Synthesis of poly(sulfonate ester)s by ADMET polymerization

Article abstract of DOI:10.1039/c4ra08788g

Many hydrocarbon polymers containing heteroatom defects in the main chain have been investigated as degradable polyethylene-like materials,including aliphatic polyesters.Here, acyclic diene metathesis (ADMET) polymerization was used for the synthesis of aliphatic poly(sulfonate ester)s.The requisite sulfonate ester containing-diene monomers with varying numbers of methylene groups was synthesized, and their polymerization in the presence of ruthenium-N-heterocyclic (Ru-NHC) alkylidene catalysts was studied. A clear negative neighboring group effect (NNGE) was observed for shorter dienes, either inhibiting polymerization or resulting in low-molecular-weight oligomers. The effect was absent when undec-10-en-1-yl undec-10-ene-1-sulfonate was employed as the monomer, and its ADMET polymerization afforded polymers with appreciable number-average molecular weights of up to 37000 g mol^-1 and a dispersity of 1.8.These polymers were hydrogenated to afford the desired polyethylene-like systems.The thermal and morphological properties of both saturated and unsaturated polymers were investigated.The incorporation of sulfonate ester groups in the polymer backbone offers an interesting alternative to other heteroatoms and helps further the understanding of the effects of these defects on the overall polymer properties.

Full text of DOI:10.1039/c4ra08788g

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Synthesis of poly(sulfonate ester)s by ADMET
polymerization†
Cite this: RSC Adv., 2014, 4, 53967
*
Rebecca R. Parkhurst, Sandor Balog, Christoph Weder and Yoan C. Simon
Many hydrocarbon polymers containing heteroatom defects in the main chain have been investigated as
degradable polyethylene-like materials, including aliphatic polyesters. Here, acyclic diene metathesis
(ADMET) polymerization was used for the synthesis of aliphatic poly(sulfonate ester)s. The requisite
sulfonate ester containing a,u-diene monomers with varying numbers of methylene groups was
synthesized, and their polymerization in the presence of ruthenium-N-heterocyclic (Ru-NHC) alkylidene
catalysts was studied. A clear negative neighboring group effect (NNGE) was observed for shorter dienes,
either inhibiting polymerization or resulting in low-molecular-weight oligomers. The effect was absent
when undec-10-en-1-yl undec-10-ene-1-sulfonate was employed as the monomer, and its ADMET
polymerization afforded polymers with appreciable number-average molecular weights of up to 37 000
g
molꢀ1 and
a dispersity Đ of 1.8. These polymers were hydrogenated to afford the desired
Received 17th September 2014
Accepted 7th October 2014
polyethylene-like systems. The thermal and morphological properties of both saturated and unsaturated
polymers were investigated. The incorporation of sulfonate ester groups in the polymer backbone offers
an interesting alternative to other heteroatoms and helps further the understanding of the effects of
these defects on the overall polymer properties.
DOI: 10.1039/c4ra08788g
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polyesters to model the effect of these hydrolyzable defects on
the crystal packing of the polyethylene-like material and
Introduction
consequently its bulk properties.10–14 Despite recent interest in
such polyesters, the corresponding poly(sulfonate ester)s have
not yet been explored. Sulfonate esters have the potential to
serve as a useful point of degradation as they are susceptible to
both thermolysis15 and hydrolysis16 (Scheme S1, ESI†).
Since the advent of the Ziegler–Natta process in the early 1950s,1
polyolens, and particularly polyethylene, have become ubiq-
uitous in applications that range from packaging,2,3 to
implants,4 to life vests.5 Polyethylene exhibits tailorable
mechanical properties, low cost, and high chemical and bio-
logical stability. However, in the light of the high rates of
polyethylene production and use, high durability also contrib-
utes to its accumulation as waste in the environment due to its
extremely slow biodegradation. Therefore, there has been great
interest in producing “degradable” polyethylene, i.e.
polyethylene-like polymers containing defects that facilitate
their breakdown into low-molecular-weight fragments that are
more easily digested by micro-organisms. The main methods
through which this has been accomplished thus far have been
the introduction of unsaturated carbon–carbon bonds, weak
sites such as ketones in both side and main chains, or other
cleavable groups such as disuldes6 or esters.7
Here, we report the use of acyclic diene metathesis (ADMET)
as an appealing technique for the synthesis of degradable,
polyethylene-like poly(sulfonate ester)s that contain sulfonate
ester groups in the backbone. Through polycondensation of
a,u-dienes, ADMET polymerization allows access to a wide
variety of both linear and hyperbranched polyolens with
unmatched architectural regularity (Scheme 1a).17,18 The devel-
opment of a wide variety of catalysts for this transformation,
particularly the later generation Ru-NHC alkylidene catalysts,
has expanded its functional group tolerance (Scheme 1b).19,20
Unlike the related ring-opening metathesis polymerization
(ROMP), which is a chain-growth process driven by the release
of ring strain, ADMET is driven by the removal of the ethylene
gas generated as a by-product. ADMET chemistry has been used
recently in the synthesis of both all-carbon polyolens21–23 as
well as polyethylene-like polymers containing heteroatoms24–30
or aromatic rings31–33 in the main chain.
