[Ph3C][B(C6F5)4]: A Highly Efficient Metal-Free Single-Component Initiator for the Helical-Sense-Selective Cationic Copolymerization of Chiral Aryl Isocyanides and Achiral Aryl Isocyanides

Article abstract of DOI:10.1002/anie.201803300

Commercially available [Ph₃C][B(C₆F₅)₄] served as a highly efficient metal-free and single-component initiator not only for the carbocationic polymerization of polar and bulky aryl isocyanides with extremely high activity up to 1.2×10⁷ g of polymer/(mol_cat. h), but also for the helical-sense-selective polymerization of chiral aryl isocyanides and copolymerization with achiral aryl isocyanides to afford high-molecular-weight functional poly(aryl isocyanide)s with good solubility as well as AIE characteristics and/or a single-handed helical conformation.

Full text of DOI:10.1002/anie.201803300

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: [Ph3C][B(C6F5)4]: Highly Efficient Metal-Free Single-Component
Initiator for Helical-Sense-Selective cationic Copolymerization of
Chiral Aryl Isocyanide and Achiral Aryl Isocyanide
Authors: Xinwen Yan, Shaowen Zhang, Pengfei Zhang, Xiaolu Wu, An
Liu, Ge Guo, Yuping Dong, and Xiaofang Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803300
Angew. Chem. 10.1002/ange.201803300
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10.1002/anie.201803300
Angewandte Chemie International Edition
COMMUNICATION
[Ph3C][B(C6F5)4]: Highly Efficient Metal-Free Single-Component
Initiator for Helical-Sense-Selective Cationic Copolymerization of
Chiral Aryl Isocyanide and Achiral Aryl Isocyanide
Xinwen Yan,[a] Shaowen Zhang,[a] Pengfei Zhang,[a] Xiaolu Wu,[a] An Liu,[a] Ge Guo,[a] Yuping Dong*[b]
and Xiaofang Li*[a]
Abstract: Commercially available [Ph3C][B(C6F5)4] serves as a
highly efficient, metal-free, and single-component initiator not only
for carbocationic polymerization of polar and bulky aryl isocyanides
with extremely high activity up to 1.2 × 107 g of polymer/(molcat.·h)
but also for helical-sense-selective polymerization of chiral aryl
isocyanides and copolymerization with achiral aryl isocyanide,
affording high molecular weight functional poly(aryl isocyanide)s with
good solubility, AIE nature, and/or one-handed helical conformation.
one example of protonic acid-coated ground glass can promote
the helical-sense-selective (co)polymerization of isocyanides.[7]
Up to now, no more metal-free catalyst is reported for the
polymerization of isocyanides as far as we know. Developing
highly efficient metal-free catalysts for the helical-sense-
selective (co)polymerization of isocyanides is still a critical
challenge.
Optically active poly(isocyanide) (PI) with single-handed helical
conformation[1] has attracted extensive attention for its excellent
properties, important functions of realizing biological activities,
and significant applications in many fields, such as molecular
recognition, photoconductivity, liquid crystal, asymmetric
synthesis and so on.[2] In general, the PIs are mainly
synthesized from the coordination-insertion polymerization of
isocyanides catalyzed by late transition metal catalysts based on
Rh, Pd, Ni and Co (Chart 1).[3] However, most of these catalysts
exhibit low to moderate activities for the (co)polymerization of
isocyanides (< 105 g of polymer/(molcat.·h)).[4] More importantly,
the issue of metal residue in the PIs significantly affects their
properties and restricts their applications as biomimetic
materials. For example, traces of transition metal residue can
serve as degradation catalysts and reduce the thermal stability
of PIs.[5] The high metal residue exceeding the standard greatly
limits the medical applications of PIs as well-defined micelles for
drug carrier and cell imaging materials in living cell.[6] Therefore,
developing metal-free catalysts with advantages of availability,
high efficiency, and no toxicity for the polymerization of
isocyanide is very essential. Some early efforts for the cationic
polymerization of isocyanides by using metal-free initiators such
as strong protonic acid-coated ground glass or Lewis acid BF3
were independently reported by Millich, Stackman, and
Yamamoto in 1960s (Chart 1).[7] In 2005, Cronelissen et al. also
Chart 1. Catalysts for the Polymerization of Isocyanides.
