Bis(β-ketoamino)copper(II)/methylaluminoxane systems for homo- And copolymerizations of methyl acrylate and 1-hexene

Article abstract of DOI:10.1002/pola.23867

Two bis(β-ketoamino)copper [ArNC(CH₃)CHC (CH ₃)O]₂Cu (1, Ar = 2,6-dlmethylphenyl; 2, Ar = 2,6-dilsopro-pylphenyl) complexes were synthesized and characterized. Homo- and copolymerizatlons of methyl aorylate (MA) and 1-hexene with bis(β- ketoamino)copper(II) complexes activated with methylalumlnoxane (MAO) were investigated in detail. MA was polymerized in high conversion (>72%) to produce the syndio-rich atactic poly(methyl acrylate), but 1-hexene was not polymerized with copper complexes/MAO. Copolymerizations of MA and 1-hexene with 1,2/MAO produced acrylate-enriched copolymers (MA > 80%) with isolated hexenes in the backbone. The calculation of reactivity ratios showed that r(MA) Is 8.47 and r(hexene) Is near to 0 determined by a Fineman-Ross method. The polymerization mechanism was discussed, and an insertion-triggered radical mechanism was also proposed.

Full text of DOI:10.1002/pola.23867

Bis(b-ketoamino)copper(II)/Methylaluminoxane Systems for Homo- and
Copolymerizations of Methyl Acrylate and 1-Hexene
HAIYANG GAO,1 XIAOFANG LIU,1 LIXIA PEI,2 QING WU1
1DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University,
Guangzhou 510275, China
2School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
Received 11 September 2009; accepted 4 December 2009
DOI: 10.1002/pola.23867
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two
bis(b-ketoamino)copper
[ArNC(CH3)CHC
acrylate-enriched copolymers (MA > 80%) with isolated hex-
enes in the backbone. The calculation of reactivity ratios
showed that r(MA) is 8.47 and r(hexene) is near to 0 deter-
(CH3)O]2Cu (1, Ar ¼ 2,6-dimethylphenyl; 2, Ar ¼ 2,6-diisopro-
pylphenyl) complexes were synthesized and characterized.
Homo- and copolymerizations of methyl acrylate (MA) and 1-
hexene with bis(b-ketoamino)copper(II) complexes activated
with methylaluminoxane (MAO) were investigated in detail.
MA was polymerized in high conversion (>72%) to produce
the syndio-rich atactic poly(methyl acrylate), but 1-hexene
was not polymerized with copper complexes/MAO. Copoly-
merizations of MA and 1-hexene with 1,2/MAO produced
mined by
a Fineman-Ross method. The polymerization
mechanism was discussed, and an insertion-triggered radical
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V
mechanism was also proposed.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 1113–1121, 2010
KEYWORDS: copolymerization; monomers; organometallic cata-
lysts; radical polymerization
INTRODUCTION The copolymerization of olefins with polar
monomers by organometallic catalysts has been an interest-
ing goal. In particular, the copolymerization of olefins with
acrylate monomers can produce new special materials:
the thermal stability, the hydrophobic character derived from
the CAC backbone, and the adhesive power introduced with
the ester groups.1–3 In the last few years, late transition
metal nickel and palladium complexes have been developed
for the direct copolymerization of ethylene and polar mono-
mers because of their greater functional group tolerance.4–14
However, the majority of the catalytic systems reported up
to now generally produce copolymers with low incorporation
of polar monomer and with low molecular weights and
branched structures.
and coworkers reported bis(salicylaldiminate)copper(II)
complexes, when activated with MAO, catalyzed the homo-
and copolymerizations of ethylene and methyl methacrylate
(MMA).24 The activity of MAO-activated [1,2-bis(4,4-
dimethyl-2-oxazolin-2-yl)ethane]copper(II) dichloride (Cu(D-
MOX)Cl2) for the polymerization of acrylates and their
copolymerization with ethylene was also reported by Sen
and coworkers,25 who investigated the reaction mechanism.
