Biomass- And calcium carbide-based recyclable polymers

Article abstract of DOI:10.1039/d0gc04170j

Biomass is a renewable source of valuable feedstock for the chemical industry of the future. A promising approach to the utilization of valuable components of biomass is the synthesis of monomers and polymers, if the overall technology is designed for a clean cycle without pollution of the environment with newly created polymers. In this work, we have developed a methodology for the recycling of polymers based on biomass and calcium carbide. First, we modified a series of biomass-derived terpene alcohols with calcium carbide followed by polymerization of the isolated vinyl ethers. Then, to study the recycling potential, the obtained polymers were subjected to pyrolysis at moderate temperatures (200-450 °C). The pyrolysis products were analyzed using TGA-MS, GC-MS, and NMR, and it was found that the polymers can be transformed quite easily. The products of the pyrolysis consisted of the starting terpenols, as well as the corresponding non-toxic ketones or aldehydes: up to 87% of the starting alcohol or up to 100% of the total sum of alcohol + aldehyde or alcohol + ketone (GC-yields). Then, the reaction mixture was hydrogenated and resulted in the formation of starting alcohol only. According to the studied pathway of polymers re-building, a terpene fragment attached to the main polyethylene chain through an oxygen atom promotes the transformation of the obtained polymers. Thus, the products of pyrolysis are environmentally friendly and can be reused in the further synthesis of monomers. The developed system has shown a unique assembling/disassembling ability and advances the concept of reusable bio-derived high value-added materials.

Full text of DOI:10.1039/d0gc04170j

Green Chemistry
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PAPER
Biomass- and calcium carbide-based recyclable
polymers†
Cite this: Green Chem., 2021, 23,
2487
Svetlana A. Metlyaeva, a Konstantin S. Rodygin,
*
a,b Kristina A. Lotsman,
a
Dmitriy E. Samoylenko a and Valentine P. Ananikov
*
a,b
Biomass is a renewable source of valuable feedstock for the chemical industry of the future. A promising
approach to the utilization of valuable components of biomass is the synthesis of monomers and poly-
mers, if the overall technology is designed for a clean cycle without pollution of the environment with
newly created polymers. In this work, we have developed a methodology for the recycling of polymers
based on biomass and calcium carbide. First, we modified a series of biomass-derived terpene alcohols
with calcium carbide followed by polymerization of the isolated vinyl ethers. Then, to study the recycling
potential, the obtained polymers were subjected to pyrolysis at moderate temperatures (200–450 °C).
The pyrolysis products were analyzed using TGA-MS, GC-MS, and NMR, and it was found that the poly-
mers can be transformed quite easily. The products of the pyrolysis consisted of the starting terpenols, as
well as the corresponding non-toxic ketones or aldehydes: up to 87% of the starting alcohol or up to
100% of the total sum of alcohol + aldehyde or alcohol + ketone (GC-yields). Then, the reaction mixture
was hydrogenated and resulted in the formation of starting alcohol only. According to the studied
pathway of polymers re-building, a terpene fragment attached to the main polyethylene chain through an
oxygen atom promotes the transformation of the obtained polymers. Thus, the products of pyrolysis are
environmentally friendly and can be reused in the further synthesis of monomers. The developed system
has shown a unique assembling/disassembling ability and advances the concept of reusable bio-derived
high value-added materials.
