Syntheses of Sequence-Controlled Polymers via Consecutive Multicomponent Reactions

Article abstract of DOI:10.1021/acs.macromol.5b00463

Multicomponent reactions have recently attracted a great deal of attention as they are considered as a powerful tool for constructing sequence-controlled polymers. Although new examples are constantly flourishing in the literature, the process that allows two or more consecutive multicomponent-reactions to react in a single operation for the syntheses of sequence-controlled polymers has not been developed until now. Here, we propose a new strategy combining multicomponent reaction of amine, thiol, and alkene conjugating and multicomponent polymerization of diyne, azide, and diamine coupling in one-pot for the synthesis of sequence-controlled polymer.

Full text of DOI:10.1021/acs.macromol.5b00463

Article
pubs.acs.org/Macromolecules
Syntheses of Sequence-Controlled Polymers via Consecutive
Multicomponent Reactions
Ze Zhang,† Ye-Zi You,† De-Cheng Wu,‡ and Chun-Yan Hong*,†
†Key Lab of Soft Matter Chemistry, Chinese Academy of Sciences, Department of Polymer Science and Engineering, University of
Science and Technology of China, Hefei 230026, Anhui, China
‡Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics & Chemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Multicomponent reactions have recently attracted a great deal of attention as they are considered as a powerful
tool for constructing sequence-controlled polymers. Although new examples are constantly flourishing in the literature, the
process that allows two or more consecutive multicomponent-reactions to react in a single operation for the syntheses of
sequence-controlled polymers has not been developed until now. Here, we propose a new strategy combining multicomponent
reaction of amine, thiol, and alkene conjugating and multicomponent polymerization of diyne, azide, and diamine coupling in
one-pot for the synthesis of sequence-controlled polymer.
remarkable control at the molecular scale is particularly
important for proteins, as it can direct the folding of a single
peptide chain into a three-dimensional nanostructure and
imbue the protein with highly specific activity.1−3 The major
challenge of polymer chemistry is to mimic such sequence
regulation in polymer synthesis.4−6 Multicomponent reaction
can connect three or more starting materials in a single
synthetic operation with high atom economy and bond-forming
efficiency, thereby increasing molecular diversity and complex-
ity in a fast and often experimentally simple fashion, which is
now a powerful strategy to regulate molecular diversity and
complexity from simple and readily available substrates in one-
pot process.5,7−13 Though the importance of sequence-control
events during polymerizations was recently emphasized by
Lutz, Liu, and Sawamoto et al.,1−3,5−7,14−16 so far, the
application of multicomponent reactions (MCRs) in polymer
synthesis is still in its infancy, and there are only a few MCRs,5
such as the Passerini three-component reaction,9,17−19 the Ugi
reaction,20−22 and metal-catalyzed multicomponent reac-
tion,12,23,24 that have been successfully employed to afford
highly functionalized polymeric materials.11 However, most of
the obtained polymers have only a few components5 whereas
the proteins have 20 amino acid components. The fewer
components limit molecular diversity and complexity for the
INTRODUCTION
■
Biopolymers, such as proteins, have well-defined sequence of
repeat units and/or functionalities along the backbone. This
Scheme 1. Outline of the Synthesis of Sequence-Controlled
Polymer via Two Consecutive Multicomponent Reactions of
Amine−Thiol−Ene Conjugating Andalkyne−Azide−Amine
Coupling in One Pot
Received: March 3, 2015
Revised: May 5, 2015
© XXXX American Chemical Society
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Figure 1. continued
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Figure 1. 1H NMR spectra (A) and 13C NMR spectra (B) for mixture of the multicomponent reaction of propargyl methacrylate, N-
(carbobenzyloxy)homocysteine thiolactone, and 4,7,10-trioxa-1,13-tridecanediamine (∗ is triethylamine).
obtained polymers. To ensure sufficient molecular diversity and
complexity, more components should be introduced into a
single chain, which needs more starting molecules to react in
one pot. The strategy that allows two or more consecutive
multicomponent reactions to react in one pot would be a very
promising method; however, it has not been developed for
sequence-controlled polymer synthesis until now.
