Generation effects on the microstructure and product distribution in ethylene polymerization promoted by dendritic nickel catalysts

Article abstract of DOI:10.1039/b511379m

Carbosilane dendrimers Gn-((ONNMe₂)NiBr₂] _m, containing up to sixteen terminal pyridylimine nickel complexes, have been studied as catalysts for polymerization of ethylene; the microstructure and the oligomer/polymer distr

Full text of DOI:10.1039/b511379m

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Generation effects on the microstructure and product distribution in
ethylene polymerization promoted by dendritic nickel catalysts{
Jose´ M. Benito, Ernesto de Jesu´s,* F. Javier de la Mata, Juan C. Flores* and Rafael Go´mez
Received (in Cambridge, UK) 9th August 2005, Accepted 5th September 2005
First published as an Advance Article on the web 22nd September 2005
DOI: 10.1039/b511379m
dendritic (n . 0) nickel complexes 6–9, as well the monometallic
6OH, are synthesized through the replacement of the labile ligand
in [NiBr2(DME)] by the corresponding chelate ligand 2–5 in
dichloromethane (Scheme 1) (see ESI{).
Carbosilane dendrimers Gn–[(ONNMe2)NiBr2]m, containing
up to sixteen terminal pyridylimine nickel complexes, have
been studied as catalysts for polymerization of ethylene;
the microstructure and the oligomer/polymer distribution are
significantly affected by the generation of the dendritic
precursor.
All of the compounds gave satisfactory elemental analyses.
The complete replacement of the chloride atom in the starting
Gn–(Cl)m (i.e., RMe2Si–Cl) by pyridylimine groups in 2–5, is
confirmed by the shift of the SiMe2 NMR resonances from d 0.4
(1H) and 0.2 (13C) to about 0.2 (1H) and 21.0 (13C). The NMR
studies of Ni(II) complexes 6–9 are not informative because of the
interfering effect caused by their paramagnetism. However, their
IR spectra show significant changes in the absorptions correspond-
ing to nCLN, indicating the coordination of the pyridylimine
bidentate ligand to the nickel center.10 The solubility of all these
compounds diminishes with the increase of dendrimer generation,
and the low solubility of the nickel derivatives limits the prepara-
tion of higher generations.
Monometallic (6, 6OH) and dendritic (7–9) nickel complexes
combined with MAO become readily soluble in toluene, and are
active in ethylene catalysis.{ They give a mixture of a toluene-
soluble fraction of oligomers together with solid polyethylene (PE),
with low to moderate activities according to Gibson’s classification
(Table 1).11 The catalytic activities are maximal at the beginning of
the reaction and steadily decrease over the time. The oligomeric
oily products are composed of even olefins up to C34. The number
average molecular weight of these oligomers is in the range
Late-transition metal complexes have been shown to be important
catalysts in the polymerization of olefins.1 Most relevant to our
present contribution are Ni(II) N/N pyridylimine-type com-
pounds.2 These and related catalysts produce ethylene derivatives
ranging from light oligomers to high molecular weight poly-
ethylene (PE), with linear to hyperbranched, or even dendritic,
microstructures as a result of the so called ‘‘chain-walking
mechanism’’.3 The polymer topology is precisely controlled
through simple variation of the reaction conditions, or by struc-
tural steric/electronic tuning of the ancillary ligands.4 Encouraged
by the fact that dendrimers can be used as well-defined supports
for active metal centers in the homogeneous phase, their catalytic
applications are being widely studied.5 Pioneering work on
dendrimer-bound metal catalysts highlighted their possible separa-
tion from the product stream by nanofiltration methods.6 In
addition, several positive dendritic effects on catalyst properties
have been observed for dendrimers with peripheral7 or embedded8
active metal centers. So far, only a few examples of metallo-
dendritic systems have been studied in oligomerization8 and
polymerization9 processes. Here, we report on a nickel catalyst
system, based on pyridylimine-ended carbosilane dendrimers, that
exhibits a significant dendritic effect on ethylene catalysis.
The pyridylimine ligand 1 (Chart 1) is synthesized by condensa-
tion in toluene of pyridine-2-carbaldehyde with 4-amino-2,5-
dimethylphenol. Subsequent silylation of the hydroxy group with
chlorosilanes Gn–(Cl)m, in the presence of NEt3, results in the
formation of derivatives 2–5. The monometallic (n 5 0) and
Chart 1
Departamento de Qu´ımica Inorga´nica, Universidad de Alcala´, Campus
Universitario, 28871, Alcala´ de Henares, Madrid, Spain.
E-mail: ernesto.dejesus@uah.es; juanc.flores@uah.es;
Fax: +34 91 885 4683
{ Electronic supplementary information (ESI) available: Synthesis and
characterizing data for the new compounds, and ethylene catalytic
reactions details. See http://dx.doi.org/10.1039/b511379m
Scheme 1 Reagents: m equiv. of: (i) 1/NEt3 in THF, (ii) [NiBr2(DME)] in
CH2Cl2.
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Table 1 Ethylene oligomerization/polymerization catalyzed by precursors 6–9a
Oligomers
Polymer
Cn branches/1000 Ci
n 5 1/2–5/¢6 Total 1023Mw
Branches/
100 Cf,g
Activityb,c
ad,e
Mn
Activityb
Tmh/uC
PDIj
f
j
Precatalyst
Generation
6OH
Monometallic
Monometallic
First
Second
Third
3.70
5.21
8.32
10.21
13.40
0.65 341.1
0.70 265.8
0.60 284.5
0.51 342.5
0.45 327.6
4
3
3
3
3
2.09
13.21
14.88
5.12
121.4
117.1
120.5
123.2
124.7
