Synthesis, characterization, and olefin (co)polymerization behavior of unsymmetrical α-diimine palladium complexes containing bulky substituents at 4-position of aniline moieties

Article abstract of DOI:10.1016/j.jorganchem.2018.09.016

Three novel unsymmetrical α-diimine Pd(II) complexes containing bulky substituents at 4-position of aniline moieties were prepared and characterized. Unfortunately, the introduction of large substituents on para- N-aryl moieties did not slow the exchange between the anti and syn forms for these complexes. These unsymmetrical α-diimine Pd(II) complexes also did not improve thermal stability and broaden molecular weight distribution in ethylene polymerization. However, these α-diimine palladium catalysts could copolymerize ethylene with biorenewable comonomer acrylic acid (AA), with comonomer incorporation in the range of 1.1–2.7% and high copolymer molecular weights. The AA units are incorporated predominately at the end of the branches. In addition, a systematic investigation on the polymerization of 1-hexene, 1-octene and 1-decene using these complexes was also performed. Changes in the ligand sterics and monomer length can influence the branch density and molecular weight of poly(α-olefins). Interestingly, these α-olefin polymers show properties characteristic of thermoplastic elastomers, i.e., good elastomeric recovery and high strain at break. Pervious work has shown that ethylene or α-olefin polymerization using some nickel α-diimine catalysts can generate elastic polyolefin materials. This work may provide an alternative and effective strategy to synthesize thermoplastic elastomers by α-diimine Pd(II) catalysts in one step using only α-olefin as the feedstock.

Full text of DOI:10.1016/j.jorganchem.2018.09.016

Journal of Organometallic Chemistry 877 (2018) 12e20
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, and olefin (co)polymerization behavior of
unsymmetrical a-diimine palladium complexes containing bulky
substituents at 4-position of aniline moieties
,
, b
Lihua Guo a *, Yanlan Liu a, Wenting Sun a, Qing Du a, Yuliang Yang a, Wenyu Kong a
,
Zhe Liu a **, Darui Chen b
,
, ***
a School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, 273165, China
b Key Laboratory of Polymeric Composite & Functional Materials of Ministry of Education, Sun Yat-Sen University, Guangzhou, 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel unsymmetrical a-diimine Pd(II) complexes containing bulky substituents at 4-position of
Received 26 June 2018
Received in revised form
1 September 2018
Accepted 19 September 2018
Available online 20 September 2018
aniline moieties were prepared and characterized. Unfortunately, the introduction of large substituents
on para- N-aryl moieties did not slow the exchange between the anti and syn forms for these complexes.
These unsymmetrical
molecular weight distribution in ethylene polymerization. However, these
a
-diimine Pd(II) complexes also did not improve thermal stability and broaden
-diimine palladium catalysts
a
could copolymerize ethylene with biorenewable comonomer acrylic acid (AA), with comonomer incor-
poration in the range of 1.1e2.7% and high copolymer molecular weights. The AA units are incorporated
predominately at the end of the branches. In addition, a systematic investigation on the polymerization
of 1-hexene, 1-octene and 1-decene using these complexes was also performed. Changes in the ligand
sterics and monomer length can influence the branch density and molecular weight of poly(
Interestingly, these -olefin polymers show properties characteristic of thermoplastic elastomers, i.e.,
good elastomeric recovery and high strain at break. Pervious work has shown that ethylene or -olefin
polymerization using some nickel -diimine catalysts can generate elastic polyolefin materials. This work
may provide an alternative and effective strategy to synthesize thermoplastic elastomers by -diimine
Pd(II) catalysts in one step using only -olefin as the feedstock.
a-olefins).
a
a
a
a
a
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
polymer molecular weight. However, conventional a-diimine Ni(II)
and Pd(II) catalysts suffer from the significant decrease of activity
and polymer molecular weight at elevated temperatures [9,10].
Brookhart and co-workers have shown that increasing N-aryl ro-
tations from perpendicular to square-planar coordination plane
results in increased associative chain transfer and the potential
decomposition arising from C-H activation of the metal center to
Since Brookhart's reported initially on Ni(II) and Pd(II) a-diimine
catalysts for olefin polymerization in 1995 [1,2], substantial efforts
have been devoted to this field in developing the novel late tran-
sition metal catalysts [3e20]. The big advantages of these catalysts
are that they can produce polyolefins with various types of
branches and have good tolerance toward polar groups [21e23].
the alkyl groups on the a-diimine ligand [24]. Thus, increasing of
The bulky ortho-aryl substituents in the
a
-diimine ligand, which
the steric bulk on the o-aryl substituents and the ligand backbone
could retard the chain transfer process and catalyst deactivation, is
essential to achieve the high polymerization activity and high
can enhance the thermal stability of these catalysts. For example,
Guan et al. reported the Pd(II) a-diimine catalysts bearing a cyclo-
phane ligand moiety (Scheme 1, I) showed the rises of thermal
stability for ethylene polymerization and incorporation of methyl
acrylate (MA) for copolymerization of ethylene and MA [25,26]. The
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: guolihua@qfnu.edu.cn (L. Guo), liuzheqd@163.com (Z. Liu),
daruichen@foxmail.com (D. Chen).
Long, Chen and Sun groups developed a series of a-diimine Ni(II)
and Pd(II) catalysts containing bulky dibenzhydryl (CHPh2) moiety
(Scheme 1, II), which can suppress the chain transfer and catalyst
deactivation at temperature ranges suitable for industrial process
https://doi.org/10.1016/j.jorganchem.2018.09.016
0022-328X/© 2018 Elsevier B.V. All rights reserved.