The metal-free fluorosubstituted borate compound (such as
[Ph3C][B(C6F5)4] and [PhNHMe2][B(C6F5)4]) and borane
compound B(C6F5)3 usually serve as activators or cocatalysts in
the coordination-insertion polymerization.[9] For example, we
have reported a series of cationic rare-earth metal alkyl species
in-situ generated from the rare-earth metal dialkyl complexes
and such activators as highly efficient catalysts for both the
highly regio-/stereoselective polymerization of conjugated dienes
and the helical-sense-selective polymerization of aryl
isocyanides.[10] Moreover, such compounds alone can also serve
as metal-free catalysts for the organic synthesis reaction.[11]
Despite these efforts, these compounds never serve as single-
component initiators for the carbocationic polymerization as far
as we known.
Herein, we present the commercially available borate
complex [Ph3C][B(C6F5)4] serves as highly efficient metal-free
single-component initiator for the carbocationic polymerization
and copolymerization of five functional aryl isocyanides
containing polar, bulky, and chiral groups with extremely high
activities, affording high molecular weight poly(aryl isocyanide)s
with good solubility, aggregation-induced emission (AIE) nature,
or one-handed helical conformation. Moreover, the optically
reported
trifluoroacetic
acid-initiated
polymerization
of
isocyanopeptides.[8] However, the activities of these metal-free
catalysts are very low (< 103 g of polymer/(molcat.·h)) and only
[a]
[b]
Dr.Xinwen Yan, Dr. Pengfei Zhang, Dr. Xiaolu Wu, Mr. An Liu, Ms.
Ge Guo, Prof. Shaowen Zhang, Prof. Xiaofang Li*
Key Laboratory of Cluster Science of Ministry of Education, School
of Chemistry and Chemical Engineering, Beijing Institute of
Technology. 5 South Zhongguancun Street, Haidian District, Beijing
100081,China
E-mail:xfli@bit.edu.cn
Prof. Yuping Dong,
Beijing Key Laboratory of Construction Tailorable Advanced
Functional Materials and Green Applications, School of Materials
Science and Engineering, Beijing Institute of Technology, Beijing
100081, China. E-mail: chdongyp@bit.edu.cn
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201803300
Angewandte Chemie International Edition
COMMUNICATION
active chiral isocyanide sequences in the copolymer chain can
induce the helical-sense-selective polymerization of achiral
isocyanide monomer to final produce the random copolymers
adopting a helical conformation with an excess screw sense in
different degrees and AIE nature (Chart 1).
Table 1. Homopolymerization of Aryl Isocyanides Catalyzed by Metal-Free
Borate and Borane Compounds[a]
Five aryl isocyanide monomers a–e containing polar ester
(4-ethoxycarbonyl phenyl isocyanide (EPI, a)), bulky naphthyl (2-
naphthyl isocyanide (NI, b)) or tetraphenylethylene (4-isocyano-
4'-(1,2,2-triphenylvinyl)-1,1'-biphenyl (ITPB, c)), or chiral ester
substituents
((1S,2R,5S)-2-isopropyl-5-methylcyclohexyl
4-
isocyanobenzoate (D-IMCI, d) and (1R,2S,5R)-2-isopropyl-5-
methylcyclohexyl 4-isocyanobenzoate (L-IMCI, e) were
synthesized and polymerized by use of single-component,
metal-free borate compounds [Ph3C][B(C6F5)4] (A) and
[PhNHMe2][B(C6F5)4] (B) or borane compound B(C6F5)3 (C). The
representative results were summarized in Table 1.
