Allylation of a lignin model phenol: A highly selective reaction under benign conditions towards a new thermoset resin platform

Article abstract of DOI:10.1039/c6ra21447a

The lack of aromatic material constituents derived from renewable resources poses a problem to meet the future demands of a more sustainable society. Lignin is the most abundant source of aromatic structures found in nature and is a highly interesting source for material applications. Development of controlled chemical modification routes of lignin structures are crucial in order to further develop this area. In this study allyl chloride is used to selectively modify a lignin phenol in the presence of other lignin functionalities, i.e. aliphatic hydroxyls and conjugated alkenes, under mild reaction conditions in quantitative yields. For this, coniferyl alcohol was used as a model compound in the present study. The modification was carried out in ethanol as the synthesis media. Studies on the effect of reaction time and temperature revealed optimum conditions allowing for a quantitative yield without any detectable levels of byproducts as studied with NMR, FT-IR and FT-Raman. The thermal stability of the formed product was determined to be up to at least 160 °C through DSC measurements. In addition, as a proof of concept, the use of the allylated monomer to form crosslinked films using free radical thiol-ene polymerization was demonstrated.

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Allylation of a lignin model phenol: A highly selective reaction
under benign conditions towards a new thermoset resin platform†
M. Jawertha, M. Lawokoa, S. Lundmarka,b, C. Perez-Berumena,c and M. Johanssona*
Received 00th January 20xx,
Accepted 00th January 20xx
The lack of aromatic material constituents derived from renewable resources poses a problem to meet the futures
demand on a more sustainable society. Lignin is the most abundant source of aromatic structures found in nature and is a
highly interesting source for material applications. Development of controlled chemical modification routes of lignin
structures are crucial in order to further develop this area. In this study allyl chloride is used to selectively modify a lignin
phenol in the presence of other lignin functionalities, i.e. aliphatic hydroxyls and conjugated alkenes, under mild reaction
conditions in quantative yields. For this, coniferyl alcohol was used as a model compound in the present study. The
modification was carried out in ethanol as the synthesis media. Studies on the effect of reaction time and temperature
revealed optimum conditions allowing for a quantitative yield without any detectable levels of byproducts as studied with
NMR, FTIR and FT-Raman. The thermal stability of the formed product was determined to be up to at least 160 ⁰C through
DSC measurements. In addition, as a proof of concept, was the use of the allylated monomer to form crosslinked films
using free radical thiol-ene polymerization demonstrated.
DOI: 10.1039/x0xx00000x
www.rsc.org/
thermoplastic and thermoset resin applications8, 10
.
Introduction
The main production source that exist for extraction of
lignins are the pulp and paper processes, which globally
11
The development efforts on new materials based on
renewable resources continuously increase to address the
futures demand on a more sustainable society1. A wide range
of resources are exploited as replacements to fossil based
material constituents in order to achieve this goal2. One of the
biggest challenges is to find alternatives to aromatic structures
currently used and only a few examples have been introduced
as replacements in conventional polymer systems3-5. Furan
dicarboxylic acid, made from sugars, is for example now
produced as an alternative to terephthalic acid6. The most
produce about 70 million tonnes of lignin every year7,
.
However only 2% are utilized for commercialization8, the rest
is combusted in a chemical recovery boiler producing energy
due to its high heating value of about 27 MJ/Kg12. The growing
consensus however, is that part of the lignin can be extracted
for production of bio-based products and create value addition
for existing industries7, 8. A few techniques, such as the
lignoboost technology, which was recently commercialized,
are emerging to allow lignin with relatively high purity to be
obtained in powder form11-13
. However, the polydispersity of
abundant source for natural aromatic structures is however
lignins found in lignocellulosic biomass3,
7,
lignin with regards to molar mass distribution is high.
Lignoboost Kraft lignin for example will have a polydispersity,
Ð, of more than six13.
8. Lignin is a
macromolecular constituent in plants that has gained an
increased interest in the past 30 years due to its low cost, its
abundance and the drive to find new material applications
derived from renewable resources7-10. It is the largest source
of naturally occurring phenolic compounds and offers a
potential alternative for non-renewable aromatic structures7.
