Well-Defined Poly(Ester Amide)-Based Homo- and Block Copolymers by One-Pot Organocatalytic Anionic Ring-Opening Copolymerization of N-Sulfonyl Aziridines and Cyclic Anhydrides

Article abstract of DOI:10.1002/anie.202015339

We report a new synthetic methodology for poly(ester amide)s by anionic ring-opening copolymerization of N-sulfonyl aziridines and cyclic anhydrides. Phosphazenes organocatalysts have been found to promote a highly-active, controlled, and selective altern

Full text of DOI:10.1002/anie.202015339

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Accepted Article
Title: Well-defined poly(ester amide)-based homo- and block
copolymers by one-pot organocatalytic anionic ring-opening
copolymerization of N-sulfonyl aziridines and cyclic anhydrides
Authors: Nikos Hadjichristidis and Jiaxi Xu
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Well-defined poly(ester amide)based homo- and block copolymers by
one-pot organocatalytic anionic ring-opening copolymerization of N-
sulfonyl aziridines and cyclic anhydrides
Jiaxi Xu and Nikos Hadjichristidis*
King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division,
KAUST Catalysis Center, Polymer Synthesis Laboratory, Thuwal 23955, Saudi Arabia
Abstract
We report a new synthetic methodology for poly(ester amide)s by anionic ring-opening
copolymerization of N-sulfonyl aziridines and cyclic anhydrides. Phosphazenes organocatalysts
have been found to promote a highly-active, controlled, and selective alternate copolymerization
in the absence of any competitive side reaction (zwitterionic mechanism and exchange
transacylations). Mechanistic studies have shown first-order dependence of the
copolymerization rate on N-sulfonyl aziridines and phosphazenes, and zero-order on cyclic
anhydrides. This one-pot methodology leads not only to homopolymers but also to poly(ester
amide)-based block copolymers. Two catalytic cycles involving ring-opening alternating
copolymerization of N-sulfonyl aziridines with cyclic anhydrides and ring-opening polymerization
of N-sulfonyl aziridines have been proposed to explain the one pot synthesis of poly(ester amide)-
based homo- and block copolymers.
1
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Poly(ester amide)s have gained an increasing interest in recent years as they combine, in the
same macromolecule, the biodegradability/biocompatibility of polyesters with excellent thermal
and mechanical properties of polyamides. Therefore, they are widely used in a variety of fields,
including drug delivery systems, hydrogels, non-viral gene carriers, composite materials, and
tissue engineering.[1] Poly(ester amide)s are traditionally produced by polycondensation
reactions from ester-containing diamines with dicarboxylic acids or their derivatives. However,
this yields by-products and low molecular weight polymers with broad distributions. In contrast
to polycondensation, ring-opening polymerization (ROP) of morpholino-2,5-diones is more atom-
economical.[2] Furthermore, the ROP of morpholino-2,5-diones is generally catalyzed by metals
or enzymes in bulk at high temperatures, resulting in uncontrolled products due to unavoidable
side reactions;[3] a significant amount of organocatalysts is required to suppress side effects.[4]
The ring-opening copolymerization of lactams and lactones can also produce copolymers with
ester and amide linkages, although not alternating and contaminated by exchange transacylation
reactions products.[5]
Inspired by the rapid advancement in polyester synthesis by ring-opening alternating
copolymerization (ROAP) of epoxides with cyclic anhydrides,[6] we thought that the ROAP of
aziridines with cyclic anhydrides is a potential synthetic methodology for poly(ester amide)s. In
addition, the functionalization of aziridines ring-nitrogen will increase the potential structural
diversity of poly(ester amide)s.
Already, a limited number of reports are available for copolymerization of aziridines with cyclic
anhydrides.[7] Although aziridines have a similar chemical structure and ring strain to epoxides,
the strong nucleophilic ring-nitrogen allows aziridines to act as nucleophilic monomers, in
contrast to the electrophilic epoxide monomers. Aziridines, even without initiators, copolymerize
with cyclic anhydrides through a zwitterionic mechanism, leading to insoluble oligomers and
copolymers with uncontrolled molecular weight.[8] Less nucleophilic N-alkyl aziridines were
designed to control their copolymerization behavior[9], but, unfortunately, copolymers with
uncontrolled molecular weight and composition were obtained. Even the use of benzyl alcohol
as an initiator for the copolymerization of N-alkyl aziridines with cyclic anhydrides has led to
uncontrolled products due to the unavoidable zwitterionic side reactions.[10] Several competing
2
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10.1002/anie.202015339
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side reactions have also been observed, such as intra- and intermolecular exchange
transacylations. To the best of our knowledge, there is no report on the controlled and selective
copolymerization to produce poly(ester amide)s from aziridine derivatives with cyclic anhydrides.
