Olefin polymerization promoted by a stereorigid bridged diiminobis(phenolate) zirconium complex

Article abstract of DOI:10.1021/ma0492115

Two diiminobis(phenolate) zirconium complexes [C₆H ₁₀-{N=CH-(3,5-^tBu₂C₆H ₂-2-O)-kO}₂]-ZrX₂ [X = CH₂Ph (1), Cl (3)] have been synthesized and characterized by NMR solution spectroscopy. Complex 1 was found conformationally stable and isolated in the octahedral C₂ symmetric form. When exposed to the light in hydrocarbon solution, 1 readily undergoes to 1,2 benzyl migration converting the diiminobis(phenolate) ligand in the corresponding benzylamidoiminobis(phenolate) . Complex 3 is more thermally stable and found in slow equilibrium between the cis-α and cis-β forms with the former largely prevalent (9:1 molar ratio). The rate of interconversion between the two diastereoisomers was estimated in 3.6 × 10^-1 s^-1 by NMR exchange spectroscopy (EXSY). 1 and 3 are active ethylene polymerization catalysts after reaction with MAO or [CPh₃] [B(C₆F₅) ₄]/Al-ⁱBu₃. The polydispersity index of polyethylene produced by 1 is about 2, suggesting the presence of a single active species and that the symmetry of the precatalyst is retained during polymerization. The catalyst is stable over 3 h, and the polymerization activity linearly increases with time. Copolymers of ethylene with propylene or 1-hexene were synthesized, and the ¹³C NMR analysis of their microstructure suggested the highly regioregular 1,2 insertion of the 1-olefin and the inability of the ligand environment to express stereochemical control during monomer insertion.

Full text of DOI:10.1021/ma0492115

Macromolecules 2004, 37, 7469-7476
7469
Olefin Polymerization Promoted by a Stereorigid Bridged
Diiminobis(phenolate) Zirconium Complex†
‡
‡
§
§
Cin zia Cu om o, Ma r ia Str ia n ese, Tom a´ s Cu en ca , Ma r tia l Sa n z, a n d
,
‡
Alfon so Gr a ssi*
Dipartimento di Chimica, Universit a` di Salerno, 84081 Baronissi, Italy, and Departamento de
Qu ı´ mica Inorg a´ nica, Universidad de Alcal a´ , Campus Universitario, Edificio de Farmacia,
2
8871 Alcal a´ de Henares, Spain
Received April 22, 2004; Revised Manuscript Received J une 24, 2004
t
ABSTRACT: Two diiminobis(phenolate) zirconium complexes [C
ZrX [X ) CH Ph (1), Cl (3)] have been synthesized and characterized by NMR solution spectroscopy.
Complex 1 was found conformationally stable and isolated in the octahedral C symmetric form. When
6 2 6 2 2
H10-{NdCH-(3,5- Bu C H -2-O)-κO} ]-
2
2
2
exposed to the light in hydrocarbon solution, 1 readily undergoes to 1,2 benzyl migration converting the
diiminobis(phenolate) ligand in the corresponding benzylamidoiminobis(phenolate). Complex 3 is more
thermally stable and found in slow equilibrium between the cis-R and cis-â forms with the former largely
prevalent (9:1 molar ratio). The rate of interconversion between the two diastereoisomers was estimated
-
1
-1
in 3.6 × 10
s
by NMR exchange spectroscopy (EXSY). 1 and 3 are active ethylene polymerization
i
catalysts after reaction with MAO or [CPh
3
6 5 4 3
][B(C F ) ]/Al- Bu . The polydispersity index of polyethylene
produced by 1 is about 2, suggesting the presence of a single active species and that the symmetry of the
precatalyst is retained during polymerization. The catalyst is stable over 3 h, and the polymerization
activity linearly increases with time. Copolymers of ethylene with propylene or 1-hexene were synthesized,
1
3
and the C NMR analysis of their microstructure suggested the highly regioregular 1,2 insertion of the
-olefin and the inability of the ligand environment to express stereochemical control during monomer
insertion.
1
-10
1
. In tr od u ction
Despite the efforts devoted over the past two decades
successfully obtained,7
and the steric hindrance at
the ortho position of the phenoxo ligand was found
critical for the living properties and stereospecificity of
the catalyst. Propylene polymerization catalyzed by the
same complexes proceeds via highly isospecific primary
insertion of the monomer.
Recently, living olefin polymerization catalysts based
on bis(iminophenolate) derivatives of group 4 metals
by scientists of industrial and academic teams in the
field of homogeneous olefin polymerization catalysis, the
performances of the heterogeneous Ziegler-Natta cata-
lysts in isotactic specific polymerization of 1-olefins are
still unequaled. The discovery of the group 4 metallocene
catalysts permitted a deep knowledge of the olefin
polymerization mechanism and the synthesis of novel
1
1
1
2-15
were achieved independently by Fujita et al.
and
1
6
Coates et al. The living properties of this new class of
titanium-based octahedral olefin polymerization cata-
lysts are very attractive, and syndiotactic polypropyl-
polymers as syndiotactic, hemi-isotactic, or stereo-block
polypropylene.1-4 However, the polymerization activity
and the stereo- and regiospecificity of the heterogeneous
Ziegler-Natta catalysts are still the highest, and the
search for novel homogeneous catalysts mimicking the
behavior of the active sites on the surface of the
heterogeneous Ziegler-Natta catalysts is thus a topic
of current interest.
1
7-19
ene
and propylene-ethylene block copolymers were
20
synthesized. Despite the C2 symmetry of the titanium-
based precatalysts, the resulting polypropylene exhibits
syndiotactic structure arising from secondary monomer
insertion and a chain end controlled mechanism.1
Some recent publications tried to rationalize these
results on the basis of QM/MM calculation and proposed
a chain end stereocontrol mediated by the site chirality.
In the frame of fast Λ-∆ configurational isomerization
of the octahedral titanium catalyst it was shown that
e.g. the re-chain ending would favor by about 5.9 kJ
8,21
Group 4 metal complexes in octahedral environment
have been synthesized using different ligand sets in
order to get the appropriate symmetry and steric
hindrance at the metal center to produce the desired
site controlled stereospecific olefin polymerization.
