Metal-Free Multicomponent Tandem Polymerizations of Alkynes, Amines, and Formaldehyde toward Structure- and Sequence-Controlled Luminescent Polyheterocycles

Article abstract of DOI:10.1021/jacs.6b12767

Sequence-controlled polymers, including biopolymers such as DNA, RNA, and proteins, have attracted much attention recently because of their sequence-dependent functionalities. The development of an efficient synthetic approach for non-natural sequence-controlled polymers is hence of great importance. Multicomponent polymerizations (MCPs) as a powerful and popular synthetic approach for functional polymers with great structural diversity have been demonstrated to be a promising tool for the synthesis of sequence-controlled polymers. In this work, we developed a facile metal-free one-pot multicomponent tandem polymerization (MCTP) of activated internal alkynes, aromatic diamines, and formaldehyde to successfully synthesize structural-regulated and sequence-controlled polyheterocycles with high molecular weights (up to 69800 g/mol) in high yields (up to 99%). Through such MCTP, polymers with the in situ generated multisubstituted tetrahydropyrimidines or dihydropyrrolones in the backbone and inherent luminescence can be easily obtained with high atom economy and environmental benefit, which is inaccessible by other synthetic approaches.

Full text of DOI:10.1021/jacs.6b12767

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Article
Metal-Free Multicomponent Tandem Polymerizations of
Alkynes, Amines, and Formaldehyde toward Structure-
and Sequence-Controlled Luminescent Polyheterocycles
Bo Wei, Weizhang Li, Zujin Zhao, Anjun Qin, Rongrong Hu, and Ben Zhong Tang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12767 • Publication Date (Web): 20 Mar 2017
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Metal-Free Multicomponent Tandem Polymerizations of Alkynes,
Amines, and Formaldehyde toward Structure- and Sequence-
Controlled Luminescent Polyheterocycles
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Bo Wei,† Weizhang Li,† Zujin Zhao,† Anjun Qin,† Rongrong Hu,*,† and Ben Zhong Tang*,†,‡
†State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou
510640, China.
‡Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Resto-
ration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong
Kong, China.
ABSTRACT: Sequence-controlled polymers, including biopolymers such as DNA, RNA and proteins, have attracted much
attention recently because of their sequence-dependent functionalities. The development of efficient synthetic approach
for non-natural sequence-controlled polymers are hence of great importance. Multicomponent polymerizations (MCPs)
as a powerful and popular synthetic approach for functional polymers with great structural diversity have been demon-
strated to be a promising tool for the synthesis of sequence-controlled polymers. In this work, we developed a facile met-
al-free one-pot multicomponent tandem polymerization (MCTP) of activated internal alkynes, aromatic diamines and
formaldehyde to successfully synthesize structural-regulated and sequence-controlled polyheterocycles with high mo-
lecular weights (up to 69 800 g/mol) in high yields (up to 99%). Through such MCTP, polymers with the in situ generated
multisubstituted tetrahydropyrimidines or dihydropyrrolones in the backbone and inherent luminescence can be easily
obtained with high atom economy and environmental benefit, which is inaccessible by other synthetic approaches.
advantages such as product structural diversity, high effi-
ciency, mild reaction condition, high atom economy, op-
INTRODUCTION
Sequence-controlled polymers, whose monomer units of
different chemical nature are arranged in an ordered fash-
ion, are of great interests to polymer scientists.1-10 Biopol-
ymers such as DNA, RNA, and proteins with ordered se-
quences of building blocks are well-known sequence-
controlled natural polymers, with their biological func-
tions highly dependent on the structures, in particular,
the strict sequence of subunits.11-14 To realize biomimic
synthesis, while the current polymerization techniques
for the precise control of molecular weights and their dis-
tributions are well-developed, the construction of non-
natural sequence-defined polymers remains to be a great
challenge for polymer chemists. The current method for
sequenced-controlled synthetic polymers are mainly
based on solid-phase synthesis. It is hence urgently de-
manded to develop new polymerization methodologies
for the efficient and convenient preparation of functional
polymers with defined sequence of their subunits. Recent-
ly, the preparation of sequence-controlled polymers have
been realized by RAFT emulsion polymerization,15 ring
opening metathesis polymerization,16,17 radical chain
polymerization,18 DNA-templated polymerization,19,20 and
assembly-line synthesis,21 etc.