L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
13
Scheme 1. (a) The strategy to slow down C-N bond rotations; (b) Interconversion between the syn- and anti-forms for unsymmetrical
a-diimine metal catalyst and some previously
reported examples. (c) Our current work.
(typically 80e100 ꢀC) [27e34]. The Gao group also provided an
alternative strategy by increasing the steric bulk of the -diimine
that bimodal molecular distributions may arise in ethylene poly-
merizations by some unsymmetrical Ni(II) -diimine catalysts due
a
a
backbone for enhancing the thermal stability of Ni(II) and Pd(II)
to the slow interconversion between the syn- and anti-conformers
catalysts (Scheme 1, III) [35e38]. The bulk of the backbone is ex-
(Scheme 1, V) [31]. The Coates group found that some bulky
pected to inhibit the N-aryl rotation of the
a
-diimine ligand
substituted a-diimine Ni(II) catalysts adopt exclusively anti con-
because of the repulsive interaction. The above-mentioned studies
clearly indicate that the inhibition of N-aryl rotation plays an
important role in enhancing the thermal stability of the catalyst
and increasing the molecular weight of the polyolefins.
formations and can produce isotactic propylene at low temperature
(Scheme 1, VI) [32]. These studies are mainly concentrated on the
nickel catalyst. The work on the properties of unsymmetrical a-
diimine Pd(II) catalysts for olefin polymerization is rarely ever done
[46].
Moreover, a great deal of work has been focused on unsym-
metrical
stereo-controlled olefin polymerization and the formation of
bimodal polyethylene [39e45]. Unsymmetrical -diimine metal
a
-diimine catalyst precursors due to their behavior of
Herein, we report the synthesis of a series of novel unsym-
metrical a-diimine Pd(II) complexes containing bulky substituents
at 4-position of aniline moieties and the influence of the bulky
ortho- and para-aryl substituents on catalytic properties in olefin
polymerization and copolymerization with polar monomers.
a
complexes, which contain two different ortho substituents on each
ring, may exist as syn- and anti-conformers (Scheme 1). The syn-
and anti-conformers may exhibit different behavior in olefin
polymerization. The interconversion between the syn- and anti-
conformers depends strongly on the steric bulkiness of the ortho
substituents of the aryl ring. When the ortho substituent are bulk
enough, the interconversion through the N-aryl rotation would
become very slow at room temperature. For example, the syn- and
2. Results and discussion
2.1. Synthesis of the ligands and Pd(II) complexes
The preparation of ligands L1-L3 and corresponding palladium
complexes 1e3 are shown in Scheme 2. Acid-catalyzed condensa-
tion reactions of anilines with 2,3-butanedione in toluene facilely
anti-conformers of Ni(II) a-diimine catalysts were isolated by Pel-
lecchia et al. and used for propylene polymerization at ꢁ45 ꢀC
(Scheme 1, IV). The propylene produced by the syn-conformer was
syndiotactic while the propylene produced by the anti-conformer
showed low stereoregularity [30]. In addition, Wu group reported
produced para-diphenylmethyl substituted
a-diimine ligands in
76e85% yields. These ligands were characterized by 1H, 13C NMR
and mass spectrometry. The 1H NMR spectrum of ligand L1 shows

14
L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
Scheme 2. Synthesis of the ligands and Pd(II) complexes.
the presence of three isomers in the molar ratio of 1:5:24 (see
supplementary materials, Fig. S7). The origin of the isomers may be
imine C¼N bond Z,E-isomerism and conformational anti/syn
isomerism, which was similar to previous observation with related
2.2. Olefin polymerization results
Ethylene polymerization was investigated by an in situ activa-
tion of the palladium complexes with 1.5 equiv. of sodium tetra-
kis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF). Results are
summarized in Table 1. The ethylene polymerization activities and
the molecular weight of the generated polyethylene decreased
when the polymerization temperature was increased from 25 to
55 ꢀC. Additionally, polymerizations conducted by complex 1 at
55 ꢀC showed a decrease of activity from 4.1 ꢂ 104 g molꢁ1 hꢁ1 to
0.6 ꢂ 104 g molꢁ1 hꢁ1 with time (see supplementary materials,
Fig. S30). These results suggest that complex 1 became unstable
and the chain transfer was also enhanced at high temperatures.
Further, increase in the bulk of the ortho substituents resulted in
increased activities and the molecular weight of polymers, which
was consistent with previous observations [1,2,4]. All of the poly-
ethylenes prepared using these palladium catalysts are highly
branched with a branching density in a narrow range of 57e67
branches per 1000 carbons, which is much lower than those by
classic Brookhart catalyst (ca. 100/1000C) [1,2]. As a result, the
polymer branching number is largely independent of polymeriza-
tion temperature and the catalyst structures, following the same
a
-diimine ligands [22,39,41,47]. Treatments of (COD)PdMeCl
(COD ¼ 1,5-Cyclooctadiene) with the corresponding ligands in
CH2Cl2 afforded the palladium complexes 1e3 in 57e72% yields.
The palladium complexes were characterized by 1H, 13C NMR, mass
spectrometry and elemental analysis. Only one Pd-Me signal and
one set of chemical resonances was observed in the 1H NMR at
60 ꢀC and ꢁ80 ꢀC for complexes 1e3 (see supplementary materials,
Figs. S15eS16). This suggested that no isomerization occurs on the
NMR time scale. However, we are unable to provide a rationale that
weather only one rotamer is present or that syn/anti interconver-
sion (N-aryl group rotation) is too fast. It is reasonable to assume
that the fast exchange between the syn (with both methyl sub-
stituents on the same side of the NePdeN plane) and anti (with
both methyl substituents on the opposite side of the NePdeN
plane) isomers exists in solution, since narrow molecular weight
distributions were observed for these Pd(II) complexes in ethylene
polymerization. Notably, a similar behavior has been reported for
analogous Pd(II) complexes [46].
