Hyperbranched polyesters by polycondensation of fatty acid-based AB: N-type monomers

Article abstract of DOI:10.1039/c6gc02294d

Widely available vegetable oils were readily derivatized into chemically pure AB_n-type monomers (n = 2 or 3). Their polymerization led to unprecedented hyperbranched polyesters. Four different AB₂/AB₃-type monomers bearing one A-type methyl ester and two or three B-type alcohol functions were purposely synthesized via two elementary steps, i.e. epoxidation of the internal double bond of the vegetable oil precursors followed by ring-opening of the epoxy groups under acidic conditions. The polycondensation of these bio-sourced monomers was performed in bulk, in the presence of an appropriate catalyst, giving access to modular hyperbranched polyesters with tunable properties. Among the catalysts tested, zinc acetate, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and sodium methoxide proved the most effective, allowing the achievement of molar masses in the range 3000-10000 g mol^-1 and dispersities varying from 2 to 15, depending on the initial conditions. The degree of branching, DB, as determined by ¹H NMR spectroscopy, was found to be between 0.07 and 0.45. The as-devised hyperbranched polyesters displayed either amorphous or semi-crystalline properties, as a function of the selected AB₂/AB₃-type initial monomers, with a glass transition temperature, T_g, ranging from -33 to 9 °C and a decomposition temperature at 5 wt% of the sample, T_d5%, varying from 204 to 340 °C.

Full text of DOI:10.1039/c6gc02294d

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Page 1 of 31
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DOI: 10.1039/C6GC02294D
Hyperbranched Polyesters by Polycondensation
of Fatty Acid-based ABn-type Monomers
Blandine Testud,a,b Didier Pintori,c Etienne Grau,a,b Daniel Tatona,b* and Henri Cramaila,b*
a. Université de Bordeaux, Laboratoire de Chimie des Polymères Organiques, UMR 5629,
Bordeaux INP/ENSCBP, 16 avenue PeyꢀBerland, Fꢀ33607 Pessac Cedex, France
b. Centre National de la Recherche Scientifique, Laboratoire de Chimie des Polymères
Organiques, UMR 5629, Fꢀ33607 Pessac Cedex, France
c. ITERG, 11 rue Gaspard Monge, Parc Industriel Bersol 2, Fꢀ33600 Pessac Cedex, France
Corresponding authors: taton@enscbp.fr, cramail@enscbp.fr
† Electronic Supplementary Information (ESI) available: [1H NMR spectra of monomers in
DMSOꢀd6/ FTꢀIR spectra of HBPEs/ 2D NMR spectra of M2HSꢀderived HBPE in CDCl3/
1H NMR spectra of M2HB, M2HU, M3HSꢀderived HBPEs and LPE in CDCl3 or DMSOꢀd6/
Catalyst dependence of DB values for M2HS and M2HBꢀbased HBPEs/ Molar mass
dependence of DB values for HBPEs prepared by TBDꢀcatalyzed polycondensation of
M2HS/ DSC curves of HBPEs and LPE/ WAXS patterns of M2HBꢀderived HBPEs/ TGA
and TGA derivative curves of HBPEs]. See DOI: 10.1039/x0xx00000x
1

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Abstract
Widely available vegetable oils were readily derivatized into chemically pure ABnꢀtype
monomers (n = 2 or 3). Their polymerization led to unprecedented hyperbranched polyesters.
Four different AB2/AB3ꢀtype monomers bearing one Aꢀtype methyl ester and two or three Bꢀtype
alcohol functions were purposely synthesized via two elementary steps, i.e. epoxidation of the
internal double bond of the vegetable oil precursors followed by ringꢀopening of the epoxy
groups in acidic conditions. The polycondensation of these bioꢀsourced monomers was
performed in bulk, in presence of an appropriate catalyst, giving access to modular
hyperbranched polyesters of tunable properties. Among the catalysts tested, zinc acetate, 1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (TBD) and sodium methoxide proved the most effective, allowing
to achieve molar masses in the range 3 000ꢀ10 000 g.molꢀ1 and dispersities varying from 2 to 15,
1
depending on the initial conditions. The degree of branching, DB, as determined by H NMR
spectroscopy, was found in between 0.07 to 0.45. The asꢀdevised hyperbranched polyesters
displayed either amorphous or semiꢀcrystalline properties, as a function of the selected AB2/AB3ꢀ
type initial monomers, with a glass transition temperature, Tg, ranging from ꢀ33 to 9°C and a
decomposition temperature at 5% wt. of the sample, Td5%, varying from 204 to 340°C.
2

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Introduction
The last three decades have witnessed the rapid development of hyperbranched polymers
(HBPs).1 The first intentional synthesis of HBPs dates back to 1987 with the polyphenylenes of
Kim and Webster,2 long after Flory had predicted in the 1950’s that ABnꢀtype monomers should
afford soluble and highly branched materials of globular shape, via a selfꢀcondensation
polymerization process.3 The field is still in expansion reaching more than 500 publications in
recent years.4 HBPs are a special class of dendritic materials and possess as common features
with dendrimers a high branching density as well as a compact and globular architecture. This
unusual structure endows them with unique properties as compared to their linear counterparts.
Of particular interest are their lower viscosity both in solution and in the molten state, an
improved solubility and a high functionality. Indeed, their large number of terminal functional
groups offers the possibility for further modifications, providing an additional method for the
variation of HBPs’ properties and broadening the scope of their potential applications. In contrast
to regular dendrimers,5 HBPs are prepared in oneꢀpot, hence they exhibit an overall structure that
is not flawless. In particular, HBPs are characterized by the presence of linear units coexisting
with dendritic and terminal units, which is mirrored by their degree of branching (DB) that is
most often lower than unity. In addition, related oneꢀpot synthetic methods to HBPs are usually
more straightforward and more economically viable, compared to dendrimer synthesis.
