Oxidative NHC-Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step-Growth Polymerization

Article abstract of DOI:10.1002/chem.201903557

The application of N-heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil-based (ethylene glycol, phthalaldehydes) and bio-based (

Full text of DOI:10.1002/chem.201903557

DOI: 10.1002/chem.201903557
Full Paper
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Polymerization |Hot Paper|
Oxidative NHC-Catalysis as Organocatalytic Platform for the
Synthesis of Polyester Oligomers by Step-Growth Polymerization
Daniele Ragno,[a] Graziano Di Carmine,[a] Arianna Brandolese,[a] Olga Bortolini,[a]
Pier Paolo Giovannini,[a] Giancarlo Fantin,[a] Monica Bertoldo,[b] and Alessandro Massi*[a]
In memory of Professor Cinzia Chiappe
Abstract: The application of N-heterocyclic carbene (NHC)
catalysis to the polycondensation of diols and dialdehydes
under oxidative conditions is herein presented for the syn-
thesis of polyesters using fossil-based (ethylene glycol,
phthalaldehydes) and bio-based (furan derivatives, glycerol,
isosorbide) monomers. The catalytic dimethyl triazolium/1,8-
diazabicyclo[5.4.0]undec-7-ene couple and stoichiometric
7.8 kgmolÀ1 as determined by NMR analysis. The synthesis
of a higher molecular weight polyester (polyethylene tere-
phthalate, PET) by an NHC-promoted two-step procedure
via oligoester intermediates is also illustrated together with
the catalyst-controlled preparation of cross-linked or linear
polyesters derived from the trifunctional glycerol. The ther-
mal properties (TGA and DSC analyses) of the synthesized
oligoesters are also reported.
quinone oxidant afforded polyester oligomers with
a
number-average molecular weight (Mn) in the range of 1.5–
Introduction
of hydroxy acids, and the step-growth polymerization of diols
and diacids and their derivatives (Figure 1).[5]
In recent years, organocatalytic polymerization strategies have
been demonstrated to effectively complement metal-catalyzed
approaches for the synthesis of macromolecular materials with
the advantage of avoiding metal contamination, which could
be unfavorable for applications in the electronic and biomedi-
cal fields.[1] Whereas organocatalytic chain-growth polymeri-
zations have been investigated in depth,[2] the application of
organocatalysts for the synthesis of polyesters (PEs), poly-
urethanes (PUs), and polycarbonates (PCs) by step-growth
polymerization is more limited in spite of the potential benefits
associated with this technique, including modularity and com-
patibility with functionalized monomers to access a wide
range of polymers for diversified applications.[3] Polyester mate-
rials are used extensively in clothing, food packaging, biomedi-
cal, and electronic devices, accounting for nearly 18% of the
world plastic production.[4] Main strategies for the synthesis of
polyesters include the ring-opening polymerization (ROP) of
cyclic esters (chain-growth process), the self-polycondensation
Figure 1. Main strategies for the synthesis of polyesters and the proposed
approach.
[a] Dr. D. Ragno, Dr. G. Di Carmine, A. Brandolese, Prof. O. Bortolini,
Dr. P. P. Giovannini, Prof. G. Fantin, Prof. A. Massi
Department of Chemical and Pharmaceutica Sciences
University of Ferrara, Via L. Borsari, 46, 44121 Ferrara (Italy)
E-mail: alessandro.massi@unife.it
Although ROP allows for an accurate control of dispersity
and molecular weight, the polycondensation of diols and acti-
vated carboxylic derivatives still represents the preferred syn-
thetic route for industry.[4,5] In this area of research, Brønsted
base and Brønsted acid organocatalysts have been established
as practical, greener alternatives to antimony-based catalysts in
a number of studies.[3,6] Lewis base organocatalysts, namely N-
heterocyclic carbenes (NHCs),[7] have also been tested in step-
growth polymerization to polyesters, but in a single investiga-
[b] Dr. M. Bertoldo
Istituto per la Sintesi Organica e la Fotoreattivitꢀ
Consiglio Nazionale delle Ricerche, Via P. Gobetti, 101-40129 Bologna (Italy)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201903557.
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tion dealing with the poly-condensation of ethyl glycolate and
the synthesis of polyethylene terephthalate (PET) by a two-
step strategy.[8]
the potential of the revealed methodology for engineering
polymers has been validated by the polycondensation of bio-
based furanic monomers, glycerol, and isosorbide to access
polyester oligomers of interest for the development of envi-
ronmentally benign macromolecular materials. Indeed, oli-
goesters can be exploited not only to prepare thermoplastic
polyesters in a subsequent polycondensation step but also as
components in thermosetting resin formulations in combina-
tion with curing agents such as isocyanates (polyurethane),
urea (polyester-urea) or anhydrides (alkydic resins) to obtain
high-performance composites, adhesives, coatings or inks.