Here, we report a first example of two consecutive
multicomponent-reactions in one pot for the preparation of
sequence-controlled polymers via using a unique monomer (A)
with two orthogonal reactive groups (alkene unit and alkyne
unit) to combine the two multicomponent reactions together in
one pot as shown in Scheme 1. First, propargyl methacrylate
(PM, A), N-(carbobenzyloxy)homocysteinethiolactone (NC,
B), and 4,7,10-trioxa-1,13-tridecanediamine (TT, C) were used
as three starting molecules in the first three-component
reaction of amine−thiol−ene conjugating to build up an
ABCBA-sequenced molecule containing two alkyne units.
Subsequently, p-toluenesulfonyl azide (PT, D) and 1,4-
phenylenediamine (1,4-PA, E) were added into the reaction
system, and the Cu-catalyzed three-component polymerization
of diyne (ABCBA), azide (D), and diamine (E) coupling
occurred immediately, giving a polymer with a DABCBADE
sequence and high molecular weight as shown Scheme 1.
EXPERIMENTAL SECTION
■
Materials. All reagents were used as received unless otherwise
stated. N,N′-Diisobutyl-1,6-hexanediamine (97%) and propargyl
methacrylate (98%) were purchased from Alfa Aesar. DL-Homocys-
teine thiolactone hydrochloride (99%), acetyl chloride (98%), and
4,7,10-trioxa-1,13-tridecanediamine (97%) were purchased from
Sigma-Aldrich. Benzyl chloroformate (96%), 1,4-butanediol (99%),
1,4-phenylenediamine (98%), and 2,2-dimethyl-1,3-propanediamine
(97%) were purchased from TCI. Triethylamine (TEA, 99.5%) and
dimethylformamide (DMF, 99.9%) were purchased from Aladdin. p-
Toluenesulfonyl azide (98%) and o-phenylenediamine (99%) were
purchased from Adamas-beta. Chloroform-d (99.8 atom % D) was
purchased from J&K. Cuprous chloride (98.5%), sodium sulfate
anhydrous (99%), sodium hydroxide (96%), n-hexane (97%), ethyl
acetate (99.5%), methanol (99.7%), dichloromethane (99.5%), and
diethyl ether anhydrous (99.7%) were purchased from Sinopharm
Chemical Reagent Co. Ltd. Propargyl methacrylate (Alfa Aesar, 98%)
was purified by small aluminum oxide (basic) chromatography to
remove inhibitor.
C
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Characterizations. All NMR spectra were recorded on a Bruker
1
AV300 NMR spectrometer (resonance frequency of 300 MHz for H
and 75 MHz for 13C) operated in the Fourier transform mode. The
samples were dissolved in chloroform-d or DMSO-d6, with
tetramethylsilane (TMS) as an internal reference. Molecular weights
and molecular weight distributions were measured by using a SEC
instrument. The system was equipped with a PL-RI differential
refractive index detector (DRI), PL-BV 400RT viscometer (Visc), and
a Precision Detectors PD2020 light scattering detector (LS). LiBr/
DMF (0.1%, w/w) solution with a flow rate of 1.0 mL/min was used
as eluent. The molecular weights were calibrated against polystyrene
standards. Mass spectrum analysis was performed by using a LC-MS
instrument (Thermo Scientific, LTQ Orbitrap XL). The system was
equipped with an ESI source, and MS data were processed using
Xcalibur software (2.1.0 SP1 built 1160).
Synthesis of N-Acetohomocysteine Thiolactone. DL-Homo-
cysteine thiolactone hydrochloride (3.07 g, 20 mmol) was mixed with
triethylamine (9.70 g, 96 mmol) in 50 mL of dichloromethane to form
a suspension in ice bath. Acetyl chloride (2.36 g, 30 mmol) was added
dropwise within 30 min. The solution was stirred overnight at room
temperature. The reaction mixture was diluted with 20 mL of
dichloromethane, filtered, washed with brine (30 mL × 2), and
extracted with dichloromethane (40 mL × 2). The organic layer was
dried with anhydrous Na2SO4. Further purification can be achieved by
silica gel column chromatography using ethyl acetate as eluent to
obtain the product as white powder. Yield was 65%. 1H NMR
spectrum (300 MHz, CDCl3): δ 1.931 (m, 1H), δ 2.029 (s, 3H), δ
2.895 (m, 1H), δ 3.301 (m, 2H), δ 4.527 (m, 1H), δ 6.171 (s, 1H).