Foley and coworkers recently found that salicylaldimine or
a-diimine copper species cannot be an active catalyst for eth-
ylene polymerization under mild conditions due to ligand
transfer to aluminum, and the resulting LAlMe2/LAlMeþ
complexes are likely the active species.26
Generally, the poly(methyl acrylate)s (PMAs) produced by
these copper(II)/MAO systems are atactic. The introduction
of ethylene into the MA or MMA homopolymerization system
resulted in the formation of acrylate-rich copolymers in
greatly reduced yields. Unlike Brookhart-type palladium
a-diimine catalyst,4,5 these systems produced highly linear
copolymers with high acrylate incorporation. Additionally,
the level of alkene incorporation in the copolymers was simi-
lar to that reported for well documented radical polymeriza-
tion systems. Besides, some copper complexes bearing [N,O]
liagnds have been developed for norbornene polymeriza-
tion.27–31 To date, reports on olefin polymerization systems
based on copper are scarce, and polymerization mechanism
In addition to nickel- and palladium-based catalyst, copper-
based systems have also been reported for homo- and
copolymerization of alkenes with polar monomers. The
researchers of Exxon-Mobil first investigated the activity of
methylaluminoxane (MAO)-activated copper(II) systems.15–18
Subsequently, several other reports have emerged of LCuCl2
or L2Cu (L: ligand) complexes using a-diimine,19 pyrazolyl-
pyrimidine,20 and pyrazolylquinoline21 ligands that are also
active for ethylene polymerization with MAO as activator. In
particular, Stibrany et al. reported good activity of bis(benzi-
midazole)copper(II)/MAO systems for the homo- and copoly-
merizations of ethylene and methyl acrylate (MA).22,23 Carlini
Correspondence to: Q. Wu (E-mail: ceswuq@mail.sysu.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1113–1121 (2010) 2010 Wiley Periodicals, Inc.
BIS(b-KETOAMINO)COPPER(II)/MAO SYSTEMS, GAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of copper complexes.
is not clearly deduced. Except for the atom transfer radical
polymerization systems, a noteworthy current limitation for
copper-based catalytic systems is the absence of copolymer-
ization of a-olefins and acrylate monomers.
geometry.34 Unlike copper complex Cu[(CH3)3CC(NH)CH-
C(O)C(CH3)3]2 adopting square planar geometry,35 bulky
copper complex 2 can be described as a distorted square
planar coordination geometry, which may result from the
steric hindrance of the substituents on the imine moieties.
The dihedral angle is 38.7ꢁ between two coordination planes,
which is larger than those of other [N,O]copper(II) com-
plexes.28–31 The bond lengths of Cu(1)AO (1.8919(13) Å)
and Cu(1)AN (1.9700(15) Å) are similar to those of bis(sali-
cylaldiminate)copper(II) complex and other [N,O]copper(II)
complexes. And the presence of N-2,6-diisopropylphenyl
group has caused an increase in the length of the CuAN
bond than that of the CuAO bond. The angle of NACuAO is
93.71(6)ꢁ, which is slightly larger than those of bis(salicylal-
diminate)copper(II) complex.28
In this article, novel bis(b-ketoamino)copper(II) complexes
were chosen and synthesized as precursors because these
copper precursors are stable, cheap, and easy to be pre-
pared. Homo- and copolymerizations of MA and 1-hexene
with novel bis(b-ketoamino)copper(II) complexes in combi-
nation with MAO were investigated. The influences of the
reaction parameters on polymerization behavior were eval-
uated, and the microstructures of the resulting polymers
were also characterized. Moreover, the polymerization mech-
anism using bis(b-ketoamino)copper complexes/MAO sys-
tems was also analyzed and commented.
MA Homopolymerization
RESULTS AND DISCUSSION
In the presence of MAO, copper complexes 1 and 2 were
investigated for MA homopolymerization. The results are
listed in Table 1. Two copper complexes 1 and 2 exhibited
moderate activity for MA homopolymerization. Steric hin-
drance of the copper complexes on aryl substituents influen-
ces their activities and the molecular weight of the produced
polymer. 1 with N-2,6-dimethylphenyl group showed higher
Synthesis of Copper Complexes and Molecular Structure
The synthetic route of the copper complexes is shown in
Scheme 1. b-Ketoamino ligands L1 and L2 were prepared by
condensation of acetylacetone and the corresponding substi-
tuted aniline following previously reported method.32 The
desired bulky copper complexes 1 and 2 were synthesized
under mild conditions in high yields (>80%) by the reaction
of Cu(OAc)2ꢀH2O with 2 equiv of the b-diketiminate ligands
in methanol.
activity for MA polymerization than
2
with N-2,6-
1H NMR analysis failed because of the paramagnetic charac-
ter of the Cu(II) complex, which is similar to other copper(II)
complexes bearing [N,O] ligands.28–31 Therefore, the IR and
elemental analysis were measured to confirm the structures
of copper complexes 1 and 2. The elemental analysis results
revealed that the components of two complexes were in
accord with the formula CuL2. Both 1 and 2 are stable in air,
and they are soluble in CH2Cl2, tetrahydrofuran (THF), and
toluene, and sparingly soluble in methanol and
hydrocarbons.
