New renewably-sourced polyesters from limonene-derived monomers

Article abstract of DOI:10.1039/c8gc02957a

The functionalisation of limonene has enabled the synthesis of two renewably-sourced monomers for the formation of terpene-derived polyesters. Three methods for the synthesis of the novel hydroxy-acid 6 are reported and their green-credentials scrutinised through comparison of their sustainability-metrics. Step-growth homo-polymerisation of 6 is demonstrated to yield a low molecular weight (2.6 kDa) novel polyester with 100% of its carbon content originating from the terpene starting material. The corresponding diol 2 is shown to act as a co-monomer with a renewable diacid. The resultant polyesters display impressive M_ns of up to 30 kDa with T_gs between -6 and 24 °C. These materials have been shown to depolymerise under basic conditions for reclamation of the diol monomer 2.

Full text of DOI:10.1039/c8gc02957a

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New renewably-sourced polyesters from
limonene-derived monomers†
Cite this: Green Chem., 2019, 21, 149
b
Megan R. Thomsett, a Jonathan C. Moore, a Antoine Buchard,
a
Robert A. Stockman *a and Steven M. Howdle
*
The functionalisation of limonene has enabled the synthesis of two renewably-sourced monomers for
the formation of terpene-derived polyesters. Three methods for the synthesis of the novel hydroxy-acid 6
are reported and their green-credentials scrutinised through comparison of their sustainability-metrics.
Step-growth homo-polymerisation of 6 is demonstrated to yield a low molecular weight (2.6 kDa) novel
polyester with 100% of its carbon content originating from the terpene starting material. The corres-
ponding diol 2 is shown to act as a co-monomer with a renewable diacid. The resultant polyesters display
impressive Mns of up to 30 kDa with Tgs between −6 and 24 °C. These materials have been shown to
depolymerise under basic conditions for reclamation of the diol monomer 2.
Received 18th September 2018,
Accepted 26th November 2018
DOI: 10.1039/c8gc02957a
rsc.li/greenchem
properties of terpenes have recently made them very attractive
for polymer research and this is highlighted by reviews which
Introduction
If the present consumption of fossil fuels continues at the have been published within the past five years.10–13 Much of
current rate, the reserves of oil feedstocks will be expended the research into the polymerisation of terpenes in their raw
within the next century. This development combined with the form has resulted only in polymers which lack the desired
associated environmental issues has resulted in a driving force properties to compete with those currently used
for scientific research to alleviate the dependence on polymers commercially.14–21 It is clear that new approaches are required
derived from fossil fuels.1 The utilisation of fossil fuels in the to unlock the potential of terpenes as monomers for renewable
manufacture of plastics currently accounts for about 7% of polymers. These observations have engendered a nascent field
worldwide oil and gas usage.2 Of the 300 million tonnes of of research into the controlled and sustainable functionalisa-
plastics produced annually worldwide, renewable polymers tion of terpenes to produce monomers that can be “dropped-
make up just 1%.1,3 The comparatively high cost of polymers in” to established and efficient polymerisation processes.
from renewable feedstocks has likely contributed to their Synthetic methods have been developed to derive terpene and
small share in the market, and severely detracts from their terpenoid based monomers which can readily undergo free
commercial viability.4–6 Renewable raw materials have the radical polymerisation.22–24 We recently reported a series of
potential to provide a wide range of monomers arguably as acryloyl and methacryloyl terpene derivatives that can be
comprehensive as those provided by the petrochemical indus- readily polymerised under standard free-radical conditions to
try.7,8 In recent years considerable research has been devoted produce materials with a diverse and attractive range of pro-
to finding renewable monomers for the synthesis of sustain- perties.24 There are also multiple examples of the ring expan-
able polymers.
