Exploring Oxidative NHC-Catalysis as Organocatalytic Polymerization Strategy towards Polyamide Oligomers

Article abstract of DOI:10.1002/chem.202004296

The polycondensation of diamines and dialdehydes promoted by an N-heterocyclic carbene (NHC) catalyst in the presence of a quinone oxidant and hexafluoro-2-propanol (HFIP) is herein presented for the synthesis of oligomeric polyamides (PAs), which are obtained with a number-average molecular weight (M_n) in the range of 1.7–3.6 kg mol^?1 as determined by NMR analysis. In particular, the utilization of furanic dialdehyde monomers (2,5-diformylfuran, DFF; 5,5’-[oxybis(methylene)]bis[2-furaldehyde], OBFA) to access known and previously unreported biobased PAs is illustrated. The synthesis of higher molecular weight PAs (poly(decamethylene terephthalamide, PA10T, M_n = 62.8 kg mol^?1; poly(decamethylene 2,5-furandicarboxylamide, PA10F, M_n = 6.5 kg mol^?1) by a two-step polycondensation approach is also described. The thermal properties (TGA and DSC analyses) of the synthesized PAs are reported.

Full text of DOI:10.1002/chem.202004296

Accepted Article
Accepted Article
Title: Exploring Oxidative NHC-Catalysis as Organocatalytic
Polymerization Strategy Towards Polyamide Oligomers
Authors: Alessandro Massi, Daniele Ragno, Arianna Brandolese,
Graziano Di Carmine, Sara Buoso, Giada Belletti, Costanza
Leonardi, Olga Bortolini, and Monica Bertoldo
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202004296
Link to VoR: https://doi.org/10.1002/chem.202004296
01/2020

10.1002/chem.202004296
Chemistry - A European Journal
FULL PAPER
Exploring Oxidative NHC-Catalysis as Organocatalytic
Polymerization Strategy Towards Polyamide Oligomers
Daniele Ragno,[a] Arianna Brandolese,[a] Graziano Di Carmine,[b] Sara Buoso,[c] Giada Belletti,[a]
Costanza Leonardi,[a] Olga Bortolini,[a] Monica Bertoldo,[a] and Alessandro Massi*[a]
[a]
Dr. D. Ragno, A. Brandolese, G. Belletti, C. Leonardi, Prof. O. Bortolini, Prof. M. Bertoldo, Prof. A. Massi
Department of Chemical and Pharmaceutical Sciences
University of Ferrara
Via L. Borsari, 46, 44121 Ferrara (Italy)
E-mail: alessandro.massi@unife.it
[b]
[c]
Dr. G. Di Carmine
School of Chemical Engineering and Analytical Science
The University of Manchester
The Mill, Sackville Street, Manchester, M13 9PL, UK
S. Buoso
Istituto per la Sintesi Organica e la Fotoreattività
Consiglio Nazionale delle Ricerche, Via P. Gobetti, 101-40129 Bologna (Italy)
Supporting information for this article is given via a link at the end of the document.
Abstract: The polycondensation of diamines and dialdehydes
promoted by an N-heterocyclic carbene (NHC) catalyst in the
presence of a quinone oxidant and hexafluoro-2-propanol (HFIP) is
herein presented for the synthesis of oligomeric polyamides (PAs),
which are obtained with a number-average molecular weight (Mn) in
the range of 1.7-3.6 kg mol-1 as determined by NMR analysis. In
particular, the utilization of furanic dialdehyde monomers (2,5-
these include catalyst stability and avoidance of metal
contamination that is particularly favorable for applications in the
electronic and biomedical fields.[4] Indeed, examples of
organocatalytic synthesis of PAs are mainly limited to the
utilization of guanidine (TBD) and phosphazene (t-BuP4)
Brønsted bases in the ROP of lactams.[5]
diformylfuran,
DFF;
5,5’-[oxybis(methylene)]bis[2-furaldehyde],
OH
O
O
OBFA) to access known and previously unreported bio-based PAs is
illustrated. The synthesis of higher molecular weight PAs
(poly(decamethylene terephthalamide, PA10T, Mn = 62.8 kg mol-1;
poly(decamethylene 2,5-furandicarboxylamide, PA10F, Mn = 6.5 kg
mol-1) by a two-step polycondensation approach is also described.
The thermal properties (TGA and DSC analyses) of the synthesized
PAs are reported.
NH2
+
+
NH2
OH
H2N
X
X
NH2
X = OH, OR, Cl
[Ru]
O
O
O
O
N
H
N
H
N
H
N
H
n
n
O
O
H2N
n
NH
Polyamides
OR
m
Introduction
O
O
+
O
H
N
Polyamides (PAs) are among the most useful class of polymers,
which find a wide range of applications as fibers, films, and high-
performance specialty materials in different types of industries.[1]
High chemical and heat resistance are important features of PAs
arising from intermolecular hydrogen bonding interactions
between the amide portions of polymeric chains. Conventional
routes for the synthesis of PAs involve the polycondensation of
diamines with diacids or their derivatives (acyl chlorides and
esters), the self-polycondensation of w-amino acid esters, and the
ring-opening polymerization (ROP) of lactams or N-carboxyamino
acid anhydrides through standard techniques (melt, interfacial,
and solution polymerization).[2] Less common is the catalytic
polyamidation of diols and diamines through iterative
dehydrogenation promoted by the Ru-based Milstein catalyst
(Figure 1).[3] The implementation of organocatalytic approaches,
which have proven to effectively complement metal-catalysis in
the production of polyesters (PEs), polyurethanes (PUs) and
polycarbonates (PCs), is more rare in the synthesis of PAs
despite the benefits associated with this polymerization strategy;
H2N
NH2
O
H
H
H
N
I
n
m
n
O
O
- mild conditions
N
H
N
H
- reyclacble reagents
- fossil- and bio-based
aldehyde monomers
n
this work
Figure 1. Main strategies for the synthesis of polyamides and the proposed
approach.
