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1

Terpolymers derived from Limonene Oxide and
Carbon

Dioxide:

Access

to

Cross-Linked

Polycarbonates with Improved Thermal Properties
Carmen Martín†and Arjan W. Kleij*,†,‡
†

Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science and
Technology, Av. Països Catalans 16, 43007 – Tarragona, Spain.

‡

Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 –
Barcelona, Spain.

Table of contents entry:

2

ABSTRACT: The formation of bio-derived materials is gaining momentum in academic and
industrial research as a consequence of depleting petroleum resources, providing mid- and longterm alternative sustainable materials. In this context, we have prepared a series of terpolymers
derived from the renewable terpene limonene producing polycarbonates with a controlled and
variable ratio of incorporated nonfunctional cyclohexene oxide (CHO) and functional limonene
oxide (LO) monomers. As catalyst, a simple binary combination of an Al(III) aminotriphenolate
complex and PPNCl (PPN = bis(triphenyl)phosphine-ammonium) has been used to afford the
targeted, partially bio-based polycarbonates with typically high carbonate content (>95%), good
conversions and a controllable amount of olefinic groups in the terpolymer backbone. Crosslinked polymers (CLP) have been easily obtained from these terpolymers through thiol-ene click
reactions allowing for the preparation of interconnected networks with improved thermal
properties with their Td in the range of 250‒280ºC and with glass transitions (Tg) of up to 150ºC.
A detailed analysis of the cross-linked polycarbonates demonstrates a clear relation between the
percentage of potential cross-linking groups in the terpolymer precursors and the physicochemical properties including solubility, rigidity and thermal stability.

3

INTRODUCTION
Polymers are ubiquitous building blocks for a range of consumer-based products providing the
necessary plastics, coatings and fibers that increase the quality of life. Therefore, the
development of new (catalytic) strategies to obtain synthetic polymers with new and improved
properties continues to attract huge interest. The conversion of natural resources into valueadded polymers1 is gaining momentum due to environmental issues, fore-casted depleting fossil
resources and more strict waste legislation. The use of carbon dioxide (CO2) as a carbon feed
stock in organic synthesis2 and the replacement of fossil fuel feed stocks for natural, renewable
ones3 in polymer synthesis have become mature areas of research. In the latter context,
polycarbonates produced by ring-opening copolymerization of epoxides and CO2 represent
promising a type of materials.4 For instance, epoxide/CO2 coupling processes have led to the
synthesis of new polymers through the use of bio-based monomers based on terpenes5 or fatty
acids.6 These new approaches enable the synthesis of polymers with diverse, new architectures
and properties potentially useful in applications that go beyond bench-scale. Thus, beside the
conventional alternating copolymerization of epoxides and CO2 (copolymers, Figure 1a), novel
polymers have been synthesized by sequential epoxide addition (CO2-based block copolymers,
Figure 1b)7,8 or by random incorporation of multiple epoxide monomers (terpolymers; Figure
1c).9,10,11 Interestingly, these block copolymers and terpolymers have been prepared with the idea
to use the pendant functional groups for post-polymerization modifications. A wide range of
such functional (ter)polymer examples exist incorporating different polymer side chain groups
such as alcohols, carboxylic acids, amines, ionic fragments and alkenes. The latter category has
been popular in this respect as it allows for easy conversion by thiol-ene “click” chemistry” to
modify the polymer properties.8,10,12

4

(a) co-polymer

(b) block co-polymer

(c) terpolymer

Figure 1. Structures of different CO2-based polycarbonates: (a) Co-polymers, (b) Block
copolymers and (c) Terpolymers. The red circles represent the carbonate units, whereas the black
and white circles are different incorporated epoxide monomers.

The easy click chemistry and other post-polymerization reactions can be used to prepare new
functionalized polymers but also offer opportunities towards the formation of cross-linked
polymers by interconnecting different polymer chains (intermolecular process) or fragments
(intramolecular process). Such cross-linking may give polymers with improved mechanical
strength and thermal stability, as well as polymer nanoparticles with many applications including
data storage technology.13,14 However, examples of cross-linked polycarbonates based on
functional monomers remain rather limited to date. An interesting example was reported by the
Coates group, describing the efficient transformation of a linear polycarbonate with pendent
alkene groups into organic nanoparticles through well-established olefin cross-metathesis
reactions (Figure 2a).9 The group of Darensbourg utilized a radical initiated thiol-ene approach to
prepare cross-linked materials using a rigid pentaerythriol tetrakis(mercaptoacetate) (Figure
2b).15 Another recent example of this thiol-ene chemistry is the formation of a rigidified polymer
reported by Pescarmona et al. that exhibited a high glass transition temperature (Tg = 130ºC,
Figure 2c).16 In this latter example a flexible dithiol linker was employed to connect the olefin
fragments present in the oligomeric carbonates. All these cross-linked polymers are based on

5

petroleum feed stocks, and thus the development of new cross-linked materials derived from
natural resources remains an interesting target to provide more sustainable alternatives.