Many studies in this field have, since the 60: s, been
Difficulties in utilizing lignin as a resource is not only due to
the molecular weight distribution but also on the abundance
of different functional groups dependant on the process
applied for the extraction7, 11, 13. The challenge in using lignin
and lignin derivatives as material building blocks thus includes
fractionation, characterization, and modification of complex
systems to obtain suitable target constituents for material
applications7, 8, 14, 15. Fractionation using solvent sequences has
therefore been considered as one route to obtain more
performed to use lignin as
a polymeric material in
homogeneous fractions to address these problems11, 13
.
Variations are also largely depending on the specie used as
the lignin source. Lignins have a composition comprised of
phenolic monomers referred to as monolignols7. There are
mainly three different monolignols distinguished by their
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degrees of substitution by methoxyl groups in the meta The allyl functional group also open up for a large variety of
positions of the aromatic ring7, 9. These are called p-coumaryl other different synthetic pathways as described in literature in
9, 15, 16
alcohol, coniferyl alcohol and sinapyl alcohol8,
respectively, Figure 1.
numerous papers21, 23, 31, 32, 33, 34. This can, if applied to lignin
structures, be considered as a material platform as shown in
Depending on plant species, the lignin contains different Scheme 1.
proportions of the three monophenolic compounds. Softwood
One other interesting route to consider in thermoset
lignins consist majorly of coniferyl alcohol, hardwood lignins of material applications is the epoxidation of the double bond to
a mixture between sinapyl alcohol and coniferyl alcohol and in form glycidyl ethers23. Around 70 % of the market of
grass the lignin is comprised of all three types of monolignols7, thermosetting resins, excluding polyurethanes, is made up of
8. The exact structure and polymerization degree of lignin in epoxy resins which is used in many different industrial
plants however remain unknown although extensively applications with high material demands15. Epoxy resins are
studied14, 17
.
versatile and can be found in coatings, adhesives and high
Lignin molecules are equipped with different performance composites. Because of the convenience of
functionalities, most of them being hydroxyl groups of epoxy resins there are high interests in finding resin systems,
phenolic or aliphatic nature. Technical lignin can however carry partially or fully, derived from renewable resources.
other kind of functionalities such as alkene, carboxyl, or Combining the epoxy functionality and the aromaticity of lignin
carbonyl groups7, 8, 11. Due to this diversity and complexity in is therefore highly interesting for future efforts.15
structure, lignin provides a challenge in terms of handling and
application7.
Allylation of lignin using different allyl compounds have
been described in the literature10, 21-23. Allyl halides in the form
Modification of lignin can be performed on the extracted of allyl bromide and allyl chloride has been utilized by Zoia et
macromolecular lignin, oligomeric lignin or mono phenols to al21 and Dournel et al10 respectively to form allyl ethers on
15,
match the various application demands7,
18. One of few lignin and lignin model compounds. Dournel et al10 examined
lignin derived pure compounds produced technically in large the allylation reaction on many low molecular weight lignin
volumes is vanillin, which have been studied for material model compounds in various yields ranging between 35 and
applications i.e. by Fache et al3, Meylemans et al4 and Harvey 100%. However, their model compounds all contain either a
et al5. Depending on end use, the modification of the phenolic phenolic hydroxyl group or an aliphatic one, never the two
hydroxyl groups present in lignin structures could be desirable types within the same compound. Zoia et al21 reported an
since these are weakly acidic and relatively easily oxidized, overall conversion yield of 80-90 % when performing the
consequently causing deterioration and discoloration of the allylation reaction on guaiacylic lignin in acetone and an excess
final material if still present19, 20. An efficient and selective of allyl bromide in relation to the total amount of hydroxyl
modification of phenols in the presence of other functional groups. Over et al22 have on the other hand examined the
groups such as aliphatic alcohols and alkenes is thus
desirable and would allow for use of more complex lignin environmentally friendly reagent. They found that 99% of the
structures as material precursors. phenolic hydroxyl groups were allylated while 71% of the
highly possibilities of using diallyl carbonate as
a
more
One specific reaction that has been performed to modify aliphatic were converted resulting in an overall yield of 82%. In
lignin and lignin model compounds is allylation to form allyl all three mentioned papers it is thus hinted that the phenolic
ethers10, 21-23. Allyl compounds, first identified, named and hydroxyl group is more prone to be modified than those of
described in 184424 have ever since been used as base aliphatic nature, however a fully selective reaction is yet to be
structures for various polymer systems either by using it shown.