Synthesis of N-(Carbobenzyloxy)homocysteine Thiolactone.
DL-Homocysteine thiolactone hydrochloride (3.07 g, 20 mmol) was
added to a suspension of NaHCO3 (8.4 g, 100 mmol) in H2O/1, 4-
dioxane (v/v, 1/1, 80 mL) at 0 °C; subsequently, the mixture was
stirred for 30 min. Benzyl chloroformate (5.1 g, 30 mmol) was added
dropwise within 30 min, and the mixture was stirred overnight at room
temperature. The reaction was terminated by diluting with brine (50
mL) and extracted with ethyl acetate (70 mL × 4). The organic layer
was dried with anhydrous Na2SO4. Further purification can be
achieved by recrystallization in ethyl acetate to obtain the product as
Figure 2. Conversion of thiolactone (A) and methacrylate (B).
double bond of propargyl methacrylate via thiol-based Michael
addition reaction while alkyne unit remained unreacted in the
absence of radicals. The quantitative conversion of the starting
molecules of A, B, and C into a ABCBA-sequenced molecule
with two alkyne unit in the three-component reaction of
amine−thiol−ene conjugating ensures the stoichiometric
balance between the in situ produced ABCBA molecule and
diamine in the following three-component polymerization of
alkyne−azide−amine coupling, which is very important for
obtaining polymer with high molecular weight because diyne
and diamine are involved in the chain-growth process of the
step-growth polymerization.12 We used NMR spectroscopy to
trace the amine−thiol−ene conjugating reaction, and the
detailed results are shown in Figure 1. The signal at 4.53
ppm (I, methine proton in N-(carbobenzyloxy)homocysteine
thiolactone) decreased with the increase of reaction time and
shifted to 4.22 ppm (I′, Figure 1A) which increased with the
increase of reaction time as shown in 1H NMR spectra (Figure
1A). Also, as shown in 13C NMR spectra, lactone unit (8,
chemical shift at 205 ppm) is transformed into amide (8′,
chemical shift at 172 ppm) (Figure 1B). The conversion of
thiolactone to thiol is shown in Figure 2A; the conversion
reached 42% after only 20 min, 89% after 6 h, 99% after 12 h,
indicating almost quantitative conversion of the starting N-
(carbobenzyloxy)homocysteine thiolactone (B) and 4,7,10-
trioxa-1,13-tridecanediamine (C) into a BCB molecule with
two thiol units after 12 h reaction.
1
white crystal. Yield was 35%. H NMR spectrum (300 MHz, CDCl3):
δ 1.986 (m, 1H), δ 2.889 (m, 1H), δ 3.274 (m, 2H), δ 4.337 (m, 1H),
δ 5.127 (s, 2H), δ 5.215 (s, 1H).
Two Consecutive Multicomponent Reactions in One Pot.
Propargyl methacrylate (2 mmol), N-(carbobenzyloxy)homocysteine
thiolactone (2 mmol), 4,7,10-trioxa-1,13-tridecanediamine (1 mmol),
and triethylamine (10 mmol) were dissolved in DMF (3 mL). Then,
the mixture was degassed with argon for 2 min, and afterward the
reaction was performed under stirring at room temperature for 12 h.
Subsequently, CuCl (0.15 mmol), 1,4-phenylenediamine (1 mmol),
and p-toluenesulfonyl azide (4 mmol) were added into the reaction
mixture under an argon atmosphere. After the polymerization has been
carried out at 70 °C for 24 h, the reaction solution was precipitated
into methanol. The obtained crude product was washed several times
using EDTA-2Na aqueous solution to remove the residual copper.
Then the obtained polymer was separated by filtration and dried under
vacuum to obtain the corresponding product as a brown solid.
RESULTS AND DISCUSSION
■
Du Prez25,26 and Endo27 et al. introduced three-component
reaction of amine−thiol−ene conjugating into polymer syn-
thesis. This conjugating reaction has high efficiency under mild
reaction conditions. The three-component reaction of prop-
argyl methacrylate (A), N-(carbobenzyloxy)homocysteine
thiolactone (B), and 4,7,10-trioxa-1,13-tridecanediamine (C)
was carried out at a molar ratio of 1:1:1 in the presence of
triethylamine (TEA) as catalyst, which is similar to previous
reports.25 4,7,10-Trioxa-1,13-tridecanediamine would cause the
ring-opening of N-(carbobenzyloxy)homocysteine thiolactone
to give a thiol intermediate, which can further react with the
Simultaneously, the in situ produced thiol intermediate is
very susceptible to reacting with the double bond of
methacrylate (A) via Michael addition reaction,28,29 yielding
the ABCBA-sequenced molecule with two alkyne units (diyne).