Figure 2. Schematic drawings of cross-linked polymers derived from petroleum sources (a-c)
and a functional terpolymer derived from the naturally occuring limonene (d). Note that here the
black and red units represent a full repeat unit based on an epoxide monomer and CO2.

6

As mentioned before, there are only few examples of 100% bio-based polycarbonates known
in the literature where natural oils have been used as co-monomers.5,6 In this regard, the first
copolymerization reaction using terpene-derived epoxides (i.e., limonene oxide, LO) and CO2
was described by Coates and co-workers utilizing highly active -diiminate (BDI) zinc catalysts,
and they succeeded in the stereoselective conversion of trans-LO.5a Furthermore, we recently
communicated the use of Al(aminotrisphenolate)/PPNCl [PPN = bis(triphenylphosphine)iminium] binary catalysts for stereoregular LO/CO2 copolymerization.17 As far as we know, the
synthesis of CO2-based terpolymers derived from renewable terpenes is still an unexplored area.
Inspired by our previous results in LO copolymerization, we set out to extend this approach
towards the preparation of (partially) renewable terpolymers. Here we disclose an efficient
catalytic method for the synthesis of new terpolymers based on cyclohexene oxide (CHO), LO
and CO2 with controllable incorporation of both functional (LO) and nonfunctional (CHO)
monomers in the polymer backbone. The olefin groups in the resultant terpolymers allow for
post-polymerization cross-linking by thiol-ene click chemistry, and our modular strategy
provides networks derived from terpenes with high thermal stability and rigidity, and offers
alternatives for polycarbonate based materials.

EXPERIMENTAL SECTION
General Considerations. All water sensitive operations were carried out under a nitrogen
atmosphere using an MBraun glovebox, standard vacuum-line and Schlenk techniques. Solvents
were purchased from Sigma-Aldrich as HPLC grade and dried by means of an MBraun
MBSPS800 purification system. All reagents were purchased from commercial suppliers
(Aldrich and Acros) and used as received unless stated otherwise. FT-IR measurements were

7

performed on a Bruker Optics FTIR Alpha spectrometer equipped with a DTGS detector, KBr
beam-splitter at 4 cm-1 resolution. Matrix-assisted laser desorption/ionization time-of-flight mass
spectrometry (MALDI-TOF-MS) was performed by the Research Support Group at ICIQ on a
BRUKER Autoflex spectrometer (more details are provided in the Supporting Information).
NMR spectra were recorded on a Bruker AV-400 spectrometer and referenced to the residual
NMR solvent signals. Glass transition temperatures (Tg) were measured under N2 atmosphere
using a Mettler Toledo equipped, model DSC822e. Samples were weighed into 40 μL aluminum
crucibles and subjected to three heating cycles covered the range from 20ºC to 150-190ºC at a
heating rate of 10ºC/min. Thermo-gravimetric analyses were recorded under a N2 atmosphere
using a Mettler Toledo equipped; model TGA/SDTA851 with a heating rate of 10ºC/min.
Molecular weights (Mn) and their distributions (Ð) of the terpolymers were determined by gel
permeation chromatography (GPC) at the Instituto de Ciencia y Tecnología de Polímeros
(CSIC), Madrid. GPC analyses were carried out with Styragel (300x7.8 mm, 5m nominal
particle size) water columns. Measurements were performed at 35ºC at a flow rate of 1 mL/min
using a RI detector (Waters, model 410) and using THF as solvent. Samples were analyzed at a
concentration of 4 mg·mL−1 after filtration through a 0.22 μm pore-size. Mn, Mw and Mw/Mn (Ɖ)
data were derived from the RI signal by a calibration curve based on polystyrene standards.
Reagents and Catalyst. Commercial CO2 was obtained from Praxair and used without further
purification. The commercially available epoxide substrates 1,2-epoxy-cyclohexane (CHO) and
(R)-limone oxide (LO) were used after distillation over calcium hydride, CaH2. Cis-limonene18
oxide was obtained following a literature procedure and purified by distillation over CaH2.
Commercially available nucleophilic additives bis(triphenylphosphine)iminium chloride
(PPNCl) was purified by recrystallization from dichloromethane and dried in vacuo at 40ºC for