directly in polymer forming reactions or as a precursor for
The present study describes a route to selectively modify
other reactive moieties25, 26. Allyl mercaptan was for example phenol groups in a lignin model compound to a very high
the monomer used when first describing the thiol-ene reaction conversion under mild reaction conditions using an
as a polymer forming reaction27. The thiol-ene reaction has environmentally friendly solvent. In addition, as a proof of
been drawing attention in recent years due to its “click” concept, crosslinked molecular networks were prepared using
characteristics under certain reaction conditions, different thiol-ene chemistry and analysed.
possible reaction mechanisms, and structural versatility28-30
.
Figure 1 The three different monolignols, from left to right: p-coumaryl alcohol,
Scheme 1
Different proposed reaction pathways for utilization of allylated
coniferyl alcohol and sinapyl alcohol labelled according to established literature.
lignin structures.
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NaBH4 (1.28 g, 33.7 mmol) was dispersed in 180 ml of
EtOAc in a 500 ml round bottom flask under agitation.
Coniferyl aldehyde (3.00 g, 16.8 mmol) was added to the
reaction vessel and the mixture was left to stir for 17.5 hours
in room temperature. 200 ml of deionized water was carefully
added to quench the reaction and the mixture was agitated for
30 additional minutes. The organic phase was extracted and
washed with brine before being dried using MgSO4 and
filtered. The EtOAc was removed in a rotary evaporator at 40
⁰C. An oily bright yellow product was obtained and re-
dissolved in warm DCM before being stored in a freezer to
recrystallize for roughly 1.5 hours. The product was filtered
and washed with heptane before being dried in vacuum oven
at 50 ⁰C overnight. The leftover mixture from the
crystallization step was dried using a rotary evaporator at 40
⁰C and the procedure was repeated and left over night to yield
more product. The result was white crystals with an overall
Experimental
Materials
All chemicals were of analytical grade and used as received.
Allyl chloride (98%), coniferyl aldehyde (98%), sodium
borohydride (NaBH4, 98%), sodium carbonate (Na2CO3, 99.5%),
sodium hydroxide (NaOH, ≥98%) and Trimethylolpropane
tris(3-mercaptopropionate) (TMP, 95%) was purchased from
Sigma Aldrich. Ethanol (96%), ethyl acetate (EtOAc),
dichloromethane (DCM), heptane and chloroform were
obtained through VWR chemicals. Magnesium sulphate
(MgSO4, 99%) was bought from Acros Organics and sodium
chloride (NaCl) from Merck. Diethyl ether was achieved
through Fisher Scientific. Irgacure 651 was obtained from BASF
chemicals.
Methods
1
Nuclear Magnetic Resonance (NMR). 1H-, 13C-, and 2D
HSQC NMR Spectra was recorded at room temperature on a
Bruker Avance III HD 400 MHz instrument with a BBFO probe
equipped with a Z-gradient coil for structural analysis. d6-
DMSO was used as solvent and the residual solvent peak was
yield of ≈ 80 %. H-NMR (400 MHz, DMSO-d6) δ 8.98 (s, 1H),
6.99 (d, J = 1.9 Hz, 1H), 6.79 (dd, J = 8.2, 2.0 Hz, 1H), 6.70 (d, J =
8.1 Hz, 1H), 6.41 (dt, J = 15.8, 1.6 Hz, 1H), 6.17 (d, J = 15.9 Hz,
0H), 4.75 (t, J = 5.5 Hz, 1H), 4.07 (td, J = 5.5, 1.7 Hz, 2H), 3.78
(s, 3H). 13C-NMR (101 MHz, DMSO-d6) δ 147.72 , 146.17 ,
129.00 , 128.52 , 127.50 , 119.44 , 115.48 , 109.70 , 61.75 ,
55.56.
