Binuclear initiators for the telechelic synthesis of elastomeric polyolefins

Article abstract of DOI:10.1021/ja1056938

Novel binuclear complexes, 4,4′-bis{[N-(2,6-diisopropylphenyl)-2-(2, 6-diisopropylphenylimino)propanamidato-K²-N,O-(trimethylphosphine) nickel(II)]methyl}-1,1′-biphenyl (2a) and 4,4′-bis{[N-(2,6- diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-4-methylpentamidato-K ²-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1′-biphenyl (2b), were synthesized by linking two nickel centers through a bis(benzyl) fragment. When activated with nickel bis(1,5-cyclooctadiene) (Ni(COD) ₂), 2a and 2b are capable of polymerizing ethylene in a quasi-living fashion, producing polymers with approximately twice the molecular weights relative to those obtained by using a structurally related mononuclear system. In addition, 2b/Ni(COD)₂ was utilized to synthesize a series of pseudo-triblock polyethylene (PE) macroinitiating copolymers, bearing atom-transfer radical polymerization (ATRP) initiators. Pseudo-pentablock copolymers were also prepared by taking advantage of a pressure-pulsing technique, wherein the ethylene pressure was increased from 100 to 500 psi in order to produce semicrystalline ethylene-rich end-blocks. Copolymers with elastomeric properties were synthesized by grafting n-butyl acrylate from the PE macroinitiators via ATRP. Examination using monotonic and step-cyclic stress-strain tests demonstrates that the materials exhibit large strains at break (1600-2000%) and excellent elastic recoveries at large strains (～80%). That materials with such desirable properties could not be attained using a mononuclear initiator demonstrates the clear advantage of growing the polymer via a telechelic mechanism.

Full text of DOI:10.1021/ja1056938

Published on Web 09/13/2010
Binuclear Initiators for the Telechelic Synthesis of
Elastomeric Polyolefins
Robert C. Coffin,†,⊥ Yanika Schneider,†,⊥ Edward J. Kramer,‡,§ and
Guillermo C. Bazan*,†,‡
Departments of Chemistry & Biochemistry, Materials, and Chemical Engineering, Mitsubishi
Chemical Center for AdVanced Materials, UniVersity of California, Santa Barbara, California 93106
Received July 12, 2010; E-mail: bazan@chem.ucsb.edu
Abstract: Novel binuclear complexes, 4,4′-bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-
propanamidato-κ2-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1′-biphenyl (2a) and 4,4′-bis{[N-(2,6-diiso-
propylphenyl)-2-(2,6-diisopropylphenylimino)-4-methylpentamidato-κ2-N,O-(trimethylphosphine)nicke-
l(II)]methyl}-1,1′-biphenyl (2b), were synthesized by linking two nickel centers through a bis(benzyl) fragment.
When activated with nickel bis(1,5-cyclooctadiene) (Ni(COD)2), 2a and 2b are capable of polymerizing
ethylene in a quasi-living fashion, producing polymers with approximately twice the molecular weights relative
to those obtained by using a structurally related mononuclear system. In addition, 2b/Ni(COD)2 was utilized
to synthesize a series of pseudo-triblock polyethylene (PE) macroinitiating copolymers, bearing atom-transfer
radical polymerization (ATRP) initiators. Pseudo-pentablock copolymers were also prepared by taking
advantage of a pressure-pulsing technique, wherein the ethylene pressure was increased from 100 to 500
psi in order to produce semicrystalline ethylene-rich end-blocks. Copolymers with elastomeric properties
were synthesized by grafting n-butyl acrylate from the PE macroinitiators via ATRP. Examination using
monotonic and step-cyclic stress-strain tests demonstrates that the materials exhibit large strains at break
(1600-2000%) and excellent elastic recoveries at large strains (∼80%). That materials with such desirable
properties could not be attained using a mononuclear initiator demonstrates the clear advantage of growing
the polymer via a telechelic mechanism.
Introduction
Ni and Pd diimine catalysts, Grubbs’s Ni salicylaldimine
catalysts, and more recently Pd phosphine sulfonate systems
Metal-mediated olefin polymerizations have attracted a great
deal of academic and industrial interest because they allow large-
scale syntheses of commodity materials.1,2 If the reaction
conditions give rise to a living polymerization, the products have
more predictable molecular weights and narrower polydispersity
indices (PDIs).3 In addition, higher-order architectures such
as block, graft, and end-functionalized materials can be
prepared by using this methodology.4 While several catalysts
are capable of polymerizing olefins in a living manner, few
can copolymerize olefins and polar comonomers with the
same degree of control.5 This difference is largely attributed
to the oxophilicity of traditional olefin polymerization
catalysts, which are composed of early transition metals that
may be readily poisoned by functional groups. Less electro-
philic metals, like Ni and Pd, must be used to prepare
functionalized polyolefin copolymers. Brookhart’s cationic
have been successful in producing random copolymers of
ethylene and functionalized comonomers.6-8
In this regard, R-iminocarboxamidato nickel precatalysts 1a,b,
shown in Figure 1a, offer unique opportunities to generate
materials with tailored, higher-order architectures.9-11 When
these initiators are activated with bis(1,5-cyclooctadiene)nickel
(Ni(COD)2), they can be used to generate a variety of materials,
ranging from random copolymers of ethylene and norbornene
acetate10 to more complex tapered diblock and tetrablock
structures.11,12 More recently, a methodology has been devel-
oped for preparing polyolefins grafted with polar chains by
copolymerizing an initiating monomer, or inimer, that is capable
(6) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267–268.
(7) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460.
(8) Guironnet, D.; Roesle, P.; Ronzi, T.; Gottker-Schnetmann, I.; Mecking,
S. J. Am. Chem. Soc. 2009, 131, 422–423.
† Department of Chemistry & Biochemistry.
‡ Department of Materials.
§ Department of Chemical Engineering.
⊥ These authors contributed equally to the manuscript.
(1) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169–1204.
(9) Diamanti, S. J.; Ghosh, P.; Shimizu, F.; Bazan, G. C. Macromolecules
2003, 36, 9731–9735.
(10) Diamanti, S. J.; Khanna, V.; Hotta, A.; Yamakawa, D.; Shimizu, F.;
Kramer, E. J.; Fredrickson, G. H.; Bazan, G. C. J. Am. Chem. Soc.
2004, 126, 10528–10529.
(2) Chen, E. Y. X. Chem. ReV. 2009, 109, 5157–5214.
(3) Coates, G. W.; Hustad, P. D.; Reinhartz, S. Angew. Chem., Int. Ed.
2002, 41, 2236–2257.
(11) Diamanti, S. J.; Khanna, V.; Hotta, A.; Coffin, R. C.; Yamakawa, D.;
Kramer, E. J.; Fredrickson, G. H.; Bazan, G. C. Macromolecules 2006,
39, 3270–3274.
(4) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.; Brookhart,
M. Prog. Polym. Sci. 2007, 32, 30–92.
(5) Boardman, B. M.; Bazan, G. C. Acc. Chem. Res. 2009, 42, 1597–
1606.
(12) Coffin, R. C.; Diamanti, S. J.; Hotta, A.; Khanna, V.; Kramer, E. J.;
Fredrickson, G. H.; Bazan, G. C. Chem. Commun. 2007, 3550–3552.