Crystal suitable for X-ray crystallography of 2 was obtained
by slow evaporation from methanol/toluene solution. ORTEP
diagram is shown in Figure 1 along with selected bond
lengths and bond angles.33 Complex 2 shows the four-coordi-
nate environment around the copper atom where the two
ligands act as bidentate N,O-chelators and lie in the trans
conformation to create two slightly distorted six-membered
coordination planes. It is well known that four-coordinated
Cu(II) complexes are usually characterized by a square pla-
nar coordination that may be distorted to pseudotetrahedral
FIGURE 1 Molecular structure of 2. The hydrogen atoms are
ꢁ
˚
omitted for clarity. Selected bond lengths (A) and angles ( ):
Cu(1)AN(1) 1.9700(15), Cu(1)AN(1)#1 1.9700(15), Cu(1)AO(1)#1
1.8919(13), Cu(1)AO(1) 1.8919(13), O(1)ACu(1)AN(1) 93.71(6),
O(1)#1ACu(1)AN(1)#1 93.71(6), O(1)#1ACu(1)AO(1) 153.53(9),
N(1)ACu(1)AN(1)#1 153.59(9).
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TABLE 1 Results of MA Homopolymerization with 1 and 2/MAO
Activitya
Mw (104 g/mol)
MWDb (Mw/Mn)
b
Run
Complex
T (ꢁC)
Al/Cu ratio (mol/mol)
Conversion (%)
1
1
1
1
1
1
1
2
2
2
2
2
2
25
45
65
65
65
65
25
45
65
65
65
65
100
100
100
50
40.5
49.1
72.6
trace
67.3
6.9
19.4
23.5
34.8
–
2.34
3.62
4.87
1.97
1.79
1.64
–
2
3
C
4
–
5
200
400
200
200
200
50
32.2
3.30
21.9
22.8
25.7
–
2.09
1.13
3.57
4.62
5.40
–
1.42
1.89
1.68
1.81
1.72
–
6
7
45.8
48.1
54.1
trace
0.6
8
9
10
11
12
100
400
0.4
–
–
3.0
1.4
–
–
b
General Condition: 20 lmol Cu complex; MA monomer: 5 mL; reaction
time, t ¼ 5 h; solvent, toluene; total volume 20 mL.
Determined by GPC relative to polystyrene standards.
Not determined.
C
a
In units of kg/(mol Cu h).
diisopropylphenyl group on the same polymerization condi-
tion (run 3 vs. run 11, run 5 vs. run 9, and run 6 vs. run
12). In contrast, the molecular weight of the obtained PMA
obtained by 1/MAO is obviously lower than those of the
obtained PMA obtained by 2/MAO. These results strongly
suggest that the ligand plays an important role in
polymerization.
suggests that MAO has a very important influence on active
center.
1H NMR and 13C NMR spectra confirmed the obtained PMAs
are atactic. The tacticity of the PMA can be calculated from
1H NMR integration of methylene reasonances.36,37 Calcula-
tion results in Table 2 indicate the obtained polymers are
syndio-rich atactic PMA. Similar result was observed by
Sen25 and coworkers for well documented radical polymer-
ization systems.38,39
The influence of temperature was also investigated. With an
increase in the reaction temperature from 25 to 65 ꢁC, the
activities increased gradually. Though both complexes
showed the highest activity for MA polymerization at 65 ꢁC,
copper complex 1 is more sensitive than 2 to the influence
of reaction temperature on the activity. Besides, raising tem-
perature also causes an increase in molecular weight of
PMA. Similar result was reported for Cu(DMOX)Cl2/MAO sys-
tem by Sen and coworkers.25
All of the obtained PMAs present about 60% of syndiotactic
triads (rr), and the number average lengths of syndiotactic
sequences (L(r)) is 2–3 times of the number average lengths
of isotactic sequences (L(m)) (see Table 2). The microstruc-
ture is affected by polymerization temperature but hardly
affected by steric hindrance of copper complexes. The syn-
diotactic triad of the produced polymers decreases when
temperature increases. On the basis of first-order Markov
model, it is noted that the persistence ratios (q) are near to
Bꢂ1 (inverse of Bernoullian parameter). This result suggests
that the control of monomer insertion is due to the stereo-
chemistry of growing chain ends.40
The activities are also sensitive to Al/Cu mole ratio. When
Al/Cu ratio was 50, only trace of polymer was obtained for
1 and 2 systems. With an increase in Al/Ni ratio, the activ-
ities for MA polymerization increased, and then decreased
markedly. An optimum Al/Ni ratio was 100 for 1 and 200
for 2. Note that Al/Cu ratio should be among very narrow
range (100–200) to achieve the high activity, which strongly
Both copper complexes were also investigated as the precur-
sors for 1-hexene homopolymerization. It was found that
TABLE 2 Methylene Tacticity of PMAs with 1,2/MAO
Diads
Triads
Run
Systems
m
r
mm
mr
rr
Bꢂ1
L(m)
L(r)
q
5
7
9
1/65 ꢁC
2/25 ꢁC
2/65 ꢁC
0.29
0.22
0.31
0.71
0.78
0.69
0.15
0.11
0.15
0.28
0.21
0.32
0.57
0.67
0.53
1.47
1.60
1.34
2.07
2.05
1.94
5.07
7.38
4.31
1.47
1.63
1.34
B ¼ [mr]/(2[mm] þ [mr]) þ [mr]/(2[rr] þ [mr]).