Also, the strong basic character of (latent) N-heterocyclic carbene
(NHC) catalysts has been exploited by DuPont in 2006 for the
polymerization of e-caprolactam[6] and later by the group of
Buchmeiser for the preparation of PA12 and PA6,12.[7] In addition
to the Brønsted base behavior, NHCs are also distinguished by
ambiphilic (a-basicity, p-acidity) and moderate nucleophilic
attributes,[8] which can be conveniently considered for the
polymerization of different monomers such as epoxides, lactones,
1
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10.1002/chem.202004296
Chemistry - A European Journal
FULL PAPER
anhydrides, carbonates, acrylates, siloxanes, and also
aldehydes.[9,10]
intermediates.[15d,g] Actually, application of the reaction conditions
previously optimized for synthesis of PEs by the same oxidative
strategy[11] (triazolium salt C1 (5 mol%) as the pre-catalyst, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU, 12.5 mol%) as the base
and quinone 4 (1 equiv.) as the external oxidant in anhydrous
THF), resulted in no formation of the target oligomer 3aa because
of the formation of a complex reaction mixture containing low
molecular weight (LMW) mixed aldehyde and imine derivatives of
2a (mainly monomers and dimers) as judged by ESI-MS analysis
(entry 1).
Our group recently contributed in this area of research describing
the unprecedented polycondensation of dialdehydes and diols
promoted by NHCs under oxidative conditions to access PEs by
the step-growth polymerization technique.[11] Key step of this
approach is the oxidation of the Breslow intermediate to generate
the corresponding acyl azolium, which is susceptible of
nucleophilic attack by the diol for completing the formal aldehyde-
to-ester conversion in an iterative fashion (Scheme 1).[12] As a
logical extension of that study, we herein report our results on the
synthesis of polyamide oligomers by the same strategy using
dialdehydes as mild acylating monomers of diamines. Actually,
downsides of common routes to PAs may be represented by the
elevated polycondensation temperatures and the need of reduced
pressures to remove the condensate, as well as the use of
hazardous halogenating reagents (e.g. thionyl chloride) in case of
more reactive diacyl chloride monomers. At the same time, the
ROP strategy may require the multi-step synthesis of starting
lactams, that is a time-consuming process especially if functional
PAs are targeted.[5b,c] Therefore, an alternative route to PAs
based on iterative, NHC-catalyzed aldehyde-to-amide conversion
occurring under mild reaction conditions from readably available
aldehyde monomers is worth of investigation (Scheme 1). In the
present contribution we describe advantages and limitations of
the envisaged strategy, which has been applied to the synthesis
of fossil-based and also bio-based polyamide oligomers. As a
matter of fact, there is an increasing interest in the utilization of
renewable resources for the development of sustainable PAs due
to their impact as high-performance materials in specific fields of
polymer chemistry such as automotive and biomedical
sectors.[1,13]
Table 1. Screening of reaction conditions for the synthesis of PETA oligomers
3aa.[a]
NHC.HX (5 mol%)
O
DBU (12.5 mol%)
Oxidant (1 equiv.)
Additive (20 mol%)
O
HN
O
NH2
NH
+
H2N
Anhydrous THF,
1a
RT, 16 h, degas (Ar)
n
3aa
O
2a
O
Me
N
t-Bu
t-Bu
t-Bu
t-Bu
NH
I
N
N
N
N
N
6
Me
N
N
C1
N
N
N
O
N
F
OH
7
4
BF4
F
F
H
N
F
N
C2
F
N
N
8
N
N
5
N
Me
CF3
CF3
9 (HFIP)
HO
Cl
Me
Me
C3
O
O
O
OH
NHC
NHC
H
H
H
Entry
NHC
HX
Oxidant/
Additive
Conv.
(%)[b]
3aa
(%)[c]
Mn
(kg/mol)[d]
Breslow
intermediate
O
O
HO
OH
1
C1
C1
C1
C2
C3
C1
C1
C1
C1
C1
C1
C1
4/-
>95
88
-
-
[O]
Ref. 11
O
O
n
PEs
2
5/-
-
-
O
O
3
5/6
4/7
4/8
4/9
4/9
4/9
4/9
4/9
4/9
4/9
>95
>95
>95
>95
>95
80
-
-
H
NHC
acyl azolium
4
-
-
O
O
5[e]
6[f]
-
-
H2N
NH2
N
H
N
H
this work
90
91
71
86
76
82
92
1.9
1.9
1.2
1.9
1.5
1.1
1.9
n
7[f,g]
8[f,h]
9[f,i]
10[f,j]
11[f,k]
12[l]
PAs
Scheme 1. NHC-based strategy to polyesters (PEs) and polyamides (PAs).
