Highly selective olefin trimerization catalysis by a borane-activated titanium trimethyl complex

Article abstract of DOI:10.1021/om401098m

Reaction of a trimethyl titanium complex, (FI)TiMe₃ (FI = phenoxy-imine), with 1 equiv of B(C₆F₅)₃ gives [(FI)TiMe₂][MeB(C₆F₅)₃], an effective precatalyst for the selective trimerization of ethylene. Mechanistic studies indicate that catalyst initiation involves generation of an active Ti^II species by olefin insertion into a Ti-Me bond, followed by β-H elimination and reductive elimination of methane, and that initiation is slow relative to trimerization. (FI)TiMe₃/B(C₆F ₅)₃ also leads to a competent catalyst for the oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer. This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants.

Full text of DOI:10.1021/om401098m

Communication
pubs.acs.org/Organometallics
Highly Selective Olefin Trimerization Catalysis by a Borane-Activated
Titanium Trimethyl Complex
Aaron Sattler, Jay A. Labinger,* and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: Reaction of a trimethyl titanium complex,
(FI)TiMe3 (FI = phenoxy-imine), with 1 equiv of B(C6F5)3
gives [(FI)TiMe2][MeB(C6F5)3], an effective precatalyst for
the selective trimerization of ethylene. Mechanistic studies
indicate that catalyst initiation involves generation of an active
TiII species by olefin insertion into a Ti−Me bond, followed by
β-H elimination and reductive elimination of methane, and
that initiation is slow relative to trimerization. (FI)TiMe3/
B(C6F5)3 also leads to a competent catalyst for the
oligomerization of α-olefins, displaying high selectivity for trimers (>95%), approximately 85% of which are one regioisomer.
This catalyst system thus shows promise for selectively converting light α-olefins into transportation fuels and lubricants.
he selective trimerization of ethylene to 1-hexene,1,2
comonomer for production of linear low-density poly-
a
hexene, turnover number (TON) ∼1.1 × 107 (mmol of olefin
oligomerized)/(mmol of Ti) at 49.3 atm) based on (FI)TiCl3
(FI = N-(5-methyl-3-(1-adamantyl)salicylidene)-2′-(2″-
methoxyphenyl)anilinato) as the precatalyst; as in most other
systems, a large excess (10000 equiv in this case) of MAO is
required for activation.18,19 In order to gain mechanistic insight
into this catalytic system, we sought to develop a precatalyst
that could be activated stoichiometrically. We report here the
synthesis and structural characterization of (FI)TiMe3, which
can selectively trimerize ethylene upon activation with 1 equiv
of B(C6F5)3. Moreover, (FI)TiMe3/B(C6F5)3 can oligomerize
LAOs with remarkable selectivity (>95%) for trimers, ∼85% of
which consist of a single regioisomer.
T
ethylene (LLDPE), has been extensively studied3 and is an
important industrial process.4 Most research in this area has
involved ill-defined catalyst systems, requiring large excesses
(typically 1000-fold or more) of an activator such as MAO.5
Stoichiometric activation of catalysts is rare.6,7 Hence, many
mechanistic aspects are at best poorly understood, in contrast
to the well-studied field of ethylene polymerization, where
stoichiometrically activated catalysts are common.8
While ethylene trimerization catalysis is well precedented,
research concerning the oligomerization of linear α-olefins
(LAOs) is scarce.9−14 In fact, to the best of our knowledge, the
only effective catalyst for selective LAO trimerization is
(R3TAC)CrCl3 (R3TAC = trialkyltriazacyclohexane), which
converts 1-decene and 1-dodecene to C30 and C36 olefins,
respectively;9 it should be emphasized that this system also
requires activation by excess MAO (100 equiv). Light LAOs are
significant components of crude oil as well as being produced in
refinery processes such as fluidized catalytic cracking, but (aside
from the aforementioned production of LLDPE) are not widely
used; upgrading light LAOs to heavier olefins by oligomeriza-
tion could be a valuable route to higher value products, such as
diesel and/or jet fuel,15 lubricants,16 and precursors to
surfactants and detergents. For example, trimerization of 1-
pentene or 1-hexene would lead to C15 and C18 hydrocarbons,
components of jet/diesel fuel. Trimerization of 1-decene is
currently used industrially for the production of synthetic
lubricants (e.g., Mobil 1),16 but these processes give
distributions of oligomers;17 selective trimerization is most
desirable, as it eliminates the need for additional energy-
intensive separations.