erational simplicity, and most importantly, well-defined
polymer structure with multiple building blocks arranged
in a strictly ordered fashion.22,23 It hence provides a great
opportunity to employ MCPs in the preparation of se-
quence-controlled polymers and further modulation of
their properties.24-29
The most widely explored MCPs are the isocyanide-
based MCPs such as Passerini three-component polymer-
ization and Ugi four-component polymerizations which
involve toxic and smelly monomers and produce polyes-
ters or polyamides that are synthetically accessible by
other methods.30-33 Alkyne-based MCPs have attracted
much attention recently because of the rich chemical
property of alkynes which leads to great structural diver-
sity and the unsaturated product structure which may
endow polymer with potential optoelectronic proper-
ties.34,35 For example, metal-catalyzed A3-polycouplings of
alkynes, aldehydes, and amines are reported to produce
poly(dipropargylamine)s;36,37 Cu(I)-catalyzed MCPs of
alkynes, sulfonyl azides, and amines/alcohols are reported
to synthesis poly(N-sulfonylamidines) and poly(N-
sulfonylimidates).38-40 Recently, we have developed a se-
ries of multicomponent tandem polymerizations (MCTPs)
to combine Sonogashira coupling reaction of alkyne and
carbonyl chloride, as well as a sequential addition or
Multicomponent polymerizations (MCP) as a rapidly
developing field in polymer chemistry enjoys a series of
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Figure 1. (A) Syntheses of compounds 4a-b and 5a-b and (B) proposed mechanisms for the MCRs.
cyclization reaction with a freshly added third component
to produce conjugated polymers such as polythiophenes
and polypyrazoles in a one-pot procedure.41-43 In such
MCTPs, the intermediate formed in the first step directly
undergoes the following reactions, avoids isola-
tion/purification and guarantees the complete conversion
of each step, which greatly simplifies the synthetic opera-
tion and improves the efficiency. These MCTPs do not
just link the functional units together in polymer chains,
they can also build new functional units embedded in the
polymer main chain at the same time, producing hetero-
atom or heterocyclic-containing conjugated polymers
with high yield, high molecular weight, high regio- and
stereoselectivity.
al-free polymerization of alkynes to avoid such problems.
There are two strategies to realize metal-free alkyne-
based MCPs. One is to use specific monomers with
unique chemical reactivity. For example, a catalyst-free
MCP of elemental sulfur, alkynes, and aliphatic amines
are reported to directly convert cheap elemental sulfur to
soluble polythioamides with well-defined structures, high
molecular weights (Mws) and high yields.45 Another strat-
egy is to use activated alkyne monomers with high reac-
tivity for a large number of metal-free multicomponent
reactions (MCRs) to conduct polymerization.46-48 For ex-
ample, ester group-activated internal alkyne, dimethyl
acetylenedicarboxylate 1a, is reported to react with 2
equivalents of aniline 2 and 2 equivalents of formaldehyde
3 at room temperature through a five molecule-reaction
to form multisubstituted tetrahydropyrimidine 4a (reac-
tion A).49 1a can also react with 2 equivalents of aniline 2
and 1 equivalent of formaldehyde 3 at elevated tempera-
ture through a four molecule-reaction to form multisub-
Currently, transition metal catalysts are required in
most alkyne-based polymerizations, however, even trace
amount of metallic residues from the catalyst may affect
the optoelectronic properties of the product and bring
biological toxicity.44 It is hence desirable to develop met-
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stituted dihydropyrrolone 5a (reaction B) (Figure 1A).50
ing to the ratio of tetrahydropyrimidines and dihydro-
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The proposed mechanism is shown in Figure 1B: in the
presence of acetic acid, 1a first reacts with aniline 2
through a hydroamination reaction to afford intermediate
A, and another half of 2 reacts with formaldehyde 3 t0
form Schiff base B at the same time. Intermediates A and
B then react together through an aza-ene type reaction to
furnish intermediate C.51,52 In the presence of excess
amount of 3, C undergoes a nucleophilic addition with 3
to afford intermediate D, followed by a cyclization reac-
tion to produce 4a; if without excess amount of 3 exists in
the system, C will directly undergoes an amidation cy-
clization at elevated temperature to generate five-
member ring-containing intermediate E, which is then
converted to 5a through an imine-enamine tautomeriza-
tion.
In this work, inspired by the highly efficient metal-free
MCRs of acetylenedicarboxylates, convenient metal-free
one-pot three/four-component tandem polymerizations
of acetylenedicarboxylates, aromatic diamines, and for-
maldehyde are developed at room temperature to
pr0duce polyheterocycles with well-defined structures,
high Mws, and high yields. Structural modulation regard-
pyrrolones in the polymer backbone is achieved by opti-
mization of polymerization temperature and monomer
loading ratio. Furthermore, sequence regularity of the
monomer building blocks are realized through adjust-
ment of the timing and order of monomer loading, fur-
nishing sequence-controlled polymer products. Last but
not least, although all the polymers contain no traditional
luminophore, interesting aggregation-induced emission
behavior is observed from them, indicating new emission
mechanism from such polyheterocycles.