Received 9th December 2020,
Accepted 18th February 2021
DOI: 10.1039/d0gc04170j
rsc.li/greenchem
the concept of a new generation of materials for the chemical
industry of the future.42–45
Introduction
Biomass is a sustainable and renewable source of essential
Although the monomers can be obtained from a renewable
monomers.1–15 Naturally derived monomers may already source, it is well recognized that polymers synthesized from
include polymerizable cycles16 or the CvC double bond, such unsaturated substrates have a common fundamental limit-
as eugenol,17–19 pinene,20 myrcene,21–25 limonene,26–28 and ation. The main polyethylene chain remains difficult to trans-
many others.29,30 Pagliaro and co-workers have reported a form or cleave, and polymers can be hardly disassembled and
powerful approach to access polycarbonates of exceptional re-built under simple conditions. Thus, a non-practical dispo-
chemical and physical properties from limonene and carbon sal is the main way to utilize used polymers at the end–of-life
dioxide using a ring-opening copolymerization reaction.31–34 stage (Fig. 1A), and the industry still lacks convenient depoly-
Several other excellent and highly promising materials were merizable recyclable polymers.46,47
obtained from renewable sources: eugenol-based functional
Chemical modification of biomass is a promising strategy
polymers,17,35–37 biorenewable networks based on terpenes for designing tailored monomers by introducing a new func-
and sunflower oil,38 biomass-derived self-healing elasto- tion and/or modifying the parent ring followed by ring
mers,9,23,25,39 rubbers,40 and multiple shape-memory opening polymerization,48–50 or incorporation of a new double
materials.41 Paramount efforts are currently spent to design CvC bond.51–56
In fact, the modification with a new double bond derived
from alkynes is the most promising way, since the addition
aInstitute of Chemistry, Saint Petersburg State University, Universitetskiy pr. 26,
reactions to unsaturated bonds are atom-economic by
definition.26,57–60 The simplest alkyne, acetylene, is the mole-
Stary Petergof 198504, Russia. E-mail: k.rodygin@spbu.ru
bZelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky pr. 47,
cule of choice to incorporate polymerizable CvC units. Within
Moscow 119991, Russia. E-mail: val@ioc.ac.ru
this approach, calcium carbide, a solid source of acetylene,
†Electronic supplementary information (ESI) available: NMR spectra; GPC, TGA,
can provide reliable and useful opportunity for introduction of
DSC investigations and detailed pyrolytic data. See DOI: 10.1039/d0gc04170j
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Scheme 1 Synthesis of the monomers from terpenols and calcium
Fig. 1 Fossil-resources-based non-renewable sequence (A); biomass-
and calcium carbide-based cycle for utilization in the synthesis of reusa-
ble polymers (bottom): vinylation (B), assembling (C), CaC2 manufactur-
ing (D), disassembling (E).
a
carbide.
Reaction conditions: terpenol (1 mmol), calcium carbide
(3 mmol), water (6 mmol), KOH (1.2 mmol), KF (1.2 mmol), 4 h. b Isolated
yields. c The reaction was carried out for 5 h.
process was for the first time performed in the present study.
Vinylation proceeded smoothly under superbasic conditions in
DMSO, and the desired VEs (1a–7a) were successfully isolated
in good yields (Scheme 1). VEs 5a–7a are new compounds,
which have not been reported earlier.78
Thus, the double CvC bond in citronellol 4 did not affect
the results; and the javanol ether 5a was isolated as a mixture
of isomers, since the starting alcohol 5 was also used as a
mixture of isomers with the same ratio (see the NMR spectra
in the ESI†). The separation procedure avoided waste-abun-
dant chromatography and similar methods with high
E-factors.58 All the monomers were distilled under vacuum
from calcium hydride before polymerization. Distillation does
not require solvents or other consumables and proceeds with a
lower E-factor.58
a CvC function (Fig. 1B). Recent development in carbide
chemistry has demonstrated amazing progress in the variety of
organic transformations61 and sustainable chemistry.62–67
Well-established procedures for the use of CaC2 in organic syn-
thesis68 allow the transformation of alcohols to vinyl ethers
(VEs)69–74 that can be easily polymerized according to standard
protocols (Fig. 1C).75 At the same time, sustainable pathways
for manufacturing calcium carbide are considered in a renew-
able way from a recycled calcium source and potentially any
carbon source63 (Fig. 1D).
In this study, we describe the synthesis of biomass-derived
monomers based on terpenols and calcium carbide, followed
by the synthesis of the corresponding well-defined polymers.
