Synthesis of new Cr(II) complexes with bidentate phosphine ligands and their behavior in the polymerization of butadiene. Influence of the phosphine bite angle on catalyst activity and stereoselectivity

Article abstract of DOI:10.1016/j.molcata.2006.11.019

Some new chromium(II) complexes were synthesized by reacting CrCl₂(thf) with various bidentate phosphines [bis(diphenylphosphino)methane (dppm); 1,2-bis(diphenylphosphino)ethane (dppe); 1,3-bis(diphenylphosphino)propane (dppp); bis(diphenylphosphino)amine (dppa)] and a new family of catalysts for the polymerization of butadiene was prepared by reacting the above chromium complexes with methylaluminoxane (MAO). These systems were found to be in some cases extremely active and they gave predominantly 1,2-polybutadienes (1,2 content >85%) having different tacticity (syndiotactic or predominantly isotactic) depending on the phosphine ligand bonded to the chromium atom. The influence of the type of ligand on catalyst activity and selectivity is discussed.

Full text of DOI:10.1016/j.molcata.2006.11.019

Journal of Molecular Catalysis A: Chemical 267 (2007) 102–107
Synthesis of new Cr(II) complexes with bidentate phosphine
ligands and their behavior in the polymerization of butadiene
Influence of the phosphine bite angle on catalyst
activity and stereoselectivity
Giovanni Ricci∗, Aldo Boglia, Tiziano Motta
CNR, Istituto per lo Studio delle Macromolecole (ISMAC), Via E. Bassini 15, 20133 Milano, Italy
Received 16 October 2006; accepted 6 November 2006
Available online 17 November 2006
Abstract
Some new chromium(II) complexes were synthesized by reacting CrCl2(thf) with various bidentate phosphines [bis(diphenylphosphino)methane
(dppm); 1,2-bis(diphenylphosphino)ethane (dppe); 1,3-bis(diphenylphosphino)propane (dppp); bis(diphenylphosphino)amine (dppa)] and a new
family of catalysts for the polymerization of butadiene was prepared by reacting the above chromium complexes with methylaluminoxane (MAO).
These systems were found to be in some cases extremely active and they gave predominantly 1,2-polybutadienes (1,2 content >85%) having
different tacticity (syndiotactic or predominantly isotactic) depending on the phosphine ligand bonded to the chromium atom. The influence of the
type of ligand on catalyst activity and selectivity is discussed.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Chromium(II) phosphine complexes; Chromium catalysts; Polymerization; 1,2-Polybutadiene; Stereoselectivity
(C4H6)Co(η5-C8H13); CoCl2(PRPh2)2–MAO, R = alkyl or
cycloalkyl group), and chromium [9] (e.g. Cr(acac)3–AlEt3,
Cr(CNPh)6/AlEt3).
1. Introduction
1,2-Polybutadiene is a polymer of industrial interest and
industrially produced. It can exist in the amorphous atactic
form and in two crystalline forms: isotactic and syndiotactic.
At present, the two stereo-isomers that are commercially most
used are the syndiotactic and atactic structures [1]. The interest
for 1,2-polybutadiene is due to the fact that it is a thermoplastic
resin showing the characteristics of both a thermoplastic and an
elastomer [1,2]. Because of this peculiarity, it has a wide range
of applications. 1,2 syndiotactic polybutadiene is used in films
(packaging breathing items for fruits, vegetables and seafood),
footwear soles, tubes, and hoses; atactic 1,2-polybutadiene is
used in the rubber and tire industry [3].
The AlEt3–Ti(OR)4 system was the first catalyst used for the
preparation of syndiotactic 1,2-polybutadiene, giving, however,
a polymer of moderate crystallinity [5]. The AlEt3–V(acac)3
system gave a polymerization product consisting of about 85%
1,2 structure, from which a highly crystalline polymeric fraction
havingasyndiotacticstructurewasisolatedbysuccessiveextrac-
tionswithdifferentsolvents[6]. Catalystsbasedonmolybdenum
were characterized by a high stereospecificity, being the syndio-
tactic crystalline fraction (residue after extraction) about 80%
[7]. All these systems based on Ti, V, and Mo were, how-
ever, characterized by a rather low activity and did not find an
industrial application. 1,2-Syndiotactic polybutadiene is instead
industrially produced with cobalt systems which exhibit without
any doubt the highest activity and stereospecificity. The system
AlR3–Co(acac)3–CS2, for instance, is by far the most active
and stereospecific catalyst for the preparation of 1,2-syndiotactic
polybutadiene [8c–f].