Treatment of a suspension of (FI)TiCl3 in Et2O at −35 °C
with MeMgBr, followed by addition of 1,4-dioxane, afforded
yellow (FI)TiMe3 (Scheme 1).20 Single-crystal X-ray diffraction
studies established the five-coordinate, pseudo-trigonal-bipyr-
amidal geometry shown in Figure 1.21 The upfield H NMR
1
chemical shift for the methoxy group (δ 2.93 ppm, Figure 2)
relative to that of (FI)TiCl3 (δ 4.09 ppm), which is six-
coordinate in the solid state,18 indicates that (FI)TiMe3 is also
Scheme 1
In 2010, Fujita et al. reported a new, highly active, and
selective ethylene trimerization system (∼92% selective for 1-
Received: November 12, 2013
© XXXX American Chemical Society
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small amounts of C14 olefins (∼5%) are also observed, with
only trace amounts of polyethylene (<3 mg) formed. The C10
and C14 olefins arise from cotrimerization of ethylene with 1-
hexene and C10 respectively; the relative amounts depend upon
catalyst concentration and the degree of conversion to which
the reaction is carried out. Overall selectivity for olefin trimers
is greater than 99%, with a turnover frequency (TOF) of ca. 2.7
× 103 (mmol of olefin oligomerized)/((mmol of Ti) h); a total
of ca. 8.3 × 103 turnovers were achieved after 3 h. The activity
of the original (FI)TiCl3/MAO system can be extrapolated
(from the high-pressure data and second-order dependence on
ethylene pressure reported18) to a TOF value of 4.6 × 103 at 1
atm, which is similar to our experimental value for (FI)TiMe3/
B(C6F5)3. Three major C10 olefins are produced, one 2-
substituted α-olefin and two α-olefins (Scheme 2).20 In order
Scheme 2
Figure 1. Molecular structure of (FI)TiMe3. H atoms on the FI ligand
are not shown for clarity.
to ascertain the relative rate of LAO and ethylene
incorporation, a competition experiment between 1-heptene
and ethylene (20/1 mole ratio) was carried out. At low
conversion (∼7% 1-heptene conversion based on the C11
yield), the ratio of 1-hexene to C11 olefins produced was
3.43/1, demonstrating that ethylene incorporation is approx-
imately 70 times faster than α-olefin incorporation, similar to
the case for the previously reported (PNP)Cr system.26
Figure 2. 1H NMR spectra of (top) (FI)TiMe3 in C6D6 and (bottom)
[(FI)TiMe2][MeB(C6F5)3] in C6D6/o-C6H4F2 at 25 °C. The aromatic
region is not shown.
The successful generation of a catalyst by stoichiometric
activation facilitates experiments aimed at insight into the
mechanism(s) of ethylene trimerization. Monitoring the
1
progress of the reaction by H NMR spectroscopy demon-
five-coordinate in solution (C6D6). Furthermore, the three
methyl groups of the TiMe3 fragment show a single H NMR
signal from 25 to −80 °C (Figure 2), indicating the fluxional
nature of (FI)TiMe3.
strates that, after addition of ethylene to [(FI)TiMe2][MeB-
(C6F5)3] in C6D6/o-C6H4F2 at room temperature, 1-hexene
formation begins rapidly (<15 min); however, at this time the
signals corresponding to the precatalyst [(FI)TiMe2][MeB-
(C6F5)3] have only decreased in intensity by ∼25% in
comparison with the spectrum acquired prior to ethylene
addition.27 Over a period of several hours, the signals of
[(FI)TiMe2][MeB(C6F5)3] decrease slowly as ethylene is
consumed and 1-hexene/C10 olefins are produced, and catalytic
activity ceases shortly after [(FI)TiMe2][MeB(C6F5)3] is no
longer observable by 1H NMR spectroscopy.20 These
observations indicate that the overall rate of trimerization is
significantly faster than the rate of initiation and that the rate of
catalyst decomposition is comparable to that for initiation.
