Synthesis of Lignin-based Phenol Terminated Hyperbranched Polymer

Article abstract of DOI:10.3390/molecules24203717

In this work, we proved the efficient synthesis of a bio-based hyper-branched polyphenol from a modified lignin degradation fragment. Protocatechuic acid was readily obtained from vanillin, a lignin degradation product, via alkaline conditions, and further polymerised to yield high molecular weight hyperbranched phenol terminated polyesters. Vanillic acid was also subjected to similar polymerisation conditions in order to compare polymerisation kinetics and differences between linear and hyperbranched polymers. Overall, protocatechuic acid was faster to polymerise and more thermostable with a degradation temperature well above linear vanillic acid polyester. Both polymers exhibited important radical scavenging activity (RSA) compared to commercial antioxidant and present tremendous potential for antioxidant applications.

Full text of DOI:10.3390/molecules24203717

molecules
Article
Synthesis of Lignin-based Phenol Terminated
Hyperbranched Polymer
Lionel Longe 1, Gil Garnier 2,* and Kei Saito 1,
*
1
School of Chemistry, Bioresource Processing Research Institute of Australia (BioPRIA), Monash University,
Clayton 3800, Australia; lionel.longe@monash.edu
Department of Chemical Engineering, Bioresource Processing Research Institute of Australia (BioPRIA),
Monash University Clayton 3800, Australia
2
*
Correspondence: gil.garnier@monash.edu (G.G.); kei.saito@monash.edu (K.S.); Tel.: +61-3-9905-4007 (G.G.);
+61-3-9905-9180 (K.S.); Fax: +61-3-9905-4597 (G.G.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Jalel Labidi
Received: 5 July 2019; Accepted: 11 October 2019; Published: 16 October 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: In this work, we proved the efficient synthesis of a bio-based hyper-branched polyphenol
from a modified lignin degradation fragment. Protocatechuic acid was readily obtained from vanillin,
a lignin degradation product, via alkaline conditions, and further polymerised to yield high molecular
weight hyperbranched phenol terminated polyesters. Vanillic acid was also subjected to similar
polymerisation conditions in order to compare polymerisation kinetics and differences between
linear and hyperbranched polymers. Overall, protocatechuic acid was faster to polymerise and more
thermostable with a degradation temperature well above linear vanillic acid polyester. Both polymers
exhibited important radical scavenging activity (RSA) compared to commercial antioxidant and
present tremendous potential for antioxidant applications.
Keywords: bio-based; hyperbranched; polyphenol; antioxidant
1. Introduction
Hyperbranched polymers (HBP) have received increasing attention over the past two decades, as
their field of the application keeps expanding every year [1,2]. The hyperbranched polymer structure
is very similar to dendrimers with a high amount of end group, often used for post functionalisation or
grafting reactions. However, HBP are often cheaper and easier to synthesise. For these reasons HBP
have replaced dendrimers in many applications, and are now commonly used in various fields of
application, including therapeutics, drug delivery, coatings, and as rheological or curing agents [
Recently, interest in the synthesis of the bio-based hyperbranched polymers has quickly grown as the
logical consequence of the success of green chemistry in the polymer field [ 14]. However, to our
3–6].
7–
knowledge, synthesis from lignin degradation products feedstock has yet to be reported.
Polyphenols are well known for their antioxidant properties. The synthesis of efficient
bio-based polymers with radical scavenging properties has seen a growing interest in the past
few years [15,16]. The most common strategy involves grafting a highly antioxidant molecule on a
polymer backbone [17,18]. Phenol-terminated molecules, such as caffeic acid, are often investigated.
To increase the phenol content, hyperbranched polymers and dendrimers are also starting to draw
attention and should drastically improve the final radical scavenging activity [19–23].