Single crystal of complex 1 suitable for X-ray diffraction analysis
was obtained by layering the n-hexane to the CH2Cl2 solution at
room temperature (Fig. 1). As expected, the Pd center adopts a
square planar geometry in the solid state. The observed bond an-
trend as some reported
be noted that Chen et al. have reported a series of
catalysts bearing both the dibenzhydryl moiety and systematically
varied ligand sterics and achieved the tuning of the branching
densities of polyethylenes though careful ligand sterics modifica-
tions [33]. Very interestingly, in contrast to the reported unsym-
a
-diimine Pd(II) catalysts [2e4,6]. It should
a
-diimine Pd(II)
gles and lengths are typical for previously reported palladium a-
diimine complexes. In addition, a C2 configuration, i.e. rac form was
observed for complex 1 in the solid state. Both phenyl rings
attached on imine groups are nearly perpendicular to the coordi-
nation plane, with the dihedral angles being 84.9ꢀ.
metrical a-diimine Ni(II) catalysts [39,41,42,45], which generated
the polyethylenes with broad even bimodal molecular weight
distributions, the Pd(II) catalysts in this system produced narrow
molecular weight distributions (less than 2.1). This result may be
attributed to two factors. First, the fast interconversion of syn- and
anti-conformers would be maintained in the cationic Pd(II) species
Table 1
Ethylene polymerization with complexes 1e3.a
Act.b
Mn
PDIc
Bd
c
Ent.
Cat.
T
Yield
(g)
(oC)
(104)
1
2
3
4
5
6
7
8
9
1
1
1
2
2
2
3
3
3
25
40
55
25
40
55
25
40
55
2.55
1.85
1.20
2.63
2.55
1.14
4.35
3.82
2.00
2.55
1.85
1.20
2.63
2.55
1.14
4.35
3.82
2.00
12.3
9.3
1.2
18.7
11.0
1.4
33.3
17.8
7.1
1.87
1.71
2.09
1.61
2.01
2.12
1.45
1.84
1.75
57
62
66
62
63
67
62
63
66
a
Fig. 1. Molecular structure of complex 1. Thermal ellipsoids are shown at the 50%
probability level. Hydrogen atoms have been omitted for clarity. The chlorine group
and methyl group attached to Pd1 are disordered over two sites, only one orientation is
shown here. Selected bond lengths (Å) and angles (deg): Pd(1)-C(38) ¼ 2.007(18),
Pd(1)-N(1) ¼ 2.101(2), Pd(1)-Cl(1) ¼ 2.288(4), C(38)-Pd(1)-Cl(1) ¼ 87.3(6), N(1)-Pd(1)-
N(1) ¼ 77.29(14).
Polymerization conditions: 10 mmol pre-catalyst; 1.5 eq. NaBAF; 50 mL of
toluene; ethylene pressure, 1.8 atm; reaction time, 10 h.
b
Activity is in unit of 104 g molꢁ1ꢃhꢁ1
.
c
Determined by GPC using universal calibration.
d
B ¼ branches per 1000 carbons, Branching numbers were determined using 1
H
NMR spectroscopy.

L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
15
during polymerization. The introduction of large substituents at 4-
position of aniline moiety did not slow this exchange through the
inhibition of C-N bond rotation. Second, the syn- and anti-forms of
the cationic Pd(II) species may show the similar catalytic properties
in ethylene polymerization.
branches, which is similar to the case of ethylene/MA copolymer-
ization and ethylene/AA copolymers reported by Jordan group [47].
The 1-hexene,1-octene and 1-decene polymerizations were also
investigated and the results were summarized in Table 3. The TOF
increased with decreasing the bulk of the ortho-substituents in the
order of 1 > 2 > 3. The sterically bulky substituents may retard the
One notable feature of a-diimine Pd(II) catalysts is their good
copolymerization ability for ethylene with polar monomers. Two
inexpensive and widely used monomers including methyl acrylate
(MA) and AA (acrylic acid) were chosen to copolymerize with
ethylene. The copolymerization results are shown in Table 2. Co-
polymers with high molecular weight were generated. In addition,
MA and AA incorporation ratios of 1.0e4.7% for complexes 1e3
were observed. When the steric hindrance of the ortho substituents
increased, the MA and AA incorporation ratio decreased. A lot of
coordination and insertion of a-olefins. In contrast to the trend of
TOF, the molecular weight of the obtained polymers was slightly
increased with the increasing the bulk of the ortho-substituents
(e.g. Table 3, Entries 1e3). This is probably due to the suppression
of the chain transfer by sterically blocking the access of the
monomer to the axial positions of the metal coordination site. 1H
NMR spectroscopy analysis showed that all the resulting polymers
were much lower branched than predicated from regular and
exclusive 1,2-monomer enchainment, i.e. 167 branches/1000C for
poly(1-hexene), 125 branches/1000C for poly(1-octene), 100
branches/1000C for poly(1-decene) (Scheme 3, regioregular poly-
mer). This is due to a fraction of 2,1 insertion followed by chain
studies on ethylene/MA copolymerization using
a-diimine Pd(II)
catalysts have been reported [2e4,6]. The 1H NMR spectra of the
copolymers suggest that the MA units are located at the end of the
branches, which result the fast chain walking after ethylene inser-
tion [21]. However, to our knowledge, very few
a
-diimine Pd(II)
walking to the primary carbon (Scheme 3, 1,
result, “chain straightening” through 1, -enchainment leads to a
more linear polymer structure. The fraction of 1, -enchainment
ranged from 42 to 54% and was comparable to the classical Broo-
khart catalyst [49], thus indicating that this class of complexes did
not have a significant selectivity for insertion fashion (1,2- vs 2,1-
u-enchainment). As a
catalysts have been reported to catalyze olefin copolymerization
with biorenewable comonomer acrylic acid, particularly because of
the poisoning of the metal centers by the active proton in eCOOH.