1,2-Polybutadiene can be prepared with various catalyst
systems [4] based on titanium [5] (e.g. AlEt3/Ti(OR)4),
vanadium [6] (e.g. AlEt3/V(acac)3), molybdenum [7] (e.g.
AlR3(Mo2(OR)2), cobalt [8] (e.g. Co(acac)3–AlEt3/CS2;
∗
Chromium catalysts, as cited above, were also used for the
preparation of 1,2-syndiotactic polybutadiene. They were found
Corresponding author. Tel.: +39 0223699376; fax: +39 0270636400.
E-mail address: giovanni.ricci@ismac.cnr.it (G. Ricci).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.11.019

G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 102–107
103
to exhibit a very low activity [9] but, unlike the other systems
based on Ti, V, Mo and Co, they were also able to give a 1,2-
isotactic polymer from butadiene, and up to now 1,2-isotactic
polybutadiene has been obtained only with chromium catalysts.
It has been recently reported, however, that more active
and stereospecific chromium catalysts for the 1,2 poly-
merization of 1,3-butadiene can be prepared by using
catalysts based on chromium(II) bidentate-phosphine com-
plexes (e.g. CrCl2(dmpe)2–MAO; dmpe = 1,2-bis(dimethyl-
phosphino)ethane) [10].
Bidentate phosphine ligands are well known in the field
of homogeneous catalysis; in many homogeneous catalytic
processes using transition metal complexes with bidentate phos-
phines as catalysts, the catalyst activity and stereoselectivity
were found to be strongly dependent on the type of phospho-
rous ligand [11]. Many ligand parameters have been proposed
in order to evaluate the relationship between ligand properties
and catalytic performance; in particular in the case of bidentate
phosphines the bite angle was used, which was reported to have
both steric and electronic effect [11].
Taking into account what above reported, we have syn-
thesized several new chromium(II) complexes by reacting
CrCl2(thf) with the following bidentate aromatic phosphines:
bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenyl-
phosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane
(dppp) and bis(diphenylphosphino)amine (dppa). The behavior
of these complexes, in association with MAO, in the polymer-
ization of butadiene was examined and compared with that of
other similar chromium systems previously reported [10]. They
were found to give predominantly 1,2-polybutadienes having
different type and degree of stereoregularity depending on the
type of phosphine ligand bonded to the chromium atom. The
catalytic activity too varied depending on the type of ligand,
being for some complexes much higher with respect to the
other chromium catalysts recently reported [10].
was refluxed over K/benzophenone for ca. 8 h, then distilled;
dichloromethane (Aldrich, 99.8% pure) was degassed under
vacuum and then dried over molecular sieves. 1,3-Butadiene
(Air Liquide, >99.5% pure) was evaporated from the container
prior to each run, dried by passing through a column packed
with molecular sieves and condensed into the reactor which
had been precooled to −20 ◦C. CrCl2 (thf) [12] was prepared
as reported in the literature, by reacting CrCl3 with chromium
powder in tetrahydrofurane; the bidentate phosphine chromium
(II) complexes were prepared as reported below, following
experimental procedures already reported in the literature for
analogous complexes [10b,13]. The elemental analyses of the
chromium complexes were performed by the analytical labo-
ratories of Polimeri Europa-Centro Ricerche Novara-“Istituto
Guido Donegani”; infrared spectra were recorded as KBr disks
with a Bruker IFS 48 instrument.
2.2. Synthesis of CrCl2 (dppm)
CrCl2(thf) (0.75 g, 3.8 mmol) and diethylether (25 ml) were
introduced in a 100 ml schlenck tube. The suspension so
obtained was kept under stirring at room temperature, then
bis(diphenylphospino)methane (1.36 g, 3.5 mmol) was added. A
green-blue precipitate was slowly formed; the suspension was
kept under stirring at room temperature for 24 h, then filtered.
The residue on the filter was dried under vacuum, transferred in
a Soxhlet apparatus and then successively extracted in continu-
ous for 24 h with boiling pentane in order to remove unreacted
diphosphine and then with boiling dichloromethane for 24 h.
The green dichloromethane solution was then concentrated and
an excess of pentane was added. The green precipitate formed
was filtered off and dried under vacuum. Yield: 0.87 g (45.1%
based on CrCl2). Anal. Calcd. for C25H22CrCl2P2: Cr, 10.25;
Cl, 13.98; P, 12.21. Found: Cr, 10.5; Cl, 13.8; P, 12.5.