Molecules 2019, 24, 3717; doi:10.3390/molecules24203717
www.mdpi.com/journal/molecules

Molecules 2019, 24, 3717
2 of 9
In this work, we present the monomer preparation and synthesis of a phenol-terminated
hyperbranched polymer. High antioxidant properties are expected, and radical scavenging activity
will be monitored to determine the efficacy of the synthesised polymer. Biomass feedstock will be
used for the monomer synthesis in order to create a bio-renewable polymer. Vanillin can be readily
obtained from lignin degradation and already has well established in industrial production and
commercial markets [24–27]. In strong caustic conditions, vanillin will yield protocatechuic acid
(PA) in high yield and high purity. We report here the synthesis of a hyperbranched polymer by
polycondensation of PA. Furthermore, similar conditions were employed with vanillic acid (VA) to
compare final polymer properties.
2. Results and Discussion
PA was readily obtained by caustic fusion of vanillin (Figure 1). Reaction temperature determines
whether demethylation occurs in addition to the oxidation of the aldehyde group. With a temperature
below 250 ◦C, vanillic acid (VA) is formed, while a higher temperature yields PA. Purity is highly
dependent on the mixing method and homogeneous heating. The reaction is total, and protocatechuic
acid and vanillic acid are obtained in high yield, 94% and 99%+ respectively.
Figure 1. Synthesis of (a) vanillic acid (VA) and (b) protocatechuic acid (PA) from vanillin.
After isolation and purification, PA and VA were polymerised by polycondensation using Steglich
esterification reaction in dimethyl formamide (DMF) (Figure 2). The Steglich esterification reaction is
well known for the associated intensive work-up to remove dicyclohexylurea (DHU), a side product
from the acid activation by N,N’-dicyclohexylcarbodiimide (DCC). In the present case, DCC was
efficiently removed by Soxhlet extraction in acetone. Gel permeation chromatography (GPC) analysis
only indicated a small difference between PA and VA-polymers. Overall, PA-polymer reached slightly
higher molecular weight than VA and, correspondingly moderately faster polymerisation kinetics
(Table 1). The presence of the methoxy group in VA is expected to reduce the phenol reactivity, due to
steric hindrance and can be the cause of that difference. However, GPC equipment was calibrated
versus linear polystyrene standards, and branched structures are known to reduce the accuracy of
molecular weight determination [28]. Therefore, no major difference in polymerisation behaviour is to
be noted at this stage.

Molecules 2019, 24, 3717
3 of 9
Figure 2. Polycondensation of (
Steglich esterification.
a) protocatechuic acid (PA) and (b) vanillic acid (VA) by
Table 1. Physical properties of polymers from protocatechuic acid and vanillic acid.
Mn (g/mol)
Đ
Tonset
Td5
PA-based polymer
VA-based polymer
7900
6400
3.1
2.6
180 ◦C
118 ◦C
164 ◦C
108 ◦C
The two polymers thermostability was compared by thermogravimetric analysis (TGA). A clear
difference between the two 5% weight loss temperature (Td5) was observed, with PA-based polymer
proving to be stable up to 164 ◦C, while VA-polymer started to deteriorate above 108 ◦C (Figure 3
and Table 1).
Figure 3. Thermogravimetric analysis of the decomposition of both PA and VA-based polymer.
Analysis by differential scanning calorimetry (DSC) of PA and VA-polymer samples could not
point out any clear glass transition temperature in the analysis range (
70 ◦C to 145 ◦C and 90 ◦C
−

Molecules 2019, 24, 3717
4 of 9
respectively), presumably due to the hyperbranched structure of PA-polymer and the low degree of
polymerisation of VA-polymer.
The overall increase in polymer molecular weight proved to be slow, possibly due to the low
reactivity of phenols group. This issue can be partially circumvented by increasing the polymerisation
temperature. High boiling point polar solvent proved to be best suited for this reaction. Polymerisation
in dimethyl sulfoxide (DMSO) proved to be more efficient than DMF as it proved to be a better solvent
for the polymer and allowed stronger heating. All other conditions were kept constant (Figure 2).