Since the first report in 2005 by Bergman and Toste,[12] among aziridines, the N-sulfonyl aziridines
have been the most suitable monomers for anionic ROP. The strong electron-withdrawing
sulfonyl group weakens the nucleophilicity of the ring-nitrogen and thus making the N-sulfonyl
aziridines electrophilic monomers. To date, however, only racemic N-sulfonyl aziridines have
been successfully polymerized to produce linear polymers.[11] The polymerization of
unsubstituted or stereoregular N-sulfonyl aziridines is challenging as they produce only insoluble
oligomers. The copolymerization of two ring-unsubstituted N-sulfonyl aziridines [N-
(methylsulfonyl)aziridine and N-(sec-butylsulfonyl)aziridine], produces soluble random
copolymers.[12] Copolymerization of unsubstituted N-sulfonyl aziridines with cyclic anhydrides
was a possible method to solve the solubility problem, but copolymerization with cyclic
anhydrides was challenging, as unsubstituted N-sulfonyl aziridines are easily undergone
homopolymerization.
Our first attempt was to copolymerize N-tosylaziridine (TAz) with phthalic anhydride (PA) in the
absence of an initiator and a catalyst (Entry 1, Table 1). There was no peaks of copolymers in the
1H NMR spectrum, indicating the absence of a spontaneous zwitterionic copolymerization
(Scheme S1, a). The ring-nitrogen in TAz does not show a nucleophilic character because the
strong electron-withdrawing toluene sulfonyl group delocalizes the lone pair electrons. Even the
addition of an initiator [BnN(H)Ts] in the mixture of TAz and PA did not promote copolymerization
(Entry 2, Table 1), indicating that the nucleophilicity of an initiator is insufficient, and the
presence of a catalyst is necessary.
Phosphazene bases are excellent organocatalysts for ROP and ROAP because they combine
high/tunable basicity and non-nucleophilic nature.[13] Although phosphazene bases exhibit high
catalytic activity in ROP of lactones and ROAP of epoxides with cyclic anhydrides, the molecular
weight distributions of the resultant polymers are broad because of the lability of esters under
the polymerization conditions, when intermolecular transesterification and intramolecular back-
3
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[14]
biting reactions are inevitable.
Compared to the high catalytic efficiency of strong basic t-
BuP4, the weaker bases t-BuP1 and t-BuP2 are more suitable for the polymerization of monomers
with ester groups.[14] Unfortunately, the highly reactive t-BuP4 causes more side reactions, and
the well-controlled/well-selective t-BuP1 and t-BuP2 are limited to catalytic reactivity in the
polymerization of ester-containing monomers.
We tried to add t-BuP4 into the mixture of TAz and PA in the presence of an initiator (Entry 3,
Table 1). The copolymerization of TAz with PA reached 96% conversion in 20 h. The molecular
weight distribution of the resulting copolymers was surprisingly narrow (Ð = 1.06, Figure 1a,
bottom). After prolonging the reaction time to 48 h (Entry 4, Table 1), the distribution remained
the same (Ð = 1.06, Figure 1a, top), indicating the absence of competitive inter- and
intramolecular exchange transacylations (Scheme S1, b). The produced negative charge on
nitrogen, at the chain-end, can be delocalized by the strong electron-withdrawing toluene
sulfonyl groups, revealing a low nucleophilic character; and thus, competitive transacylation
exchanges are suppressed. This is the first time a super-strong base, t-BuP4, has been used to
promote a high-reactive, high-controlled, and high-selective organocatalytic polymerization of
ester-containing monomers.