Room-temperature EPR spectroscopy shows the formation
of a TiIII species (g = 1.958) in the reaction of B(C6F5)3 with
(FI)TiMe3;28 the yield is ∼20% (quantified by double
integration relative to Cp2VCl2 as an external standard),
which is consistent with the 80% NMR yield of [(FI)TiMe2]-
[MeB(C6F5)3] (see above). Upon addition of ethylene, the
EPR signal increases during trimerization catalysis, reaching a
maximum intensity around 5 times the initial value after 2 h (by
which time ∼80% of 1-hexene production has occurred) and
remaining there for several hours after catalysis has stopped.
These observations strongly suggest that the TiIII species is a
decomposition product, not an active trimerization catalyst or
precatalyst.
1
Treatment of a solution of (FI)TiMe3 in C6D6/o-C6H4F2 (ca.
0.6 mL/0.1 mL) with 1 equiv of B(C6F5)3 at 25 °C immediately
generates a new species, characterized by NMR spectroscopy
(Figure 2) as [(FI)TiMe2][MeB(C6F5)3] (Scheme 1), in
approximately 80% yield. The downfield chemical shift of the
OMe group (δ 3.69 ppm, Figure 2) in comparison to the signal
for (FI)TiMe3 indicates coordination to Ti, while the 1H NMR
chemical shift of [MeB(C6F5)3]− (δ 1.19 ppm, Figure 2) and
the Δδ(m,p-F) value of 2.5 ppm in the 19F NMR spectrum are
consistent with a noncoordinating solvent-separated anion,
rather than a coordinating tight ion pair.22 Interestingly,
although five-coordinate complexes are typically fluxional23 on
the NMR time scale (e.g. (FI)TiMe3), the two methyl groups
of [(FI)TiMe2] are chemically inequivalent and static in the 1H
NMR spectrum at 25 °C (δ 1.56 and 1.72 ppm, Figure 2);
however, a slow exchange process can be observed by exchange
spectroscopy (EXSY). We suggest that the tridentate FI ligand
inhibits Berry pseudorotation;24 loose coordination of a solvent
molecule may also be involved.
[(FI)TiMe2][MeB(C6F5)3] is an effective precatalyst for
ethylene trimerization.25 Exposure of a solution of (FI)TiMe3/
B(C6F5)3 (7 μmol, 7 mM) in o-C6H4F2 (1 mL) to 1 atm of
ethylene at 25 °C produces primarily 1-hexene and C10 olefins;
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Treatment of [(FI)TiMe2][MeB(C6F5)3] with a 1/1 C2H4/
C2D4 mixture at 25 °C produces only four isotopologues of 1-
hexene, namely C6H12, C6H8D4, C6H4D8, and C6D12, which
rules out a Cossee type mechanism due to the absence of
scrambling and is in accord with the metallacycle mecha-
nism.29−31 Such a mechanism implicates a reduced species,
presumably TiII; in order to investigate its formation,
[(FI)TiMe2][MeB(C6F5)3] was treated with C2D4 at 25 °C.
The immediate formation of CH3D was observed by 1H NMR
spectroscopy, along with catalytic production of 1-hexene-d12,
implying that initiation proceeds via a sequence of initial
ethylene (C2D4) insertion(s) into Ti−CH3, followed by β-D
elimination and reductive elimination of CH3D, which gives the
aforementioned TiII species (Scheme 3). We have not yet been
selective β-H elimination (favored mechanism, Scheme 4).