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RESULTS AND DISCUSSION
Metal-free MCTP of Acetylenedicarboxylates, Ar-
omatic Diamine, and Formaldehyde. To explore the
metal-free MCTP, 4,4'-methylenedianiline 6 was selected
as a bifunctional monomer to react with 1a and 3. The
polymerization was conducted in methanol at room tem-
perature in a tandem manner: 1a and 6 were first reacted
for 30 min, 3 and acetic acid were then added to react for
another 16 h to afford polymer P1a (Figure 2A). After a
series of optimization on polymerization condition such
as monomer concentration, loading ratio, reaction time,
Figure 2. Synthesis and Characterization of P1a-b. (A) The synthetic routes of P1a-b. IR spectra of (B) 1b, (C) 6, (D) 4b, and
(E) P1b. 1H NMR spectra of (F) 1b, (G) 6, (H) 4b, and (I) P1b in CDCl3.
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the amount of acetic acid, as well as solvent, only poly- sentative new peaks emerged at δ 4.27 and 4.91 in the
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meric precipitates with low Mw were obtained, owing to
the poor solubility of P1a in the reaction solvents such as
methanol, ethanol, acetic acid, etc (Table S1). However,
considering the solubility of acetylenedicarboxylate 1a, 6,
and the aqueous solution of formaldehyde, these hydro-
philic solvents which are miscible with water are required.
To improve the solubility of the polymer product in
methanol and further increase its Mw, monomer 1b with
hydrophilic 2-(2-methoxyethoxy)ethyl groups was de-
signed and synthesized (Figure S1). Based on the afore-
mentioned procedure of MCTP, product P1b with better
hydrophilicity and good solubility in methanol can be
expected. The monomer concentrations of 1b, 6, and 3
were first optimized while the monomer loading ratio was
fixed as [1b]:[6]:[3] = 1:1:3 (Table S2). When the concentra-
tion of 1b gradually increases from 0.08 M to 0.30 M, the
Mw of P1b generally increases from 30 600 g/mol to 69
800 g/mol, while keeping high yields of up to 99%. Com-
pared with the reported polymers prepared from multi-
component polymerizations, the molecular weights of
these polymers are among the highest.
spectrum of 4b, attributed to the CH2 protons on the
newly formed tetrahydropyrimidine ring, noted as He and
Hd, respectively (Figure 2F-H). Similarly, the CH2 peaks at
δ 4.20 and 4.83, and the splitted peaks from the two 2-(2-
methoxyethoxy)ethyl groups all emerged in the spectrum
of P1b, suggesting the formation of tetrahydropyrimidine
rings (Figure 2I). Moreover, the NH2 resonance of 6 at δ
3.55 has disappeared in the spectrum of P1b, proving the
total consumption of the monomer.
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Interestingly, the CH2 resonances of 6 at δ 3.78 have
splitted into three peaks at δ 3.87, 3.83 and 3.79 in the
spectrum of P1b, suggesting three different chemical envi-
ronments of the methylenedianiline moieties in the pol-
ymer chain. There are two different nitrogen atoms exist
on the tetrahydropyrimidine ring: Na between two CH2
groups and Nb between a C=C bond and a CH2 group.
During the MCTP, the two NH2 groups of 6 react random-
ly with 3 or 1b, which eventually converted to Na and Nb,
respectively. As a result, the methylenedianiline moieties
from 6 may possess three different forms in the polymer
chain: those with two Na atoms, with two Nb atoms, or
with one Na atom and one Nb atom, and the correspond-
ing middle CH2 protons are noted as Ha, Hb and Hc, re-
spectively (Figure 2A). Luckily, Ha, Hb and Hc can be dis-
To characterize the chemical structure of the polymers,
model compounds 4b and 5b were synthesized (Figure 1A
1
and Figure S2-S3). The IR, H and 13C NMR spectra of
1
monomers 1b and 6, model compound 4b, and polymer
P1b were compared and analyzed. The absorption peak at
1726 cm-1 in the IR spectra of 1b associated with the
stretching vibration of carbonyl groups was splitted into
two peaks at ~1694 and 1741 cm-1 in the spectra of 4b and
P1b after the tetrahydropyrimidine ring forms, owing to
the different chemical environments of the two types of
carbonyl groups in 4b and P1b. Meanwhile, the absorp-
tion bands of 6 at 3423 and 3322 cm-1 associated with the
N-H stretching vibrations disappeared in the spectra of
4b and P1b, suggesting the complete consumption of
amine in the model reaction and polymerization (Figure
2B-E).
tinguished from H NMR spectra and their ratio is calcu-
lated to be 1.1: 1.2: 1.0.