Polyesters such as poly(lactic acid) and poly-
hydroxyalkanoates are therefore oen mentioned as environ-
mentally benign alternatives to the use of polyolens.8,9
Numerous reports have also investigated long-chain aliphatic
ADMET offers an interesting alternative for the incorpora-
tion of aliphatic sulfonate esters into the polymer backbone,
which is a rare occurrence in the literature. The poly-
condensation of disulfonyl chlorides with diphenols has been
Adolphe Merkle Institute, University of Fribourg, Rte de l'Ancienne Papeterie, 1723
Marly, Switzerland. E-mail: yoan.simon@unifr.ch
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra08788g
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(ESI), electron ionization (EI), or matrix-assisted laser
desorption/ionization (MALDI). Polymer molecular weights
were measured by gel permeation chromatography (GPC, 40 ꢁC,
1 mL minꢀ1) in CHCl3 unless otherwise noted, and are reported
versus polystyrene standards. A Polymer Laboratories 5 mm
mixed-C guard column and two GPC columns were employed
with an Agilent Technologies series 1200 HPLC instrument.
Wyatt Technology Corp. provided both the detector (Optilab
REX interferometric refractometer) and soware (ASTRA) for
analysis.
Thermogravimetric analysis (TGA) was performed using a
Mettler-Toledo STAR thermogravimetric analyzer under N2 with
a heating rate of 10 ꢁC minꢀ1. Differential scanning calorimetry
(DSC) measurements were conducted on a Mettler-Toledo STAR
instrument. These experiments were done under N2 with
Scheme 1 (a) ADMET polymerization of a,u-dienes and (b) structures
of Grubbs catalyst 2nd generation (GII) and Hoveyda–Grubbs catalyst
2nd generation (HGII).
ꢀ1
ꢁ
heating/cooling rates of 10 C min for the specied temper-
ature range. Dynamic mechanical analysis39 was performed
with a TA Instruments DMA Q 800 under N2. The heating rate
employed was 3 ꢁC minꢀ1, ranging from ꢀ100 ꢁC to the melting
point of the sample. Films of P3 for thermomechanical analysis
used in the synthesis of aromatic poly(sulfonate ester)s which
possess interesting physical, chemical, electrical and thermal
properties.34,35 However, the number-average molecular weights
of the resulting polymers tend to be rather low (ranging from
2000 to 16 000 g molꢀ1) and the dispersities high (up to 29).
These conditions also oen require a phase-transfer catalyst
and produce stoichiometric amounts of salt by-products. To the
best of our knowledge, the only example of aliphatic poly-
(sulfonate ester)s to date have been synthesized by the ring-
opening polymerization of 1,3-propanesultone.36 The present
strategy allows access to new polyethylene-like materials,37
which exhibit properties complementing the range of charac-
teristics covered by previously available materials.
ꢁ
were solution cast from PhCH3 at 30 C.
Small- and wide-angle X-ray scattering (SAXS and WAXS,
respectively) spectra were recorded by a NanoMax-IQ camera
(Rigaku Innovative Technologies, Auburn Hills, MI USA).
Samples were kept at room temperature in vacuum during the
measurements. Raw data were processed according to standard
procedures, and the isotropic scattering spectra are presented
as a function of the momentum transfer q ¼ 4plꢀ1 sin (q/2),
where q is the scattering angle and l ¼ 0.1524 nm is the photon
wavelength. Films of HDPE, P3 and HP3 for SAXS/WAXS anal-
ysis were prepared by melt-processing and quenching in iced
water (HDPEꢁ) or by placing the freshly melted samples in a
freezer (ꢀ20 C) (P3/HP3).
Experimental section
Materials and methods
All commercially available chemicals were used as received
without further purication. 1-Bromo-6-hexene, 5-hexen-1-ol,
11-bromo-1-undecene, and oxalyl chloride ((COCl)2) were
purchased from ABCR, and 11-undecen-1-ol was purchased
Synthesis of hex-5-en-1-yl hex-5-ene-1-sulfonate (M2)
Hex-5-ene-1-sulfonyl chloride (2). 1-Bromo-6-hexene (500
from TCI Chemicals. High density polyethylene (HDPE) was mg, 3.07 mmol) and sodium sulte (Na2SO3, 560 mg, 4.45
received from DSM (Stamylan 7048). All other chemicals were mmol) were combined in water (4.0 mL). The mixture was
purchased from Sigma-Aldrich. Dimethylformamide (DMF) was heated to reux overnight with stirring. Aer cooling to room
dried by passing the solvent through alumina columns. A Bio- temperature, water was removed under reduced pressure. The
tage®IsoleraOne with pre-packed silica gel columns was resulting white solid was taken up in phosphorus(V) oxychloride
employed for ash column chromatography. Monomer M1 was (POCl3, 4.0 mL), tted with a reux condenser, and ushed with
synthesized according to the procedure reported by Le Flohic N2 for 10 min. Aer heating to 120 ꢁC for 100 min, the reaction
et al.38 Microwave reactions were performed on
tage®Initator Microwave Synthesizer.
a
Bio- was cooled to room temperature. The reaction mixture was then
poured over ice and the POCl3 was allowed to hydrolyze for 1 h.
