Crystalline Isotactic Polar Polypropylene from the Palladium-Catalyzed Copolymerization of Propylene and Polar Monomers

Article abstract of DOI:10.1002/anie.201600819

Moderately isospecific homopolymerization of propylene and the copolymerization of propylene and polar monomers have been achieved with palladium complexes bearing a phosphine-sulfonate ligand. Optimization of substituents on the phosphorus atom of the ligand revealed that the presence of bulky alkyl groups (e.g. menthyl) is crucial for the generation of high-molecular-weight polypropylenes (M_w≈10⁴), and the substituent at the ortho-position relative to the sulfonate group influences the molecular weight and isotactic regularity of the obtained polypropylenes. Statistical analysis suggested that the introduction of substituents at the ortho-position relative to the sulfonate group favors enantiomorphic site control over chain end control in the chain propagation step. The triad isotacticity could be increased to mm=0.55–0.59, with formation of crystalline polar polypropylenes, as supported by the presence of melting points and sharp peaks in the corresponding X-ray diffraction patterns.

Full text of DOI:10.1002/anie.201600819

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201600819
German Edition: DOI: 10.1002/ange.201600819
Polymerization
Crystalline Isotactic Polar Polypropylene from the Palladium-
Catalyzed Copolymerization of Propylene and Polar Monomers
Yusuke Ota, Shingo Ito, Minoru Kobayashi, Shinichi Kitade, Kazuya Sakata, Takao Tayano, and
Kyoko Nozaki*
Abstract: Moderately isospecific homopolymerization of
propylene and the copolymerization of propylene and polar
monomers have been achieved with palladium complexes
bearing a phosphine-sulfonate ligand. Optimization of sub-
stituents on the phosphorus atom of the ligand revealed that the
presence of bulky alkyl groups (e.g. menthyl) is crucial for the
generation of high-molecular-weight polypropylenes (Mw
ꢀ 104), and the substituent at the ortho-position relative to the
sulfonate group influences the molecular weight and isotactic
regularity of the obtained polypropylenes. Statistical analysis
suggested that the introduction of substituents at the ortho-
position relative to the sulfonate group favors enantiomorphic
site control over chain end control in the chain propagation
step. The triad isotacticity could be increased to mm = 0.55–
0.59, with formation of crystalline polar polypropylenes, as
supported by the presence of melting points and sharp peaks in
the corresponding X-ray diffraction patterns.
fins.[7] Despite the substantial progress in this area, most
catalytic systems are still restricted to the copolymerization of
ethylene and polar monomers, and examples of the copoly-
merization of propylene and polar monomers remain scarce.
The first copolymerization of propylene and a polar mono-
mer, methyl acrylate, was reported by Brookhart and co-
workers, who used palladium/a-diimine catalysts.[8] However,
the obtained copolymer contained significant amounts of
“chain-straightening” units, generated by a formal 3,1-inser-
tion of propylene, that is, a 2,1-insertion of propylene
followed by b-hydride elimination and a reinsertion in the
opposite direction. We have recently disclosed a novel
palladium/imidazo[1,5-a]quinolin-9-olate-3-ylidene
(Pd/
IzQO) catalyst system,[9] which represents the first general
method for the polymerization of propylene and the copoly-
merization of propylene with various polar monomers in the
absence of any formal 3,1-insertion of propylene. However,
the obtained polypropylenes exhibited an atactic stereoregu-
larity, which may render this catalytic system less attractive. In
the present study, we accomplished the moderately isospecific
homopolymerization of propylene and the copolymerization
of propylene with polar monomers through the use of
palladium/phosphine-sulfonate catalysts. The incorporation
of bulky menthyl substituents,[10] derived from (À)-menthol,
into the catalysts generates polypropylenes with high molec-
ular weights and high levels of regioregularity (Scheme 1).
Furthermore, the introduction of substituents at the ortho-
position relative to the sulfonate group favors enantiomorph-
ic site control over chain end control in the chain propagation
step, thereby leading to crystalline isotactic polar polyprop-
ylene (iPPP) with high triad ratios (mm ꢁ 0.59).