1
used as an internal standard. 128 scans were used for the H-
NMR measurement and 475 scans for the 13C-NMR. The data
was analyzed using MestReNova software. Data were
processed with MestreNova (Mestrelab Research) using 90°
shifted square sine-bell apodization window; baseline and
phase correction was applied in both directions.
Synthesis of Coniferyl Alcohol Allyl Ether (CAAE)
NaOH (0.32 g, 7.77 mmol) was dissolved in 30 ml of ethanol
under agitation while heated to 55 ⁰C. A condenser was
applied to prevent evaporation of solvent and allyl chloride.
When the temperature had reached 55 ⁰C and the NaOH was
dissolved, CA (1 g, 5.55 mmol) was added to the mixture and
dissolved. Allyl chloride (0.63 ml, 7.77 mmol) was carefully
provided to the solution using a syringe through a septum and
the reaction was left to stir for 17.5 hours. The reaction was
terminated by stopping the flow in the condenser and open up
the system for 30 min to let excess allyl chloride evaporate.
Ethanol was rotaevaporated at 40 ⁰C. The yellow viscous
product was re-dissolved in diethyl ether and extracted
towards a 10 wt% Na2CO3 water solution. The organic phase
was dried using MgSO4 and filtered. Diethyl ether was
removed using a rotary evaporator at 30 ⁰C. The product was
dried in vacuum oven at 50 ⁰C over night and the yield was
determined to 80 %. 1H-NMR (400 MHz, DMSO-d6) δ 7.05 (t, J =
1.1 Hz, 0H), 6.89 (d, J = 1.4 Hz, 1H), 6.53 – 6.39 (m, 0H), 6.34 –
6.19 (m, 0H), 6.11 – 5.92 (m, 0H), 5.38 (dd, J = 17.3, 1.8 Hz,
0H), 5.24 (dd, J = 10.5, 1.7 Hz, 0H), 4.81 (t, J = 5.5 Hz, 0H), 4.53
(dt, J = 5.4, 1.5 Hz, 1H), 4.09 (td, J = 5.4, 1.6 Hz, 1H), 3.78 (s,
1H). 13C-NMR (101 MHz, DMSO-d6) δ 149.16 , 147.10 , 133.87 ,
130.21 , 128.68 , 128.50 , 119.03 , 117.45 , 113.46 , 109.44 ,
68.95 , 61.63 , 55.48.
Fourier Transform Infrared Spectroscopy (FT-IR) was
performed on a Perkin-Elmer Spectrum 2000 FT-IR equipped
with a MKII Golden Gate, heat controlled single reflection ATR
system of Specec LTD. Spectra were recorded in the range of
600 to 4000 cm-1 with 32 scans averaged at 4.0 cm-1 resolution
at 80 ⁰C and at room temperature.
Fourier Transform Raman Spectroscopy (FT-Raman) was
performed on a Perkin-Elmer Spectrum 2000 NIR FT-Raman
instrument at room temperature using 800-1500 mW laser
power and 32 scans.
Differential Scanning Calorimetry (DSC) was performed
using a Mettler-Toledo DSC equipped with a sample robot and
a cryo-cooler and Mettler Toledo STARe software V9.2. The
heating and cooling rates were 10 K/min for all measurements.
All measurements were performed under N2-atmosphere. The
modified model compound were analysed in cycles of 0 ⁰C to
100 ⁰C, 100 ⁰C to -40 ⁰C to remove thermal history of the
sample, -40 ⁰C to 300 ⁰C and 300 ⁰C back to 0 ⁰C. The
crosslinked thiol-ene films were analysed by heating from 25
⁰C to 100 ⁰C to remove thermal history before being measured
in cycles between 100 ⁰C and -60 ⁰C
3
times.