q ¼ 2[m][r]/[mr].
L(r) ¼ 1 þ 2[rr]/[mr].
L(m) ¼ 1 þ 2[mm]/[mr].
BIS(b-KETOAMINO)COPPER(II)/MAO SYSTEMS, GAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Copolymerizations of MA and Hexene with 1,2/MAO
MA/Hexene in
MAb Incorporation
s(mol%)
Mw
MWDc
c
Run
Complex
Feed (mol/mol)
Yield (g)
Activitya
(104 g/mol)
(Mw/Mn)
13
14
15
16
17
18
19
20
1
1
1
1
2
2
2
2
20.0/80.0
44.4/55.6
58.2/41.8
80.0/20.0
20.0/80.0
44.4/55.6
58.2/41.8
80.0/20.0
0.76
2.39
3.14
3.75
0.24
1.62
2.60
3.47
3.17
9.94
13.1
15.6
0.99
6.76
10.8
14.5
a
80.4
87.0
90.9
94.3
83.3
88.7
92.0
95.2
2.77
5.87
6.89
12.4
3.58
6.70
9.99
18.4
1.78
1.98
2.18
2.61
2.05
1.98
2.58
2.60
General condition: 20 lmol Cu complex; monomer total addition (MA þ
hexene): 95.6 mmol; temperature: 45 ꢁC; Al/Cu ¼ 200; reaction time, t ¼
12 h; solvent: toluene; total volume: 20 mL.
In units of kg/(mol Cu h).
Determined by integration of 1H NMR.
b
c
Determined by GPC relative to polystyrene standards.
complexes 1 and 2 were inactive for 1-hexene homopoly-
merization in the presence of MAO, and no polyhexene,
oligomers, or isomer of hexene (determined by GC-MS) were
obtained under the similar polymerization condition with 1-
hexene instead of MA monomer.
As the hexene feed content increases, both the activity and
the molecular weight of the product decreases. Although
increasing the hexene feed contents raises the hexene incor-
poration into the copolymer, the hexene content incorporated
into the copolymer is always lower than the hexene content
in the monomer feed. The molecular weight distributions
(MWDs) are close to 2 and appear as a unimodal in the GPC
chromatogram (see Fig. 2) suggesting that copolymerizations
occur at the single active site and that the polymerization
products are the true copolymers and not mixtures of the
homopolymers.
Copolymerizations of MA and 1-Hexene
Bis(b-ketoamino)copper complexes activated with MAO were
found to be able to efficiently copolymerize MA and 1-hex-
ene. Copolymerizations of MA and hexene were carried out
with 1 and 2 activated with MAO at moderate reaction con-
dition (T ¼ 45 ꢁC, and Al/Cu ¼ 200) with various comono-
mer feed contents. All results of copolymerizations are listed
in Table 3.
To determine the monomer reactivity ratios, a series of
experiments were performed with 2/MAO system and
stopped at the short time to achieve the low conversions
(<10%). The copolymer compositions were determined by
1H NMR integration of the methoxy protons versus methyl
protons. The reactivity ratios r(MA) ¼ 8.47 and r(hexene) ¼
0.14 determined by a Fineman-Ross method illustrates the
much higher reactivity of MA. These are in good agreement
with those reported in the literature for radical copolymer-
ization of the two monomers.42 Besides, r(hexene) is near to
0, indicating that there is no hexene self-propagation.
