Highly 2,3-Selective Polymerization of Phenylallene and Its Derivatives with Rare-Earth Metal Catalysts: From Amorphous to Crystalline Products

Article abstract of DOI:10.1002/anie.201707601

Rare-earth metal complexes (Flu-CH₂-Py)Ln(CH₂SiMe₃)₂(THF)_n (Ln=Sc(1), Lu(2), Tm(3), Y(4) and Gd(5)), upon the activation of [Ph₃C][B(C₆F₅)₄] and AlⁱBu₃, were employed to catalyze the polymerization of allene derivatives under mild conditions. The Gd, Y, Tm, Lu metal based precursors exhibited distinguished 2,3-selectivity (>99.9 %) for phenylallene (PA) polymerization, whereas the smallest Sc metal based precursor showed a moderate 2,3-selectivity. The activity increased with the central metal size following the trend of Gd(5)>Tm(4)>Y(3)>Lu(2)>Sc(1). Moreover, Gd(5) also realized the purely 2,3-selective polymerizations of polar or nonpolar allene derivatives, para-methylphenylallene, para-flourophenylallene and para-methoxyphenylallene, regardless of electron-donating or -withdrawing substituents. Owing to the highly regular backbones, these polymers (except PPA) were crystalline, thus being the first crystalline polymers based on allene derivatives.

Full text of DOI:10.1002/anie.201707601

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Accepted Article
Title: Highly 2,3-Selective Polymerization of Phenylallene and
Derivatives with Rare-Earth Metal Catalyst: from Amorphous to
Crystalline Products
Authors: Fei Lin, Zhaohe Liu, Tiantian Wang, and Dongmei Cui
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707601
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COMMUNICATION
Highly 2,3-Selective Polymerization of Phenylallene and Its
Derivatives with Rare-Earth Metal Catalysts: from Amorphous to
Crystalline Products
Fei Lin, Zhaohe Liu, Tiantian Wang and Dongmei Cui*
Abstract:
Rare-earth
SiMe (THF)
metal
complexes
(Flu-CH
2
-
shown their power on selective and controlled coordination
polymerizations of a wide range of monomers such as dienes,
alfa-olefins, styrenes, cyclic esters, and even functional
monomers,[6] thus, the highly selective and efficient
polymerization of allene derivatives using rare-earth metal
catalysts is a challenging but a promising project.
Py)Ln(CH
2
3
)
2
n
(Ln = Sc(1), Lu(2), Tm(3), Y(4) and Gd(5)),
i
upon the activation of [Ph
3
C][B(C
6
F
)
5 4
] and AlBu
3
, were firstly
employed to catalyze the polymerization of allene derivatives under
mild conditions. The Gd, Y, Tm, Lu metal based precursors exhibited
distinguished 2,3-selectivity (>99.9%) for phenylallene (PA)
polymerization, whereas the smallest Sc metal based precursor
showed a moderate 2,3-selectivity. The activity increased with the
central
metal
size
following
the
trend
of
Gd(5)>Tm(4)>Y(3)>Lu(2)>Sc(1). Moreover, Gd(5) also realized
unprecedentedly the purely 2,3-selective polymerizations of polar or
nonpolar allene derivatives, para-methylphenylallene, para-
flourophenylallene and para-methoxyphenylallene, regardless of the
electron donating or withdrawing substituents. Owing to the highly
regular backbones, these polymers except PPA were crystalline,
which represent the first crystalline allene derivatives based
polymers.
Chart 1. Possible Microstructures of Polymers Based on Allene Derivatives.
On other hand, whether the newly developed polymers
have strongly potential application depends on their unique
properties, of which the basic and mostly important physical and
mechanical properties are mainly determined by crystallinity as
well as the crystal structures.[7] More than 80% commodity
polymers such as polyethylene and isotactic polypropylene are
crystallizable.[8] Unfortunately, almost all the previously reported
polyallenes are amorphous owing to their low regularity and low
molecular weights except poly(allene),[9] because their polymer
chains are inherently regular and highly symmetric.
To create new materials arising from new monomer
resources has been a long-standing target of polymer science.