Tremendous synthetic efforts have been made in the past decades to access a wide range of
HBPs, from the polycondensation of ABnꢀtype monomers to less conventional polymerization
methods.4,6–11 In this context, hyperbranched polyesters have been the most investigated. This is
due to the wide availability of various suitable monomers and the relative ease of synthesis of this
subclass of HBPs. The success of hyperbranched polyesters (HBPEs) has resulted in the
marketing of miscellaneous BoltornTM products based on 2,2ꢀbis(methylol)propionic acid (bisꢀ
3

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MPA).12,13 As a matter of fact, HBPEs have been nearly exclusively designed starting from fossil
resources.14–18 In a context of price volatility and uncertain supply of oil and gas, combined with
environmental concerns (global warming, waste production, etc.), there is yet an urgent need to
release the chemical industry from its dependence on petroleum resources.19 In recent years,
particular attention has been paid to biomass as a sustainable source of carbon.20,21 Among
available renewable resources, vegetable oils and their derivatives, i.e. fatty acid methyl esters
(FAMEs), represent a promising feedstock for the polymer industry owing to their abundant
availability, relative low cost and inherent degradability.22–28 Interests in FAMEs are also
motivated by the presence of both ester and double bonds enabling countless derivatizations and
design of a wide variety of functional building blocks and related materials.
Synthesis of vegetable oilꢀbased linear polyesters has been extensively studied in the past
decade,29–31 including by our group.32 In contrast, only a handful of studies have been dedicated
to HBPEs derived from plant oils. Related examples have involved the polycondensation of ABnꢀ
type monomer precursors (n ≥ 2). For that purpose, selective modifications/derivatizations of
hydroxyꢀcontaining fatty acid derivatives have been implemented. For instance, Meier et al. and
Li et al. have resorted to the thiolꢀene addition of 1ꢀthioglycerol onto methyl 10ꢀundecenoate, as
a means to access an AB2ꢀtype monomer, the polycondensation of which, in presence of organic
or metallic catalysts, has led to bioꢀbased HBPEs.33,34 Meier et al.33 have also reported on the
polycondensation of the same AB2ꢀtype monomer using glycerol as a core molecule, and 1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (TBD) as a catalyst at 120 °C, resulting in HBPEs with moderate
ꢀ1
34
ŭ
molar masses (Mn = 3 500ꢀ4 400 g.mol , Đ = 1.87ꢀ2.85). Li et al. have achieved higher molar
ꢀ1
ŭ
masses (Mn = 11 400ꢀ60 400 g.mol ) though with broader dispersities (Đ = 5.2ꢀ25.3), upon using
a metallic catalysis based on Ti(OBu)4, Sb2O3 or Zn(OAc)2 operating at 160ꢀ170 °C. DB values
13
determined by quantitative C NMR have been found in the range 0.38ꢀ0.54, in agreement with
the value of 0.5 predicted by theory.35 As for Petrović et al., they have developed two synthetic
4

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pathways to HBPEs from hydroxylated FAMEs obtained either by (i) hydrogenation36 of
epoxidized soybean oil or by (ii) hydroformylation.37,38 High molar masses (up to 42 000 g.molꢀ1)
have been obtained only in the latter case, with a weight average hydroxyl functionality of the
polymer up to 23.2. Note, however, that no DB value has been reported. Two other reports have
dealt with HBPE synthesis via the A2 + B3 approach, through the copolymerization of various
α,ωꢀdiacids with glycerol.39,40
In this contribution, we describe a platform of new ABnꢀtype building blocks based on
FAMEs and the synthesis of related vegetable oilꢀbased HBPEs with tunable properties. For that
purpose, sunflower, castor and rapeseed oils were used as raw materials for the synthesis of
chemically pure monomers of ABnꢀtype (n = 2 or 3), featuring an ester (A) and two or three
alcohol (B) moieties. The polytransesterification of these monomers was optimized, by varying
1
the initial experimental conditions. Insights into their fine structure are provided both by H
NMR spectroscopy and MALDI ToF mass spectrometry. Their crystallization ability and thermal
stability are also assessed by DSC and TGA analyses.
Experimental
Materials.
1,5,7ꢀTriazabicyclo[4.4.0]decꢀ5ꢀene (TBD, 98%), metaꢀchloroperbenzoic acid (mꢀcpba, < 77%),
anhydrous zinc acetate (Zn(OAc)2, 99.99% trace metals basis), sodium methoxide (powder,
95%), glycerol (99%), anhydrous tertꢀbutanol (ACS reagent, ≥ 99.0%), phosphoric acid (85%)
and hydrogen peroxide (30%) were obtained from Sigma Aldrich. Methylꢀ10ꢀundecenoate (>
96.0%) was supplied by TCI Europe. All products and solvents were used as received except
otherwise mentioned. The solvents were of reagent grade quality and were purified when
necessary according to methods reported in the literature. Methyl 9,10ꢀdihydroxystearate
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(M2HS), methyl 9,10,12ꢀtrihydroxystearate (M3HS) and methyl 13,14ꢀdihydroxybehenate
(M2HB) were synthetized by ITERG (Pessac, France), and their synthesis is described further.
Methods.