In addition, the C≡C resonance of 1b at δ 75.04 has
disappeared, while the carbonyl peak of 1b at δ 151.84 has
splitted and shifted to δ 165.40/163.85 and δ 165.48/163.93
in the 13C NMR spectra of 4b and P1b, respectively (Figure
S4). Meanwhile, two new peaks emerged at δ 100.60 and
47.54 in the spectrum of 4b, and at δ 100.12 and 47.74 in
the spectrum of P1b, representing the carbons on the tet-
rahydropyrimidine ring. Similar with the 1H NMR spectra,
the CH2 resonance of 6 at δ 40.20 has splitted into three
peaks at δ 40.25, 40.56, and 40.86, corresponding to the
carbon atoms bonded with Ha, Hc, and Hb, respectively.
The structural analysis results suggested that P1b with
100% tetrahydropyrimidine structure in the main chain
was obtained from the MCTP at room temperature with
the monomer ratio of [3]/[1b] = 3:1.
In the 1H NMR spectra, the five peaks of 1b resonanced
at δ 4.37, 3.73, 3.64, 3.54, and 3.38, corresponding to the
CH2 or CH3 groups, are generally splitted into two peaks
for each in the spectrum of 4b, owing to the different
chemical
environments
of
the
two
2-(2-
To explore the monomer applicability, we further tried
this MCTP with five additional bis-aromatic amines 7-11 with
methoxyethoxy)ethyl groups in 4b. Meanwhile, the repre-
Figure 3. The MCTP with different aromatic diamine monomers.
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different electronic property and steric hindrance (Figure 3).
mentioned MCTP based on reaction A.
1
1
2
3
4
5
6
7
8
Their polymerization results are summarized in Table S3,
which indicate that this MCTP applies to all these aromatic
diamines. The electron-rich aromatic diamine monomers 7-8
are more suitable than electron-deficient aromatic diamines
9 or diamine monomer 11 with high steric hindrance, owing
to their reactivity difference in the hydroamination and
Schiff base formation reactions.
The structures of P2a-b were characterized by IR, H
13
NMR, and C NMR spectra through comparison with the
spectra of model compounds 4b and 5b, as well as P1b. In
the IR spectra, a N-H stretching vibration peak of 5b
emerges at 3288 cm-1, representing the remaining N-H
group of 5b, which can not be observed in the spectra of
P1b, but becomes obvious in the spectra of P2a-b. In the
meantime, the two carbonyl peaks in the spectrum of 5b
at 1639 and 1702 cm-1 are different from those two peaks in
the spectrum of 4b at 1694 and 1741 cm-1. From the spectra
of P1b to that of P2a-b, the two peaks at 1694 and 1741 cm-
1 have gradually changed to 1639 and 1700 cm-1, respective-
ly, suggesting that the content of dihydropyrrolone in the
polymer backbone is increased in P2a-b (Figure 4B-F).
The ratio of dihydropyrrolone structure and tetrahy-
dropyrimidine structure in the polymer backbone can be
Structural Modulation of Tetrahydropyrimidines
and Dihydropyrrolones. To modulate the polymer
backbone structure according to the mechanism, the
MCTP of 1b, 6, and 3 was then conducted at elevated
temperature with decreased loading ratio of formalde-
hyde to form dihydropyrrolones (Figure 4A). When the
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o
polymerization temperature was raised to 70 C and the
[3]/[1b] ratio was decreased to 3:2, polymer P2a with 90%
yield and a Mw of 14 000 g/mol was obtained. When the
[3]/[1b] ratio was further decreased from 1:1 to 1:2, contin-
uous reduction of Mws and yields were observed from P2b
to P2e (Table S4). Such results suggested that the MCTP
based on reaction B is less efficient than the above-
1
calculated from the H NMR spectra. The representative
CH2 resonance of 4b at δ 4.91 (Hd) and the CH2 resonance
of 5b at δ 4.56 (Hf) both emerged in the spectra of P2a-b
(Figure 4G-K). The integration ratio of Hd peak and Hf
Figure 4. Synthesis and Characterization of P2a-b. (A) The synthetic routes of P2a-b. IR spectra of (B) 4b, (C) 5b, (D) P1b,
(E) P2a, and (F) P2b. 1H NMR spectra of (G) 4b, (H) 5b, (I) P1b, (J) P2a, and (K) P2b in CDCl3.
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peak can be calculated to be 100:0 (P1b), 50:50 (P2a), and two different roles in the MCTP and eventually converted
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18:82 (P2b), suggesting that the dihydropyrrolone content
ratio in the polymer chain can be tuned from 0% to 82%.