Eisen claimed the synthesis of isotactic poly(1-hexene)
and polypropylene via the enantiomorphic site con-
trolled mechanism using racemic C2 symmetric bis-
-
1
mol the formation of the Λ complex which, in turn,
dictates the coordination-insertion of propylene with
the si enantioface (si-propylene), with the latter favored
by about 7.9 kJ mol over the re-propylene. This could
explain why, despite the C2 symmetric structure of the
precatalysts in the solid state, syndiotactic polypropyl-
ene is produced. The QM/MM calculation also showed
that, by passing from titanium to zirconium, a complex
combination of steric and electronic factors would favor
the primary insertion of propylene in the case of
(
amidinate) zirconium complexes.5,6
-1
22
Kol obtained zirconium complexes with the same
symmetry properties using the diamine bis(phenolate)
ligand: isospecific living 1-hexene polymerization was
†
Dedicated to Prof. Adolfo Zambelli on the occasion of his 70th
birthday.
‡
Universit a` di Salerno.
Universidad de Alcal a´ .
23
zirconium. An experimental comparison of the perfor-
mances in propylene polymerization of bis(iminophenol-
ate) complexes of zirconium and titanium, namely
§
*
To whom correspondence should be addressed. E-mail:
agrassi@unisa.it.
1
0.1021/ma0492115 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/10/2004

7
470 Cuomo et al.
Macromolecules, Vol. 37, No. 20, 2004
Sch em e 1. Geom etr ic Ster eoisom er s of Octa h ed r a l
Com p lexes w ith N,N-Br id ged Diim in obis(p h en ola te)
Liga n d
ence of two singlets at 1.85 and 1.41 ppm and only one
t
signal at 8.04 ppm for the Bu and aldimino groups,
respectively, confirms the proposed structure for 1. The
benzyl zirconium complex is stable for months in solid
state when stored in a drybox protected from light but
in solution is readily, quantitatively, and selectively
converted to the imino-amido derivative [C6H10-
t
{
NdCH-(3,5- Bu2C6H2-2-O)-κO}{N-CH(CH2Ph)-(3,5-
F igu r e 1. 1H NMR spectrum (C
D
, 25 °C, δ in ppm) of
t
t
6
6
Bu2C6H2-2-O)-κO,κN}]Zr(CH2Ph) [(Bn Bu4salcyen)ZrBn,
t
(
4 2
Bu salcyen)ZrBn (1).
(
2)] (toluene, 1 day at 25 °C; 90 min at 35 °C) via benzyl
migration from zirconium to the imine carbon (Scheme
bis[N-(3-methylsalicylidene)-2,3,4,5,6-pentafluoroaniline]-
MCl2, confirmed the prevailingly primary insertion of
propylene with the zirconium-based catalysts (with
about 3.5 mol % of regioinversion) where secondary
insertion is remarkably favored for titanium.24 Despite
the C2 symmetry of the zirconium complex in the solid
state, the stereospecificity was found poor, and partly
syndiotactic polypropylene was obtained.
2
). This intramolecular migration was previously ob-
28
29
served by Floriani et al. and Scott et al. for the
neutral bis(aryliminophenolate)benzyl complexes of
titanium and zirconium. A radical mechanism was
hypothesized promoted by the presence of a stable
radical leaving group (i.e., benzyl): different alkyl
complexes bearing e.g. neopentyl groups were found
30
thermally stable for days in solution. A kinetic study
Here we report on the synthesis and catalytic activity
in olefin polymerization of stereorigid bridged diimino-
bis(phenolate) zirconium complexes with C2 symmetry.
In our opinion the presence of the stereorigid bridge
would hamper site isomerization during the polymeri-
zation process preserving the symmetry of the precata-
lysts during the polymerization process and yield mono-
modal, narrow polydispersion of the molecular weight.
As a matter of fact, it was previously shown that uni-,
bi-, and multimodal polyethylene were produced with
bis(iminophenolate) zirconium complexes as a result of
of the thermal decomposition of 1 yielding the single
1
product 2 was undertook by H NMR spectroscopy at
1
3
08 K using the H-imine signal and the residual protio
signal of benzene-d6 as reference. The expected first-
order kinetics corresponding to the intramolecular 1,2-
migration of the benzyl group was actually observed
-2 -1
with a specific kinetic constant of 3.6 × 10
s
(see
Supporting Information).
The growth of a single crystal from solution of 1 is
thus not reliable as well as further characterization by
X-ray diffraction. Compound 2 was fully characterized
2
5
the fluxional behavior of the ligand environment.
1
13
1
by H and C NMR analysis. The H NMR (benzene-
d6, 25 °C) exhibits four singlets at 1.71, 1.67, 1.36, and
2
. Resu lts a n d Discu ssion
t
1
.35 ppm for the Bu groups, in agreement with lower
2
.1. Syn th esis of th e Liga n d a n d Com p lexes 1-3.
symmetry of the pentacoordinate monoalkyl species.
Two complex AB patterns for the methylene protons of
the two different benzyl groups, namely the one mi-
grated to the imine carbon (two doublets of doublets at
The preligand N,N′-(1R,2R)-cyclohexylenebis(3,5-di-tert-
butylsalicylidene imine) ( Bu4salcyenH2) was synthe-
sized using a modified literature procedure consisting
of condensation of 3,5-di-tert-butyl-2-hydroxybenzalde-
t
2
3
3.03 and 2.73 ppm; J HH ) 12.3 Hz, J HH ) 6.6 and 3.3
hyde with (1R,2R)-cyclohexanediamine in methanol
Hz) and the Zr-CH2Ph (two doublets at 2.56 and 2.36
catalyzed by formic acid.2
6,27
The reaction of the preli-
ppm; J HH ) 10.5 Hz). A doublet of doublets at 4.71 ppm
2
gand with ZrBn4 in toluene at room temperature readily
yields the corresponding C2 symmetric benzyl complex,
C6H10-{NdCH-(3,5- Bu2C6H2-2-O)-κO}2]Zr(CH2Ph)2
( Bu4salcyen)ZrBn2, (1)] with high selectivity through
alkane elimination reaction. The expected product was
isolated as a red-orange microcrystalline powder after
distillation of the solvent. During the workup a partial
decomposition of 1 was monitored by H NMR analysis,
and the formation of a minor product characterized by
for the methine protons of the amide group and a
doublet at 7.89 ppm (J ) 1.8 Hz) for the imine proton
were additionally detected. In the aromatic region, four
signals at 7.81, 7.56, 7.31, and 7.13 ppm were assigned
to not equivalent protons of the phenoxo groups. The
stereoselective formation of 2 further confirms the
proposed structure for 1. Actually, the benzyl migration
in the trans and cis-â forms of 1 would produce two
diastereoisomers resulting (at least formally) from the
attack of the two prochiral faces of the ligand or the
formation of two CR-amido carbons with opposite con-
figurations.