[e]
[e]
Run Cat.[b] [Mon.]/ Mon. Solvent Temp. Time Yield Act.[d] Mn Mw/Mn
[Cat.][c]
(oC) (min) (%)
(104)
1
2
A
A
A
A
B
C
A
A
A
A
A
A
A
A
A
A
100:1
100:1
100:1
100:1
100:1
100:1
500:1
1000:1
2000:1
2000:1
2000:1
2000:1
100:1
100:1
100:1
100:1
a
a
a
a
a
a
a
a
a
a
a
a
b
c
d
e
toluene 25
1
1
87
97
83
97
45
49
79
68
54
922
6.4
5.47
4.84
4.92
7.52
The experiment results showed that the structure and type of
catalyst had a profound impact on the EPI polymerization (Table
1, runs 2, 5–6). The isolated ion pair borate A [Ph3C][B(C6F5)4]
PhCl
25
25
1035 8.1
886 7.3
1035 6.7
3
PhCl2
1
4
C2H2Cl4 25
1
exhibited
a
very high activity up to 1.0
×
106
g
of
5
6
7
8
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
25
25
25
25
25
50
70
90
180
720
5
10
15
1
2.7
0.73
842
702
18.8 2.64
7.6
9.0
9.6
3.91
6.30
6.85
polymer/(molcat.·h) for the polymerization of EPI at room
temperature in chlorobenzene within 1 min, suggesting that the
cation Ph3C+ is an excellent cationic initiator for the EPI
polymerization under mild conditions (Table 1, run 2). While the
similar isolated ion pair borate B [PhNHMe2][B(C6F5)4] showed
lower activity ca. 2.7 × 103 g of polymer/(molcat.·h) under similar
conditions within 3 h, implying that the more charge dispersion
on the proton of cationic initiator [PhNHMe2]+ due to strong
electron-donating ability of the nitrogen atom of the coordinated
PhNMe2 slowed down the polymerization rate to some extent
(Table 1, run 5). If neutral borane C B(C6F5)3 was used as
catalyst, the lowest activity ca. 7.3 × 102 g of polymer/(molcat.·h)
was obtained under similar conditions within 12 h, which might
attribute to the weakly electropositive Lewis acid and the large
steric hindrance around the boron atom of B(C6F5)3 which
significantly limited the coordination of isocyanide monomer with
B(C6F5)3 in PhCl (Table 1, run 6). Other important factors such
as solvents, molar ratio of monomer to catalyst and reaction
temperature also played important roles in the EPI
polymerization (Table 1, runs 1–4, 7–12). The highest activity up
to 1.15 × 107 g of polymer/(molcat.·h) was achieved at 70 oC
under molar ratio of [EPI]/[[Ph3C][B(C6F5)4]] ca. 2000 : 1 within 1
min in PhCl (Table 1, run 11). In contrast, [Ph3C][B(C6F5)4]
exhibited a very low activity ca. 64 g of polymer/(molcat.·h) for NI
polymerization at 70 oC (Table 1, run 13). This might be
attributed to both of the big steric hindrance of naphthalene and
the weak electronegativity of the carbon atom of the C≡N triple
bond due to the strong conjugated effect of naphthalene,
resulting in the weak attraction for the strong electropositive
carbon cation catalyst. In the polymerization of ITPB and D/L-
IMCI, the high activities ca. 5.1–8.8 × 105 g of polymer/(molcat.·h)
were achieved by using [Ph3C][B(C6F5)4] at 25 oC for 3 min
(Table 1, runs 14–16).
9
745 16.8 3.86
10
11
12
13
14
15
16
55 11419 9.2
55 11502 6.1
54 11115 5.1
3.20
3.30
3.00
-
1
1
70 1440 10 0.064
-
25
25
25
3
3
3
100
94
88
877
4.0f 4.09f
539 12.7f 3.24f
506 10.4f 3.18f
[a] Conditions: 8.5 μmol of catalyst, 5 mL of solvent. [b] Catalysts: A =
[Ph3C][B(C6F5)4], B = [PhNHMe2][B(C6F5)4], C = B(C6F5)3. [c] Molar ratio of
monomer to catalyst. [d] Activity: kg of polymer/(molcat.·h). [e] Determined by
GPC in CHCl3 at 25 oC against polystyrene standard. [f] Determined by GPC
in THF at 25 oC against polystyrene standard.