1H NMR, 13C NMR, and ESI-MS spectra of the obtained diyne
D
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Figure 3. continued
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1
Figure 3. SEC curve of the sequence-controlled polymer (A), H NMR spectrum (B), and 13C NMR spectrum (C) of the obtained sequence-
controlled polymers with via sequential multicomponent reactions (x is methanol).
Table 1. Results of the Obtained Sequence-Controlled
Polymers with via Sequential Multicomponent Reactions
Scheme 2. Outline of the Synthesis of Sequence-Controlled
Polymer via Two Consecutive Multicomponent Reactions of
Amine−Thiol−Ene Conjugating and Alkyne−Azide−Diol
Coupling in One Pot
entry
A
B
C
D
E
Mn
PDI
a
b
c
f
d
e
1
2
3
4
5
PM
PM
PM
PM
PM
NC
TT
PT
PT
PT
PT
PT
1,4-PA
1,4-PA
1,2-PA
DD
72 600
130 700
153 300
36 400
1.8
1.5
1.9
2.0
1.4
i
NT
NT
NT
NT
DD
DD
TT
DD
g
h
BD
163 200
a
b
PM is propargyl methacrylate. NC is N-(carbobenzyloxy)-
c
homocysteinethiolactone. TT is 4,7,10-trioxa-1,13-tridecanediamine.
d
e
f
PT is p-toluenesulfonyl azide. 1,4-PA is 1,4-phenylenediamine. DD
g
is 2,2-dimethylpropane-1,3-diamine. 1,2-PA is 1,2-phenylenediamine.
h
i
BD is butane-1,4-diol. NT is N-acetohomocysteine thiolactone.
demonstrate the formation of ABCBA with two alkyne units. As
is apparent from Figure 1A, the signal of methacrylate double
bond protons (δ = 5.75, 6.1 ppm) decreased with the increase
of reaction time, which demonstrates the formation of thioether
between the electron-deficient carbon−carbon double bond of
propargyl methacrylate and the thiol, released from the
aminolysis of N-(carbobenzyloxy)homocysteine thiolactone.
13C NMR results showed that the signals at 126.8 and 136.9
ppm (carbons of double bond) shifted to 34.5 and 36.1 ppm,
respectively, after thiol−Michael addition. The conversion of
electron-deficient carbon−carbon double bond reached 28% in
20 min, 86% after 6 h, almost 98% in 12 h as shown in Figure
2B. The high-resolution spectrum of the isolated the formed
new intermediate molecule of ABCBA with two alkyne units is
shown in Figure S3; the main peak with mass value of 993.3959
corresponds to the sum of the mass of the intermediate
molecule (970.41) and the mass of sodium cation (22.99).
Subsequently, p-toluenesulfonyl azide (D), 1,4-phenylenedi-
amine (E), and CuCl were directly added into the above
reaction mixture; a large number of bubbles were released from
the mixture, indicating that the multicomponent polymerization
of ABCBA,12,30 1,4-phenylenediamine, and p-toluenesulfonyl
azide occurs via azide−alkyne−amine coupling. After reaction
for 24 h, the reaction mixture became dark brown. In this
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coming from propargyl methacrylate (A monomer), the peaks
at 5.0 ppm (i protons) and 1.75 ppm (h protons) coming from
N-(carbobenzyloxy)homocysteine thiolactone (B monomer),
the peaks at δ 2.3 ppm (u protons) and 7.0−8.0 ppm (w, v
protons) coming from p-toluene sulfonylazide (D monomer),
and the signals at δ 7.0−8.0 ppm ascribed to 1,4-phenylenedi-
amine (E monomer) ppm are all present in the obtained
polymer. On the other hand, the signals at δ 10.4 ppm for the
proton of amidine, formed from the multicomponent polymer-
ization of azide−alkyne−amine, also appeared in the obtained
polymer, which demonstrates that all the starting molecules are
cooperated into the sequence-controlled polymer. In Figure
3C, the peaks coming from the starting molecules A, B, C, D,
and E are all present in the 13C NMR spectrum of the obtained
sequence-controlled polymer, demonstrating the obtained
polymer having a DABCBADE sequence in the structure.