Allene compounds have been recognized as highly reactive and
versatile building blocks in organic synthesis, enabling
numerous efficient transformations to access new organic
compounds.[
1]
Therefore, polymers arising from allene
compounds have been significantly attractive materials, since
they contain functional methylene double bonds attached to the
polymer backbone, which, for instance, arrange in high order to
render the materials unique chiral properties.[2] However,
investigating the polymerization of allene compounds has not
progressed smoothly, because the cumulative double bonds in
these compounds are highly active and multi-selective in the
Herein, we report unprecedentedly the purely 2,3-selective
coordination polymerizations of phenylallene and its polar or
nonpolar derivatives by using rare-earth metal based precursors
1-5 (Chart 2). Owing to the highly ordered microstructures, the
first crystalline allene derivative based polymers have been
achieved.
polymerization
process
prone
to
give
uncontrolled
microstructures (Chart 1) or even insoluble products. Radical
and ionic polymerizations of allene compounds could only
accomplish low conversions without selectivity and afford
[
3]
products with molecular weights smaller than 3000 Da. With
respect to the coordination polymerization, a powerful manner to
provide regio- and stereoselectivity, indeed, more controllable
polymerizations of allene derivatives bearing alkoxyl, amido,
lactam, fluorescent, or even sugar etc. electron-donating
functional groups have been achieved, whereas the catalysts
are limited to a few Ni[2b, 4] and Ti[5] complexes of moderate
activities. To date no other group metals based catalytic system
has been reported. Recently, rare-earth metal complexes have
Chart 2. Precursors Used in This Study.
The Sc precursor 1, which was reported previously to have
the highest activity and excellent selectivity in styrene (St)
polymerization,[6a] was firstly attempted to catalyze phenylallene
[
a]
b]
F. Lin, Z. Liu, T. Wang and Prof. D. Cui
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, China
E-mail: dmcui@ciac.ac.cn
F. Lin, Z. Liu and T. Wang
(
PA) polymerization. Delightedly, upon activation of
i
[
Ph
3
C][B(C
6
5
F )
4
] and AlBu
3
, Sc(1) could transfer 500 equiv. PA
to 94.1% conversion in 10 h (Table 1, entry 1) with 2,3-selectivity
of 62.0%. Switching to a slightly larger Lu based complex 2,
under the same conditions, the PA conversion reached to 98.3%,
[
University of Chinese Academy of Sciences, Changchun Branch
Changchun 130022, China
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Angewandte Chemie International Edition
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COMMUNICATION
and Gd(5) 89.75(8)°),[6a,6f] facilitates to accommodate bulky PA
coordination-insertion, and therefore, provides higher activity
Table 1. Polymerization of PA catalyzed by precursors 1-5.
Entry[a]
2, 3[b]
(%)
[c]
[c]
[d]
and 2,3-selectivity as compared with the reported [(π-
allyl)NiOCOCF ] /PPh catalytic system.
3 2 3
Complex
t
h)
Conv.
(%)
M
n
M /M
w n
T
g
-
4
o
[4a]
(
(×10 )
( C)
Intrigued by the high acitivity and distinguished 2,3-
selectivity of 5(Gd) for PA polymerization, we employed it to
investigate the polymerizations of the substituted allenes bearing
the electron donating and withdrawing functional groups. When
using an electron donating monomer p-methylphenylallene
1
2
3
4
5
S
C
(1)
10
10
6
94.1
98.3
100
100
100
62.0
2.1
2.2
2.6
2.5
1.59
1.64
1.66
1.65
1.74
74.9
61.8
61.6
61.3
61.5
Lu(2)
Tm(3)
Y(4)
>99.9
>99.9
>99.9
>99.9
(
pMPA), even under a low monomer concentration (toluene :
4
pMPA = 3:1), the polymerization could reach completeness in 1
h (Table 2, entry 1). Switching to PA bearing an electron-
withdrawing halogen group, by contrast, the para-chlorine and
para-bromine substituted monomers p-chlorophenylallene
Gd(5)
2
2.6
i
[
a] Polymerization conditions: Ln 10 μmol, [PA]/[Ln]/[Al Bu
3
3
]/[Ph C][B(C
F
6 5
4
) ] =
5
00/1/10/1 (mol/mol), toluene/PA = 1/1(v/v), T
at 20 oC. [c] Determined by GPC in THF at 40 o
against polystyrene standard. [d] Determined by DSC.
