Phenol-yne Click Polymerization: An Efficient Technique to Facilely Access Regio- and Stereoregular Poly(vinylene ether ketone)s

Article abstract of DOI:10.1002/chem.201702966

Alkyne-based click polymerizations have been well-established. However, in order to expand the family to synthesize polymers with new structures and novel properties, new types of click polymerizations are highly demanded. In this study, for the first tim

Full text of DOI:10.1002/chem.201702966

DOI: 10.1002/chem.201702966
Communication
&
Click Polymerization
Phenol-yne Click Polymerization: An Efficient Technique to
Facilely Access Regio- and Stereoregular Poly(vinylene ether
ketone)s
Yang Shi,[a] Tianwen Bai,[a] Wei Bai,[a] Zhe Wang,[a] Ming Chen,[a] Bicheng Yao,[a] Jing Zhi Sun,[a]
Anjun Qin,*[a, b] Jun Ling,*[a] and Ben Zhong Tang*[a, b, c]
from reported efficient organic reactions have been promoted.
However, on account of the rather complicated process of
polymerization, the number of organic reactions for further de-
velopment to polymerizations is limited. To make organic reac-
tions suitable for developing into polymerizations, several es-
sential criteria should be considered and satisfied: a) high effi-
ciency and excellent yields of organic reactions; b) satisfying
physical and chemical stabilities of catalytic system; c) mild re-
action conditions, together with d) simple preparation proce-
dures and general availability of monomers.[1]
Abstract: Alkyne-based click polymerizations have been
well-established. However, in order to expand the family
to synthesize polymers with new structures and novel
properties, new types of click polymerizations are highly
demanded. In this study, for the first time, we established
a new efficient and powerful phenol-yne click polymeri-
zation. The activated diynes and diphenols could be facile-
ly polymerized in the presence of the Lewis base catalyst
of 4-dimethylaminopyridine (DMAP) under mild reaction
conditions. Regio- and stereoregular poly(vinylene ether
ketone)s (PVEKs) with high molecular weights (up to
35200) were obtained in excellent yields (up to 99.0%).
The reaction mechanism was well explained under the as-
sistance of density functional theory (DFT) calculation. Fur-
thermore, since the vinyl ether sequence acts as a stable
but acid-liable linkage, the polymers could be decom-
posed under acid conditions, rendering them applicable
in biomedical and environmental fields.
Among the well-known organic reactions, click chemistry is
the most ideal candidate to be developed into a powerful tool
for efficient polymerizations.[2] Click chemistry represents a
novel chemical reaction that possesses the attractive advantag-
es of high efficiency, good selectivity, mild reaction conditions,
functional group tolerance, simple work-up procedures, and
no or benign byproducts.[2a,c] Therefore, with the enthusiastic
efforts of polymer scientists, click reactions have been success-
fully developed into powerful click polymerizations, which in-
clude the CuI- and RuII-catalyzed azide–alkyne click polymeri-
zations (AACPs),[3] metal-free azide–alkyne click polymerizations
(MFCPs),[1a] thiol-yne click polymerization,[4] amino-yne click
polymerization,[5] and so on.[6]
Establishment and development of new polymerization tech-
niques to facilely synthesize functional polymeric materials is
of vital importance to the polymer and materials sciences. Cur-
rently, an increasing number of polymerizations developed
Although all of these polymerizations are effective, some
drawbacks are unavoidable, such as the insecurity of the azide
monomers used in the AACPs and MFCPs, the strongly pun-
gent aroma of thiol compounds in the thiol-yne click polymeri-
zations, and the existence of stereoisomer mixtures in the
MFCPs. Thus, other efficient and powerful alkyne-based click
polymerizations that are free from such drawbacks remain to
be developed.