10/18/48
12/36/97
17/22/62
13/11/38
12/10/36
76
145
101
62
5.88
1.64
2.16
10.62
17.07
6.61
3.58
2.77
11.25
14.06
6
7
8
9
a
4.82
58
b
Conditions: 50 ml toluene; n(Ni) 5 14 mmol; Al/Ni 5 1000; tp 5 24 h; Tp 5 20 uC; pCH 5 2 bar; activities ¡7%. Activity: g/(mmolNi
h
2
4
bar). Corrected to consider the lower olefin lost during work-up. Schulz–Flory parameter a. Determined by GC. Determined by 1H
c
d
e
f
NMR. Total number of methyl per 100 C. Determined by DSC (second heating run). Determined by 13C NMR. Determined by GPC.
g
h
i
j
Mn 5 265–340, and they are characterized by Schulz–Flory chain
length distributions with a values from 0.45 to 0.70.§ A relatively
low branching is observed by 1H NMR (three branches per 100 C,
for C4 to C34 oligomers).12 In contrast, a higher degree of
branching is found in the solid polymers that makes some of them
to be considered hyperbranched.4 Tm values of all the samples are
in agreement with their level of chain branching, and are lower
than those typically found for HDPE (¢135 uC).13a Analysis by
GPC of the solid materials shows that weight average molecular
weights of the polymers are low (Mw 5 1600–17 000) with wide
distributions (PDI 5 3–14). The quantitative 13C NMR analyses
of the polymers formed show branching densities in the range of
60–170 total branches per 1000 main-chain carbons and an
abundant amount of branches are relatively long (¢6 C).13 These
findings imply that polymerization active species walk along the
chain much further than the oligomerization ones.
this reduction in the number of branches is larger for the longest
branches than for the methyl ones.
The production of oligomers and polymers by eventual
formation of mononuclear nickel species by the split of the Si–O
bond after the addition of the cocatalyst, is excluded because of the
observed performance of compound 6OH (Table 1), which behaves
differently than the reference compound 6 and related dendrimers.
Therefore, protection at the phenolic oxygen, even with a very
simple carbosilane group (Me3Si–), seems to be positive in terms of
polymerization activity (compare the performance of 6OH vs. 6).
Although, the cooperative effect of other metal centers in close
proximity to the active site could play an important role, these
results might be interpreted as a consequence of the combination
of the ‘‘steric pressure’’ on the growing chains and ‘‘microenviron-
ment protection’’ of higher generation dendrimers on the poly-
merization species. Their bulkiness enhances chain transfer
processes, and favors the performance of the oligomerization
species, as reflected by lowering in a parameter and increasing in
activity, respectively. A larger size also affects the polymerization
active species encumbering the catalyst chain walking process
from going too deep into the coiled polymer chain, leading to less
Several trends arise when analyzing the data in Table 1 with
regard to the metallic nuclearity or generation of the precursors.
The activity leading to oligomers slightly increases whereas the
activity producing polymer diminishes for higher generations
(Fig. 1). Monometallic compound 6 is superior yielding PE than
oligomer, while the contrary is observed for polynuclear dendrimer
9, with sixteen Ni centers (Fig. 2). The Schulz–Flory parameter (a)
also decreases with increasing nuclearity. Conversely, higher
molecular weights (and PDIs) are observed for the polymers
produced by higher generations. The branching degree of the
polymers is also generation-dependent and decreases progressively
on going from the monometallic complex 6 to the G3 dendrimer 9;
Fig. 1 Activities by precursors 6OH and Gn–[(ONNMe2)NiBr2]m.
5218 | Chem. Commun., 2005, 5217–5219
Fig. 2 Metallodendrimer G3–[(ONNMe2)NiBr2]16 (9).
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produced by more protected active centers in larger dendrimers,
with lower activity, but higher Mw (and PDI). For instance, the
later is reasonable if back-folding of terminal active species into the
dendrimer matrix becomes significant at higher generation.
In summary, we have shown an example in which the size of
the dendrimer regulates the production of ethylene insertion
products (oligomer vs. polymer), the oligomer chain-length
distribution (a), and the branching density, molecular weight,
and polydispersity of the polymers. It is noteworthy that such
dendritic effect has been obtained with peripheral metallodendri-
mers when core- or focal-point-dendrimers are a priori more suited
systems for these purposes.
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We thank financial support from the DGI-Ministerio de Ciencia
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Madrid (project GR/MAT/0733/2004). In addition, we express
our gratitude to Carmen Mart´ınez and Carlos Mart´ın, REPSOL
YPF, for their valuable assistance with 13C NMR polymer
analyses, and Ma Teresa Rodr´ıguez Laguna, Dpto. Qu´ımica F´ısica
(UAH), for her significant help with GPC polymer analyses.
Notes and references
{ We found the starting material [NiBr2(DME)] to be totally inactive under
the same reaction conditions.
§ Schulz–Flory parameter a 5 kp/(kp + kct) 5 mol of Cn+2/mol of Cn;
kp 5 rate of propagation, kct 5 rate of chain transfer.
1 (a) V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283; (b)
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This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 5217–5219 | 5219