Even in DMSO, the increase of the polymers molecular weight proved to be slow. Nevertheless, with
long reaction time, ultra-high molecular weight polymers could be obtained (Figure 4). Samples
1
with Mn of up to 2,000,000 g.mol−1 were successfully isolated. However, H NMR analysis of the
synthesised polymer evidence a side reaction, due to interaction with the solvent (Figure S2). Moffat
and Burdon previously reported that DCC and DMSO could react in the presence of acid catalyst—also
known as Pfitzner-Moffatt conditions—and react with phenolic compounds [29]. Several compounds
can be created from this reaction. The main reported behaviour was the alkylation of the phenols
in the available ortho positions with thiomethoxymethyl groups. Binding of dicyclohexylurea to the
molecules was also reported in the same position. Formation of cyclic hemithioacetal structures was
also reported. In 13C NMR analysis, the signal in the
DHU molecules to the polymer structure.
δ = 25–50 ppm clearly indicated the binding of
Figure 4. Evolution of polymers molecular weight during the polymerisation of protocatechuic acid and
vanillic acid by Steglich esterification in DMSO, measured by gel permeation chromatography (GPC).
The formation of methylthiomethyl esters and N-acylureas by the reaction of the carboxylic acid
group has been reported in the literature [30] (Figure S1). However, considering the overall molecular
weight of the final polymer sample, this reaction either (a) was not favourable or (b) was reversible in
a way that does not prevent polycondensation. The latter case could, however, explain the overall
slow kinetics of the polymerisation. Nevertheless, due to the high polymerisation degree, this reaction
cannot be detected by NMR analysis.
The thermal stability of both polymers synthesised in DMSO was analysed by thermogravimetric
analysis (Figure 5). Polymerisation in DMSO did not change the degradation temperature for
VA-polymer, but remaining ash content greatly increased from 10% to 50%. Similarly, degradation
of PA-polymer now also yields about 50% of the mass of ashes. The degradation temperature for
◦
PA-polymer, however, now greatly increased, and is now thermostable to up to 312 C, probably
partially, due to the 200-fold increase in molecular weight. During differential scanning calorimetry
(DSC) analysis over a large range of temperatures (
temperature could be detected. Glass transition temperature of branched polymers can be hard to
detect using DSC so other detection methods should be used in future work.
−
70 ◦C to Td5 20 ◦C) no clear glass transition
−

Molecules 2019, 24, 3717
5 of 9
Figure 5. Thermogravimetric analysis of polymer from protocatechuic acid and vanillic acid.
Due to the presence of the phenolic group, antioxidant properties are expected from both polymers.
The radical scavenging activity is measured by the determination of the amount of substrate that
will bleach half the amount of a stable radical compound, 2,2-diphenyl-1-picrylhydrazyl (DPPH).
The procedure is adapted from the recommendation of Kedare et al. [31] (Figure 6). As expected, on
account of the higher amount of phenol group, due to its hyperbranched structure, PA-based polymer is
almost twice as efficient as VA-polymer, with an EC50 (efficient concentration) of 0.08 mg/mol (Table 2).
Antioxidant bio-based polymer represents a high interest in research and for industrial application.
By grafting an increasing amount of phenol on the polymer backbone, the antiradical efficacy can be
greatly improved (Table 2). Chitosan, a linear biopolymer with no phenol group is often studied as a
polymer backbone for grafting antiradical compounds to create bio-based antioxidant polymers, and
was compared here to linear VA-polymer. PA-polymer RSA was also compared to a bio-based polymer
with similar structure (Figure S4). Chitosan grafted with molecules with multiple phenol group (caffeic
acid), or dendritic structure (chitosan graft tannic acid, dendritic phenol) were selected for comparison.
Additionally, RSA of two widely used industrial antioxidant (Irganox® 1010 and Irganox® 1098) are
also reported and provide a benchmark for commercial antioxidant. Overall, our HBP have higher
antiradical efficacy than all other biopolymers, but one, and is also two to four time more efficient than
the commercial non bio-based options.
Figure 6. Inhibition percentage of (
a) PA-polymer and (b) VA-polymer versus concentration. This was
measured by bleaching of DPPH at 515 nm during radical scavenging assays.

Molecules 2019, 24, 3717
6 of 9
Table 2. Antioxidant properties of synthesised polymers compared to literature and commercially
available antioxidant.