Spectroscopic data: IR (KBr) ␯ (cm−1) 3059 (m), 1485 (m),
1438 (s), 1364 (m), 1138 (s), 1095 (s), 1046 (m), 1015 (m), 920
(w), 868 (s), 783 (s), 740 (s), 690 (s), 520 (s).
In the present paper we report on the synthesis of the various
chromium complexes, on their use in combination with MAO
in the polymerization of butadiene and on the characterization
of the polymers obtained; the influence of the ligand on catalyst
activity and stereoselectivity is also discussed.
2.3. Synthesis of CrCl2 (dppe)
CrCl2(thf) (0.33 g, 1.69 mmol) was suspended in diethylether
(25 ml) and 0.74 g of 1,2-bis(diphenylphosphino)ethane
(1.86 mmol) in diethylether (25 ml) were added. The suspen-
sion so obtained was kept under stirring for about 24 h, after
which the formation of a turquoise precipitate was observed.
The solvent was removed under vacuum and the residue was
extracted in continuous with boiling pentane for 24 h in order
to separate unreacted phosphine. The residue on the filter was
then extracted in continuous with boiling diethylether for ca.
72 h. The diethylether solution was then concentrated and the
blue precipitate was separated by filtration and dried under
vacuum. Yield: 0.264 g (30% based on CrCl2). Anal. Calcd.
For C26H24CrCl2P2: Cr, 9.97; Cl, 13.60; P, 11.88. Found: Cr,
9.9; Cl, 13.6; P, 11.5.
2. Experimental
2.1. Materials and methods
Bis(diphenylphosphino)methane (dppm) (Strem, 97% pure),
1,2-bis(diphenylphosphino)ethane (dppe) (Strem, 99% pure),
1,3-bis(diphenylphosphino)propane (dppp) (Strem, 98% pure),
bis(diphenylphosphino)amine (dppa) (Strem, 98% pure) and
methylaluminoxane (MAO) (Crompton, 10 wt.% solution in
toluene) were used as received. Toluene (Fluka, >99.5% pure)
was refluxed over Na for ca. 8 h, then distilled and stored over
molecular sieves under dry nitrogen; pentane (Prolabo, >99%
pure) was refluxed over Na/K alloy for ca. 8 h, then distilled
over molecular sieves under dry dinitrogen; diethylether(Lab-
Scan, >99.8% pure) was refluxed over Na/K/benzophenone for
ca. 8 h, then distilled; tetrahydrofurane (Baker, >99.8% pure)
Spectroscopic data: IR (KBr) ␯ (cm−1) 3052 (m), 1484 (m),
1435 (s), 1371 (w), 1159 (m), 1125 (m), 1099 (s), 1027 (w),
1000 (w), 861 (m), 741 (s), 693 (s), 550 (w), 517 (m), 488 (w).

104
G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 102–107
2.4. Synthesis of CrCl2 (dppp)
2.7. Polymer characterization
CrCl2(thf) (0.86 g, 4.4 mmol) and the phosphine (2.25 g,
5.5 mmol) were introduced in a 100 ml flask together with
diethylether (50 ml). A green suspension was rapidly formed;
it was kept under stirring for one night, and then filtered. The
residue on the filter was dried under vacuum, transferred in a
Soxhlet, andextractedfirstwithpentaneforabout24 hinorderto
remove the excess of phosphine and then with dichloromethane
for about 48 h. A green solution was formed which was con-
centrated under vacuum. A large amount of pentane was then
added causing the formation of a green precipitate which was
filtered off and dried in vacuum. Yield: 1.57 g (66.7% based on
CrCl2). Anal. Calcd. For C24H21CrNCl2P2: Cr, 10.23; Cl, 13.95;
P, 12.19. Found: Cr, 10.1; Cl, 13.4; P, 11.9.
The infrared spectra were performed with a Bruker IFS 48
instrument, using polymer films on KBr disks. The films were
obtained by deposition from solutions in benzene or hot (ca.