characterized by use of UV/Vis transmittance spectroscopy, UV
absorption spectroscopy, and fluorescence spectroscopy (Figure
2 and ESI). The FT-IR, 1H NMR spectra and GPC curves
demonstrated the isocyanide monomers had successfully been
transformed into PIs with moderate to high molecular weights
(Mn = 4.0–18.8 × 104 g/mol) and moderate to broad molecular
weight distributions (Mw/Mn = 2.64–7.52). CD spectra indicated
that both poly(L-IMCI) and poly(D-IMCI) were optically active,
and the obvious positive and negative CD signals at 364 nm
demonstrated that they had opposite single-handed helical
conformations[12] (Figure 1A). The dropping transmittance in the
UV/Vis transmittance spectroscopy and the presence of light-
scattering tails above 500 nm in the UV absorption spectroscopy
of the poly(ITPB) demonstrated the formation of nanoaggregates
when the water fraction in THF/water mixture increased from 0
to 90%. The fluorescence spectrum showed that the
fluorescence intensities of poly(ITPB) increased with the
increasing water fraction in THF/water mixture from 0 to 30%,
suggesting the poly(ITPB) having a AIE feature similar to ITPB
monomer (Figure 2A–B).[13] When the water content was higher
than 40%, the aggregation states having a more loose stacking
structure were rapidly formed from the polymer chains. For such
loose aggregates, the intrinsic rotational confinement of the
rotation of the four phenyl rings of TPE units was low. Moreover,
All of the resulting polyisocyanides showed good solubility in
commonly used organic solvents such as THF, toluene and
CHCl3. Among them, the poly(EPI), poly(ITPB) and poly(D/L-
IMCI) were characterized by using FT-IR, 1H NMR spectroscopy
and GPC analysis. Poly(D/L-IMCI)s were also characterized by
using CD and SEM (Figure 1 and ESI). Poly(ITPB) was also
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10.1002/anie.201803300
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COMMUNICATION
the loss of non radiation energy was still very large. As a result,
the luminescence became weak until the luminescence
disappeared completely. The blue shift of the maximum
emission wavelength was due to the non polarity of the four
phenyl rings of TPE units and polymer main chains in the interior
of the aggregates. SEM spectra showed that the optically active
poly(D/L-IMCI)s had good single-handed helical conformations.
Table 2 Helical-Sense-Selective Random Copolymerization of Achiral ITPB
with Chiral D/L-IMCI by Using [Ph3C][B(C6F5)4][a]
15
10
5
25
20
15
10
5
poly(D-IMCI-co-ITPB)s
poly(L-IMCI-co-ITPB)s
L-IMCI cont.: 100 mol%
L-IMCI cont.: 74 mol%
L-IMCI cont.: 70 mol%
L-IMCI cont.: 39 mol%
L-IMCI cont.: 17 mol%
L-IMCI cont.: 12 mol%
ITPB cont.: 100 mol%
D-IMCI cont.: 11 mol%
D-IMCI cont.: 19 mol%
D-IMCI cont.: 37 mol%
D-IMCI cont.: 69 mol%
D-IMCI cont.: 76 mol%
D-IMCI cont.: 100 mol%
[d]
[e]
Run IMCI ITPB [IMCI]/ Mon.YieldAct.[b]IMCI cont.[c] Mn[d] Mw/Mn
△ε364
(mmol) (mmol) [ITPB] (%)
(mol%) (104)
(M-1 cm-1)
3.3 4.78 -0.59
0
0
1
2
3
4
5
6
7
8
9
0.023 0.23
0.046 0.23
1:10 c+d 100 938
1:5 c+d 97 965
1:1 c+d 91 1324
5:1 c+d 99 746
11
19
37
69
76
12
17
39
70
74
-5
-5
4.4 4.67
5.6 4.16
4.3 3.06
6.8 2.70
2.2 6.83
1.9 7.28
4.8 3.88
4.3 2.70
5.1 2.94
-1.24
-3.88
-13.07
-16.13
0.61
1.14
4.60
12.58
15.47
-10
-15
-20
-25
-10
-15
0.23
0.23
0.23 0.046
0.23 0.023 10:1 c+d 99 658
0.023 0.23
0.046 0.23
300
400
500
600
0
20
40
60
80
100
1:10 c+e 100 938
1:5 c+e 100 1000
1:1 c+e 92 1342
5:1 c+e 92 693
0.23
0.23
Figure 1. (A) CD spectra of poly(D/L-IMCI), poly(ITPB) and poly(D/L-IMCI-co-
ITPB)s in THF (0.2 mg/mL) at 25 oC. (B) Plots of △ε364 value of the copolymers
as a function of the D/L-IMCI content.