9
10.1021/ja1056938  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 13869–13878 13869

A R T I C L E S
Coffin et al.
Figure 1. Structures of 1a,b (a) and 2a,b (b), and cartoon representations of graft copolymers prepared using initiators 1b (c) and 2b (d).
of undergoing atom transfer radical polymerization (ATRP).13,14
Commodity monomers such as methyl methacrylate, n-butyl
acrylate (nBA), and tert-butyl acrylate have been grafted from
the resulting macroinitiators using ATRP. A cartoon representa-
tion of these structures is shown in Figure 1c.
prepared using 1b/Ni(COD)2. Graft copolymers generated from
macroinitiators with the highest inimer content exhibited the
greatest elongation at break and elastic recoveries. However,
these grafts also suffered from insufficient crystallinity, which
resulted in nonuniform deformation behavior.
One area of polymer science that takes advantage of precisely
engineered materials is the field of thermoplastic elastomers (TPEs).
For example, while polyolefin elastomers show superior mechanical
properties relative to conventional rubber materials,15-20 they are
nonpolar and, as such, suffer from poor adhesion and low
compatibility with polar plastics. Further, their tendency to
absorb significant quantities of oil is problematic because it has
been associated with reduced mechanical performance.21 On the
other hand, TPEs prepared from acrylic monomers have
enhanced compatibility and oil resistance in comparison to
polyolefin elastomers but suffer from low elongation at break.22,23
In a recent report, we demonstrated how these two problems
can be addressed by covalently linking polyacrylates and
polyolefins in the form of a graft copolymer (see Figure 1c).4,14
This class of materials was prepared by grafting nBA from
polyethylene (PE) macroinitiators with varying inimer content,
It seemed reasonable that a superior elastomer could be
prepared by placing the amorphous inimer-rich segment in the
center of the molecule, similar to conventional TPEs (as shown
in Figure 1d). In addition, crystallinity could be increased by
using a pressure-pulsing technique, wherein inimer incorporation
at the chain ends is drastically reduced by rapidly increasing
the ethylene pressure. Achieving this target structure requires
that both chain ends are capable of incorporating additional
monomer after the inimer copolymerization has taken place.
Such a requirement is nicely addressed with a telechelic initiator,
in which two chain ends propagate independently.
Despite being widely reported for other living methodologies
such as anionic24 and free-radical,25,26 “inside-out” or telechelic
polymerization of olefins by binuclear organometallic catalysts
remains a relatively unexplored synthetic strategy.4,27,28 The
majority of bimetallic olefin polymerization catalysts29-36
feature a multidentate ligand capable of coordinating two metal
(13) Schneider, Y.; Azoulay, J. D.; Coffin, R. C.; Bazan, G. C. J. Am.
Chem. Soc. 2008, 130, 10464–10465.
(24) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. ReV.
2001, 101, 3747–3792.
(14) Schneider, Y.; Lynd, N. A.; Kramer, E. J.; Bazan, G. C. Macromol-
ecules 2009, 42, 8763–8768.
(25) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921–2990.
(26) Ruehl, J.; Nilsen, A.; Born, S.; Thoniyot, P.; Xu, l.; Chen, S.; Braslau,
R. Polymer 2007, 48, 2564–2571.
(15) Ruokolainen, J.; Mezzenga, R.; Fredrickson, G. H.; Kramer, E. J.;
Hustad, P. D.; Coates, G. M. Macromolecules 2005, 38, 851–860.
(16) Harney, M. B.; Zhang, Y. H.; Sita, L. R. Angew. Chem., Int. Ed. 2006,
45, 2400–2404.
(27) Desurmont, G.; Tanaka, M.; Li, Y.; Yasuda, H.; Tokimitsu, T.; Tone,
S.; Yanagese, A. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4095–
4109.
(17) Edson, J. B.; Wang, Z.; Kramer, E. J.; Coates, G. W. J. Am. Chem.
Soc. 2008, 130, 4968–4977.
(28) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085–3100.
(29) Tomov, A.; Kurtev, A. J. Mol. Catal. A: Chem. 1995, 103, 95–103.
(30) Champouret, Y. D. M.; Fawcett, J.; Nodes, W. J.; Singh, K.; Solan,
G. A. Inorg. Chem. 2006, 45, 9890–9900.
(18) Hotta, A.; Cochran, E.; Ruokolainen, J.; Khanna, V.; Fredrickson,
G. H.; Kramer, E. J.; Shin, Y. W.; Shimizu, F.; Cherian, A. E.; Hustad,
P. D.; Rose, J. M.; Coates, G. W. Proc. Natl. Acad. Sci. U.S.A. 2006,
103, 15327–15332.
(31) Dohler, T.; Helmar, G.; Walther, D. Chem. Commun. 2000, 945–946.
(32) Li, L.; Metz, M. V.; Li, H.; Chen, M.-C.; Marks, T. J.; Liable-Sands,
L.; Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 12725–12741.
(33) Li, H.; Marks, T. J. Proc Natl. Acad. Sci. U.S.A. 2006, 103, 15295–
15302.
(19) Bensason, S.; Nagarenko, S.; Chum, S.; Hiltner, A.; Baer, E. Polymer
1997, 38, 3515–3520.
(20) Wang, H. P.; Khariwala, D. U.; Cheung, W.; Chum, S. P.; Hiltner,
A.; Baer, E. Macromolecules 2007, 40, 2852–2862.
(21) Hofmann, W. Rubber Technology Handbook; Hanser: Munich,
Germany, 1989; p 162.
(34) Zhang, S.; Vystorop, I.; Tang, Z.; Sun, W.-H. Organometallics 2007,
26, 2456–2460.
(22) Tong, J.-D.; Moineau, C.; Leclere, Ph.; Bredas, J. L.; Lazzaroni, R.;
Jerome, R. Macromolecules 2000, 33, 470–479.
(35) Guo, N.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2008, 130,
2246–2261.
(23) Yamamoto, M.; Nakano, F.; Doi, T.; Moroishi, Y. Int. J. Adhes. Adhes.
2002, 22, 37–40.
(36) Rodriguez, B. A.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc. 2009,
131, 5902–5919.
9
13870 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Telechelic Synthesis of Elastomeric Polyolefins
A R T I C L E S
Scheme 1. Synthesis of Binuclear Complexes 2a,b
centers, where the proximity of the metals results in cooperative
behavior. In contrast, we designed the binuclear complexes 2a,b
shown in Figure 1b, such that the two initiating sites are
coValently linked through a bis(benzyl) fragment. Unlike
previously reported bimetallic olefin polymerization initiators,
the metal centers in 2a,b propagate away from each other as
the polymer grows.
Here, we disclose the synthesis and characterization of 2a,b.
We show that 2a, in the presence of Ni(COD)2, can prepare PE
in a quasi-living fashion, as indicated by a linear dependence
of the number-average molecular weight (Mn) and narrow
polydispersity indices (PDIs). The molecular weights of the
products obtained are approximately double those achieved with
the mononuclear analogue 1a, indicating that the two propagat-
ing sites behave independently of each other. Subsequently, we
use 2b to prepare triblock and pentablock PE copolymers with
grafted nBA chains that show a significant improvement in
elastomeric performance relative to the materials prepared using
mononuclear initiator 1b (Figure 1c vs 1d).