The peculiar stereoelectronic features of NHCs make these
molecules a special class of organocatalysts with three major
attributes: ambiphilicity (s-basicity and p-acidity), moderate
nucleophilicity, and strong basicity.[7] In polymer chemistry, all
types of catalytic activation modes by NHCs are conveniently
exploited for the preparation of polymers from diverse mono-
mers such as lactones, anhydrides, carbonates, epoxides, lac-
tams, siloxanes, acrylates, and aldehydes.[9] In this latter case,
the ambiphilic character of NHCs accounted for the polyben-
zoin condensation of terephthalaldehyde[10] and the iterative
Stetter reaction of dialdehydes and enones affording polymeric
1,4-diketones (Scheme 1, routes a,b).[11] This umpolung (polarity
Results and Discussion
The polycondensation of ethylene glycol 1a and terephthal-
aldehyde 2a was selected as a benchmark for the optimization
of the reaction conditions with the inexpensive triazolium salt
A and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); in our previ-
ous studies, in fact, this pre-catalyst/base couple proved to be
effective in the NHC-promoted esterification of aldehydes
under different oxidative conditions.[15] A prerequisite of the
present investigation was the utilization of a slight excess of
diol 1a (1.1 equiv) to promote the formation of bis-hydroxyl-
terminated polymers.
Initially, the biomimetic system of electron-transfer media-
tors (ETMs) developed by Bꢁckvall[16] and Sundꢂn[17] groups
was applied to evaluate the possible use of air as the terminal
oxidant in our polymerization procedure (Table 1). Accordingly,
a catalytic amount of the low-cost alcohol 4 generated the qui-
none oxidant 5 (ETM’) in situ, which was responsible for the
oxidation of the Breslow intermediate with formation of the re-
active acyl azolium and the reduced species 5’. This latter
could be re-oxidized to 5 by atmospheric oxygen in the pres-
ence of catalytic iron(II) phthalocyanine 6 (ETM’’). Unfortunate-
ly, the above oxidation conditions were only moderately effec-
tive, furnishing low aldehyde conversions (20–22%) in anhy-
drous tetrahydrofuran (THF) after 16 hours at room tempera-
1
ture or 608C as determined by H NMR analysis (entries 1 and
2).
Scheme 1. NHC umpolung (Routes a, b) and oxidative (Route c) catalysis in
polymer chemistry.
Gratifyingly, the utilization of a stoichiometric amount of oxi-
dant 5 represented a major step forward, allowing a good re-
action conversion (75%) in 2 hours and the isolation of 3aa
(precipitation into excess of methanol) with a number-average
reversal) chemistry relies on the formation of the nucleophilic
Breslow intermediate as the key catalytic species;[7] oxidative
protocols, however, have recently enriched the toolbox of
NHC-based strategies through formation of acyl azolium inter-
mediates to perform a number of normal polarity transforma-
tions including the direct aldehyde-to-ester conversion.[12]
Hence, we present herein the unprecedented polycondensa-
tion of dialdehydes and diols promoted by NHCs under oxida-
tive conditions as an alternative, atom-economical synthetic
route to polyesters by using the step-growth polymerization
technique (Figure 1; Scheme 1, Route c).
1
molecular weight (Mn) of 4.2 kgmolÀ1 based on H NMR spec-
troscopic analysis (entry 3). An increase of oligomer length
(Mn =6.5 kgmolÀ1) was achieved by extending the reaction
time to 16 hours (entry 4). Disappointingly, the addition of mo-
lecular sieves to prevent competitive nucleophilic attack by
water, heating of the reaction mixture (608C), and the replace-
ment of THF with anhydrous DCM or with the biomass-derived
Me-THF did not improve the polymer growth (entries 5–8). Fur-
thermore, unsatisfactory reaction outcomes were detected
with different oxidants such as phenazine 7 (entry 9) and azo-
benzene 8 (entry 10), with the latter being completely ineffec-
tive. Finally, the optimized conditions detailed in entry 4 were
successfully applied to the gram-scale synthesis of 3aa (1.69 g,
88% yield) starting from 10 mmol of aldehyde 2a (entry 11).
The combination of environmental concerns and depletion
of fossil resources have stimulated extensive research on the
polymerization of monomers derived from renewable feed-
stocks.[6,13,14] Therefore, after a propaedeutic study on the syn-
thesis of PET and polyethylene isophthalate (PEI) oligomers,
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Subsequently, thanks to the ability of NHCs to work as effec-
tive transesterification agents[9] and mimicking the approach
for the production of commercial PET, the synthesis of high
molecular weight PET was addressed by an NHC-promoted
two-step procedure in analogy with the studies of Way-
mouth,[8] Hedrick,[8] Sardon,[18] and their co-workers (Scheme 2;
Table 1. Screening of reaction conditions for the synthesis of PET oligo-
mers 3aa.[a]
Scheme 2. NHC-promoted two-step procedure for the synthesis of high mo-
lecular weight PET 3aa’.