This ratio can be further increased to 92%, with the sacri-
fice of the yield and Mw in P2d-e (Table S4). Meanwhile,
the unique N-H resonance of 5b at δ 8.03 (Hg) and the
aromatic proton resonance at δ 7.78 both emerge in the
spectra of P2a-b, proving the formation of such dihydro-
pyrrolone structures.
to Na and Nb in P1b at room temperature, respectively.
The random distribution of Na and Nb leads to the
abovementioned three different chemical environments
of the monomer moieties in P1b. To avoid such uncertain-
ty and synthesize polymers with well-defined structure,
diamine monomer 6 was added in two separate steps and
each portion serves one specific role (Figure 5A). Firstly, 1
equivalent of 6 was reacted with 2 equivalent of 1b at
room temperature for 30 min to afford double addition
product; another equivalent of 6 and 3 were then added
to react for 16 hours to afford P3a. Through such strategy,
the NH2 groups on the first portion of 6 all converted to
Nb, and the NH2 groups on the second half of 6 all con-
verted to Na. Hence the structure of P3a is well-defined
with Ha-containing diamine moieties and Hb-containing
diamine moieties take a strict alternative order in the pol-
ymer chain. Similarly, P3b with well-defined structure
was obtained when diamine monomer 7 without CH2 pro-
tons was used instead of 6 in this MCTP (Figure 5B).
Similarly, from the 13C NMR spectra of P1b to that of
P2a-b, the two carbonyl peaks of 4b at δ 165.40 and 163.85,
as well as the two characteristic peaks of 4b at δ 100.60
and 47.54, are gradually changed to the carbonyl peaks of
5b at δ 164.46 and 163.88, as well as the characteristic
peaks of 5b at δ 103.22 and 48.41, respectively (Figure S5).
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13
In the analysis of IR, H and C NMR spectra of P1b, P2a
and P2b, the intensities of the characteristic peaks of 5b
generally increase from P1b to P2a, then to P2b, accom-
panying with the decrease in the intensities of the repre-
sentative peaks of 4b, suggesting the increasement of di-
hydropyrrolone content from P1b to P2a-b.
If two different diamine monomers are used in each
step, the four-component tandem polymerization can
afford sequence-controlled polymers P3c-d (Figure 5C-D).
For example, one equivalent of 7 was first reacted with
Synthesis
of
Sequence-controlled
Poly(tetrahydropyrimidines)s. As discussed in the
mechanism, the NH2 groups of diamine monomer 6 serve
Figure 5. Synthesis of sequence-controlled polymers P3a-d. The synthetic routes of (A) P3a, (B) P3b, (C) P3c, and (D) P3d.
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Careful analysis of the 1H NMR spectra of P3c-d can prove
two equivalent of 1b, and then one equivalent of 6 and 3
1
2
3
4
5
6
7
8
the sequence-controlled structures. Take P3c for example,
were added to complete the MCTP and furnish P3c. In the
polymer chain of P3c, moieties from 6 and 7 take an al-
ternative order and the CH2 protons from 6 are all Ha-
type. Alternatively, when the feed sequence of 7 and 6 was
exchanged, P3d can be obtained and the CH2 protons
from 6 are all Hb-type. The polymerization yields of P3a-d
are almost quantitative, and the Mws of P3c-d are general-
ly smaller compared with that of P3a-b.
1
the H NMR peak for the CH2 proton from monomer 6 only
emerges at  3.81, indicating that nitrogen atoms from mon-
omer 6 are all Na-type; The integration ratio of Ha peak at 
3.81 and Hd peak at  4.82 are strictly 1:2 in the 1H NMR spec-
trum of P3c, which are the same as in structure I (Figure 7A);
The number of Na- and Nb-type nitrogen atoms are the same
in the polymer structure, which equals to half of the number
of Hd atoms according to the polymer structure; Hence the
number of Ha atom equals to the number of Na atom or Nb
atom, indicating that Na are all from monomer 6 and the
nitrogen atoms from monomer 7 are all Nb-type. In another
word, moieties from monomer 7 only exist as structure II and
structures III-IV with Na from monomer 7 do not exist in the
polymer product, otherwise the integration ratio of Ha and
Hd should be less than 1:2. The only possible combination of
structure I and II are the proposed alternative structure of
9
Such sequence-controlled polymer structure can be
proved by spectroscopic analysis. For example, P3a-d
share similar IR spectra profile with P1b, which possess
the representative two carbonyl peaks at ~1741 and 1691
cm-1, suggesting similar structure with P1b (Figure S6).