Ethylene Homopolymerization. As shown in Scheme 2, a
series of ethylene (C2H4) homopolymerizations was performed
to confirm that the nickel centers in the 2a,b system behave as
independent polymerization sites and that the polymer propaga-
tion proceeds in a telechelic fashion. Under these conditions,
the PE prepared from a binuclear system should exhibit twice
the Mn value as that from an analogous mononuclear system.
Polymerizations of C2H4 with 2a/Ni(COD)2 were carried out
under conditions similar to those used for 1a/Ni(COD)2, which
has been investigated previously.9,39 In a typical reaction, 5 µmol
of 2a was combined with 25 µmol of Ni(COD)2 in 30 mL of
toluene in a steel autoclave reactor. The vessel was pressurized
with 100 psi of C2H4 to initiate the reaction, and the polymer-
ization was terminated by quenching with acetone. The results
from this study are summarized in Table 1, together with
previously reported data for 1a.39
Entries 3-6 show that the Mn of the PE prepared by 2a
increases linearly with time, as determined by gel permeation
Results and Discussion
Synthesis and Characterization of 2a,b. Complexes 2a,b were
prepared according to the series of steps shown in Scheme 1.
In THF at room temperature, 2 equiv of Ni(COD)2 undergoes
oxidative addition with 1 equiv of 4,4′-bis(chloromethyl)biphe-
nyl in the presence of 4 equiv of trimethylphosphine to yield
4,4′-bis{[trans-bis(trimethylphosphine)chloronickel]methyl}-
1,1′-biphenyl (3) in 85% yield. This compound must be prepared
under dilute conditions, as more concentrated conditions lead
to undesirable side reactions. Compound 3 was subsequently
reacted with 2 equiv of potassium [N-(2,6-diisopropylphenyl)-
2-(2,6-diisopropylphenylimino)propanamidate (a) or potassium
[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-4-
methylpentanamidate (b) at -35 °C in toluene, leading to 2a
(79%) or 2b (73%), respectively. After recrystallization from
Figure 2. Molecular structure of complex 2a with thermal ellipsoids set
at 50% probability. Hydrogen atoms and cocrystallized pentane are omitted
for clarity.
1
toluene, compounds 2a and 2b were characterized by H, 13C,
Scheme 2. Ethylene Homopolymerization Using 2a,ba
and 31P NMR spectroscopies. The NMR spectra of 2a,b
complexes are similar to those of the corresponding mononuclear
1a,b analogues.2 Single crystals of 2a suitable for X-ray
diffraction studies were obtained by vapor diffusion of pentane
into a concentrated toluene solution. The ORTEP depiction of
the resulting molecular structure is shown in Figure 2. Metrical
parameters show that the ligand is bound in the desired N,O-
coordination mode, as opposed to the thermodynamically
favored and polymerization-inactive N,N-coordination mode.37
Bond lengths and angles for 2a are similar to those observed
for 1a.38 Additional details are provided in the Supporting
Information (Table S1). These data demonstrate that the desired
binuclear connectivity can be achieved without significant
perturbations in the coordination sphere of the nickel center.
a Ar ) 2,6-diisopropylphenyl and R ) methyl or isobutyl.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13871

A R T I C L E S
Coffin et al.
Table 1. Summary of Ethylene Polymerizationsa
For example, MN 1-15 denotes a copolymer prepared from 1b
that has t1 ) 1 min and t2 ) 15 min. Additionally, pseudo
“pentablock” structures were produced by increasing the po-
lymerization pressure from 100 to 500 psi at the completion of
t2 and allowing the polymer to grow until t3. The physical
properties of the PE macroinitiators are summarized in Table
entry
precatalyst
time (min)
activity (kg/mol·h)
Mn (kg/mol)
PDI
1
2
3
4
5
6
7
8
1a
1a
2a
2a
2a
2a
1b
2b
5
10
2.5
5
10
12.5
44
22
153
180
96
146
180
175
310
350
23
44
17
40
80
92
230
240
1.4
1.4
1.4
1.4
1.4
1.5
1.4
1.5
1
2, with the H NMR spectra and DSC thermograms provided
in the Supporting Information (Figures S3-S8).
When 1b/Ni(COD)2 is exposed to a 0.6 M concentration of
4 at t2, in the presence of 100 psi of C2H4, a diblock copolymer
with Mn ) 62 kg/mol and an inimer content of 15 mol % is
produced (MN 1-15). The molecular weight, PDI, and thermal
properties of MN 1-15 are similar to those of a 16 mol %
copolymer prepared previously.14 When 2b is employed under
identical conditions, a triblock macroinitiator with Mn ) 97 kg/
mol and an inimer content of 14 mol % is generated (BN 1-15).
While the molecular weight of the macroinitiator obtained from
2b/Ni(COD)2 is not quite double the Mn achieved using 1b/
Ni(COD)2, the thermal characteristics of the products are
comparable (see Table 2). To confirm the proposed structure
of BN 1-15, and also to demonstrate that it is fundamentally
different from any structure that can be prepared using the 1b
system, we synthesized a higher Mn diblock macroinitiator.
Specifically, by shifting t2 from 15 to 30 min, we achieved an
increase in Mn from 62 to 88 kg/mol. Comparison of MN 1-30
and BN 1-15 will enable us to elucidate key structural
differences between macroinitiators prepared from 1b vs 2b.
A different macroinitiator structure was prepared by using
the pressure-pulsing technique, as shown in Scheme 3.10 GPC
traces of the triblock and pentablocks are provided in Figure 3.
As t3 is increased from 0 to 6 min, the Mn increases in a
predictable manner, while the PDIs remain relatively low (PDI
e 1.5). With the addition of a PE block produced at an increased
C2H4 pressure (t3 ) 2 min) to the triblock BN 1-15, the resulting
pentablock BN 1-15-2 exhibits a decrease in Tg from 35 to 32
°C and an increase in crystallinity from 0.1 to 0.3%. This trend
is expected for a material with ethylene-rich segments incor-
porated at the chain ends. When t3 is increased to 6 min,
macroinitiator BN 1-15-6 exhibits an Xc of nearly 1%, which
represents a 9-fold increase in the crystalline content compared
to BN 1-15. These results indicate that the pressure-pulsing
technique can be used to effectively modify the physical
properties of PE macroinitiators. Furthermore, the structure of
the products prepared by this method is akin to an ABABA
pentablock copolymer where the A-blocks are PE-rich segments
and the B-blocks are a random copolymer of C2H4 and 4.
Synthesis and Characterization of Graft Copolymers. Sem-
icrystalline PE does not become soluble until it is heated to
more than 100 °C in chlorinated aromatic or hydrocarbon
solvents.40 Meanwhile, the “grafting-from” technique is prefer-
ably performed at lower temperatures to minimize unwanted
reactions that may result in cross-linking.41 Specific precautions
thus need to be taken to mitigate these conflicting requirements.