[d]
Entry
Oxidant
[mol%]
Time
[h]
Conv.[b]
[%]
3aa[c]
[%]
Mn
[kgmolÀ1
]
Table 2, entry 2). Hence, isolated 3aa (Mn =6.5 kgmolÀ1) was
heated at 2508C for 2 hours under vacuum in the presence of
triazolium pre-catalyst A (5 mol%) and DBU (5 mol%), afford-
ing PET 3aa’ with high molecular weight, as evidenced by the
1
air, 4/6[e]
air, 4/6[e]
5 (100)
5 (100)
5 (100)
5 (100)
5 (100)
5 (100)
7 (100)
8 (100)
5 (100)
16
16
2
16
16
16
16
16
2
20
22
75
>95
>95
>95
>95
>95
61
10
11
61
82
80
71
80
78
44
–
n.d.
n.d.
4.2
6.5
6.5
6.2
6.3
5.8
1.9
–
2[f]
3
4
5[g]
6[f]
7[h]
8[i]
9
1
disappearance of the end group signals in the H NMR spec-
trum (4:1 CDCl3-TFA; see the Supporting Information).
Polyethylene isophthalate oligomers 3ab (Mn =6.5 kgmolÀ1
;
77% yield) were readily obtained by the optimized protocol
starting from diol 1a and isomeric isophthalaldehyde 2b
(Table 2, entry 3).
10
11[j]
2
16
–
>95
88
6.5
[a] Conditions: 1a (0.88 mmol), 2a (0.80 mmol),
A (0.08 mmol), DBU
(0.20 mmol), THF (6.0 mL). [b] Detected by 1H NMR analysis of the crude
reaction mixture (durene as internal standard). [c] Isolated yield via pre-
cipitation into excess methanol (see the Experimental Section). [d] Calcu-
As anticipated, we also aimed to exploit the organocatalytic
procedure for the synthesis of bio-based polymers from renew-
able monomers, including the abundant and inexpensive glyc-
erol. Polyesters derived from glycerol find important applica-
tions in pharmaceutical and biomedical fields as drug-delivery
agents,[19] surgical adhesives,[20] and tissue engineering scaf-
folds.[21] Given that the physical properties of poly(glycerol
esters) strongly depend on the linear or branched microstruc-
ture of the glycerol unit,[22] the development of a selective,
metal-free strategy for the synthesis of glycerol PEs with a con-
trolled degree of acylation is highly desirable.[23,24] In this direc-
tion, our optimized polymerization procedure was performed
with terephthalaldehyde 2a and glycerol 1b and we observed
the rapid and quantitative formation of poly(glycerol tere-
phthalate) (PGT) 3ba’ as a gel-like material, which was insolu-
ble in common organic solvents (DCM, THF, DMSO, DMF) likely
because of the high content of triacylglycerol units [Scheme 3,
Eq. (a)]. The synthesis of linear PGT was next addressed and,
after a long period of experimentation with different azolium
salt promoters and conditions, we found that the more ste-
1
lated by H NMR analysis after precipitation of the polymer. [e] ETM’/ETM’’
system: 4 (10 mol%), 6 (2.5 mol%). [f] T=608C. [g] Reaction run in the
presence of 4 ꢃ molecular sieves. [h] Anhydrous DCM as solvent. [i] Me-
THF as solvent. [j] Conditions: 1a (11.0 mmol), 2a (10.0 mmol),
(1.00 mmol), 5 (20 mmol), DBU (2.5 mmol), THF (60 mL).
A
In parallel, the possibility of recycling both the DBU base
and the oxidant 5 was investigated for the setup of a more
sustainable polymerization procedure; accordingly, the reac-
tion mixture containing 3aa was concentrated and the result-
ing solid residue was triturated several times with Et2O for the
solubilization of DBU and alcohol 5’, which was formed quanti-
tatively at complete reaction conversions. After washing with
acidic aqueous solution, the protonated DBU and 5’ could be
separated and alcohol 5’ quantitatively re-oxidized to quinone
5 with air in the presence of catalytic phthalocyanine 6 (see
the Supporting Information for details).