EC50 (mg/mL)
Antiradical Efficacy (mmol) a
PA-polymer
VA-polymer
0.08
0.22
0.76
0.30
Chitosan
/
/
/
/
/
/
n.d. [17]
0.16 [17]
5.8 [18]
0.36 [19]
0.18 [32]
0.36 [19]
Chitosan graft caffeic acid
Chitosan graft tannic acid
Dendritic phenol
Irganox® 1010
Irganox® 1098
a: DPPH equivalents per gram of material; n.d.: not detected.
3. Materials and Methods
Chemicals and solvents: Chemicals were purchased from Sigma Aldrich (Sydney, NSW, Australia)
(vanillin, 4-dimethylaminopyridine (DMAP), N,N’-Dicyclohexylcarbodiimide (DCC), lithium bromide),
from Merck PTY (Melbourne, VIC, Australia) (Potassium hydroxide, anhydrous manganese sulphate)
and from TCI (Tokyo, Japan) (deuterated dimethyl sulfoxide or DMSO) and used as received. Reagent
grade solvents (anhydrous dimethylformamide or DMF, diethyl ether, hydrochloric acid 37% in water,
Tetrahydrofuran or THF) were purchased from Thermo Fisher Scientific (Melbourne, VIC, Australia).
The molecular weight of polymers samples was analysed by size exclusion chromatography (SEC)
in DMF or water. SEC in DMF was performed at 40 ◦C on a Tosoh EcosHLC-8320 equipped with
double detectors (RI, UV 280 nm) using Tosoh alpha 4000 and 2000 columns. DMF/LiBr 0.1 M was
used as a mobile phase (flow rate of 1 mL/min). Polystyrene standards were used for the calibration.
1H NMR and 13C qNMR experiments were performed in deuterated DMSO on a Bruker Avance
400 MHz NMR spectrometer equipped with a CryoProbe Prodigy 5 mm 1H/1H-13C/15N probe. NMR
experiments were performed with the sample held at 25
±
0.1 ◦C. Chemical shifts for all experiments
are referenced using the Unified Scale relative to 0.3% tetramethylsilane in deuterated chloroform.
Radical scavenging activity was measured through the measurement of the bleaching of DPPH at
515 nm by UV-visible spectroscopy. A fresh solution of DPPH at 2.65 mg/mL in methanol is prepared
and kept away from light in the fridge. The solutions of the substrate in DMSO are also prepared at a
known concentration. In a test tube, 60 µL of the DPPH solution is added with a known volume of
the substrate solution. The volume is then adjusted to 3 mL with DMSO. The test tube is then sealed
closed and kept away from light to react for 3 h. After that time, the absorption of the solution at
λ = 315 nm is measured. The absorption of the substrate alone is also measured to subtract its effect on
the measurement. This is repeated as much as needed with different volumes of the substrate solution
in order to gather data before and after the total quenching of DPPH. The residual absorption of totally
quenched DPPH is also measured for more accurate results. Ascorbic acid was selected as standard,
and its antiradical efficacy was also measured and compared to literature to confirm measurements
(Figure S3, Table S1) [16]. UV measurements were performed on an Agilent Cary 60 UV-Vis cell reader
spectrophotometer in quartz cells.
Degradation temperatures were measured by thermogravimetric analysis (TGA) performed on a
Simultaneous Thermal Analyser (STA) 8000 from PerkinElmer. The heating rate was fixed to 30 ◦C/min
under nitrogen flow.
Glass transition temperatures were measured by differential scanning calorimetry (DSC) on a
DSC 8000 from PerkinElmer fitted with a dual-stage heat-exchanger cooling system Intracooler 2.
Samples were analysed in aluminium pans and under nitrogen atmosphere. The heating rate was set
to 10 ◦C/min and cooling rate 150 ◦C/min. Scanning ranges were set between 70 ◦C and up to the
−
polymer degradation temperature, determined by TGA, minus 20 ◦C.

Molecules 2019, 24, 3717
7 of 9
3.1. PA Synthesis
Protocatechuic acid was obtained by the caustic fusion of vanillin. In a nickel crucible potassium
hydroxide (33 g, 0.588 mol) is added with a small volume of water (4 mL). A crucible is placed in
an oil bath, and an overhead mechanical stirrer is installed. Heating is set to 260 ◦C and stirring is
set to ca. 100 rpm. After a few minutes, the salt mixture becomes a homogeneous viscous mixture.