[a] Y. Shi, T. Bai, W. Bai, Z. Wang, Dr. M. Chen, Dr. B. Yao, Prof. J. Z. Sun,
Prof. A. Qin, Prof. J. Ling, Prof. B. Z. Tang
MOE Key Laboratory of Macromolecular Synthesis and Functionalization
Department of Polymer Science and Engineering
Zhejiang University, Hangzhou 310027 (P.R. China)
E-mail: qinaj@zju.edu.cn
Inspired by our recently established spontaneous thiol-yne
click polymerization and amino-yne click polymerizations, we
wondered whether the polyhydroalkoxylation of alkynes would
occur if sulfur in the mercapto-group and nitrogen in the
amino group are replaced by oxygen atom in the hydroxyl
group since they are from the same group or period in the pe-
riodic table, respectively. Surprisingly, to the best of our knowl-
edge, only Endo and co-workers have reported the polymeri-
zation of activated diynes with aliphatic diols in the presence
of the organophosphorus catalyst of nBu3P.[7] However, the
scope of the diols, and the unpleasant smell of the catalyst as
well as the harsh anaerobic conditions have limited the appli-
cation of this polymerization.
lingjun@zju.edu.cn
[b] Prof. A. Qin, Prof. B. Z. Tang
Guangdong Innovative Research Team, State Key Laboratory of
Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640 (P.R. China)
E-mail: msqinaj@scut.edu.cn
[c] Prof. B. Z. Tang
Department of Chemistry, Hong Kong Branch of
Chinese National Engineering Research Center for Tissue Restoration and
Reconstruction, The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
E-mail: tangbenz@ust.hk
Supporting information and the ORCID identification numbers for the
authors of this article can be found under https://doi.org/10.1002/
chem.201702966.
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Herein, different from Endo’s work, we succeeded in estab-
lishing a novel polymerization of aromatic diols and diynes
under the catalyst of a solid Lewis base of 4-dimethylamino-
pyridine (DMAP). Simply mixing the diynes and diphenols in
THF in the presence of DMAP at ambient conditions readily
produced regio- and stereoregular poly(vinylene ether ketone)s
(PVEKs) in excellent yields after only 4 h. This polymerization
has many remarkable advantages, for instance, the avoidance
of expensive transition-metal catalyst,[8] the mild reaction con-
ditions, the high efficiency, and no byproducts. Thus, we suc-
cessfully established a new kind of click polymerization, that is,
the phenol-yne click polymerization.
and N,N-dimethylformamide (DMF), we assigned THF as the
most suitable solvent when considering the balance of solubili-
ty, Mw and yields of the products (Table S2).
Thirdly, the monomer concentration investigation showed
that with the concentration of 0.1m, the best Mw (15400) and
yield (92.9%) of the product was obtained (Table S3). So 0.1m
was chosen as the optimal monomer concentration.
Fourthly, we studied the effects of the concentration of
DMAP on the polymerization results. When the concentration
of DMAP increased from 0.008 to 0.02m, the Mw and yield of
the product were almost constant (Table S4). Thus, the catalyst
concentration of 0.008m was used in our experiments.
Finally, the temperature investigations suggested that it
exerts little influence on the polymerization results (Table S5).
To save energy and simplify the operation, 258C was selected
as the preferable temperature.
Furthermore, the PVEKs prepared by the phenol-yne click
polymerization are thermally stable at temperatures as high as
2598C with only 5% loss of weight. Thanks to their containing
vinyl ether groups, the PVEKs can be rapidly hydrolyzed in
strongly acidic solutions with a concentration of hydrochloric
acid [c(HCl)] higher than 10À3.2 molLÀ1, while remaining chemi-
cally stable below this value.
With the optimized reaction conditions in hand, we set out
to explore the universality of the polyhydroalkoxylation of the
activated bis(aroylacetylene)s 1a, 1b and bisphenols 2a, 2b
(Scheme 1). All the polymerizations proceeded smoothly and
PVEKs of P1a2a–P1b2b with high Mw (up to 35200) were pro-
duced in excellent yields (up to 99.0%; Table 1). Generally, the
To develop the phenol-yne click polymerization, the activat-
ed diyne of aroylacetylene 1a and another commercially avail-
able bisphenol A (2a) were initially used as model monomers
to optimize the reaction conditions such as reaction time, the
reaction solvent, the concentrations of monomer and catalyst,
and the reaction temperature (Scheme 1). Monomer 1a was
synthesized according to the reported methods as shown in
the Supporting Information.[9]
Table 1. Phenol-yne click polymerizations of bis(aroylacetylene)s 1 and
diphenols 2.[a]
[b]
[b]
Entry
Monomer
Polymer
Yield [%]
Mw
Mw/Mn
The time course investigation indicated that the weight-
average molecular weights (Mw) of PVEKs no longer increased
after successive reactions for 4 h (Table S1). Thus, 4 h was se-
lected as the optimized polymerization time.
