyst bearing methyl groups (3) showed
conversions of 61%, the phenyl analogue (7) gave conversions of
50%. A similar trend was observed for the nickel and cobalt cata-
lysts (Fig. 3). A possible reason for this decrease in activity with in-
creased steric bulk could be reduced accessibility of the monomer
to the vacant metal center. This hypothesis is also supported by the
low activity of the crowded tridentate-bound catalyst 9 (64%) in
comparison to the more accessible bidentate-bound analogue, 1,
(90%).
The molecular weight of the polynorbornene produced was also
regulated by the structure of the catalysts. An increase in steric
bulk from methyl group in 1 to tert-butyl group in 4 resulted in
an increase in molecular weight from 5.13 Â 105 to 5.33 Â 105. This
is normal for addition polymerization reactions and results from
reduced b-hydride elimination with increase in steric hindrance
[17]. A more interesting observation made in this work was the ex-
treme high molecular weights of polymers obtained using catalyst
9 containing the tridentate ligand L5 which shields the metal atom
by 59.6% [9e]; hence promoting chain propagation over chain ter-
mination as compared to the smaller bidentate ligands L1–L5
which shields the metal atom by 38.8–46.5% [11].
All the polymers obtained were slightly soluble in benzene,
chlorobenzene and other chlorinated solvents indicating low ste-
reoregularity. Attempts to determine melting points and glass
transition temperatures were not successful. TGA-DSC analyses
showed the polymers decomposed before melting. Thus attempts
to determine melting points and glass transition temperatures of
the polymers were not successful.
The polymer microstructure was investigated by 1H NMR, 13C
NMR and IR spectroscopy. All the polymers showed very similar
NMR and IR spectra, indicating that the type of polynorbonene
produced is independent of the catalyst system and reaction

L.L. Benade et al. / Polyhedron 30 (2011) 2878–2883
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2883
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spectra, typical of vinyl-addition type polynorbonene [20]. The
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The authors would like to thank the National Research Founda-
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in Catalysis (c⁄change) South Africa and University of Johannesburg
for financial support.
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