Jordan and co-workers described ethylene/AA copolymrization
u
u
behavior of a series of (N,N0-diaryl-
a-diimine) Pd(II) catalysts that
contain bulky diarylmethyl moiety in one N-aryl rings and sec-
ondary amide (-CONHMe) or tertiary amide (-CONMe2) groups on
the other N-aryl rings [48]. However, by using the highly sterically
insertion) of
ligand from the ortho-methyl to ortho-isopropyl substituents
resulted in a lower branched polymer with higher degree of 1,
enchainment. When monomer length increases from C6 to C10, the
fraction of 1, -enchainment for 1-hexene, 1-octene and 1-decene
a-olefins. Specially, increasing the steric bulk of the
u
-
hindered
a-diimine Pd(II) catalysts containing naphthalene or
benzothiophene units on both N-aryl rings, no AA incorporation
was observed in ethylene-AA copolymerization [30]. As a result, the
ability of the unsymmetrical a-diimine Pd(II) catalysts in this sys-
tem to copolymerize ethylene with AA may be attributed to the
right steric hindrance of the unsymmetrical ligands, which can
narrow the gap in the insertion barriers between ethylene and AA.
The 1H NMR and 13C NMR spectra of the ethylene/AA copolymer
u
is slightly changed. However, the branching density could be
dramatically reduced from 91 to 109 branches/1000C to 51e66
branches/1000C. This difference in branching density translated
into greater differences in polymer melting points (11e16 ꢀC versus
57e59 ꢀC). Most interestingly, the microstructure analysis of
poly(1-hexene) based on 13C NMR spectroscopy showed the pres-
ence of only methyl, butyl, and long chain branches (ꢄhexyl) for all
the catalysts (Fig. 3a). In addition, poly(1-octene) and poly(1-
decene) made with these catalysts showed only methyl and long
chain branches (ꢄhexyl) (Fig. 3b and c). The typical peaks of methyl
(1B1 at ca. 19.9 ppm) and long chain branches (1B6þ at ca. 14.3 ppm
and 2B6þ at ca. 22.9 ppm) can be safely identified in the 13C NMR
spectra. No ethyl, propyl, or adjacent methyl branches were
detectable, indicating that the insertion occurred only into primary
Pd-alkyl bonds.
indicate the incorporation of the AA monomer (Fig.
2 and
Figs. S44e47 in supplementary materials). The AA radical homo-
polymer is insoluble in CDCl3, while the ethylene/AA copolymer is
soluble. The characteristic peak for AA homopolymer generated via
radical polymerization was at ca. d 2.38 in CD3OH, corresponding to
-CH2CH(COOH), and was very broad because of the irregularity of
the repeating units [32]. This peak was not observed in the 1H NMR
of ethylene/AA copolymer. These results suggested that no AA
radical homopolymerization occurred. The triplet at 2.34 ppm was
assigned to the methylene hydrogen of the -CH2COOH moiety. The
AA units are incorporated predominately at the end of the
The mechanical properties of these poly(a-olefins) were exam-
ined by tensile tests (Fig. 4). The product generated from 1-hexene
and 1-octene polymerization showed very low stress at break
values. For comparison, poly(1-decene) sample showed high stress
and strain at break values (6.27 Mpa and 1340%). These results are
attributed to longer methylene sequences in the 1-decene polymer
backbone. Clearly, the polymer microstructures including molecu-
lar weights and branching densities, which can be modulated by
monomer length, have a significant influence on the mechanical
properties of these polymers.
The elastic recovery, i.e., the capability to return to the initial
state once the force is removed was also investigated (Fig. 5).
Polymer samples were subjected to hysteresis testing where each
sample was extended to 300% strain over 10 cycles. The strain re-
covery values (SR) can be calculated by SR ¼ 100 (εa ꢁ εr)/εa, where
εa is the applied strain and εr is the strain in the cycle at zero load
after 10th cycle. These polymer samples exhibit a certain amount of
unrecovered strain after the first cycle, followed by minimal
deformation on each subsequent cycle. A permanent structural
change happens during the first cycle, after which better elasto-
meric properties are created. The elastic recovery behavior of 1-
Table 2
Ethylene-polar monomer copolymerization with complexes 1e3.a
d
Ent.
Cat.
Monomer
Xb
Yield
(g)
Act.c
Mn
PDId
Be
(%)
(104)
1
2
3
4
5
6
1
2
3
1
2
3
MA
MA
MA
AA
AA
AA
4.7
3.4
1.0
2.7
2.2
1.1
0.39
0.52
0.57
0.83
0.94
1.24
10.83
14.44
15.83
23.06
26.11
34.44
1.0
1.2
1.4
1.8
3.3
5.8
1.52
1.53
1.58
1.80
1.45
1.48
50
60
60
53
55
62
a
Conditions: total volume of CH2Cl2 and polar vinyl monomer, 50 mL; 30 mmol
pre-catalyst; 1.5 eq. NaBAF; polar monomer concentration, 1 M; ethylene pressure,
1.8 atm; reaction time, 12 h; reaction temperature, 30 ꢀC.
b
Determined by 1H NMR spectroscopy.
c
Activity is in unit of 102 g molꢁ1ꢃhꢁ1
.
d
e
Determined by GPC using universal calibration.