1
2
3
4
1a+2a
1a+2b
1b+2a
1b+2b
P1a2a
P1a2b
P1b2a
P1b2b
96.8
99.0
86.5
93.2
18600
35200
17500
35000
1.72
2.05
2.00
1.84
Secondly, we studied the effect of solvent on the polymeri-
zation results. After testing several solvents including tetrahy-
drofuran (THF), dichloromethane (DCM), toluene, chloroform
[a] Carried out in THF at 258C in air for 4 h; [M]0 =0.10m; [DMAP]=
0.008 m. [b] Estimated by gel-permeation chromatography (GPC) in THF
on the basis of polystyrene calibration. Mw =weight-average molecular
weight; Mn =number-average molecular weight; Mw/Mn =polydispersity
index (PDI).
Mw values of polymers generated from 2b are higher than
those from 2a probably due to the larger molecular weight of
the former and the easier deprotonation. The results demon-
strate the general applicability of this powerful polymerization
methodology.
All the obtained polymers are completely soluble in com-
monly used organic solvents, such as THF, chloroform, DCM
and DMF. The PVEKs are also thermally stable; 5% loss of
weights was observed at temperatures higher than 2598C
under nitrogen (Figure S1).
Owing to the good solubility of P1a2a–P1b2b, the struc-
1
tures of all polymers were characterized by FT-IR and H NMR
spectra (see the Experimental Section and Figures S2–S8 in
Supporting Information for details). For example, in the FT-IR
spectra of polymer P1a2a and its monomers 1a and 2a in Fig-
ure S2, the stretching vibrations of ꢀCÀH and CꢀC in 1a and
OÀH group in 2a appeared at 3210, 2090 and 3344 cmÀ1, re-
spectively. These characteristic peaks, however, could not be
observed in the spectrum of P1a2a, indicating that the ethynyl
Scheme 1. Syntheses of poly(vinylene ether ketone)s by organocatalytic
phenol-yne click polymerizations of bis(aroylacetylene)s 1 and diphenols 2.
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anti-Markovnikov products with E- and Z-configurations, and
one Markovnikov product.[4b,5,10] To assign the new peak and to
check the regio- and stereoselectivity of our developed poly-
merization, we carried out the model reaction using benzoyl-
acetylene 11 and phenol 12 as the starting materials under ex-
actly the same conditions as the polymerization. The gas chro-
matography-mass spectrometry (GC-MS) measurement of the
crude product showed only one isomer with molecular weight
of 223 (Figure S9). After purification, only one product 13 was
indeed obtained in 92% yield. The 1H NMR spectrum of 13
showed distinct doublet peaks assigned to the proton reso-
nances of the linear vinyl groups at d=6.72 and 8.00 ppm,
representing a linear anti-Markovnikov product (Figure 1D).
The 13C NMR spectrum also confirmed the vinyl ether ketone
structure of 13 (Figure S10).
Moreover, the coupling constants (JHH) of these two peaks
are both 12.8 Hz, suggesting the vinyl groups own an E-iso-
meric configuration.[11] These results also agree well with previ-
ous reports.[12] Thus, this reaction is regio- and stereoselective.
Combined with the high efficiency, this reaction is a new kind
1
of click reaction. Compared with the H NMR spectrum of the
model compound 13, the new formed peak of P1a2a (d=
7.97 ppm) is readily assignable to its E-isomers (Figure 1c) al-
though another peak at d=6.69 ppm overlaps with that of
benzene rings. It is worth noting that no other isomeric reso-
1
nance was observed in the H NMR spectrum of P1a2a. Similar
results were obtained in the H NMR spectra of other PVEKs
Figure 1. 1H NMR spectra of: A) monomer 1a, B) monomer 2a, C) polymer
P1a2a, and D) model compound 13 in [D1]CHCl3. The solvent peaks are
marked with asterisks.