Amazingly, the synthesized polymers can be disassembled
back to monomers using pyrolysis under relatively mild con-
ditions. Thus, at the end-of-life stage, the used polymeric
materials can be easily re-built. We show that the terp-O- func-
tion facilitates the design of a unique class of materials posses-
sing assembling/disassembling features (Fig. 1E).
Polymerization and characterization of the obtained polymers
Cationic polymerization was chosen for the polymerization of
the VEs, since free-radical techniques are hardly available for
O-vinyl substrates.75 The cationic mechanism provides the flex-
ible properties of the desired polymers and materials derived
from them.79 Changing the molecular mass and weight distri-
bution, various demanded polymers can be obtained as a
result. The aim of this work was not to obtain a particular
Results and discussion
Synthesis of monomers
Menthol (1), (−)-borneol (2), fenchol (3), citronellol (4), javanol polymer with strictly defined characteristics; therefore, the
(5), cis-myrtanol (6) and rosaphen (7) were chosen as initial process was not considered with a narrow focus (Scheme 2).
substrates. These substrates are extracted from biomass, and Important to note is that high enough yields were obtained with
do not contain a CvC double bond except citronellol. the used procedure and the process was totally suitable for
Citronellol was added to the scope, since we were interested in further investigation. The polymerization of O-vinyl derivatives
the use of substrates with both double bond and vinyl func- is a well-established procedure75,80–85 and the desired polymers
tion. We also expanded the scope of the reaction using a terpe- can be produced in the required amounts and properties.
nyl-free substrate derived from biomass – rosaphen (7) to
The resulting polymers P4 and P7 were liquids, and the
demonstrate the tolerance of the methodology. The terpenols others were solids, depending on the nature of the starting
1–4 have already been successfully vinylated previously71,76 or ethers. The polymeric nature of the products was verified
the corresponding VEs were isolated according to the other using NMR and GPC (see the ESI† for details). Then, we
strategy;77 however, re-cycling/re-building properties of the carried out DSC measurements (Fig. 2) to determine the glass
obtained monomers and polymers have not been studied. The transition and melting temperatures as well as the stability
terpenols 5–7 have not been vinylated previously and the ranges. DSC measurements were carried out up to 300 °C,
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Scheme 2 Synthesis of biomass-based polymers from the vinyl ethers
(isolated yields are given).
Fig. 3 TGA thermograms of the studied polymers P1–P7.
DSC and TGA measurements gave a good estimate about
specific temperature limits that were then used during the pyr-
olytic re-building of the polymers (described below).
Plasticity of the selected polymers
The resulting polymers should have valuable mechanical pro-
perties, in particular, the ability to melt without decompo-
sition. It was difficult to expect such stable properties in case
of polymers based on terpenols due to the active nature of the
initial substrates in synthetic transformations. However, we
were surprised with a reasonable overall stability of all the
polymers. As a representative example the myrtanol-based
polymer (P6) was selected and melted in a drying oven at
120 °C. The polymer was stable and melted smoothly (Fig. 4a
and b). Moreover, it became a viscous liquid and was easy to
shape (Fig. 4c and d). Upon cooling, the polymer became solid
and fragile again (Fig. 4e). We re-melted the sample several
times in air to ensure that no decomposition occurred. After
seven melts, the polymer remained unchanged and pliable on
slight heating and still solid on cooling (Fig. 4f). Of course, in
Fig. 2 DSC thermograms of polymers P1–P7 (corresponding bio-
derived starting materials and the numbers are given according to
Scheme 2).
since all the polymers began to decompose at higher values (in
agreement with TGA (Fig. 3) and pyrolysis data). All the poly-
mers had glass transition temperatures (except P1): P2 –
205 °C, P3 – 204 °C, P4 – 215 °C, P5 – 121 °C, P6 – 68 °C, P7 –
214 °C (Fig. 2). Melting temperatures were determined for two
polymers: P3 – 241 °C, P6 – 110 °C. Note, P6 was further
melted at 120 °C in a drying oven to study plasticity properties.