However, it is possible that a head-to-tail coupling occurs first,
followed by selective 1,2-insertion into the more substituted
segment of the metallacycle and selective β-H elimination
(alternate mechanism, Scheme 4). Although we cannot
distinguish between these two pathways at this point, we
favor the former in light of the propensity for metal-
lacyclopentanes to form tail-to-tail dimers,33 while the alternate
pathway requires a 1,2-addition of the third olefin into the more
sterically encumbered side of the metallacycle.
In conclusion, [(FI)TiMe2][MeB(C6F5)3] is a precatalyst for
selective olefin trimerization. This stoichiometrically activated
catalyst system facilitates mechanistic studies, which demon-
strate that catalyst activation occurs via ethylene insertion(s),
followed by β-H elimination and reductive elimination of
methane, to form the active TiII catalyst. Most importantly,
[(FI)TiMe2][MeB(C6F5)3] can oligomerize LAOs to trimers
with extremely high selectivity, giving 85% of a single
regioisomer, offering the potential for upgrading inexpensive
and readily available feedstocks to value-added products, such
as diesel/jet fuel and lubricants.
Scheme 3
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental details,
characterization data, and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
able to observe any corresponding α-olefin (e.g., CH3CD
CD2); most likely it is incorporated in the first trimerization
cycle, which can give several different C7 olefins. Furthermore,
if the rates of ethylene insertion and β-H elimination are
similar, olefins higher than propylene would be involved in the
first trimerization cycle, making it difficult to experimentally
observe these products. These results, along with the slow rate
of initiation (see above), suggest that the slow step in this
system is the initial insertion of ethylene into a Ti−Me bond of
the cationic titanium dimethyl complex. Previous DFT
calculations for a related system ([(Ph(CMe2)Cp)TiMe2]+)19
predict an initiation pathway that involves methane elimination,
in accord with our experimental observations.32
[(FI)TiMe2][MeB(C6F5)3] is also a precatalyst for the
selective trimerization of LAOs, affording trimers in greater
than 95% selectivity at 25 °C, with TONs of ca. 350 for 1-
pentene and 1-hexene and ca. 100 for 1-decene (Scheme 4).
Even more remarkably, among the trimers produced, ca. 85%
are one regioisomer, strongly suggesting a single site active
catalytic species.20 The major olefin product is proposed to
form by a tail-to-tail coupling, followed by 1,2-insertion and
AUTHOR INFORMATION
Corresponding Authors
*E-mail for J.A.L.: jal@caltech.edu.
*E-mail for J.E.B.: bercaw@caltech.edu.
■
Notes
The authors declare the following competing financial
interest(s): A provisional patent application partially based on
this work has been filed.
ACKNOWLEDGMENTS
■
We thank David VanderVelde for help setting up several NMR
experiments, Larry Henling for assistance with crystallographic
work, Angelo Di Bilio, Loi Do, and John Anderson for help
with EPR experiments, and Harry B. Gray and Jay R. Winkler
for helpful discussions. This work was funded by BP through
the XC2 program.
REFERENCES
■
(1) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674−675.
(2) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.;
Wass, D. F. Chem. Commun. 2002, 858−859.
Scheme 4
(3) For reviews, see: (a) McGuinness, D. S. Chem. Rev. 2011, 111,
2321−2341. (b) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H.
J. Organomet. Chem. 2004, 689, 3641−3668. (c) Agapie, T. Coord.
Chem. Rev. 2011, 255, 861−880.
(4) Reagen, W. K.; McDaniel, M. P. (Phillips Petroleum Company).
U.S. Patent 5,382,738, 1995.
(5) The inherent problems of activation with large excesses of MAO
have been noted previously. For example, see: Stennett, T. E.;
Haddow, M. F.; Wass, D. F. Organometallics 2012, 31, 6960−6965.
Difficulties using activators other than MAO have been reported. See:
McGuinness, D. S.; Overett, M.; Tooze, R. P.; Blann, K.; Dixon, J. T.;
Slawin, A. M. Z. Organometallics 2007, 26, 1108−1111.