Scheme 1. Copolymerizations of N-sulfonyl aziridines with PAa
4
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Table 1. Copolymerizations of N-sulfonyl aziridines with PAa
c
b
Entry Monom. Base
(M)
[TAz]0/[PA]0/[I]0/ T/oC
[Base]0
t/h
Conv.b/ Mn,theor
/
Mn,NMR
/
Đd
%
kg.mol-1
kg.mol-1 Mw/Mn
1
TAz
TAz
TAz
-
-
15/15/0/0
25
25
25
24
24
20
48
24
48
24
48
1
-
-
-
-
2
15/15/ 1/ 0
-
-
-
-
3
t-BuP4 15/15/1/0.3
t-BuP2 15/15/1/0.3
t-BuP1 15/15/1/0.3
t-BuP4 15/15/1/0.3
96
99
95
99
59
92
92
99
96
98
99e
95
94
99
5.23
5.39
5.18
5.39
3.32
5.03
5.03
5.39
16.8
34.1
5.39f
5.04
4.98
5.27
5.35
5.57
5.38
5.50
3.48
5.10
5.13
5.58
17.2
35.7
5.56
5.12
5.02
5.31
1.06
1.06
1.09
1.08
1.10
1.09
1.07
1.07
1.04
1.02
1.07
1.07
1.07
1.08
4
5
TAz
TAz
TAz
25
25
50
6
7
8
9
10
11
12
13
14g
15h
16i
24
2.5
12
3
TAz
TAz
TAz
TAz
TAz
TAz
t-BuP4 50/50/1/0.3
t-BuP4 100/100/1/0.3
t-BuP2 40/15/1/0.3
t-BuP2 15/15/1/0.3
t-BuP2 15/15/1/0.3
t-BuP2 15/15/1/0.6
50
50
25
25
25
25
24
24
24
5
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17j
18k
19l
20
TAz
TAz
TAz
BAz
NAz
t-BuP2 15/15/1/0.9
t-BuP2 15/15/1/0.3
t-BuP2 15/15/1/0.3
t-BuP4 15/15/1/0.3
t-BuP4 15/15/1/0.3
25
25
25
25
25
24
24
24
20
20
99
5.30
5.39
3.47
6.35
N.A.m
5.39
5.41
3.31
5.95
N.A.m
1.07
1.07
1.12
1.07
N.A.m
99
62
99
21
N.A.m
aThe copolymerizations were initiated by BnN(H)Ts at 25oC in THF ([PA]0 = 1.0 M). bTAz conversion was determined
1
c
by H NMR in CDCl3 using integrals of the characteristic signals. Calculated as follows: ([PA]0/[I]0) × conv. of TAz ×
(M.W. of N-sulfonyl aziridines + M.W. of PA) + (M.W. of Initiator). dDetermined by SEC traces at 35oC in THF (1.0 mL
min-1) using PSt standards. ePA conversion was determined by in-situ FTIR. fCalculated as follows: ([PA]0/[I]0) × conv.
g
h
of PA × (M.W. of N-sulfonyl aziridines + M.W. of PA) + (M.W. of Initiator). Initiated by benzoic acid. Initiated by
i
j
k
l
benzyl alcohol. Initiated by 1,4-benzenedimethanol. Initiated by 1,3,5-benzenetrimethanol. In DMF. In CH2Cl2.
mCopolymers dissolved in solvent.
Figure 1. (a) Bottom: SEC traces of copolymers of TAz and PA in 20 h (Entry 3, Table 1); Top: the reaction was
1
prolonged to 48 h (Entry 4, Table 1); (b) FTIR spectrum of the copolymers powder; (c) H NMR spectrum of
copolymers. (d) MALDI-ToF MS spectrum of the copolymers. (e) SEC traces of copolymers with different molecular
weights (Entry 9, 11, and 12, Table 1).
FTIR characterization was used to confirm the structure of copolymers (Figure 1b). The
absorption peaks at 1721 cm-1 and 1691 cm-1 correspond to the stretching vibration of C=O in
6
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esters and C=O in amide units, respectively. The FTIR spectrum exhibits peaks at 1273 cm-1 and
1166 cm-1, corresponding to C-N and C-O stretching vibrations. The results confirm the presence
of the ester and amide linkages in the copolymers. The chemical structures of the copolymers
1
were further analyzed by H NMR and 13C NMR spectroscopy (Figure 1c and Figure S1). The
characteristic signal of the methylene protons of the TAz moves to a low field from 2.37 ppm to
4.45 ppm and 4.21 ppm after the formation of esters and amides linkages (e and f in Figure 2c).
Peaks corresponding to the methylene of BnN(H)Ts are observed at 5.03 ppm (a in Figure 2c),
indicating BnN(H)Ts as an effective initiator. We did not observe any amine linkage signals at
3.15-3.34 ppm, showing its perfectly alternating structure.
