Ultra-rigid metallocenes for highly iso- and regiospecific polymerization of propene: The search for the perfect polypropylene helix

Article abstract of DOI:10.1002/chem.201103547

Ultra-rigid metallocenes are studied for highly iso- and regiospecific polymerization of propene. A single-crystal X-ray structure is obtained of the rac- and meso- forms of the complex and its rac-Hafnium analogue. For a better understanding typical stru

Full text of DOI:10.1002/chem.201103547

DOI: 10.1002/chem.201103547
Ultra-Rigid Metallocenes for Highly Iso- and Regiospecific Polymerization of
Propene: The Search for the Perfect Polypropylene Helix
Alexander Schçbel,[a] Eberhardt Herdtweck,[b] Matthew Parkinson,[c] and
Bernhard Rieger*[a]
The catalytic polymerization of propene has undergone
a fast development since the discovery in 1954 by Ziegler
and Natta. The ZN systems have been further developed
and are used today to produce isotactic polypropylene (iPP)
with isotacticities of up to 99% mmmm leading to melting
transitions (Tm) of 1668C.[1] The first stereoselective homo-
geneous catalysts able to produce iPP were the C2-symmet-
ric ansa-metallocenes, introduced by Ewen, Brintzinger, and
Kaminsky.[2–5] Compared to the known Ziegler–Natta sys-
tems, these early C2-symmetric ansa-metallocene dichlorides
activated with methylaluminoxane (MAO) produced iPP
with relatively low molecular weight and isotacticity.[6,7]
Changing the ligand substitution pattern of these ansa-bisin-
denyl-based metallocenes, catalysts that are able to produce
polypropylene with high melting transitions and high molec-
ular weights have been obtained (1–3). The 2- and 4-posi-
tions on the indenyl moiety have proven to be crucial
(Table 1).[8–10] The 2-methyl group is known to increase mo-
lecular weight due to the inhibition of the bimolecular chain
release by b-hydride transfer to the next coordinated mono-
mer. Additionally substitution in 4-position increases molec-
ular weight and isotacticity.
Further improvements in 2-, 4-, and 7-substitution gave
metallocene catalysts able to polymerize propene with high
stereo- and regioregularity. At low polymerization tempera-
tures of 0 or À308C, these complexes can produce iPP with
a Tm of 163 and 1688C, respectively.[1,11] Besides metallo-
cenes, titanium salalen complexes have been reported re-
cently that are able to produce iPP also with high isotactici-
ties and melting transitions.[12]
In addition to the mentioned 2- and 4-substitution, which
influences stereo- and regioselectivity and helps to control
molecular weight, a substituent in the 7-position increases
the stereorigidity of the complex and affords the preferred
formation of the rac- over the meso-form of the complex.[13]
The known synthesis route for the 2-alkyl-4-aryl-substituted
complexes, which allows the introduction of aryl substituents
À
in 4-position by a C C coupling reaction, is at least a five-
step procedure up to the indanone.[9,14] Furthermore, there is
a lack of easily accessible 2,4,7-substituted indenyl ligands
able to control steric and electronic properties. Here, we
report on a short and convenient strategy, that produces ex-
actly such structures (Scheme 1).
Table 1. Effect of 2,4-substitution on the indenyl moiety of different
ansa-bisindenyl metallocenes.[9]
The synthesis of indanone 5 is achieved from two simple
reactants in two steps. Compound 5 is a building block with
two different functionalities: the hydroxyl group can easily
be reacted with electrophiles and the bromo substituent,
[a]
[b]
Metallocene
Mw
Tm
mmmm[c]
A[d]
rac-[Me2Si(2-Me-4-Ph-Ind)2ZrCl2] (1)
729
195
36
157
145
137
95.2
88.5
81.7
755
99
190
rac-[Me2Si
rac-[Me2Si
ACHTUNGTRENNUNG
A
À
which is prone to C C coupling, affords a variety of inda-
[a] In kgmolÀ1. [b] In8C. [c] In%. [d] Activity in kg(PP)mmolÀ1(Zr)hÀ1
.