(6) (a) Agapie, T.; Day, M. W.; Henling, L. M.; Labinger, J. A.;
Bercaw, J. E. Organometallics 2006, 25, 2733−2742. (b) Schofer, S. J.;
Day, M. W.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E.
C
dx.doi.org/10.1021/om401098m | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Organometallics 2006, 25, 2743−2749. (c) Agapie, T.; Labinger, J. A.;
Bercaw, J. E. J. Am. Chem. Soc. 2007, 129, 14281−14295. (d) Otten,
E.; Batinas, A. A.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2009,
131, 5298−5312.
(7) For examples of MAO-free tantalum trimerization catalysis, see:
(a) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J. Am.
(b) Pellecchia, C.; Pappalardo, D.; Oliva, L.; Mazzeo, M.; Gruter, G.-J.
Macromolecules 2000, 33, 2807−2814.
(26) Do, L. H.; Labinger, J. A.; Bercaw, J. E. Organometallics 2012,
31, 5143−5149. The relative rate of ethylene/1-heptene incorporation
was reported there to be 1/1, but that was the result of an arithmetic
error; the actual value is 52/1.
(27) The half-life of [(FI)TiMe2][MeB(C6F5)3] is significantly
greater in the absence of ethylene (>12 h) than in the presence (∼1
h), indicating that catalyst decomposition is promoted by reaction with
ethylene.
Chem. Soc. 2001, 123, 7423−7424. (b) Arteaga-Muller, R.; Tsurugi,
̈
H.; Saito, T.; Yanagawa, M.; Oda, S.; Mashima, K. J. Am. Chem. Soc.
2009, 131, 5370−5371. (c) Chen, Y.; Callens, E.; Abou-Hamad, E.;
́
Merle, N.; White, A. J. P.; Taoufik, M.; Coperet, C.; Le Roux, E.;
(28) Reduction of similar compounds to TiIII upon addition of
B(C6F5)3 has been observed previously. For examples, see: (a) Ewart,
S. W.; Baird, M. C. Top. Catal. 1999, 7, 1−8. (b) Grassi, A.; Zambelli,
A.; Laschi, F. Organometallics 1996, 15, 480−482. (c) Williams, E. F.;
Murray, M. C.; Baird, M. C. Macromolecules 2000, 33, 261−268.
(d) Mahanthappa, M. K.; Waymouth, R. M. J. Am. Chem. Soc. 2001,
123, 12093−12094.
Basset, J.-M. Angew. Chem., Int. Ed. 2012, 51, 11886−11889.
(8) Chen, E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391−1434.
(9) (a) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm, S.;
̈
̈
Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 4337−
4339. (b) Wasserscheid, P.; Grimm, S.; Kohn, R. D.; Haufe, M. Adv.
Synth. Catal. 2001, 343, 814−818.
̈
(10) McGuinness, D. S. Organometallics 2009, 28, 244−248.
(11) (a) Bowen, L. E.; Charernsuk, M.; Wass, D. F. Chem. Commun.
2007, 2835−2837. (b) Bowen, L. E.; Charernsuk, M.; Hey, T. W.;
McMullin, C. L.; Orpen, A. G.; Wass, D. F. Dalton Trans. 2010, 39,
560−567.
(29) Belov, G. P.; Dzhabiev, T. S.; Kolesnikov, I. M. J. Mol. Catal.
1982, 14, 105−112.
(30) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 1304−1305.
(31) (FI)TiCl3/MAO gives the same isotopic distribution when
treated with a 1/1 C2H4/C2D4 mixture, thereby indicating that this
system also operates through a metallacycle mechanism. Laskowski, C.
A.; Labinger, J. A.; Bercaw, J. E. Unpublished results.
(32) Tobisch, S.; Ziegler, T. Organometallics 2003, 22, 5392−5405.
(33) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1980,
102, 5610−5618.
(12) For examples of α-olefin oligomerization by group IV metal
complexes, see: (a) Lian, B.; Beckerle, K.; Spaniol, T. P.; Okuda, J.