t
[
[
t
1
1
a well-defined H NMR pattern (see infra for more
details) was assessed. When the reaction was repeated
in pentane and the manipulation of the reaction solution
at room temperature is fast, 1 was isolated analytically
pure. The H NMR (benzene-d6, 25 °C) exhibits the
typical AB pattern (two doublets at 2.77 and 2.70 ppm;
J HH ) 7.6 Hz; see Figure 1) expected for the diaste-
reotopic methylene protons of the two cis benzyl groups
diagnostic of cis-R structure (see Scheme 1). The pres-
To get more stable catalyst precursors, we synthesized
the chloride analogue of 1 via metathesis of ZrCl4(THF)2
1
t
with the sodium salt of the preligand Bu4salcyenH2.
2
This reaction produced in toluene at room temperature
a yellow microcrystalline solid exhibiting elemental
analysis closely corresponding to that of the expected

Macromolecules, Vol. 37, No. 20, 2004
Olefin Polymerization 7471
Sch em e 2
1
product. However, the H NMR analysis revealed that
the reaction mixture consists of two isomers of general
carried out in toluene at room temperature in the
presence of triethylamine (Zr/N 1:2 molar ratio) pro-
duced 3a ,b (9:1 molar ratio) in 66% yield. The composi-
tion of the mixture of 3a and 3b resulted independent
of the synthetic route used and even after repeated
crystallizations in different solvents and conditions.
Thus, we investigated by EXSY (exchange spectroscopy)
NMR techniques the possibility that the cis-R and cis-â
stereoisomers could be involved in an equilibrium
reaction. Actually, the presence of EXSY correlation
t
formula [C6H10-{NdCH-(3,5- Bu2C6H2-2-O)-κO}2]ZrCl2
( Bu4salcyen)ZrCl2 (3a ,b)] and structurally correspond-
ing to the cis-R (3a ) and cis-â (3b) in a 9:1 molar ratio
Scheme 2). The H NMR spectrum (benzene-d6, 25 °C)
of the mixture exhibits a symmetric pattern for 3a
consisting of two singlets at 1.71 and 1.36 ppm for the
Bu groups and only one signal at 7.78 for the aldimino
groups. An asymmetric pattern for 3b containing four
t
[
1
(
t
t
t
singlets at 1.76, 1.70, 1.31, and 1.29 ppm for the Bu
peaks found for the Bu signals of the two stereoisomers
groups and two different signals at 7.77 and 7.68 ppm
for the aldimino groups was additionally detected (see
Figure 2). Attempts to synthesize 3 through different
suggested isomerization between this two forms with a
-
1
-1
kinetic rate of 3.6 × 10
s
(see Supporting Informa-
3
1
tion). We also investigated the behavior of the 3a ,b
1
mixture by VT (variable temperature) H NMR spec-
t
troscopy. At 100 °C in toluene the line width of the Bu
signals due to the cis-R form results little bit increased
t
(
∆ν1/2 ) 2 Hz), whereas the Bu signals of the cis-â form
reaches coalescence likely with trans stereoisomer. This
suggests that the barrier to the conversion of the cis-R
form with the other possible stereoisomers is higher
-
1
than 84 kJ mol . Fujita et al. reported on the basis of
DFT calculations that the cis-R form of similar un-
bridged bis(iminophenolate) zirconium complex is more
-
1
-1
stable of about 20 kJ mol and 33 kJ mol than the
2
5
cis-â and trans isomers, respectively. Thus, it is not
surprising the obtaining of a mixture of the two isomers
3
a ,b with the most thermodynamically favored isomer
largely prevailing. The stereoselective synthesis of 1 by
protonolysis route could be likely favored by steric
t
interaction of the bulky benzyl groups with the Bu ortho
substituents of the phenoxo groups.
2
.2. Olefin P olym er iza tion Ca ta lyzed by 1 a n d
F igu r e 2. 1H NMR spectrum (C
3. Activation of 1 with B(C6F5)3, [CPh3][B(C6F5)4], or
MAO produces active ethylene polymerization catalysts
under a range of experimental conditions (see Table 1).
The perfluoroborane is an activator less efficient than
trityl salt and MAO (compare entries 2, 3, and 5), in
agreement with previous reports on the polymerization
6
D
6
, 25 °C, δ in ppm) of
t
(
4 2
Bu salcyen)ZrCl (3).