Curing of thiol-ene films were done by using a Blak Ray® B-
100 AP (100 W, λ= 365 nm) high intensity UV lamp with an
irradiance of 21 mW/cm2 as a light source for 30 minutes.
Conversion as a Function of Time
The experimental procedure was the same as for the first
CAAE synthesis but in 2.77 mmol (≈0.5 g) scale.
Reduction of Coniferyl Aldehyde to Coniferyl Alcohol (CA)
Coniferyl alcohol (CA) was prepared from coniferyl aldehyde
based on procedures previously reported by Ralph et al.35, 36
0.7 ml of the solution was removed, dried using vacuum
1
and analysed by H-NMR. This was repeated during the course
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of the reaction to be able to determine the conversion for
each point in time.
Effect of Reaction Temperature
The experimental procedure was the same as for the first
CAAE synthesis but in 1.66 mmol (≈0.3 g) scale with respect to
everything but the solvent, 15 ml was used for practical
reasons.
The reaction proceeded for 6 hours before 0.7 ml was
1
removed, dried using vacuum and used for H-NMR analysis to
Scheme 2: A selective modification of coniferyl alcohol (CA) using allyl chloride to
form coniferyl alcohol allyl ether (CAAE).
determine the conversion. This was repeated when 24 hours
had past. The procedure was performed for the temperatures
25, 35, 45, 55, 65, 75 and 85 ⁰C to evaluate the effect of
temperature on the reaction rate. The NMR-tubes were cooled
with dry ice in acetone before each sample was extracted to
quickly reduce the reaction speed. Vacuum was applied to
remove the residual allyl chloride and ethanol and the dried
mixture were redissolved in DMSO before the NMR analysis
was performed.
atmospheric oxygen has no effect on the reaction. This would
also mimic the atmosphere in true technical processes.
Reaction conditions, including time and temperature
dependences, were studied for the synthesis. The synthesized
monomers thermal properties were also examined and
analysed for both characterization purposes and to determine
how the product can be handled for further functionalization
and utilization. The in house synthesis of the model
compound, CA, as described above, granted purity of the
reagent36 since coniferyl alcohol has an intrinsic instability in
the structure making it prone to oxidation to coniferyl
aldehyde when stored, also observed in our laboratory.
Thermoset Crosslinking through Thiol-Ene Coupling
CAAE, TMP and Irgacure 651 (≈3 wt%) was added to a glass vial
and mixed using ≈100 mg chloroform. The resin was spread
out on a glass substrate and chloroform left to evaporate for
10-15 minutes before the mixture was covered using a glass
cover. The resins were placed under UV-light for 30 minutes to
cure. The resulting films was removed from the glass and
retrieved for analysis. Three films were prepared with different
thiol/ene molar ratios with regards to reacting groups (+10
thiol, equimolar ratio, and -10% thiol). The cured films were
analysed by FT-IR, FT-Raman and DSC.
As a proof of concept, crosslinked thermoset films were
prepared through free radical thiol-ene crosslinking using TMP
as a trifunctional thiol functional crosslinker, Scheme 3. The
modified model compound would in this case act as a bi-
functional monomer where both the allyl ether as well as the
α-β double bond allow crosslinked thermosets to be made.
Synthesis of CAAE
Characterization and validation for the synthesis were carried
out by 1H-, 13C & 2D HSQC NMR, FT-IR, FT-Raman and GC-MS.
1H- & 13C-NMR
Result and Discussion
1H-NMR reveals the difference in structure between CA and
The purpose of this study was to establish a synthesis route to
selectively modify the phenol group in
a lignin model
the formed product, Figure 2. One can first notice that the
compound in the presence of primary alcohols and conjugated
alkenes, reaction scheme in Scheme 2. This will provide a tool
for modification of more complex lignin derivatives with an
overall aim to create new thermoset resin platforms. Coniferyl
alcohol was selected as a model since it is the main monolignol
in softwood7 and contain 3 important lignin functionalities; a
conjugated alkene, a phenol and an aliphatic hydroxyl. In
addition, it could be easily synthesized from the cheaply
available aldehyde. Allyl chloride was chosen for the
modification since it has suitable reactivity as will be discussed
later. In addition, it is a well-established chemical used for
allylation of aliphatic structures in an industrial scale37. Ethanol
was used as a solvent for several reasons: all reagents are
soluble; it is environmentally friendly and easy to handle.