1
Nuclear magnetic resonance (NMR) spectroscopy. H and 13C NMR experiments were
conducted on Bruker Avance 400 spectrometer (400.20 MHz or 400.33 MHz and 100.63 MHz
1
for H and 13C, respectively) at room temperature, in CDCl3 as solvent except otherwise
mentioned. Chemical shifts (δ) are reported in parts per million relative to the known solvent
residual peak (δ = 7.26 ppm). DEPTꢀ 135 (Distortion Enhanced Polarization Transfer) and twoꢀ
1
1
dimensional analyses such as Hꢀ1H COSY (Homonuclear correlation spectroscopy), Hꢀ13Cꢀ
1
HSQC (Heteronuclear single quantum coherence) and Hꢀ13CꢀHMBC (Heteronuclear multiple
bond correlation) were also performed.
Size exclusion chromatography (SEC) analyses were performed in THF as the eluent
(1 mL/min) at 40°C, on a PLꢀGPC 50 plus Integrated GPC from Polymer laboratoriesꢀVarian
with a series of four columns from TOSOH [TSKgel TOSOH: HXLꢀL (guard column 6.0 mm ID
x 4.0 cm L); G4000HXL (7.8 mm ID x 30.0 cm L); G3000HXL (7.8 mm ID x 30.0 cm L) and
G2000HXL (7.8 mm ID x 30.0 cm L)]. The elution times of the filtered samples were monitored
using RI detectors with a calibration curve based on low dispersity polystyrene standards (PS).
Trichlorobenzene was added as flow marker.
Differential scanning calorimetry (DSC) measurements were carried out on DSC Q100
apparatus from TA Instruments. For each sample, two cycles from ꢀ100 to 150 °C (except
otherwise mentioned) were performed at 10 °C.minꢀ1. Glass transition and melting temperatures
were calculated based on the second heating run.
6

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Thermogravimetric analyses (TGA) were performed on two different apparatus from TA
Instruments depending on the availability: TGA Q50 and Q500 at heating rate of 10 °C.minꢀ1
under nitrogen atmosphere from room temperature to 700 °C.
MALDI ToF mass spectrometry (MS) analyses were performed by the Centre d’Etude
Structurale et d’Analyse des Molécules Organiques (CESAMO, Bordeaux, France) on a Voyager
mass spectrometer (Applied Biosystems). The instrument is equipped with a pulsed N2 laser (337
nm) and a timeꢀdelayed extracted ion source. Spectra were recorded in the positiveꢀion mode
using the reflectron and with an accelerating voltage of 20 kV. Samples were dissolved in THF at
10 mg/ml. The IAA matrix (transꢀ3ꢀindoleacrylic acid) was prepared by dissolving 10 mg in 1 ml
of THF. A methanol solution of cationization agent (NaI, 10 mg/ml) was also prepared. The
solutions were combined in a 10:1:1 volume ratio of matrix to sample to cationization agent. One
to two microliters of the obtained solution was deposited onto the sample target and vacuumꢀ
dried.
Synthetic procedures.
Synthesis of methyl 9,10ꢀdihydroxystearate (M2HS).
Epoxidation step. Methyl esters of high oleic sunflower oil (40 kg, 133.3 mol) and formic acid
(1.7 kg, 37.8 mol) were added in a reactor equipped with a mechanical stirrer, a dropping funnel
and a condenser. The resulting mixture was heated at 40 °C for 1 hour under stirring. Hydrogen
peroxide (35%, 24.4 kg, 251.6 mol) was added dropwise to the reactor at 40 °C, using a dropping
funnel to maintain the temperature in the reactor close to 70–75°C. As the reaction is exothermic,
a cooling system was used to cool down the reactor. The reaction was monitored by gas
chromatography and epoxide titration. The reaction mixture was then cooled down to room
temperature and the aqueous phase was discarded. The organic layer was washed with an
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aqueous solution of sodium hydroxide (0.1 N) until the pH became neutral. The organic phase
was then dried under vacuum at 60 °C to afford a clear and slightly yellow liquid.
Hydroxylation step. Epoxidized fatty acid methyl esters (10 kg) were placed along with an
aqueous solution of phosphoric acid (12 % w/w, 5 kg) and tertꢀbutanol (3 kg) as solvent in a
reactor equipped with a condenser and a mechanical stirrer. The resulting mixture was heated at
90 °C under vigorous stirring. The reaction was monitored by gas chromatography. When the
reaction was completed, the aqueous phase was discarded at 50°C. tertꢀButanol was eliminated
under vacuum distillation. The organic phase was then washed with hot water until the pH
reached 6ꢀ7 and dried under vacuum to afford a white solid. M2HS was then recrystallized in
cyclohexane (3 times, 40ꢀ60 g.Lꢀ1) and dried under vacuum to afford a white solid powder. The
product was then dissolved in a minimum of dichloromethane (DCM) and injected in a Flash
chromatography apparatus from Grace. The constituents were separated on a silica column, using
a dichloromethaneꢀmethanol gradient and an Evaporating Light Scattering Detector (ELSD). Two
fractions were collected, corresponding to M2HS and its acid form, respectively. M2HS purity
was determined by GC (97.8%).
The procedure followed to prepare methyl 13,14ꢀdihydroxybehenate (M2HB) and methyl
9,10,12ꢀtrihydroxystearate (M3HS) was nearly identical to that described for M2HS; details are
provided hereafter.
Synthesis of methyl 13,14ꢀdihydroxybehenate (M2HB): methyl ester of refined erucic acid
rapeseed oil. After dihydroxylation, the crude mixture was purified by means of neutralization
with potassium hydroxide and recrystallization in cyclohexane, to afford M2HB as a white
powder (purity: 94%). Yield of the overall synthesis: 80%.