routes always produced the same mixture of 3a and 3b
with the same relative ratio but in lower yields. For
example, the reaction of the preligand with ZrCl4(THF)2

7
472 Cuomo et al.
Macromolecules, Vol. 37, No. 20, 2004
Ta ble 1. Eth ylen e P olym er iza tion Ca ta lyzed by 1 a n d 3a ,b
entrya
catalyst
cocatalyst
E (atm)
time (min)
yield (g)
activityf
Mw/Mng
Mw (×10 )
4
b
1
2
3
1
1
1
1
1
1
1
1
B(C6F5)3/AlMe3
B(C6F5)3/AliBu3
[CPh3][B(C6F5)4]/AliBu3
5
5
5
5
5
5
5
5
1
5
5
5
1
1
1
1
60
60
60
100
60
30
6
0.06
0.08
0.80
13.0
6.30
3.30
0.98
0.30
0.47
0.30
2.85
0.65
0.62
0.31
0.25
0.11
1.2
1.6
16
156
126
132
196
120
9
6
57
13
62
b
2.11
1.64
2.52
2
280
22
c
4
5
6
7
8
MAO
MAO
MAO
MAO
MAO
MAO
1.53
1.84
218
109
3
d
9
1
60
60
60
60
60
45
30
15
1
1
1
1
1
1
1
0c
1
3a ,b
3a ,b
3a ,b
3a ,b
3a ,b
3a ,b
3a ,b
[CPh3][B(C6F5)4]/AliBu3
MAO
2
3
4
5
MAO/AlMe3
MAO
e
e
MAO
MAO
MAO
41
50
44
2.95
2.54
3.30
96
92
100
e
e
6
a
-4
Polymerization conditions: catalyst ) 50 µmol ([Zr] ) 5.0 × 10 M); toluene ) 100 mL; Al/Zr molar ratio ) 500; temperature ) 30
c
d
-3
°
C. b B/Zr molar ratio ) 1; Al/Zr molar ratio ) 6. B/Zr molar ratio ) 1; Al/Zr molar ratio ) 40. Catalyst ) 50 µmol ([Zr] ) 1.0 × 10
M); toluene ) 50 mL. Catalyst ) 10 µmol ([Zr] ) 2.0 × 10 M); toluene ) 50 mL. f Activity ) gPE mmolcat
e
-4
-1 -1
h
.
g
Determined by GPC.
performances of the diaminobis(phenolate)benzylzir-
In all considered cases polyethylene produced is linear
and exhibits melting points in the range 131-139 °C.
Noteworthy the polydispersity index (PDI) of the poly-
ethylene samples (PDI ) 2.5-3.3) by 3a ,b is larger that
than found with 1 although the propagating species
during the polymerization catalyzed by these com-
pounds are expected to be the same and the bulkiness
of the growing polymer chain would hamper site isomer-
ization between the C2 to C1 symmetric species during
ethylene polymerization. The polymerization activity of
1 and 3a ,b dramatically decreases by passing from
ethylene to propylene. This is not surprising considering
data reported for the unbridged bis(iminophenolate)
catalysts carrying not perfluorinated N-aromatic sub-
7
i
conium catalysts, whereas the [CPh3][B(C6F5)4]/Al- Bu3
cocatalyst shows productivity values higher than former
of nearly 1 order of magnitude. The use of trimethyl-
aluminum as scavenger produces a decrease of the
polymerization activity likely as a result of fast reaction
with the imino functionality. Thus, solid MAO contain-
ing low amount of “free” (not coordinated) trimethyl-
aluminum (see Experimental Section) was explored and
found as the best activator (entries 4-9). Interestingly,
the MAO activated catalyst is 8-fold more active than
trityl activated catalyst (compare entries 3 and 5). A
possible explanation is that the methyl cationic complex,
resulting from reaction of 1 with MAO, is more ther-
mally stable than the corresponding benzyl complex.
Considering Scott’s hypothesis of a fast benzyl vs alkyl
migration from zirconium to the imino carbon of the
ligand via radical mechanism,30 this would produce a
higher concentration of the active sites during the
activation reaction. The polymerization activity of 1/MAO
increases almost linearly with polymerization time up
to 100 min (entries 4-8). The polydispersity of the
polyethylene samples produced with MAO and trityl
activated catalysts (entries 3 and 7) is monomodal and
narrow (Mw/Mn < 2), indicating the presence of a single
active species. High molecular weights were found also
at very short polymerization time.
1
6,34
6
stituents
as well as the performances of the bis-
11
(amidinate) and diaminobis(phenolate) octahedral
zirconium complexes in propylene polymerization. The
reason probably arises from electronic properties of the
metal center bearing this ligand. Traces of isotactic
polypropylene were recovered from propylene polymer-
ization experiments likely resulting from a minor or-
ganometallic species produced by the ligand leaching
in the presence of high concentration of MAO: actually
1
3
polypropylene with similar C NMR fingerprints were
produced by ZrCl4(THF)2/MAO under similar condi-
3
3
tions.
To gain information on the stereo- and regiochemistry
of the propylene insertion, copolymerizations of ethylene
with propylene were conducted under a range of experi-
mental conditions in the presence of the bridged diimi-
nobis(phenolate) complexes 1 and 3a ,b. The results are
summarized in Table 2. Ethylene-propylene copolymers
with propylene concentration up to 56 mol % were
obtained. The copolymerization rate is rather low and
decreases when propylene concentration in the feed is
The 3a ,b catalysts exhibits comparable performances.
The activity of 3a ,b/MAO is lower than that found for
1
under the same conditions (compare entries 5 and 11)
likely as a result, among other possible explanations,
of precatalyst 3a ,b composition, isomerization between
the cis-R and cis-â forms (where the latter is expected
to be less active than former), and slow alkylation of
the chloride derivatives followed with competitive de-
composition of the active species. Addition of trimethyl-
aluminum to solid MAO was found to produce a noxious
effect (compare entries 11 and 12) analogously to the
findings previously reported for the bis(iminophenol-
1
3
increased. The C NMR analysis of the copolymer
obtained in run 19 (P mol % ) 43) revealed that the
regiospecificity of the propylene insertion is highly
1
3
primary because of the lacking of C NMR signals in
2
4,32
35
ate)
and other nonmetallocene zirconium olefin
the region 35.7-34.9 ppm due to SRâ carbons. Indeed,
3
3
polymerization catalysts. The polymerization activity
increases with polymerization time as long as 60 min,
but the average molecular weights are nearly constant,
indicating a fast chain transfer reaction with aluminum
of MAO (entries 13-16). The [CPh3][B(C6F5)4]/Al- Bu3
cocatalyst resulted efficient in activating the chloride
derivative (entry 10).