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The H NMR spectra of P3a and P1b are also very much
alike, except that in the region of δ 3.79-3.87, instead of
observing three peaks as in the spectrum of P1b, only two
peaks emerged at δ 3.79 and 3.87 with equal integration
area in the spectrum of P3a, suggesting equal amount of
Ha protons and Hb protons in P3a. In the same region, no
peak is observed in the spectrum of P3b because no CH2
proton exists in monomer 7; only Ha peak at δ 3.81 is ob-
served in the spectrum of P3c, and only Hb peak at δ 3.87
emerged in the spectrum of P3d, proving the expected
structure of P3a-d (Figure 6A-D). Similarly, in the 13C
NMR spectra, besides the common representative peaks
of P3a-d located at about δ 165.55, 164.00, 100.00, and
48.00, P3a possesses two peaks at δ 40.29 and 40.90, rep-
resenting the two carbons bonded with Ha and Hb, re-
spectively; P3b has no peak in such region; P3c has a sin-
gle peak at δ 40.29 and P3d has a single peak at δ 40.90,
which also prove the expected well-defined and sequence-
controlled structure of P3a-d (Figure 6E-H).
1
P3c shown in Figure 5c. Similarly for P3d, the H NMR peak
for the CH2 proton from monomer 6 only emerges at  3.87,
and the integration ratio of Hb peak at  3.87 and Hd peak at
 4.81 are strictly 1:2, proving the well-defined polymer struc-
ture with predesigned subunit sequence of P3d.
We have also designed and synthesized two low molecular
weight oligomers 13 and 14 with precise structure and se-
quence and fully characterized the structure to further com-
pare with P3c-d and prove the sequence of the polymers
(Figure 7B). Diamine 6 and mono-amine 12 were added into
the reaction system with different order to produce com-
pounds 13-14 quantitatively, respectively, suggesting high
specificity of these reactions. The structures of 13-14 are fully
1
characterized by H NMR, 13C NMR, IR, as well as high-
resolution MS spectra, proving the expected structure shown
1
in Figure 7B (Figure S7-8). The representative H and 13C
NMR peaks of 13 and 14 can be correlated with the character-
istic peaks in that of P3d and P3c, respectively, proving the
sequence-controlled polymer structures (Figure S9-11).
E
A
C-H
C-H
H
H
a
b
a
b
F
B
G
H
C
D
H
a
H
b
170
3.2
130
90
50 41
40
7.2
6.8 4.8
4.4
4.0
3.6
Chemical shift (ppm)
Chemical shift (ppm)
1
Figure 6. NMR spectra of sequence-controlled polymers P3a-d. H NMR spectra of (A) P3a, (B) P3b, (C) P3c, and (D) P3d in
CDCl3. 13C NMR spectra of (E) P3a, (F) P3b, (G) P3c, and (H) P3d in CDCl3.
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Figure 7. (A) Structures of I, II, III, and IV. (B) Synthesis of model oligomers 13-14.
A random copolymer P3ab was also prepared for compar-
The photophysical properties of the model com-
ison by adding monomer 6 and 7 with 1:1 ratio at the first
step. The 1H NMR spectrum of P3ab was compared with that
of P3a-d as shown in Figure S12. The peaks in spectra of P3ab
is much broader compared with the other spectra. Most im-
portantly, the characteristic peaks of CH2 from monomer 6
emerge as broad multiple peaks, mainly composed of peaks
for Ha, Hb, and Hc, suggesting a random structure of P3ab, in
comparison with the regular structure of P3c and P3d with
strict sequence of the subunits.
The thermal properties of these polymers are studied.
Thermogravimetric analysis suggests that the polymers gen-
erally possess good thermal stability, with their decomposi-
tion temperature under nitrogen at a 5% weight loss ranging
from 234258 oC (Figure S13). Differential scanning calorime-
try measurement suggests that P1b, P2ab, and P3ad with
hydrophilic 2-(2-methoxyethoxy)ethyl groups possess glass
transition temperature ranging from 30-54 oC (Figure S14-15).
The Luminescence Behaviors. There is no classical
luminogen exists in the model compounds and no report
has indicated the emission from the solution or amor-
phous state of their derivatives, although it is reported
that the crystal of 4a can emit light, owing to the crystal
confinement of the structure which may form possible
“heteroatom cluster” to serve as luminogen.53 In this work,
luminescence is generally observed from the model com-
pounds and polymers, and their luminescence behaviors
are systematically investigated.
pounds and polymers are summarized in Figure 8 and
Table S5. The absorption maxima of these compounds
were generally located at 300-316 nm with similar absorp-
tion profile. In dilute THF solutions, no emission can be
observed from them, however, the solid state and the
PMMA-doped thin film state of these model compounds
and polymers exhibit different emission behaviors (Figure
8B and Figure S16). They generally possess wide emission
peaks with the emission maxima located at 493-513 nm in
the solid state and at 484-514 nm in the thin film state.