8

24 h. Tris(3,5-dimethyl-2-hydroxybenzyl)amine and its aluminium complex (AlMe) were
prepared as described previously19 and used after drying under vacuum at 40ºC for 24 h.
General Catalytic Procedure for the Copolymerization of CO2 and Epoxides. All reactions
were prepared in a glovebox using a 30 mL stainless-steel Berghof reactor. In a typical
experiment, the catalyst (0.05 mmol), co-catalyst/nucleophile (0.025 mmol) and 10 mmol of
epoxide (0.98 g of CHO or 1.52 g of LO) were added into the teflon reaction vessel. Three cycles
of pressurization and depressurization of the reactor (5 bar) were carried out before finally
stabilizing the pressure at 15 bar. The reaction was stirred at 45ºC (40ºC inside the reactor) for 48
h. Finally, the reaction was stopped and the autoclave was allowed to cool down to room
temperature before venting. The crude reaction mixture was analyzed by 1H NMR spectroscopy
to determine the conversion to polymer and the amount of ether linkages.
General Catalytic Procedure for the CHO-LO-CO2 Terpolymerization Reactions. The
experiments were performed as mentioned above for the copolymerization processes using 0.05
mmol of catalyst, 0.025 mmol of co-catalyst and 10 mmol of the corresponding mixture of LO
and CHO. For example, the terpolymer containing 30% of the LO monomer incorporated was
prepared from a mixture of 5 mmol of CHO (0.49 g) and 5 mmol of LO (0.76 g) at 15 bar, 45ºC
(40 ºC inside the reactor) during 48 h.
Kinetic Experiments. All reactions for the polymerization control experiments and kinetic
studies were prepared in the glovebox using a 30 mL stainless-steel Berghof reactor. The teflon
reaction vessel was charged with 0.50 mol % of AlMe catalyst, 0.25 mol % of co-catalyst
(PPNCl) and 3.5 mmol of the epoxide mixture (1:1 ratio; 0.17 g of CHO and 0.27 g of LO).
Three cycles of pressurization and depressurization of the reactor (5 bar) were carried out before
finally stabilizing the pressure at 15 bar. Parallel reactions were stirred at 45ºC (40ºC inside the

9

reactor) for different reaction times (from 2 to 48 h). For each reaction time, stirring was stopped
and the autoclave was cooled down to room temperature before venting. The conversion of the
substrate was examined by 1H NMR spectroscopy (CDCl3) of an aliquot taken from the crude
reaction mixture and using as internal standard 1,4-dimethoxybenzene (typically 5.0 mol % with
respect to the epoxide substrate).
Isolation of the Polymers. The crude reaction mixture was extracted with dichloromethane
and the solution allowed to evaporate. The crude mixture was then re-dissolved in a minimal
amount of dichloromethane and the polymer precipitated with a 1 M solution of hydrochloric
acid in methanol. Upon settling, the supernatant solution was decanted and discarded. Then the
polymer was washed with a solution of methanol. Finally the purified polymer was dried in
vacuo at 40°C overnight and analyzed by 1H and 13C{1H} NMR, IR, DSC, TGA and GPC. The
MALDI-TOF assignment for various terpolymers was done taking into account the relative ratios
of CHO/LO monomers incorporated as determined by 1H NMR spectroscopy.
General Procedure for the Cross-Linking Process through Thiol-Ene Click Chemistry.
All reactions were carried out under inert conditions using standard vacuum-line and Schlenk
techniques. A radical initiator (0.20 equiv. of AIBN; 2,2’-azobis(2-methylpropionitrile) vs
olefins units in polymers) and 150 mg of the corresponding polymer were transferred into a
Schlenk tube equipped with a Teflon stirring bar. Then, the solvent (tetrahydrofuran, 1.0 mL)
and 1,2-ethanedithiol (0.50 equiv. vs olefin units in the polymers) were added.20 The reaction
mixture was heated in an oil bath at 70ºC and after 12 h the volatiles were removed in vacuo.
Afterward, the crude reaction mixture was washed three times with methanol. Finally, the
purified polymer was dried in vacuo and analyzed by 1H and

13

C{1H} NMR, IR, GPC, DSC,

TGA.

10

Figure 3. Previously developed LO/CO2 copolymerization process using the binary catalyst
AlMe/PPNCl.

RESULTS AND DISCUSSION
Terpolymer Synthesis using the Binary Catalyst AlMe/PPNCl. Previously we reported the
use of Al-based amino(triphenolate) complexes with PPNCl as binary catalysts for the coupling
reaction between CO2 and limonene oxide (LO) (Figure 3).17 The aluminum complex with
peripheral methyl substituents has proven to be an efficient catalyst for the synthesis of
stereoregular LO-based polycarbonates with Mn´s up to around 10.6 kg/mol (Ɖ ~ 1.4) after 24 h
at 45ºC and 5 bar operating pressure under neat conditions using 1.0 mol % of AlMe and 0.50 mol
% of PPNCl with respect to the substrate. Furthermore, the use of this catalytic system has
shown kinetic differences between the conversions of the cis- and trans-LO monomer, with the
latter one being more slowly consumed. Based on these observations, we decided to probe a
similar approach towards terpolymers based on CO2, CHO and the renewable substrate cis-LO.
Initially we assessed the potential terpolymerization process under similar conditions used for the
LO/CO2 copolymerization process, i.e. using 1.0 mol % of AlMe and 0.50 mol % of PPNCl, and
performing the reaction at 45ºC and 10 bar pressure. Upon using a starting ratio of 1:1 between
both epoxides, we clearly observed the formation of a polymer having both monomers