As shown in Table 3, the complex 1 shows higher copoly-
merization activity than 2. This suggests that the structure
of the copper complexes influences their copolymerization
activities. The introduction of bulky steric hindrance
decreases the copolymerization activities, but increases the
molecular weights of MA-hexene copolymers. This result is
identical to that of MA homopolymerization, indicating that
homopolmerization active species is same to copolymeriza-
tion active species.
The structure of MA-hexene copolymers was also analyzed
by NMR spectroscopy. Comparing 1H NMR spectrum (Fig. 3)
of MA-hexene copolymer with those of MA homopolymer, the
methoxyl peak at 3.67 ppm, the MA methine proton at 2.34
ppm, and the methyl of hexene peak at 0.90 are easily distin-
guished suggesting hexene is effectively incorporated.
Copolymerization activities and the molecular weights of the
copolymers are also affected by the hexene feed content. An
increase in the initial hexene feed content leads to a gradual
decrease in the activity with respect to MA homopolymeriza-
tion. Besides, it is noteworthy that increasing the hexene
feed content up to 55.6 mol % decreased copolymerization
activity, but obviously increased molecular weight of copoly-
mers with respect to MA homopolymerization. This result
indicates that 1-hexene plays an important role in the chain
termination reaction, and hexene can prohibit effectively
chain transfer or termination reaction. Sen and coworkers
also reported similar MA-ethylene or propylene copolymer-
ization results.25 Higher hexene feed content leads to
decrease in molecular weight of copolymer when compared
with PMA, which can be reasonably explained from the
kinetic aspect.41
The 13C NMR spectrum is much more definitive, as shown in
Figure 4. The peak at 175.0 ppm, which is the same as
in homopolymer of MA, is assigned to the carbonyl carbon in
long consecutive MA runs. The peaks for the methoxyl car-
bon (51.8 ppm), methylene carbon (41.3 ppm), and methine
carbon (35.0 ppm) in MA unit are assigned on the basis of
the 13C NMR spectrum of PMA. In addition, two peaks (14.1,
and 23.0 ppm) can be assigned to the methyl carbon and
methylene carbon in the side chain of hexene (butyl group of
hexene). On the other hand, the presence of MA-hexene
dyads is signaled by the resonances at 37.8 ppm. At same
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ARTICLE
FIGURE 2 GPC curves of MA-hexene copolymers using 1, 2/MAO.
time, the resonance from hexene-hexene dyads at 43.0 ppm
is not observed, indicating that hexene is randomly distrib-
uted in the copolymer chains. This result is good agreement
with reactivity ratios result (r(hexene) ꢃ 0). Therefore, it is
concluded that the polymer produced in copolymerization of
MA, and hexene is a copolymer with long MA sequences and
isolated hexenes in the backbone.
1-hexene using bis(b-ketoamino)copper/MAO systems reveal
the incorporation of 1-hexene in copolymer at levels compa-
rable to those observed with radical initiator system.42 The
calculated reactivity ratios, r(MA) ¼ 8.47 and r(hexene) ¼
0.14, also support a radical mechanism.44 1-Hexene cannot
be polymerized or oligomerized with bis(b-ketoamino)cop-
per/MAO system, and the hexene units are isolated in the
backbone of the copolymer, which are in sharp contrast with
typical insertion/coordination polymerization.
Mechanistic Consideration
Two possible mechanisms for the copper-mediated homo-
and copolymerization of acrylates with alkenes are radical
and insertion. Some authors have already studied polymer-
ization mechanism for acrylates and alkenes with copper(II)/
MAO system. Stibrany et al. postulated a coordination/inser-
tion mechanism for bis(benzimidazole)copper/MAO sys-
tems.22 Shibayama thought that the active species for MMA
polymerization with Cu(II) amidinate/MAO system is a Cu(I)
complex.43 Carlini et al. also suggested that homo- and
copolymerization MMA with ethylene proceeded through a
Cu(I)-mediated polymerization mechanism, but the author
did not completely exclude that radical species may also par-
ticipate in the polymerization.10 Sen and coworkers pointed
out that the use of radical traps, such as galyinoxyl, DPPH,
and TEMPO as probes for a radical mechanism in the copper
complex, such as Cu(DMOX)Cl2 using MAO can lead to the
wrong conclusion.25 The radical traps may fail to intercept
radical reactions that proceed in the presence of MAO. Foley
and coworkers recently presented that salicylaldimine or
a-diimine copper species cannot be an active catalyst for
ethylene polymerization because of ligand transfer to
aluminum.26
To probe the polymerization mechanism of this system, sev-
eral experiments were designed and carried out. First, when
only MAO was introduced into solution of MA in toluene
without copper complex, no polymer was obtained. This
result suggests that MAO alone cannot initiate the polymer-
ization of MA under the adopted reaction conditions. Second,
mixing MAO or trimethylaluminum (TMA) with b-ketoamino
ligand in solution of MA in toluene also results in no poly-
mer suggesting that chains grow on aluminum center for MA
Our observations consistent with a radical mechanism are as
follows. The tacticity of mm, mr, and rr stereotriads of the
obtained atactic PMA is very similar to those of PMA with a
free radical initiator, such as AIBN. Besides, control experi-
ments involving the polymerization of MA in the presence of
FIGURE 3 1H NMR spectra of PMA and MA-hexene copolymer.
