Block copolymers of highly isotactic polypropylene via controlled Ziegler-Natta polymerization

Article abstract of DOI:10.1021/ma048144b

The block copolymerization polypropylene under highly isotactic stereocontrol, using a new catalyst obtained by means of rational ligand design, was discussed. Moderately isotactic polypropylenes of very low average molecular mass in reasonable yields was

Full text of DOI:10.1021/ma048144b

Macromolecules 2004, 37, 8201-8203
8201
Block Cop olym er s of High ly Isota ctic
P olyp r op ylen e via Con tr olled Ziegler -Na tta
P olym er iza tion
Vin cen zo Bu sico,*,† Rober ta Cip u llo,†
Nic F r ied er ich s,‡ Sa r a Ron ca ,† Giova n n i Ta la r ico,†
Ma r ia Togr ou ,† a n d Bin g Wa n g‡
Dipartimento di Chimica, Universita` di Napoli
“Federico II”, Via Cintia, 80126 Naples, Italy, and SABIC
EuroPetrochemicals, Research & Development Centre,
P.O. Box 319, 6160 AH Geleen, The Netherlands
Received September 10, 2004
Revised Manuscript Received September 16, 2004
In tr od u ction . A polymerization is controlled1 if chain
initiation is rapid relative to propagation, and chain
termination and transfer are “negligible” in the time
scale of the experiment. When this holds, polymer
molecular mass increases linearly with monomer con-
version, and Mw/Mn is 1 or slightly higher, similarly to
what happens under truly living conditions.
For most catalytic olefin polymerizations, at practical
temperatures (>20 °C) a transient-controlled regime is
observed for the first few seconds (with metallocene
catalysts2) or fractions of a second (with “classical”
Ziegler-Natta systems3). With a limited number of
catalysts, though, the said regime extends over several
minutes or even hours (which has often been described
as a “living” character).4 Combining this with a high
stereoselectivity is a challenging target and can open
the way to novel polyolefin architectures and in the first
place to block copolymers.
F igu r e 1. QM/MM transition states for propene insertion
with the re (top, left) and si (top, right) enantioface, and for
â-H transfer to the monomer (bottom, left), at an active cation
with Λ configuration. The growing chain is simulated with an
ibutyl (in light green). Bulky R1 substituents (generically
represented with yellow spheres) disfavor the latter two
processes due to the repulsive contacts evidenced with circles.
The extreme case of â-H transfer to the monomer when R1 )
1-adamantyl is explicitated (bottom, right).
Ch a r t 1
For the 1-alkene of highest market significance,
namely propene, block copolymerization has been re-
peatedly achieved under a syndiotactic stereocontrol,4
which is of rather limited practical applicability; herein,
we describe the first block copolymerization of propene
under highly isotactic stereocontrol, using a new catalyst
obtained by means of rational ligand design.
Kol and co-workers had reported5 on the living and
highly isotactic polymerization of 1-hexene promoted by
the C2-symmetric complex 1 of Chart 1 (Bn ) benzyl),
after activation by B(C6F5)3.
In our hands, 1/B(C6F5)3 turned out to be inactive
toward propene; on the other hand, with 1/methyl-
alumoxane (MAO) and 1/[HMe2N(C6H5)][B(C6F5)4]/
Al(ibutyl)3 we obtained moderately isotactic polypropy-
lenes of very low average molecular mass in reasonable
yields.6 13C NMR end group analysis of samples pre-
pared at different propene concentrations revealed a 1,2
insertion regiochemistry and pointed to trans-alkylation
by the Al-alkyl and â-H transfer to the monomer as the
dominating chain transfer pathways.6
us to conclude that the alkyl substituent R1 (Chart 1)
is crucial not only for the enantioselectivity (which is
expected, based on the well-known growing chain
orientation mechanism of stereocontrol6,9) but also on
the ease of monomer-induced chain transfer. Indeed, a
bulkier R1 makes chain misorientation (i.e., with the
first C-C bond pointing toward the nearest-in-space
R1, rather than away from it) more difficult and at the
same time enhances the steric pressure on the space-
demanding six-center transition state of â-H transfer
to propene,10 which is therefore severely destabilized
(Figure 1).
In Table 1, we summarize the main results of our
calculations, for the cases of R1 ) methyl, tbutyl,
1-adamantyl, 9-anthracenyl, and cumyl. In particular,
for each model system we report the difference in
internal energy between the transition states of 1,2
The former process was easily suppressed by reacting
MAO or Al(ibutyl)3 with a sterically hindered phenol.7,8
Contrasting the latter and improving the stereoselec-
tivity, instead, required a fine-tuning of the ancillary
ligand, as is reported in this communication.
Resu lts a n d Discu ssion . Simple quantum mechan-
ics/molecular mechanics (QM/MM) calculations (see
Supporting Information) on models of active cations led
propene insertion with opposite enantiofaces (∆E#
)
enantio
and that (∆E#T/P) between the transition states of chain
transfer to the monomer and of chain propagation via
1,2 insertion with the favored enantioface. The values
of ∆E# are scaled to the case with R1 ) R2 ) H, used
T/P
† Universita` di Napoli “Federico II”.
‡ SABIC EuroPetrochemicals.
* Corresponding author. E-mail: busico@unina.it.
as a reference (∆E#
) 0) and computed at full-QM
T/P
level; this means that such values can be used to predict
10.1021/ma048144b CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/29/2004