100 ◦C) solutions in 1,2,4-trichlorobenzene. 13C NMR measure-
ments were performed with a Bruker AM 270 instrument. The
spectra were obtained in C2D2Cl4 at 103 ◦C (hexamethyldisilox-
ane, HMDS, as internal standard). The concentration of polymer
solutions was about 10 wt.%. Differential scanning calorime-
try (DSC) scans were carried out on a Perkin Elmer Pyris 1
instrument. Typically, ca. 10 mg of polymer were analyzed in
each run, while scan speed was ca. 20 K/min under a dinitro-
gen atmosphere. The molecular weight averages (MW) and the
molecular weight distribution (MWD) were obtained by a high
temperature GPCV 2000 system (from Waters) using two on-
line detectors: a differential viscometer and a refractometer. The
experimental conditions consisted of three Olefi columns from
PSS, o-dichlorobenzene as mobile phase, 0.8 mL/min of flow
rate and 145 ◦C of temperature. The calibration of the GPC sys-
tem was constructed by 18 narrow MWD polystyrene standards
with the molar mass ranging from 162 to 3.3 × 106 g/mol. The
polybutadiene microstructure was determined according to what
already reported in the literature [8e,14].
Spectroscopic data: IR (KBr) ␯ (cm−1) 3054 (ms), 2934 (m),
1484 (m), 1437 (s), 1403 (mw), 1313 (w), 1245 (w), 1157 (s),
1121 (s), 1100 (s), 1072 (ms), 1028 (m), 998 (m), 972 (mw),
829 (mw), 744 (s), 721 (s), 695 (s), 551 (s), 510 (s).
2.5. Synthesis of CrCl2 (dppa)
CrCl2(thf) (2.58 g, 13.2 mmol) was suspended in diethylether
(70 ml). Bis(diphenylphosphino)amine (5 g, 13 mmol) was then
added and the reaction was kept under stirring at room tem-
perature for ca. 24 h. The formation of a light green precipitate
was observed while the supernatant solution was green. The
green precipitate was separated by filtration, dried under vac-
uum, transferred in a Soxhlet and then extracted in continuous
with boiling pentane for 48 h; in this way the unreacted phos-
phine could be separated in the pentane solution. The residue
was then extracted in continuous with boiling dichlorometane
for 48 h; at the end the dark green solution thus obtained
was concentrated, a large amount of pentane was added caus-
ing the formation of a green precipitate which was separated
by filtration and then dried under vacuum. Yield: 2.372 g
(35.4% based on CrCl2). Anal. Calcd. For C24H21CrNCl2P2:
Cr, 10.23; Cl, 13.95; P, 12.19. Found: Cr, 10.4; Cl, 14.2;
P, 12.3.
3. Results and discussion
The chromium complexes were prepared according to a gen-
eral experimental procedure already reported in the literature
[10b,12,13]. CrCl2(thf) was reacted with the various biden-
tate phosphines using diethylether as solvent. A suspension is
always formed; the precipitate was separated by filtration and
then worked up as described in the experimental part. The com-
plexes were found to have, by elemental analysis, the general
formula CrCl2(L) (L = dppm, dppe, dppp, dppa).
The results obtained in the polymerization of 1,3-butadiene
with the systems obtained by combining the above complexes
with MAO are reported in Table 1 and can be summarized as
follows:
Spectroscopic data: IR (KBr) ␯ (cm−1) 3227 (mw), 3148
(mw), 3053 (m), 1483 (m), 1436 (s), 1410 (w), 1184 (ms), 1126
(s), 1099 (s), 1026 (m), 999 (m), 944 (m), 741 (s), 693 (s), 618
(w), 592 (w), 540 (s), 515 (s).
(i) The systems CrCl2(Ph2P(CH2)nPPh2)–MAO (n = 1, 2, 3)
give from butadiene polymers having an essentially 1,2
structure. The 1,2 content varies in the range 80–90%,
slightly increasing with decreasing the length of the bridge
(n) between the two phosphorous atoms. The type of lig-
and has, however, a much stronger influence on catalyst
stereoselectivity. As a matter of fact, CrCl2(dppm)–MAO
gives 1,2-polybutadienes in which isotactic sequences
are predominant (see Fig. 1c), while CrCl2(dppe)–MAO
and CrCl2(dppp)–MAO give essentially syndiotactic 1,2-
polybutadienes (see Fig. 1b). The catalyst activity too,
is greatly affected by the type of ligand, being the sys-
tem CrCl2(dppm)–MAO much more active than the other
systems CrCl2(dppe)–MAO and CrCl2(dppp)–MAO (see
Table 1).