1
(Figures S6–S8).
These results suggest that such polymerization could furnish
unitary products with high regio- and stereoregularity. Conse-
quently, it is a new kind of click polymerizaiton, that is, a
phenol-yne click polymerization.
groups in 1a and the phenol groups in 2a have reacted. Simi-
lar results were obtained from FT-IR spectra of P1a2b–P1b2b.
Figure 1 shows the H NMR spectra of P1a2a and its mono-
mers in CDCl3 as an example. The ethynyl and phenol protons
of 1a and 2a resonated at d=3.43 and 4.65 ppm, respectively.
These peaks, however, almost disappeared in the spectrum of
P1a2a, further substantiating the conclusion drawn from the
FT-IR analysis. Moreover, all the peaks are assignable when
compared with the monomers except for the new peak at d=
7.97 ppm.
1
For this click polymerization, the organocatalyst of DMAP
plays a critical role, therefore three other catalysts were chosen
to confirm the indispensable nucleophilicity of the catalyst,
and comparable results were obtained in Table S6. Triethyla-
mine (TEA), a base with weak nucleophilicity, can catalyze this
reaction. However, the non-nucleophilic bases of 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) and inorganic KOH were ineffec-
tive, although their pKa values are similar to that of DMAP.[13]
We thus propose a plausible mechanism for this polymeri-
zation based on related reports[7,13,14] and density functional
Theoretically, as shown in Scheme 2, the reaction of phenol
group with aroylacetylene will generate three isomers, two
Scheme 2. The possible isomeric structures of the phenol-yne click reaction.
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Figure 2. DFT calculated profiles of nucleophilic addition and proton transfer steps of E- (black) and Z-configuration pathways (red), as well as the transforma-
tions between anionic intermediates (blue) and vinyl products (olive). All numbers are given as relative Gibbs free energy in kcalmolÀ1
.
theory (DFT) calculations.[15] Similar with starting materials of
the model reaction, we used the benzoylacetylene 11 and
phenol 12 to demonstrate this process. Two elementary reac-
tions were discovered, which include nucleophilic-addition and
proton-transfer reactions. Moreover, two transformation path-
ways between anionic and stable intermediates were ob-
served. These routes are shown in Figure 2 and Scheme 3 with
selected 3D geometries and coordinates in the Supporting In-
formation (Figure S11).
In the first step, phenol anion is formed from 12 through hy-
drogen-abstraction reaction with DMAP. This anion attacks the
terminal carbon of 11 to produce an anionic transition inter-
mediate. Due to the steric hindrance of anion with both
phenyl and carbonyl, a transition state in E-configuration is
Scheme 3. Proposed reaction mechanism.
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forbidden, which was not observed in calculations after many
attempts. The addition step exhibits a Gibbs energy barrier of
14.42 kcalmolÀ1 in Z-configuration (Z_TS1) route. Subsequent-
ly, a configuration exchange through allene-type TS occurs
Àlgc(HCl) higher than 3.2, the kobs drops below 10À3 sÀ1 and
signifies a sluggish proton transfer to b-carbon of vinyl group.
Kresge’s and co-workers[17a] have presented the same evidence.
In reference to these results, the Àlgc(HCl) value of vinyl ether
cleavage suggested is specific and reliable. Thus, these acid-re-
sponsive PVEKs are promising for application in biomedical
and environmental fields.[18]
with a negligible Gibbs energy barrier of only 0.18 kcalmolÀ1
and almost all Z-configuration intermediates (Z_1) transfer to
E-isomers (E_1) due to the energy benefit of 2.04 kcalmolÀ1
,
.
In the second step, stable products are obtained through an
intermolecular-proton-transfer pathway. If potential (Z)-13 ki-
netically formed, it would transform to (E)-13, which is evi-
denced by the “oxygen-activated double bond reaction”. The
(E)-13 is more stable than (Z)-13 with a Gibbs energy differ-
ence of 3.79 kcalmolÀ1; this indicates the complete formation
of favored (E)-13 product.