Methyl 9,10,11ꢀtrihydroxystearate (M3HS): methyl ester of castor oil. M3HS was purified by
recrystallization in cyclohexane followed by a flash chromatography performed on a silica
8

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column using a dichloromethane:methanol gradient (95:5). The AB3ꢀtype monomer was obtained
as a white powder in rather low yield (25%). Purity: 98.1%.
Synthesis of methyl 10,11ꢀdihydroxyundecanoate (M2HU).
Epoxidation step. Methyl undecenoate (15 g, 0.076 mol) and mꢀcpba (39.2 g, 0.227 mol) were
stirred at room temperature in DCM (20 mL.gꢀ1 of product) overnight. 3ꢀchlorobenzoic acid
formed as side product precipitated in DCM. The reaction mixture was first filtered to remove 3ꢀ
chlorobenzoic acid, washed with aqueous sodium sulfite Na2SO3 (3 x 50 mL), aqueous sodium
bicarbonate NaHCO3 (4 x 50 mL) and brine (2 x 50 mL) until the pH became neutral. The
organic phase was then dried over anhydrous magnesium sulfate, filtered and DCM was removed
on a rotary evaporator to afford methyl 10ꢀepoxyundecenoate. Yield: 92%.
Hydroxylation step. Methyl 10ꢀepoxyundecenoate (2.5 g) was charged in a roundꢀbottom flask
equipped with a mechanical stirrer, an oil bath and a condenser. The epoxidized intermediate was
dissolved in 50 mL of a 1:1 (v/v) mixture of water and tertꢀbutanol under stirring. After the
addition of phosphoric acid (85% w/w, 3 wt%), the reaction flask was heated under reflux at
90°C. 4 hours later, the aqueous phase was discarded at 50°C and tertꢀbutanol was removed
under vacuum distillation. After the addition of 50 mL DCM, the organic phase was washed
twice with water (2 x 50 mL) and brine (1 x 50 mL), dried over anhydrous magnesium sulfate,
filtered and the solvent was removed on a rotary evaporator. M2HU was obtained as a white
powder (purity: 99%). Yield: 57%.
Synthesis of hyperbranched polyesters.
Polytransesterification reactions were performed in bulk, in a Schlenk flask equipped with a
magnetic stirrer, a nitrogen inlet tube and an oilꢀbath heating system. ABnꢀtype monomers (n = 2
or 3) were dried at 90 °C, i.e. above their melting point, under dynamic vacuum prior to use. This
9

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preꢀdrying step took one hour after the reaction mixture was placed under nitrogen blowing.
Temperature was then raised to 120 °C and the catalyst was introduced at a loading of 1.5 wt%
relatively to the monomer. The mixture was allowed to react under stirring at 120 °C for 2 hours.
In a second stage, the temperature was raised to T2 (°C) and dynamic vacuum was applied in
order to remove the released methanol. Reaction conditions were optimized for each monomer as
follows. M2HS, M3HS and M2HB were preferably polymerized at T2 = 160 °C during 15ꢀ24
hours and M2HU at T2 = 140 °C during 8ꢀ10 hours. HBPEs prepared from M2HS and M3HS
were obtained as colorless and highly viscous oils, while polycondensation of M2HB and M2HU
afforded whitish waxes. The observed change in texture was the first sign of the semiꢀcrystalline
character of these HBPEs.
Results and discussion
1- Synthesis of bio-based ABn-type monomers
Unsaturated FAMEs serving as raw plant oil materials were first chemically modified, so as
to achieve different precursors of AB2/AB3ꢀtype consisting of methyl ester (A) and alcohol (B)
reactive functions. The conversion of Aꢀtype FAMEs into ABnꢀtype monomers (n = 2 or 3) was
accomplished following a twoꢀstep straightforward procedure, involving (i) epoxidation of the
double bond and (ii) subsequent ringꢀopening of the asꢀformed epoxide under acidic conditions.
Derivatization of C=C double bonds into 1,2ꢀdiols through epoxidation has been widely applied
to vegetable oil derivatives.41 Upon using methyl oleate, it leads to the formation of methyl 9,10ꢀ
dihydroxystearate (M2HS), i.e. an AB2ꢀtype monomer (Scheme 1). Other substrates of interest
were also considered, on the basis of the same strategy. Thus, methyl euricate, a C22ꢀcarbon
atomꢀcontaining precursor (instead of C18 for methyl oleate), allowed accessing methyl 13,14ꢀ
dihydroxybehenate (M2HB). The latter monomer was expected to provide HBPEs with a higher
10

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alkyl chain length between branching points compared to M2HS (11 CH2 vs. 7 CH2). Similar
modification of methyl ricinoleate, where a hydroxyl function is already present, yielded the
AB3ꢀtype monomer precursor denoted as M3HS. Lastly, methyl undecenoate (C11:1) was
selected to prepare methyl 10,11ꢀdihydroxyundecanoate (M2HU), an AB2ꢀtype monomer, the
polycondenation of which should lead to HBPEs free of dangling chains.
Scheme 1. Synthetic route to novel ABnꢀtype fatty acidꢀbased monomers and related
hyperbranched polyesters.
All monomer precursors were obtained as white powders in yields higher than 60%, except
for M3HS (≈ 25%). The low yield in the latter case could be explained by the hydroxylation of
methyl ricinoleate which resulted in a mixture of four diastereoisomers, only one of them being
able to crystallize.42–45 The structure of the soꢀformed AB2ꢀ or AB3ꢀtype monomers was assessed
1
by H NMR spectroscopy (see ESI Figure S1). GC analyses revealed a chemical purity higher
than 94% in all cases.