S
ince the discovery of chromium[1] and titanium[2] catalysts
for propylene polymerization, polypropylene has been one of
the most widely used commodity plastics in the world.[3] The
most important parameter to control the macroscopic proper-
ties of the resultant polypropylene is its stereoregularity. For
example, highly isotactic crystalline polypropylene is used as
a rigid material and exhibits high toughness.[3] Polypropylene
with low levels of isotacticity shows good elastomeric proper-
ties,[4] while syndiotactic polypropylene displays other unique
properties, such as high impact strength and optical clarity.[5]
Therefore, control over regio- and stereoregularity is crucial
in any type of propylene polymerization. Accordingly, it is
hardly surprising that substantial efforts have been devoted to
the understanding and control of mechanistic details in the
metal-catalyzed polymerization of propylene.[3a,6]
In addition to the homopolymerization of ethylene or
propylene, the late transition metal catalyzed copolymeriza-
tion of olefins and polar monomers has recently emerged as
a powerful method for the synthesis of functional polyole-
[*] Y. Ota, Dr. S. Ito, Prof. K. Nozaki
Department of Chemistry and Biotechnology
Graduate School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 (Japan)
E-mail: nozaki@chembio.t.u-tokyo.co.jp
Dr. M. Kobayashi, Dr. S. Kitade, K. Sakata, T. Tayano
Japan Polychem Corporation
1 Toho-cho, Yokkaichi, Mie, 510-0848 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201600819.
Scheme 1. Copolymerization of propylene and polar monomers with
palladium/phosphine-sulfonate catalysts.
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Table 1: Propylene polymerization using palladium/phosphine-sulfonate catalysts.[a]
[g]
Entry Catalyst Yield
[g][b]
Activity
Mn
Mw/
Mn
[mm][d] [mr][d] [rr][d] 4[mm][rr] 2[rr]/ regiodefects
Tg[f]
Tm
DH[h]
[c]
[gmmolÀ1 hÀ1
]
[kgmolÀ1 [c]
]
/[mr]2
[mr] [%][e]
[8C]) [8C]
[JgÀ1
]
1
1a
1b
1c
1d
1e
0.03
0.41
0.48
1.81
2.38
0.1
1.7
2.0
7.5
9.9
0.7
0.6
1.0
14
1.2
1.2
1.8
2.1
2.0
0.23
0.12
0.27
0.49
0.57
0.47. 0.30 1.25
1.28 nd
1.74 nd
1.04 nd
0.55 0.8
0.69 0.9
nd
nd
nd
nd
nd
nd
–
nd
nd
nd
–
2
0.47 0.41 0.89
0.48 0.25 1.17
0.40 0.11 1.35
0.32 0.11 2.45
3[i]
4
À10.2
5
24
À9.3 43.5,
11.6
62.0
6
7
8
1 f
1g
1h
1.83
1.41
1.47
7.6
5.9
6.1
20
21
7.0
2.0
1.9
2.2
0.55
0.59
0.51
0.33 0.12 2.42
0.29 0.12 3.37
0.37 0.12 1.79
0.73 1.3
0.83 0.9
0.65 1.3
À9.9 44.4,
61.6
3.7
17.8
nd
À9.6 46.6,
63.9
nd
nd
[a] A mixture of catalyst 1 (0.020 mmol) and propylene (6.0 g) in toluene (10 mL) was stirred for 12 h at 508C in a 50 mL stainless-steel autoclave; nd:
not determined. [b] Yields of isolated products after passing the reaction mixture through a short pad of silica gel (entries 1–3) or after evaporation of
solvents and reprecipitation from methanol (entries 4–8). [c] Molecular weights determined by size-exclusion chromatography analysis using
polystyrene standards and corrected by universal calibration. [d] Triad ratios determined by 13C NMR analysis. [e] Regiodefects determined by
quantitative 13C NMR analysis. [f] Glass transition temperatures determined by DSC analysis. [g] Melting temperatures determined by DSC analysis.
[h] Heats of fusion determined by DSC analysis. [i] As a result of the strong overlap of signals, triad ratios are only approximate.