the secondary propylene insertion would produce a
resting state of the catalyst which would favor ethylene
insertion followed by primary propylene insertion. The
sequence of these events produces a PEP triads which
apparently corresponds to the tail-to-tail propylene
enchainment of a PP diad characterized by the presence
of the SRâ carbons shown in Scheme 3. Moreover, the
i

Macromolecules, Vol. 37, No. 20, 2004
Olefin Polymerization 7473
Ta ble 2. Olefin Cop olym er iza tion Ca ta lyzed by 1 a n d 3a ,b Activa ted w ith MAO
comonomer comonomer
concn in the feed
(mol/L)
ethylene concn in
the feed (mol/L)
time
(h)
yield
(g)
concn in the
polymer (mol %)
Mw
entrya
catalyst
comonomer
Mw/Mnf
(×10 )
63
3
1
1
1
2
2
2
2
2
2
2
7
8
9
0
1
2
3
4
5
6
1
1
1
1
3a ,b
1
1
1
propene
propene
propene
propene
propene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
0.6
1.3
2.4
2.4
0.5
3.2
6.4
6.4
neat
3.2
0.005
0.013
0.32
0.17
0.005
0.20
0.20
0.10
0.20
0.20
1
1
1
1
1
3
3
1
3
3
0.26
0.06
0.62
0.08
0.07
2.56
2.74
0.14
0.33
0.08
19
26
43
56
10
12
33
38
22
9
2.2
b
b
b
2.0
55
c
c
d
e
c
2.1
4.4
81
25
1
3a ,b
a
-3
Polymerization conditions: catalyst ) 50 µmol ([Zr] ) 1.0 × 10 M); Al/Zr molar ratio ) 500; toluene ) 50 mL; temperature ) 25
-4
d
°
C. b Catalyst ) 50 µmol ([Zr] ) 5.0 × 10 M); toluene ) 100 mL. c Toluene ) 30 mL; 1-hexene ) 20 mL. Ethylene pressure ) 0.5 atm.
e
f
1
-Hexene ) 50 mL; cocatalyst ) MMAO (7 wt % solution in heptane). Determined by GPC.
Sch em e 3
of SRγ of the EPEPE, this picture suggests aspecific
propylene insertion, producing m- and r-PP diads
1
3
completely at random. Moreover, in the C NMR
spectrum of the sample 20 (P mol % ) 56) PPP triads
were produced, and two more signals with the same
intensity at 38.9 and 38.7 ppm were detected and
r
r
m
r
attributed to the SRγ of the P P PEP and P P PEP
pentad (Figure 4). This finding confirms the lack of any
chemical shift of the SRγ carbons of the PPEP tetrad is
dramatically affected by the steric relationship between
the two methyl groups of the propylene diad flanking
ethylene. Actually the chemical shifts of the r-SRγ and
r
m
m-SRγ carbons of the P PEP and P PEP tetrads fall at
3
8.8 and 37.9 ppm,36 respectively, with the latter
overlapping the resonance of the SRγ of the EPEPE
alternating pentad. In the 13C NMR spectrum of sample
1
9 (Figure 3) two resonances due to the SRR carbons were
F igu r e 4. 13C NMR spectral region (C
ppm) of SRγ: (a) P mol % ) 43 (run 19, Table 2) and (b) P mol
) 56 (run 20, Table 2).
2
2 4
D Cl , 110 °C, δ in
%
stereocontrol in the formation of propylene homose-
quences. Unfortunately, the check for the same splitting
for the P P PEP and P P PEP pentads is not possible
for the overlap of these resonances with that of SRγ of
EPEPE at 37.8 ppm.
Aiming to obtain copolymers with higher 1-olefins and
get information on their microstructure, copolymeriza-
tion of ethylene with 1-hexene was carried out using
the 1 and 3 catalysts. Ethylene-co-1-hexene polymers
with 1-hexene concentration up to 38 mol % were
obtained (entry 24, Table 2). Copolymerization runs
carried out in neat 1-hexene using MMAO cocatalyst
F igu r e 3. 13C NMR spectrum (C
2 2 4
D Cl , 110 °C, δ in ppm) of
r
m
m
m
ethylene/propylene copolymer (entry 19, Table 2).
detected with the same intensity at 45.5 and 45.7 ppm
and attributed to the PP diads with r and m stereo-
chemistry, respectively.35 The r-SRγ signal is detected
at 38.8 ppm with the expected 2:1 intensity ratio of
r-SRR. Noteworthy, the signal due to the m-Pâδ carbon
is observed with the same intensity of r-SRγ at 20.8 ppm.
Considering that the signal of r-Pâδ would overlap the
signal of Pδδ at 20.1 ppm and that of m-SRγ overlaps that

7
474 Cuomo et al.
Macromolecules, Vol. 37, No. 20, 2004
to prevent catalyst precipitation yielded copolymers with
goes to 1,2 benzyl migration from the metal to the imino
carbon, converting the diiminobis(phenolate) ligand in
the corresponding benzylamidoiminobis(phenolate). This
decomposition pathway reduces the number of the
active sites and the polymerization activity as a conse-
quence. When complexes 1 and 3 were activated with
MAO, ethylene polymerization catalysts with moderate
activity were found: the polydispersity of the polyeth-
ylene samples obtained in a range of experimental
conditions is monomodal and close to 2, suggesting the
presence of a single active species. The catalyst is stable
over 3 h, and the polymerization activity linearly
increases with time. The ligand leaching can be ruled
out because the ZrCl4(THF)2/MAO catalyst produces
polyethylenes with broader molecular weight distribu-
tion (PDI > 5) under comparable conditions.33 Ethyl-
ene-1-olefin copolymerization experiments confirmed
the stereo- and regiochemistry of 1-olefin insertion
previously reported for the corresponding unbridged
complexes. The regiospecificity is highly primary, but
stereospecificity is scarce because of inefficient control
of the ligand framework and low steric interaction of
the coordinated monomer with the chiral methine of the
last unit in the growing polymer chain.
1
3
lower incorporation of 1-hexene (entry 25, Table 2).