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2- Synthesis of hyperbranched polyesters by self-polycondensation of AB2- and AB3–type
monomers
All polymerizations were carried out in bulk, in line with many of the Principles of Green
Chemistry,46 following a twoꢀstepꢀoneꢀpot methodology illustrated in Scheme 1. After a twoꢀhour
oligomerization stage at 120 °C, the temperature was raised to 160 °C, and dynamic vacuum was
applied. Polycondensations were thus performed in the melt, and crude polymers were analyzed
without further purification (Table I).
Table I. Screening of various catalysts for the stepꢀgrowth polymerization of M2HS
xa
a
ŭ
Mn
Entry Catalyst
Đa
(g.mol-1)
1 100
1 200
3 500
1 100
1 200
4 100
1 000
(%)
44
1
2
Ti(OBu)4
Ti(OiPr)4
Zn(OAc)2
DBTO
1.09
1.08
2.71
1.09
1.08
2.46
1.10
48
3
98
4
39
5
Sb2O3
45
6
TBD
100
21
7
mꢀTBD
DABCO
DBU
8
No polymerization
9
10
NaOMe
100
6 100
3.08
(a) determined by SEC in THF; calibration with PS standards. Procedure: 2 hours at 120 °C
under N2 followed by 13 hours at 160 °C under dynamic vacuum
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Various commercially available transesterification catalysts were screened with a loading
of 1.5 wt.% relatively to the monomer. A more systematic investigation was undertaken with
M2HS (Table I). Because transition metal catalysts currently dominate condensation
polymerization processes at the industrial scale, a selection of zinc, tin, titanium and antimonyꢀ
based catalysts was considered first. Organic catalysts were also investigated in line with the
development of environmentally friendly metalꢀfree processes.47,48 Guanidines including 1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (TBD), its Nꢀmethyl derivative, namely, 7ꢀmethylꢀ1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (mꢀTBD), the amidine base 1,8ꢀdiazabicyclo[5.4.0]undecꢀ7ꢀene
(DBU) and the tertiary amine 1,4ꢀdiazabicyclo[2.2.2]octane (DABCO) were thus examined.
Finally, sodium methoxide (NaOMe) was also tested as it is widely used in oleochemistry as
transesterification catalyst of crude vegetable oils.49
Most of the organometallic catalysts tested, including titanium (IV) butoxide and
isopropoxide, Ti(OBu)4 and Ti(OiPr)4, respectively, dibutyltin(IV) oxide (DBTO) and antimony
trioxide (Sb2O3), along with the organic base mꢀTBD showed a poor efficiency toward the
polycondensation of M2HS. DABCO and DBU did not show any activity at all. Much better
results were achieved with zinc acetate, Zn(OAc)2, TBD and NaOMe. Different reaction
outcomes were thus noted from one catalyst to another, which might be correlated to the reaction
mechanism in each case. Regarding the difference between mꢀTBD and TBD used as
organocatalyst, one can hypothesize that TBD acts as an efficient dual catalyst due to both its Hꢀ
donor and Hꢀacceptor capability.47ꢀ48 In contrast, MTBD is known to behave as a Bronsted base
that appears poorly active under the conditions tested here.
Based on this catalyst screening, Zn(OAc)2, TBD and NaOMe were selected for further
polymerization/optimization utilizing the other ABnꢀtype bioꢀsourced monomers.
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Table II. Molar masses, dispersity and thermal properties of HBPEs obtained by selfꢀ
polytransesterification of M2HS, M2HB, M2HU and M3HS
e
Td
b
d
d
d
Ta
Mn
Tg
Tm
Tc
c
Entry Monomer
Catalyst
Đb DBFrey
5wt.%
(°C) (g.mol-1)
(°C) (°C) (°C)
(°C)
P1
P2
Zn(OAc)2 160
Zn(OAc)2 170
3 500
6 300
4 100
5 300
7 600
6 100
3 000
5 600
5 600
9 200
2 400
2 400
3 800
2.71
>15
2.46
3
0.07
0.26
0.35
0.38
0.41
0.29
0.09
0.18
0.33
0.30
nd.
ꢀ27
ꢀ30
ꢀ24
ꢀ24
ꢀ22
ꢀ20
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
246
252
320
324
338
340
nd.
P3
TBD
TBD
160
170
170
160
M2HS
P4
P5
P6
P7
TBD6
NaOMe
>12
3.08
1.93
>11
3.05
3.27
2.33
2.67
2.58
Zn(OAc)2 160
nd. nd.
f
P8
Zn(OAc)2 160
ꢀ
20
22
23
61
54
ꢀ
14
12
12
44
36
ꢀ
260
298
332
204
230
254
M2HB
P9
TBD
160
160
ꢀ
P10
NaOMe
ꢀ
P11
Zn(OAc)2 140
ꢀ
M2HU
P12
TBD
140
0.25
nd.
ꢀ
P13
M3HS
Zn(OAc)2 160
ꢀ3
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P14
P15
TBD
160
160
5 600
4 600
4.05
nd.
nd.
ꢀ1
3
ꢀ
ꢀ
ꢀ
ꢀ
305
299
NaOMe
2.83
(a) Temperature of polymerization. (b) SEC in THF ꢀ calibration PS standards. (c) 1H NMR. (d)
DSC – 10 °C.minꢀ1. (e) determined by TGA – 10 °C.minꢀ1 under N2. (f) Duration of
polymerization increased from 15 to 24 hours. nd. = not determined.