sion of oxygen containing terpenes to produce lactones and
Terpenes, terpenoids and resin acids are a group of non- lactams for use in ring opening polymerisation (ROP) to form
polar small molecules typically produced biosynthetically by polyesters and polyamides, respectively.25–31
many classes of trees and plants.9 Terpenes are an abundant
Limonene (1) is one of the most abundant terpenes
waste stream, are inexpensive, and do not directly compete (present in more than 300 plants) and is a prominent waste
with food sources; this makes them ideal small molecule stream of the citrus industry, with the (R)-enantiomer being
building blocks for many applications.9 The many desirable produced on a scale of over 70 KTA.32 With two double bonds,
methods for its functionalisation are broad and well estab-
lished in organic chemistry, as summarised by Bessière and
aSchool of Chemistry, University Park, University of Nottingham, Nottingham,
co-workers.33 In the field of polymer chemistry however, the
NG7 2RD, UK. E-mail: robert.stockman@nottingham.ac.uk; Fax: +44 (0)115 951356
majority of research has stemmed from the epoxidation of the
endocyclic double bond to produce cis- and trans-limonene
oxide. Coates and co-workers performed seminal studies on
bDepartment of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc02957a
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the copolymerisation of these epoxides with CO2 to produce formed using an Agilent 1260 Infinity Series HPLC (Agilent
linear polycarbonates.34 They went on to show that the (1 : 1) Technologies, USA) in THF (HPLC grade, Fisher Scientific) as
combination of amorphous (S)- and (R)-poly(trans-limonene eluent at room temperature using two Agilent PL-gel mixed-E
oxide) produces a stereocomplexed, semi-crystalline polymer columns in series with a flow rate of 1 mL min−1. A differen-
with enhanced mechanical properties beyond those observed tial refractometer (DRI), was used for sample detection. Infra-
for its individual components.35 Greiner and co-workers devel- red spectra were recorded using a Bruker Tensor 27 FT-IR
oped a stereoselective synthesis of limonene oxide, which was spectrophotometer using either an ATR attachment or a solu-
shown to be more efficient and to produce high molecular tion cell using CHCl3 as the solvent (νmax is reported in
weight polymers (Mn > 100 kDa) with attractive thermal pro- cm−1). High-resolution mass spectra were obtained using a
perties (e.g. glass transition temperature (Tg) = 130 °C).36
Bruker MicroTOF mass spectrometer operating in electrospray
However, approaches using step-growth polymerisation are ionisation (ESI) mode. Dynamic mechanical analysis (DMA)
few. Currently, the only method to incorporate limonene into a was used to determine the Tg of the polymers. Measurements
step-growth polymer uses thiol–ene chemistry and has been were performed on a Triton Technologies DMA (now Mettler
used to produce polyesters,37 polyamides and polyurethanes.38 Toledo DMA1) using the powder pocket accessory. The
Whilst highly atom-efficient, this approach relies on the coup- sample (40 mg 5 mg) was weighed into a powder pocket.
ling of terpenes with polymerisable functionalities, which are Samples were measured at 1 and 10 Hz in single cantilever
not necessarily derived from renewable sources. In recent bending geometry between −100–100 °C. The value of the Tg
years, a wide range of terpene derived polymers have been was taken as the peak of the tan delta (tan δ) curve. DFT calcu-
reported, but some of the synthetic strategies employed might lations details (methods, data and output files) can be found
be considered step-intensive and not sustainable. There is a in the ESI.†
lack of terpene derived monomers for step-growth polymeris-
ation, and studies into end of life solutions are few. Designing
a closed-loop thermoplastic which can be depolymerised and
then re-polymerised to feed back into the system would
capture the embodied energy and value currently contained in
polymers, and could also reduce leakage into natural systems.
This is crucial to the concepts of a circular economy and
allows us to move away from the “take, make, dispose” status
quo.39
Herein we report the efficient synthesis of two terpene-
derived monomers, which are accessed in three steps or less
from limonene, to produce bio-derived polyesters via step-
growth polymerisation. These materials have been shown to
depolymerise under basic conditions to allow reclamation of
the terpene derived component, opening up the potential for
recyclability of this novel bio-derived polymer and creating a
closed, circular system.