The fluorescence images of the polymers were taken un-
der UV irradiation and yellow emission can be generally
observed by naked eyes from their solid powders (Figure
S17). The dihydropyrrolone-containing model compounds
5a-b generally possess blue-shifted emission maxima and
decreased emission efficiencies compared with the tetra-
hydropyrimidine-containing small molecules and poly-
mers. Although the emission efficiency of the polymers
lies in the range of 1.6-5.3%, aggregation-induced emis-
sion (AIE) characteristics of the model compounds and
polymers are suggested, considering that their solutions
are non-emissive, especially for model compound 4a
whose solid state emission efficiency is 40%.
The PL spectra of P1a-b in THF/hexane solutions with
different hexane contents were then investigated to study
the AIE behavior of the polymers as examples (Figure 8C-
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Journal of the American Chemical Society
D). In dilute THF solutions, P1a-b are nonemissive. Grad- the polymer. The precipitate was allowed to stand over-
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6
7
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ual addition of hexane into the THF solution results in
aggregation of the polymer chains because hexane is a
nonsolvent for P1a-b, and new emission peaks emerged at
507 nm (P1a) and 512 nm (P1b), respectively, which were
further enhanced upon addition of hexane while keeping
the spectral profile unchanged, demonstrating typical AIE
phenomenon. In the aggregated state, the large amount
of carbonyl groups as well as heteroatoms in the polymer
chains are clustered together to form “heteroatom
clusters”, which might serve as the luminogen and are
responsible for such fluorescence enhancement.54
night. The product was filtered and washed with n-
hexane (3 × 20 mL), and then dried under vacuum at 40
oC to a constant weight.
General procedure for the synthesis of P2a-b. The
synthetic procedure of P2a is given below as an example.
4,4'-Methylenedianiline 6 (119 mg, 0.6 mmol), bis[2-(2-
methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6
mmol) and 1 mL of methanol were added into a 10 mL
polymerization tube equipped with a magnetic stir bar.
The mixture was stirred at room temperature for 30 min
in air. Then aqueous solution of formaldehyde 3 (37 wt%,
68 μL, 0.9 mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL
of methanol were added and the mixture was allowed to
react at 70 oC for 16 h. After the reaction, the polymeriza-
tion mixture was diluted with 5 mL of dichloromethane
and the solution was added dropwise to 100 mL of n-
hexane through a cotton filter to precipitate the polymer.
The precipitate was allowed to stand overnight. The
product was filtered and washed with n-hexane (3 × 20
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1.6
4a
4b
4a
4b
A
B
5a
5a
5b
5b
1.2
0.8
0.4
0.0
P1a
P1b
P2a
P2b
P3a
P3b
P3c
P3d
P1a
P1b
P2a
P2b
P3a
P3b
P3c
P3d
o
mL), and then dried under vacuum at 40 C to a constant
weight.
General procedure for the synthesis of P3a-d. The
synthetic procedure of P3a is given below as an example.
4,4'-Methylenedianiline 6 (59 mg, 0.3 mmol), bis[2-(2-
methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6
mmol) and 1 mL of methanol were added into a 10 mL
polymerization tube equipped with a magnetic stir bar.
The mixture was stirred at room temperature for 30 min.
Then 4,4'-methylenedianiline 6 (59 mg, 0.3 mmol), aque-
ous solution of formaldehyde 3 (37 wt%, 135 μL, 1.8
mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL of metha-
nol were added and the mixture was allowed to react at
room temperature for 16 h. After the reaction, the
polymerization mixture was diluted with 5 mL of di-
chloromethane and the solution was added dropwise to
100 mL of n-hexane through a cotton filter to precipitate
the polymer. The precipitate was allowed to stand over-
night. The product was filtered and washed with n-
hexane (3 × 20 mL), and then dried under vacuum at 40
oC to a constant weight.
270
300
330
360
390
420 420
480
540
600
660
Wavelength (nm)
Wavelength (nm)
hexane fraction
(vol %)
hexane fraction
(vol %)
C
D
90
80
70
60
50
30
0
90
80
70
60
50
30
0
410
470
530
590
650 410
470
530
590
650
Wavelength (nm)
Wavelength (nm)
Figure 8. Photophysical Properties of the model compounds
and polymers. (A) Absorption spectra of THF solutions and
(B) PL spectra of solid state of the model compounds and
polymers (4b is liquid at room temperature). Concentrations
of 4a-b, 5a-b and P1a-b: 1 × 10−5 M; concentrations of P2a-b
and P3a-d: 5.4 mg/L. Excitation wavelength: 340 nm. PL spec-
tra of (C) P1a and (D) P1b in THF/hexane mixtures with dif-
ferent hexane contents. Concentrations: 1 × 10−5 M. Excitation
wavelength: 350 nm.