1H and 13C NMR spectra were measured on a Bruker Avance The aqueous layer was extracted with dichloromethane (CH2Cl2,
spectrometer at either 360 MHz (1H) and 90 MHz (13C), or 2 ꢂ 25 mL). The combined organic layers were dried over
500 MHz (1H) and 125 MHz (13C) as noted. All spectra were anhydrous sodium sulfate (Na2SO4), ltered, and the solvent
recorded in CDCl3, and chemical shis (d) are reported in parts was removed under reduced pressure. The crude product was
per million (ppm) referenced to the residual solvent peak puried by ash column chromatography (1 : 3 CH2Cl2-
(7.27 ppm and 77.0 ppm, respectively). Infrared (IR) spectra
: Hexane, Rf ¼ 0.25) to yield 2 as a yellowish oil (356 mg, 64%).
were recorded on a PerkinElmer Spectrum 65 FT-IR Spectrom- 1H NMR (360 MHz, CDCl3), d (ppm): 5.78 (m, 1H, CH]CH2),
eter using attenuated total reection (ATR) sampling.
5.04 (m, 2H, CH]CH2), 3.69 (m, 2H, CH2–SO2Cl), 2.10 (m, 4H,
Mass spectrometry (MS) data for small molecules were CH2–CH] , CH2–CH2SO2Cl), 1.60 (m, 2H, CH2–CH2CH]). 13C
provided by the analytical services at the University of Fribourg NMR (90 MHz, CDCl3), d (ppm): 137.0 (CH]CH2), 115.9
Chemistry Department using either electrospray ionization (CH]CH2), 65.2 (CH2–SO2Cl), 32.8 (CH2–CH]), 26.6
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(CH2–CH2SO2Cl), 23.6 (CH2–CH2CH]). MS (MALDI): m/z solvent was removed under reduced pressure. The crude
([M ꢀ H]ꢀ) ¼ 181.06.
product was puried by ash column chromatography (1 : 4
Hex-5-en-1-yl hex-5-ene-1-sulfonate (M2). 5-Hexen-1-ol, 4- CH2Cl2 : Hexane, Rf ¼ 0.21) to yield M3 as a colorless oil (789
dimethylaminopyridine (DMAP), and triethylamine (Et3N) were mg, 64%). 1H NMR (500 MHz, CDCl3), d (ppm): 5.82 (m, 2H,
dissolved in CH2Cl2 and cooled to 0 ꢁC in an ice/water bath. CH]CH2), 4.98 (m, 4H, CH]CH2), 4.21 (t, J ¼ 5.0 Hz, 2H, CH2–
Compound 2 was then added drop-wise and the reaction mixture OSO2), 3.08 (m, 2H, CH2–SO2–O) 2.05 (m, 4H, CH2–CH]), 1.84
was stirred for 75 min. The reaction mixture was allowed to cool (m, 2H, CH2–CH2OSO2), 1.71 (m, 2H, CH2–CH2SO2–O),
then washed with water, 1 N HCl, and NaHCO3(aq) (15 mL each). 1.45–1.29 (m, 24H, CH2–CH2–CH2). 13C NMR (125 MHz, CDCl3),
The organic layer was dried over magnesium sulfate (MgSO4),
d
(ppm): 139.2 (CH]CH2), 139.1 (CH]CH2), 114.17
ltered, and the solvent was removed under reduced pressure. The (CH]CH2), 114.15 (CH]CH2), 69.7 (CH2–OSO2), 50.4
crude product was puried by ash column chromatography (1 : 1 (CH2–SO2–O), 33.8 (2 ꢂ CH2–CH]), 29.35, 29.33, 29.31, 29.20,
CH2Cl2 : Hexane, Rf ¼ 0.18) to yield M2 as a colorless oil (326 mg, 29.18, 29.05, 29.01, 29.00, 28.96, 28.87, 28.85, 28.2, 25.4, 23.5
88%). 1H NMR (360 MHz, CDCl3), d (ppm): 5.78 (m, 2H, CH]CH2), (CH2–CH2–CH2). MS (ESI): m/z ([M + Na]+) ¼ 409.3.