Synthesis of 2a and 2b
To a solution of limonene (1) (16.1 mL, 100 mmol) in THF
(30 mL) cooled to 0 °C was added borane (100 mL, 100 mmol,
1 M in THF) dropwise over 25 minutes. Once the addition was
complete the reaction was allowed to warm to room tempera-
ture and stirred (2 h). The reaction was cooled to 0 °C and a
solution of NaOH(aq) (350 mL, 350 mmol, 1 M) was added fol-
lowed by H2O2 (34 mL, 350 mmol, 30% w/v in H2O). The reac-
tion mixture was stirred (30 min) at 0 °C and then at room
temperature (30 min). EtOAc (400 mL) was added to the reac-
tion mixture and the organic layer separated and washed with
water (200 mL), Na2CO3(aq) (200 mL), NaCl(aq) (200 mL), dried
over Na2SO4, and concentrated under reduced pressure to give
the crude material as a yellow oil. Purification by silica gel
column chromatography (40% EtOAc in petroleum ether)
afforded 2a and 2b (3 : 1) as a pale yellow oil (16.7 g,
97.0 mmol, 97%).
Experimental
Synthesis of 4
Materials
To a solution of 2a and 2b (3 : 1) (17.95 g, 104 mmol) in
CH2Cl2 (1000 mL) was added a solution of 2,2,6,6-tetra-
methyl-1-piperidinyloxy (TEMPO) (6.5 g, 41.7 mmol) in
CH2Cl2 (20 mL) dropwise, bisacetoxyiodobenzene (BAIB)
(100.4 g, 312 mmol) was subsequently added in portions over
15 min. The reaction was stirred at room temperature (16 h).
5% Na2S2O3(aq) (540 mL) then NaHCO3(aq) (1050 mL) was
added and the mixture was stirred vigorously (15 min). The
organic layer was separated and the aqueous layer was
extracted with CH2Cl2 (3 × 500 mL). The organic extracts
were combined, washed with NaCl(aq) (500 mL), dried over
Benzyl alcohol (BnOH) was distilled from CaH2 prior to use;
THF was freshly distilled from sodium metal and benzophe-
none; water was deionised before use. Limonene was pur-
chased from Sigma Aldrich. All other reagents were purchased
from a chemical supplier and used without further purifi-
cation. Experiments carried out under inert atmosphere used
argon or nitrogen, and glassware was flame dried prior to use.
All solutions are saturated unless stated otherwise.
General methods and instrumentation
1H and 13C NMR spectra were recorded in CDCl3, at ambient Na2SO4 and concentrated under reduced pressure.
temperature using Bruker AV400 (400 MHz), AV3400 Purification by silica gel column chromatography (5–7%
(400 MHz), AV3400HD (400 MHz) and AV3500 (500 MHz) spec- EtOAc in petroleum ether) afforded 4 as a pale-yellow oil
trometers. Gel permeation chromatography (GPC) was per- (5.8 g, 34.5 mmol, 33%).
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Synthesis 5a and 5b
Results and discussion
To a solution of 2a and 2b (3 : 1) (5.00 g, 29.1 mmol) in MeCN
(300 mL) was added [Cu(MeCN)4]OTf (546 mg, 1.4 mmol). A
solution of TEMPO (210 mg, 1.4 mmol), 2,2-bipyridyl (210 mg,
1.4 mmol) and 1-methylimidazole (240 mg, 2.91 mmol) in
MeCN (9 mL) was added dropwise, and the mixture stirred at
room temperature (20 h). The reaction mixture was passed
through a plug of silica and the product eluted with Et2O. The
solvent was removed under reduced pressure to give the crude
product, purification by silica gel chromatography (15% EtOAc
in petroleum ether) afforded 5a and 5b (3 : 1) as a colourless
oil (3.83 g, 22.5 mmol, 77%).