>95
>95
>95
>95
Results and Discussion
The polycondensation of ethylenediamine (EDA) 1a with
terephthalaldehyde 2a was selected as the model reaction using
a slight excess of diamine 1a (1.1 equiv.) to promote the formation
of oligomeric poly(p-ethylene terephthalamide) (PETA)[14] 3aa
with terminal amine groups (Table 1). At the beginning of this
research, we were aware that the direct oxidative N-acylation with
aldehydes[15] may be precluded in the case of primary amines
because of the competing imine formation and that this issue can
be addressed by the addition of nucleophiles as additives[15a,b,c,f]
or by the execution of a two-step procedure with activated ester
[a]Conditions: 1a (0.88 mmol), 2a (0.80 mmol), NHC.HX (0.08 mmol), DBU (0.20
mmol), oxidant (1.60 mmol), additive (0.32 mmol), THF (6.0 mL). [b]Detected by
1H NMR of the crude reaction mixture (durene as internal standard). [c]Isolated
yield via precipitation technique (see the Experimental Section). [d]Calculated by
1H NMR after precipitation of the polymer. [e]Imidazole 8: 1.76 mmol. [f]HFIP 9:
2.40 mmol. [g]Reaction time 24 h. [h]Reaction time 8 h. [i]Reaction run in the
presence of 4 Å molecular sieves. [j]Temperature: 50 °C. [k]Anhydrous DCM as
solvent. [l]Conditions: 1a (11.0 mmol), 2a (10.0 mmol), C1 (1.00 mmol), 4 (20
mmol), DBU (2.5 mmol), 9 (30.0 mmol), THF (60 mL).
2
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The utilization of phenazine 5 as oxidant had no effect on the
reaction outcome producing the same results as the quinone 4
(entry 2). Addition of the nucleophilic co-catalyst 1,2,4-triazole 6
(20 mol%) to the couple C1/5 as reported by Connon and co-
workers[15m] for amidation of primary amines suppressed imine
formation but, unexpectedly, it produced a mixture of mono- and
bis-amide derivatives of 2a without evidence of oligomeric
species (1H NMR and ESI-MS analyses, entry 3). Also,
unsatisfactory results with lack of chain elongation were detected
applying the oxidative amidation procedures of primary amines
developed by the groups of Rovis[15b] and Bode[15a] involving,
respectively, the utilization of C2 (20 mol%) in combination with
1-hydroxy-7-azabenzotriazole (HOAt) 7 (20 mol%; entry 4), and
of C3 (20 mol%) with imidazole 8 (1.1 equiv.; entry 5).
the highly reactive diester V with formation of the amide bond and
release of HFIP in an iterative manner yielding PAs 3.
N
O
O
N
N
O
O
CF3
CF3
H
H
I
H
O
2
2nd Catalytic
Cycle
IV
DBU
-H
C1
O
O
O
OH
CF3
O
O
CF3
CF3
N
N
H
H
N
N
F3C
O
O
N
N
II
F3C
F3C
-2e-, -H
V
OH
III
acyl
azolium
Breslow
Intermediate
9
9
H2N
NH2
1
O
O
In light of the above disappointing findings, the one-pot two-step
amidation strategy via hexafluoroisopropyl esters proposed by
Studer and co-workers[15d] was next investigated. Accordingly, the
mixture of aldehyde 2a, C1 (5 mol%), DBU (12.5 mol%), 4 (1
equiv.), and hexafluoro-2-propanol (HFIP) 9 (1.5 equiv.) in
anhydrous THF was reacted for two hours, then diamine 1a (1.1
equiv.) was added and the mixture stirred under inert atmosphere
(Argon) for an additional 16 hours (entry 6). Gratifyingly, this
procedure allowed for the isolation of 3aa (precipitation
technique) in 90% yield with a number-average molecular weight
4
N
H
N
H
n
3
Scheme 2. Proposed mechanism for the synthesis of PAs 3.
Oligomeric PETA 3aa is typically prepared by polycondensation
of terephthaloyl acid/chloride with ethylenediamine and, in virtue
of its thermal stability and flame retardancy, is conveniently used
as charring agent for the synthesis of halogen-free flame retardant
polypropylene-based composites.[14c,d] In order to establish
strengths and limitations of the proposed methodology, other
semi-aromatic and fully-aromatic PAs 3 of synthetic relevance
were addressed by the iterative N-acylation of diamines 1 with
dialdehyde monomers 2 (Table 2). High molecular weight (HMW)
poly(decamethylene terephthalamide (PA10T) displays excellent
heat and mechanical resistance together with low water
adsorption and good dimensional stability.[1] Because of these
favorable features and easy accessibility of 1,10-decanediamine
(1,10-DDA) 1b from renewable castor oil, bio-based PA10T is
produced by several companies and widely employed in
electronics and automotive industry.[16] Implementation of our
optimized protocol with diamine 1b and terephthalaldehyde 2a
afforded PA10T oligomers 3ba in good isolated yield (88%) and
satisfactory molecular weight (Mn = 2.6 kg mol-1; entry 2).
1
(Mn) of 1.9 kg mol-1 based on H NMR spectroscopic analysis.
Extending the reaction time to 24 hours did not increase the
oligomer length (entry 7), while a shorter time (8 hours) afforded
3aa with lower yield (70%) and Mn (1.2 kg mol-1; entry 8).
Disappointingly, the polymer growth could not be improved either
by adding molecular sieves to avoid water nucleophilic attack onto
acyl azolium (entry 9) or by heating the reaction mixture to 50 °C
(entry 10). The replacement of THF with anhydrous DCM
determined a significant decrease of Mn (1.1 kg mol-1) likely
because of the lower solubility of the growing polymer chain in this
halogenated solvent (entry 11). Satisfyingly, the optimized
conditions of entry 6 could be applied to the gram-scale synthesis
of 3aa (1.75 g, 92% yield) from 10 mmol of 2a (entry 12). Of note,
this experiment also served to demonstrate that the cost and the
environmental impact of the disclosed polyamidation procedure
can be considerably reduced thanks to the simple recovery and
reutilization of both HFIP 9 (evaporation) and quinone 4
(regeneration by air oxidation of the reduction product o,o′-di-tert-
butyl-p-bisphenol; see the Supporting Information for details).