p
= 20°C. [b] Measured by 1
H
(
pClPA) and p-bromophenylallene (pBrPA) could not be
NMR and 13C NMR in CDCl
C
3
polymerized (Table 2, entries 2 and 3). Surprisingly, the para
fluorine substituted monomer p-fluorophenylallene (pFPA) could
be polymerized with a satisfactory 77% conversion of 1000
and strikingly, an over 99.9% 2,3-regulated PPA with moderate
molecular weight (M
distribution (M /M
equiv. pFPA by prolonging the polymerization time to 8 h, and a
4
n
= 2.2 × 10 ) and narrow molecular weight
4
high molecular weight PpFPA (M
n
= 2.8×10 , M
/M
w n
= 1.58) was
w
n
= 1.64) was isolated (Table 1, entry 2).
generated (Table 2, entry 4). This represents the first successful
coordination polymerization of an allene monomer containing an
electron withdrawing substituent, in particular, the strong fluorine
atom. The different polymerization performance of the halogen
substituted allenes was attributed to the overall effect of halogen
groups on the electron density of the allene double bonds, the
balance between the electron-withdrawing and the back-feeding
of the lone electron pair. The electron-withdrawing effect
reduces the electron density of the allene cumulative double
bonds, which is not favored by a coordination-insertion catalysts,
whilst the back-feeding of the lone electron pair affects
oppositely. Fluorine at the para position is strong in both electron
withdrawing and lone-pair back-feeding, comparatively, bromine
and chlorine are weak in back-feeding. In all, the cumulative
double bonds in pFPA changed less as compared to PA, but
those in pClPA and pBrPA were electron deficient. Since, the
Employing much larger central metals based complexes Tm(3),
Y(4) and Gd(5), both pure 2,3-selectivity (>99.9%) and complete
conversion (100%) were achieved at shorter polymerization
times (Table 1, entries 3-5). To distinguish more clearly the
catalytic activities of these complexes, the PA polymerizations
were performed under a low monomer concentration and
quenched at a low conversion (Table S1). The results showed
that the activity following the trend of Sc(1) < Lu(2) < Tm(3) <
Y(4) < Gd(5) increased with the central rare-earth metal ionic
radius (Sc3+ 0.745 Å< Lu3+ 0.861 Å<Tm3+ 0.88 Å< Y3+ 0.901 Å<
Gd3+ 0.938 Å).[10] Obviously, Gd(5) was the most active complex
with the activity of 29 kg/(molLn•h). This catalytic trend is in
striking contrast to that for styrene polymerization, in which the
activity increases with the decrease of the central rare-earth
metal size,[6a] suggesting different polymerization mechanisms
between PA and St monomers. Different from the single olefinic
structure of styrene, phenylallene bears cumulative C=C bonds
that coordinate to the central metal more easily but more
sterically demanding. The gradually more opening coordination
environment of these complexes from Sc(1) to Gd(5), which is
attributed to the smaller bite angle (Flucent-Ln-N: Sc(1)•THF
+
-
active metal alkyl species M R in the coordination-insertion
polymerization facilitates the electron-enriched double bond
coordination, therefore, the activity order of pMPA > PA >
pFPA> pClPA and pBrPA was reasonable. This could be further
confirmed by the polymerization of an electron-donating p-
methoxyphenylallene (pMOPA) that preformed faster than
96.29(12)°, Lu(2) 92.23(7)°, Tm(3) 91.71(10)°, Y(4) 92.01(9)°
Table 2. Polymerization of substituted PA catalyzed by precursor 5(Gd).
Entry[a]
Monomer
pMPA
t(h)
1
Conv. (%)
2, 3[b] (%)
>99.9
-
M
n
[c]
(×10-4)
M
/M
[c]
T
g
[d](oC)
T
m
[d]
(oC)
∆H[d](J/g)
w
n
1
2
3
4
5
100
0
2.2
-
1.74
-
91.1
-
158.7/188.2/207.5
20.24
pBrPA
pClPA
pFPA
24
24
8
-
-
-
-
0
-
-
-
-
77
82
>99.9[e]
>99.9
2.8[f]
4.0
1.58
2.09
81.5
83.1
168.1/192.4/211.8
183.2/197.64
3.78
13.95
pMOPA
8
i
[
a] Polymerization conditions: Gd 20 μmol, [Gd]/PA/[Ph
3
C][B(C
6
F )
5 4
]/[AlBu
3
] = 1:1000:1:10 (mol/mol), toluene/monomer = 3/1 (v/v) for entry 1 and toluene/PA =
1
/1 (v/v) for entries 2-5, T
p
= 20°C. [b] Measured by 1H NMR and 13C NMR in CDCl
3
at 20 oC. [c] Determined by GPC in THF at 40 oC against polystyrene
standard. [d] Determined by DSC. [e] Measured by H NMR and 13C NMR in C
1
D Cl
at 20 oC. [f] Determined by GPC in 1,2,4-trichlorobenzene at 150 oC against
2
2 4
a polystyrene standard.