nones with variable substitution in 4- and 7-positions. Re-
duction of 5 to the indene 7 proceeds with high yield and si-
lylation of 8 to the ligand precursor 9 is perfectly regioselec-
tive. In addition the 7-alkoxy group gives rise to the pre-
dominant formation of the desired rac-form (up to rac/
meso=9:1) of the complexes most presumably to repulsive
interactions of these groups in the meso isomers.[13]
[a] A. Schçbel, Prof. Dr. B. Rieger
Technische Universitꢀt Mꢁnchen
WACKER-Lehrstuhl fꢁr Makromolekulare Chemie
Lichtenbergstr. 4, 85748 Garching (Germany)
Fax : (+49)89-289-13562
A single-crystal X-ray structure was obtained of the rac-
(Figure 1) and meso-forms (in the Supporting Information)
of complex 10a and its rac-Hafnium analogue 10b (in the
Supporting Information).[15] Compound rac-10a crystallizes
in the orthorhombic space group Pbcn.
For a better understanding typical structural parameters
are compared from similar compounds. The bite angle is
58.18, which is smaller than that of complex 1 (59.28).[9] The
dihedral angle of the phenyl and the indenyl ring is 40.68
and is therefore smaller than the corresponding one in
rieger@tum
[b] E. Herdtweck
Technische Universitꢀt Mꢁnchen
Anorganisch-Chemisches Institut
Lehrstuhl fꢁr Anorganische Chemie
Lichtenbergstr. 4, 85747 Garching (Germany)
[c] M. Parkinson
Borealis Polyolefine GmbH
Sankt-Peter-Strasse 25, 4021 Linz (Austria)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103547.
4174
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Chem. Eur. J. 2012, 18, 4174 – 4178

COMMUNICATION
1 (44.88).[9] In case of hafnocene
10b both the bite angle (57.88)
and the dihedral angle (40.28)
are even smaller. Due to steric
repulsion, the dihedral angle
between the indenyl and phenyl
groups in the 4-position would
have a theoretical optimum at
908. In contrast the contribution
of the delocalization effect of
the p-electrons would lead to
a coplanar structure. Therefore
the expected dihedral angle of
complex 10a (compared to 1)
should be the same or even
larger if there is a repulsive
contribution of the tert-butyl
groups with the indenyl frag-
ment. The smaller angle in this
case can be explained as
a result of steric interaction of
the tert-butyl groups of both
phenyl substituents at least in
the solid state. In addition the
smaller bite angle in 10a in-
creases this repulsion and
therefore the dihedral angle is
Scheme 1. Synthesis of 2-, 4-, and 7-substituted bisindenyl metallocenes.
even further reduced. The sili-
con methyl groups are in close
contact to the oxygen atom resulting in a repulsive interac-
tion.[16] This leads to a distortion of the SiMe2 group out of
the plane defined by the SiMe2 and ZrCl2 fragments (see
the Supporting Information). Therefore the observed small-
er bite angle is assumed to be a result of this steric interac-
tion between the methoxy substituent and the silicon bridge,
a structure element that is indicative for the high steric ri-
gidity of the ansa-metallocene frame.
Both complexes (10a, b) show moderate to good polymer-
ization activities after treatment with triisobutylaluminium/
[CPh3][B(C6F5)4]. The polymer products are highly isotactic
ACHTUNG-
T
ACHTUNGTRENNUNG
polypropylenes with ultrahigh molecular weights and show
melting transitions (ex reactor) up to 1718C (Table 2,
entry 4), which to the best of our knowledge is the highest
reported melting transition for untreated (not extracted or
annealed) iPP. The stereoregularity of the polypropylenes
produced with the complexes 10a, b are extraordinarily high
(entries 1–8). Given the extremely high stereoregularity ob-
served by 13C NMR spectroscopy, the experimental condi-
tions of the NMR experiment itself limit the precision of the
Figure 1. Ortep style plot of compound rac-10a in the solid state. Hydro-
gen atoms are omitted for clarity. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths [ꢃ] and bond angles [8]:
À
À
Zr1 Cl1 2.4180(4), Zr1 Cg1 2.2334; Cl1-Zr1-Cl1a 99.44(1), Cl1-Zr1-Cg1
105.14, Cl1-Zr1-Cg1a 106.47, Cg1-Zr1-Cg1a 130.18. Cg denotes the
centre of gravity of the Cp-part of the indenyl ligand. Symmetry opera-
tion to equivalent atom positions a: 1Àx, y, 0.5Àz. The molecule is locat-
ed on a twofold axis bisecting the Cl1-Zr1-Cl1a angle.