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Table 2. Scope of the oxidative polycondensation of diols 1 and dialdehydes 2.[a]
[c]
Entry
1
Diol
Dialdehyde
Polyester
Polyester acronym
Yield[b]
[%]
Mn
[kgmolÀ1
]
PET
82
78
77
6.5
–
2[d]
PET (high Mn)
3
PEI
6.5
4
PGT (cross-linked)
90
–
5[e]
PGT (linear)
71
68
1.5
1.5
6
PEF
7
8
PEOBF
73
81
3.5
3.1
PBHMT
9
PBHMF
78
71
7.8
2.8
10
PIT
11
PIF
72
2.7
1
[a] Conditions: 1 (0.88 mmol), 2 (0.80 mmol), A (0.08 mmol), DBU (0.20 mmol), 5 (1.60 mmol), THF (6.0 mL). [b] Isolated yield. [c] Calculated by H NMR anal-
ysis after precipitation of the polymer. [d] See Scheme 2 and the Experimental Section for reaction conditions. [e] Performed with pre-catalyst B
(0.08 mmol), Et3N (0.20 mmol), THF (60 mL).
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rectly starting from DFF and OBFA dialdehydes 2c,d in place of
the corresponding acids FDCA and OBFC (entries 6 and 7).
The scope of the polycondensation was next extended to
the use of 2,5-bis(hydroxymethyl)furan (BHMF) 1c,[30] which is
an HMF-derived diol that is widely employed to prepare PEs
and PUs.[31] Thus, polymerization of 1c with terephthalalde-
hyde 2a produced poly(2,5-furandimethylene terephthalate)[32]
(PBHMT) oligomers 3ca (Mn =3.1 kgmolÀ1) with good efficien-
cy (81% yield) under the optimized oxidative conditions
(entry 8).
Satisfyingly, the replacement of 2a with 2,5-diformylfuran 2c
afforded the fully furan-based polyester poly(2,5-furandi-meth-
ylene 2,5-furandicarboxylate) (PBHMF)[31] 3cc with the highest
molecular weight registered in this study (Mn =7.8 kgmolÀ1
;
entry 9). It is important to stress the mild polycondensation
procedure presented herein appears well suited for the utiliza-
tion of BHMF, as this diol has been reported to suffer from low
thermal stability in solution polymerizations.[31a]
Scheme 3. Optimized conditions for the synthesis of cross-linked and linear
poly(glycerol esters) 3ba’ and 3ba.
Secondary alcohols were also effective in the NHC-promoted
polyesterification with dialdehydes. Indeed, additional mono-
mer combinations involved isosorbide (IS) 1d, which is a re-
newable diol (1,4:3,6-dianhydro-d-glucitol) available from glu-
cose and cellulose with high efficiency. Isosorbide has recently
been attracting increasing interest in polymer chemistry by
virtue of its rigid structure, chirality, and nontoxicity.[33] In par-
ticular, poly(isosorbide)terephthalate[31a,34] (PIT) and poly(isosor-
bide 2,5-furandicarboxylate)[35] (PIF), which are the isosorbide-
based counterparts of PET and PEF, respectively, display excel-
lent thermal and structural properties, making them suitable
for packaging applications.[36] In our study, PIT oligomers 3da
rically hindered pre-catalyst B, in combination with triethyl-
amine in dilute solution, produced linear PGT oligomers 3ba
(Mn =1.5 kgmolÀ1) in 71% yield after precipitation [Eq. (b)]. Ac-
tually, following the detailed study by Slawko and Taylor on
the characterization of glycerol-based PEs,[24] the formation of
triacyl and 1,2-diacylglycerol motifs in 3ba could be excluded
by quantitative 13C NMR analysis ([D6]DMSO) using chromi-
um(III) acetylacetonate as relaxation agent.
Although high molecular weight linear PGT 3ba could not
be obtained by using our strategy, this proof-of-concept study
on glycerol demonstrated that the structural variation of NHC
catalyst might allow the regioselective activation of the polyol
substrate to produce polyesters with a well-defined architec-
ture; an additional advantage is the maintenance of polyester
microstructure thanks to the mild reaction conditions that pre-
clude acyl group migration during the polycondensation.
The compatibility of the oxidative polycondensation proto-
col with respect to variation of the bio-based dialdehyde was
then verified by the utilization of furan aldehydes derived from
5-hydroxymethyl furfural (HMF); namely, 2,5-diformylfuran
(DFF) 2c and 5,5’-oxybis(methylene)bis2-furaldehyde (OBFA)
2d (Table 2). In recent years, DFF and difuranic OBFA have
been employed as monomers to prepare furan-urea resins and
imine-based polymers,[25,26] but also as suitable precursors of
the corresponding diacids, that are 2,5-furandicarboxylic acid
(FDCA)[27] and 5,5’-oxybis(methylene)bis2-furancarboxylic acid
(OBFC).[28] Today, FDCA is widely recognized as an effective
equivalent of terephthalic acid for the synthesis of PEs from re-
newable resources; in particular, poly(ethylene furanoate) (PEF)
has emerged as an innovative alternative to PET with attractive
mechanical and barrier properties,[29] and some companies are
presently building plants for its production on a large scale.