2.6. Polymerization of butadiene
All operations were carried out under an atmosphere of dry
dinitrogen. A standard procedure is reported. 1,3-Butadiene was
condensed into a 25 mL dried glass reactor kept at −20 ◦C,
then toluene was added and the solution so obtained was
brought to the desired polymerization temperature. Methyla-
luminoxane (MAO) and the chromium compound were then
added, as toluene solutions, in the order. The polymerization
was terminated with methanol containing a small amount of
hydrochloric acid; the polymer was coagulated and repeat-
edly washed with methanol, then dried in vacuum at room
temperature.
(ii) As reported by van Leeuwen [11], the effect of bite angle
(βn) in catalytic reactions can be separated in steric bite

G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 102–107
105
Table 1
Polymerization of butadiene with catalysts based on different Cr(II) phosphine compoundsa
b
Run
Cr-compound
βn (◦)
Al/Cr
Time (min)
Conv. (%)
Nc (h−1
)
1,2-Polybutadienes d (%)
rr/mr/mme
mp (◦C)
(molar ratio)
(molar ratio)
1
2
3
4
5
6
7
8
9
10
11
12
13
CrCl2 (dppm)
CrCl2 (dppm)
CrCl2 (dppm)
CrCl2 (dppm)
CrCl2 (dppe)
CrCl2 (dppe)
CrCl2 (dppp)
CrCl2 (dppp)
CrCl2 (dppa)
CrCl2 (dppa)
CrCl2 (dppa)
CrCl2 (dppa)
CrCl2 (dppa)
72
72
72
72
85
85
91
91
1000
500
100
50
1000
100
1000
100
1000
500
100
50
20
25
24
55.7
52.1
38.4
22.7
15.3
10.5
17.6
13.7
37.5
42.7
84.0
35.0
9.2
4,330
3,241
2,463
1,104
23
11
18
14
11,667
6,642
4,356
1,815
80
89
87
88
88
88
87
80
80
91
89
89
88
90
18/48/34
19/48/33
20/48/32
19/48/33
61/34/5
59/33/8
64/32/4
66/31/3
66/30/4
64/32/4
64/33/3
63/32/5
73/27/0
–
–
–
–
95
32
1020
1440
1440
1440
5
10
30
30
180
89
106
105
104
100
98
99
137
1000
a
Polymerization conditions: butadiene, 2 ml; toluene, total volume 16 ml; MAO; Cr, 1 × 10−5 mol; +20 ◦C (−30 ◦C in run 13).
Natural bite angles βn are taken from Ref. [11].
N = moles of butadiene polymerized per mol of Cr per hour.
Determined by 1H NMR.
Determined by 13C NMR. The molecular weights (MW) of the polymers obtained with the various chromium systems were in the range 250,000–300,000 with
b
c
d
e
a MW/MN of about 2.
angle effect and electronic bite angle effect. The first
that, at least in the series CrCl2(Ph2P(CH2)nPPh2)–MAO
(n = 1, 2, 3), the effect of the ligand on activity, chemose-
lectivity and stereoselectivity would be mainly due to steric
factors; an increased hindrance of the ligand, from dppm to
dppe and dppp, seems indeed to cause a decreasing in cata-
lystactivity(thecoordinationofthemonomerislikelymore
difficult when a hindered ligand is bonded to the metal)
and a radical change in stereoselectivity, from isotactic to
syndiotactic.
one is related to the steric interactions (ligand–ligand or
ligand–substrate) generated when the bite angle is modi-
fied by changing the backbone and keeping the substituents
at the phosphorous donor atom the same. This likely means
The fact that CrCl2(dppm)–MAO gives 1,2-polybuta-
dienes in which isotactic sequences are predominant, while
CrCl2(dppe)–MAO and CrCl2(dppp)–MAO give predom-
inantly syndiotactic 1,2-polybutadienes, can be interpreted
according to the previously suggested diene polymeriza-
tion mechanism [15]. The interpretation already given for
the results obtained in the polymerization of butadiene
with the two systems CrCl2(dmpm)2–MAO [10b] and
CrCl2(dmpe)2–MAO [10a] and with the cobalt systems
CoCl2(PRPh2)2–MAO (R = methyl, ethyl, normal-propyl,
iso-propyl and cyclohexyl) [8i] is still valid and applicable
to the chromium systems of this work. The catalytic site
in the polymerization of butadiene with the chromium
systems of this work is most probably that shown in
Fig. 2. The incoming monomer and the butadienyl unit
can assume two different orientations (exo–exo and
exo–endo). By insertion of the incoming monomer at C3
of the butadienyl group a 1,2 polymer is obtained in both
cases, isotactic from an exo–endo situation (Fig. 2-right),
syndiotactic from an exo–exo situation (Fig. 2-left). An
orientation like that in Fig. 2-right is favored when no
ligand or a low hindered ligand (low βn value) is bonded
to the metal while an orientation like that in Fig. 2-left is
the favored one when an hindered ligand (higher βn value)
is present. This is the reason why predominantly isotactic
Fig. 1. 13C NMR spectrum (olefinic region, C2D2Cl4, HMDS as internal stan-
dard, 103 ◦C) of the 1,2-polybutadienes obtained with (a) CrCl2(dppa)-MAO
(Table 1, run 9); (b) CrCl2(dppe)-MAO (Table 1, run 5); (c) CrCl2(dppm)-MAO
(Table 1, run 1).