NMR analysis of the copolymer microstructure of sample
3 suggested that 1-hexene was mainly incorporated as
C
2
1
3
isolated units producing C NMR signals at 38.1 and
4.5 ppm attributed to the methine and methylene
SRδ+) of the main-chain carbons, respectively, and
3
(
signals at 34.1, 29.3, 23.4, and 14.3 ppm attributed to
the carbon atoms in the butyl branch (see Supporting
Information). However, additional signals at 35.0 and
2
4.5 ppm were attributed to SRγ and Sââ carbons of
4
0
13
alternating triad HEH. In the C NMR spectrum of
sample 26, signals due to the HH diads were detected
with very low intensity, but unfortunately no conclusion
on the stereochemistry of 1-hexene insertion could be
reached. Noteworthy, the gel permeation chromatogra-
phy (GPC) curve of the ethylene-1-hexene and ethyl-
ene-propylene copolymers is monomodal and the PDI
is close to 2, comparable to that found for polyethylene,
indicating the homogeneity of the copolymers and that
they result from a single catalytic species.
The results herein reported for the stereo- and regio-
chemistry of 1-olefin insertion with bridged diiminobis-
(phenolate) catalysts 1 and 3 are in qualitative agree-
ment with experimental data reported for the correspond-
ing unbridged zirconium complexes. However, the bridged
catalysts exhibit higher regiospecificity, and propylene
insertion occurs exclusively in a primary fashion. De-
spite the C2 symmetry of the 1 and 3 catalyst the
stereochemistry of propylene insertion is aspecific,
indicating that both ligand environment and chirality
of the last unit in the growing polymer chain are unable
to express face selectivity in the monomer coordination/
insertion likely as a result of a too large mouth of the
catalyst and the distance of the chiral methine of the
growing polymer chain from the metal of the active
species. Thus, the regiospecificity of the propylene
insertion seems mainly due to electronic factors and not
to the steric hindrance at the metal center.
4
. Exp er im en ta l Section
4.1. Gen er a l In for m a tion . All organic preparations were
carried out under normal atmospheric conditions, while orga-
nometallic manipulations were performed under a dry nitrogen
atmosphere using standard Schlenk line techniques and a
glovebox. Solvents (toluene, hexane, pentane, tetrahydrofuran)
were refluxed and distilled over sodium benzophenone before
use. (1R,2R)-Cyclohexanediamine, 3,5-di-tert-butyl-2-hydroxy-
benzaldehyde, HCOOH, n-BuLi, ZrCl
4
, NaH, AlMe
3
, AliBu
3
(
Aldrich), [CPh ][B(C ] (Boulder SPA Co.), and B(C F )
3
6
F
5
)
4
6 5
3
(Strem Chemicals) were used as received. Triethylamine and
1-hexene (Aldrich) were distilled from CaH . Ethylene and
propylene (S.O.N., polymerization grade) were dried by passing
over activated 4 Å molecular sieves. Methylalumoxane (MAO)
2
(
5
Witco, 10 wt % solution in toluene) was dried in a vacuum at
0 °C to remove the uncoordinated AlMe and used as a solid
after washing hexane. Modified methylalumoxane (MMAO-
3
The fluxional behavior of the unbridged bis(iminophe-
nolate) catalysts could explain the higher polymeriza-
tion activity of these complexes in propylene polymer-
ization compared to that of the title catalysts.
3
A) (Akzo Nobel, 7 wt % solution in heptane) was used as
37
38
received. ZrBn
4
and ZrCl
4
(THF)
2
were prepared according
to the literature procedures. Elemental analyses were per-
formed with a Perkin-Elmer 240-C instrument.
3
. Con clu sion s
4.2. Syn th esis of N,N′-(1R,2R)-Cycloh exylen ebis(3,5-d i-
t
ter t-bu tylsa licylid en e im in e) ( Bu
4
sa lcyen H
2
). The preli-
We reported the synthesis and characterization of
t
gand Bu
4 2
salcyenH was readily synthesized by a modified
2
6,27
diiminobis(phenolate) zirconium complexes [C6H10-{Nd
literature procedure.
To a solution of 3,5-di-tert-butyl-2-
t
CH-(3,5- Bu2C6H2-2-O)-κO}2]ZrX2 [X ) CH2Ph (1), Cl
hydroxybenzaldehyde (10.75 g, 0.46 mmol) in methanol (240
mL) 0.3 mL of formic acid was added under stirring followed
by the addition of a solution of (1R,2R)-cyclohexanediamine
(3)]. The chloride derivative is thermally stable and
found in slow equilibrium between the cis-R and cis-â
forms with the former largely prevalent (9:1 molar
ratio). The kinetic barrier to the isomerization has been
estimated higher than 84 kJ mol by NMR methods,
and the kinetic constant for the interconversion between
(
2.62 g, 23 mmol) in methanol (20 mL). The resulting mixture
was dilute with methanol to reach a total volume of 400 mL
and stirred overnight at room temperature. A yellow precipi-
tate was recovered by filtration. Further crops were collected
by crystallization of the mother liquor at -10 °C. The yellow
solid was dried overnight in a vacuum at 65 °C (11.0 g, yield
88%).
-
1
-
1
-1
the two diastereisomers is 3.6 × 10
s . The corre-
sponding benzyl zirconium complex is conformationally
more stable and pure and has been isolated with C2
symmetry. Thus, the steric interaction between the
zirconium ancillary ligands and the ortho substituent
at the aryloxo groups bound to zirconium plays a
relevant role in stabilizing the C2 symmetry of this
zirconium complex. Assuming that the growing polymer
chain is a sterically demanding group, the C2 symmetry
of the catalyst would further be stabilized by the
presence of this bulky group bound to the metal of the
active center and retained during polymerization. The
benzyl complex 1 is light sensitive and readily under-
4.3. Syn th eses of 1, 2, a n d 3a ,b. 4.3.1. Syn th esis of
(tBu
salcyenH
dropwise to a stirred suspension of ZrBn
sa lcyen )Zr Bn
(1). A yellow solution of the Bu
t
-
4
2
4
2
(1.0 g, 1.83 mmol) in pentane (60 mL) was added
(0.84 g, 1.83 mmol)
4
in pentane (20 mL) for 10 min at room temperature, affording
a red solution. Quick removal of the solvent in a vacuum gave
the expected product as a red-orange powder (1.4 g, 1.77 mmol,
yield 93%).