0.23 0.046
10 0.23 0.023 10:1 c+e 92 614
[a] Conditions: 2.3 μmol of [Ph3C][B(C6F5)4], 5 mL of PhCl as solvent, 25 oC, 3
min. [b] Activity: kg of polymer/(molcat.·h). [c] Determined by 1H NMR spectra in
CDCl3 at room temperature. [d] Determined by GPC in THF at 25 oC against
polystyrene standard. [e] Determined by CD spectra in THF (c = 0.2 mg/mL,
0.1 cm path length).
5000
0
8000
6000
4000
2000
0
0
4500
4000
3500
3000
2500
2000
1500
1000
500
10%
20%
30%
40%
50%
60%
70%
80%
85%
90%
95%
97.5%
99%
10%
20%
30%
40%
50%
60%
70%
80%
90%
Solvent separation experiments of these copolymerization
products in toluene indicated negligible quantities of poly(ITPB)
impurities. The GPC curves showed that the molecular weight
distributions became narrow as the D/L-IMCI contents of the
copolymers increased from 11 to 76 mol%. The reactivity ratios
(r(ITPB) = k(IPTB)(ITPB)/k(ITPB)(IMCI) and r(IMCI) = k(IMCI)(IMCI)/k(IMCI)(ITPB)) for
the ITPB/D-IMCI system (r(ITPB) = 1.335 and r(D-IMCI) = 0.310) and
the ITPB/L-IMCI system (r(ITPB) = 1.623 and r(L-IMCI) = 0.247) were
obtained from the Fineman-Ross plots at very low monomer
conversion (< 5%).[14] These results suggested a preference for
the reaction of ITPB over those of D/L-IMCI with this carbon
0
300
400
450
500
550
600
350
400
450
3500
3000
2500
2000
1500
1000
500
3000
2500
2000
1500
1000
500
0
0
10%
20%
30%
40%
50%
60%
70%
80%
90%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0
0
300
300
350
400
450
350
400
450
cation. The product of the reactivity ratios (r(ITPB) (IMCI) = 0.414
r
and 0.401, respectively) demonstrated a tendency to form
random copolymers, in agreement with the microstructures of
the resulting copolymers determined by 1H NMR analysis. CD
spectra displayed that all of the copolymers adopted a helical
conformation with an excess screw sense in different degrees,
which were significantly affected by the D/L-IMCI contents of the
Figure 2. Plots of fluorescence intensity vs water fraction in THF/water mixture
(0.01 mg of ITPB unit/mL). (A) ITPB monomer; (B) poly(ITPB) (Table 1, run
14); (C) poly(D-IMCI-co-ITPB) (Table 2, run 5); (D) poly(L-IMCI-co-ITPB)
(Table 2, run 9) (conditions: EX wavelength: 320 nm for A, 290 nm for B-D; EX
slit: 2.5 nm for A, 5 nm for B-D; EM slit: 5 nm; 650 V for A, 700 V for B-D).
copolymers (Figure 1A). Moreover,
a
typical nonlinear
More importantly, [Ph3C][B(C6F5)4] could also promote the
helical-sense-selective random copolymerization of achiral ITPB
(c) with chiral D-IMCI (d) or L-IMCI (e) at 25 C in PhCl under
relationship was found between the strength of CD signals at
364 nm and the D/L-IMCI contents in the corresponding
copolymers, which means the helical sense selective random
copolymerization of chiral D/L-IMCI and achiral ITPB monomer
by using metal free carbon cation (Figure 1B).[15] The
fluorescence spectra revealed that some of poly(D/L-IMCI-co-
ITPB)s also had AIE nature similar to poly(ITPB) (Figure 2D,
Figure S45, B, C, E, F in ESI). SEM images showed that the
o
different molar ratio of D/L-IMCI to ITPB, giving a series of
random poly(D/L-IMCI-co-ITPB)s with unique properties and
excess single-handed helical conformations in different degrees.
The experimental results were summarized in Table 2.