The reaction mechanism hypothesized for the NHC-promoted
polyamidation relies on an ionic pathway in agreement with
Studer proposal[15d] and the general mechanism of oxidative NHC-
catalysis (Scheme 2).[12] Hence, deprotonation of C1 pre-catalyst
by DBU, followed by nucleophilic attack of the generated carbene
I on the dialdehyde 2 affords the Breslow intermediate II, which in
turn is oxidized by the quinone 4 to the corresponding acyl
azolium III. Next, nucleophilic attack by HFIP 9 results in the
formation of the hexafluoroisopropyl monoester IV and the
release of the NHC catalyst. The aldehyde functionality of
monoester IV is then involved in a second catalytic cycle affording
the hexafluoroisopropyl diester V (the concurrent oxidative
esterification of both aldehyde functionalities of 2 seems to be
unlikely but it cannot be excluded). Diester V is, however, the key
substrate of the subsequent amidation step as confirmed by NMR
and IR analyses (see the Supporting Information for details). At
this stage, the diamine 1 introduced in the reaction mixture attacks
Next, by mimicking the two-step polycondensation approach
applied for the production of HMW PAs,[16a,c,d,17] oligomeric 3ba
was utilized as the prepolymer for the synthesis of HMW PA10T
3ba’ (Scheme 3 and Table 2, entry 3).
O
HN
O
8
HN
n
3ba (Mn = 2.6 Kg/mol)
LiCl (5 equiv.)
DIPEA (3 equiv.)
O
HN
O
+
DMA
8
HN
0 °C to RT, 48 h
Cl
O
n
85%
3ba’ (Mn = 62.8 Kg/mol)
O
Cl
10
(0.9 equiv.)
Scheme 3. Two-step procedure for the synthesis of high molecular weight
(HMW) PA10T 3ba’.
3
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10.1002/chem.202004296
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Accordingly, terephthaloyl chloride 10 was added at 0 °C to a
mixture of isolated oligomers 3ba, N,N-diisopropylethylamine
(DIPEA) and LiCl in N,N-dimethylacetamide (DMA), and the
polymerization continued at room temperature for 48 hours
affording 3ba’ (85%) with increased molecular weight (Mn = 62.8
kg mol-1), as evidenced by the diminished integral values of the
end group signals at 3.04 ppm in the 1H NMR spectrum (1:1
CDCl3-HFIP, Figure 2).
molecular structures of these polymers.[16e,18b] The resulting Mn for
3ba’’ was 22.3 kg mol-1, a value which was in good agreement
with that determined by NMR analysis (Mn = 20.2 kg mol-1).
(
)
]
2 [η!" − ln η#$%
ꢀ
[ ]
η =
(eq 1)
ꢁ
!
ꢃ
[ ]
η
ꢂ&
=
(eq 2)
ꢄ
Oligomeric poly(hexamethylene terephthalamide) (PA6T)[16d] 3ca
(Mn = 2.5 kg mol-1) and poly(hexamethylene isophthalamide)
(PA6I)[20] 3cb (Mn = 1.7 kg mol-1) were readily prepared with good
efficiency (84% and 78% yield) by the polycondensation of 1,6-
hexanediamine (1,6-HDA) 1c with terephthalaldehyde 2a and
isophthalaldehyde 2b, respectively (entries 4-5).
Furan-based polyamides are promising sustainable alternatives
to polyphthalamides featuring improved solubility and enhanced
processability, which make them of great commercial interest as
innovative, high performance materials for diversified
applications.[21,22] This special class of PAs is suitably accessed
by chemical and enzymatic methods using diamines and 2,5-
furandicarboxylic acid (FDCA)[23] monomers, the latter serving as
an effective equivalent of terephthalic acid. Therefore, the
compatibility of the disclosed oxidative step-growth polyamidation
methodology with respect to furanic-aliphatic PAs was also
demonstrated by the synthesis of oligomeric poly(decamethylene
furanamide) (PA10F)[17,21] 3bc and poly(hexamethylene
furanamide) (PA6F)[17,21c,d,22] 3cc starting from 2,5-diformylfuran
(DFF) 2c.[24] This bio-based dialdehyde monomer is produced
from the platform chemical 5-hydroxylmethyl furfural (HMF) and it
has been recently employed to prepare furan-urea resins,[25]
imine-based polymers,[26] and polyesters.[27] Satisfyingly,
oligomeric polyamides 3bc (Mn = 1.6 kg mol-1) and 3cc (Mn = 1.7
kg mol-1) were obtained in good yields (81% and 72%) by our
complementary oxidative strategy using DFF 2c in the place of
the corresponding acid FDCA (entries 6-7). In analogy with the
synthesis of HMW PA10T 3ba’, oligomeric 3bc was utilized as the
prepolymer to prepare chain-extended CE-PA10F 3bc’ (Table 2,
entry 8). Accordingly, 2,5-furandicarbonyl dichloride 11 was
added at 0 °C to a mixture of 3bc, DIPEA and LiCl in DMA, and
the polymerization let to proceed at room temperature for 48
hours affording 3bc’ (76%) with increased molecular weight (Mn
= 6.5 kg mol-1) as determined by NMR analysis (Scheme 4).
Figure 2. 1H NMR spectra of oligomeric PAT10 3ba and HMW PA10T 3ba’ (1:1
CDCl3-HFIP).