Initially, we focused on the homopolymerization of
propylene with palladium/phosphine-sulfonate catalysts 1a–
1d to investigate the effect of the phosphorus-bound sub-
stituents on the catalytic performance (Table 1). Palladium
catalyst 1a, widely used in the copolymerization of ethylene
with polar monomers,[7d] yielded merely oligopropylenes
(entry 1). As a consequence of the low molecular weight, it
was difficult to accurately determine their tacticity and
regiodefects,[11] but the triad ratio was consistent with an
almost atactic stereoregularity (Figure S19). Catalyst 1b,
bearing bulky biaryl groups on the phosphorus atom,[12] also
furnished oligomers, and an increase in the molecular weight
could not be observed (entry 2). Replacing the substituents
with alkyl groups (1c)[13] led to a slightly increased catalytic
activity and molecular weight, although no improvement in
terms of tacticity was observed (entry 3, Figure 1a). In
contrast, the use of catalyst 1d with bulky menthyl groups[10]
dramatically improved the catalytic performance[14] and
afforded high-molecular-weight polypropylene (Mn > 104;
entry 4).[15] Analysis of the obtained polymer by 13C NMR
spectroscopy revealed that the control over regioregularity
was almost absolute.[16,17] According to our preliminary
theoretical calculations,[18] the high regioregularity can be
attributed to the selective 1,2-insertion of propylene. It is
worth noting that other, previously reported, propylene
polymerization catalysts based on Group 10 metals, for
example, nickel complexes,[19,20] require low temperatures
(< À608C) to achieve control over the regioregularity.
Moreover, the stereoregularity of the polypropylene obtained
with 1d (entry 4) was much improved (mm = 0.49; Figure 1b)
relative to that obtained with 1a or 1c (entries 1 or 3).
We carried out further optimizations on the phosphine-
sulfonate ligand to increase the molecular weight as well as
the stereoregularity of the resultant polypropylene. Previous
studies on the substituent effects on the phenylene linker in
phosphine-sulfonate ligands have shown that the substituent
at the ortho-position of the sulfonate group has a strong
influence on the catalytic activity and the molecular weight of
the polymer.[21] Encouraged by these reports, we synthesized
1e–1h[22] bearing various substituents at the ortho-position
relative to the sulfonate group (Table 1, entries 5–8). When
catalysts 1e and 1 f, which contain a trimethylsilyl (entry 5) or
a diphenylmethylsilyl (entry 6) group, were used, the molec-
ular weight and mm triad ratio increased relative to that with
1d (entry 4). The highest triad ratio (mm = 0.59) was
observed with 1g bearing a phenyl group (entry 7, Figure 1c),
even though the molecular weight and the catalytic activity
was lower compared to that with 1e (entry 5). Replacing the
phenyl group with a 1,2,3,4,5-pentafluorophenyl group (1h)
resulted in a significant decrease in the molecular weight and
mm triad ratio (entry 8). The favorable effects of the
substituent at the ortho-position relative to the sulfonate
group could be explained by control over the orientation of
the -SO2- moiety.[23,24]
As the triad ratios were affected by the substituent at the
ortho-position of the sulfonate group (Table 1), we wanted to
ascertain if the tacticity was determined by chain end control
or by enantiomorphic site control. A common method to do
Figure 1. 13C NMR spectra (d=18–23 ppm; -(CH(CH3)CH2)n-) of poly-
propylenes obtained with a) 1c (entry 3; Table 1), b) 1d (entry 4;
Table 1), and c) 1g (entry 7; Table 1).
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this is the Bernoullian test: if the value of 4[mm][rr]/[mr]2
presence of methyl acrylate, a propylene/methyl acrylate
approaches 1, the stereoregularity is determined by chain end
control.[25] In addition, another complementary triad test was
performed: If the stereoregularity is determined by enantio-
morphic site control, the value for 2[rr]/[mr] should approach
1.[26] In the case of catalysts 1e–1h (entries 5–8), 4[mm]-
[rr]/[mr]2 values of 1.79–3.37 were calculated, and these are
substantially larger than that obtained with 1d (entry 4; 1.35).