Chem. Eur. J. 2012, 18, 4174 – 4178
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4175

B. Rieger et al.
Table 2. Polymerization of propene with complexes 10a (Zr) and 10b (Hf) activated with triisobutylaluminum/ACTHNUTRGENN[UG CPh3][BACHTUNGTNERN(UGN C6F5)4] in toluene.
[c]
[d]
[f]
Complex
n[a]
p[b]
Tp
Tm
mmmm[e]
2,1 insertion[e]
3,1 insertion[e]
Mw
A[g]
1
2
3
4
5
6
7
8
10a
10a
10a
10b
10b
10b
10b
10b
2
1
1
2
2
2
2
2
4
4
6
3
4
4
6
4
30[h]
50[i]
70[h]
0
165
163
158
171
170
165
163
160
99.6
99.4
99.0
99.9
99.5
99.3
99.0
98.4
0.09
0.17
0.23
–
–
–
0.04
–
–
0.02
0.03
0.06
700000
480000
420000
5800000
1700000
1100000
600000
61000
103000
129000
1800
27000
17000
54000
33000
30[h]
50
–
0.06
0.04
0.04
70
70
410000
[a] In mmol. [b] p=pAr +ppropene in bar. [c] Polymerization temperature in 8C Æ18C. [d] In 8C. [e] In% . from 13C NMR spectroscopy (100 MHz), deter-
mined assuming the enantiomorphic site model.[19] [f] In gmolÀ1. [g] Activity in kg(PP)molÀ1
ACHTUNGTRENNUNG
determined isotacticity. When only isolated stereo defects
are observed, the detection threshold of the mmrr signal
limits the accuracy of the quantification. For the extremely
isotactic materials, for which no stereo defects were ob-
served under standard NMR conditions (8k/NOE/3s),
a pentad isotacticity of 100.0% is observed by inspection
(Figure 2). However, use of automated spectral analysis
merization temperatures (entries 5 and 6) the amount of the
mmmm-pentad is increased in the same way as before, while
the Tm increases more significantly. The change of the so far
linear behavior is due to the now perfect regioregular poly-
merization behavior at these temperatures. With higher tac-
ticities a further increase in stereoregularity results in an
only slight increase of Tm reaching a maximum value of
about 1718C, which we assume to be kinetically controlled
by the applied conditions during the DSC measurement.
This is underlined by short annealing experiments resulting
in fractions of the polymer that melt close to the so far ex-
pected equilibrium melting point of 1868C (see the Support-
ing Information).[20,21]
A significant increase of molecular weight, independent
of the ligand substitution pattern, is found for iPP produced
with hafnium complexes, like the hafnocene 10b. This effect
is known as the hafnium effect and is most presumably due
À
À
to a more stable Hf C bond compared to the Zr C
bond.[3,22] The decrease of molecular weight with higher
polymerization temperature is a known effect for metallo-
cene catalysts, as solubility of propene is decreased together
with the propagation rate and additionally the rate of the
termination reaction is increased (Table 2). As molecular
weight definitely has an influence on the mechanical behav-
ior of the polymer, high stereo- and regioregular iPP togeth-
er with a high molecular weight is important.
Figure 2. Quantitative 13C{1H} NMR spectrum of iPP produced with com-
plex rac-10b at 08C including BHT as stabilizer, illustrating no detectable
stereo defects and only the expected 13C satellites and artifacts (spinning/
decoupling sidebands) under standard measurement conditions.
non-zero baseline and phasing issues, results in non-zero in-
tegrals and thus pentad isotacticities <100.0%. With the
signal-to-noise ratio of the mmmm signal known and
a signal-to-noise ratio of 2.5 assumed as the detection
threshold of the mmrr signal, the minimum pentad isotactic-
ity of sample given in entry 4 in Table 2 can be estimated to
be at least 99.90% using the enantiomorphic site control
model (see the Supporting Information).