Validation of the method for Mn determination by NMR analysis
was performed for HMW PA10T by viscometry. Indeed, size
exclusion chromatography (SEC) of semi-aromatic PAs is
complicated by their low solubility in common organic solvents
and the requirement of sophisticated equipment compatible with
pure HFIP as eluent. On the other hand, viscometry is widely
recognized as a reliable method for the determination of
molecular weight for this class of polymers.[16e,18] In particular,
Mathias and co-workers found a correlation between the number-
average molecular weight estimated by NMR and the intrinsic
viscosity ([η]) of a series of PAs[18a] with Mn in the range of ca. 2-
25.2 kg mol-1.[18] Hence, our study started with the synthesis of an
authentic sample of PA10T with a molecular weight to fall in the
calibration range (3ba’’, Mn = 20.2 kg mol-1. See the Supporting
Information). Then, 3ba’’ was fully solubilized in sulfuric acid
(96%) at a concentration of 0.5 g dL−1 (12 hours mixing). Single-
point intrinsic viscosity measurements were performed by means
of an Ubbelohde viscometer in a 25 °C controlled water bath.
Specific viscosity (ηsp) and relative viscosity (ηrel) were obtained
from flow time data of sulfuric acid and 3ba’’ (average of four
values, ±0.2 s). Single point intrinsic viscosity ([η]) was then
determined using the Solomon-Ciutǎ equation 1[19] ([η] = 1.93).
The number-average molecular weight of 3ba’’ was estimated
with the Mark-Houwink equation 2 considering K = 5.58 ´ 10-4 dL
g-1 and a = 0.81. These constants were initially calculated for
PA12T[18a] and subsequently adopted for PA10T due to the similar
O
HN
O
O
8
HN
n
O
3bc (Mn = 1.6 Kg/mol)
LiCl (5 equiv.)
HN
O
O
DIPEA (3 equiv.)
+
8
DMA
0 °C to RT, 48 h
HN
O
O
n
76%
3bc’ (Mn = 6.5 Kg/mol)
Cl
Cl
11
(0.9 equiv.)
Scheme 4. Two-step procedure for the synthesis of CE-PA10F 3bc’.
4
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10.1002/chem.202004296
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FULL PAPER
Table 2. Scope of the oxidative polyamidation of diamines 1 with dialdehydes 2.[a]
C1 (5 mol%)
DBU (12.5 mol%)
4 (1 equiv.)
HFIP 9 (1.5 equiv.)
O
O
O
O
+
NH2
H2N
N
H
N
H
Anhydrous THF
RT, degas (Ar)
H
H
n
1
2
3
Polyamide acronym Yield (%)[b]
Mn (kg/mol)[c]
Entry
1
Diamine
Dialdehyde
Polyamide
O
O
HN
O
NH2
NH
PETA
90
1.9
H2N
O
n
1a (EDA)
2a
3aa
O
O
O
O
HN
O
2
PA10T
88
85
2.6
H2N
NH2
8
8
O
O
HN
1b (1,10-DDA)
n
2a
2a
3ba
HN
3[d]
HMW PA10T
62.8
H2N
NH2
8
8
O
HN
1b (1,10-DDA)
n
3ba’
3ca
O
O
HN
PA6T
PA6I
84
78
2.5
1.7
4
5
6
H2N
NH2
4
4
O
O
HN
1c (1,6-HDA)
n
2a
HN
H2N
NH2
O
4
4
O
HN
O
1c (1,6-HDA)
2b
3cb
O
n
O
O
O
HN
O
O
O
H2N
NH2
PA10F
PA6F
81
72
1.6
1.7
8
8
HN
HN
n
1b (1,10-DDA)
2c (DFF)
3bc
O
O
HN
O
O
O
O
7
H2N
NH2
4
4
n
1c (1,6-HDA)
2c (DFF)
O
3cc
O
O
O
HN
O
O
8[e]
CE-PA10F
PA10FF
76
95
6.5
3.2
H2N
NH2
8
8
HN
n
1b (1,10-DDA)
2c (DFF)
3bc'
O
O
O
O
O
HN
O
O
O
O
O
O
O
O
O
9
H2N
NH2
8
8
4
HN
HN
1b (1,10-DDA)
2d (OBFA)
3bd
n
O
HN
O
O
O
O
O
90
86
3.6
1.9
10
11
NH2
H2N
PA6FF
PAFAT
4
n
1c (1,6-HDA)
2d (OBFA)
3cd
HN
O
H2N
O
O
O
H
N
O
NH2
O
n
1d (BAF)
2a
3da
[[a]Conditions: 1 (0.88 mmol), 2 (0.80 mmol), C1 (0.08 mmol), DBU (0.20 mmol), 4 (1.60 mmol), HFIP 9 (2.40 mmol), THF (6.0 mL). [b]Isolated yield. [c]Calculated by
1H NMR after precipitation of the polymer. [d]See Scheme 3 and the Experimental Section for reaction conditions. [e]See Scheme 4 and the Experimental Section for
reaction conditions.
5
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Unfortunately, Mn evaluation of 3bc’ by viscosity
measurements was not possible due to the absence of known
Mark-Houwink coefficients for PA10F and the degradation of
the sample in concentrated sulfuric acid likely because of
instability of the furan ring under strong acidic conditions for the
prolonged time necessary for viscosity analysis.