Vanillin (12 g, 0.079 mol) is then carefully added into the solution. After 45 min, the reaction is stopped
and allowed to cool down. The reaction mixture is dissolved in water (200 mL), and the product is
then recovered by acidification/precipitation with hydrochloric acid. Filtration in a Buchner funnel
yields 9.5 g of protocatechuic acid (0.061 mol). The aqueous phase is further extracted three times with
diethyl ether (50 mL). The organic phase in then dried over anhydrous MgSO4 and evaporated under
1
reduced pressure to recover an additional 2.1 g of protocatechuic acid (0.013 mol, 94% yield). H-NMR
(400 MHz, d6-DMSO): δ 7.32 (s, H, ArH), δ 7.27 (d, 1H, ArH), δ 6.79 (d, 1H, ArH).
3.2. VA Synthesis
Synthesis of vanillic acid is achieved by an identical procedure, only maintaining reaction
temperature at 150 ◦C. Potassium hydroxide (33 g, 0.588 mol) and water (4 mL) mixture are brought to
a gel in the nickel crucible by maintaining heating at 150 ◦C and constant stirring ca. 100 rpm. Vanillin
(12 g, 0.079 mol) is then added, and left to react for 45 min. After allowed to cool down, the mixture is
dissolved in water (200 mL), and vanillic acid is crushed out by acidification with hydrochloric acid.
10.3 g of vanillic acid can be recovered (0.067 mol). Extraction of the aqueous phase with diethyl ether
1
further yields 1.9 g of vanillic acid (0.012 mol, 99%+ yield). H-NMR (400 MHz, d6-DMSO):
(s, 1H, ArH), δ 7.43 (d, 1H, ArH), δ 6.84 (d, 1H, ArH), δ 3.92(s, 3H, CH3).
δ 7.44
3.3. Polymerisation of VA and PA
Vanillic acid and protocatechuic acid are polymerised by polycondensation using Steglich
esterification reaction. The phenol compound (3 mmol) is added to a 50 mL round bottom flask and
dissolved in 30 mL of solvent (DMF or DMSO). 3.6 mg of DMAP (0.03 mmol) and 680 mg of DCC
(3.3 mmol) are further added to the flask. Magnetic stirring is set up ca. 300 rpm until complete
homogenisation of the solution. The round bottom flask is then fitted with a condenser, and the
reaction is allowed to carry on in reflux condition. After the reaction, the mixture is left to cool down
and is then filtered to remove insoluble side product dicyclohexylurea. Complete removal of DHU is
reached by overnight Soxhlet extraction with acetone. The soluble fraction is then dissolved in DMSO
and added dropwise to a large volume of THF to crush out high molecular weight fractions.
4. Conclusions
In this work, we reported the synthesis and polymerisation of a bio-based monomer from lignin,
PA. Its AB2 configuration was compared to another monomer with simple AB structure, VA. During
the polymerisation study side reaction with the solvent was detected. However, in those conditions,
the final synthesised polymer gained better thermal properties and was able to reach a high degree of
polymerisation. Both polymers exhibited high antioxidant properties, due to the presence of phenolic
groups in the structure. NMR spectroscopy was used to characterise the different side reactions
and their effect on the polymer structure. In future work, we ought to improve the PA-polymer
characterisation, focussing on the structure (degree of branching) and physico-mechanical properties.
This polymer is expected to have a great impact as a 100% bio-based polymer with antioxidant
properties. Applications are expected for paint, packaging, emulsion, formulation or drug delivery
application. In addition, the phenol terminated structure allows easy post-modification and will be
considered in future work to create further applications for this polymer.

Molecules 2019, 24, 3717
8 of 9
Supplementary Materials: The following are available online: DPPH analysis of ascorbic acid standard and
NMRs of the synthesised polymer are available in supporting information.
Author Contributions: L.L. carried out all the experiment and data analysis and wrote the draft manuscript. G.G.
and K.S. devised the project and supervised L.L. and reviewed and edited the final manuscript.