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pFPA to give high molecular weight PpMOPA (M
n
= 4.0×104)
(
Table 2, entry 5). However, the electron donating but polar
pMOPA polymerization was slower than PA, since the polar
methoxyl group had priority to coordinate to the active species
thus impeded the coordination of double bond.[11] Thereby, in the
polymerizations of PA and its derivatives, the coordination of the
monomers via its double bonds to the active metal center should
be the rate-determination step.
The microstructure of PPA obtained with complexes 2-5 is
perfect 2,3-regularity. As shown in Figure 1, the newly generated
methylene protons H
methine proton H shows a peak at δ = 6.36 ppm. The phenyl
protons Ph-H appear as multi peaks ranging from δ = 7.09 to
.18 ppm. The integral intensity ratio of the resonances for these
a
give resonance at δ = 2.97 ppm and the
b
7
phenyl, olefinic and aliphatic protons is 5:1:2, while the
Figure 3. 1H NMR spectrum of PpMOPA (Table 2, entry 5) (400 M Hz, CDCl
20 C).
methylene protons of 1,2-polyphenylallene at δ = 5.1 ppm isn’t
3
,
o
observed, indicating the pure 2,3-regularity (>99%)[4a],[12]
.
Correspondingly, carbons C
3
a b c
, C and C give sharp peaks at δ
9.45, 128.86 and 137.37 ppm in the 13C NMR spectrum,
respectively (Figure 2), which also indicated that aryl
substituents arrange in the isotactic fashion. Noted that, the 2.3-
regularity of PPA isolated from the scandium complex 1
Figure 4. 13C NMR spectrum of PpMOPA (Table 2, entry 5) (100 M Hz, CDCl
3
,
o
20 C).
decreased to 62.0%, meanwhile, this polymer has higher T
glass transition temperature, 74.9 °C) (Table 1, entry 1) than
,3 PPA (T = 61.5 °C) (Table 1, entry 5) obtained by using
complexes 2-5, which might be contributed to the higher internal
rotation barrier in the 1,2-PPA backbone.
g
(
2
g
Figure 1. 1H NMR spectrum of PPA (Table 1, entry 5) (500 M Hz, CDCl
3
,
o
20 C).
More delightedly, we found the methyl and even polar
fluorine or methoxy substituted groups, despite of their electronic
nature, affected the polymerization activity of the corresponding
monomers PpMPA, PpFPA and PpMOPA but didn’t disturb the
polymerization selectivity to afford the 2,3-regulated products.
As shown in Figures 3 and 4, the H and 13C NMR spectra of
1
polymers pMPA, pFPA and pMOPA show no resonances from
the 1,2-units, respectively.[
13]
Therefore, the highly selective
polymerizations of polar and nonpolar allene derivatives by
using the rare-earth metal catalytic systems have been realized
for the first time.
For any given polymers, the crystallization behavior is
determined mainly by the highly ordered polymer chains.[14] The
perfect 2,3-regularity of the polymers obtained from the above
allene derivatives directed us to explore their crystallinity.
Figures 5 and 6 are the differential scanning calorimetry (DSC)
thermograms and wide angle X-ray diffraction (WAXD)
Figure 2. 13C NMR spectrum of PPA (Table 1, entry 5) (125 M Hz, CDCl
3
,
o
20 C).
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COMMUNICATION
profiles[15] of PPA, PpMPA, PpFPA and PpMOPA. For PPA only
In conclusion, we have demonstrated that rare-earth metal
based precursors are the most powerful catalytic systems
towards the polymerizations of phenylallene and its derivatives.