The use of anisole as the solvent results in a controlled ATRP
process, as does the addition of a small amount of deactivator,
CuBr2.13,42 Accordingly, we prepared a series of graft copoly-
mers in the manner illustrated in Scheme 4. Briefly, the
macroinitiator was dissolved in an anisole solution containing
CuBr and CuBr2 ligated with N,N,N′,N′′,N′′-pentamethyldieth-
a Polymerization conditions: 20 °C, 100 psi, 10 µmol of 1a or 5 µmol
of 2a and 25 µmol of Ni(COD)2; 5 µmol of 1b or 2.5 µmol of 2b and
12.5 µmol of Ni(COD)2.
chromatography (GPC) at 150 °C in 1,2,4-trichlorobenzene.
Comparison of entries 1 and 4 with entries 2 and 5 reveals that
the Mn of the products obtained with 2a is indeed nearly double
that of 1a. Entry 6 demonstrates that the Mn growth departs
from linearity after 10 min, which is also observed with 1a/
Ni(COD)210 and is presumed to be a result of product precipita-
tion. In addition to PE, copolymers of C2H4 and 5-norbornen-
2-yl acetate were prepared using 2a/Ni(COD)2, and the evolution
of Mn over time is shown in the Supporting Information (Figure
S2). Although the molecular weights of these copolymers are
less than double those observed using 1a, the products have
narrow polydispersities and molecular weights that increase
linearly with time, indicating a significant degree of control over
the copolymerization process.
The reactivities of 1b and 2b were also examined in the
presence of C2H4, and the results are shown in entries 7 and 8
in Table 1. As demonstrated with 2a, binuclear complex 2b
produces a polymer with the same molecular weight as
mononuclear 1b, in half the time. It should also be noted that
initiators with an isobutyl backbone (1b) have been previously
shown to be more reactive than those with a methyl backbone
(1a).39 Our results are consistent with these observations, which
show a nearly 2-fold increase in activity with the isobutyl
analogue 2b. Further, the isobutyl variant is more tolerant of
functionalized norbornenes and can thus produce PE macroini-
tiators with higher molecular weights and narrower PDIs in
comparison to the methyl counterpart. We therefore utilized 1b
and 2b in the synthesis of PE macroinitiators discussed in the
next section.
Synthesis and Characterization of Triblock and Pentablock
PE Macroinitiators. A series of PE macroinitiators was prepared
by copolymerizing C2H4 with 5-norbornen-2-yl 2-bromo-2-
methylpropanoate (4), as shown in Scheme 3. Compound 4 is
capable of initiating ATRP and will be referred to as an inimer
henceforth. A steel autoclave reactor was charged with 2.5 µmol
of 2b or 5 µmol of 1b and 12.5 µmol of Ni(COD)2 in 30 mL
of toluene. Because Ni(COD)2 decomposes in the presence of
4, it is necessary to initiate the polymerization by growing a
short PE block (approximately 12 kg/mol for t1 ) 1 min) prior
to introducing 4 into the reaction mixture; this technique has
been shown to produce PE macroinitiators in a quasi-living
fashion.13 The copolymers are named according to the complex
used in their synthesis (MN is 1b and BN is 2b) and also the
length of reaction time of each block, as measured in minutes.
(37) Rojas, R. S.; Wasilke, J. C.; Wu, G.; Ziller, J. W.; Bazan, G. C.
Organometallics 2005, 24, 5644–5653.
(38) Lee, B. Y.; Bazan, G. C.; Vela, J.; Komon, Z. J. A.; Bu, X. J. Am.
Chem. Soc. 2001, 123, 5352–5353.
(40) Jackson, J. F.; Mandelkern, L. Macromolecules 1968, 1, 546–554.
(41) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921–2990.
(42) Matyjaszewski, K.; Nanda, A. K.; Tang, W. Macromolecules 2005,
38, 2015–2018.
(39) Azoulay, J. D.; Itigaki, K.; Wu, G.; Bazan, G. C. Organometallics
2008, 27, 2273–2280.
9
13872 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Telechelic Synthesis of Elastomeric Polyolefins
A R T I C L E S
Scheme 3. Synthesis of Triblock and Pentablock PE Macroinitiators Using 2b
Table 2. Physical Properties of PE Macroinitiatorsa
macroinitiator
t1 (min)
t2 (min)
t3 (min)
mol % 4b
Mn (kg/mol)c
PDIc
Tg (°C)d
Tm (°C)d
Xc (%)d
MN 1-15
MN 1-30
BN 1-15
BN 1-15-2
BN 1-15-6
1
1
1
1
1
15
30
15
15
15
0
0
0
2
6
15
15
14
12
8
62
88
97
112
128
1.3
1.4
1.4
1.4
1.5
48
38
35
32
27
114
116
113
111
0.1
0.2
0.1
0.3
0.9
94, 110
a Copolymerization conditions: 20 °C,100 psi, 0.6 M [4]0, 5 µmol of 1b or 2.5 µmol of 2b, 12.5 µmol of Ni(COD)2. b Determined by 1H NMR
spectroscopy. c Determined by GPC. d Determined by DSC.
Scheme 4. Synthesis of Graft Copolymersa
Figure 3. GPC traces of triblock and pentablock PE macroinitiators.
ylenetriamine (PMEDTA). A specified amount of degassed nBA
was then introduced. The mixture was then heated to 85 or 95
°C to initiate the polymerization, followed by quenching with
methanol. Due to the considerable inimer content of the
macroinitiators, all except BN 1-15-6 were soluble in anisole
at 85 °C; BN 1-15-6 requires heating to 95 °C to achieve a
homogeneous solution. The results of the grafting experiments,
along with the physical properties of products, are summarized
in Table 3.
a PMDETA ) N,N,N′,N′′,N′′-pentamethyldiethylenetriamine.
conditions, a lower nBA mol % is observed due to the decreased
fractional content of inimer in the parent macroinitiators (G4,
G5). Likewise, increasing [nBA]0 from 0.46 to 0.7 M leads to
a nearly 2-fold increase in nBA incorporation (G5, G6). The
effect of grafting was also monitored by high-temperature GPC;
the results are shown in Table 3, and the trace for G4 is provided
in the Supporting Information (Figure S11). All grafts show an
increase in Mn relative to that of the parent macroinitiators,
without a substantial increase in the PDI, verifying the controlled
nature of the grafting process. For example, when BN 1-15-2
When nBA is grafted from both diblock (MN 1-15 and MN
1-30) and triblock (BN 1-15) macroinitiators, the products
G1-G3 exhibit an nBA incorporation of approximately 30 mol
1
%, as determined by high-temperature H NMR spectroscopy
(Figures S9 and S10). However, when pentablock copolymers
(BN 1-15-2 and BN 1-15-6) are subject to the same grafting
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13873

A R T I C L E S
Coffin et al.