8202 Communications to the Editor
Ta ble 1. Ca lcu la ted Va lu es of ∆E#
Defin ition s, See Text) for P r op en e P olym er iza tion a t
Mod el Active Ca tion s Der ived fr om 1 to 5 of Ch a r t 1a
Macromolecules, Vol. 37, No. 22, 2004
a n d ∆E#
(for
en a n tio
T/P
tion). They were then tested in propene polymerization
at 25 °C, comparatively with the known5-7 complexes
1 and 2. The results, summarized in Table 2, are in nice
agreement with the QM/MM-predicted trends, with one
partial exception (vide infra).
precursor
R1
R2
∆E#
∆E#
enantio
T/P
reference
H
H
0
0
2
1
3
4
5
methyl
methyl
0
0.2
1.8
3.5
-1.5
2.2
The catalytic performance of 3, in particular, is truly
remarkable. It produces an isotactic polypropylene (iPP)
which, at the 13C NMR characterization (see Supporting
Information and Figure 2a), reveals only very low
amounts of rr stereodefects (0.3 mol %) and of isolated
2,1 regiodefects in erythro configuration (0.4 mol %).9
The melting temperature of 151 °C and enthalpy of 110
J g-1 are among the highest ever reported for “single-
site” iPP.9 Moreover, 13C NMR end group analysis of
samples obtained at reaction times up to 3 h indicated
a linear increase of Mn (9.5 × 103 Da at 1 h, 2.0 × 104
Da at 2 h, 2.8 × 104 Da at 3 h), with the initial benzyl
ends and the saturated ends deriving from reaction
quenching with acidified methanol in 1:1 mole ratio
within the experimental error (see again Figure 2a);
consistently, Mw/Mn values e1.3 were measured by GPC
(Supporting Information). Traces of terminal vinylidenes
tbutyl
tbutyl
1.7
4.1
1.0
3.7
1-adamantyl
9-anthracenyl
cumyl
methyl
methyl
methyl
aAll energy values in kcal/mol.
relative trends but not absolute average polymerization
degrees. In all cases, R1, R2 ) alkyl (Chart 1) were
modeled at the MM level, and the growing polypropy-
i
lene chain was simulated with an butyl.
From the table, it is immediate to note that for
“spherical” R1s ∆E#
and ∆E#
follow parallel
enantio
T/P
trends. Indeed, both parameters increase regularly with
increasing van der Waals radius, the highest values
corresponding to R1 ) 1-adamantyl. At the other
extreme, system 4, with the big but flat R1 ) 9-anthra-
cenyl, was predicted to afford a nonstereoregular poly-
mer of lower average molecular mass than that with
R1 ) methyl. With its bulky albeit conformationally
flexible R1 ) cumyl (R,R-dimethylbenzyl), system 5
instead would lie in between 1 and 3.
1
became detectable in the H NMR spectra for reaction
times in excess of 3 h.
Compared with the original catalyst 1, the above
corresponds to an increase in enantioselectivity from
96% to 99.7%, in average chain growth time (at 25 °C
and under the conditions described) from less than 0.5
Complexes 3-5 of Chart 1 were prepared on purpose
(see Experimental Section in the Supporting Informa-
Ta ble 2. Selected Resu lts of P r op en e P olym er iza tion (a t 25 °C, [C3H6] ) 1.36 M) P r om oted by 1-5 (for Exp er im en ta l
Deta ils, see Su p p or tin g In for m a tion )
a
precursor
Rp
kp, M-1 s-1
ft,m,b s-1
tcg,c min
Mn,d kDa
[mmmm]e
Tm,f °C
2g
0.9
1.6
4.1
32
70
n.d.
n.d.
n.d.
4
4
0.02
0.80
0.985
0.02
0.89
ami
123
151
ami
136
1g,h
3g
0.045
0.053
n.d.
6 × 10-4
3 × 10-5
n.d.
28 ( 5
550 ( 150
n.d.
110 ( 40
0.8
180 ( 60
4g
5g
0.85
3 × 10-4
55 ( 20
Productivity, in kg (polymer)/[mol (Zr) × [C3H6] × h]. Frequency of chain transfer to the monomer. c Average chain growth time.
a
b
Upper limit of Mn, calculated as M1kp[C3H6]/ft,m, and reached approximately at tp >3tcg.
e 13C NMR fraction of isotactic pentads in fully
d
regioregular sequences. f Maximum of the melting endotherm in second DSC heating scan for polypropylene samples at the upper limit
g
h
i
of Mn. Cocatalyst, [HMe2N(C6H5)][B(C6F5)4]/Al(iButyl)3/2,6-di-tbutylphenol. Data from ref 7. Amorphous. n.d. ) not determined (Mn
and stereoregularity too low for accurate measurements).
F igu r e 2. 100 MHz 13C NMR spectra (in tetrachloroethane-1,2-d2 at 120 °C) of (a) an iPP sample prepared with 3/[HMe2N-
(C6H5)][B(C6F5)4]/Al(ibutyl)3/2,6-di-tbutylphenol at 25 °C and a reaction time of 1 h and (b) and of the sample of iPP-block-PE (see
Experimental Section). The chemical shift scale is in ppm downfield of TMS. In (a), resonances labeled with a, b, c, d are due to
i
benzyl ends, butyl ends, 2,1 regiodefects, and rr stereodefects, respectively;7,9 peaks not explicitly assigned in (b) arise from the
regio- and stereodefects in the iPP block (refer to (a)).