Conversely, the corresponding 2[rr]/[mr] values (0.65–0.83)
were much closer to 1, compared to that with 1d (0.55). These
results suggest that the introduction of substituents at the
ortho-position relative to the sulfonate group enhances
enantiomorphic site control rather than chain end control.[27]
We also carried out copolymerizations of propylene and
various polar monomers to synthesize iPPP (Table 2). For
these copolymerizations, 1e was used as the catalyst, as it
showed the highest catalytic activity for the homopolymeri-
zation of propylene and furnished polypropylene with the
highest molecular weight. The quantities of polar monomers
in the feed were adjusted according to their copolymeriz-
ability to achieve approximately 1% incorporation of the
polar monomers (entries 1, 4, 7, and 8). For example, allyl
acetate (2a) was successfully incorporated into polypropylene
(entries 1–3) with molecular weights of the copolymers (Mn =
7.2–8.9 ” 103 gmolÀ1; entries 1 and 2) comparable to that
obtained using the Pd/IzQO catalyst system (Mn = 11 ”
103 gmolÀ1).[9] Increasing the amount of allyl acetate twofold
resulted in the incorporation ratio also increasing by a factor
of about 2, although the molecular weight and catalytic
activity decreased (cf. entries 1 and 3). Copolymerization with
allyl cyanide (2b) furnished the corresponding copolymers
containing cyano groups (entries 4–6). The catalytic activity
was similar to that of allyl acetate, while the molecular weight
of the copolymer slightly decreased (cf. entries 1 and 4).
Copolymerization with allyl chloride (2c) provided the
corresponding halogenated polypropylene (entry 7). In the
copolymer was obtained (entry 8). When the amount of
methyl acrylate was increased twofold (entry 9), the incorpo-
ration ratio was again increased by a factor of about 2 (cf. with
entry 8).
The structure of the propylene/polar monomer copoly-
mers was examined by NMR analysis. All the obtained
copolymers consisted of regioregular polypropylene back-
bones with approximately 1% 1,2-/2,1-regiodefects, and with
the polar monomers incorporated into the polymer main
chain. The stereoregularity was characterized by triad values
of mm = 0.55–0.57,[28] when 1e and 1g were used as catalysts
(Table 2). The mm values are almost identical to those of the
corresponding propylene homopolymers (Table 1). All the
propylene/allyl monomer copolymers exhibited a polymer
structure that is consistent with the insertion of allyl mono-
mers 2a–2c with the same regioselectivity as that of
propylene (Figure 2), which suggests that 2a–2c underwent
exclusive 1,2-insertion. Conversely, the NMR spectra of the
propylene/methyl acrylate copolymer indicated that methyl
acrylate underwent a 2,1-insertion.[29]
Figure 2. 13C NMR spectrum of the propylene/allyl cyanide copolymer.
Table 2: Copolymerization of propylene and polar vinyl or allyl monomers using 1e and 1g.[a]
[h]
Entry Co-mono-
mer
Time Monomer Yield Activity
Mn
Mw/ Incorp. [mm][e] Regiodefects Tg[g]
Tm
[8C]
DH[i]
[c]
[h]
[mL]
[g][b] [gmmolÀ1 hÀ1
]
[kgmolÀ1 [c]
]
Mn
[%][d]
[%][f]
[8C]
[JgÀ1
]
1
2a
(CH2OAc)
12
0.5
0.18 0.75
8.9
2.7
1.7
0.55
1.0
À11.7 43.4,
6.7
60.4
2
3
4
5
2a
2a
48
12
0.5
1.0
1.0
1.0
0.22 0.22
0.09 0.38
0.19 0.79
0.49 0.51
7.2
5.8
7.5
10
3.4
2.6
1.9
2.4
1.7
2.9
1.6
1.3
0.56
0.55
0.55
0.57
1.1
1.3
1.1
1.0
nd
nd
nd
nd
nd
nd
nd
nd
nd
10.1
2b (CH2CN) 12
2b
48
48
12
À10.0 45.4,
61.8
6[j]
7
2b
1.0
0.19 0.20
0.14 0.58
0.20 0.83
0.14 0.58
6.3
11
2.3
2.2
2.2
1.9
1.4
1.7
1.6
3.3
0.57
0.56
0.56
0.55
1.0
1.1
0.9
1.3
À12.0 43.1,
62.1
À10.0 42.9,
15.5
11.0
11.4
nd
2c (CH2Cl)
0.25
0.25
0.5
61.7
8
2d (CO2Me) 12
2d 12
4.6
2.9
À14.3 43.3,
61.8
nd
9
nd
[a] A mixture of catalyst 1e (0.020 mmol), propylene (6.0 g), and monomers 2a–d in toluene (10 mL) were stirred at 508C in a 50 mL stainless-steel
autoclave; nd: not determined. [b] Yields of isolated products after passing the reaction mixture through a short pad of silica gel. [c] Molecular weights
determined by size-exclusion chromatography using polystyrene standards and corrected by universal calibration. [d] Molar incorporation ratios of
polar monomers determined by 1H NMR analysis. [e] Triad ratios determined by 13C NMR analysis. [f] Regiodefects determined by quantitative
13C NMR analysis. [g] Glass transition temperatures determined by DSC analysis. [h] Melting temperatures determined by DSC analysis. [i] Heats of
fusion determined by DSC analysis. [j] Copolymerization performed using 1g.