a: PMA (run 8); b: Copolymer (run 17).
BIS(b-KETOAMINO)COPPER(II)/MAO SYSTEMS, GAO ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE
4
13C NMR spectra of
PMA and MA-hexene copolymer.
a: PMA (run 8); b: Copolymer
(run 17).
polymerization can be ruled out.26 In addition, bis(b-keto-
amino)copper complex cannot polymerize MA without MAO.
Therefore, it is concluded that bis(b-ketoamino)copper(II)
complex and MAO cooperate in MA polymerization process.
For copolymerization of MA and 1-hexene, 1-hexene is iso-
lated in backbone because r(hexene) is near to zero. When
1-hexene monomer is added into the growing chain, macro-
molecule radical with the new added 1-hexene is disfavored
for chain termination and chain propagation. Therefore, it is
found that an increase in the hexene feed content decreased
the activity, but obviously increased molecular weight of
copolymers with respect to MA homopolymerization.
Besides, bulky steric hindrance of the ligand leads to a great
barrier of the interaction between MA and Cu(I) complex,
thus decreases the concentration of the initial radical initia-
tor. And bulky steric hindrance of the ligand can also stabi-
lize copper metal radical, and prohibit chain termination
between copper metal radical and macromolecules radical.
These can explain why the ligand plays a very important
role on the activity and molecular weight of the obtained
In addition, it was found that b-ketoamino ligands and CuBr
can in situ polymerize MA in the presence of MAO with simi-
lar activity (conditions: 20 lmol CuBr, 20 lmol L2, 200
equiv MAO, 5 mL MA, 5 h, toluene, 20 mL). This result
clearly indicates that Cu(I) is true active species. Therefore,
it is deduced that MAO as a reducing agent reduces cop-
per(II) to copper(I). Besides, our experimental results of
influence of Al/Ni ratio and previous reports also show that
MAO can stabilize active species (e.g., MAO can remove radi-
cal traps),25 and may promote polymerization rate and
increase activity as Lewis acid.42
polymers. For copper(II)/MAO system,
a
reasonable
A possible insertion-triggered mechanism that reconciles our
observations was proposed. Mixing MAO and copper(II) com-
plex leads to a copper(I) complex by the reduction of the
precursor, which is consistent with our and other group’s
result determined by EPR.10,22 The next step involves the
interaction of MA and copper(I) complex, thereby producing
a copper(I)–carbon bond. Then the homolysis of this cop-
per(I)–carbon bond happens, thus, producing radical initia-
tor. The bond homolysis step is reversible and the concentra-
tion of radical is low at any given time. This explains our
inability to detect the radical by EPR. Similar pathway for
radical generation has been observed in cobalt-mediated rad-
ical polymerization45 and palladium-mediated radical poly-
merization.11,39,46–49 The actual polymerization occurs by
successive addition of acrylate monomer to the alkyl radical.
Chain termination majorly occurs by recombination with
copper metal radical and growing radical chain, and then
undergoing b-H elimination. Radical disproportionation and
bimolecular coupling terminations may also be termination
pathways. However, the stability of acrylate radicals and con-
trol of diffusion of macromolecule radical make these two
pathways less favorable than other radical species.47,50
explain for no activity toward other traditional radical
monomers, such as styrene, is that there is no interaction
between monomer and copper(I) complex or homolysis
cannot happen, thus no radical species can produce in the
system.
Radical mechanism of the palladium-based system for acry-
late monomer polymerization has been proved.46–49 Albeniz
and coworkers monitored the insertion of MA into the
PdAC6F5 bond and homolytic cleavage of the new PdAC
bond with NMR. Authors presented that homopolymerization
of MA and copolymerization of MA and 1-hexene proceed by
the insertion-triggered radical polymerization.48,49 Micro-
structures of PMA and MA-hexene copolymer produced by
palladium-based system11,39,46–49 are very similar to those
produced by bis(b-ketoamino)copper(II)/MAO system.