Table 3. Physical Properties of Graft Copolymersa
100 MPa, which is in line with the moderately branched,
semicrystalline structure of the PE produced.43 The slightly
higher stress at break of the PE obtained from 2b may be
explained by its higher melting point and crystallinity, relative
to the product from 1b.
yield
mol %
graft macroinitiator (mg) Mn (kg/mol) PDI nBAc Tg (°C)d Tm (°C)d Xc (%)d
G1 MN 1-15
G2 MN 1-30
G3 BN 1-15
G4 BN 1-15-2
G5 BN 1-15-6
89
90
92
95
94
110
122
143
178
195
239
1.5 30
1.6 31
1.5 28
1.4 26
1.5 23
1.4 40
-28
-27
-30
-30
-31
-35
114
115
107
112
101
101
0.1
0.2
0.1
0.3
0.8
0.4
As can be seen in Figure 4, both diblock and triblock
macroinitiators are significantly less extensible than the PE
homopolymers, with a strain-at-break of roughly 200%, con-
sistent with the incorporation of a high Tg norbornene unit.44
Increasing the molecular weight from 62 to 88 kg/mol does not
significantly alter the mechanical properties of the resulting
macroinitiator (MN 1-15 vs MN 1-30 in Table 4). The slight
increase in the melting point and degree of crystallinity of the
higher Mn copolymer correlates with a slightly higher Young’s
modulus. The triblock macroinitiator prepared from the 2b
system exhibits properties comparable to those of the two
materials prepared using the 1b system. This is not unexpected
since the mechanical properties of BN 1-15, as well as those
prepared from the monometallic system, are dominated by the
glassy, norbornene-rich segments.
G6 BN 1-15-6b 126
a Grafting conditions: 0.46 M nBA, 85-95 °C, 1 h. b [nBA]0 ) 0.70
c
M. Determined by H NMR spectroscopy. d Determined by DSC.
1
As described above, although there were minimal differences
in the stress-strain response of diblock and triblock macroini-
tiators, the effect of the polymer structure, and thereby the
impact of the telechelic polymerization on the mechanical
properties of the products, will become evident upon examina-
tion of the graft copolymers. In addition to monitoring the
deformation behavior under monotonic stress, the elastic
recovery was also investigated by using step-cyclic stress-strain
tests, wherein the material was elongated in successive loading
and unloading cycles and the elastic recovery from a maximum
applied strain (εmax) was recorded. The results from both tests
are summarized in Table 5. The nominal stress vs nominal strain
plots are provided in the Supporting Information (Figure S16).
As shown in Figure 5, the true stress vs true strain relationship
of G1 under monotonic elongation reveals necking behavior
presumed to be caused by insufficient crystallinity.13 Compari-
son of G1 and G2, prepared from a mononuclear complex,
demonstrates that increasing the Mn of the macroinitiator has
little effect on the mechanical properties of the graft copolymer,
with a similar strain at break (1095% and 1025%) and true stress
(5 MPa) observed for the two materials (Table 5). In contrast,
when triblock macroinitiator BN 1-15 is grafted with nBA, the
product G3 shows a 2-fold increase in the strain at break to
2000%, while the true stress at break increases to 20 MPa.
Further, the elastomeric performance of G3 is also improved,
as shown in Figure 6, where elastic recoveries (ER) are plotted
against εmax. Unlike the grafts G1 and G2, G3 exhibits high
ER from applied stress, even at strains above 500%, with an
average ER of 79%. These findings reveal an improvement in
the mechanical properties of the elastomers that cannot be
achieved simply by increasing the Mn of the parent macroini-
tiator. Therefore, the value of the telechelic initiator as a
convenient tool to access polymer structures with enhanced
mechanical properties becomes apparent.
Figure 4. Monotonic stress vs strain curves of PE homopolymers and
diblock and triblock macroinitiators.
Table 4. Mechanical Properties of PE Homopolymers, Diblock and
Triblock Macroinitiators
strain at break
macroinitiator Tg (°C)a Tm (°C)a Xc (%)a Young’s modulus (MPa)b (%)b (MPa)b
MN PE
BN PE
MN 1-15
MN 1-30
BN 1-15
131
133
114
116
113
50.2
44.7
0.1
0.2
0.1
106
115
125
136
118
1390
1310
210
250
200
23
27
20
19
22
48
38
35
a Determined by DSC. b Determined by monotonic stress-strain tests.
is grafted with nBA, the Mn shifts from 112 to 178 kg/mol.
Thermal properties of grafts were also examined using DSC,
and the thermograms are provided in the Supporting Information
(Figures S12-S15). All graft copolymers exhibit a softening
relative to the parent macroinitiators, as evidenced by a decrease
in Tg from an average of 35 °C to an average of -28 °C.
Increasing nBA content from 23 to 40 mol % results in a Tg
decrease from -31 to -35 °C for grafts G5 and G6. Altogether,
the thermal properties change in accord with the anticipated
structural modification from the grafting reactions.
Mechanical Properties of Diblock and Triblock Macroini-
tiators and Their Grafts. Mechanical properties of diblock and
triblock macroinitiators and their grafts were investigated using
monotonic tensile stress-strain tests; the results are provided
in Figure 4 and summarized in Table 4. These studies were
carried out to highlight how molecular structure, and thereby
the choice of initiator in the polymerization, translates into
materials with optimized properties. PE prepared from 1b/
Ni(COD)2 and 2b/Ni(COD)2 was also examined as a baseline
measurement. As expected, both PE homopolymers display
deformation behavior typical of a plastically deforming material,
with an initial elastic regime at low strains, followed by strain
hardening at approximately 800% strain. Both materials exhibit
an elongation at break of 1300% and a Young’s modulus of
Mechanical Properties of Pentablock Macroinitiators and
Their Grafts. Having demonstrated significant differences
between diblock and triblock graft copolymers, we next
examined the effect of the pressure-pulsing technique on the
mechanical properties of the pentablock macroinitiators and their
(43) Kennedy, M. A.; Peacock, A. J.; Mandelkern, L. Macromolecules 1994,
27, 5297–5310.
(44) Scrivani, T.; Benavente, R.; Perez, E.; Peren˜a, J. Macromol. Chem.
Phys. 2001, 202, 2547–2553.
9
13874 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Telechelic Synthesis of Elastomeric Polyolefins
A R T I C L E S
Table 5. Mechanical Properties of Diblock and Triblock Graft Copolymers
% ERa
graft macroinitiator at 250% strain at 400% strain average Young’s modulus (MPa)b nominal strain at break (%)b nominal stress at yield (MPa) true stress at break (MPa)c
G1 MN 1-15
G2 MN 1-30
G3 BN 1-15
80
78
76
73
75
74
78
72
79
0.4
0.6
0.4
1095
1025
2000
0.8
0.8
1.4
5
5
20
a ER ) elastic recovery, determined by monotonic stress-strain tests. b Determined by step cyclic stress-strain tests. c Maximum stress (engineering
stress).
Figure 7. Monotonic stress vs strain curves of triblock and pentablock
macroinitiators.
Figure 5. True stress vs true strain of diblock and triblock graft copolymers.
Table 6. Mechanical Properties of Triblock and Pentablock
Materials
Young’s
modulus
strain at
stress at
block
Tg (°C)a Tm (°C)a Xc (%)a (MPa)b break (%)b break (MPa)b
BN 1-15
BN 1-15-2
BN 1-15-6
35
32
27
113
111
94, 110
0.1
0.3
0.9
118
57
70
200
470
540
22
19
26
a Determined by DSC. b Determined by monotonic stress-strain tests.