As summarized in Table II, M2HS, M3HS and M2HB were efficiently polymerized at 160
°C, monomer conversion over 95% being reached within 15 hours. SEC analyses indicated the
formation of HBPEs with molar masses (Mn) in between 3 000 and 6 100 g.molꢀ1, corresponding
ŬŬŬŬŬ
to an average number degree of polymerization (DPn) ranging from 10 to 12. However, since
these molar masses were determined using linear PS standards, they are only apparent values and
should be taken with caution, real values being likely much higher. HBPEs are indeed expected
to display much smaller hydrodynamic radii than that of a linear homologues or of PS standards
of same molar mass. Dispersity values higher than 2 were obtained, as expected for
hyperbranched materials prepared by polycondensation of ABnꢀtype monomers.50 Higher molar
masses, Mn = 7 600 g.molꢀ1, were reached by increasing either the reaction time, from 15 to 24
hours, and/or the reaction temperature up to 170 °C, as highlighted with M2HS and M2HB
(Table II, P1 vs. P2, P3 vs. P4 and P5, P7 vs. P8). The impact was even more important on
dispersities, significantly broader values being obtained (> 10) in agreement with the theory.
However, higher polymerization temperatures (≥ 200 °C) and longer reaction times (over 40
hours) were found to favor the formation of insoluble gelꢀlike materials.
In contrast to the other bioꢀbased monomers, M2HU was preferably polymerized at 140 °C.
Due to the higher reactivity of its primary alcohols, it was indeed noted that gelation occurred
more rapidly at 160 °C, i.e. in less than 10 hours. Yet, further decrease of the polymerization
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temperature to 120 °C yielded oligomers only (Mn < 900 g.molꢀ1). Within 8 hours at 140 °C, full
conversions were achieved using both zinc acetate and TBD as catalysts, leading to molar mass
around 2 500 g.molꢀ1.
All these experiments revealed the importance of suitable experimental conditions to
achieve soluble HBPEs. According to Flory’s theory, the statistical polycondensation of ABnꢀ
type monomers yields HBPs without any risk of gelation provided that (i) A reacts exclusively
with B and (ii) reactions do not involve internal cyclization.3 Though, in practice, side reactions
can take place during HBP synthesis and can also be responsible for the formation of insoluble
material due to crossꢀlinking. Syntheses involving M2HS were further scrutinized by analyzing
M2HSꢀbased HBPEs by MALDIꢀToF mass spectroscopy (Figure 1).
Figure 1. MALDIꢀToF MS analysis of M2HSꢀbased HBPE (Matrix: transꢀ3ꢀidoleacrylic acid,
entry P3, Table II)
Four distinct families of populations were eventually detected with an expected peakꢀtoꢀ
peak mass increment of 298.1 g.molꢀ1 matching the repeating unit. Population F1 corresponds to
the targeted HBPE chains derived from M2HS, i.e. n x (Munit) + MMeOH + MNa where Munit
=
MM2HS ꢀ MMeOH = 298.1 g.molꢀ1. Population F2 shows a mass difference of ꢀ32 g.molꢀ1, compared
to the former regular HBPE structure. This was ascribed to the presence of lactone units (n x
(Munit) + MNa), as the result of an esterification reaction occurring by an intramolecular pathway.
The latter side reaction could either involve the carbonyl group of the focal point and a hydroxyl
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function of the same HBPE molecule (F2, Scheme 2), or could take place via a hydroxylꢀester
interchange reaction between groups of the same branch (F’2, Scheme 2). As for population F3,
it was characterized by m/zꢀ18 g.molꢀ1 with regards to F1 (Scheme 2), and it could be explained
by a loss of a water molecule. In other words, this population reflected the occurrence of
intramolecular etherification between two hydroxyl groups: n x (Munit) + MMeOH + MNa ꢀ MH2O. It
is noteworthy that, if intramolecular cyclizations through ester (F2) or ether (F3) bond formation
might reduce achievable molar masses of these bioꢀbased HBPEs, they did not yield crossꢀlinked
materials. Gelation observed experimentally would thus be due to extensive etherification side
reactions taking place intermolecularly, and thus leading to more than one focal group per
macromolecule, i.e. at n x (Munit) + 2x MMeOH + MNa ꢀMH2O. Accordingly, a series of peaks (=
population F4) was observed, at m/z+14 g.molꢀ1 (= MMeOH ꢀ MH2O), and assigned to ether bridges
between polymeric chains (Scheme 3).
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Scheme 2. Proposed side reactions during bioꢀbased HBPE synthesis and related populations:
cyclization by intramolecular esterification (F2), by intramolecular hydroxylꢀester interchange
between groups of the same branch (F’2) and by intramolecular etherification (F3).
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Scheme 3. Intermolecular etherification accounting both for the presence of population F4 and
for gel formation.
Although these side reactions could not be quantified, they appeared to occur whatever the
polymerization catalyst employed (Zn(OAc)2 and TBD), and thus compete with the
transesterification reaction during the whole polymerization process. These observations were
supported by SEC analyses, as illustrated in Figure 2, comparing SECꢀRI traces of aliquots of the
TBDꢀcatalyzed polycondensation of M2HS. In the early stages of reaction, indeed,
chromatograms displayed the pattern of a typical stepꢀgrowth polymerization of ABnꢀtype
monomers. Observation of multimodal distributions was consistent with the formation of
ŬŬŬŬŬ
intermediate oligomers of increasing degrees of polymerization (DPn). The disappearance of the
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peak at 27.9 minutes, assigned to M2HS, evidenced a complete conversion within the first 4
hours of polymerization. After 15 hours of reaction, signals attributed to dimers and trimers at
26.4 and 25.3 minutes, respectively, were split into two discrete peaks, as the result of lactone
formation by cyclization, as discussed above. In absence of focal point constituted of a methyl
ester function, these cyclic oligomers cannot react further during polymerization and remain as
such till the last stage of the reaction, thereby contributing to the broadening of the molar mass
distribution. Beyond 20 hours of polymerization, a shoulder was observed in the higher molar
mass region, likely as the consequence of both intermolecular transesterification and
etherification side reactions. This intermolecular etherification reaction can create a second focal
point in the polymer chain and ultimately lead to crosslinking.