To further confirm the structure and terminal functional groups of poly(TAz-alt-PA), matrix-
assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used
(Figure 1d). Two main series of peaks are observed in accordance with sodium-cationized
poly(TAz-alt-PA), which contains BnN(H)Ts at one chain-end, and TAz or PA at the other chain-
end. The molecular mass of adjacent peaks showed a fixed interval of 345.07, which is the
molecular mass of a repeating unit of TAz and PA. The results show that the copolymers have a
perfectly alternating nature having at one chain-end BnN(H)Ts and at the other chain-end either
TAz or PA. No molecular mass corresponding to chains from inter- and intramolecular
nucleophilic attacks was observed, indicating the absence of competing side reactions. At high
conversion when the concentration of the monomers is low, a slightly decreasing alternating
degree was observed in MALDI-TOF MS, but 1H NMR spectrum was not observed amine linkage
signals and SEC traces remained narrow.
The less basic phosphazene bases, t-BuP1 and t-BuP2, caused lower reaction rates (Entry 5-8,
Table 1). The alternating degree was not affected, but a slightly broader Ð was found by SEC
(Figure S10-S13). The results show that the basicity of phosphazens mainly affects the rates and
not molecular weight or Ð. Increasing the reaction temperature to 50 °C significantly accelerates
ROAP (Entries 9 and 10, Table 1). Narrow and unimodal traces of SEC (Figure S14-S15) remain,
even with prolonged reaction time. Increasing the reaction temperature to 50 °C significantly
accelerates ROAP (Entries 9 and 10, Table 1). Narrow and monotropic traces of SEC (Figure S14-
S15) remain, even with prolonged reaction time. The ROAP of TAz with PA is fast, highly-
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controlled, and highly-selective at 50oC without competitive side reactions. For further
evaluation of the catalytic activity, the monomer/initiator feed ratios were increased to 50 and
100 (Entry 11 and 12, Table 1). Well-controlled molecular weights with narrow distributions (Ð =
1.04 and 1.02) were observed, indicating a highly controlled polymerization behavior.
Given the homopolymerizability of TAz, excess TAz as the “third” monomer could perform a one-
pot synthesis of P(TAz-alt-PA)-b-PTAz type diblock copolymers since homopolymerization of TAz
will happen after complete consumption of PA. To closely monitor the copolymerization progress,
we used a low-reactive t-BuP2 as the organocatalyst (Entry 13, Table 1). Excess TAz accelerated
the progress of ROAP; complete conversion of PA was reached in 3 h. As demonstrated by in-situ
FTIR (Figure 2a), the consistency of the decrease of PA and increase of esters and amides groups
1
indicates an alternating copolymerization. As monitored by H NMR spectroscopy (Figure S9),
PTAz peaks appeared and increased only after the complete consumption of PA. The molecular
weight was increased by maintaining unimodal and narrow distributions (Figure S16), indicating
the formation of a block sequence rather than mixtures of P(TAz-alt-PA) and PTAz.
8
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Figure 2. (a-b) In-situ FTIR surface image and the conversion of PA overtime for ROAP at a ratio of
[TAz]0/[PA]0/[BnN(H)Ts]0/[t-BuP2] = 40/15/1/0.3. (c) Molecular weight (Mn,NMR) and molecular weight distribution (Ð)
over TAz conversion for the ROAP; (d-f) Kinetics of the ROAP of TAz with PA in-situ FTIR at fixed time intervals of 1
min.
Kinetic studies were performed to monitor the copolymerization progress (Figure 2). The
conversion of PA shows a linear relationship with time (Figure 2b), indicating a zero-order
dependence on the concentration of PA. As shown in Figure 2d-2e, the kinetics reveal that the
rates is a first-order dependence of TAz concentration, a zero-order dependence of the
concentration of PA, and a first order dependence of t-BuP2 concentration. Together, the data
support a complete rate law of d[PA]/dt = kobs = kp[TAz][t-BuP2]. The results indicate that the ring-
opening of TAz is the rate determined step. The evolution of molecular weight against TAz
conversion showed a linear correlation with narrow distribution (Ð < 1.10, Figure 2c), suggesting
a living behavior.