Macromolecules, Vol. 37, No. 22, 2004
Communications to the Editor 8203
to 9 h and in the limiting value of Mn from 4 to 110
kDa. This opens the door to the synthesis of block
copolymers of highly isotactic polypropylene.
investigation of the structure and performance of the
new catalysts, and the synthesis and characterization
of a variety of novel iPP-block-polyolefin materials with
different compositions and block lengths, will be re-
ported in due course.
In Figure 2b, we show the fully assigned quantitative
13C NMR spectrum of a sample of iPP-block-PE (PE )
polyethylene), prepared as described in the Experimen-
tal Section, with block lengths at the upper limit for end
group and juncture detectability. From peak integration,
by averaging over the said sets of resonances (in 1:1:1
integral ratio within the experimental error), we calcu-
lated a number-average polymerization degree of 2.4 ×
102 for the iPP block and of 2.9 × 102 for the PE block;
consistently, the GPC characterization gave a Mn value
of 22 kDa and a Mw/Mn ratio of 1.3. The sample showed
two DSC melting peaks, at 126 °C (∆hm ) 65 J g-1) and
152 °C (∆hm ) 62 J g-1), for the PE and iPP block,
respectively (for GPC and DSC traces, see Supporting
Information).
Ack n ow led gm en t. This research was cofunded by
the Italian Ministry for University (PRIN 2002). M.T.
and S.R. acknowledge the European Community (RTN
HPRN-CT2000-00004) for Postdoctoral Fellowships. The
authors are grateful to the Centro Interdipartimentale
di Metodologie Chimico-Fisiche, Universita` di Napoli
Federico II, for use of the computing and NMR facilities.
Su p p or tin g In for m a tion Ava ila ble: Experimental sec-
tion, computational details, and GPC and DSC traces of iPP-
block-PE. This material is available free of charge via the
Internet at http://pubs.acs.org.
Let us now give a look at the performance of the other
two new catalysts. In agreement with the QM/MM
prediction, 4 turned out to afford an oligomeric, oily
polypropylene, with just a slight enrichment in syndio-
tactic diads ([r] ) 0.6, measured by 13C NMR) due to a
weak stereocontrol exerted by the growing chain,9
similarly to 2.6
The only system that performed partly at odds with
the forecast of our relatively simple theoretical approach
is 5. Indeed, if the experimental enantioselectivity in
propene insertion is intermediate between those of 1 and
3, as anticipated, the molecular weight capability turned
out to be higher than that of 3, due to a faster chain
transfer combined with a much faster chain propagation.
The obvious conclusion is that the conformationally
flexible cumyl fragment would require a more sophis-
ticated computational analysis taking into account the
contributions of all accessible transition state struc-
tures; this however was out of the scope of the paper.
In conclusion, by elaborating on a known ancillary
ligand framework, we have developed the first catalyst
for the highly isotactic controlled polymerization of
propene and used it successfully in block copolymeri-
zation. Further developments, including a more detailed
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