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Figure 2. SECꢀRI traces of aliquots withdrawn at different reaction times, during the TBDꢀ
catalyzed polycondensation of M2HS.
Characterization of HBPEs
The chemical structure of the asꢀformed HBPEs was first confirmed by FTꢀIR analysis (see ESI
Figure S3). In all spectra, absorption bands characteristic of both the ester and the hydroxyl
functions were observed at 1 734 and 3 400 cmꢀ1, respectively. To gain a better insight into their
fine structure, and in particular to determine the DB value of these HBPEs, analysis by 1H NMR
spectroscopy was realized. Fréchet et al.51 first expressed the DB as follows:
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[D] + [T]
(1)
DBFréchet
=
[D] + [T] + [L]
where [D], [T] and [L] correspond, respectively, to the molar proportion of dendritic, terminal
and linear units. DB is thus equal to 1 for perfectly branched structures (e.g. for dendrimers),
whereas HBP usually exhibit DB values lower than 1. Assuming that this equation is only valid
for high molar mass HBPs, Frey et al.35 extended equation (1) and proposed the following
expression for DB:
2[D]
DBFrey
=
(2)
2[D] + [L]
Both definitions lead to similar results at high conversions. In the present study, equation (2) was
used for the determination of DB.
Importantly, wellꢀseparated signals assigned to the three different subunits, T, L and D,
could be clearly distinguished by 1H NMR spectroscopy, allowing the DB value to be accurately
determined. As an illustration, Figure 3 (see also Fig. S4 in ESI) depicts the 1H NMR spectrum
of P3 of Table II . The steep decrease in intensity of the peak assigned to the methoxy group at
3.66 ppm confirmed the polyester synthesis. Consistently with the 1H NMR spectrum of M2HS,
the peak at 3.38ppm was attributed to the terminal unit (T), i.e. to the protons adjacent to the
vicinal diol on αꢀcarbon atoms (Ha). After the first substitution, protons Hb and Hc, which are
characteristic of the linear unit (L), are no longer equivalent and thus display downꢀfielded
signals at 3.56 and 4.81 ppm, respectively. It should be noted that chemical shifts are equivalent
after esterifying either the 9 or the 10ꢀposition. Finally, the peak at 4.99 ppm was unambiguously
assigned to the protons Hd of the dendritic unit (D).
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1
Similarly, HBPEs derived from M2HB and M2HU were successfully characterized by H
NMR spectroscopy (see ESI, Figures S5ꢀ6 and Table S1). In the case of the AB3ꢀtype monomer,
however, asꢀformed HBPEs are not characterized by three but by four different units, including
the terminal (T), the linear (L), the semiꢀdendritic (sD) and the perfectly dendritic (D) units.
Since the three hydroxyl groups of M3HS are not chemically equivalent, the number of
accessible configurations is increased to 8, leading to a complex NMR spectrum (see ESI, Fig.
S7) that did not allow us to determine the DB value in this case.
The DB was found to dramatically depend on the type of catalyst employed with both AB2ꢀ
type monomers investigated in this work. While zinc acetate appeared to favor the formation of
weakly branched structures (DB < 0.26), both TBD and NaOMe gave HBPEs with a DB up to
0.45 (see ESI, Figure S8).
Figure 3. 1H NMR spectrum of P3 prepared by polycondensation of M2HS in CDCl3 (Table II)
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We also attempted to determine the DB value as a function of the conversion, on the basis of
previous works by Hölter and Frey35 and by Yan, Müller et al,52 who expressed the conversion
dependence of the DB as follows:
2x
DB =
ɴ
(3)
5 ꢀ x
k = k
L
D
where x corresponds to the conversion of the focal point A. This equation describes the ideal
random polycondensation of AB2ꢀtype monomers during which the formation of linear (kL) and
dendritic (kD) units takes place at equal reaction rates (kL = kD, Scheme 4). Overall, DB values
were found in the range 0.07 to 0.45 (Table II), which remained lower than the value of 0.5
predicted by theory at full conversion. This somehow limited value of DB can be easily
correlated to the position of the two alcohol groups of our bioꢀbased AB2ꢀtype monomers. Once a
linear unit is formed, the reactivity of the remaining (nonꢀreacted) alcohol (ꢀOHL) is indeed
significantly reduced due to steric hindrance, limiting the formation of dendritic units. This
phenomenon has been called by Galina et al.53 as the negative substitution effect, on the basis of
kinetic investigations.