The polymers were restructured in one-step according to
TGA (Fig. 3): from 237 °C to 501 °C. Polymer P1 was restruc-
tured from 297 °C to 498 °C. The thermal stability of P4 was
the lowest. The weight loss of the polymers begins at
130–299 °C, with 3.5–7.5% weight loss; then, it reaches about
89–99%. Above 501 °C, all the samples were continuously
decomposed and a thermally stable residue was formed in all
the cases in small amounts (0.05–3.4%).
Fig. 4 Simple mechanical treatment of the myrtanol-based polymer: a
– initial polymer as a white powder; b – melted polymer; c–f – various
forms of the polymer after re-melting.
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the solid state, the polymer became brittle, and the compo-
sition may be further optimized (with well-developed tailoring
using plasticizers or modifiers, for example) to reach the
desired properties.
Pyrolysis of the starting alcohols, vinyl ethers, polymers, and
determination of pyrolysis products
Scheme 3 The disassembling of the bio-based polymers from calcium
carbide. The final products are in blue (see the detailed scheme of each
polymer re-building in the ESI†).
General information about the pyrolytic experiments. We
used TGA-MS for the mass loss determination and GC-MS
coupled with a pyrolytic unit for product separation on a GC
column. Both methods involved heating the sample with MS
analysis of the products. We, then, compared the MS spectra
and combined the overall information discussed below. All the
following suggestions are based on the TGA-MS and GC-MS
data. We also performed pyrolytic experiments on larger scale
in a quartz tube placed in an oven. The pyrolysis products
were, then, collected and identified with NMR (more infor-
mation including NMR-spectra can be found in the ESI†).
Pyrolysis of the initial alcohols. First, we investigated the
decomposition of the starting substrates at 300 °C in order to
identify major products and record MS spectra. Of note, only
major products were identified; however, the scope of the pro-
ducts was rather narrow. The starting alcohols menthol (1),
fenchol (3) and rosaphen (7) were stable under pyrolysis con-
ditions and remained unchanged. Similar results were obtained
for borneol (2) (94% of the initial alcohol and 5% of iso-borneol
as the products of pyrolysis); and for javanol (5) (97% of the
initial alcohol). Both citronellol (4) (63% of the initial alcohol)
and myrtanol (6) (only 12% of the initial alcohol) were less
stable; the rest of the products were presented as an un-
identified mixture. Thus, terpenols showed good stability under
examined conditions and the starting alcohol was the main
pyrolysis product in all cases except for myrtanol.
chains are stable and difficult to disassemble (if at all poss-
ible). Our idea was to investigate if a terpenyl function and
ether linkage can facilitate depolymerization ability. The poly-
mers were heated at different temperatures: 200, 250, 300, 350,
400, and 450 °C under an argon atmosphere. The resulting
composition of each obtained mixture depended on the temp-
erature and the nature of the polymer (Scheme 3). The initial
alcohols and corresponding ketones or aldehydes are desirable
products that are valuable for further processing (we indicated
selected runs in Table 2, and all the data were collected in the
ESI†).
The composition of each mixture depended on the temp-
erature and the sum of valuable products could reach 100%,
which is a very promising result in the case of recycling of poly-
mers. In some other cases, the starting alcohol content could
reach 87%, which is very interesting for the re-use and re-
cycling of polymers, since the alcohols can be re-vinylated and
the corresponding VEs can be re-polymerized. Selected poly-
mers P1, P2 and P6 were disassembled in a quartz tube
(Table 2). The pyrolysis was rapid and led to the formation of a
liquid phase that was collected and examined with NMR and
GC-MS. The composition of the mixtures was the same as for
Vinyl ether pyrolysis. The VEs, in contrast to the initial alco-
hols, were found unstable under pyrolysis conditions, even
those featuring low boiling points and high volatility (Table 1).
The starting alcohols were mostly found as pyrolysis pro-
ducts in all cases. The main products of borneol VE (2a) and
citronellol VE (4a) pyrolysis were iso-borneol and citronellol.