9
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Scheme 2. Plausible copolymerization pathways of TAz with PA towards poly(ester amide)-based homo-
and block copolymers.
Based on the above studies, a plausible mechanism of the copolymerization of TAz with PA is
proposed (Scheme 2). The super Brønsted base t-BuP4 (pKa = 42.6 in MeCN)[13] abstracted a
proton from the initiator, forming an active nitrogen anion, as supported by NMR titration
experiments (Figure S30). The nitrogen anion attacks and ring-opens the TAz, and the active
anion center is transformed into amine chain end. Then, the amine anion easily attacks the higher
reactive PA. The propagation begins through the anionic chain-end as activating species to
copolymerize the monomers (Scheme 2, Cycle 1). The self-propagation of TAz occurs after full PA
consumption (Scheme 2, Cycle 2).
The influence of the initiator structure was investigated (Entry 14-17, Table 1). Benzoic acid and
benzyl alcohol showed practically the same catalytic efficiency, indicating that the carboxyl group
and primary hydroxyl groups are the actual initiators. High polar DMF causes a faster reaction
rate without breaking the alternating degree, and the low polar CH2Cl2 affords a lower reaction
rate along with a slight decrease in the alternating degrees (Entry 18-19, Table 1). Other N-
sulfonyl aziridines were synthesized and examined for the ROAP with PA. A higher active N-
brosylaziridine (BAz) was copolymerized with narrow molecular weight distribution (Entry 20,
Table 1, Figure S23). However, the higher activity of N-(4-nitrobenzenesulfonyl)aziridine (NAz)
10
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from PA, produced an insoluble white powder, due to the presence of insoluble blocks of
repeating units of NAz,[15] indicating lower alternating degrees.
The resulted poly(ester amide)s were analyzed by thermogravimetric analysis and differential
scanning calorimetry (Figure 3), showing a Td 5% of 265 °C and a Tg of 114 °C. The thermal stability
is almost the same as that of the poly(propylene oxide-alt-PA) (Td 5% of 269 °C),[16] while the Tg of
poly(TAz-alt-PA) significantly increase than the poly(propylene oxide-alt-PA) (Tg of 55 °C),[17]
because of amide linkages. Compared with poly[(3S)-3-benzyl 6-methyl-morpholine-2, 5-dione]
from ROP of (3S)-3-benzyl 6-methyl-morpholine-2, 5-dione (Td 5% of 295 °C, Tg of 100 °C),[4a] the
decreasing in thermal stability was observed, and the Tg of poly(TAz-alt-PA) increases under
influence of the benzene ring in backbone.
Figure 3. (a) TGA thermogram for poly(TAz-alt-PA) under N2; (b) DSC thermogram for poly(TAz-alt-PA) under N2.
In conclusion, we reported a new synthetic methodology of poly(ester amide)s from ROAP of N-
sulfonyl aziridines with cyclic anhydrides. The commercially available phosphazenes
organocatalysts were used to perform a highly-active, highly-controlled, and highly-selective
copolymerization without any competing side reactions (zwitterionic mechanism and exchange
transacylations). Poly(ester amide)s are obtained in an energy-saving way (temperature  50oC)
with perfectly alternating sequence distribution, highly controlled molecular weight, and narrow
molecular weight distribution (Ð < 1.10). The copolymerization progress is first-order
dependence on N-sulfonyl aziridines concentration, a zero-order dependence on cyclic
11
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anhydrides concentration, and a first-order dependence on phosphazenes concentration. The
effects of temperature, solvent, initiator, and basicity of phosphazenes were studied. Two
catalytic cycles involving ROAP of N-sulfonyl aziridines with cyclic anhydrides and ROP of N-
sulfonyl aziridines were proposed to explain the synthesis of alternating copolymers and diblock
copolymers in one-pot.
Acknowledgements
The research work is supported by King Abdullah University of Science and Technology (KAUST),
Thuwal, Saudi Arabia.
Conflict of interest
The authors declare no conflict of interest.
Keywords: anhydrides  anionic copolymerization  phosphazenes  poly(ester amide)s  N-
sulfonyl aziridines
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Entry for the Table of Contents
Phosphazenes organocatalysts have been found to promote a highly-active, controlled, and
selective alternating ring-opening copolymerization of N-sulfonyl aziridines and cyclic anhydrides.
This one-pot methodology leads not only to homopolymers but also to poly(ester amide)-based
block copolymers.
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