B ¼ branches per 1000 carbons, Branching numbers were determined using 1
H
NMR spectroscopy.

16
L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
Fig. 2. 1H NMR spectra of the ethylene-AA copolymer obtained by complex 3 (CDCl3, sample from Table 2, entry 6).
Table 3
a
break values. These samples exhibit high SR values, which are
comparable with previously reported elastic polyolefin materials
obtained by reported nickel -diimine catalysts [43,50e55]. As a
result, -olefin polymerization in this system can generate ther-
-olefin polymerization with complexes 1e3.a
TOFb
Mn
PDIc
Bd
1,ue
Tm
c
f
a
Ent.
Cat.
[M]
Yield
(g)
a
(hꢁ1
)
(104)
(%)
(oC)
moplastic elastomers with good elastomeric recovery and high
strain at break. The current palladium catalytic system may provide
a rare route to the synthesis of thermoplastic elastomers in one step
1
2
3
4
5
6
7
8
9
1
2
3
1
2
3
1
2
3
1-Hexene
1-Hexene
1-Hexene
1-Octene
1-Octene
1-Octene
1-Decene
1-Decene
1-Decene
4.01
3.24
2.66
2.10
1.70
1.68
3.34
2.50
2.30
159
128
105
62
51
50
79
59
55
14.4
14.7
15.2
6.7
7.6
8.0
10.8
11.0
13.1
1.88
1.68
1.60
1.90
1.90
1.99
1.82
1.89
1.69
109
101
91
76
72
69
66
64
51
46
50
54
47
50
51
42
43
54
15
16
11
35
36
31
57
59
58
using only a-olefin as the feedstock [53,56].
3. Conclusions
In summary, three unsymmetrical
a-diimine palladium com-
a
plexes containing bulky substituents at 4-position of aniline moi-
eties were synthesized and characterized. The aim was to
investigate the effect of bulky substituents at 4-position of aniline
moieties on the polymerization processes as well as the properties
of the resulting polymer products. The Pd(II) catalysts in this sys-
tem produced polyethylenes with narrow molecular weight dis-
tributions (less than 2.1). The introduction of large substituents on
para- N-aryl moieties did not slow the exchange between the anti
Polymerization conditions: total volume of toluene and monomer, 50 mL;
monomer concentration, 3 M; 30 mmol pre-catalyst; 1.5 eq. NaBAF; reaction tem-
perature, 25 ꢀC, reaction time, 10 h.
b
Turnover frequency ¼ moles of substrate converted per mole of catalyst per
hour.
c
Determined by GPC using universal calibration.
d
B ¼ branches per 1000 carbons, Branching numbers were determined using 1
H
NMR spectroscopy.
e
1,u-enchainment calculated from the equation according to reference [49]: %
¼ [(1000-(n-2)B)/(1000þ2B)] ꢂ 100, R stands for the total methyl groups per
1,
u
and syn forms for these complexes. However, these
a-diimine
1000 methylene groups, n stands for the monomer length, i.e. the carbon atom
number of monomer.
palladium catalysts could copolymerize ethylene with bio-
renewable comonomer acrylic acid, with high comonomer incor-
poration and high copolymer molecular weights and the AA units
are incorporated predominately at the end of the branches. In
f
Determined by differential scanning calorimeter (DSC).
hexene polymer was not determined due to its very low strain at
break values. Overall, monomer length plays an important role on
the elastic properties of these polymer samples. The unrecovered
strain from 300% after 10th cycle is 159% for the polymer sample
produced using 1-octene as monomer (SR ¼ 47%), while higher
unrecovered strain (208%) is observed for the polymer sample
produced using 1-decene as monomer (SR ¼ 31%). This trend is
opposite to the effect of monomer length on stress and strain at
addition, the polymerization behavior of higher
these catalysts was also investigated. The tuning in ligand sterics
and monomer length enables the tuning of the polymer micro-
a-olefins using
structures such as branching density through the fraction of 1,
enchainment. The most abundant branches of these polymers are
methyl and longer than butyl branches. Interestingly, these poly(
olefins) catalyzed by -diimine Pd(II) catalysts in this system dis-
played properties characteristic of thermoplastic elastomers. Thus,
u-
a
-
a

L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
17
Scheme 3. Modes of monomer insertion and enchainment in Pd(II) catalyzed a-olefin polymerization.
Fig. 4. Stress-strain curves for polymers generated by complex 1.