Fenchyl VE (3a) was stable under pyrolysis conditions. Thus, the
VEs were less stable at heating compared with the alcohols.
Re-building of the polymers. As already pointed out in the
introduction, several known polymers with polyethylene
Table 2 The main products of pyrolytic disassembling of the polymersa
Polymer T, °C
Alcohol, % Aldehyde, % Ketone, % Total,b %
P1
350
400
87
73
—
—
—
—
—
—
—
10
27
35
3
21
4
44
37
35
39
8
95
93
83
96
83
69
78
86
93
87
89
88
57
100
85
83
76
20
10
59
47
44
52
—
—
—
—
—
—
—
—
—
—
Tubec 73
P2
P3
P4
450
37
Tubec 36
400
450
300
350
400
300
350
25
26
76
66
52
86
67
P5
P6
Table 1 Main products of pyrolysis of the vinyl ethers (VEs)
Tubec 53
P7
300
350
400
450
56
48
48
37
Initial
Corresponding Glycerotri-
Squalene,
%
VE alcohol, % VE, % acetate, %
caprilate, %
1a
2a
3a
4a
5a
6a
7a
6
—
—
97
—
26
16
—
—
8
—
12
47
9
27
—
—
—
—
—
6
26
4a
—
41
17
21
4
a A mixture after the pyrolysis contained an initial alcohol 1–7, the
corresponding aldehyde 4b–7b or ketone 1c–3c and some other pro-
ducts (see the ESI† for detailed information). b Total sum of alcohol +
aldehyde + ketone amounts according to GC-MS, excluding gaseous
undetectable products. c 25 mg of a polymer was placed in a quartz
tube and heated up to 750 °C followed by collection of the liquid
phase (see page S23 in the ESI† for more details).
—
—
—
—
77
—
a 41% of iso-borneol was observed instead of borneol.
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analytic pyrolysis. Since the tube was not cooled after the
Obviously, it is impossible to completely re-build a polymer
heating zone, the yields of the products were slightly lower due without disassembling the main polyethylene chain.
to the high volatility of the liquid products. Note, the pyrolysis Presumably, the main chain should be depolymerized into
in a tube was not optimized and was performed to collect smaller units that would have to be converted into stable com-
larger amounts of samples and compare the products with the pounds with various numbers of carbon atoms. To elucidate
analytical pyrolysis results. Thus, the compositions of the mix- the main disassembly processes, we pyrolyzed 150 mg of
tures were the same for both analytical and laboratory borneol-derived polymer P2 in a quartz tube according to the
experiments.
standard procedure, but with the analysis of not only liquid
but also gaseous products. (Note that the solid residue was
only 2–3%.) The pyrolysis off-gases were passed first through
Identification of the pyrolysis products
Note, the stereochemical aspect was omitted due to the wide an ice-cooled trap, then through a deuterochloroform trap,
range of possible isomerization processes (Scheme 3; see the and then through deuterobenzene trap. Cooling the gases was
detailed schemes in the ESI†). In the case of polyvinyl menthol necessary to increase their solubility, and two absorbers with
(P1) pyrolysis, the main products are menthol (1), menthone different deuterium solvents were supposed to capture as
(1c), and squalene. The yield of the initial alcohol and squalene many compounds as possible. After bubbling, the corres-
decreased with increasing temperature, while the yield of the ponding solutions were analyzed using NMR and GC-MS. The
respective ketone increased. During pyrolysis in a tube, the composition of the exhaust gases can be characterized based
1
main product is also the alcohol (73%), which is confirmed on the GC-MS and H NMR spectra; the amount of dissolved
with NMR spectra (see in the ESI†). The main pyrolysis products compounds in solutions was insufficient to record 13C NMR
of polyvinyl borneol (P2) are borneol (2), camphor (2c), and spectra – less than 10 mg. In the fraction with chloroform,
toluene as a stable compound. The yield of toluene in the toluene (51%), camphene (10%), 2-methyl-3-methyl-
mixture decreases with an increase in the pyrolysis temperature, enecyclohexene (10%) were identified; in the fraction with
and the yields of borneol and camphor increase. Under optimal benzene, tetradecane (21%), heptadecane (14%), octadecane