5.02 (m, 4H, CH]CH2), 4.22 (t, J ¼ 7.2 Hz, 2H, CH2–OSO2), 3.10
(m, 2H, CH2–SO2–O) 2.11 (m, 4H, CH2–CH]), 1.85 (m, 2H,
Polymer synthesis
CH2–CH2OSO2), 1.74 (m, 2H, CH2–CH2SO2–O), 1.53 (m, 4H,
CH2–CH2CH]). 13C NMR (90 MHz, CDCl3), d (ppm): 137.9
General procedure for bulk polymerization. A 10 mL Schlenk
(CH]CH2), 137.5 (CH]CH2), 115.4 (CH]CH2), 115.2 (CH]CH2), ask was charged with monomer and ushed with N2 for 20
69.4 (CH2–OSO2), 50.2 (CH2–SO2–O), 33.0 (2 ꢂ CH2–CH]), 28.6 min. The chosen Ru-NHC alkylidene catalyst (1 mol%) was
(CH2–CH2OSO2), 27.3 (CH2–CH2SO2–O), 24.6 (CH2–CH2CH]), 22.9 added under N2 to the stirred monomer, and the reaction vessel
(CH2–CH2CH ¼). MS (ESI): m/z ([M + Na]+) ¼ 269.1.
was placed under dynamic vacuum (ꢃ0.01–0.1 mbar) at room
temperature. The evolution of a gas could immediately be
observed. Aer 2.5 h, bubbling had slowed and the temperature
was then slowly raised to 80 ꢁC. The reaction mixture was stirred
Synthesis of undec-10-en-1-yl undec-10-ene-1-sulfonate (M3)
Undec-10-ene-1-sulfonyl chloride (3). 11-Bromo-1-undecene at this temperature overnight. The reaction mixture was
(1.000 g, 4.29 mmol) and Na2SO3 (865 mg, 6.86 mmol) were subsequently cooled to room temperature and dissolved in a
combined in tetrahydrofuran (THF, 4.0 mL), ethanol (EtOH, 8.0 minimal volume of a 1 : 99 v/v mixture of ethyl vinyl ether and
mL), and water (8.0 mL) in a microwave vial. Aer 5 min of pre- toluene (PhCH3). The polymer was then precipitated in cold
stirring, the mixture was microwaved for 30 min at 160 ꢁC. The methanol (MeOH) and collected by vacuum ltration to yield
solvent was then removed under reduced pressure and the the desired polymer.
residue was further dried at 60 ꢁC in a vacuum oven overnight.
Solution polymerization of M3. Monomer M3 (100 mg, 0.26
The resulting white solid was suspended in cold (COCl)2 and mmol) was dissolved in o-dichlorobenzene (200 mL) and the
ꢁ
stirred at 0 C under N2 for 30 min. The suspension was then system was ushed with N2 for 10 min. Catalyst GII (2.2 mg,
warmed to room temperature. DMF was added drop-wise and 1 mol%) was added under a ow of N2 with stirring. The solu-
the reaction mixture was stirred for 3 h. The solvent was then tion was submitted to dynamic vacuum maintained between
removed by distillation. The remaining solid was taken up in 40–150 mbar by a regulator. The reaction mixture was heated to
ꢁ
diethyl ether (Et2O, 50 mL), ltered, and the ltrate was washed 45 C and stirred overnight. Visible gas evolution commenced
with Et2O (50 mL). The combined organic layers were concen- immediately. Aer cooling to room temperature, the solution
trated under reduced pressure. The crude product was puried was precipitated into cold MeOH and collected by vacuum
by ash column chromatography two times (1 : 3 CH2Cl2- ltration to yield P3 as a light gray solid (92 mg, 99%).
: Hexane, Rf ¼ 0.28) to yield 3 as a colorless oil (1.043 g, 24%).
Polymer P2 (bulk polymerization). 1H NMR (360 MHz,
1H NMR (500 MHz, CDCl3), d (ppm): 5.81 (m, 1H, CH]CH2), CDCl3), d (ppm): 5.40 (broad m, 2H, –CH]), 4.21 (t, J ¼ 5.4 Hz,
4.97 (m, 2H, CH]CH2), 3.67 (m, 2H, CH2–SO2Cl), 2.05 (m, 4H, 2H, CH2–OSO2), 3.08 (m, 2H, CH2–SO2–O) 2.04–1.47 (m, 12H,
CH2–CH], CH2–CH2SO2Cl), 1.50 (m, 2H, CH2–CH2CH]), 1.40– –CH2–). 13C NMR (90 MHz, CDCl3), d (ppm): 130.0 (m, –CH]),
1.26 (m, 10H, CH2–CH2–CH2). 13C NMR (125 MHz, CDCl3) 139.1 69.6 (CH2–OSO2), 50.1 (CH2–SO2–O), 31.8 (CH2–CH]), 28.6,
(CH]CH2), 114.2 (CH]CH2), 65.4 (CH2–SO2Cl), 33.7 27.9, 22.9 (CH2–CH2–CH2).
(CH2–CH]), 29.2, 29.1, 29.0, 28.9, 28.8, 27.6, 24.2
(CH2–CH2–CH2). MS (EI): m/z (M+) ¼ 252.11.