Figure 6. Elastic recoveries of diblock and triblock graft copolymers.
graft copolymers. Results from these studies are depicted in
Figure 7 and summarized in Table 6. With the addition of an
ethylene-rich end-block to the triblock BN 1-15, a significant
transformation is observed, including a 2-fold decrease in
Young’s modulus, from 118 to 57 MPa, and a 2-fold increase
in strain-at-break for the pentablock BN 1-15-2. When t3 is
increased from 2 to 6 min, the elongation at break shifts from
470 to 540% strain, while Young’s modulus increases to 70
MPa. Additionally, BN 1-15-6 shows a higher stress at break
relative to BN 1-15-2, confirming that longer ethylene end-
blocks give rise to a tougher material. These results correlate
well with the gradual increase in the crystalline content of the
pentablock macroinitiators demonstrated by the DSC analysis
discussed earlier.
Figure 8. True stress vs true strain plots of triblock and pentablock graft
copolymers.
that were performed on the diblock and triblock graft copoly-
mers. The true stress vs true strain results are depicted in Figure
8, while the elastic recoveries are shown in Figure 9, and all
results are summarized in Table 7. Grafts generated from
macroinitiators with increasing C2H4 content show an increase
in Young’s modulus and true stress at break and a slight decrease
in the strain at break (G4, G5). Again, this trend is consistent
with a shift in the crystalline content of the materials. Com-
Grafts prepared from the pentablock macroinitiators were
subject to the same monotonic and step-cyclic stress-strain tests
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13875

A R T I C L E S
Coffin et al.
norbornene segments. Pentablock materials exhibit an increase
in Young’s modulus and strain at break, which is expected for
materials containing semicrystalline PE end-blocks.
Upon grafting with nBA, the advantage of the binuclear
catalyst system becomes apparent. When compared to the
grafted diblock copolymers (G1, G2), the grafted triblock
copolymer shows a 2-fold increase in elongation at break (G3).
Additionally, the true stress at break increases 4-fold, while the
average elastic recovery remains quite high (at nearly 80%) and
does not decrease at higher strains, as is evident with G1 and
G2. Grafts prepared from the pentablock macroinitiators (G4,
G5) show an increase in Young’s modulus and true stress at
break and a slightly lower strain at break compared to G3,
consistent with increased crystallinity of the PE-rich segments.
A comparison of pentablock grafts with different nBA content
reveals that longer nBA side chains lead to improved elastic
properties, as evidenced by increased strain at break and elastic
recovery. These results indicate that, under identical reaction
conditions, the telechelic initiator system allows access to
polymer structures with improved mechanical properties over
those obtained from the mononuclear system. In addition, the
collected set of observations demonstrate that telechelic po-
lymerization of olefins using well-defined organometallic initia-
tors provides unique opportunities to access new materials with
higher-order architectures, wherein the individual components
can be used to modulate useful physical properties.
Figure 9. Elastic recoveries of triblock and pentablock graft copolymers.
parison of G5 and G6 reveals that increasing nBA content leads
to improvement in elastomeric performance. As nBA content
is varied from 23 to 40 mol % nBA, the strain at break increases
from 1600 to 1800%, while the average elastic recovery shifts
from 74 to 80%. Likewise, increased nBA content results in
the softening of the product, as indicated by a decrease in
Young’s modulus from 0.7 to 0.4 MPa and a decrease in true
stress at break from 30 to 16 MPa. These observations confirm
that it is possible to tailor the “softness” and “elasticity” of the
graft copolymer by modulating the nBA incorporation, thereby
highlighting the versatility of this system.
Experimental Section
General Remarks. All manipulations were performed under an
inert atmosphere using standard glovebox and Schlenk-line tech-
niques. Toluene and THF were distilled from sodium benzophenone
ketyl, and pentane from Na/K alloy. The toluene used in ethylene
polymerizations was purchased from Aldrich (anhydrous grade) and
further dried over Na/K alloy. Ethylene (research grade, 99.99%)
was purchased from Matheson Trigas and purified by passing
through Agilent moisture and oxygen traps. Bis(1,5-cyclooctadiene)
nickel (Ni(COD)2) was purchased from Strem. Unless otherwise
specified, all other reagents were purchased from Aldrich and used
without further purification.
Synthesis of 4,4′-Bis{[trans-bis(trimethylphosphine)chloron-
ickel(II)]methyl}-1,1′-biphenyl (3). The synthesis of 3 was carried
out under minimal light exposure. To a solution of Ni(COD)2 (550
mg, 1 mmol) in 125 mL of THF at -35 °C was added a solution
of trimethylphosphine (304 mg, 2 mmol) in 1 mL of THF, followed
immediately by a solution of 4,4′-bis(chloromethyl)biphenyl (504
mg, 1 mmol) in 10 mL of THF. The reaction was stirred for 2 h at
room temperature, after which time the solvent was removed in
vacuo. The residue was redissolved in 100 mL of toluene and
filtered through Celite. The solvent was removed until about 50
mL remained, and then the solution was stored at -35 °C overnight,
resulting in the precipitation of a red-pink powder (555 mg, 82%
yield).
Summary, Discussion, and Conclusions
In summary, we report the synthesis of two new binuclear
Ni complexes, 2a and 2b. These complexes are the first
examples of initiators capable of quasi-living telechelic poly-
merization of ethylene and copolymerization of ethylene and
functionalized norbornenes. Unlike the majority of bimetallic
olefin polymerization catalysts in the literature, the Ni centers
in 2a,b are covalently linked and are thus designed to propagate
away from one other as the polymerization proceeds. NMR
spectroscopy and single-crystal X-ray diffraction studies confirm
that the binuclear structure does not significantly alter the
coordination sphere about Ni. Ethylene polymerizations per-
formed by activating 2a,b with Ni(COD)2 reveal that, for a given
reaction time, the molecular weights are approximately double
those obtained from analogous mononuclear systems (1a,b).
This observation indicates that the Ni centers behave indepen-
dently in the bimetallic system and operate with a level of
control similar to that observed with the monometallic analogues.
In addition, the 2b/Ni(COD)2 system can be employed in the
synthesis of triblock and pentablock PE macroinitiating copoly-
mers, containing C2H4 and a norbornene comonomer, which
bears a functionality useful for initiating ATRP (4). Diblock
macroinitiators were synthesized using 1b/Ni(COD)2 for com-
parison. Pentablock macroinitiators were prepared by using a
pressure-pulsing technique to produce ethylene-rich end-blocks,
following the growth of the PE-co-4 segment. All products
generated using the 2b system have narrow PDIs, consistent
with a controlled copolymerization sequence, as well as an
increase in Mn and predictable inimer content. Monotonic
stress-strain analyses of the diblock and triblock macroinitiators
indicate that they have similar mechanical behavior and fail at
relatively low strains of 200%, due to the high Tg of the
1H NMR (399.95 MHz, benzene-d6, 295 K): δ ) 7.66 (d, 4H,
3JHH ) 7.6 Hz, Bn), 7.61 (d, 4H, 3JHH ) 7.6 Hz, Bn), 1.80 (s, 4H,
CH2Ph), 1.04 (s, 36H, P(CH3)3). 31P NMR (162 MHz, benzene-d6,
295 K): δ ) -16.7. Accurate elemental analysis was not possible
due to the reactive nature of this compound.