We initially set out to investigate the formation of a limonene
derived monomer that could homo-polymerise. The target was
the polymerisation of hydroxy-acid 6 to yield the polyester 7,
which contains an exclusively terpene-derived backbone. To
the best of our knowledge a material of this nature is yet to be
reported. Step-growth polycondensation of hydroxy-acids is
well known. However, there are very few examples where a (car-
bocyclic) ring is incorporated into the backbone of the
polymer. We hypothesised that this feature might impart struc-
tural rigidity and perhaps lead to desirable properties such as
a high Tg. Using Brown’s conditions,40 limonene (1) was con-
verted to its corresponding 1,5-syn and 1,5-anti diol derivatives
2a and 2b, respectively (Scheme 1). The reaction proceeded
Synthesis of 6a and 6b
To a solution of 5a and 5b (3 : 1) (3.00 g, 17.5 mmol) in tBuOH/ smoothly in an excellent yield of 97%, yielding 2a and 2b in a
H2O (2 : 1, 80 mL) was added NaH2PO4 (8.19 g, 52.5 mmol) 3 : 1 ratio, each as a 1 : 1 mixture of C-8 epimers. Three syn-
and the resulting solution was cooled to 0 °C. H2O2 (6.83 mL, thetic routes (labelled as Route A, B and C) were investigated
87.5 mmol, 30% w/v in H2O) was added followed by NaClO2 for the conversion of these diols to the targeted hydroxy-acid 6
(11.87 g, 105 mmol, 80%) in portions over 45 min. The reac- (Fig. 1).
tion was allowed to stir at 0 °C (30 min) and then extracted
Route A involved global oxidation followed by ketone
into EtOAc (80 mL). The organic extract was washed with H2O reduction, Route B comprised oxidative lactonisation and sub-
(5 × 50 mL), NaCl(aq) (50 mL), dried over Na2SO4 and concen- sequent hydrolytic ring opening, and Route C utilised a two-
trated under reduced pressure to yield 6a and 6b (3 : 1) as a step chemoselective primary alcohol to carboxylic acid oxi-
white semi-solid (2.2 g, 11.8 mmol, 68%).
dation. The optimised two-step routes were compared in terms
of yield, operational simplicity and sustainability. The latter
was investigated using the metric tool Green Motion™
(Fig. 1).41 One benefit of Route A is its operational simplicity;
the synthetic sequence could be carried out in a single
working day, and following aqueous work up, no further puri-
fication of the products was required. However, this route did
require the use of super-stoichiometric Jones’ reagent, and
produced a mixture of eight diastereomers, complicating ana-
lysis of the reaction mixtures considerably.
Route B had a similar overall yield (32% vs. 38%), but puri-
fication of the lactone 4 via chromatography was required and
each step took longer than a day for completion. Route C was
the highest yielding, affording the hydroxy-acid 6 in 52% over
two steps. Other benefits to Route C are the incorporation of a
catalytic protocol for aldehyde formation that utilises mole-
cular oxygen as the terminal oxidant and the mild conditions
employed for the Pinnick oxidation. One drawback of this
route was that it required purification of 5a and 5b using silica
gel chromatography.
Synthesis of 7
6a and 6b (3 : 1) (500 mg, 2.68 mmol) and Sn(oct)2 (8.5 μL,
1 mol%) were added to a flame dried flask under a flow of N2,
which was heated to either 120 or 180 °C and stirred for the
required time period. The temperature was then set to 180 °C
and the system put under vacuum for 24 h. The polymerisation
was stopped by cooling to −5 °C and the resulting polymer
dried under vacuum for 24 h.
Synthesis of 10
All reagents were dried under vacuum at 25 °C for at least 24 h
prior to use. The mixture of 2a and 2b (3 : 1) (1.50 g, 8.7 mmol)
was added to a 50 mL round bottomed flask equipped with a
condenser and trap cooled to 0 °C. Succinic acid (9) (1.13 g,
9.57 mmol) was added, and the reaction heated to 190 °C
(6 h). The metal catalyst (Sn(oct)2 or Ti(OBu)4) was added
(1 mol%), the temperature increased and the system put under
vacuum. After an elapsed time the polymerisation was stopped
by cooling to −5 °C and the polymer washed with methanol.
Degradation studies
10 (96 mg) was added to a mixture of NaOH(aq) (4 mL, 3 M)
and THF (4 mL) and heated in a 95 °C oil bath (10 days). The
solvent was removed under reduced pressure and the residue
acidified with HCl(aq) (1 M), the aqueous phase was extracted
with CH2Cl2 (4 × 15 mL), dried over Na2SO4 and concentrated
under reduced pressure to yield 2a and 2b as a yellow oil
(59 mg, 0.43 mmol).