Funding: The financial support of the ARC Industrial Transformation Research Hub – Bioprocessing Advanced
Manufacturing Initiative (BAMI) and Monash University is gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
Nabae, Y.; Kakimoto, M.A. Design and synthesis of hyperbranched aromatic polymers for catalysis. Polymers
2018, 12, 1344. [CrossRef] [PubMed]
Sun, F.; Luo, X.; Kang, L.; Peng, X.; Lu, C. Synthesis of hyperbranched polymers and their applications in
analytical chemistry. Polym. Chem. 2015, 6, 1214–1225. [CrossRef]
Yates, C.R.; Hayes, W. Synthesis and applications of hyperbranched polymers. Eur. Polym. J. 2004
40, 1257–1281. [CrossRef]
,
Jeon, I.-Y.; Noh, H.-J.; Baek, J.-B. Hyperbranched macromolecules: From synthesis to applications. Molecules
2018, 23, 657. [CrossRef]
Rimondino, G.N.; Oksdath-Mansilla, G.; Brunetti, V.; Strumia, M.C. More than just size: Challenges and
opportunities of hybrid dendritic nanocarriers. Curr. Pharm. Des. 2017, 23, 3142–3153. [CrossRef]
Zheng, Y.; Li, S.; Wenga, Z.; Gao, C. Hyperbranched polymers: Advances from synthesis to applications.
Chem. Soc. Rev. 2015, 44, 4091–4130. [CrossRef]
Testud, B.; Pintori, D.; Grau, E.; Taton, D.; Cramail, H. Hyperbranched polyesters by polycondensation of
fatty acid-based ABn-type monomers. Green Chem. 2017, 19, 259–269. [CrossRef]
Pramanik, S.; Hazarika, J.; Kumar, A.; Karak, N. Castor oil based hyperbranched poly(ester amide)/polyaniline
nanofiber nanocomposites as antistatic materials. Ind. Eng. Chem. Res. 2013, 52, 5700–5707. [CrossRef]
Thakur, S.; Karak, N. Ultratough, ductile, castor oil-based, hyperbranched, polyurethane nanocomposite
using functionalized reduced graphene oxide. ACS Sustainable Chem. Eng. 2014, 2, 1195–1202. [CrossRef]
10. Zhang, T.; Howell, B.A.; Dumitrascu, A.; Martin, S.J.; Smith, P.B. Synthesis and characterization of
glycerol-adipic acid hyperbranched polyesters. Polymer 2014, 55, 5065–5072. [CrossRef]
11. Huang, Y.; Wang, D.; Zhu, X.; Yana, D.; Chen, R. Synthesis and therapeutic applications of biocompatible or
biodegradable hyperbranched polymers. Polym. Chem. 2015, 6, 2794–2812. [CrossRef]
12. Žagara, E.; Žigon, M. Aliphatic hyperbranched polyesters based on 2,2-bis(methylol)propionic
acid—Determination of structure, solution and bulk properties. Prog. Polym. Sci. 2011, 36, 53–88.
[CrossRef]
13. Bayan, R.; Karak, N. Renewable resource modified polyol derived aliphatic hyperbranched polyurethane as
a biodegradable and UV-resistant smart material. Polym. Int. 2017, 66, 839–850. [CrossRef]
14. Abbina, S.; Vappala, S.; Kumar, P.; Siren, E.M.J.; La, C.C.; Abbasi, U.; Brooks, D.E.; Kizhakkedathu, J.N.
Hyperbranched polyglycerols: Recent advances in synthesis, biocompatibility and biomedical applications.