4,4′-Bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-
propanamidato-K2-N,O-(trimethylphosphine)nickel(II)]methyl}-1,1′-
biphenyl (2a). To a stirred suspension of 3 (250 mg, 0.372 mmol)
in 5 mL of toluene at -35 °C was added a chilled solution of
potassium N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimi-
no)propanamidate (350 mg, 0.78 mmol) in 5 mL of toluene. The
reaction was stirred at room temperature for 2 h, resulting in an
orange-brown solution. The solution was filtered through Celite,
and the solvent was removed. The residue was then dissolved in a
9
13876 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010

Telechelic Synthesis of Elastomeric Polyolefins
A R T I C L E S
Table 7. Mechanical Properties of Graft Copolymers Prepared from Triblock and Pentablock Macroinitiators
% ERa
graft macroinitiator at 250% strain at 400% strain average Young’s modulus (MPa)b nominal strain at break (%)b nominal stress at break (MPa)c true stress at break (MPa)b
G3 BN 1-15
76
80
77
83
74
83
67
83
79
77
74
80
0.4
0.5
0.7
0.4
2000
1960
1600
1800
1.4
1.6
2.0
1.1
20
25
30
16
G4 BN 1-15-2
G5 BN 1-15-6
G6 BN 1-15-6d
a Determined by monotonic stress-strain tests. b Determined by step cyclic stress-strain tests. c Maximum engineering stress. d Increased nBA
content relative to G5.
minimal amount of toluene (∼2 mL), then layered with 5 mL of
pentane, and cooled to -35 °C for 24 h, yielding 365 mg (79%) of
reddish orange crystals.
and cooled to -35 °C for 24 h, yielding 365 mg (73%) of reddish
orange crystals. Due to the limited solubility of 2b in C6D6, 13C
spectra were obtained in THF-d8.
1H NMR (500 MHz, benzene-d6, 295 K): δ ) 7.51 (d, 4H, 3JHH
) 7.9 Hz, Bn), 7.35 (d, 4H, 3JHH ) 8.1 Hz, Bn), 7.27 (d, 4H, 3JHH
) 7.5 Hz), 7.19-7.11 (m, 2H), 7.08-6.99 (m, 6H), 3.68 (sept,
1H NMR (500 MHz, benzene-d6, 295 K): ) 7.79 (d, 4H, 3JHH ) 8
Hz, Bn), 7.54 (d, 4H, 3JHH ) 8 Hz, Bn), 7.40 (d, 4H), 7.30-7.08 (m,
8H, ph-H), 3.79-3.63 (m, 8H, iPr-CH), 2.87-2.81 (m, 6H,
iBut-CH and iBut-CH2), 1.77-1.64 (br s, 12H, iPr-CH3), 1.62-1.52
3
3
4H, JHH ) 6.8 Hz, iPr-CH), 3.47 (sept, 4H, JHH ) 6.8 Hz,
iPr-CH), 2.20 (s, 6H, CH3 ligand backbone), 1.58-1.38 (br m,
3
(br m, 24H, iPr-CH3), 1.40 (d, 12H, iPr-CH3 JHH ) 6.8 Hz), 1.11
3
3
24H, iPr-CH3), 1.32 (d, 12H, JHH ) 6.7 Hz, iPr-CH3), 1.13 (d,
(br d, 4H, benzyl-CH2), 1.06 (d, 12H, iBut-CH3, JHH ) 6.5 Hz),
12H, 3JHH ) 6.8 Hz, iPr-CH3), 1.11 (s, 4H, benzyl CH2) 0.47 (d,
0.53 (d, 18H, PCH3, 2JHP ) 10.1 Hz). 13C NMR (125.7 MHz, THF-
d8, 295 K): ) 184.6 (carbonyl), 165.69 (imine), 148.09, 141.03, 138.78,
129.8, 126.20, 124.87, 122.51, 42.08 (imine, Ph-C), 35.21, 29.86,
23.37, 21.68, 14.56, 12.18 (d, 1JCP ) 28 Hz, PCH3). 31P NMR (161.0
MHz benzene-d6, 295 K): δ ) -7.95 ppm.
18H, JHP ) 10.2 Hz, P(CH3)3). 13C NMR (125.7 MHz, benzene-
2
d6, 295 K): δ ) 181.37 (carbonyl), 164.99 (imine), 149.86, 147.69,
141.75, 140.44, 138.68, 130.57, 129.66, 128.89, 126.03, 122.86,
1
29.78, 29.23, 24.52, 24.26, 23.06, 21.77, 20.38, 12.45 (d, JCP
)
38 Hz, PCH3). 31P NMR (161.0 MHz, benzene-d6, 295 K): δ )
-8.2. Anal. Calcd for C74H104N4Ni2O2P2: C, 70.48; H, 8.31; N,
4.44. Found: C, 70.51; H, 8.46; N, 4.10.
Synthesis of PE and PE Macroinitiators. N-(2,6-Diisoprop-
ylphenyl)-2-(2,6-diisopropylphenylimino)methylamidato]-Ni(η1-
CH2Ph)(PMe3)38 (1a), N-(2,6-diisopropylphenyl)-2-(2,6-diisopro-
pylphenylimino)isobutanamidato]-Ni(η1-CH2Ph)(PMe3)39 (1b), and
5-norbornen-2-yl 2-bromo-2-methylpropanoate (4)13 were synthe-
sized as previously reported. Ethylene homopolymerizations were
carried out in a steel autoclave reactor, loaded with 10 µmol of 1a
or 5 µmol of 2a and 25 µmol of Ni(COD)2 in 30 mL of toluene,
under an inert atmosphere. In the case of isobutyl analogues, 5 µmol
of 1b or 2.5 µmol of 2b and 12.5 µmol of Ni(COD)2 in 30 mL of
toluene were employed in the reaction. The polymerization was
initiated by introducing a continuous feed of ethylene at 100 psi to
the mixture of initiator and Ni(COD)2 at 20 °C. The polymerization
was terminated by quenching the product with acetone. The polymer
was collected by filtration, and dried under high vacuum overnight
until a constant weight was achieved. The polymerization activities
were calculated from the mass of the product obtained.
PE macroinitiators were synthesized in a steel autoclave reactor,
equipped with an addition funnel, using 5 µmol of 1b or 2.5 µmol
of 2b and 12.5 µmol of Ni(COD)2 in 30 mL of toluene. The addition
funnel was charged with a 0.6 M solution of 4 in toluene and
prepressurized with ethylene at 150 psi for 3 min. The polymeri-
zation was initiated by introducing a continuous feed of ethylene
at 100 psi to the mixture of initiator and Ni(COD)2 at 20 °C, for a
specified amount of time (t1). At t1, 4 was added to the reaction by
opening the addition funnel, and the polymerization was allowed
to proceed until t2. Pentablocks were prepared by increasing the
polymerization pressure from 100 to 500 psi at the completion of
t2 and allowing the polymer to grow until t3. Polymerizations were
terminated by quenching with acetone. The polymer was collected
by filtration and dried under high vacuum overnight until a constant
weight was achieved.