Through solvent sequence fractionation has also been shown
that more than 40% of lignoboost Kraft lignin is soluble in
ethanol and render a fraction with a Ð of about 2, which is
peak corresponding to the phenol proton at
disappeared in the product while the primary alcohol triplet
peak at = 4.75 ppm is still present, only shifted to = 4.81
ppm. Three additional peaks of interest have appeared, one
double triplet at = 4.53 ppm, two double quadruplets at
5.24 ppm and = 5.38 ppm and one characteristic multiplet at
= 6.03 ppm. These are peaks assigned to the newly formed
" = 8.98 ppm has
"
"
"
" =
"
"
allyl ether. It is also worth noticing that the double bond
promising for both future academic work as well as for
industrial purposes7,
13
. Furthermore, the reaction was
Scheme 3: Thiol-ene crosslinking using TMP and CAAE
performed under ambient atmosphere to verify that
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5.24/118.02 ppm, and δ13H/δ13C = 5.38/117.92 ppm.
The purity and molecular weight of CAAE was further
analysed by GC-MS, details are found in the supporting
information.
FT-IR & Raman
FT-IR and FT-Raman measurements of the CA and CAAE
provide complementary data on different structural features.
Spectra of the two compounds are very useful both to confirm
the results of the present study but will also be of great value
when using the CAAE in further modifications. Functional
groups such as phenols and alcohols have strong signals in FT-
IR while peaks coupled to carbon-carbon double bonds are
stronger in the FT-Raman spectra. It is for example seen in the
FT-IR spectra, Figure 4, that the hydroxyl peak around 3300
cm-1 representing both the aliphatic alcohol and the phenol in
CA changes to a more well-defined, although broad, peak in
the spectrum for CAAE.
Figure 2 1H-NMR of CA and CAAE, water peak at " = 3.33ppm has been removed
from these spectra.
As can be seen in FT-Raman spectra, Figure 5, the
conjugated double bond between the α and β carbons is found
at 1657 cm-1 in both the reagent as well as in the product. It
can also be realised that a peak at 1651 cm-1 is present in the
product spectra but absent in the CA, indicating the addition of
a new allyl double bond. The peaks around 1600 cm-1
correspond to the aromatic vibrations of the rings.
between the α (" = 6.41 ppm) and β (" = 6.18) carbons are
preserved and not affected by the reaction conditions.
1H-NMR results suggests that a selective reaction occurs
without affecting the aliphatic hydroxyl group or the Cα – Cβ
double bond.
13C-NMR analysis was also performed for further
characterization and future reference, Figure 3. The spectra of
CA correspond very well with assignments found in literature
Conversion as a Function of Time
38. The product 13C-NMR reveal three newly formed peaks at
= 68.95ppm, = 117.45 ppm and = 133.87 ppm belonging to
"
Samples from the reaction mixture was retrieved at specific
intervals and analysed with NMR to determine the conversion
over time. The specimens were cooled, the solvent evaporated
and the dried residue re-dissolved in d6-DMSO before analysis.
This procedure results in samples where the phenols are
present in their deprotonated state. When CA is deprotonated
several peaks shifts in the 1H-NMR spectra. The peak
corresponding to the methoxy group at 3.78 ppm shifts
downwards compared to the protonated CA methoxy peak.
This allows for calculation of conversion through peak integrals
using the deprotonated CA methoxy peak and the methoxy
peak corresponding to the CAAE structure. For the same
purpose the peak at 4.07 ppm belonging to the two protons on
the γ carbon can be used, both methoxy as well as γ peaks are
"
the newly formed allyl ether strengthening the claim that a
"
selective allylation have occurred.