1
6 6
H NMR data (C D , 25 °C, δ in ppm): δ 8.04 (s, 2H, CHd
N), 7.92 (d, 2H, J ) 2.4 Hz, ArH), 7.39 (d, 2H, J ) 2.4 Hz,
ArH), 6.63 (t, 4H, m-Ph), 6.52 (t, 2H, p-Ph), 6.45 (d, 4H, o-Ph),
2.77-2.70 (AB pattern, 4H, J ) 7.6 Hz, CH
2
2
Ph), 2.18 (br, 2H,

Macromolecules, Vol. 37, No. 20, 2004
Olefin Polymerization 7475
t
t
cyclohexyl CH), 1.85 (s, 18H, Bu), 1.41 (s, 18H, Bu), other
venting the reaction vessel and pouring the polymerization
mixture into ethanol acidified with aqueous HCl. The coagu-
lated polymer was recovered by filtration, washed with an
excess of fresh ethanol, and dried in a vacuum at room
temperature. The polymerization runs with monomer fed at
atmospheric pressure were performed by introducing toluene
and the alkylaluminum reagent into a 100 mL flask equipped
with a magnetic bar. The nitrogen atmosphere was replaced
with ethylene, and the polymerization run was started after
equilibration of the solution with ethylene at the desired
temperature by injection of a solution of the precatalyst in
toluene.
Cop olym er iza tion P r oced u r e. Copolymerizations of eth-
ylene with propylene were performed following the procedures
above-described, the only difference being the feeding of the
reactor. In entries 17 and 21, ethylene and propylene were
mixed at atmospheric pressure with appropriate compositions
using a Brooks gas flowmeter, and the gas phase let flow
through the polymerization solution with a rate of 500 mL/
min. The inlet and outlet gas-phase compositions were moni-
tored by gas chromatography analysis. In entries 18-20, 25
mL of propylene was condensed in the toluene solution
containing MAO, and thus ethylene was fed at 1.3 (entry 19)
and 0.7 atm (entry 20).
Copolymerizations of ethylene with 1-hexene were per-
formed in a 100 mL flask equipped with a magnetic stir bar.
The glass reactor was charged with toluene, 1-hexene, and
MAO in that order, and the nitrogen atmosphere was replaced
with ethylene fed at atmospheric pressure. When the solution
reached thermodynamic equilibrium with the gas phase, the
reaction was started by injection of a toluene solution of the
precatalyst. The polymerization run was stopped by addition
of ethanol acidified with aqueous HCl and the copolymers
isolated according to the conventional procedure.
1
3
cyclohexyl resonances obscured. C NMR (C
ppm): δ 163.7 (CHdN), 162.1 (C-O), 145.4 (C-CHdN), 139.7,
39.3, 131.1, 130.9, 128.5 (Ar), 125.6, 124.5, 120.5 (Ph), 72.3
cyclohexyl CH), 67.0 (CH Ph), 35.9, 34.4 (CMe ), 31.7, 30.54
C(CH ), 29.4, 24.6 (cyclohexyl CH ).
Anal. Found: C, 73.02; H, 8.71; N, 3.67. Calcd for C50
Zr: C, 73.3; H, 8.13; N, 3.42.
6 6
D , 25 °C, δ in
1
(
(
2
3
3
)
3
2
66 2 2
H N O -
t
4
4
.3.2. Syn th esis of (Bn Bu sa lcyen )Zr Bn (2). A red
solution of benzyl complex 1 (0.11 g, 0.13 mmol) in toluene (8
mL) was stirred for 1.5 h at 35 °C, during which time the
solution slowly turned red/yellow. Distilling off the solvent in
a vacuum, a glassy yellow solid was obtained (0.09 g, 0.11
mmol, yield 82%).
1
6 6
H NMR data (C D , 25 °C, δ in ppm): δ 7.89 (d, 1H, J )
1
)
)
.8 Hz, CHdN), 7.81 (d, 1H, J ) 1.7 Hz, ArH), 7.56 (d, 1H, J
2.4 Hz, ArH), 7.31 (d, 1H, J ) 1.7 Hz, ArH), 7.13 (d, 1H, J
2.4 Hz, ArH), 7.02 (m, 2H, m-Ph), 6.84 (m, 1H, p-Ph), 6.78
(
m, 4H, m′-Ph and o-Ph), 6.63 (t, 1H, p′-Ph), 6.55 (d, 2H, o′-
3
Ph), 4.71 (dd, 1H, J ) 6.6 and 3.3 Hz, N-CH) 3.03-2.73 (dd,
2
3
2
H, AB pattern, J ) 12.3 Hz, J ) 6.6 and 3.3 Hz, CH
2
Ph
migrated to the imine carbon), 2.56-2.36 (d, 2H, AB pattern,
2
J ) 10.5 Hz, Zr-CH
2
Ph), 2.28 (br, 1H, cyclohexyl CHNdC),
t
13
1
.71, 1.67, 1.36, 1.35 (s, 9H, Bu). C NMR (C D , 25°C, δ in
6 6
ppm): δ 160.5 (CHdN), 159.8 (C-O), 154.8 (C-O), 146.1 (C-
CHdN), 141.3 (C-CHdN), 141.1, 139.6, 136.9, 133.4, 130.3,
1
6
3
3
29.2 (Ar), 129.1, 128.9, 127.4, 125.1(Ph), 122.9, 120.9 (Ar),
9.8, 69.4 (cyclohexyl CH), 67.3, 58.4 (CH
5.73 (CMe ), 32.0 (C(CH ), 31.8 (CMe ), 31.6 (C(CH
0.7 (CMe ), 30.4, 30.2 (C(CH ), 27.1, 25.3, 25.1, 23.0 (cyclo-
).