Furthermore, SEC was used to characterize the molecular
weight of the formed sequence-controlled polymer, the
obtained polymer has a molecular weight of 72.6 kDa and
PDI of 1.8 as shown in Figure 3A.
Moreover, this strategy is not limited in using above starting
molecules; for example, if 2,2-dimethylpropane-1,3-diamine
(DD) was used in the ring-opening of thiolactone instead of
4,7,10-trioxa-1,13-tridecanediamine (C), the obtained polymer
with a DCBABCDE sequence has molecular weight of 130.7
kDa and PDI of 1.5. The molecular weight is much higher than
that of the obtained polymer using 4,7,10-trioxa-1,13-
tridecanediamine in the ring-opening of thiolactone, which
may result from that 2,2-dimethylpropane-1,3-diamine has very
short space, making the corresponding polymer chain more
rigidity to suppress cyclization.12 On the other hand, in the
multicomponent polymerization of azide−alkyne−amine stage,
the polymers obtained using phenylenediamines have much
higher molecular weight than that of polymer produced using
alkyldiamines as shown in Table 1, which is due to that the
polymers produced from phenylenediamines are more rigid
than the polymer produced from alkyldiamine, which can
suppress entanglement and cyclization although phenylenedi-
amine possesses weaker nucleophilicity than alkyldiamines.12
Compared with those sequence-controlled polymers prepared
via single multicomponent polymerization, the polymers
prepared via two consecutive multicomponent reaction not
only have more components but also have much higher
molecular weight. Therefore, this strategy is a promising
method for the synthesis of sequence-controlled polymer with
high molecular weight.
On the other hand, diol is also a nucleophile, if diol (butane-
1,4-diol, BD) is used in the above multicomponent polymer-
ization instead of diamine, poly(imidate) macromolecules will
be formed by an analogous Cu-catalyzed multicomponent
polymerization.31 Because of the weaker nucleophilicity of
diols, they could be competed by even a small amount of H2O
which reacts with ketenimine intermediately to terminate the
polymerization;24 therefore, anhydrous and highly purified
DMF is used as the solvent in the synthesis, and the
polymerization is prolonged to 72 h, a high-molecular-weight
polymer is obtained as shown in Scheme 2 and Figure 4.
1
Figure 4. H NMR spectrum (A), 13C NMR spectrum (B), and SEC
curve (C) for the obtained sequence-controlled polymervia two
consecutive multicomponent reactions of amine−thiol−ene conjugat-
ing and alkyne−azide−diol coupling in single pot (X is diethyl ether).
multicomponent polymerization of azide−alkyne−amine cou-
pling, Cu-catalyzed cycloaddition of azide−alkyne (CuAAC)
first occurred, and subsequently, nucleophilic amine attack to
ring-open the cycloaddition product of azide−alkyne, forming
poly(amidine) macromolecules. Figure 3 shows the spectra of
1H NMR and 13C NMR in DMSO-d6. Based on the H NMR
1
CONCLUSIONS
spectrum, all the starting molecules are present in the targeting
sequence-controlled polymer. Particularly, the peaks at δ 3.2−
3.6 ppm (a and b protons) and 1.6 ppm (c protons) originating
from 4,7,10-trioxa-1,13-tridecanediamine (C monomer), the
peaks at δ 1.1 ppm (p protons) and 4.2 ppm (q protons)
■
In conclusion, the process that allows two consecutive three-
component reactions toward sequence-controlled polymers in
one-pot was reported, in which first, the three-component-
reaction of propargyl methacrylate, N-(carbobenzyloxy)-
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ASSOCIATED CONTENT
* Supporting Information
NMR spectra of the prepared monomers and the obtained
polymers; SEC curves for the obtained polymers. The
Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b00463.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail hongcy@ustc.edu.cn (C.-Y.H.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from the National Natural Science
Foundation of China (51273187, 21374107, and 21474097),
the Fundamental Research Funds for the Central Universities
(WK2060200012), and the Program for New Century
Excellent Talents in Universities (NCET-11-0882).
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