2D HSQC NMR was also performed to determine and verify
the structure of the product, spectrum is found in supporting
information Figure S1. The cross peak references for α, β, γ
and the aromatics are found at δαH/δαC = 6.48/128.91 ppm,
δβH/δβC = 6.27/129.07 ppm, δγH/δγC = 4.11/62.11 ppm, δ2H/δ2C
= 7.06/109.86 ppm, δ5H/δ5C = 6.89/113.87 ppm, δ6H/δ6C
6.89/119.48 ppm. The allyl group is found at δ10H/δ10C
4.52/69.57 ppm, δ11H/δ11C = 6.03/134.4 ppm, δ12H/δ12C
=
=
=
Figure 4 FT-IR in the range between 2600 and 3700 cm-1
.
Figure 3 13C-NMR of CA and CAAE.
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Figure 7: Conversion calculated from NMR integrals, reaction temperature 55
⁰C.
Figure 5 FT-Raman in the range between 1680 and 1570 cm-1
.
represented in Figure 6.
minimal. The chloride represent a suitable reactivity as a
leaving group in relation to the phenolate as a nucleophile.
The possibility to use ethanol as a solvent for these reactions
opens up new opportunities since ethanol is renewable,
abundant and exhibit very good solvent properties. The used
ethanol also contained 4% water demonstrating that water
can be present without having any significant effects on the
reaction. We can thus conclude that the reaction indeed is
very selective, relatively fast, and goes to almost quantitative
conversion under ambient atmosphere.
To give as accurate description as possible, the conversion
was calculated using both the peaks corresponding to the
methoxy groups as well as the γ-protons. The mean values are
presented in Figure 7 (and Table S1). It can be seen that the
conversion is well above 90 % after 24 hours which really
emphasis the selectivity of this reaction.
Effect of Reaction Temperature
In Figure 8, the conversion achieved after 6 and 24 hours were
calculated in the same way as above and shown for the
different reaction temperatures; 25, 35, 45, 55, 65, 75, and 85
⁰C. It can clearly be seen that temperature has a great
influence over the reaction rate. At 85 ⁰C however a slight
drop in conversion is observed compared to the lower
temperature. 85 ⁰C is well above the boiling point of both allyl
chloride and ethanol and this result in a possible loss of
reagent from the reaction vessel through the septum used. It
should however be noted that a conversion of +98% could be
obtained under optimal reaction conditions which is close to
the detection limit of residual CA in the system. Another
observation is that the reaction was performed in ethanol as a
solvent providing a high concentration of primary aliphatic
alcohols. The reactivity of allyl chloride towards aliphatic
alcohols under the present reaction conditions must thus be
Thermal Properties
Thermal analysis provides useful information on phase
transitions but also about the thermal stability of the different
chemical entities. It can be seen in Figure 9 that the allylation
reduced the melting temperature of from 76 ⁰C for CA to 60 ⁰C
for CAAE. The crystallization temperature also shifted from 53
to 14 ⁰C. An exotherm can furthermore be observed for the
allylated species with a minimum at 233 ⁰C suggesting a
reaction occurring for CAAE. This analysis also provide practical
information about synthesis and processing limitations since
the newly formed monomer is shown to be stable up to at
least 160 ⁰C under inert conditions.
2D HSQC NMR analyses were performed on CAAE before
and after heating to 300 ⁰C in the DSC, Figure S1 & S2. The
Figure 6 Utilizing integral calculation of the peaks the conversion can be
Figure 8 Conversion vs Temperature for 6 & 24 hour reactions.
achieved. Here, 55 ⁰C after 6 hours.
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Figure 10: FT-Raman spectra of uncured CAAE and TMP mixture and a crosslinked film.
Thiol peak (2578 cm-1) and ene (1655 cm-1) peak has decreased and disappeared during
curing.
Figure 9 DSC thermograms of the second heating scan. Samples heated from -30
to 300⁰C. Both CAAE and CA are present in the graph for comparison.
the dominating crosslinking reaction for this monomer and
that full conversion of the both the double bonds and the
thiols have been achieved.
samples retrieved from the DSC were difficult to dissolve in
DMSO however some useful information was obtained.