11

incorporated. The conversion (24 h; 75 %) of the substrate and the percentage of LO
incorporated (30%; by signal integration) could be calculated by 1H NMR spectroscopic analysis
of an aliquot taken from the reaction mixture (Supporting Information, Figure S23). Thermal
analysis showed several glass transition temperatures for this CO2-based terpolymer (Tg = 54, 72
and 126ºC, respectively, Supporting Information Figure S57). The presence of different Tg values
observed by differential scanning calorimetry analysis (DSC) may be a result of a poor control
over the terpolymerization process leading to a rather heterogenous combination of terpolymeric
species. With the aim to gain improved control over the coupling product between CO2 and both
epoxides, we decided to use lower amounts of AlMe and PPNCl (0.50 and 0.25 mol %,
respectively) as well as a slightly lower reaction temperature (40ºC) and higher initial pressure
(p(CO2)º = 15 bar). Under these latter conditions, the reaction seems to proceed more selectively
though a longer reaction time was necessary to obtain appreciable monomer conversions.
Importantly, the selective formation of a polycarbonate containing 30% of incorporated LO and
with only one Tg (95ºC) was noted. However, the molecular weight that was observed was rather
modest Mn = 4.0 kg/mol with a narrow polydispersity (Ɖ = 1.48; see Table 1 entry 4). On the
basis of this result, we used similar conditions to access various CHO/LO based terpolymers
with different percentages of LO incorporated (Table 1). Variation of the relative concentrations
of both CHO and LO monomers in reactions led to different polycarbonates with percentages of
incorporated LO monomer ranging from 1040% (Table 1). Additionally, the CHO/CO2 and
LO/CO2 copolymers were also prepared under the same conditions to compare their properties
with the terpolymer ones.21

12

Table 1. Terpolymerization of CHO, cis-LO and CO2 using AlMe as Catalyst and PPNCl as Nucleophilic Additive.a

Entry

Polymer

CHO : LOb

Conv.

carbonate linkages

n:m

(%)c

(%)d

Mne,f

Ɖe

(%)d

Tg

Td

(ºC)g

(ºC)h

1

P-0i

1:0

77

99

100 : 0

11.9

1.49

114

249

2

T-10

2:1

70

99

90 : 10

8.22

1.24

111

263

3

T-20

1.5 : 1

68

98

83 : 17

5.82

1.31

109

248

4

T-30

1:1

75

95

69 : 32

3.96

1.48

96

210

5

T-40

1:2

73

95

56 : 42

3.58

1.34

83

228

6

P-100i

0:1

65

93

0 : 100

3.69

1.34

72

226

a

Reaction conditions: CHO and LO (10 mmol in total, molar ratio indicated in the table), AlMe 0.50 mol %, PPNCl 0.25 mol %, 40ºC,
p(CO2)º = 15 bar, 30 mL autoclave, neat conditions, 48 h. bCHO and LO molar ratios used in the catalytic reaction. cDetermined by 1H
NMR analysis of the crude reaction mixture. dDetermined by 1H NMR analysis of the isolated polymer. eDetermined by gel
permeation chromatography (GPC) analysis in THF using polystyrene standards. fIn kg/mol. gDetermined by differential scanning
calorimetry (DSC). hDecomposition temperature (Td) determinated by thermogravimetric analysis (TGA) at 10% weight loss of the
polymer analyte. iCHO/CO2 and LO/CO2 copolymers (P-0 and P-100, respectively) were prepared under the same conditions.

13

The data in Table 1 show that the percentage of LO incorporated in these terpolymers has a
significant effect on the molecular weight (Mn) as well as on the glass transition temperature (Tg).
At higher percentages of LO incorporated in the terpolymer products, lower molecular weights
are found and concomitantly lower Tg values. These observations are to be expected when the
amount of LO is increased in the initial reaction mixture as this monomer typically shows lower
conversion kinetics compared to CHO.
In order to gain more insight into the terpolymerization process, the CO2/CHO/LO coupling
process was followed in time. Such kinetic experiments are usually performed to assess whether
diblock copolymers or terpolymeric structures are formed.22 This type of analysis is useful to get
more detailed information about the reactivity of all involved monomers and the monomer
composition in the chain as to discriminate between a homogeneous or a gradient type of
terpolymer.15,23 We thus carried out a set of experiments with an equimolar CHO/LO mixture at
40ºC and 15 bar and followed the terpolymerization process in time. Reactions were stopped at
different time intervals and the reaction mixtures were analyzed by 1H NMR spectroscopy to
determine the consumption of each epoxide monomer (Supporting Information, Table S1).
Figure 4 shows the conversion of both CHO and LO in this terpolymerization process. Clearly,
CHO reacts much faster than LO, delivering a polycarbonate with an incorporation ratio
CHO/LO of 74:26 determined by 1H NMR signal integration. Despite the fact that CHO reacts
significantly faster in this terpolymerization, LO incorporation into the polymer backbone occurs
from the early stage (12% on average; after 2 h, 14% total monomer conversion) to the end of
the process (26% on average; after 48 h with a 70% total monomer conversion). Therefore, the
polymer seemingly has the functional monomer LO reasonably distributed throughout the entire
chain. To further substantiate a random distribution of the LO monomer, the ratio between the