Similarly, the difuranic-diacid OBFC is under investigation for
the synthesis of novel polyester-ether materials, including the
promising poly(ethylene 5,5’-(oxybis(methylene)bis(2-furancar-
boxylate))) (PEOBF).[28] Hence, PEF and PEOBF oligomers 3ac
(Mn =1.5 kgmolÀ1) and 3ad (Mn =3.5 kgmolÀ1) were readily
prepared by using our complementary oxidative strategy, di-
(Mn =2.8 kgmolÀ1) and PIF oligomers 3dc (Mn =2.7 kgmolÀ1
)
were prepared in good yields (71–72%; entries 10 and 11) with
the terminal isosorbide units enchained through the more nu-
cleophilic C5 hydroxyl group, as proven by 1H NMR analysis
and comparison with authentic samples of C2 and C5 benzoyl-
ated isosorbide derivatives (see the Supporting Information).
Scope and limitation were further investigated by analysis of
challenging combinations involving ortho-difunctionalized or
tri-functionalized aromatic monomers as well aromatic alco-
hols, which are known to be poorly reactive substrates in NHC-
promoted esterifications (Scheme 4).[12] Whereas the polymeri-
zation of o-phthaldialdehyde 2e with ethylene glycol 1a pro-
duced a complex mixture of compounds with no evidence of
ester linkage formation (NMR analysis), the reaction of ben-
zene-1,3,5-tricarboxaldehyde 2 f with 1a afforded the target
polyester 3af in 79% yield after purification [Eq. (a)]. This poly-
mer was insoluble in all common organic solvents because of
its highly cross-linked structure, as confirmed by thermal analy-
sis (see below). Hydroquinone 1e reacted efficiently with tere-
phthalaldehyde 2a, furnishing the fully aromatic 3ea in 88%
yield but with low molecular weight [Mn =0.4 kgmolÀ1,
Eq. (b)].[37] Disappointingly, the aromatic triol 1,3,5-trihydroxy-
benzene (phloroglucinol) 1 f was found to be unreactive in the
polycondensation with 2a under the disclosed polymerization
conditions.
All synthesized polyesters were characterized by thermogra-
vimetric analyses (TGA). The collected relevant parameters,
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Figure 2. Comparison of the TGA curves of polyesters prepared from tere-
phthalaldehyde/isophthalaldehyde/benzene-1,3,5-tricarbaldehyde and ethyl-
ene glycol/glycerol (3aa, 3aa’, 3ab, 3ba, 3ba’, 3af).
Scheme 4. Use of tricarboxaldehyde 2 f and hydroquinone 1e in the dis-
closed polycondensation procedure.
displayed no detectable CH2-OH groups (see the Supporting
Information).
namely the onset of thermal degradation (Tdi), the temperature
of maximum degradation rate (Td), and the residuum after deg-
radation at 8508C, are summarized in Table 3.
Polyesters 3aa and 3aa’ both exhibited melting and crystal-
lization peaks in the heating and cooling steps of DSC analysis
(Figures 3 and 4). Indeed, the solid-state PET with a regular se-
quence is well known to be a semicrystalline polymer with a
melting temperature between 245 and 2658C, depending on
Table 3. Onset of thermal degradation (Tdi), temperature of maximum
degradation rate (Td), and residuum after degradation (residuum) from
TGA analyses; glass transition temperature (Tg), melting temperature (Tm),
crystallization temperature (Tc), and melting enthalpy (DHm) from DSC
analyses of polyesters 3.
TGA
DSC
[a]
[c]
[d]
Tdi [8C] Td [8C] Res.[b] [%] Tg [8C] Tm [8C] Tc [8C] DHm [JgÀ1
]
3aa
3aa’
3ab
3ac
410
419
404
327
444 5.3
447 7.3
442 4.1
354 10.2
65
83
53
52
242
244
–
209
214
–
53
42
–
186
139
48
3ad <200 342/392 25.6
3af <200 442 13.4
247 258/364 7.3
–
60
91
176
136
–
181
–
–
–
–
142
–
–
–
–
39
–
–
–
3ca
3ba
3ba’
3da
205
212
399
407 29.0
422 1.1
421 1.9
333 6.9
409 3.3
540 18.9
Figure 3. Second heating step in the DSC thermograms of polyesters 3 indi-
cated in the legend. Plots have been arbitrary shifted vertically for clarity
and a selection of significant transitions have been indicated as examples.