Polymer P3 (solution polymerization). 1H NMR (360 MHz,
CDCl3), d (ppm): 5.39 (broad m, 2H, –CH]), 4.21 (t, J ¼ 4.5 Hz,
Undec-10-en-1-yl undec-10-ene-1-sulfonate (M3). 10- 2H, CH2–OSO2), 3.08 (m, 2H, CH2–SO2–O) 1.98 (m, 4H,
Undecen-1-ol (0.64 mL, 3.18 mmol), DMAP (39 mg, 0.32 mmol), CH2–CH]), 1.85 (m, 2H, CH2–CH2OSO2), 1.74 (m, 2H,
and Et3N (0.53 mL, 3.82 mmol) were combined in CH2Cl2 CH2–CH2SO2–O), 1.41–1.29 (m, 24H, CH2–CH2–CH2). 13C NMR
(20 mL) and cooled to 0 ꢁC in an ice/water bath with stirring. A (90 MHz, CDCl3), d (ppm): 130.3 (m, –CH]), 69.7 (CH2–OSO2),
solution of compound 3 (965 mg, 3.82 mmol) in CH2Cl2 (10 mL) 50.4 (CH2–SO2–O), 32.6 (CH2–CH]), 29.7, 29.6, 29.40, 29.38,
was then added drop-wise to the reaction mixture. The reaction 29.3, 29.1, 29.04, 28.99, 28.2, 25.4, 23.4 (CH2–CH2–CH2).
mixture was stirred at 0 ꢁC for 30 min, allowed to warm to room
Procedure for catalytic hydrogenation. Polymer P3 (350 mg)
temperature, and washed with water, 1 N HCl, and brine (25 mL and 10% Pd/C (17.5 mg) were combined in PhCH3 (20 mL). The
each). The organic layer was dried over MgSO4, ltered, and the heterogenous mixture was then stirred at room temperature
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under a hydrogen pressure of 30 bar for 72 h. Following resulting sodium sulfonate product exhibited extremely low
hydrogenation, the reaction mixture was diluted with CHCl3 solubility in water due to the longer alkenyl chains, and there-
(40 mL) and ltered over celite. The solvent was removed under fore the reaction did not reach high conversion even aer
reduced pressure, yielding HP3 as an off-white solid (258, 73%). reuxing for several days. Instead, a microwave procedure using
1H NMR (300 MHz, CDCl3), d (ppm): 4.21 (t, J ¼ 6.0 Hz, 2H, a 1 : 2 : 2 mixture of THF, EtOH, and water was employed.42 The
CH2–OSO2), 3.08 (m, 2H, CH2–SO2–O), 1.86 (m, 2H, chlorination procedure using POCl3 was similarly not appli-
CH2–CH2OSO2), 1.74 (m, 2H, CH2–CH2SO2–O), 1.41–1.26 (m, cable for the synthesis of sulfonyl chloride 3, as it resulted in
32H, CH2–CH2–CH2). 13C NMR (75 MHz, CDCl3), d (ppm): 69.7 low and inconsistent yields and undesired by-products. To
(CH2–OSO2), 50.4 (CH2–SO2–O), 29.7, 28.6, 29.54, 29.51, 29.4, circumvent these issues, (COCl)2 with a catalytic amount of
29.3, 29.2, 29.04, 29.99, 28.2, 25.4, 23.4 (CH2–CH2–CH2).
DMF was used as the chlorinating agent to form sulfonyl
chloride 3 in reasonable and reproducible yields. Monomer M3
was subsequently synthesized using a condensation procedure
analogous to that used for the preparation of M1 and M2.
Results and discussion
Monomer synthesis
The negative neighboring group effect (NNGE) in bulk ADMET
polymerization, a phenomenon in which a coordinating group
in the monomer can bind the catalyst and inhibit the catalytic
cycle, has been well-documented in the literature.40 This effect
generally occurs when fewer than two methylene spacers sepa-
rate the coordinating group and the alkene, and it has been
observed for a wide variety of functional groups, including
ethers,24 thioethers,26 phosphates,30 esters,25 and carbonates.27
With the aim of circumventing this potential problem, three
different sulfonate ester-containing a,u-diene monomers were
synthesized, M1–M3, in which the sulfonate ester group was
separated from the alkene with m ¼ 2, 4, and 9 methylene
groups, respectively. The synthesis of M1 has been reported
previously, and the procedure was readily adapted to the
synthesis of M2 (Scheme 2a).38,41 Reuxing 6-bromo-1-hexene in
water in the presence of Na2SO3 generated the hexenyl sodium
sulfonate intermediate, which was subsequently chlorinated
using POCl3 to yield sulfonyl chloride 2. Finally, monomer M2
was synthesized by the condensation of 2 with 5-hexen-1-ol
(Scheme 2c).
Polymer synthesis
With the sulfonate ester-containing a,u-diene monomers in
hand, poly(sulfonate ester)s are accessible via ADMET poly-
condensation (Scheme 3). Ru-NHC alkylidene catalysts were
chosen to achieve polymerization as they tend to display more
functional group tolerance than Mo-based metathesis cata-
lysts.43 From the series of Grubbs catalysts, the second genera-
tion catalysts, both Grubbs (GII) and Hoveyda–Grubbs (HGII),
were employed as their increased thermal stability is compat-
ible with the high temperatures necessary as viscosity increases
in bulk polymerizations.29
The various ADMET conditions screened for monomers
M1–3 are summarized in Table 1. In every case, 1 mol% of
catalyst was employed. Despite the two methylene spacers
between the alkene and coordinating sulfonate group, mono-
mer M1 did not polymerize efficiently with either catalyst.