conditions pyrolysis mainly resulted in the alcohol and ketone, (10%), and dodecane (6%) were identified. The saturated
1
and the main signals of the alcohol were observed in H and hydrocarbons can be considered as the products of polyethyl-
13C NMR spectra, while signals of toluene were not found. ene chain conversion. Thus, the gas mixture consisted of
During the pyrolysis of polyvinyl fenchol (P3), unlike the other toluene and various stable hydrocarbons. Hydrocarbons were
polymers, a large amount of acetaldehyde was formed especially formed as a result of rearrangements of fragments of the
at low temperatures (see the ESI†). When the temperature rises polymer chain. We also do not exclude the presence of gaseous
to 400 °C, the polymer was mainly converted to fenchone (3c) hydrogen in the off-gases. The liquid phase after the pyrolysis
and the initial alcohol 3; and the yields of both products consisted of camphor (47%), borneol (37%), and toluene
increased significantly with increasing temperature. Pyrolysis of (16%). The resulting liquid fraction, without any purification
polyvinyl citronellol (P4) led to fewer main products – only citro- and processing, was studied in the next stage – hydrogenation.
nellol and citronellal, while the yield of alcohol 4 decreased
Total reductive recycling of the polymers
with increasing temperature, and the yield of aldehyde 4b
increased. The pyrolysis of polyvinyl javanol (P5) occurred Despite the fact that in all the cases another main product of
mainly with the formation of javanol 5; the yields of the corres- polymer re-building is an eco-friendly and valuable corres-
ponding aldehyde 5b and other products were much lower. The ponding aldehyde or ketone, working with mixtures is compli-
pyrolysis of polyvinyl myrtanol (P6) led to myrtanol (6), myrtanal cated and accompanied by additional wastes for the separation
(6b), and toluene. In this case, the yield of the alcohol, alde- of individual components. In addition, direct re-vinylation of an
hyde, and toluene decreased with increasing temperature, while alcohol/aldehyde(ketone) mixture obtained after re-building is
the scope of products increased (many products with 5% yield not possible. Such a process would lead to the formation of a
and less; see the ESI†). Pyrolysis in a tube resulted mainly in large amount of condensation products under the required
myrtanol 6 (53%). Interestingly, the pyrolysis of polyvinyl rosa- basic conditions. Therefore, we decided to find out whether it is
phen (P7) led to complete conversion to rosaphen 7 and the possible to hydrogenate an aldehyde or ketone to the initial
corresponding aldehyde 7b at 300 °C. As the pyrolysis tempera- alcohol directly in the reaction mixture using sodium boro-
ture increases, the alcohol yield decreases.
hydride.86 We selected the borneol/camphor system since the
Thus, the main products of polymer pyrolysis were the expected product of camphor hydrogenation should be borneol
corresponding alcohol and aldehyde/ketone, depending on and iso-borneol as a mixture with the ratio 1 : 4. Borneol after
whether it was
a primary alcohol or a secondary one the pyrolysis and iso-borneol after the sodium borohydride
1
(Scheme 3). In addition, squalene or toluene was formed in treatment are easily distinguished by means of H NMR spec-
many cases as a stable by-product (see the detailed scheme in troscopy. According to NMR data, it would be easy to find out
the ESI†). It can be noted that in the cases of myrtanol, the amount of the camphor-derived iso-borneol. In addition,
menthol, citronellol and javanol, the corresponding alcohol the tolerance of starting borneol would be tested in the pres-
was predominantly formed, and for borneol and fenchol – the ence of borohydride as well. A model mixture of commercially
ketone.
available borneol and camphor was tested, and the treatment
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considered as renewable raw materials derived from biomass.