Scheme 1 Synthesis of diols 2a and 2b from limonene (1) using a
Brown hydroboration/oxidation sequence.
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Fig. 1 Comparison of Route A (top), Route B (middle) and Route C (bottom) for the synthesis of hydroxy-acid 6 and subsequent analysis of the
three routes using the metric tool Green Motion™.
Using Green Motion™ we evaluated each route with respect
to overall sustainability; the lower the impact to the environ-
ment the higher the rating (100 being ideal). This metric com-
bines the twelve principles of green chemistry with a penalty
point system. The use of the toxic Jones’ reagent in Route A
resulted in a score of zero with respect to “hazard and toxicity”
and subsequently, the lowest overall score of 38. Route B
scores a slightly higher 41 however falls down due to the use
of halogenated solvents for the oxidative lactonisation. The
highest yielding approach (Route C), which employs a catalytic
protocol and benign reagents, also scored the highest on
Green Motion™ with a score of 53. According to the guidelines
laid out within the metric,41 this score validates that the route
can be considered “green”. For this reason, Route C was
adopted for further investigations.
Table 1 Polymerisation of hydroxy-acids 6a and 6b
Entry
T/°C
t
Mn/kDa
Đ
Tg/°C
1
180
180
120
120
24 h
24 h
24 h
1 month
2.6
0.3
1.2
2.2
1.4
1.1
3.5
2.4
44
N/A
—
2a
3
4
44
a Reaction performed using 6a only.
With the hydroxy-acid 6 in hand, initial studies into its
polymerisation were performed. These were carried out using
bulk polymerisation, with Sn(oct)2 as a catalyst at 180 °C or
120 °C followed by heating at 180 °C under vacuum (Table 1).
The consumption of the monomer was followed by 1H NMR
spectroscopy, using the resonances obtained from the respect-
ive C-1 environments. Initially the polymerisation was
attempted using a (3 : 1) mixture of 6a and 6b (Table 1, entry 1).
1H NMR analysis after 20 min indicated that the concentration
of 6a had decreased and that the major component was
the lactone 4. After 3 hours, no significant change could be
observed. However, after 24 h the relative concentration of 6b
began to decrease and
a resonance at approximately
4.65–4.90 ppm emerged, indicating formation of the polyester
7. At this stage vacuum was applied to drive the polymerisation
to high conversion and remove lactone 4 from the reaction
mixture. The broad peaks at approximately 4.40–4.55 and
4.65–4.90 ppm were tentatively assigned to the respective axial
and equatorial protons alpha to the oxygen in the ester bond
(Fig. 2).
GPC analysis revealed molecular weights of up to 2.6 kDa,
but with a large proportion of the reaction mixture being com-
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sation with 6b (ΔG = as low as −3.9 kcal mol−1). The poly-
merisation of 6b is marginally favoured thermodynamically
(ΔG = between −0.8 and −1.9 kcal mol−1 depending on the
conformation(s) of 6b units in the polymer linkage – e.g.
axial/axial, equatorial/equatorial, axial/equatorial or equatorial/
axial). This somewhat explains the long reaction times and low
molecular weights obtained from the polymerisation of 6a and
6b. One potential solution to this is to utilise 6a and 6b as
comonomers in the synthesis of copolyesters.
It was also intriguing to investigate whether the lactone 4,
which had served as an intermediate in the synthesis of 6a,
could itself be utilised as a monomer for synthesis of the poly-
ester 7 via ring opening polymerisation (ROP). It is well known
that six membered (delta) lactones are typically stable to ring
Fig. 2 1H NMR of the monomers 6a and 6b and the resulting opening, due in part to low ring strain.43 It is also recognised
polymer 7.
that substituents on the ring decrease the equilibrium
monomer conversion. This is especially true for lactones sub-
stituted in the alpha and delta positions.44,45 The terpene
derived delta-lactone 4 is substituted in both the alpha and
prised of low molecular weight oligomers. It is well established delta positions suggesting that its ring opening polymerisation
that sterically hindered secondary alcohols can be slow to pro- may be thermodynamically unfavourable. However, 4 is also a
pagate in polyesterifications.42 We infer that lactonisation bridged bicyclic system (oxabicyclo[3.3.1]nonane) and we had
occurs through the disfavoured 1,5-diaxial conformer of 6a, speculated that this may provide the additional ring strain
whereby (unlike 6b) the close proximity of the endo-alcohol required to facilitate efficient ROP.
functionality with the carboxylic acid would be sufficient to
The polymerisation of 4 was attempted under a wide range
allow ring closing. To test our hypothesis, the hydroxy-acid 6a of conditions that are well precedented for the ROP of lactones.