DPX 500 spectrometers using TMS as an internal standard and
CDCl3 as solvent. 13C NMR spectra were acquired in 5 mm NMR
tubes at 298 K on Bruker DPX 500 or Bruker Ascend Tm 400
spectrometers. 13C NMR chemical shifts were internally referenced
to CHCl3 (77.16 ppm) for chloroform-d1. Elemental analyses were
performed on a Vario EL microanalyzer. Mass spectra were ob-
tained using electro spray ionization (ESI) LCMS-2010A for the
anilines and L1-L3. Matrix assisted laser desorption ionization time
of flight mass (MALDI-TOF) were performed on Bruker ultra-
fleXtreme for complexes 1e3. The molecular weight and the mo-
lecular weight distribution of the polymers were determined by gel
permeation chromatography (GPC, Tosh, Tokyo, Japan) equipped
with two linear Styragel columns at 40 ꢀC using THF as a solvent
and calibrated with polystyrene standards, and THF was employed
as the eluent at a flow rate of 0.35 mL/min. At least three specimens
of each polymer were tested. DSC was performed by a DSC Q20
from TA Instruments. Samples were quickly heated to 100 ꢀC and
kept for 5 min to remove thermal history, then cooled to ꢁ70 ꢀC at a
rate of 10 K/min, and finally reheated to 100 ꢀC at the same rate
under a nitrogen flow (50 mL/min). The maximum points endo-
therm (heating scan) were taken as the melting temperature (Tm).
Stress/strain experiments were performed at 10 mm/min by means
of a Universal Test Machine (UTM2502) at room temperature.
Polymers were melt-pressed at 30e35 ꢀC above their melting point
Fig. 3. 13C NMR spectra of polymers of (a) 1-hexene, (b) 1-octene, (c) 1-decene poly-
merized by complex 1. The large peak at 30 ppm is (CH2)n. Common signals are only
labeled on one of the four spectra for clarity. For example, 1B1, which is present in all
four spectra, is only labeled in (a).
this work demonstrates the great potential of synthesizing ther-
moplastic elastomers by
a-diimine Pd(II) catalysts in a single step
using only -olefin as the feedstock.
a
4. Experimental section
4.1. General procedures
Analytically pure solvent (toluene, n-hexane, dichloromethane)
were dried before use. All other reagents were purchased from
commercial sources and used without purification. The synthetic
route for anilines A1-A3 was shown in supporting information. 1H
NMR spectra were acquired in 5 mm NMR tubes at 298 K on Bruker

18
L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
Fig. 5. Plots of hysteresis experiments of ten cycles at a strain of 300% for samples generated by complex 1, (a) 1-octene (Table 3, entry 4) (b) 1-decene (Table 3, entry 7).
to obtain the test specimens, which have 14-mm gauge length, 2-
mm width, and thickness of 0.5 mm.
J ¼ 1.6 Hz, 2H, Ar-H), 5.41 (s, 2H, CHPh2), 5.23 (s, 2H, CHPh2), 2.50
(m, 2H, CH(CH3)2), isomer 1: 1.55 (s, 1H, N¼CMe), isomer 2: 1.30 (s,
5H, N¼CMe), isomer 2: 1.14 (d, J ¼ 6.9 Hz, 5H, CH3), isomer 2: 1.09
(d, J ¼ 6.8 Hz, 5H, CH3), isomer 1: 1.04 (dd, J ¼ 6.8, 3.3 Hz, 1H, CH3),
isomer 1: 1.01 (dd, J ¼ 6.7, 1.8 Hz, 1H, CH3). 13C NMR (126 MHz,
4.2. Synthesis of ligands
CDCl3)
d
169.14 (N¼CMe), 145.05, 144.65, 143.82, 142.60, 138.38,
4.2.1. Preparation of L1
134.85, 131.92, 129.70, 129.43, 128.87, 128.47, 128.22, 128.01, 126.32,
126.10, 125.09, 56.64 (CHPh2), 52.57 (CHPh2), 52.05 (CHPh2), 28.47
(CH(CH3)3), 27.85 (CH(CH3)3), 24.14 (CH3), 23.21 (CH3), 22.99 (CH3),
22.55 (CH3), 16.84 (N¼CMe), 16.48 (N¼CMe). ESI-MS (m/z): calcd for
A
solution of A1 (3.8 g, 8.6 mmol), 2,3-butadione (0.37 g,
4.3 mmol) and p-toluenesulfonic acid (20 mg) in toluene (50 mL)
was stirred at 80 ꢀC until there was one main point on the TLC plate.
The solvent was partially evaporated under reduced pressure until
the formation of a yellow solid. The yellow solid was filtered,
washed three times by 20 mL methanol and dried under high
vacuum. A amount of the product (3.02 g) was obtained as yellow
powder in 75.6% yield. 1H NMR (500 MHz, CDCl3) three isomers.
C
74H69N2: 985.5618, found: 985.5447[MþH]þ.
4.5. Synthesis of the palladium complexes
isomer 1: isomer 2: isomer 3 ¼ 1:5:24 (molar ratio),
d
7.22e6.89 (m,
4.5.1. Preparation of complex 1
40H, Ar-H), 6.83 (d, J ¼ 15.3 Hz, 2H, Ar-H), 6.57 (d, J ¼ 35.0 Hz, 2H,
Ar-H), 5.38 (s, 2H, CHPh2), 5.27 (s, 2H, CHPh2), isomer 1: 2.50 (s,
0.2H, CH3), isomer 3: 1.91 (s, 4.8H, CH3), isomer 2: 1.86 (s, 1H, CH3),
isomer 2: 1.57 (s, 1H, N¼CMe), isomer 3: 1.28 (s, 4.8H, N¼CMe),
A mixture of L1 (0.93 g, 1 mmol), (COD)PdMeCl (0.27g, 1 mmol)
and CH2Cl2(20 ml) was stirred for 2 days at room temperature.