Heating M1 to 80 ꢁC overnight in the presence of HGII, resulted
in oligomers with a number-average molecular weight of Mn ¼
800 g molꢀ1, as determined by NMR end-group analysis (Fig. S1,
ESI†), whereas GII appeared to be inactive and the monomer
was recovered. It was possible to form polymers from monomer
Accessing monomer M3, however, proved to be non-trivial
(Scheme 2b). The rst step in the original synthesis relies on
the dissolution of the alkenyl sodium sulfonate salt in water as
it is generated. In the case of 11-bromo-1-undecene, the
M2 using both catalyst HGII and GII (Mn ¼ 9,24 ꢂ 103 g molꢀ1
,
Đ ¼ 1.8 and Mn ¼ 7.03 ꢂ 103 g molꢀ1, Đ ¼ 1.7, respectively).
However, the relatively low molecular weight resulted in a sticky
material that was difficult to isolate and purify by precipitation.
Consequently, our focus turned to the longer chain mono-
mer M3. Undecenyl chains are commonly used in ADMET
monomers as the competing formation of cyclic monomers is
disfavored when chain lengths exceed ten atoms.44 Under
identical bulk polymerization conditions, GII proved to produce
polymers of higher molecular weight (Mn ¼ 2.07 ꢂ 104 g molꢀ1
)
than HGII (Mn ¼ 1.75 ꢂ 104 g molꢀ1). Both catalysts afforded
polymers with a dispersity (Đ) of 2.2, which is consistent with
the theoretical
Đ value of 2 for step-growth addition
Scheme 2 Synthesis of (a) sulfonyl chlorides 1 and 2 (m ¼ 2 and m ¼ 4, Scheme 3 ADMET polycondensation of sulfonate ester-containing
respectively), (b) sulfonyl chloride 3 (m ¼ 9), (c) monomers M1–M3.
a,u-dienes (M1–M3) to unsaturated poly(sulfonate ester)s.
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Table 1 Conditions for ADMET polycondensation of sulfonate ester-containing a,u-dienes
Trial
Monomer
Conditions
Cat. (1 mol%)
NMRa Mn (g molꢀ1
)
GPCb Mn (g molꢀ1
)
Mw (g molꢀ1
)
Đ
1
2
3
4
5
6
7
8
M1
M1
M2
M2
M3
M3
M3
M3
Vac, 80 ꢁC, overnight
Various conditions
Vac, 80 ꢁC, overnight
Vac, 80 ꢁC, overnight
Vac, 80 ꢁC, overnight
Vac, 80 ꢁC, overnight
N2, 80 ꢁC, 24 h
HGII
GII
HGII
GII
HGII
GII
GII
800
—
—
—
—
—
—
—
—
—
—
—
9.24 ꢂ 103
7.03 ꢂ 103
1.75 ꢂ 104
2.07 ꢂ 104
6.57 ꢂ 103
3.66 ꢂ 104
1.62 ꢂ 104
1.21 ꢂ 104
3.92 ꢂ 104
4.54 ꢂ 104
1.99 ꢂ 104
6.71 ꢂ 104
1.8
1.7
2.2
2.2
3.0
1.8
7.5 ꢂ 103
Vac, 50 wt% in o-DCB, 45 ꢁ
C
GII
—
a
Determined by end-group analysis where possible. b Determined by GPC in CHCl3 vs. polystyrene standards.
polymerizations.45 Based on the successful procedure reported
for the polymerization of the analogous undecenyl ester-
containing a,u-diene,29 we also conducted the polymerization
reaction under a constant ow of N2 to remove ethylene, rather
than application of reduced pressure. This procedure however,
carried out at 80 ꢁC and with GII as the catalyst as before,
resulted in lower molecular weights (Mn ¼ 6.57 ꢂ 103 g molꢀ1),
and an unexpectedly high Đ of 3.0.
As the reaction mixtures in the above-described bulk poly-
merizations became highly viscous to the point of inhibiting
magnetic stirring, solution polymerization of M3 was also
conducted under reduced pressure in o-dichlorobenzene, which
was used as an inert solvent with low volatility.46 In this case, the
vacuum was carefully maintained between 40–150 mbar by
means of a regulator to mitigate the evaporation of the solvent.
Gratifyingly, these conditions afforded P3 of rather high
molecular weight (Mn ¼ 3.66 ꢂ 104) and low dispersity (Đ ¼ 1.8).
The use of a solvent also made it possible to conduct the reac-
ꢁ
tion at lower temperatures (45 C), which likely decreases the
amount of double bond migration and isomerization by GII.47
Lower reaction temperatures may also increase the lifetime of
the active catalyst, resulting in higher molecular weights.
Polymer P3 was exhaustively hydrogenated to access the fully
unsaturated poly(sulfonate ester) HP3 (Scheme 4). Catalytic
hydrogenation (H2, Pd/C) provided HP3 cleanly. Slight discol-
oration resulting in light grey materials likely due to trace
amounts of catalyst is apparent in the high molecular saturated
polymer P3. This coloration disappears upon hydrogenation to
HP3 indicating that the trace catalyst had remained bound to
the alkenes in the main chain. Fig. 1a shows an overlay of the 1H
NMR spectra of M3, P3, and HP3. The two terminal alkene
peaks converge into an internal alkene peak aer polymeriza-
tion, which appears to contain overlapping cis and trans signals.