CONCLUSIONS
In summary, we present a metal-free one-pot multicom-
ponent tandem polymerization of ester group-activated
alkynes, aromatic diamines, and formaldehyde with high
efficiency and convenience, which enables facile prepara-
tion of polyheterocycles with well-defined structures,
regulated sequence, high molecular weights, and high
yields. Most of the polymerizations only link the mono-
mer building blocks together, few of them can build new
functional groups in situ. In these MCTPs, functional het-
erocycles are constructed directly from the polymeriza-
tion of commercially available, inexpensive, simple mon-
omers, and the resultant polymer structures are inacces-
sible by other synthetic approaches. Moreover, disubsti-
tuted internal alkynes were used as monomers in these
MCTPs rather than traditional terminal alkynes, which
greatly enriched the monomer variety, product diversity
EXPERIMENTAL SECTION
General procedure for the synthesis of P1a-b. The
synthetic procedure of P1b is given below as an example.
4,4'-Methylenedianiline 6 (119 mg, 0.6 mmol), bis[2-(2-
methoxyethyoxy)ethyl] but-2-ynedioate 1b (191 mg, 0.6
mmol) and 1 mL of methanol were added into a 10 mL
polymerization tube equipped with a magnetic stir bar.
The mixture was stirred for 30 min at room temperature
in air. Then aqueous solution of formaldehyde 3 (37 wt%,
135 μL, 1.8 mmol), acetic acid (205 μL, 3.6 mmol) and 1 mL
of methanol were added and the mixture was reacted at
room temperature for 16 h. After the reaction, the
polymerization mixture was diluted with 5 mL of di-
chloromethane and the solution was added dropwise to
100 mL of n-hexane through a cotton filter to precipitate
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and functionalities. Interesting and unique inherent fluo-
1
2
3
4
5
6
7
8
rescence behavior was observed from the model com-
pounds and the polyheterocycles even without typical
luminophore existed in the structure, indicating new lu-
minescence mechanism and luminophores. Such findings
may be correlated with and reveal the underlying reason
for the commonly observed but mysterious autofluores-
cence phenomenon in biological systems. These MCTPs
provide a great opportunity for the facile construction of
synthetic sequence-controlled polymers, which can pave
the way for future investigation of sequence-dependent
functionalities of polymer materials.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Materials/Instrumentation, synthetic procedures and charac-
terization data; effect of monomer concentration, loading
ratio, and diamine monomers on the polymerization; HRMS
spectra of 1b, 4b, 5b, and 13-14; 13C NMR spectra of the model
1
compounds and polymers; IR spectra of P3a-d; H and 13C
1
NMR, and IR spectra of 13-14; H NMR spectra of P3ab; TGA
and DSC spectra of the polymers; PL spectra of the PMMA-
doped films, fluorescence photos of the solid state, and the
photophysical properties of the model compounds and pol-
ymers.
AUTHOR INFORMATION
Corresponding Author
*msrrhu@scut.edu.cn
*tangbenz@ust.hk
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
(30) Sehlinger, A.; Dannecker, P.-K.; Kreye, O.; Meier, M. A. R.
Macromolecules 2014, 47, 2774–2783.
(31) Kreye, O.; Tóth, T.; Meier, M. A. R. J. Am. Chem. Soc. 2011,
133, 1790–1792.
(32) Wang, Y.-Z.; Deng, X.-X.; Li, L.; Li, Z.-L.; Du, F.-S.; Li, Z.-C.
Polym. Chem. 2013, 4, 444–448.
(33) Kakuchi, R. Angew. Chem. Int. Ed. 2014, 53, 46–48.
(34) Hu, R.; Li, W.; Tang, B. Z. Macromol. Chem. Phys. 2015, 217,
213–224.
This work was partially supported by the National Science
Foundation of China (21404041, 21490573 and 21490574), the
Natural Science Foundation of Guangdong Province
(2016A030306045 and 2016A030312002), the National Basic
Research Program of China (973 Program; 2013CB834701), the
Innovation and Technology Commission of Hong Kong (ITC-
CNERC14SC01), and the Guangdong Innovative Research
Team Program (201101C0105067115).
(35) Matsumura, Y.; Fukuda, K.; Inagi, S.; Tomita, I. Macromol.
Rapid Commun. 2015, 36, 660–664.
(36) Liu, Y.; Gao, M.; Lam, J. W. Y.; Hu, R.; Tang, B. Z. Macro-
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