The present complexes produce extraordinarily high ster-
eoregularities also at increased polymerization temperatures
and therefore also the melting transitions stay quite high
(entry 7). This underlines their temperature stability with re-
spect to stereoselectivity based on their high rigidity.[9,17,18]
There is a clear correlation between melting transition and
tacticity (entries 4–8). Due to the fact that the amount of re-
gioerrors is similar for entries 4, 5 and 6–8, the change in Tm
is only influenced by the mmmm-pentad concentration. A
nearly linear increase of Tm can be seen in the case of the
lower tacticities (entries 6–8). Going further to lower poly-
Comparing the two different metals (Hf and Zr) one can
see higher melting transitions for the iPP produced with the
hafnocene 10b compared to the zirconocene 10a at the
same conditions. Additionally to the high mmmm-pentad
concentration a low amount of regioerrors and 3,1-insertions
helps to improve the Tm further. The difference between the
zirconocene 10a and the hafnocene 10b is the amount of re-
gioerrors (Table 2, entries 1–3 and 5–7). For [Cp2MCl2] (M=
Zr, Hf) theoretical investigations show the same trend as
the energy difference between the transition states of a 1,2
and a 2,1-insertion is higher for the hafnocene compared to
the corresponding zirconocene species.[23]
Additionally, there is a clear dependency of propene con-
centration on the amount of stereodefects (entries 7 and 8).
The formation of a stereoerror is known to proceed either
through the choice of the “wrong” enantioface of the mono-
mer or due to a chain end epimerization process (CEP).[24]
A key step of the CEP is b-hydride elimination and there-
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Chem. Eur. J. 2012, 18, 4174 – 4178

Ultra-Rigid Metallocenes
COMMUNICATION
fore the amount of stereoerrors and the molecular weight of
the produced polypropylenes are influenced directly by the
propene concentration. Higher propene concentration sup-
presses the b-hydride elimination due to a faster coordina-
tion and insertion reaction and therefore results in polypro-
pylenes with higher stereoselectivity together with higher
molecular weights.
Acknowledgements
This work was supported by Borealis Polyolefine GmbH. The authors
thank Dr. Luigi Resconi for many fruitful discussions; Gerhard Hubner
and Lisa Marie Steiner are thanked for undertaking the NMR measure-
ments.
A crucial difference of the presented complexes 10a,b
with respect to metallocenes known for the production of
highly isotactic PP is the combination of high stereo- and re-
gioselectivity, also present at elevated polymerization tem-
peratures. Especially the effect of Hf in combination with
the presented ligand structure increases the regioselectivity
of the complex 10b at elevated temperatures. Hence poly-
propylenes with high melting points can be obtained also at
high polymerization temperatures just by increasing the pro-
pene concentration (Table 2 entries 7 and 8).
Keywords: high melting transition · metallocenes · propene
polymerization · regioselectivity · stereoselectivity
[1] N. Kashiwa, S.-i. Kojoh, J.-i. Imuta, T. Tsutsui, in Metalorganic Cata-
lysts for Synthesis and Polymerization (Ed.: W. Kaminsky), Springer,
Berlin, Heidelberg, 1999, pp. 30–37.
[2] J. A. Ewen, J. Am. Chem. Soc. 1984, 106, 6355–6364.
[3] J. A. Ewen, L. Haspeslagh, J. L. Atwood, H. M. Zhang, J. Am.
Chem. Soc. 1987, 109, 6544–6545.
[4] F. R. W. P. Wild, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Orga-
nomet. Chem. 1982, 232, 233–247.
[5] F. Wild, M. Wasiucionek, G. Huttner, H. H. Brintzinger, J. Organo-
met. Chem. 1985, 288, 63–67.
Theoretical investigations concerning regioregularity in
case of 4-alkyl-substituted SiMe2ACTHNUTRGNEUNG(Ind)2 metallocenes show
a decrease in regioselectivity for methyl and ethyl groups.
Isopropyl is known to be more selective and a tert-butyl sub-
stituent again decreases regioselectivity.[25,26] Symmetrically
substituted bisindenyl complexes (e.g., 4,7-dimethyl substi-
tuted ones) are also known to have lower regioselectivity
compared to the unsubstituted catalysts.[27] Values deter-
mined for the amount of regioerrors, especially in case of
hafnium-based metallocenes, are generally rare. However, in
case of the highly 4-substituted ansa-bisindenyl metallocenes
they are either not available (Hf) or rather rare (Zr). For
[6] W. Kaminsky, K. Kulper, H. H. Brintzinger, F. Wild, Angew. Chem.