2
Ph), 41.0 (CH-N),
3
3
)
3
3
3 3
)
), 30.8,
3
3 3
)
hexyl CH
2
Anal. Found: C, 73.02; H, 8.25; N, 3.40. Calcd for C50
66 2 2
H N O -
Zr: C, 73.3; H, 8.13; N, 3.42.
t
4
.3.3. Syn th esis of ( Bu
4
sa lcyen )Zr Cl
2
(3a ,b). The syn-
thesis of the complex 3a ,b was carried out by two different
Ch a r a cter iza tion of th e P olym er s. NMR Characteriza-
routes.
1
13
tion. H and C NMR spectra of the organometallic compounds
t
Method A. A toluenic solution (10 mL) of Bu
4
salcyenH
2
(0.40
were recorded with a Bruker ADVANCE 400 spectrometer
g, 0.73 mmol) and NEt
3
(0.20 mL, 1.5 mmol) was added to a
(THF) (0.28 g, 0.74 mmol) in
toluene (10 mL). The mixture was stirred at room temperature
overnight. The yellow solution was filtered, and the yellow
solid was isolated after distillation of the solvent. The solid
was washed with hexane and dried in a vacuum (0.34 g, 0.48
1
13
13
(
400 MHz for H and 100 MHz for C). The C NMR spectra
stirred suspension of ZrCl
4
2
of the polymer and copolymer samples were recorded with an
1
AM 250 Bruker spectrometer (250 MHz for H and 63 MHz
13
for C) at 110 °C using 1,1,2,2-tetrachloroethane-d
(
2
as solvent
1
0.5 mL, 20 wt %). The H NMR chemical shifts were referred
to TMS as external standard using the residual protio impuri-
mmol, yield 66%).
13
ties of the deuterated solvents as reference. The C NMR
t
Method B. To a solution of Bu
in toluene (25 mL) solid NaH (0.06 g, 2.5 mmol) was added
under nitrogen, and the resulting mixture was stirred over-
4
salcyenH
2
(0.61 g, 1.11 mmol)
chemical shifts were calibrated using the Sδδ signal of poly-
ethylene at 30.0 ppm as internal reference. The copolymer
compositions were estimated using the method proposed in the
night at room temperature. The yellow suspension of the
35,39,40
literature.
t
sodium salt Na
sion of ZrCl
2
Bu
2
4
salcyen was added dropwise to a suspen-
(0.42 g, 1.11 mmol) in toluene (20 mL) and
stirred for 12 h. The reaction mixture was filtered and the
expected product was isolated by crystallization in toluene at
GPC analysis of the polymers was carried out by high-
temperature GPC at 140 °C using 1,2,4-trichlorobenzene as
solvent and narrow MWD polystyrene standard sample as
reference. The measurements were performed on a PL-GPC210
with four PL-Gel Mixed A columns, RALLS (light scattering)
detector (PD2040), H502 viscometer (Viscotek), refractive
detector, and DM400 data manager (Viscotek).
Differential scanning calorimetry (DSC) measurements were
carried out with a TA Instrument DSC 2920 at a heating rate
of 10 °C/min.
4
(THF)
4
°C as yellow solid (0.75 g, 1.1 mmol, yield 96%).
1
6 6
H NMR data (C D , 25 °C, δ in ppm) for 3a : δ 7.82 (d, 2H,
J ) 2.3 Hz, ArH), 7.78 (s, 2H, CHdN), 7.26 (d, 2H, J ) 2.3
t
Hz, ArH), 3.53 (m, 2H, cyclohexyl CH), 1.71 (s, 18H, Bu), 1.36
t
(
s, 18H, Bu), 0.70 (m, 4H, cyclohexyl CH
2
), 0.53 (m, 4H,
). For 3b: δ 7.77, 7.68 (s, 1H, CHdN), 7.24 (d,
H, J ) 2.6 Hz, ArH), 7.08 (d, 2H, J ) 2.5 Hz, ArH), 1.76,
cyclohexyl CH
2
2
1
t
13
6 6
.70, 1.31, 1.29 (s, 9H, Bu). C NMR (C D , 25 °C, δ in ppm)
for 3a : δ 165.5 (CHdN), 160.0 (C-O), 142.3 (C-CdN), 139.7,
32.5, 131.3, 124.8 (Ar), 67.5 (cyclohexyl CH), 36.2, 34.8
CMe ), 31.9, 30.5 (C(CH ), 28.0, 24.3 (cyclohexyl CH ). Anal.
Zr:
Ack n ow led gm en t. Financial support for this re-
search from the European Commission (Contract HPRN-
CT2000-00004), DGICYT (Project MAT2001-1309), and
Ministero dell’Universit a` e della Ricerca Scientifica
1
(
3
3
)
3
2
Found: C, 61.45; H, 7.63; N, 3.93. Calcd for C36
C, 61.16; H, 7.41; N, 3.96.
2 2 2
H52Cl N O
(MURST, Roma, Italy; PRIN-2002: “Fine tuning by
4
.4. P olym er iza tion P r oced u r e. The polymerization runs
organometallic catalysts of microstructure and chemical
and physical properties of hydrocarbon homopolymers
and copolymers”) is gratefully acknowledged. The au-
thors are also grateful to Dr. Patrizia Oliva for NMR
analyses of the polymers.
were carried out following a standard procedure. A sealed glass
vial containing the appropriate amount of precatalyst was
introduced into a 500 mL B u¨ cki glass autoclave equipped with
a mechanical stirrer. The autoclave was evacuated and
charged with the desired amount of alkylaluminum compound
dissolved in toluene. After the equilibration of the solution with
ethylene or propylene at the appropriate pressure and tem-
perature, the polymerization run was started by breaking the
vial with the mechanical stirrer. The run was terminated by
Su p p or tin g In for m a tion Ava ila ble: Kinetic study of
thermal decomposition of complex 1, EXSY NMR spectrum of
complex 3, and 13C NMR spectrum of the ethylene-co-1-hexene

7
476 Cuomo et al.
Macromolecules, Vol. 37, No. 20, 2004
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M.; Tanaka, H.; Fujita, T. Macromol. Rapid Commun. 2003,
2
4, 452.
(
21) Lamberti, M.; Pappalardo, D.; Zambelli, A.; Pellecchia, C.
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