14

net consumption of CHO and LO monomers was followed in time providing relative
incorporation kinetics (Figure 5). This graph thus gives an impression of the relative rates of
incorporation of both monomers, and the terpolymer that was produced has apparently a high
CHO/LO incorporation ratio (7:1) at the initial stage of the reaction (first 2 h period), which then
quickly decays to lower CHO/LO incorporation rates in subsequent one hour time intervals (t =
3, 4, 5 and 6 h).

Figure 4. Kinetic profiles for the CHO and cis-LO (denoted as LO) consumption and the
terpolymer formation. In green, the average % of LO incorporation throughout the reaction.
Reaction conditions: CHO/LO (1:1 ratio, 3.5 mmol in total), AlMe 0.50 mol %, PPNCl 0.25 mol
%, p(CO2)º = 15 bar, neat, 40ºC. Polycarbonate conversion, and CHO and LO consumption were
determined by 1H NMR analysis of the crude reaction mixture using 1,4-dimethoxybenzene as
internal standard.

15

Notably, after a 10 h reaction time the overall reaction kinetics become rather slow likely as a
consequence of the observed increasing viscosity of the mixture. It is thus clear that the
terpolymer starts with a low content of incorporated LO monomer (12%) but after 10 h, when the
CHO monomer conversion reaches around 95% (Figure 4) this becomes significantly higher with
CHO/LO incorporation ratios < 0.5 (for more details see Table S2, Supporting Information).
Overall, this results in a terpolymer structure with a clear gradient in the pendant olefin groups.

Figure 5. The change in the relative CHO/LO incorporation kinetics in time for the
terpolymerization process carried out at 40ºC, 15 bar and using 1:1 ratio of CHO and LO (3.5
mmol in total).

16

T-30

n = 7+x, m = 3
x = 1,2,3,4

n = 7+x, m = 3
x = 1,2,3,4

+196

n = 14x, m = 6
x = 1,2,3,4

+196
196

n = 14x, m = 6
x = 1,2,3

196

Figure 6. MALDI-TOF mass spectrum recorded for terpolymer T-30. The upper part shows the
10005000 m/z region, whereas the lower part displays an enlarged portion between 17003400
with some of the assigned peaks explained together with the most dominant end-groups
observed.

17

The terpolymer structures T-m (m = 10, 20, 30 or 40; Table 1 entries 2-5) were further
investigated by MALDI-TOF mass spectrometry (Figure 6, and Supporting Information: Figures
S53-S56) using similar conditions that were previously successfully used for LO/CO2
copolymers.17 As a representative example the characteristics of T-30 (having an approximate
30% average of LO/CO2 carbonate repeat units) are discussed here.24 The full trace (Figure 6,
upper part) and the selected, enlarged MALDI-TOF region (m/z 17003400) show a complicated
combination of peaks. Despite this complexity, we were able to assign most of the peaks by
considering (1) the preference for specific end-groups such as –OH and those having H2O
eliminated (see terpolymer schematic insets at the top of Figure 6),25 and (2) the average % of
LO incorporated in the terpolymer T-30 determined by 1H NMR analysis. For instance, the peak
at m/z = 1722 could be readily assigned to a terpolymer based on seven CHO/CO2 and three
LO/CO2 repeat units with a cyclohexanol end-group (calculated: m/z = 1721), and subsequent
incorporation of four CHO/CO2 fragments (m/z = 1864, 2006, 2147 and 2291; m = 142, equal
to a CHO/CO2 repeat unit) was recognized. At various stages also the incorporation of new
LO/CO2 repeat units (196) was detected, for instance from m/z = 1722 to 1918 and from m/z =
2291 to 2485.
Apart from all the possible combinations of n and m (i.e., the ratio of CHO/CO2 vs LO/CO2
repeat units for T-30 being an average of around 30%), also the presence of different end-groups
in these polymer distributions add to the complexity of the analysis. The peak at m/z = 1756
(calculated: 1757) was assigned to a terpolymer having the same n/m ratio as found for the peak
at m/z = 1722 but with a different end-group based on a dehydrated LO fragment (upper part of
Figure 6, green schematic inset). Propagation through consecutive CHO/CO2 insertions is again
easily recognized giving rise to polymers with increasing n up to 11 (m/z = 2323, calculated:

18

2324), and the incorporation of LO/CO2 repeat units can also be observed from, for instance, the
peak at m/z = 2038 (n = 9, m = 3) to give a polymer chain with increasing m (n = 9, m = 4: m/z =
2236, calculated: 2234).
In the higher mass region up to m/z 3300 similar type of distributions can be found, and the
MALDI-TOF analysis therefore provides evidence for a complex mixture of polymer structures
with distinct n/m ratios, but more importantly it provides further proof for the early incorporation
of the functional monomer LO into the polymer backbones. This corroborates with the kinetic
profiles for CHO and LO consumption and the synthesis of terpolymeric structures with a clear
gradient in the presence of the olefin groups from relatively low (12% in the first 2 h) to high
(>26%) at the end of the terpolymerization process.