3.2 kg mol-1) and 3cd (PA6FF; Mn = 3.6 kg mol-1), respectively,
displaying the unusual polyamide-ether repeating unit (entries
9-10). Finally, the polyamidation of 2,5-bis(aminomethyl)furan
(BAF)[28] 1d with terephthalaldehyde 2a furnished the furanic-
aromatic polyamide 3da (PAFAT; Mn = 1.9 kg mol-1) endowed
with a stiffer backbone structure (entry 11).[29]
The synthesis of novel furan-based polyamides was also
addressed in our study by considering unprecedented
monomer combinations to access PAs with unexplored
potential in the polymer field. Hence, the polycondensation of
5,5’-[oxybis(methylene)]bis[2-furaldehyde] (OBFA)[27] 2d with
diamines 1b and 1c gave the oligomers 3bd (PA10FF; Mn =
In order to evaluate the thermal stability of the prepared PAs,
thermogravimetric analysis was carried out under standard
conditions in nitrogen atmosphere (Table 3). The PA10T
oligomer 3ba displayed a very good thermal stability exhibiting
a main degradation process with Td at 486 °C (Figure 3).
Table 3. Decomposition temperature at 5% weight loss (Td,5%), temperature of maximum degradation rate (Td) and residuum after degradation (Res.) from TGA
analyses; glass transition temperature (Tg), melting temperature (Tm), crystallization temperature (Tc) and melting enthalpy (DHm) from DSC analyses of polyamides
3.
Td,5% (°C)
Td[a] (°C)
Res.[b] (%)
Tg[c] (°C)
Tm[d] (°C)
Tc (°C)
DHm (J/g)
Mn (kg/mol)
REF
PETA (3aa)
264
250
431
450
31.0±0.5
n.a.
n.d.
n.a.
n.d.
106
117
117
n.a.
132.3
n.d.
n.a
n.d.
n.a
n.d.
n.a
1.9.
4.2
PETA (lit.)
31
32
~420
377
~450
486
20-30
2.5±0.5
1.8±0.5
2.5±0.5
n.a.
n.d.
n.d.
n.d
>10
2.6
PA10T (3ba)
220
185
19
PA10T (3ba’’)
HMW PA10T (3ba’)
PA10T (lit.)
364
482
235
198
15.3
3.2, 43.4[e]
90
20.2
62.8
4.2
291
465
200, 287, 296
304, 317
302, 313
279
177, 273
n.a.
n.a.
n.a.
16e[f]
30b
33
436
491
n.a.
287.7
n.a.
100
11
n.a
n.a
n.a.
n.a.
14
420-440
426
479
< 5
132.6
113
146
n.a.
170
n.a.
105
132
90
313
276
n.a.
21
30a
33
482
n.a.
290
n.a.
n.a.
26
PA6T (3ca)
355
472
6.5±0.5
2.4-5.2
n.a.
277
217
16
2.5
PA6T (lit.)
>380
245
480-484
471
374-379[f]
270
n.a.
108-146
n.d.
3.8-3.9
15.6
>10
1.7
34
22a
32
n.d..
n.d
428
~450
475
~5
368
n.d.
PA6I (3cb)
176
3.6±0.5
n.a.
n.d.
n.d.
n.d.
PA6I (lit.)
409
455
n.d.
n.d.
n.d.
15.7
1.6
22b
PA10F (3bc)
CE-PA10F (3bc’)
PA10F (lit.)
159
462
11.5±0.5
21.8±0.5
n.a.
n.d.
n.d.
n.d.
310
455
132
71
n.d.
n.d.
n.d.
6.5
n.a
350-450
473
n.d.
n.d.
n.d.
5.3
21d
21d
366
n.a
98
135
n.a.
n.a..
n.d.
13.4
1.7
PA6F (3cc)
171
404
34.5±0.5
90-110
95
n.d.
n.d.
PA6F (lit.)
380
2.4
22b
21b
21d
22a
n.a.
n.a.
n.a
350-410
460
110
119
86-64
69
n.d
162
n.d.
n.d.
n.d.
n.d.
n.d.
n.a
n.d.
n.a
5.2
322
13.4
30-62
3.2
n.a.
287-309
292
355-408
456
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
PA10FF (3bd)
PA6FF (3cd)
PAFAT (3da)
21.0±0.5
28.0±0.5
19.5±0.5
225
357
73
3.6
157
349
n.d.
1.9
[a]Temperature of the peak minimum in the DTGA plots; [b]Residuum at 850 °C; [c]Values at the midpoint. [d]Values at the peak maximum. [e]Total area of the peaks
with maximum at 287 °C and 296 °C; n.d. = not detected; n.a. = not available; [f]data collected at 20 °C/min as scan rate.
6
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Surprisingly, the value is higher than that of PA10T samples of
higher Mn (3ba’ and 3ba’’), which were obtained by
subsequent chain extension with terephthaloyl chloride, and
the stability decrease is highest for the polymer with the highest
Mn. In any case, the values are comparable to those of PA10T
obtained by other polymerization methods (Table 3 and
references therein) and the differences lie within the range
often observed among similar samples prepared in different
batches.[34] The differences among the temperature at the
maximum degradation rate reflect the differences among the
temperature of 5% mass loss. Regarding the mass loss at low
temperature, this is usually associated to the chain ends. In the
case of the high molecular weight polymers the shift of the
temperature can be associated to the increase of the
polydispersity (PDI) in the second step process. Indeed, step
growth polymerization is known to give polymers with PDI ≥ 2,
depending on the reagent ratio. The control over the reagent
ratio can be difficult for laboratory scale preparation, and it
becomes critical over multistep processes. In fact, it can be
usually overcome by scaling to industrial production where
appropriate measure to reduce reagent volatilization can be
implemented.
still higher molecular weight and these peaks can be ascribed
to crystals made of HMW chains. The low temperature peak
can be assigned to the low molecular weight (LMW) sample
fraction or to crystals including many chain ends. Indeed,
polymers obtained by step-growth polymerization are known to
have polydispersity index larger than 2. And the value is as
much high as the reagent ratio in the feed differs from 1. This
event may occur also if the two reagents have different volatility
so that the reagent ratio may change during the ongoing of the
polymerization under vacuum as in the case of the PA10T
oligomeric precursor and terephthaloyl chloride. The high
polydispersity of the sample is also supported by the observed
difference between the characteristic temperatures of the
polymer prepared by the herein proposed method and by salt
condensation. Indeed, in the latter mentioned case, the
characteristic temperature values are a little bit higher, thus
supporting the lower polydispersity. The consistency of the
data herein presented is corroborated by the intermediate
characteristic temperatures observed for PA10T 3ba’’ having
intermediate molecular weight with respect to 3ba and 3ba’.