During stirring, the solid was completely dissolved and the color of
the solution was changed from yellow to red. The solvent was
removed, and the resulting powder was washed with n-hexane
(3 ꢂ 10 mL) and dried under vacuum to obtain a red solid (0.70 g,
64%). A single crystal of complex 1 was obtained by layering n-
hexane onto the CH2Cl2 solution at room temperature.1H NMR
isomer 1: 1.11 (s, 0.2H, N¼CMe). 13C NMR (126 MHz, CDCl3)
d 168.98
(N¼CMe), 146.26, 144.35, 143.42, 142.55, 138.35, 132.73, 129.67,
129.39, 128.86, 128.52, 128.22, 128.02, 126.15,124.18, 56.35 (CHPh2),
53.46 (CHPh2), 52.50 (CHPh2), 51.91 (CHPh2), 18.07 (CH3), 17.95
(CH3), 16.38 (N¼CMe), 15.85 (N¼CMe). ESI-MS (m/z): calcd for
(500 MHz, CDCl3)
d 7.24e6.99 (m, 38H, Ar-H), 6.95 (s, 3H, Ar-H),
C
70H61N2: 929.4835, found: 929.4827 [MþH]þ.
6.86 (s, 1H, Ar-H), 6.66 (s, 1H, Ar-H), 6.53 (s, 1H, Ar-H), 6.15 (s, 1H,
CHPh2), 5.89 (s, 1H, CHPh2), 5.43 (d, J ¼ 10.4 Hz, 2H, CHPh2), 2.25 (d,
J ¼ 9.4 Hz, 6H, N¼CMe), 0.78 (s, 3H, CH3), 0.61 (s, 3H, CH3), 0.56 (s,
4.3. Preparation of L2
3H, Pd-CH3). 13C NMR (126 MHz, CDCl3)
d
176.69 (N¼CMe), 171.61
The same procedure as above was employed. Yeild 84.9% (3.49 g,
yellow powder). 1H NMR (500 MHz, CDCl3) two isomers. isomer 1:
(N¼CMe), 144.24, 144.00, 143.65, 142.56, 142.34, 142.07, 141.68,
141.25, 135.73, 135.57, 130.48, 130.27, 129.77, 129.55, 129.48, 129.38,
129.34, 129.30, 129.23, 128.76, 128.33, 127.91, 126.45, 126.19, 56.25
(CHPh2), 52.50 (CHPh2), 52.21 (CHPh2), 19.02 (CH3), 18.53 (CH3),
18.30 (N¼CMe), 17.98 (N¼CMe), 2.88 (Pd-CH3). MALDI-TOF-MS (m/
z): calcd for C70H60N2Pd: 1033.3807, found: 1033.3311, [M-Me-Cl]þ.
Anal. Calcd for C71H63ClN2Pd: C, 78.51; H, 5.85; N, 2.58; Found: C,
78.41; H, 5.76; N, 2.60.
isomer 2 ¼ 1:5 (molar ratio),
d 7.22e6.86 (m, 42H, Ar-H), 6.53 (s, 2H,
Ar-H), 5.40 (s, 2H, CHPh2), 5.25 (s, 2H, CHPh2), 2.21 (ddt, J ¼ 22.3,
15.0, 7.5 Hz, 4H, CH2), isomer 1: 1.56 (s, 1H, N¼CMe), isomer 2: 1.29
(s, 5H, N¼CMe), isomer 2: 1.12 (t, J ¼ 7.5 Hz, 5H, CH3), isomer 1: 1.01
(t, J ¼ 7.4 Hz, 1H, CH3). 13C NMR (126 MHz, CDCl3)
d 169.02
(N¼CMe), 145.78, 144.55, 143.67, 142.59, 138.37, 132.18, 129.69,
129.43, 128.50, 128.22, 128.03, 127.40, 126.16, 56.53 (CHPh2), 52.48
(CHPh2), 51.93 (CHPh2), 24.65 (CH2), 24.03 (CH2), 16.62 (N¼CMe),
16.17 (N¼CMe), 13.88 (CH3), 13.59 (CH3). ESI-MS (m/z): calcd for
4.6. Preparation of complex 2
C
72H65N2: 957.5148, found: 957.5128 [MþH]þ.
Using the same procedure as for the synthesis of complex 1,
complex 2 was obtained as a red powder (0.63 g, 57%). 1H NMR
4.4. Preparation of L3
(500 MHz, CDCl3)
d
7.23e6.97 (m, 39H, Ar-H), 6.95 (d, J ¼ 7.7 Hz, 2H,
Ar-H), 6.91 (s, 1H, Ar-H), 6.65 (s, 1H, Ar-H), 6.52 (s, 1H, Ar-H), 6.23 (s,
1H, CHPh2), 5.94 (s, 1H, CHPh2), 5.45 (d, J ¼ 11.3 Hz, 2H, CHPh2), 2.77
(dd, J ¼ 15.3, 7.6 Hz,1H, CH2), 2.64 (dd, J ¼ 14.8, 7.4 Hz,1H, CH2), 2.37
(m, 2H, CH2), 1.41 (t, J ¼ 7.5 Hz, 3H, CH3), 1.33 (t, J ¼ 7.5 Hz, 3H, CH3),
The same procedure as above was employed. Yeild 78.7% (3.33 g,
yellow powder).1H NMR (500 MHz, CDCl3) two isomers. isomer 1:
isomer 2 ¼ 1:5 (molar ratio),
d 7.24e6.87 (m, 42H, Ar-H), 6.50 (d,

L. Guo et al. / Journal of Organometallic Chemistry 877 (2018) 12e20
19
0.80 (s, 3H, N¼CMe), 0.62 (s, 3H, N¼CMe), 0.55 (s, 3H, Pd-CH3). 13
C
pressure was kept at a constant value of 1.8 atm by continuous
feeding of gaseous ethylene throughout the reaction. The poly-
merization was terminated by the addition of a large amount of
methanol after continuous stirring for 12 h. Then the methanol was
decanted off, and the sticky polymer was redissolved in petroleum
ether. The polymer solution was filtered through alumina or silica
to remove catalyst residues. After evaporation, the resulting poly-
mer was collected and dried under vacuum at 40 ꢀC to a constant
weight. MA or AA incorporation was determined by 1H NMR. MA
% ¼ 1/3 IOCH3/[IOCH3/3 þ [(ICH3 þ ICH2þCH)-3]/4] ꢂ 100% [57]. AA
% ¼ 1/2 Ia-CH2/[Ia-CH2/2 þ (ICH3 þ ICH2þCH þ 1/2 Ia-CH2)/4] ꢂ 100%
[32].