Therefore, it is believable that homopolymerization and
copolymerization of MA with 1-hexene proceed through
insertion-triggered radical mechanism. According to our pro-
posed mechanism, all of observations can be reasonably
explained, although a direct evidence for metal-carbon bond
homolysis is lack and hard to be determined.
1118
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ARTICLE
EXPERIMENTAL
0.146 mol), and catalytic amount of p-toluenesulfonic acid in
toluene(150 mL) were combined and heated to reflux 5 h,
whereas water was removed as a toluene azeotrope under
130 C using a water separator. The ligands L1 and L2 were
collected by vacuum distillation and recrystallization from
hexane.
All manipulations involving air- and moisture-sensitive com-
pounds were carried out under an atmosphere of dried and
purified nitrogen with standard vacuum-line, Schlenk, or
glovebox techniques.
ꢁ
Materials
L1, yield: 76%. Anal. Calc. For C13H17NO: C, 76.81; H, 8.43;
N, 6.89. Found: C, 76.54; H, 8.42; N, 6.93. FAB-MS (m/z):
204, 188, 160, 146, 105. 1H NMR (CDCl3, 500 MHz), d
(ppm): 11.9 (1H, ANH), 7.07–7.13(3H, AC6H3), 5.21(1H,
CH¼¼), 2.20(3H, ACH3), 2.11 (3H, ACH3), 1.63 (6H, A(CH3)2).
13C NMR (CDCl3, 125 MHz), d (ppm): 195.9, 162.8, 136.5,
136.1, 128.1, 127.4, 96.1, 29.0, 18.8, 18.2.
Toluene was dried over sodium metal and distilled under
nitrogen. 1-Hexene (Acros) was purified by distillation over
potassium. MA (Guangzhou Chemical) was dried over CaH2
and then freshly distilled in vacuum before use. MAO wꢁas
prepared by partial hydrolysis of TMA in toluene at 0–60 C
with Al2(SO4)3ꢀ18H2O as water source. The initial [H2O]/
[TMA] molar ratio was 1.3. Other commercially available
reagents were purchased and used without purification.
L2, yield: 65%. Anal. Calc. For C17H25NO: C, 78.72; H, 9.71;
N, 5.40. Found: C, 79.01; H, 9.60; N, 5.26. FAB-MS (m/z):
260, 244, 202. 1H NMR (CDCl3, 500 MHz), d (ppm):
12.06(1H, ANH), 7.16–7.31(3H, AC6H3), 5.21(1H, CH¼¼),
3.00–3.06(2H, ACH(CH3)2), 2.13(3H, CH3C(O)A), 1.64(3H,
ACH3), 1.22(6H, CH(CH3)2), 1.16(6H, CH(CH3)2). 13C NMR
(CDCl3, 125 MHz), d (ppm): 195.8, 163.3, 146.2, 133.5,
128.2, 123.5, 95.5, 29.0, 28.4, 24.5, 22.6, 19.1.
Polymerization
In a typical procedure, the appropriate MAO solid was intro-
duced into the round-bottom glass flask, and then the solu-
tion of monomer in toluene (1-hexene and/or MA) was
added via a syringe. Toluene and the solution of copper com-
plex were syringed into the well-stirred solution in order,
and the total reaction volume was kept at 20 mL. The poly-
merization reaction was continuously stirred for an appro-
priate period at the polymerization temperature, which was
controlled with an external oil bath. The polymerization was
terminated by the addition of 200 mL of acidic ethanol (95:5
ethanol/HCl). The resulting precipitated polymers were col-
lected and treated by filtration, washing with ethanol several
Synthesis of Copper Complexes
Ligand L1 (2.424 g, 11.92 mmol) was allowed to react with
Cu(OAc)2ꢀH2O (0.905 g, 4.55 mmol) in 150 mL methanol at
the refluxing temperature for 2 h. After cooling, the solid
was collected by removing solvent in vacuum and recrystal-
lized from the toluene/methanol solution to give copper
complex 1 as black crystals in 85% yield. The copper com-
plex 2 as brown crystals was prepared by the same proce-
dure in 89% yield.
ꢁ
times, and drying in vacuum at 40 C to a constant weight.