3cc <200
3dc
3ea
373
224
140
389
–
–
–
–
84.6
445[e]
[a] Temperature of the peak minimum in the DTGA plots. [b] Residuum at
8508C. [c] Values at the midpoint. [d] Values at the peak maximum.
[e] Maximum of the observed endothermic peak.
The PET oligomer 3aa showed a degradation plot (Figure 2)
that was comparable to those reported for a commercial PET
and for the same polymer prepared under different condi-
tions.[36,38] The thermal stability of 3aa was high, with a Tdi
value of the main degradation process at 4108C. The weight
loss measured between room temperature and 3008C was
minimal (3.1%) and mainly due to a step with onset tempera-
ture at 1008C, which was attributed to the loss of moisture
and to the dehydration of terminal hydroxyl groups.[37] Accord-
ingly, this step disappeared completely after chain-elongation
Figure 4. Cooling step in the DSC thermograms of polyesters 3 indicated in
the legend. Plots have been arbitrary shifted vertically for clarity. Two crystal-
lization peaks have been indicated as examples.
1
in 3aa’, in good agreement with its H NMR spectrum, which
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the cooling protocol, the macromolecular features (e.g., molec-
ular weight, existence of defects), and the presence of residua
or additives.[39]
mechanism but further investigation is needed to clarify this
effect.
As previously reported,[24] cross-linked and linear poly(glycer-
ol esters) 3ba’ and 3ba were found to be amorphous, exhibit-
ing only a heat capacity jump while crossing the Tg during
DSC analysis and no peaks in the heating-cooling scan (Fig-
ures 3 and 4). The observed glass transition temperature of
3ba’ and 3ba were quite different to that of 3ba, which is
lower by 858C, thus revealing a higher chain mobility in this
sample in agreement with the presence of a negligible
number of crosslinks.
The melting peaks of 3aa and 3aa’ were observed at the
minimum of the range noted above, in accordance with the
adopted measurement protocol. In particular, the melting peak
of 3aa’ is shifted at little higher temperature and does not in-
clude the shoulder toward low temperatures that is instead
present in 3aa (Figure 3).
This behavior is consistent with the higher molecular weight
of 3aa’ and thus with the presence of a lower number of
chain ends. In fact, chain ends usually behave like defects in
polymer crystallization, affecting the kinetics and the thermo-
dynamics of the process and, consequently, the melting tem-
perature.[40]
The oligoester 3da from terephthalaldehyde and isosorbide
was also found to be an amorphous solid with high Tg and
good thermal stability, as previously reported for the corre-
sponding polymer prepared by polycondensation of isosorbide
and terephthaloyl dichloride.[34a] The thermal stability of 3da
was similar to that of PET oligomer 3aa, being shifted by only
108C toward lower temperature (Figure 5).
Furthermore, the glass transition temperature of 3aa’ was
observed at ca. 808C, in good agreement with the expected
value for this polymer in the semi-crystalline form.[41] On the
other hand, the lower Tg value of 3aa (658C) indicates a plasti-
cization effect by the high number of chain ends present in
the sample because of its low molecular weight.
Poly(ethylene isophthalate) (PEI) is a polymer similar to PET
except for the lower ability to crystallize.[42] Accordingly, PEI
oligomers 3ab exhibited a thermal stability under nitrogen at-
mosphere similar to that of 3aa (Figure 2) and only a glass
transition temperature at 538C and no detectable melting and
crystallization peaks by DSC analysis (Figures 3 and 4). These
features are consistent with the results by Lee and co-workers,
who reported a Tg value of 568C for a PEI with a molecular
weight of 21000 kgmolÀ1 and residual crystallinity of only
2%.[43]
The insoluble polymer 3af, derived from the polycondensa-
tion of ethylene glycol and the tricarboxaldehyde 2 f, showed
a thermal stability comparable to those of PET and PEI, with
the onset of the main degradation step at 4198C (Figure 2).
Additional weight loss steps were also observed at lower tem-
peratures likely because of the degradation of the low molecu-
lar weight branches or the presence of impurities that could
not be efficiently removed from the rigid cross-linked structure.
Indeed, based on DSC analysis, the polymer did not show any
thermal transition in the analyzed range (up to 2808C), thus
suggesting a higher value of the glass transition due to the ri-
gidity of the aromatic cross-linker (Figure S1).
Figure 5. Comparison of TGA curves of oligoesters prepared from
terephthalaldehyde/diformylfuran and ethylene glycol/isosorbide (3aa, 3da,
3ac, 3dc).