As shown in Table 2, when the molar ratios of chiral D/L-
IMCI to achiral ITPB increased from 1:10 to 10:1, the activities
firstly increased from 9.4 × 105 g of polymer/(molcat.·h) to 13.4 ×
105 g of polymer/(molcat.·h) then decreased to 6.1–6.6 × 105 g of
polymer/(molcat.·h) (Table 2, runs 1–10). Correspondingly, the
D/L-IMCI contents of these copolymers also gradually increased
from 11 to 76 mol% (Table 2, runs 1–10).
optically active poly(D/L-IMCI-co-ITPB)s adopted
a helical
conformation with an excess screw sense in different degrees.
High resolution ESI-MS spectrum of the NI oligomers
obtained by [Ph3C][B(C6F5)4] in chlorobenzene confirmed the
end groups by a series of peaks assigned to Ph3C(C=N-C10H7)1-
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10.1002/anie.201803300
Angewandte Chemie International Edition
COMMUNICATION
6C10H6N≡C + H+, H(C=N-C10H7)1-5C10H6N≡C + H+ respectively
(Figure 3, ESI). Based on these results, a possible carbocationic
polymerization mechanism was suggested in Scheme 1. The
Conflict of interest
The authors declare no conflict of interest.
cation Ph3C+ serves as
a truly cationic initiator for the
Keywords: isocyanide • (co)polymerization • metal-free •
helical-sense-selective • cationic mechanism
polymerization of isocyanides. Moreover, the presence of
C10H6NC end group demonstrates that the termination process
undergoes a C-H activation of one carbon-hydrogen bond on the
ortho-position of the aryl isocyanide monomer. As a result, the
H+ proton is produced as new cationic initiator for further
polymerization of isocyanide in combination with an polymer
chain as Ph3C(C=N-C6H4R)m+1C10H6N≡C. Similarly, the polymer
chains as H(C=N-C6H4R)nC10H6N≡C will also be synthesized in
the following polymerization process. Such C-H activation of aryl
isocyanide monomer has never been observed in the
polymerization of isocyanide by using late transition metal
catalyst, identifying the high catalytic activity of these cationic
initiators.
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1177-1183.
Figure 3. ESI-MS spectrum of NI oligomer obtained by [Ph3C][B(C6F5)4].
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Scheme 1. Possible Carbocationic Polymerization Mechanism.
[8]
[9]
In summary, commercially available [Ph3C][B(C6F5)4] serves
as a highly efficient metal-free cationic initiator for the helical-
sense-selective carbocationic polymerization of chiral aryl
isocyanides and copolymerization with achiral aryl isocyanide
with extremely high activity (up to 107 g of polymer/(molcat.·h)),
affording high molecular weight functional PIs with good
solubility, AIE nature or single-handed helical conformation.
Such highly efficient metal-free catalyst is quite rare for the
polymerization of isocyanides, let alone its high helical sense
selectivity in both polymerization and copolymerization of
isocyanides. Further studies on the (co)polymerization of other
functional isocyanide monomers by use of metal-free borate
catalyst are in progress.
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Acknowledgements
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Financial support for this work was provided by the National
Natural Science Foundation of China (Nos. 21490570,
21490574, 21774014) and the 111 Project (B07012).
This article is protected by copyright. All rights reserved.

10.1002/anie.201803300
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Commercially available
Xinwen Yan,[a] Shaowen Zhang,[a]
Pengfei Zhang,[a] Xiaolu Wu,[a] An
Liu,[a] Ge Guo,[a] Yuping Dong*[b]
and Xiaofang Li*[a]
[Ph3C][B(C6F5)4] serves as a
highly efficient, metal-free, and
single-component initiator not only
for carbocationic polymerization of
polar and bulky aryl isocyanides
with extremely high activity up to
1.2 × 107 g of polymer/(molcat.·h)
but also for helical-sense-selective
polymerization of chiral aryl
isocyanides and copolymerization
with achiral aryl isocyanide,
affording high molecular weight
functional poly(aryl isocyanide)s
with good solubility, AIE nature,
and/or one-handed helical
Page No. – Page No.
[Ph3C][B(C6F5)4]: Highly Efficient
Metal-Free Single-Component
Initiator for Helical-Sense-
Selective Cationic
Copolymerization of Chiral Aryl
Isocyanide and Achiral Aryl
Isocyanide
conformation.
This article is protected by copyright. All rights reserved.