Furthermore, chain extended PA10Ts showed glass transition
temperature larger than the oligomer precursor, as expected,
and the value is comparable to the one of PA10T exhibiting a
similar melting value.[33] In fact, this last value is a little bit lower
with respect to the other reported values, but this is strongly
affected by the conditions adopted for sample isolation, as well
as by the analysis conditions, such as heating rate, isothermal
steps, etc..[16c]
PA6T 3ca was observed to degrade between 3ba’ and its
precursor oligomer 3ba, with a 5% weight loss at 355 °C (Table
3). The corresponding onset temperature of 3ca is 455 °C,
value that is close to the ones reported for PA6T obtained by
solid-state polycondensation from salt precursors[16a] or by
interfacial polycondensation.[30]
Figure 3. Comparison among the TGA curves of terephthalaldehyde- and
isophthalaldehyde-derived polyamides.
Figure 4. DSC analysis at 10°C/min of PA10T 3ba (red lines), 3ba’ (black
lines) and 3ba’’ (blue lines). Dashed lines are cooling steps performed after
pre-heating. Solid lines are 2nd heating steps accomplished after the cooling
ones.
In spite to the observed differences among samples having
different molecular weight, all synthetized PA10Ts as well as
PA6T have good thermal stability (Figure 3). This feature is
also connected to their semicrystalline structure evidenced by
DSC analysis (Table 3). Indeed, all above mentioned polymers
exhibited melting and crystallization peaks with a higher value
for PA6T compared to PA10T (Table 3), in agreement with
literature.[35] PA10T 3ba’’ was found to have a higher melting
temperature and enthalpy than its precursor 3ba, as expected,
based on the higher molecular weight (Figure 4). Extra peaks
at higher temperatures were also observed in 3ba’, having a
All others synthesized polymers 3 did not show any melting
peaks in the analyzed temperature range. This result is in
accordance with the literature in the case of PA10F and PA6F:
For both polymers, melting peaks were observed only for
samples with molecular weight larger than the ones described
in the present paper (Table 3). In the case of PA10FF, PA6FF
and PAFAT no data on the thermal behavior have been
previously reported.
7
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10.1002/chem.202004296
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All amorphous polymers, except PETA 3aa and PAFAT 3da,
showed glass transition processes ranging in between 69 °C
and 146 °C. The temperature at which the transition was
observed is in good accordance with previous reported data in
the case of terephthalate polymers, when polymers with
comparable molecular weight are considered (Table 3).
In the case of PETA 3aa and of the fully aromatic PAFAT 3da
neither melting or glass transition processes could be observed
(Table 3). These polymers are characterized by a very rigid
main-chain structure and thus most likely they have transitions
at temperatures above the investigated temperature range, or
transitions occur with a very low change in specific heats cp, so
that they cannot be properly identified.
the data for PA10F 3dc’, which is much more stable than the
precursor 3bc’ at lower molecular weight. In fact, this polymer
was previously reported to give crystallization with melting
temperature at 138 °C, which is close to the glass transition. In
any case, the still scarce data on these polymers do not allow
to unambiguously explain the observed thermal behavior.
Conclusion
In summary, we have presented a novel strategy for the
synthesis of oligomeric polyamides (PAs) relying on the step-
growth polycondensation of diamines and dialdehydes
promoted by an N-heterocyclic carbene (NHC) catalyst in the
presence of a quinone oxidant. While the optimized procedure
required the addition of hexafluoro-2-proponal (HFIP) as a
nucleophilic additive to guarantee the oligomer growth, the mild
polyamidation conditions (ambient temperature) together with
the simple recovery of both external oxidant and HFIP may
represent operational advantages of the proposed
methodology. The results of our study have also demonstrated
that the obtained oligomeric PAs (Mn = 1.7-3.6 kg mol-1) are
compatible with a subsequent chain-elongation step to achieve
PAs with higher molecular weight, as exemplified by the
synthesis of chain extended PA10T (Mn = 62.8 kg mol-1) and
PA10F (Mn = 6.5 kg mol-1). Finally, important benefit of the
disclosed iterative amine-to-aldehyde condensation may
consist in the utilization of readily available dialdehyde
monomers including those belonging to the furanic platform,
thus allowing for the easy access to known and novel bio-
based PAs of interest for the development of environmentally
benign macromolecular materials.
Comparison
terephthalaldehyde-
among
the
and
thermal
isophthalaldehyde-derived
stability
of
polyamides (Figure 3) showed large differences among
amorphous and semicrystalline PAs. Indeed, both PETA 3aa
and PA6I 3cb degrade by a substantial two-step process, the
first in the 200-300 °C temperature range, and the second at
around 400 °C. The former step accounts for the degradation
of the samples corresponding to the LMW fraction of the
polymers.[32] An additional difference between the degradation
plots of PAs in Figure 3 is the residuum after thermal
degradation (Table 3), which is highest for PETA, medium for
PA6T and PA6I, and lowest for PA10T, in agreement with
previous literature finding (Table 3 and references therein).