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Finally, an investigation of the physical properties of the
Acknowledgements
obtained (co)polymers revealed that they are crystalline.
Differential scanning calorimetry (DSC) analysis of the
polypropylene obtained with 1d (mm = 0.49; entry 4 in
Table 1) showed a glass-transition temperature (Tg) at
À108C, but no melting endotherm. Conversely, the polyprop-
ylene obtained with 1g (mm = 0.59; entry 7 in Table 1), which
exhibited the highest triad ratio, revealed a Tg value of À108C
and two melting endotherms at Tm = 47 and 648C (Fig-
ure S52). These are features that are typically observed for
low-molecular-weight and/or moderately isotactic polypropy-
lenes.[30] An X-ray diffraction analysis (Cu-Ka; l = 0.1541 nm)
was subsequently conducted to confirm the origin of the
endotherms. The diffraction pattern displayed broad signals
with relatively sharp peaks at 2q = 14.1, 16.7, and 21.18
(Figure 3, top). These signals could be assigned to the (111),
(008), and (202) + (026) reflections of the g-crystal form.[31]
The peak shape suggested that the obtained polypropylene
crystallized predominantly in the g-crystal form with low
crystallinity. The degree of crystallinity was estimated to be
about 12% on the basis of a comparison of the observed heat
of fusion (DH = 18 JgÀ1) with literature values (145–
150 JgÀ1).[32] The propylene/allyl cyanide copolymer (entry 6
in Table 2) showed similar Tg (À128C) and Tm (43/628C)
values (Figure S109), as well as similar X-ray diffraction
signals (Figure 3, bottom).[33]
In summary, we accomplished the palladium-catalyzed
regio- and stereocontrolled homopolymerization of propyl-
ene and the copolymerization of propylene and polar mono-
mers. The use of menthyl-substituted phosphine-sulfonate
ligands is crucial for the formation of polypropylenes with
high molecular weight. The introduction of substituents at the
ortho-position relative to the sulfonate group increased the
molecular weight of the obtained polypropylene, and favored
enantiomorphic site control over chain end control in the
chain propagation step. Under optimum conditions, various
functional groups such as acetoxy, cyano, chloro, and alkox-
ycarbonyl moieties could be incorporated at 1–3 mol% into
the polymer main chain to afford moderately crystalline iPPP.
This work was supported by CREST of JST and the “Nano-
technology Platform” (project No.12024046) of MEXT,
Japan. The theoretical calculations were performed using
computational resources provided by the Research Center for
Computational Science, National Institutes of Natural Scien-
ces, Okazaki, Japan. Y.O. thanks the Japan Society for the
Promotion of Science (JSPS) for their Program for Leading
Graduate Schools (MERIT), as well as for a Research
Fellowship for Young Scientists. S.I. is grateful for financial
support from the TonenGeneral Sekiyu Foundation.
Keywords: homogeneous catalysis · isospecificity · palladium ·
polymerization · propylene
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7505–7509
Angew. Chem. 2016, 128, 7631–7635
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those of linear ethylene/a-olefin copolymers and isotactic
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inherently low crystallinity of the polypropylenes obtained in
this study.
Received: January 24, 2016
Revised: April 10, 2016
Published online: May 10, 2016
Angew. Chem. Int. Ed. 2016, 55, 7505 –7509
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 7509