Since many elegant studies have reported the vinyl ether
linkage can be cleaved through acid hydrolysis, it means that
our prepared PVEKs possess hydrolytic degradability.[16] The
corresponding mechanism (Scheme S1) indicates that the acid-
catalyzed vinyl ether process involving the intramolecular
proton transfer to b-carbon is the rate-determining step. Sever-
al previous reports also provide second-order hydrolysis rate
constants of simple vinyl ethers with different substituents at
a or b positions.[17] Indeed, the PVEKs are highly acid respon-
sive with ultrafast hydrolytic degradability in aqueous HCl/THF
solution with a Àlgc(HCl) value below 3.2. For example, the
Mw value of P1a2a dramatically decreased from 11000 to
nearly 2000 in a few tens of seconds (Table S7).
In summary, we have established the first example of
phenol-yne click polymerization. Under very mild reaction con-
ditions, the Lewis base of DMAP could efficiently catalyze the
polymerization of bis(aroylacetylene)s 1 and diphenols 2, and
regio- and stereoregular PVEKs with high molecular weights
could be produced in excellent yields at room temperature
after 4 h. Strong evidence suggests that the nucleophilicity of
base catalyst plays a crucial role for this polymerization.
1H NMR spectra revealed that solely anti-Markovnikov additive
products with 100% E-isomer were obtained. DFT calculation
unveiled the intrinsic mechanism of this phenol-yne click poly-
merization. The obtained PVEKs are completely soluble in com-
monly used solvents and are thermally stable. Furthermore,
the polymers are chemically stable in neutral and basic condi-
tions but could be swiftly decomposed when the Àlgc(HCl)
value is lower than 3.2. Thus, these PVEKs are promising for ap-
plication in biomedical and environmental fields, and the
phenol-yne click polymerization will open new avenues in the
synthesis of functional polymers.
The molecular-weight evolution of P1a2a in different acidic
solutions was monitored by gel permeation chromatography
(GPC) by using THF as eluent. As shown in Figure 3, the Mw
value of the example polymer P1a2a sharply decreases under
strong acid conditions, whereas it remains almost unchanged
even after 10 min when the Àlgc(HCl) value is higher than 3.2
(Figure S12). In addition, the degradation degree determined
by integration of the peak areas confirmed that the polymer
was fully degraded.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21525417 and 21490571); the
key project of the Ministry of Science and Technology of China
(2013CB834702); The National Program for Support of Top-
Notch Young Professionals; the Fundamental Research Funds
for the Central Universities (2015ZY013) and the Innovation
and Technology Commission of Hong Kong (ITC-CNERC14S01).
A.J.Q. and B.Z.T. thank the support from Guangdong Innovative
Research Team Program (201101C0105067115).
Kirby and co-worker[16d] have extensively studied the appar-
ent rate constant (kobs) and second-order rate constant (kH) of
E-vinyl ether structure which is similar to PVEK units. With
Conflict of interest
The authors declare no conflict of interest.
Keywords: organocatalysts
· polymers · polymerization ·
poly(vinylene ether ketone)s · regio- and stereoselectivity
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Manuscript received: June 28, 2017
Accepted manuscript online: July 2, 2017
Version of record online: && &&, 0000
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Chem. Eur. J. 2017, 23, 1 – 7
www.chemeurj.org
6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
Cut to the click: The first example of
phenol-yne click polymerization was es-
tablished. Bis(aroylacetylene)s and di-
phenols could be efficiently polymer-
ized in the presence of organic bases
under mild conditions, and regio- and
stereoregular polymers with high mo-
lecular weights could be produced in
excellent yields. An intrinsic mechanism
is proposed with the assistance of DFT
calculations. The polymers could be
completely decomposed when the
Àlgc(HCl) is lower than 3.2, but are
chemical stable above this value.
&
Click Polymerization
Y. Shi, T. Bai, W. Bai, Z. Wang, M. Chen,
B. Yao, J. Z. Sun, A. Qin,* J. Ling,*
B. Z. Tang*
&& – &&
Phenol-yne Click Polymerization: An
Efficient Technique to Facilely Access
Regio- and Stereoregular
Poly(vinylene ether ketone)s
Chem. Eur. J. 2017, 23, 1 – 7
www.chemeurj.org
7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