This parameter, in fact, depends on the N/C ratio and, typically,
the higher the ratio, the more this parameter increases.
All furan-based PAs synthesized herein displayed a general
low thermal stability with the exception of PA10F 3bc’ and
PA10FF 3bd, which are stable at least up to 310 °C (Table 3).
All the other furan-based PAs start to degrade in between 130
and 206 °C (Figure 5).
Experimental Section
General procedure for the synthesis of polyamides 3
A mixture of pre-catalyst C1 (0.08 mmol) and hexafluoro-2-propanol
(HFIP) (2.40 mmol) in anhydrous THF (6.0 mL) was degassed under
vacuum and saturated with argon (by an Ar-filled balloon) three times.
Then, DBU was added (0.20 mmol), and the reaction was stirred at
room temperature for 5 minutes. Later, oxidant 4 (1.60 mmol) and
aldehyde 2 (0.80 mmol) were added, and the reaction mixture was
stirred at room temperature for 2 h. After complete consumption of
aldehyde (verified by 1HNMR analysis), diamine 1 (0.88 mmol) was
added and the reaction was stirred for 16 h at the same temperature.
After this period the mixture was concentrated and the resulting residue
triturated with fresh portions of Et2O (3 ´10 mL) and centrifuged. The
organic solutions were collected for the recovery of DBU and oxidant 4
while the solid precipitate was dissolved in the minimum amount of
appropriate solvent, and precipitated by dropwise addition into a poor
solvent at 0 °C.
Figure 5. Comparison among the TGA curves of furan-based polyamides.
Synthesis of high molecular weight (HMW) PA10T 3ba’
The reasons for the higher thermal stability of PA10FF 3bd
cannot be easily explained. Although this polymer did not show
any crystallization and melting peaks during cooling and the
subsequent heating (up to 250 °C), it may have some
crystallinity phase with melting temperature above the
degradation. However, further investigation is needed to
confirm this hypothesis. The same hypothesis cannot support
To a 10 mL round bottom flask, under Ar atmosphere were added 200
mg of 3ba (Mn
= 2.6 kg/mol, 0.077 mmol), anhydrous N,N-
dimethylacetamide (1 mL), lithium chloride (16 mg, 0.38 mmol) and
N,N-diisopropylethylamine (40 µl, 0.23 mmol). The resulting mixture
was stirred at room temperature until most of the prepolymer was
dissolved, then the mixture was cooled to 0 °C. In a different flask,
terephthaloyl chloride 10 (15 mg, 0.073 mmol) was dissolved in 0.5 mL
of anhydrous N,N-dimethylacetamide. The solution was transferred in
8
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10.1002/chem.202004296
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an air tight syringe and injected in the prepolymer solution at 0 °C. After
10 minutes, the ice bath was removed and the mixture was stirred at
room temperature for 48 h. After this period, the solution was slowly
added to water (15 mL). The resulting solid precipitate was filtered and
washed with methanol (15 mL) and diethyl ether (15 mL). Finally, the
polymer was dried in vacuum at 100 °C for 4 h (183 mg, 85% yield, Mn
= 62.8 kg/mol).
Böhl, S. Epple, C. Bonten, M. R. Buchmeiser, Macromolecules 2013,
46, 8426-8433.
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Selected recent reviews: a) S. J. Ryan, L. Candish, D. W. Lupton,
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Synthesis of chain-extended CE-PA10F 3bc’
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To a 10 mL round bottom flask, under Ar atmosphere were added 200
mg of 3bc (Mn
= 1.6 kg/mol, 0.068 mmol), anhydrous N,N-
dimethylacetamide (1 mL), lithium chloride (14 mg, 0.34 mmol) and
N,N-diisopropylethylamine (36 µl, 0.20 mmol). The resulting mixture
was stirred at room temperature until most of the prepolymer was
dissolved, then the mixture was cooled to 0 °C. In a different flask, 2,5-
furandicarbonyl dichloride 11 (13 mg, 0.065 mmol) was dissolved in 0.5
mL of anhydrous N,N-dimethylacetamide. The solution was transferred
in an air tight syringe and injected in the prepolymer solution at 0 °C.
After 10 minutes, the ice bath was removed and the mixture was stirred
at room temperature for 48 h. After this period, the solution was slowly
added to water (15 mL). The resulting solid precipitate was filtered and
washed with methanol (15 mL) and diethyl ether (15 mL). Finally, the
polymer was dried in vacuum at 100 °C for 4 h (162 mg, 76% yield, Mn
= 6.5 kg/mol).
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Acknowledgements
We gratefully acknowledge Paolo Formaglio for NMR
experiments and Tatiana Bernardi for MS analyses.
Keywords: carbenes • organocatalysis • oxidation •
polyamides • polymerization
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Entry for the Table of Contents
triazolium (5 mol%)
DBU (12.5 mol%)
quinone (1 equiv.)
HFIP (1.5 equiv.)
H2N
O
NH2
O
O
O
+
N
H
N
H
THF, RT
n
Y = 72-95%
Mn = 1.7-3.6 kg/mol
H
H
O
CHO
OHC
H2N
H2N
NH2
m
O
O
O
O
O
CHO
O
NH2
CHO
Organocatalytic step-growth polymerization: fossil- and bio-based polyamides are obtained under mild ambient conditions by
polycondensation of diamines and dialdehydes promoted by an N-heterocyclic carbene in the presence of recyclable external oxidant
and nucleophilic additive
11
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