NMR (126 MHz, CDCl3)
d
176.64 (N¼CMe), 171.57 (N¼CMe), 144.3,
144.10,143.76, 142.77, 142.61, 142.44, 141.83, 141.47, 141.19, 135.66,
135.39,133.78,132.72,130.45,130.21,129.83,129.44,129.08,128.78,
128.35, 127.95, 127.34, 126.81, 126.48, 126.18, 56.42 (CHPh2), 52.65
(CHPh2), 52.37 (CHPh2), 24.53 (CH2), 23.90 (CH2), 19.51 (CH3), 18.39
(CH3), 14.35(N¼CMe), 13.24 (N¼CMe), 3.32 (Pd-CH3). MALDI-TOF-
MS (m/z): calcd for C72H64N2Pd: 1061.4120, found: 1061.3390, [M-
Me-Cl]þ. Anal. Calcd for C73H67ClN2Pd: C, 78.69; H, 6.06; N, 2.51;
Found: C, 78.42; H, 6.16; N, 2.64.
4.7. Preparation of complex 3
Using the same procedure as for the synthesis of complex 1,
complex 3 was obtained as a red powder (0.82 g, 72%).1H NMR
4.10. General procedure for a-olefin polymerization
(500 MHz, CDCl3)
d 7.24e7.06 (m, 28H, Ar-H), 7.05e6.98 (m, 12H,
Polymerization was carried out in a 100 mL round-bottomed
Schlenk flask. The reactor was first dried by heating at 110 ꢀC and
Ar-H), 6.95 (d, J ¼ 7.1 Hz, 2H, Ar-H), 6.63 (s, 1H, Ar-H), 6.50 (d,
J ¼ 12.7 Hz, 1H, Ar-H), 6.20 (s, 1H, CHPh2), 5.98 (s, 1H, CHPh2), 5.45
(m, 2H, CHPh2), 2.92 (m, 2H, CH(CH3)2),1.48 (dd, J ¼ 12.3, 6.7 Hz, 4H,
CH3), 1.35 (d, J ¼ 6.6 Hz, 2H, CH3), 1.21 (t, J ¼ 9.2 Hz, 6H, CH3), 0.86 (s,
3H, Pd-CH3), 0.62 (m, 6H, N¼CMe). 13C NMR (126 MHz, CDCl3)
cooled to room temperature under vacuum. Pre-catalyst(30 mmol),
NaBAF(1.5 eq.), freshly distilled toluene, monomer and a magnetic
stirrer were transferred into the reactor vessel in that order. The
total volume of toluene and monomer were kept at 50 mL. They
was stirred for 10 h at 25 ꢀC. Polymerization was quenched with
ethanol. The solid polymer was filtered, washed with ethanol
several times, and dried in vacuum at 40 ꢀC to a constant weight.
d
176.63 (N¼CMe), 171.62 (N¼CMe), 144.41, 144.12, 143.77, 142.82,
142.64, 141.87 (s), 141.52, 140.68, 140.43, 138.84, 137.53, 135.49,
135.31, 130.36, 130.17, 129.82, 129.57, 129.48, 129.37, 129.33, 129.30,
129.29, 128.78, 128.49, 128.41, 128.25, 127.97, 126.82, 126.48, 126.16,
125.76, 124.87, 56.49 (CHPh2), 56.47 (CHPh2), 52.80 (CHPh2), 52.54
(CHPh2), 28.43 (CH(CH3)2), 28.16 (CH(CH3)2), 24.87 (CH3), 23.94
(CH3), 23.54 (CH3), 22.63 (CH3), 20.05 (N¼CMe), 18.60 (N¼CMe),
3.70 (Pd-CH3). MALDI-TOF-MS (m/z): calcd for C74H68N2Pd:
1090.4417, found: 1090.3704, [M-Me-Cl]þ. Anal. Calcd for
Acknowledgements
This work was supported by Shandong Provincial Natural Sci-
ence Foundation (ZR2018MB023), the National Natural Science
Foundation of China (Grant No. 21671118), The Key Laboratory of
Polymeric Composite & Functional Materials of Ministry of Edu-
cation (PCFM-2017-01), Excellent experiment project of Qufu
Normal University (jp201705) and the Taishan Scholars Program.
C
75H71ClN2Pd: C, 78.86; H, 6.27; N, 2.45; Found: C, 78.88; H, 6.19; N,
2.51.
4.8. General procedure for ethylene polymerization
Appendix A. Supplementary data
Polymerization was carried out in a 250 mL round-bottomed
Schlenk flask. The reactor was first dried by heating at 110 ꢀC and
cooled to room temperature under vacuum. A 250 mL round-
bottomed Schlenk flask was charged with the required amount of
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2018.09.016.
pre-catalyst(10 mmol), NaBAF(1.5 eq.), 50 mL of freshly distilled
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