1985, 97, 507–508; Angew. Chem. Int. Ed. Engl. 1985, 24, 507–508.
[7] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100,
1253–1345.
[8] W. Spaleck, M. Antberg, J. Rohrmann, A. Winter, B. Bachmann, P.
Kiprof, J. Behm, W. A. Herrmann, Angew. Chem. 1992, 104, 1373–
1376; Angew. Chem. Int. Ed. Engl. 1992, 31, 1347–1350.
[9] W. Spaleck, F. Kuber, A. Winter, J. Rohrmann, B. Bachmann, M.
Antberg, V. Dolle, E. F. Paulus, Organometallics 1994, 13, 954–963.
[10] U. Stehling, J. Diebold, R. Kirsten, W. Roll, H. H. Brintzinger, S.
Jungling, R. Mulhaupt, F. Langhauser, Organometallics 1994, 13,
964–970.
example, rac-[Me2Si
{2-Me-4-(1-naphthyl)Ind}2ZrCl2] or rac-
[11] H. Deng, H. Winkelbach, K. Taeji, W. Kaminsky, K. Soga, Macro-
molecules 1996, 29, 6371–6376.
[12] K. Press, A. Cohen, I. Goldberg, V. Venditto, M. Mazzeo, M. Kol,
Angew. Chem. 2011, 123, 3591–3594; Angew. Chem. Int. Ed. 2011,
50, 3529–3532.
[13] J. Schulte, T. Sell, M. G. Thorn, A. Winter, A. Dimeska, 2009,
WO2009054831 A1.
[14] V. V. Izmer, A. Y. Lebedev, M. V. Nikulin, A. N. Ryabov, A. F. Asa-
chenko, A. V. Lygin, D. A. Sorokin, A. Z. Voskoboynikov, Organo-
metallics 2006, 25, 1217–1229.
[Me2Si{2-nPr-4-(9-phenanthryl)Ind}2ZrCl2] are among the
ACHTUNGTRENNUNG
most stereoselective metallocenes reported so far, and pro-
duce polypropylene (508C, 1 bar propene) with a total
amount of regioerrors (2,1- + 3,1-insertion) of 0.34 and
0.22%, respectively.[1] Complexes 10a,b are both even more
regioselective.
The metal itself in combination with this particular substi-
tution pattern in 2-, 4-, and 7-positions together with the
tert-butyl groups on the 4-phenyl ring evidently play an im-
portant role on the stereo- and regioselectivity. The easy
and convenient synthesis route with the possibility of an in-
dependently substitution pattern especially in the 4- and 7-
positions affords the superior control of stereoregularity in
combination with regioselectivity in the final complexes.
Hence, further investigations for differently substituted 2-,
4-, and 7-indenyl moieties in order to study the electronic
and steric influences and if possible to further improve
stereo- and regioselectivity especially at higher polymeri-
zation temperatures are in progress. Within the frame of
measurement precision, the complexes presented here al-
ready afford the formation of perfect isotactic polypropy-
lene helices. However, their packing, especially of the high
molecular weight products, in the solid state is currently
a matter of intensive investigation.[28–31] A solution to the
question how the extreme molecular parameters of the pres-
ent polymer products allow a technical processing, will
decide on the future of this new polypropylene family.
[15] Single crystal X-ray structure determinations: rac-10a: orange brick,
C52H66Cl2O2SiZr, Mr =913.26; orthorhombic, space group Pbcn (No.