Comparison of the TGA plots of terephthalaldehyde-based
oligoesters 3aa and 3da with those of diformylfuran-based oli-
goesters 3ac and 3dc (Figure 5) reveals a lower thermal stabili-
ty of the latter two because of the higher instability of the
furan moiety.[44] Instead, the replacement of ethylene glycol
with an isosorbide unit improves the thermal stability of isosor-
bide-based oligoesters 3da and 3dc compared with ethylene
glycol-derived oligoesters 3aa and 3ac. This behavior is likely
due to a different mechanism of degradation resulting from
the change of chemical structure and not to chemical-physical
differences: both isosorbide-based oligoesters 3da and 3dc
are amorphous materials with very similar glass transition tem-
perature values (Table 3), and both ethylene glycol-based oli-
goesters 3aa and 3ac are semicrystalline. Indeed, 3ac is a low
molecular weight PEF exhibiting both melting and crystalliza-
tion peaks in DSC analysis (Figures 3 and 4). The detected
values were a little lower than those reported for PEF, which
has been described as a semicrystalline material with Tg =70–
908C, Tm =200–2158C, and Tc =150–1658C;[31a,35c,44,45] however,
this decreasing trend with the reduction of molecular weight
has already been observed for other polymers.[46]
Substitution of ethylene glycol with glycerol in the polycon-
densation with terephthalaldehyde afforded the linear oligo-
mer 3ba, which displayed lower thermal stability (Figure 2)
compared with 3aa in the region in which the dehydration of
hydroxyl groups is expected to occur (150–4008C). Actually,
the Tdi of 3ba was found at 2058C; this functional oligomer,
however, did not degrade completely in nitrogen atmosphere,
leaving a quite high residuum even at 9008C (ca. 30%;
Table 3). In contrast, the insoluble cross-linked analogue 3ba’
exhibited similar thermal behavior before 4008C but a plot
more comparable to those of PET and PEI above this tempera-
ture, with negligible residuum at 9008C. This result indicates
that the presence of the secondary free hydroxyl groups in
each repeating unit of the regular 3ba affects its degradation
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Oligoesters 3cc, 3ad, and 3ca displaying a 5-substituted
furan-2-yl-moiety as a repeating unit were all found to have
thermal stability lower than PEF 3ac (Figure 6). The more
stable oligoester of this series was 3ca, derived from tere-
phthalaldehyde; in spite of its lower molecular weight, 3ca
was found to be a semicrystalline polymer with Tg higher than
room temperature (608C) and Tm of ca. 1808C, thus indicating
this oligoester as a promising candidate to prepare transparent
thermoplastic films.
the disclosed mild procedure proved to be effective for the
preparation of a series of synthetically relevant bio-based poly-
esters from furanic monomers, glycerol, and isosorbide. Addi-
tionally, the results of the present study have demonstrated
the compatibility of the obtained oligoesters with a subse-
quent NHC-catalyzed chain-elongation step to achieve PEs
with higher molecular weight as well as the possibility of con-
trolling the microstructure (linear or cross-linked) of poly(glyc-
erol esters) by modulation of the steric hindrance of the NHC
catalyst. Further investigation aimed at extending this method-
ology to other polymer classes (e.g., polyamides) is under way
in our laboratories and results will be reported in due course.
Experimental Section
General procedure for the synthesis of polyesters 3
A mixture of diol 1 (0.88 mmol), aldehyde 2 (0.80 mmol), oxidant 5
(1.60 mmol), and pre-catalyst A (0.08 mmol) in anhydrous THF
(6.0 mL) was degassed under vacuum and saturated with argon
(by using an Ar-filled balloon) three times. DBU was then added
(0.20 mmol), and the reaction was stirred at RT for 16 h. The mix-
ture was concentrated and the resulting residue was triturated
with fresh portions of Et2O or DCM (3ꢄ10 mL) and centrifuged.
The organic solutions were collected for the recovery of DBU and
oxidant 5; the solid residue was dissolved in the minimum amount
of appropriate solvent, and precipitated by dropwise addition into
a poor solvent at 08C (see the Supporting Information for details).
Figure 6. Comparison of the TGA curves of PEF oligomer 3ac, oligoesters
displaying the 5-substituted furan-2-yl-moity (3cc, 3ad, 3ca), and the fully
aromatic oligoester 3ea.
The fully aromatic polyester 3ea was observed to degrade
by a quite complex process consisting of several steps, the
largest one having onset temperature at 5158C and the lowest
at 2258C (Figure 6). Nevertheless, 3ea can be considered to
show a good thermal stability in relation to its low molecular
weight, since the degradation processes at low temperature
can be attributed to the loss of the chain-end groups. The DSC
analysis of 3ea showed a heat capacity jump at 3908C and an
endothermic peak with a maximum at 4558C (Figure S2). The
former can be easily attributed to a glass transition process,
whereas the latter can be due to a melting even if the corre-
sponding crystallization was not observed on cooling. It must
be pointed out that, in the case of the present sample, the
first heating step was carried out up to 4008C to eliminate any
process responsible for the observed weight loss step in TGA
that occurred before this temperature. In fact, the assignment
of the above-mentioned peak to any degradation process is
unlikely because no weight loss processes were detected by
TGA in this range. The observed characteristic temperatures (Tg
and Tm) of 3ea are a little different from those reported in the
literature;[47] this discrepancy, however, can be explained by
the difference of molecular weight and chain-end groups.
