Angewandte
Chemie
Table 2: Ethylene/MA copolymerization.[a]
branching numbers well above 80.
Therefore, the low branching num-
bers in the 20s observed with com-
plexes 5–8 are extremely unusual.
Because of the high molecular
weight and low branching density,
the polyethylene generated by these
complexes is semicrystalline, with
a melting temperature approaching
1008C. Polyethylene with these
properties has rarely been synthe-
sized previously with palladium a-
diimine complexes.
[b]
[c]
[e]
Entry Cat. [MA]
T
Yield
Activity
XMA
[%]
Mn
PDI
B[d]
Tm
[8C]
[m]
[8C] [g]
[103 g(molPd)À1 hÀ1
]
(10À3
)
1
2
3
4
5
6
7
8
9
5
5
5
5
5
6
6
6
6
6
A
A
A
A
A
–
1
1
1
2
–
1
1
1
2
–
1
1
1
2
20
20
40
60
40
20
20
40
60
40
20
20
40
60
40
19.3
128
3.7
2.1
1.4
2.1
118
4.3
2.9
1.3
1.7
28
–
98.0
18.9
3.8
3.0
3.0
89.5
10.8
3.53
3.87
3.43
28
2.13
1.78
3.45
1.88
1.81
2.46
1.53
2.28
1.81
1.91
32 73
0.55
0.32
0.21
0.32
17.7
0.65
0.44
0.19
0.25
4.21
0.05
0.13
trace
0.03
0.4
1.1
1.7
1.8
–
0.5
1.6
2.9
3.3
–
34 71
36 61
50 51
40 52
31 75
35 72
42 60
43 52
46 53
10
11
3.66 105
–
–
–
–
–
As mentioned above, all previ- 12
0.33
0.87
–
3.2
3.3
–
3.1
2.7
–
2.5
1.51
1.76
–
93
93
–
13
14
15
ous modifications of complex A led
to reduced activity, lower polymer
molecular weight, and similar or
0.20
5.8
1.89 105
[a] Reaction conditions: precatalyst (0.010 mmol), NaBAF (1.2 equiv), total volume of toluene and MA:
25 mL, 1 atm, 15 h. [b] Amount of MA incorporated (mol%). [c] Molecular weight was determined by
GPC in trichlorobenzene at 1508C with polystyrene standards. [d] Number of branches per 1000 carbon
atoms, as determined by 1H NMR spectroscopy. Branches ending with a functional group were added to
the total number of branches. [e] Melting temperature was determined by differential scanning
calorimetry (DSC).
higher branching density. In this
sense, complexes 5–8 possess
unique and quite surprising proper-
ties in ethylene polymerization. As
compared to complex A, these
complexes
demonstrate
much
greater thermal stability, higher
activity by up to an order of magnitude, and the generation
of polymers with higher molecular weight by up to an order of
magnitude and lower branching density by a factor of
approximately 4. The property of greater thermal stability
can be understood readily. The diphenylmethyl groups block
the axial position more efficiently than the isopropyl groups in
complex A, thus slowing down the potential catalyst-decom-
position pathways.[5,6] This strategy has been successfully
applied previously, for example, in cyclophane-based palla-
dium(II) a-diimine complexes (Scheme 2, B) and the “sand-
wich” diimine palladium catalyst D, all of which showed
enhanced thermal stability. However, both B and D showed
lower activity and provided polyethylene with a lower
molecular weight. For example, the reduction in activity by
a factor of approximately 10 and reduction in polymerization
molecular weight by a factor of about 2 found with complex D
as compared to the classic complex A was attributed to the
significantly slower ethylene insertion rate by complex D.[16]
Also, the sterically highly bulky complexes B and D
generated polyethylene with branching numbers (ca. 110/
1000 C) higher than that observed with the classic complex A.
Currently, the opposite trend observed with complexes 5–8 is
not fully understood, and it clearly cannot be explained
simply on the basis of a steric effect. It is possible that the
unique structures of these complexes facilitate the ethylene-
trapping and -enchainment step, which therefore outcompete
the chain-transfer and chain-walking steps. As the catalyst
spends more time enchaining ethylene than chain walking
along the polymer chain, polymers with higher molecular
weight, higher activity, and lower branching density are
obtained.
order of magnitude higher, the copolymer molecular weight
was up to 6 times higher (see Figures S6 and S7 for
a comparison of the copolymer yield and molecular weight
with these catalysts), and the copolymer branching density
was about 3 times lower. Also, much greater thermal stability
was observed. Complexes 5 and 6 showed appreciable activity
at 608C, whereas complex A completely decomposes under
these conditions.[19] The MA-incorporation ratio is lower for
complexes 5 and 6 than for complex A. However, the
differences became smaller at higher temperatures. The low
MA-incorporation ratio probably originates from the steric
bulk of the ligands, which makes monomer binding more
unfavorable for MA. Because of the high molecular weight
and low branching density of these copolymers, a melting
temperature of 50–708C was observed. Again, such a high
melting temperature has not been observed previously for E–
MA copolymers synthesized with a palladium a-diimine
catalyst (the difference between the copolymers obtained
with complexes 5 and 6 (semicrystalline solid) and complex A
(sticky oil) can be clearly seen in Figure S8). Complexes 7 and
8 are not suitable for the copolymerization. Presumably, the
electron-withdrawing groups make the Pd center more
electrophilic and therefore more prone to catalyst poisoning
by the polar groups.
With the classic a-diimine palladium complex A and
complex D, hyperbranched (branch on branch) and even
dendritic polymers are produced, with randomly distributed
branches, including Me, Et, iPr, nBu, sBu, and long-chain
branches. Furthermore, the catalytic activity, molecular
weight of the polymer, branching density, and distribution
of short-chain branches are relatively independent of ethyl-
ene pressure.[2a,3a,b,16] As a result, it is very difficult to control
the polymerization process and the properties of the resulting
(co)polymer by the use of different polymerization condi-
tions. In contrast, the catalytic activity and polymer molecular
In the ethylene/MA copolymerization study (Table 2),
similar trends were observed to those for ethylene homopo-
lymerization. As compared to complex A under the same
conditions, the activity of complexes 5 and 6 was up to an
Angew. Chem. Int. Ed. 2015, 54, 9948 –9953
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