The open spatial environment of the constrained-geometry-
o
a single T
g
at 61.5 C was observed and no diffraction peaks was
detected, suggesting that PPA is amorphous. While all the other
o
polymers have higher T
8
g
than PPA (T
g
= 91.1 C for PpMPA,
o
o
1.5 C for PpFPA and 83.1 C for PpMOPA) and show multiple
endothermic peaks within the range of 155-215 C, e.g. 158.7 C, (Ln = Gd, Y, Tm and Lu), and their large central metal sizes,
88.2 C and 207.5 C for PpMPA. The multiple melting points
2 2 3 2
configuration catalysts (CGC), (Flu-CH -Py)Ln(CH SiMe ) (THF)
o
o
o
o
1
facilitate the coordination of phenyl substituted cumulative
double bonds to give PPA with perfect 2,3-regularity (>99.9%).
Whilst the small Sc active species shows decrease in 2,3-
selectivity (62.0%) because of its crowded coordination sphere.
Further investigating the polymerization performances of Gd
precursor towards phenylallene derivatives bearing electron
are possibly arising from the various crystal morphologies or
different thicknesss of the lamella. Furthermore, PpMPA, PpFPA
and PpMOPA show sharp diffraction peaks, e.g. 2θ values of
1
4.7o, 16.8o and 26.7o for PpFPA, confirming their high
crystallinity. These polymers represent the first example of
crystalline polyallenes, meanwhile, have excellent thermal
donating (CH
3 3
and CH O) and withdrawing (F, Br and Cl)
o
stability (less than 5% weight loss at up to 350 C, Figures S8-
substituents, respectively, reveals that the electron donating
substituent accelerates polymerization via increasing the
electron density of the reaction double bond, on the contrary, the
electron withdrawing one reduces the activity. Moreover, the
system is tolerant to polar groups such as methoxy and fluorine,
although the activities of pMOPA and pFPA are lower than
pMPA and PA. Noted that the substituents affect the activity
significantly but slightly change the excellent 2,3-selectivity of
this catalytic system. Remarkably, owing to the highly ordered
microstructures of these obtained polymers, PpMPA, PpFPA
11).
PPA
PpMPA
exo
PpFPA
and PpMPPA are crystalline polymers with high melting points
o
(
T
m
> 200 C). We are currently making great efforts to realize
polymerization of more allene derivatives using these rare earth
catalysts.
PpMOPA
Acknowledgements
30
60
90
120
150
180
210
240
This work was supported by the NSF China for Projects Nos.
o
Temperature( C)
21634007, 21374112 and 21574125, and MST for “973” project
No. 2015CB654702.
Figure 5. DSC traces of polymers: PPA (Table 1, entry 1), PpMPA (Table 2,
entry 1), PpFPA (Table 2, entry 4) and PpMOPA (Table 2, entry 5).
Keywords: allene • rare earth catalyst • selective polymerization
•
polyallene • crystalline polymer
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PpMPA
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5
10
15
20
/deg
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2
Figure 6. WAXD profiles of polymers: PPA (Table 1, entry 1), PpMPA (Table 2,
entry 1), PpFPA (Table 2, entry 4) and PpMOPA (Table 2, entry 5). The
o
samples were crystallized isothermally at 150 C for 3h.
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Angewandte Chemie International Edition
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COMMUNICATION
e) Y. P. Pan, W. F. Rong, Z. B. Jian, D. M. Cui, Macromolecules 2012,
[11] D. T. Liu, C. G. Yao, R. Wang, M. Y. Wang, Z. C. Wang, C. J. Wu, F.
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[12] The small peaks around the sharp and strong proton resonances are
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[
9]
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& the Lanthanides: Inorganic &
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COMMUNICATION
COMMUNICATION
Rare-earth metal complexes (Flu-CH
2
-
Fei Lin, Zhaohe Liu, Tiantian Wang and
Dongmei Cui*
Py)Ln(CH SiMe (THF) (Ln = Sc, Lu,
2
3
)
2
n
Tm, Y and Gd), upon the activation of
i
[
Ph
3
C][B(C
6
5
F )
4
] and AlBu
3
, were
Page 1 – Page 5
firstly employed to catalyze the 2,3
selective polymerization of
Highy 2,3 Selective Polymerization of
Phenylallene and Its Derivatives with
Rare-Earth Metal Catalysts: from
Amorphous to Crystalline Products
phenylallene (PA) and its derivatives
bearing polar or nonpolar substituent
groups. Owing to the highly regular
backbones, these obtained polymers
except PPA were crystalline, which
represent the first crystalline allene
derivatives based polymers.
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