J. Mater. Chem. B 2017, 5, 9249–9277. [CrossRef]
15. Pu, S.; Li, J.; Sun, L.; Zhong, L.; Ma, Q. An in vitro comparison of the antioxidant activities of chitosan and
green synthesized gold nanoparticles. Carbohydr. Polym. 2019, 211, 161–172. [CrossRef] [PubMed]
16. Omidi, S.; Kakanejadifard, A. Modification of chitosan and chitosan nanoparticle by long chain pyridinium
compounds: Synthesis, characterization, antibacterial, and antioxidant activities. Carbohydr. Polym. 2019
15, 477–485. [CrossRef] [PubMed]
,
˙
17. Ilyasog˘lu, H.; Nadzieja, M.; Guo, Z. Caffeic acid grafted chitosan as a novel dual-functional stabilizer for
food-grade emulsions and additive antioxidant property. Food Hydrocoll. 2019, 95, 168–176. [CrossRef]
18. Jing, Y.; Diao, Y.; Yu, X. Free radical-mediated conjugation of chitosan with tannic acid: Characterization and
antioxidant capacity. React. Funct. Polym. 2019, 135, 16–22. [CrossRef]
19. Li, C.; Wei, Y.; Shi, W.; Wang, J.; Wang, B. Antioxidant capacity and kinetics of dendritic hindered phenols
using DPPH assay. Prog. React. Kinet. Mec. 2015, 40, 279–290. [CrossRef]

Molecules 2019, 24, 3717
9 of 9
20. Wei, L.; Li, Q.; Chen, Y.; Zhang, J.; Mi, Y.; Dong, F.; Guo, Z. Enhanced antioxidant and antifungal activity
of chitosan derivatives bearing 6-O-imidazole-based quaternary ammonium salts. Carbohydr. Polym. 2019
206, 493–503. [CrossRef]
,
21. Wang, S.; Kaneko, D.; Kan, K.; Jin, X.; Kaneko, T. Syntheses of hyperbranched liquid-crystalline biopolymers
with strong adhesion from phenolic phytomonomers. Pure Appl. Chem. 2012, 84, 2559–2568. [CrossRef]
22. Wang, S.; Tateyama, S.; Kaneko, D.; Ohki, S.Y.; Kaneko, T. Synthesis of well-defined hyperbranched
polymers bio-based on multifunctional phenolic acids and their structure–thermal property relationships.
Polym. Degrad. Stab. 2011, 96, 2048–2054. [CrossRef]
23. Kaneko, D.; Kinugawa, S.; Matsumoto, K.; Kaneko, T. Terminally-catecholized hyper-branched polymers
with high performance adhesive characteristics. Plant Biotechnol. 2010, 27, 293–296. [CrossRef]
24. Hocking, M.B. Vanillin: Synthetic flavoring from spent sulfite liquor. J. Chem. Educ. 1997, 74, 1055–1059.
[CrossRef]
25. Fargues, C.; Mathias,
Á.; Rodrigues, A. Kinetics of vanillin production from kraft lignin oxidation. Ind. Eng.
Chem. Res. 1996, 35, 28–36. [CrossRef]
26. Collis, B.C. Manufacture of vanillin from lignin. U.S. Patent No. 2692291, 1954.
27. Schulz, L. Manufacture of vanillin. U.S. Patent No. 2187366, 1940.
28. Gaborieau, M.; Castignolles, P. Size-exclusion chromatography (SEC) of branched polymers and
polysaccharides. Anal. Bioanal. Chem. 2011, 4, 1413–1423. [CrossRef]
29. Burdon, M.G.; Moffatt, J.G. Sulfoxide-carbodiimide reactions. IV. acid-catalyzed reactions of phenols with
sulfoxides and carbodiimides. J. Am. Chem. Soc. 1966, 88, 5855–8564. [CrossRef]
30. Lerch, U.; Moffatt, J.G. Carbodiimide-sulfoxide reactions. XIII. Reactions of amines and hydrazine derivatives.
J. Org. Chem. 1971, 36, 3861–3869.
31. Kedare, S.B.; Singh, R.P. Genesis and development of DPPH method of antioxidant assay. J. Food Sci. Technol.
2011, 48, 412–422. [CrossRef] [PubMed]
32. Avelino, F.; Silva, K.T.; Mazzetto, S.E.; Lomonaco, D. Tailor-made organosolv lignins from coconut wastes:
Effects of green solvents in microwave-assisted processes upon their structure and antioxidant activities.
Bioresour. Technol. Repor. 2019, 7. [CrossRef]
Sample Availability: Samples of the compounds PA, VA, PA-polymer and VA-polymer both polymerised in
DMSO are available from the authors.
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).