Angew. Chem., Int. Ed. 2007, 46, 8507−8510. (b) Xu, T.; Liu, J.; Wu,
G.-P.; Lu, X.-B. Inorg. Chem. 2011, 50, 10884−10892. (c) Londaitsbe-
here, A.; Cuenca, T.; Mosquera, M. E. G.; Cano, J.; Milione, S.; Grassi,
A. Organometallics 2012, 31, 2108−2111. (d) Dagorne, S.; Bellemin-
Laponnaz, S.; Romain, C. Organometallics 2013, 32, 2736−2743.
(13) For α-olefin dimerization, see: (a) Broene, R. D.; Brookhart, M.;
Lamanna, W. M.; Volpe, A. F., Jr. J. Am. Chem. Soc. 2005, 127, 17194−
17195. (b) Small, B. L.; Marcucci, A. J. Organometallics 2001, 20,
5738−5744. (c) Small, B. L. Organometallics 2003, 22, 3178−3183.
(d) Tellmann, K. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.
Organometallics 2004, 24, 280−286.
(14) For cotrimerization of ethylene and styrene, see: (a) Bowen, L.
E.; Wass, D. F. Organometallics 2006, 25, 555−557. (b) Reference
11b.
(15) (a) Wright, M. E.; Harvey, B. G.; Quintana, R. L. (The United
States of America as Represented by the Secretary of the Navy). U.S.
Patent 8,395,007, 2013. (b) Beech, J. H., Jr.; Owen, H.; Ramage, M. P.;
Tabak, S. A. (Mobil Oil Corporation). U.S. Patent 4,720,600, 1988.
(16) (a) Hamilton, L. A.; Seger, F. M. (Mobil Oil Corporation). U.S.
Patent 3,149,178, 1964. (b) Brennan, J. A. (Mobil Oil Corporation).
U.S. Patent 3,382,291, 1968.
(17) Shubkin, R. L.; Baylerian, M. S.; Maler, A. R. Ind. Eng. Chem.
Prod. Res. Dev. 1980, 19, 15−19.
(18) Suzuki, Y.; Kinoshita, S.; Shibahara, A.; Ishii, S.; Kawamura, K.;
Inoue, Y.; Fujita, T. Organometallics 2010, 29, 2394−2396.
(19) For related Ti-based trimerization systems, see: (a) Reference
6d. (b) Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem., Int.
Ed. 2001, 40, 2516−2519. (c) Deckers, P. J. W.; Hessen, B.; Teuben, J.
H. Organometallics 2002, 21, 5122−5135. (d) Hessen, B. J. Mol. Catal.
A: Chem. 2004, 213, 129−135.
(20) See the Supporting Information for experimental details.
(21) For other examples of structurally characterized complexes with
the [TiMe3] fragment, see: (a) Nikiforov, G. B.; Roesky, H. W.;
Heisen, B. C.; Grosse, C.; Oswald, R. B. Organometallics 2008, 27,
2544−2548. (b) Ghesner, I.; Fenwick, A.; Stephan, D. W. Organo-
metallics 2006, 25, 4985−4995. (c) Buil, M. L.; Esteruelas, M. A.;
Lop
565.
́
ez, A. M.; Mateo, A. C.; Onate, E. Organometallics 2007, 26, 554−
̃
(22) Δδ(m,p-F) values of 3−6 ppm indicate coordination of
[MeB(C6F5)3]− to the metal center. See: Horton, A. D. Organo-
metallics 1996, 15, 2675−2677.
(23) Muetterties, E. L. Acc. Chem. Res. 1970, 3, 266−273.
(24) Berry, R. S. J. Chem. Phys. 1960, 32, 933−938.
(25) A related system, Cp*TiMe3/B(C6F5)3, has also been reported
to have minor trimerization activity. See: (a) Pellecchia, C.;
Pappalardo, D.; Gruter, G.-J. Macromolecules 1999, 32, 4491−4493.
D
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