Characterization
Elemental analyses were performed on a Vario EL microana-
lyzer. Mass spectra were measured on a VG ZAB-HS instru-
ment using fast atom bombardment (FAB). 1H NMR and 13C
NMR spectra were carried out on a Varian Mercury-Plus 500
MHz instrument at room temperature in chloroform-d solu-
tions. GPC analyses of the molecular weight and MWD of the
polymers were performed on a Waters 150C instrument
with standard polystyrene as the reference and with THF as
Complex 1: Anal. Calc. For C26H32N2O2Cu: C, 66.71; H, 6.89;
N, 5.98. Found: C, 66.58; H, 7.15; N, 5.86. FTIR (KBr, cmꢂ1):
3418(w), 2911(w), 1575(s), 1514(s), 1404(s), 1279(w),
1186(w), 1020(w), 847(w), 762(m).
Complex 2: Anal. Calc. For C34H48N2O2Cu: C, 70.37; H, 8.34;
N, 4.83. Found: C, 70.01; H, 8.37; N, 4.82. FTIR (KBr, cmꢂ1):
3433(m), 2960(s), 2869(w), 1582(s), 1515(s), 1445(s),
1414(s), 1322(w), 1271(w), 1181(w), 1016(w), 940(w),
800(w), 765(m).
ꢁ
the eluent at 40 C. FTIR spectra were recorded on a Nicolet
Nexus 670 FTIR spectrometer. Analysis of hexene homopoly-
merization product was performed by GC-MS on a Finnigan
Voyager GC-8000 TOP gas chromatograph-mass spectrometer
with DB-5MS GC column.
CONCLUSIONS
We have discovered new bis(b-ketoamino)copper(II)/MAO
systems for the homopolymerization of MA and their copoly-
merization with 1-hexene. Copper complexes activated with
MAO can polymerize MA to produce the syndio-rich atactic
PMA, but cannot polymerize 1-hexene. With an increase in
the polymerization temperature, the activity and molecular
weight of the produced PMAs increased. The bulky steric
hindrance of the copper complex leads to low activity and
high molecular weight of the produced PMA. Copolymeriza-
tions of MA with 1-hexene with 1,2/MAO produce acrylate-
enriched copolymers (MA > 80%) with isolated hexenes in
the backbone. As the hexene feed content increases, both the
activity and the copolymer molecular weight decreases, but
the incorporation of hexene in the copolymer increases. The
Crystal Structure Determination
Crystal data obtained with the x-2y scan mode were col-
lected on a Bruker SMART 1000 CCD diffractometer with
graphite-monochromated Mo Ka radiation (k ¼ 0.71, 073 Å)
at 293 K. The structure was solved using direct methods,
whereas further refinements with full-matrix least squares
on F2 were obtained with the SHELXTL program package. All
nonhydrogen atoms were refined anisotropically. Hydrogen
atoms were introduced in calculated positions with the dis-
placement factors of the host carbon atoms.
Synthesis of Ligands
Acetylacetone (15.0 mL, 0.146 mol), 2,6-dimethylaniline
(18.0 mL, 0.146 mol) or 2,6-diisopropylaniline (27.5 mL,
BIS(b-KETOAMINO)COPPER(II)/MAO SYSTEMS, GAO ET AL.
1119

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
20 Bushuev, M. B.; Krivopalov, V. P.; Semikolenova, N. V.;
Peresypkina, E. V.; Virovets, A. V.; Sheludyakova, L. A.;
Lavrenova, L. G.; Zakharov, V. A.; Larionov, S. V. Russ J Coord
Chem 2006, 32, 208–213.
reactivity ratios r(MA) is 8.47, and r(hexene) is close to 0.
An insertion-triggered radical mechanism has been proposed,
and radical initiator is produced by homolysis of Cu(I)Acarbon
bond.
21 Larionov, S. V.; Savel’eva, Z. A.; Semikolenova, N. V.;
Klevtsova, R. F.; Glinskaya, L. A.; Boguslavskii, E. G.; Ikorskii, V.
N.; Zakharov, V. A.; Popov, S. A.; Tkachev, A. V. Russ J Coord
Chem 2007, 33, 436–448.
The financial supports by NSFC (Projects 20604034,
20974125, 20674097, and 20734004), the Science Foundation
of Guangdong Province (Project 8251027501000018), and
Ministry of Education of China (Foundation for Ph.D. Training)
are gratefully acknowledged.
22 Stibrany, R. T.; Schulz, N. D.; Kacker, S.; Patil, A. O.; Baugh,
L. S.; Rucker, S. P.; Zushma, S.; Berluche, E.; Sissano, J. A.
Macromolecules 2003, 36, 8584–8586.
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˚
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ꢁ
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˚
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BIS(b-KETOAMINO)COPPER(II)/MAO SYSTEMS, GAO ET AL.
1121