Scheme 4. Rate constants of the formation of linear and dendritic units: kL and kD
Here, both the Zn(OAc)2 and the TBDꢀcatalyzed polytransesterifications of M2HS were
1
monitored by H NMR spectroscopy. Theoretical and experimental data are compared in Figure
4. The discontinuity of the curves showing the evolution of DB with the ester group conversion
(x) is explained by the fact that the first three data were collected during the oligomerization stage
at 120 °C (see above), while subsequent data were obtained when the polymerization took place
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at 160 °C under dynamic vacuum. This curve clearly indicates that the two hydroxyl functions of
M2HS do not exhibit the same reactivity, irrespective of the catalyst, experimental points always
remaining below the theoretical line. The maximum DB value of 0.4 with TBD and of 0.2 with
Zn(OAc)2 denotes a reactivity ratio r (r = kL/kD) equal to 0.5 and to 0.1, respectively.53
0.50
TBD
0.40
Zinc acetate
0.30
0.20
0.10
0.00
0
0.2
0.4
0.6
0.8
1
x
Figure 4. Conversion dependence of the degree of branching (DB) for the polycondensation of
M2HS using TBD or zinc acetate as catalyst (solid lines: theoretical data, symbols: NMR data)
Thermal properties
Thermal properties of our bioꢀbased HBPEs were assessed by DSC analysis. Glass
transition and melting temperatures were thus determined from the second heating scan at 10
°C.minꢀ1 and crystallization temperatures from the first cooling scan at the same rate (Table II).
As expected, all HBPEs derived from M2HS and M3HS proved amorphous. Compounds
obtained from M3HS showed higher glass transition temperatures, i.e. in between ꢀ10 and 9 °C,
compared to M2HSꢀbased polyesters (ꢀ32.5 ≤ Tg ≤ ꢀ20 °C), which was obviously explained by
the higher functionality of the AB3ꢀtype monomer (M3HS vs. M2HS). Unlike linear polymers
(see ESI Fig. S9), the Tg value of HBPs is greatly affected both by the nature and the content of
endꢀgroups (T units).11 The number of polar endꢀgroups increasing with the monomer
functionality, more interactions likely develop within M3HSꢀbased HBPEs, compared to M2HSꢀ
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based homologues of similar molar masses. Note however that the branching density of these
renewable HBPEs did not seem to impact Tg values.
Interestingly, both HBPEs derived from M2HB and M2HU displayed semiꢀcrystalline
properties. DSC thermograms are given in ESI (Fig. S10). The higher flexibility of M2HBꢀ
derived HBPEs, which consist of a higher chainꢀlength between branching points compared to
M2HS (11 carbon atoms vs. 7), allowed the development of crystalline zones. The semiꢀ
crystalline character of M2HBꢀbased HBPEs was confirmed by wideꢀangle Xꢀray diffraction
(WAXS) measurements (see ESI; Fig. S11). The absence of pendant alkyl chains acting as
plasticizers also appeared to endow the HBPEs derived from M2HU with semiꢀcrystalline
properties. In this case, complex melting and crystallization patterns were observed, probably due
to the unsymmetrical nature of the initial diol that could affect the chain mobility and prevent the
formation of stable crystals.
Lastly, thermal stabilities of these HBPEs were investigated by TGA under nonꢀoxidative
conditions, and at a heating rate of 10 °C.minꢀ1 (Table II). Typical degradation profiles are given
in ESI (Fig. S12ꢀ13). HBPEs thus showed typical thermal stabilities for oilyꢀderived polymers,32
with a 5% weight loss (Td5%) up to 332 °C. HBPEs derived from M2HU were found to exhibit
ꢀ
ŭ
the lowest heat resistance of this series, likely due to their lower molar masses (Mn ≤ 2 500 g.mol
1). Overall, HBPEs prepared using zinc acetate (P1, P2, P7, P8, P11 and P13) appeared less stable
than their homologues obtained with TBD or NaOMe. The thermal degradation of the former
compounds was found to be accelerated at high temperatures, likely due to residual traces of
Zn(OAc)2 catalyzing their depolymerization (see ESI Fig. S14), as already reported for more
conventional polyesters, such as poly(ethyleneterephthalate)54 or poly(tetramethylene
succinate).55
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Conclusion
Sunflower, castor and rapeseed oils can serve as raw materials to be readily derivatized into
AB2ꢀ or AB3ꢀtype monomer substrates featuring methyl ester and alcohol functions, by means of
epoxidation followed by ringꢀopening of epoxyꢀrings. Subsequent polycondensation of these bioꢀ
based monomers can be performed in bulk, giving access to modular hyperbranched polyesters
through repeated transesterification reactions. Zinc acetate, TBD and sodium methoxide ꢀthree
catalytic systems operating by a specific activation mechanismꢀ, are the most effective, achieving
molar masses in the range 3 000ꢀ10 000 g.molꢀ1 and dispersities varying from 2 to 15. Depending
on the initial conditions and on the monomer considered, either amorphous or semiꢀcrystalline
hyperbranched polyesters, with a Tg value in the range ꢀ33 °C to 9 °C, a thermal stability above
300 °C, and with a degree of branching that could be varied from 0.07 to 0.45, can thus be
synthesized. Due to the presence of numerous hydroxyl groups both in linear and terminal units,
such hyperbranched polyesters are likely amenable to facile postꢀpolymerization modification,
allowing for further tuning the properties of these renewable branched materials.
Funding Sources
This work was performed, in partnership with the SAS PIVERT, within the frame of the French
Institute for the Energy Transition (Institut pour la Transition Energétique (ITE) P.I.V.E.R.T.
(www.institutꢀpivert.com) selected as an Investment for the Future (“Investissements d’Avenir”).
This work was supported, as part of the Investments for the Future, by the French Government
under the reference ANRꢀ001ꢀ01.
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Acknowledgments
The authors thank University of Bordeaux, Bordeaux INP, CNRS, Aquitaine Regional Council,
the SAS PIVERT and ITERG for the support of this research. Besides, the writers acknowledge
P. Castel and C. Absalon from CESAMO for MALDIꢀToF MS experiments and A.Bentaleb from
CRPP for the WAXS measurements.
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