Synthesis of high molecular weight PET 3aa’
Isolated 3aa (126 mg, 0.66 mmol of repeating unit, Mn =
6.5 kgmolÀ1) was heated at 2508C for 2 hours under vacuum in
the presence of triazolium pre-catalyst A (7.5 mg, 0.03 mmol) and
DBU (4.5 mL, 0.03 mmol). After this period the flask was cooled, the
reaction mixture was triturated with DCM and centrifuged (3ꢄ
10 mL). Final drying under vacuum afforded 3aa’ with high molec-
1
ular weight, as confirmed by H NMR analysis.
Synthesis of cross-linked PGT 3ba’
A mixture of glycerol 1b (65 mL, 0.88 mmol), terephthalaldehyde
2a (107 mg, 0.80 mmol), oxidant 5 (654 mg, 1.60 mmol), and pre-
catalyst A (18 mg, 0.08 mmol) in anhydrous THF (6.0 mL) was de-
gassed under vacuum and saturated with argon (by using an Ar-
filled balloon) three times. DBU was then added (30 mL,
0.20 mmol), and the reaction was stirred at RT for 16 h. Solvent re-
moval under reduced pressure, trituration of the reaction mixture
with DCM and subsequent centrifugation (3ꢄ10 mL) followed by
drying under vacuum afforded 3ba’ (160 mg, 90%) as an off-white
solid.
Synthesis of linear PGT 3ba
A mixture of glycerol 1b (65 mL, 0.88 mmol), terephthalaldehyde
2a (107 mg, 0.80 mmol), oxidant 5 (654 mg, 1.60 mmol), and pre-
catalyst B (29 mg, 0.08 mmol) in anhydrous THF (60 mL) was de-
gassed under vacuum and saturated with argon (by using an Ar-
filled balloon) three times. Et3N was then added (28 mL,
0.20 mmol), and the reaction was stirred at RT for 1 h. After solvent
removal under reduced pressure, trituration with DCM and subse-
quent centrifugation (3ꢄ10 mL), the resulting solid was dissolved
Conclusions
We have described a novel strategy for the synthesis of poly-
esters (PEs) based on the polycondensation of diols and dialde-
hydes promoted by an N-heterocyclic carbene (NHC) catalyst
in the presence of an external oxidant. While PEs of relatively
low molecular weight were obtained (Mn =1.5–7.8 kgmolÀ1),
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in a minimum amount of DCM and MeOH (4:1, v/v) and precipitat-
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washing with cold 2-propanol and drying under vacuum afforded
3ba (126 mg, 71%, Mn =1.5 kgmolÀ1) as an off-white solid.
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Synthesis of polyester 3af
A mixture of diol 1a (74 mL, 1.32 mmol), trialdehyde 2 f (130 mg,
0.80 mmol), oxidant 5 (981 mg, 2.40 mmol), and pre-catalyst A
(18 mg, 0.08 mmol) in anhydrous THF (6.0 mL) was degassed under
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three times. DBU was then was added (30 mL, 0.20 mmol), and the
reaction was stirred at RT for 16 h. Solvent removal under reduced
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Acknowledgements
We gratefully acknowledge Sara Buoso for experimental work.
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Keywords: carbenes · organocatalysis · oxidation · polyesters ·
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Chem. Eur. J. 2019, 25, 1 – 11
www.chemeurj.org
10
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Metal-free polymerization: Fossil- and
bio-based polyesters are obtained
under mild ambient conditions by poly-
condensation of diols and dialdehydes
promoted by N-heterocyclic carbenes in
the presence of an external oxidant. The
catalyst-controlled definition of poly-
ester architecture (cross-linked or linear)
is also illustrated with the trifunctional
glycerol monomer (DBU=1,8-diazabicy-
clo[5.4.0]undec-7-ene).
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Polymerization
D. Ragno, G. Di Carmine, A. Brandolese,
O. Bortolini, P. P. Giovannini, G. Fantin,
M. Bertoldo, A. Massi*
&& – &&
Oxidative NHC-Catalysis as
Organocatalytic Platform for the
Synthesis of Polyester Oligomers by
Step-Growth Polymerization
Chem. Eur. J. 2019, 25, 1 – 11
www.chemeurj.org
11
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