106
G. Ricci et al. / Journal of Molecular Catalysis A: Chemical 267 (2007) 102–107
4. Conclusions
A new class of chromium catalysts, CrCl2(Ph2P–X–
PPh2)–MAO [X = (CH2), (CH2)2, (CH2)3, NH] have been pre-
pared and used in the polymerization of butadiene to essentially
1,2 polymers. Isotactic or syndiotactic polymers were obtained
depending on the type of ligand. A relationship between stere-
oselectivity and ligand bite angle has been suggested.
The systems CrCl2(dppa)–MAO and CrCl2–(dppm)–MAO
gave syndiotactic and predominantly isotactic 1,2-polybuta-
dienes, respectively, and exhibited an activity much higher with
respect to the other syndiospecific and isospecific chromium
systems up to now known.
Moreover, these catalysts appear of interest from the indus-
trial point of view, since their activities and stereospecificities
are comparable with those of the cobalt catalysts actually used
for the industrial production of 1,2-polybutadiene.
Fig. 2. Possible reciprocal orientation (exo–exo and exo–endo) of the butadiene
entering monomer and the allylic unit of the polymer growing chain.
Acknowledgements
polymers when L = dppm (βn = 72◦) and essentially
syndiotactic polymers when L = dppe or dppp (βn = 85◦
and 91◦, respectively) are obtained.
We thank Mr. Giulio Zannoni and Mr. Alberto Giacometti
Schieroni for their assistance in NMR and GPC analyses of
the polymers, respectively. This work has been carried out with
the financial support of Fondazione Cariplo, “Progettazione di
nanocompositi a matrice elastomerica funzionalizzata”.
(iii) We have also investigated the effect of Al/Cr molar ratio
value on catalyst activity and selectivity. It was found to
have practically no influence on catalyst chemo- and stere-
oselectivity, while catalyst activity was found to increase
with increasing the Al/Cr molar ratio. A larger number of
active sites is likely formed at higher Al/Cr ratio.
References
(iv) We have also examined the polymerization of butadiene
with CrCl2(dppa)–MAO. This system gave highly syndio-
tactic 1,2 polymer (percentage of syndiotactic triads, [rr],
of about 65%) and exhibited a very high activity. If we
compare this system with other syndiospecific chromium
catalysts previously reported (e.g. CrCl2(dmpe)2–MAO)
[10], we observe that the system CrCl2(dppa) is less
stereospecific (percentage of syndiotactic triads [rr] of
65% versus 85% for CrCl2(dmpe)2–MAO) but largely
more active (N = moles of butadiene polymerized per
mol of Cr per hour = 11667 h−1 versus 1024 h−1 for
CrCl2(dmpe)2–MAO). The presence of a potential
donor nitrogen atom in the backbone determining weak
interactions with the metal center and, as reported by
van Leeuwen [11a], leading to an enlargement of the
bite angle, could in some way account for the high
activity and syndiospecificity of the system CrCl2(dppa)–
MAO.
(v) The influence of the polymerization temperature on
selectivity was also evaluated and, as already reported for
other chromium systems [10], the polymer syndiotacticity
was found to increase with decreasing the polymerization
temperature. The system CrCl2(dppa)–MAO gives a
1,2-polybutadiene with a melting point of 137 ◦C and a
syndiotactic index [rr] of about 73% at −30 ◦C (Table 1,
run 13), while the polymer obtained at room temperature
has a melting point of 104 ◦C and a syndiotactic index of
66% (Table 1, run 9).
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