60),
4868.7(2) ꢃ3, Z=4,
1.246 gcmÀ3, T=123(1) K, F
AHCTUNGTRENNUNG
a=17.2603(5),
b=13.5761(4),
(MoKa)=0.71073 ꢃ, m=0.398 mmÀ1
(000)=1928, qmax: 25.378, R1=0.0249
c=20.7772(6) ꢃ,
V=
l
G
,
1calcd
=
(4243 observed data), wR2=0.0663 (all 4412 data), GOF=1.105,
395 parameters, D1max/min =0.28/À0.28 eꢃÀ3. meso-10a: red needle,
C52H66Cl2O2SiZr·C5H12, Mr =985.40; monoclinic, space group P21/n
(No. 14), a=14.3365(4), b=13.4062(4), c=27.4537(7) ꢃ, b=
94.1569(12)8, V=5262.7(3) ꢃ3, Z=4,
lACHTUNGTRENNUNG
0.373 mmÀ1, 1calcd =1.244 gcmÀ3, T=123(1) K, F
A
:
25.428, R1=0.0270 (8480 observed data), wR2=0.0676 (all 9323
data), GOF=1.059, 613 parameters, D1max/min =0.36/À0.41 eꢃÀ3. rac-
10b: yellow block, C52H66Cl2HfO2Si, Mr =1000.53; orthorhombic,
space group Pbcn (No. 60), a=17.2305(6), b=13.5746(4), c=
20.7612(7) ꢃ, V=4856.0(3) ꢃ3, Z=4,
lACHTUNGTRENNUNG
2.320 mmÀ1, 1calcd =1.369 gcmÀ3, T=123(1) K, F
A
:
25.408, R1=0.0177 (3888 observed data), wR2=0.0425 (all 4468
data), GOF=1.147, 272 parameters, D1max/min =0.70/À0.44 eꢃÀ3
.
CCDC-841805 (rac-10a), CCDC-841804 (meso-10a), and CCDC-
841806 (rac-10b) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Chem. Eur. J. 2012, 18, 4174 – 4178
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4177

B. Rieger et al.
[16] M. Mantina, A. C. Chamberlin, R. Valero, C. J. Cramer, D. G. Truh-
lar, J. Phys. Chem. A 2009, 113, 5806–5812.
[17] W. A. Herrmann, J. Rohrmann, E. Herdtweck, W. Spaleck, A.
Winter, Angew. Chem. 1989, 101, 1536–1538; Angew. Chem. Int. Ed.
Engl. 1989, 28, 1511–1512.
[18] W. Rçll, H. H. Brintzinger, B. Rieger, R. Zolk, Angew. Chem. 1990,
102, 339–341; Angew. Chem. Int. Ed. Engl. 1990, 29, 279–280.
[19] L. Resconi, A. Fait, F. Piemontesi, M. Colonnesi, H. Rychlicki, R.
Zeigler, Macromolecules 1995, 28, 6667–6676.
[20] K. Yamada, M. Hikosaka, A. Toda, S. Yamazaki, K. Tagashira, Mac-
romolecules 2003, 36, 4790–4801.
[21] K. Yamada, M. Hikosaka, A. Toda, S. Yamazaki, K. Tagashira, Mac-
romolecules 2003, 36, 4802–4812.
[22] J. A. Ewen, R. L. Jones, A. Razavi, J. D. Ferrara, J. Am. Chem. Soc.
1988, 110, 6255–6256.
[23] A. Laine, M. Linnolahti, T. A. Pakkanen, J. R. Severn, E. Kokko, A.
Pakkanen, Organometallics 2010, 29, 1541–1550.
[25] M. Toto, L. Cavallo, P. Corradini, G. Moscardi, L. Resconi, G.
Guerra, Macromolecules 1998, 31, 3431–3438.
[26] T. Yoshida, N. Koga, K. Morokuma, Organometallics 1996, 15, 766–
777.
[27] L. Resconi, F. Piemontesi, I. Camurati, D. Balboni, A. Sironi, M.
Moret, H. Rychlicki, R. Zeigler, Organometallics 1996, 15, 5046–
5059.
[28] F. J. Medellꢄn-Rodrꢄguez, M. Mata-Padilla, S. Sꢅnchez-Valdes, S.
Vega-Dꢄaz, O. Dꢅvalos-Montoya, J. Polym. Sci. Part B 2008, 46,
2188–2200.
[29] S. Z. D. Cheng, J. J. Janimak, A. Zhang, H. N. Cheng, Macromole-
cules 1990, 23, 298–303.
[30] J. J. Janimak, S. Z. D. Cheng, P. A. Giusti, E. T. Hsieh, Macromole-
cules 1991, 24, 2253–2260.
[31] H. B. Lu, J. L. Qiao, Y. B. Xu, Y. L. Yang, J. Appl. Polym. Sci. 2002,
85, 333–341.
Received: November 10, 2011
[24] V. Busico, R. Cipullo, J. Am. Chem. Soc. 1994, 116, 9329–9330.
Published online: March 1, 2012
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Chem. Eur. J. 2012, 18, 4174 – 4178