Scheme 1. Synthesis of Cross-Linked Polymers through Radical-Initiated Thiol-Ene Click
Chemistry. Here m Denotes the % of LO/CO2 Repeat Units determined in the Different
Terpolymers by 1H NMR, whereas T-m is the Terpolymer Substrate and CLP-m the CrossLinked Polymer.

Terpolymer Cross-Linking Reactions. Having prepared a varied range of polycarbonates
containing a controlled number of the functional monomer LO, we next turned our focus on post-

19

polymerization modifications with the aim to improve their thermic properties. Since the pendent
olefin groups are reasonably distributed throughout the polymer structures, we envisioned that
cross-linking of the alkene units by thiol-ene chemistry would be an ideal strategy to prepare
thermally more stable terpolymeric architectures. Thus, the partial bio-based terpolymers
described in Table 1 were treated with 1,2-ethanedithiol in the presence of 2,2’-azobis(2methylpropionitrile) (AIBN) used as radical initiator in catalytic amounts (0.20 equiv. vs amount
of olefin groups) in THF. The thiyl radicals generated by heating the reaction mixture to 70ºC
should attack the least hindered carbon center of the olefin unit (Scheme 1) and enable the
connection of two separate LO/CO2 repeat units either via intra- or inter-molecular pathways. It
should be noted that the reaction stoichiometry is likely a decisive factor to control the ratio
between mono- and di-reacted dithiol linkers which may influence the overall polymer
properties.
The thiol-ene addition reaction leads to a gradual disappearance of the olefin groups in the
polymer backbone, a change that could be easily followed and confirmed by IR and NMR
spectroscopy (see Figures 7 and 8). For instance, evidence for thioether formation is provided by
changes in the typical IR bands for the copolymer P-100 substrate located at 1645 cm-1 (C=C)
and 887/837 cm1 (C=C−H) that disappear in time (Figure 7).
1

H NMR analysis of T-20 treated with 1,2-ethanedithiol to give CLP-20 provided additional

confirmation of olefin to thioether conversion in the polymer structure (Figure 8). For example,
the appearance of a peak at 0.98 ppm can be ascribed to thioether linkage formation causing a
clear upfield shift of the methyl resonance of the incorporated LO unit (colored green; in the
yellow sphere). Also, the presence of various peaks in the region 2.53.0 ppm supports the
presence of thioether linkages.26 When a (larger) excess of 1,2-ethanedithiol is used, a peak with

20

increasing intensity at 2.73 ppm is noted which can be ascribed to the presence of cumulative
amounts of mono-coupled dithiol species (cf., presence of –CH2SH pendant fragments). In
addition to these observation done by 1H NMR, the disappearance of the quaternary and
secondary carbon peaks pertinent to the olefin groups at 148.6 and 108.6 ppm, respectively, of
T-20 was also noticed in the 13C NMR (see Supporting Information: Figure S25).

Figure 7. Comparative IR spectra for the copolymer P-100 and its corresponding cross-linked
polymer CLP-100 (top) after the thiol-ene reaction.

21

(a)

(b)
Me

(c)
Me

(d)
2.73

Me

Figure 8. Comparative 1H NMR spectra in CDCl3 at 25ºC for: (a) isolated T-20 terpolymer, and
(bd) prepared cross-linked polymers CLP-20 using different 1,2-ethanedithiol molar equiv. in
the cross-linking process: (b) 0.50 equiv., (c) 0.70 equiv. and (d) 1.5 equiv. The colored spheres
represent the relevant protons attached to the indicated carbon centers in the schematic structure
above.