X-ray crystallography, single-crystal data for 2a: empirical
formula (including cocrystallized pentane), C42H64N2NiOP, M )
702.63; monoclinic, space group P2(1)/c; a ) 12.593(4), b )
15.007(5), and c ) 17.257(7) Å; R ) 90°, ꢀ ) 91.519(0)°, γ
) 90°, V ) 4159(2) Å3, Z ) 4, Dcalc ) 1.122 Mg/m3, µ ) 0.563
mm-1, Mo KR, λ ) 0.71073, T ) 150 K; R(F242s) ) 0.0412, Rw
(F2, all data) ) 0.1248, goodness-of-fit )1.071 for all 2132 unique
data (6509 measured, 1105 refined), Rint ) 0.1200, 2yo 54.98.
Deposited with CCDC as no. 736656. The single crystal was
mounted on a glass fiber and transferred to a Bruker CCD platform
diffractometer. The SMART45 program package was used to
determine the unit-cell parameters and for data collection (25
s/frame scan time for a sphere of diffraction data). The raw frame
data were processed using SAINT46 and SADABS47 to yield the
reflection data file. Subsequent calculations were carried out using
SHELXTL48 program. The structure was solved by direct methods
and refined on F2 by matrix least-squares techniques. Analytical
scattering factors of 49 for neutral atoms were used throughout the
analysis. Hydrogen atoms were located from a difference Fourier
map and refined (x, y, z, and Uiso).50
4,4′-Bis{[N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)-
4-methylpentanamidato-K2-N,O-(trimethylphosphine)nickel(II)]methyl}-
1,1′-biphenyl (2b). To a stirred suspension of 3 (200 mg, 0.3 mmol)
in 5 mL of toluene at -35 °C was added a chilled solution of
potassium N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino))-
4-methylpentanamidate18 (300 mg, 0.6 mmol) in 5 mL of toluene.
The reaction was stirred at room temperature for 2 h, resulting in
an orange-brown solution. The solution was filtered through Celite,
and the solvent was removed. The residue was then dissolved in a
minimal amount of toluene (∼2 mL), layered with 5 mL of pentane,
Synthesis of Graft Copolymers. A copper salt stock solution
was prepared by dissolving CuBr (45 mg, 3.07 × 10-4 mol), CuBr2
(3.5 mg, 1.25 × 10-5 mol), and PMDETA (115 mg, 6.6 × 10-4
mol) in 10 mL of anisole under an inert atmosphere. In a typical
reaction, a 10 mL round-bottom flask was charged with 50 mg of
macroinitiator, 2 mL of stock solution, and 3 mL of anisole. The
flask was equipped with a septum, and degassed nBA (0.4 or 0.6
mL) was added via syringe. The flask was further degassed by
bubbling argon for 10 min and then placed in an 85 °C oil bath to
initiate the polymerization. When grafting with BM 1-15-6, it was
necessary to heat the sample to 95 °C to ensure a homogeneous
solution. After 1 h of reaction time, the graft copolymer was
(45) SMART Software Users Guide, Version 5.1; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 1999.
(46) SAINT Software Users Guide, Version 6.0; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 1999.
(47) Sheldrick, G. M. SADABS, Version 2.05; Bruker Analytical X-Ray
Systems, Inc.: Madison, WI, 2001.
(48) Sheldrick, G. M. SHELXTL, Version 6.12; Bruker Analytical X-Ray
Systems, Inc.: Madison, WI, 2001.
(49) International Tables for X-ray Crystallography, Vol. C; Kluwer
Academic Publishers: Dordrecht, 1992.
(50) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13877

A R T I C L E S
Coffin et al.
precipitated into methanol, collected by filtration, and dried in a
vacuum oven at 50 °C overnight.
Tensile Testing. Samples were elongated either until fracture
or to a given tensile strain εmax using an Instron (Norwood, MA)
1123 testing instrument. Two types of mechanical tests were
performed: (1) samples were stretched monotonically until fracture,
and stress-strain curves were recorded; or (2) step-cycle tests were
performed that combine a stepwise stretching with unloading-
reloading cycles. In each step, the sample was extended step-by-
step up to strains of 10, 50, 100, 250, 400, 500, 750 and 1000 (%).
Once the sample reached the appropriate strain, the crosshead
direction was reversed, and the sample strain was decreased at the
same rate until zero stress was achieved. The sample was then
extended again at the same constant strain rate until it reached the
next targeted step-strain. The step-cycle test was performed until
the sample fractured, or until the final step of 1000% was reached.
The elastic recovery (ER) was calculated from the step-cycle tests,
and is defined as the strain recovered upon unloading divided by
the maximum strain (εmax) reached during the step. Likewise, in
Figures 5 and 8, the true stress versus true strain are provided,
wherein true stress (σt) is defined as σn/L0, while true strain is
[ln(1 + ∆L/L0)], where σn is the nominal stress, L0 the initial length
of the specimen, and ∆L the change in the length the specimen.
Characterization of Materials. Gel Permeation Chromatog-
raphy (GPC). GPC analysis of polymers was performed on a
Polymer Laboratories high-temperature GPC system (model PL-
220) equipped with a refractive index detector. Samples were run
at 150 °C in spectrophotometric grade 1,2,4-trichlorobenzene,
stabilized with BHT (0.5 g of BHT/4 L of solvent). Molecular
weights were calculated by using a universal calibration from
narrow polystyrene standards in the molecular weight range of from
580 to 7.5 million g/mol. Mark-Houwink parameters of R ) 0.7
and κ ) 47.7 were utilized to correct for polyethylene.
1
Nuclear Magnetic Resonance (NMR). H, 13C and 31P NMR
spectra of complexes 3 and 2a,b were obtained using Varian 400
and 500 MHz spectrometers in benzene-d6 and THF-d8 at room
temperature. 1H NMR spectra of PE macroinitiators and graft
copolymers were obtained using a Bruker 500 MHz spectrometer
in 1,1,2,2-tetrachloroethane-d2 at 115 °C.
Differential Scanning Calorimetry (DSC). DSC was used to
determine the thermal characteristics of the PE macroinitiators and
graft copolymers using a TA Instruments differential scanning
calorimeter (model Q-20). The DSC measurements were performed
on 5 mg polymer samples at a rate of 10 °C/min in the temperature
range of -85 to 160 °C. The melting point (Tm) and heat of fusion
(Hf) were calculated using the data from the third heating/cooling
cycle. The degree of crystallinity (Xc) was determined by dividing
the measured Hf by 288 J/g, the accepted value for 100% crystalline
high-density polyethylene (HDPE) material.51
Acknowledgment. The authors are grateful to Dr. Gang Wu
for assistance with the crystallographic studies and to the Depart-
ment of Energy and the Mitsubishi Chemical Center for Advanced
Materials for financial support.
Supporting Information Available: Crystallographic data, 1H
NMR spectra, DSC thermograms, and other information. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Mechanical Analysis of Materials. Sample Preparation.
Polymer samples were compression-molded at 110 °C into a film
0.2-0.5 mm thick using a Carver lab press (model M). Film
samples were dried at 50 °C under a high vacuum overnight to
remove any remaining solvent. Samples then were cut into
specimens of 4-7 mm long × 2 mm wide using a dogbone die.
JA1056938
(51) Krupe, I.; Luyt, A. S. J. Appl. Polym. Sci. 2001, 81, 973-980.
9
13878 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010