Proton-carbon correlation of the allyl ether can be identified
and found in both the monomer as well as in the thermally
formed product. The signals have however shifted suggesting
that the thermal treatment affected the allyl group. One other
thing easily determined is that the H-C5 correlation of the
aromatic ring is no longer present suggesting that a Claisen re-
arrangement have occurred as a main reaction. During the
Claisen re-arrangement the phenolic group is reformed, now
with an allyl group ortho to the phenol21, mechanism
represented in Figure S3. Depending on application and
material preferences it is thus crucial to have an understanding
of this aspect when this compound is subjected to elevated
temperatures. Further extensive work is needed to find the full
mechanisms of these thermally driven reactions as well as the
complete structure of the formed products including the
insoluble residues.
The thermal properties of the material was determined
using DSC. The Tg of the crosslinked film with an equimolar
ratio thiol/alkene was determined to 17 °C upon heating and
13 °C upon cooling. No crystallinity was observed in the
thermograms. The relatively low observed Tg is due to the
flexible thio ether bonds formed during curing which is a
wellknown feature for thiol-ene systems42. Aromatics on the
other hand should increase the Tg transition suggesting that
larger lignin structures would give higher Tg´s in this type of
systems8. The off stoichiometry based thermoset films
exhibited slightly lower Tg:s (around 13 °C upon heating)
indicating the presence of non-reacted excess functional
groups. This is also in accordance with the statement that the
thiol-ene reaction is dominating in this system.The plotted DSC
curves can be found in Figures S7-S9.
Conclusions
Crosslinking through Free Radical Thiol-Ene Coupling
The free radical thiol-ene reaction is a versatile synthesis
The present paper demonstrates a versatile route for selective
modification of lignin phenols to form aryl allyl ethers. It is
shown that a selective modification of the phenol in coniferyl
alcohol to form coniferyl alcohol allyl ether can be performed
in quantitative yields, using allyl chloride in the presence of
aliphatic alcohols, alkenes, and water under aerobic
conditions. Renewable, environmentally friendly protic
solvents, such as ethanol, can be used without affecting the
reaction. It was also determined that the reaction rate is
increased when the temperature is increased without reduced
selectivity of the reaction.
route towards crosslinking of
a wide variety of alkene
monomers.39 The well documented thiol-ene reaction is one
straight forward way to show the practicality of the
modification and could be an interesting subject for lignin
utilization in materials in the future40, 41. Crosslinked films
using CAAE were made using TMP as a trifunctional thiol
crosslinker in either equimolar thiol/alkene or ±10% thiol ratio.
The thermoset films were analysed using FT-Raman, FT-IR and
DSC to determine the final structure as well as the thermal
properties of the material.
In FT-Raman spectra, Figure 10, one can clearly see the
disappearance of the thiol peak around 2578 cm-1 as well as
the ene peaks around 1655 cm-1. Only very small differences in
the FTIR or FT-Raman spectra were seen on the samples
having either 10% thiol or ene excess making a quantitative
analysis of residual thiols or alkenes impossible. The FTIR and
FT-Raman spectra thus confirm that the thiol-ene reaction is
It was furthermore demonstrated that the formed product,
CAAE, is stable up to at least 160 ⁰C, in inert conditions, thus
allowing for further chemical transformations to be made.
Claisen re-arrangement and other reactions start to occur at
temperatures
above
175
⁰C.
In addition to the selective modification it was also proven
that CAAE can be used as a monomer in thermosets using free
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I. Volf, I. Ignat, M. Neamtu and V. I. Popa, Chemical
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L. Zoia, A. Salanti, P. Frigerio and M. Orlandi, BioResources
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Thermoset films with a full conversion of the double bonds
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Acknowledgements
The authors are grateful to the Knut and Alice Wallenberg
2014, 9, 6540-6561.
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8 | J. Name., 2012, 00, 1-3
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Highly selective functionalization of a lignin model compound to form thermoset resins under benign
conditions.