22

The optimal reaction conditions for the cross-linked polymer synthesis were then further
examined with particular attention dedicated to influence of the cross-linking reagent on the
polymer properties (Table 2). We performed a series of reactions with terpolymer T-20 and
varied the concentration of 1,2-ethanedithiol. Moreover, reactions were performed under
concentrated conditions to maximize intermolecular cross-linking (i.e., 1.0 mL of solvent per 150
mg of polymer substrate). The final product may incorporate thiol pendant groups (m’ repeat
units in Scheme 1). Indeed, we were able to detect such pendant groups by 1H NMR
spectroscopy (Figure 8) and their amount depended on the molar equiv. of 1,2-ethanedithiol used
in the cross-linking process. Figure 8 suggest that the percentage of free, unreacted −SH groups
should be minimal upon using a stoichiometric amount of dithiol reagent (i.e., 0.50 equiv.).
The post-modified CLP´s were analyzed by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). These thermal analyses showed that interconnected networks
CLP-20a and CLP-20b derived after the cross-linking using an excess (> 0.50 molar equiv.) of
dithiol reagent exhibited the lowest Tg and Td values among the series (Table 2, entries 1 and 2).
Conversely, the use of a stoichiometric amount of dithiol (0.50 equiv., entry 3) resulted into the
formation of a cross-linked polymer CLP-20c with better thermal properties. The reaction was
then also carried out in the presence of a smaller than stochiometric amount of dithiol (0.30
equiv., entry 4). Under these conditions, the resulting CLP-20d did not show any improvement
relative towards CLP-20c. Additionally, no observable reaction was noted under dithiol-free
conditions in the presence of AIBN as radical initiator (entry 5, Table 2). From the combined
results it can be concluded that the use of a stoichiometric amount of dithiol produces a crosslinked polymer with optimized thermal features.

23

Table 2. Synthesis of Cross-Linked Polymers via Thiol-Ene Click Chemistry varying the
Amount of Dithiol and using Terpolymer T-20.a,b

Entry

dithiol (equiv)c

CLPd

Tg (ºC)e

ΔTg(ºC)f

Td (ºC)g

ΔTd(ºC)h

1

1.5

CLP-20a

107

7

264

16

2

0.70

CLP-20b

109

6

272

24

3

0.50

CLP-20c

118

18

279

31

4

0.30

CLP-20d

112

9

272

24

5

0



100



248



a

Reaction conditions: 150 mg of terpolymer T-20, AIBN (0.20 equiv. vs olefin groups), 12 h,
THF (1.0 mL), 70ºC. bTg and Td values for the sample T-20 used in these studies were 100ºC and
248ºC, respectively. cEquivalents of 1,2-ethanedithiol used vs amount of olefin groups. dCrosslinked polymer (CLP) containing a maximum m % of linkages. eDetermined by differential
scanning calorimetry (DSC). fΔTg is the difference between the Tg of the cross-linked polymer
and the T-20 terpolymer substrate. gDecomposition temperature (Td) determined by
thermogravimetric analysis (TGA) at 10% weight loss of the polymer. hΔTd is the difference
between Td values of the cross-linked polymer and the T-20 terpolymer substrate.
With these optimized conditions (0.50 equiv. dithiol, 0.20 equiv. of AIBN, 12 h, 70ºC, THF),
we then further investigated the formation of cross-linked terpolymers with different amount of
olefin groups (T-m; m = 10, 20, 30, 40% and P-100; Table 3).27 For the CLP´s derived from T10 and T-20 we observed the quantitative conversion of the olefin groups and formation of
soluble networks CLP-10 and CLP-20.28 However, a further increase in reactive olefin groups
in the ter/copolymer substrate (T-30, T-40 and P-100) resulted in the formation of materials that

24

are virtually insoluble in organic solvents. From Table 3 it can be deduced that the cross-linking
reaction had a pronounced positive effect on the thermal stability of the resultant CLP polymers
increasing the decomposition temperatures (Td) up to 54ºC (for CLP-30, entry 3). Interestingly,
this cross-linking process also enhanced the glass transition temperature and the CLP´s exhibited
single Tg´s from 116ºC (CLP-30, entry 3) up to 150ºC (CLP-100, entry 5). As expected, the Tg
values increased as a function of the amount of cross-linkable olefin groups with CLP-40 (entry
4) and CLP-100 (entry 5) displaying the highest Tg. In the latter case, the Tg increased from 73 to
150ºC (Tg = 77ºC) despite the rather modest molecular weight (Mn = 3.69 kg/mol; Table 1,
entry 6) of the copolymer substrate P-100. Copolymer P-100 was also reacted with an excess of
dithiol (Table 3, entry 6) towards the formation of a CLP with a larger amount of unreacted thiol
groups as observed for T-20 (cf., formation of CLP-20a and CLP20b, entries 1 and 2 in Table
2). The resulting cross-linked polymer CLP-100a showed a much lower Tg and also a lower Td
values than obtained for CLP-100 thus supporting the view that stoichiometric amounts (0.50
molar equiv.) of the dithiol reagent are also ideal for converting polymers substrates with higher
cross-linkable olefin densities into thermally more stable materials. Under more dilute conditions
(entry 7), the CLP-100b produced from T-100 was fully soluble and showed virtually no change
in the Tg value. This indicates that most of the olefin groups had either reacted in an
intramolecular fashion and/or had led to a considerable concentration of unreacted mono-thiol
groups in the polymer backbone. GPC analysis of this post-modified polymer CLP-100b showed
a considerable increase in molecular weight (Mn = 9.51 kg/mol) and polydispersity (Ð = 3.15)
compared with P-100 (Mn = 3.69 kg/mol, Ð = 1.34). Thus, the cross-linking process carried out
under diluted conditions favors to a larger extent olefin functionalization and/or introduction of
intramolecular linkages though some level of intermolecular cross-linking is maintained.