(accessed through the hydrolysis of 4) was subjected to the Various Brønsted46 and Lewis47 acids, organocatalysts,48,49 and
polymerisation conditions in the absence of 6b (Table 1, entry 2). metal alkoxides47 were investigated under a range of tempera-
Quantitative conversion to the lactone 4 was observed, with tures (see ESI†). Disappointingly however, even with extended
no polymeric species being detected. In an effort to promote reaction times of up to 6 months, only initiation to the benzyl
the formation of linear polymers, the polymerisation was ester 8 could be observed and no evidence of propagation
repeated at 120 °C (Table 1, entry 3). Again, GPC analysis could be detected (Scheme 2).
revealed that the material was mainly composed of low mole-
To better understand why lactone 4 was not undergoing
cular weight oligomers. Since the reduced temperature was ring opening polymerisation DFT calculations were conducted
likely contributing to a slower rate of reaction, the polymeris- (protocol rM062X-D3/6-311++g(2d,p)/cpcm = ethylethanoate).
ation was subsequently repeated for a much longer period of DFT analysis indicated that the most stable conformation of 4
1 month (Table 1, entry 4). 1H NMR analysis suggested that occurs when the cyclohexane ring is in a chair conformation.
the reaction had gone to full conversion and consumption of The driving force for most ROP is the release of ring strain.
the monomer was confirmed by high resolution mass spec- The enthalpy of polymerisation (ΔH) can be used as a measure
trometry (HRMS). GPC analysis showed a broad peak (typical of the ring strain and in general for polymerisation to be
of step growth polymerisation) and a significantly greater pro- favourable, it is required that ΔH < 0.50 The ring strain for the
portion of higher molecular weight species (Mn = 2.2 kDa) predicted structure was calculated to be at least −4.7 kcal mol−1
albeit with a higher dispersity (Đ = 2.4) than the polymer syn- which implied that polymerisation may be enthalpically
thesised at 180 °C. Both polymers were investigated using favourable. However, the overall ΔG of the polymerisation was
DMA to study their thermal properties. The analysis indicated calculated as at best +1.4 kcal mol−1. This indicates the entro-
that 7 is an amorphous polymer with a Tg of 44 °C, which is pic factor of the ROP is counteracting the ring opening, and
relatively high for polyesters with molecular weights in the
region of 2.2–2.6 kDa. This lends support to our prior hypoth-
esis that the presence of a carbocyclic ring in the backbone
would impart rigidity to the polymer.
One potential explanation for the slow rate of polymeris-
ation observed with the hydroxy-acids 6a and 6b is the some-
what sterically hindered environment of the secondary alco-
hols. Using DFT analysis it could be inferred that the formation
of the lactone via 6a was favoured (ΔG = −8.3 kcal mol−1
)
over homopolymerisation (ΔG −0.9 kcal mol−1) or copolymeri- Scheme 2 Attempted ROP of lactone 4.
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Table 2 Polymerisation studies of the diols 2a and 2b with succinic
acid (9)
Entry
Catalyst
T/°C
t/h
Mn/kDa
Tg/°C
Đ
1
2
3
4
5
6
7
8
9
Ti(OBu)4
Sn(oct)2
N/A
Ti(OBu)4
Sn(oct)2
N/A
Ti(OBu)4
N/A
Sn(oct)2
190
190
190
230
230
230
230
230
230
24
24
24
24
24
24
3
18.4
20.7
9
11.2
10.1
9.2
30.4
9.3
28.3
6
21
−7
4
15
19
16
6
2.4
2.5
1.7
1.5
1.5
1.4
2.5
1.3
2.3
3
3
23
Fig. 3 GPC traces for the synthesis of polymer 10 (Table 2, entries
7–9).
that the enthalpic driving force is not large enough to over-
come it.