This alkene peak completely disappears upon hydrogenation.
Fig. 1 a) 1H NMR spectra of monomer M3, unsaturated poly(sulfonate
ester) P3, and saturated poly(sulfonate ester) HP3 in CDCl3. (b) Overlay
of GPC chromatograms in CHCl3 of the poly(sulfonate ester) before
(P3) and after (HP3) catalytic hydrogenation.
The elution time of the polymer by GPC does not change aer
hydrogenation, indicating that the sulfonate ester groups are
stable to the reaction conditions (Fig. 1b).
Polymer properties
The thermal properties of the higher molecular weight poly-
(sulfonate ester)s made from monomer M3 (both the as
prepared P3 and the hydrogenated HP3) were studied to
investigate the effects of sulfonate ester defects in the poly-
ethylene backbone on the crystallinity and bulk properties of
the material. Thermogravimetric analysis (TGA) traces of both
P3 and HP3 display a sharp, rapid weight loss with an onset of
decomposition (Td) at 247 ꢁC and 261 ꢁC, respectively (Fig. 2). A
Scheme 4 Catalytic hydrogenation of P3 leads to the fully unsaturated
poly(sulfonate ester) HP3.
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Table 2 Summary of thermal properties of P3 and HP3 as compared
HDPE
comparison with the TGA trace of HDPE of a comparable
molecular weight (Mn ¼ 2.14 ꢂ 104 g molꢀ1), which was used as
ꢁ
a reference and does not decompose until well above 450 C,
Tm (ꢁC) Tc (ꢁC) Tg (ꢁC) Td (ꢁC) DHm (J gꢀ1
)
Cryst.a (%)
*
shows that both poly(sulfonate ester)s exhibit a much lower
thermal stability than the corresponding hydrocarbon polymer.
This observation is consistent with the decomposition of the
sulfonate ester moiety by a thermally activated elimination
mechanism.15 This process has been successfully employed as a
decrosslinking reaction orthogonal to the generation of photo-
crosslinked polymers.49–53 The relatively small increase in Td
when comparing unsaturated HP3 to saturated P3 indicates
P3
HP3
HDPE 133.5
41.8
79.7
28.5
68.2
114.5
ꢀ15.8 247.5
31.6
54.6
155.1
11
19
54
—
—
261.0
459.3
a
Percent crystallinity vs. 100% crystalline HDPE (DHm ¼ 287.3 J gꢀ1).48
*
that the alkene defect does not signicantly impact the thermal Additionally, as the starting monomer is asymmetric, the
stability of the polymer. resulting polymer should have a regioirregular structure, which
Thermal transitions in P3 and HP3 were elucidated via would even further prevent crystallization. We note that no glass
ꢁ
differential scanning calorimetry (DSC) and dynamic mechan- transition could be discerned in the DSC traces above ꢀ80 C.
ical analysis. The DSC traces of both poly(sulfonate ester)s show The DMA trace of a solution cast lm (PhCH3) of P3 (Fig. S2,
sharp endotherms that are diagnostic of crystalline to melt ESI†), however, clearly reveals a Tg at ꢀ15.8 ꢁC, where the
transitions (Fig. 3). As expected, both the double bonds and material transitions from a glassy state with a tensile storage
sulfonate ester moieties act as defects inuencing the crystal- modulus E0 of 1228 MPa at ꢀ100 ꢁC to a rubbery plateau with a
lization of the polymers, resulting in a decrease in the melting room-temperature E0 of 85 MPa. Unfortunately, lms of HP3
ꢁ
ꢁ
ꢁ
temperature from 134 C (HDPE) to 80 C (HP3) or 42 C (P3, were too brittle for mechanical testing.
Table 2). A corresponding decrease of the heat of fusion is also
observed, indicative of a decrease in crystallinity (Table 2). et al.10 found that the ester groups are likely included in the
crystalline phase as packing defects, as has been demonstrated
for methyl-substituted polyethylene by Wagener and
coworkers.21,54 A comparison of the literature data suggests that
an increase of the fraction of methyl groups has a more
signicant inuence on the melting point of the corresponding
polymer than an increase of the ester content. In addition, an
increase of the methyl content was found to cause a change of
the morphology from an orthorhombic to a hexaganol crystal
structure,54 whereas the orthorhombic structure was main-
tained in long-chain polyesters.10 Mecking and coworkers
propose two possible explanations for this effect; the decreased
steric hindrance of the ester moiety versus the methyl group,
and the inuence of dipole–dipole interactions between ester
groups on the packing. We surmised that the investigation of
In their study on long-chain aliphatic polyesters, Mecking
the present long-chain poly(sulfonate ester)s might contribute
Fig. 2 Thermogravimetric analysis traces of P3, HP3, and HDPE, which
was used as a reference. Samples were heated at a rate of 10 ꢁC minꢀ1
under nitrogen.
to a better understanding of this situation, as sulfonate esters
are also capable of dipole–dipole interactions but are more
sterically demanding than carbonate esters due to their tetra-
hedral geometry.