25

Table 3. Synthesis of Various Cross-Linked Polymers (CLPs) via Thiol-Ene Click Chemistry.a

Entry

T-mb

Tg

Td

(ºC)c

(ºC)d

Tg

ΔTg

Td

ΔTd

(ºC)c

CLP-me

(ºC)f

(ºC)d

(ºC)g

1

T-10

102

263

CLP-10

117

15

282

19

2

T-20

100

248

CLP-20

118

18

279

31

3

T-30

100

210

CLP-30

116

16

264

54

4

T-40

97

228

CLP-40

119

22

256

28

5

P-100

73

226

CLP-100

150

77

245

19

6h

P-100

73

226

CLP-100a

83

10

239

13

7i

P-100

75

226

CLP-100b

74

0

233

7

a

Reaction conditions: 150 mg of terpolymer, 1,2-ethanedithiol (0.50 equiv. vs olefin groups),
AIBN (0.20 equiv. vs olefin groups), 12 h, 70ºC. bTerpolymer containing m % of LO monomer
incorporated. cDetermined by differential scanning calorimetry (DSC). dDecomposition
temperature (Td) determined by thermogravimetric analysis (TGA) at 10% weight loss of the
polymer. eCross-linked polymer (CLP) containing a maximum of m % of linkages. fΔTg is the
difference between the Tg values of the cross-linked polymer CLP and the terpolymer T-m
substrate. gΔTd is the difference between the Td values of the cross-linked polymer CLP and the
terpolymer T-m substrate. hUsing 1.0 equiv. of 1,2-ethanedithiol vs olefin groups. iUsing 20 mL
of THF as solvent.

The increase of the rigidity in the CLP´s with increasing cross-linking density could be
observed by DSC analysis as shown in Figure 9. The CLP´s having a higher amount of linker
units (i.e., CLP-30, CLP-40 and CLP-100) displayed more flattened glass transitions and this

26

behavior may be expected of highly rigid29 and intermolecular connected polymer chains, in line
with the poorer solubility observed for the CLP´s.

Figure 9. Selected region of the differential scanning calorimetry (DSC) analysis of CLP-10,
CLP-20, CLP-30, CLP-40 and CLP-100. The heating rate was 10ºC/min. In each case, the trace
of the second cycle is shown.

27

CONCLUSION
In summary, we describe here the first catalytic process towards the formation of terpolymers
based on the naturally occurring terpene limonene oxide (LO) with a controlled number of
functional repeat units. The control over the number of olefin units enables the post-modification
of these terpolymers through thiol-ene chemistry providing materials with improved thermal
properties exhibiting much higher Tg and Td values than their corresponding terpolymer
precursors. These new cross-linked polymers (CLP´s) thus provide a partial bio-based
alternative for those systems that are derived from fossil fuel based feed stocks only. The
combined analytical data (IR, NMR, TGA, DSC and GPC) shows good control in the radical
initiated thiol-ene cross-linking process when stoichiometric amount of dithiol reagent (0.50
equiv. vs olefin groups) is utilized and emphasizes the importance of the appropriate reaction
stoichiometry, and the influence of the amount of functional groups present in the ter/copolymer
backbone enabling the synthesis of new cross-linked materials with Tg´s of up to 150ºC. Due to
the vast increase in bio-renewable materials in industry and academic settings, our current focus
is now on the design of new and stable (partially) bio-based co- and terpolymers with different
architectures and novel properties using terpenes as accessible and modular scaffolds.30

ASSOCIATED CONTENT
Supporting Information. Further experimental details, spectra of isolated polymer samples
(NMR and IR), and relevant GPC, MALDI-TOF-MS, DSC and TGA traces. This material is
available free of charge via the Internet at http://pubs.acs.org.

28

AUTHOR INFORMATION
Corresponding Author
akleij@iciq.es
Notes
The authors declare no competing financial interest.

ACKNOWLEDGMENTS
We acknowledge financial support from ICIQ, ICREA, the Spanish MINECO (project CTQ2014-60419-R) and the Severo Ochoa Excellence Accreditation 2014–2018 through project
SEV-2013-0319. C. M. is grateful to the Marie Curie COFUND action from the European
Commission for co-financing a postdoctoral fellowship. The authors also thank Dr. Marta
Giménez and Cristina Rivero for assistance with high pressure experiments and Dr. Noemí
Cabello for the MALDI-TOF MS analyses. Simona Curreli, María José Hueso and Marta
Serrano are thanked for their help regarding the thermal analyses (TGA/DSC).

29

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C NMR) were carried out by using 2D experiments

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37