Following our studies into the synthesis of an entirely
terpene derived polymer, we moved to investigate the potential
of the limonene derived diol 2 as a comonomer. An analogous
approach has been reported recently by Roth and co-workers
whereby a borneol derived diol was employed in step-growth
polymerisations.51 We envisaged that the copolymerisation of
2 with a suitable diacid would produce polymers with a higher
degree of flexibility than 7, which may possess lower Tgs and
be suitable for different applications. Succinic acid (9) was
selected as co-monomer due to its established availability
from renewable resources.52 The co-polymerisation of 2 and 9
was anticipated to yield the entirely bio-based polyester 10
(Table 2).
One of the most commonly employed catalysts for conden-
sation polymerisation is Sn(oct)2, however the use of this
species raises concerns with respect to sustainability. One
class of catalyst that can be considered a greener alternative
are the titanium alkoxide complexes,7,53 whose environmental
benefits arise predominantly from the fact that titanium is
generally non-toxic and earth abundant.54 The polymerisation
of 2a and 2b with succinic acid (9) was investigated using Sn
(oct)2 and Ti(OBu)4 and the results were compared by GPC
analysis (Table 2). Initially, esterification was performed for
6 hours under an inert atmosphere at 190 °C. At this stage the
catalyst was added and polymerisation was induced via the
application of vacuum. Pleasingly, GPC analysis revealed that
similar results could be obtained using both Sn(oct)2 and Ti
(OBu)4; in each case high molecular weight species were pro-
duced. Furthermore, in the absence of catalyst, polymerisation
was still observed to occur. However, in this case significantly
lower molecular weight species (ca. 9 kDa) were obtained
(Fig. 3). By optimising the polymerisation, we found that at a
temperature of 230 °C in the presence of catalyst (Table 2,
were achieved and the materials also exhibited low Tgs
(6–23 °C).
It is becoming increasingly important that chemists design
end of life solutions into their molecules. The polyesters syn-
thesised from limonene diol (2) and succinic acid (9) should
be susceptible to hydrolytic degradation at a variety of pHs and
temperatures. Three samples of polymer 10 were submerged in
aqueous buffer solutions at pH 3, 7 and 11 and heated to
50 °C for two weeks. Analysis by GPC indicated no observable
change in the molecular weight and it was hypothesised that
this was due to the observed insolubility of the polymer in
aqueous media. In an attempt to encourage degradation, the
reaction was repeated using a mixture of THF and NaOH(aq)
(3 M) (1 : 1) and heated in a 95 °C oil bath for 10 days
(Scheme 3). Analysis by GPC revealed that the crude mixture
extracted from the degradation study contained no high-mole-
cular weight species, as demonstrated by the disappearance of
1
the peak at a retention time of ∼32 min (Fig. 4). H NMR ana-
lysis indicated that the polymer had fully depolymerised back
to monomer (2a and 2b), which was recovered in excellent
purity and with an identical d.r. of 3 : 1 by simple extraction
into CH2Cl2 (Fig. 5). This opens up the possibility that polymer
10 could be easily and continuously recycled via a closed, cir-
cular system.
entries 4–9), significantly higher molecular weights (30 kDa) Scheme 3 Degradation of 10 under basic conditions.
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derived completely from renewable resources and a high per-
centage of its carbon content can be sourced from terpene
waste streams. Finally, the polymer 10 was shown to degrade
under basic conditions into its substituent monomer 2,
demonstrating that the system has a high potential for
recyclability.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to thank the EPSRC for funding this
work (MT, Centre for Doctoral Training in Sustainable
Chemistry); EP/L015633/1 and (JM) EP/N019784/1. Ms Alice
Haddleton for assistance with thermal analysis. AB thanks the
University of Bath HPC for computing resources and the Royal
Society for funding (Fellowship UF140207).
Fig. 4 GPC traces showing the degradation of polymer 10 through the
disappearance of the peak representing high molecular weight species.
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