To further understand the effect of the sulfonate ester defect
on morphology, melt-processed lms of the materials were
analyzed via small- and wide-angle X-ray scattering (SAXS and
WAXS, respectively) (Fig. 4). For HDPE, the sharp scattering
maxima, centered at q ꢃ 15 and 16.7 nmꢀ1 in the WAXS spectra,
are associated with the (110) and (200) planes of the ortho-
rhombic unit cell.55–59 The corresponding scattering maxima
observed for HP3 and P3 are centered at lower q values, which
indicates an increased lattice constant and, as expected, a lower
degree of crystallinity. In the case of P3, only the rst scattering
peak is evident. These crystalline peaks also overlap with a
signicant amorphous halo. The shis in the WAXS spectra
between HDPE, and P3 and HP3 were also conrmed by IR
spectroscopy (Fig. S3, ESI†). The methylene rocking and scis-
soring vibrations indicative of an orthorhombic crystal
Fig. 3 Comparison of the second heating curves from the DSC of P3,
HP3, and HDPE. All experiments were performed at a heating/cooling
rate of 10 ꢁC minꢀ1
.
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Conclusions
The synthesis of a new class of polymers, poly(sulfonate ester)s,
has been achieved. Sulfonate ester-containing a,u-dienes
expand the library of functional monomers for ADMET poly-
merization. The long-chain monomer M3 and its sulfonyl
chloride precursor, necessary to achieve high molecular
weights, were synthesized for the rst time with the aid of a
microwave reactor. Solution polymerization with GII at high
concentrations under reduced pressure proved to be the
optimal conditions for polymerization. Comparison of the
thermal properties of these materials demonstrates the ease of
thermal degradation of poly(sulfonate ester)s as well as the
effects of the introduced functional group on crystallinity.
Access to this previously unexplored class of polymers elabo-
rates on our understanding of what materials may constitute
“degradable polyethylene.” Due to the highly versatile nature of
olen metathesis, studying the metathesis activity of sulfonate
ester-containing a,u-dienes also leaves open the opportunity to
tune material properties through the use of a wide variety of
comonomers.
Fig. 4 a) SAXS and (b) WAXS spectra of films of P3, HP3, and HDPE. In
all cases films were melt-processed and quenched at low
temperature.
structure, at 719/730 cmꢀ1 and 1463/1472 cmꢀ1 respectively, are
not present in the spectra of P3 and HP3. Thus, it appears that
the inclusion of sulfonate esters in the polymer backbone cau-
ses a signicant disturbance on the crystal structure of the
present polymers, which does not mirror the above-discussed
behavior of long-chain aliphatic polyesters.10 The asymmetry-
caused regioirregularity of the polymer would also surely
result in a disturbance of the crystalline structure. However, this
issue also arises in the long-chain aliphatic polyesters and is
therefore likely not a signicant factor.
Acknowledgements
This work was supported by the European Research Council
(ERC-2011-AdG 291490-MERESPO). The authors thank Mr Felix
Fehr for NMR analysis acquired on a 500 MHz spectrometer and
Mr Fredy Nydegger for mass spectrometry, both of the Univer-
sity of Fribourg chemistry department analytical services. The
authors also thank Dr Lucas Montero for fruitful discussion on
ADMET polymerization.
A distinct feature of the SAXS spectra of P3 and HP3 is the
scattering maxima at q ꢃ 3 and 6 nmꢀ1. The center positions of
these scattering maxima correspond to a separation length of
approximately 2 nm and suggest a highly correlated arrange-
ment of clusters of sulfonate ester groups segregated from the
polymer phase, possibly owing to dipole–dipole interactions.
This implies a rather well-dened folding of the polymer chains.
Similar structural features have been observed for precise ion-
containing copolymers based on polyethylene and have been
attributed to a regular layer-like packing of ion-containing
clusters.60 In our data, the presence of crystallites is more
evident than in other studies in which data were collected at
temperatures exceeding the Tm of the polymer. In HP3, an
increased proximity of clusters is suggested as compared to P3,
as the peaks are sharper and centered at larger q values.
The features of the SAXS spectra at low angles primarily
originate from the scattering contrast found between the crys-
talline and amorphous domains. The center of the so-called
crystalline peak is usually interpreted as the measure of the
average distance between crystalline domains. HDPE shows a
separation length of approximately 21 nm, whereas that length
is smaller for HP3 at ꢃ7.8 nm. The crystalline peak of P3 is not
as distinct as in the case of the samples above. This is likely due
to the decreased level of crystallinity caused by additional
unsaturation in the backbone. It is clear from the analysis of the
thermal, mechanical, and morphological properties, that while
the inclusion of sulfonate ester groups does disturb the bulk
properties as compared to HDPE, this material ts in nicely with
the series of polyethylene-like polymers reported to date.
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