Since the developed methodology involves the synthesis of
polymers with variable masses and distributions, the pro-
perties of the resulting polymers and materials can vary over a
wide range, which opens up new prospects for the reuse of
polymers in materials science. The polymers obtained in this
work have sufficient plasticity, and are able to roll out and
change shape when heated many times.
Promising results were obtained by studying the disassem-
bling feature of the obtained polymers. Even at relatively low
temperatures of 300–400 °C, many polymers were transformed
into the starting alcohols and a number of other non-toxic,
valuable and separable components: the corresponding alde-
hyde and ketone, as well as squalene and toluene. The yields
of the recovered alcohols varied depending on the temperature
and nature of the polymer and can reach 86%. Thus, the
monomer can be collected and reused again in re-vinylation
reaction followed by re-polymerization and re-use.
Interestingly, after the procedure, practically no solid residues
of pyrolysis (no more than 3% by weight) remained that also
looks promising in the challenge of solid waste disposal. Of
course, the scope of the pyrolysis products is quite large, and
we were unable to identify all the components with a content
below 1–2 mass%; however, the results obtained revealed the
possibility of recycling polymeric materials to recover starting
components in fairly high yields that constitutes a step
forward in the efficient conversion of used polymers.
Scheme 4 The overall sequence involving reductive recycling of the
bornyl-based polymer.
with borohydride led to the hydrogenation of camphor with the
formation of iso-borneol. The starting borneol remained in the
mixture unchanged. Comparing the ratio of borneol/iso-borneol
in the mixture using NMR spectra, we found that sodium boro-
hydride selectively and quantitatively hydrogenated camphor to
the corresponding alcohol leaving initial borneol intact. Then,
the real mixture after the pyrolysis of polymer P2 (borneol-
based) was treated with sodium borohydride under the same
conditions, which led to the complete hydrogenation of
camphor (2c) to iso-borneol (2d) (Scheme 4). No camphor was
observed in the reaction mixture according to GC-MS.
The present study gives a new conceptual recognition to the
idea of designing bio-derived materials, which simultaneously
In this case, as in the blank experiment, the initial borneol 2
remained unchanged as well. Thus, after the re-building of the
polymer, only the initial alcohol 2 and 2d (borneol + iso-
borneol) were present in the mixture and the mixture can be re-
vinylated again immediately. Both alcohols can be vinylated
under the same conditions resulting in the same VE 2a when
treated with carbide. Of course, this is a simple model system
for testing the entire sequence and illustrating the possibility of
a closed cycle. Separation of isomers may be performed when
selective involvement of a single type of monomer is required.
To check the total recyclability, we pyrolyzed a sample of polyvi-
nyl menthol (P1), collected the liquid reaction mixture, reduced
it with sodium borohydride, vinylated with calcium carbide,
and polymerized again. The same polymer was isolated and
characterized using standard techniques (NMR and GPC).
One can assume that the hydrogenation of the other
ketones and aldehydes occurs in a similar way.87–90 It should
be noted that an appropriate industrial-scale technology may
involve direct reduction with hydrogen according to well-devel-
oped environmentally benign protocols.91–96 Thus, complete
quantitative recovery of the initial alcohols can be easily
achieved with environmentally benign hydrogenation by
simple treatment with reduction reagents.
incorporate
the
disassembling/re-building
possibility.
Synthesis of the monomers via atom-economic addition reac-
tion and purification of the VEs by distillation are important
green advantages of the process.
Experimental data
Materials and methods
Materials. Calcium carbide (granulated, particle size
0.1–1 mm, 75% of acetylene (gas-volumetric as indicated by
the supplier)), menthol (99%), (−)-borneol (97%), and fenchyl
alcohol (≥96%) were purchased from Sigma-Aldrich and used
without further purification. Citronellol (95%) and boron tri-
fluoride etherate (48%) were purchased from Acros Organics
and used as received. KOH and KF were purchased from
Vekton (Russia). cis-Myrtanol, javanol, and rosaphen were pur-
chased from a local perfume company and checked with NMR
and GC before using. KF was dried and ground before use.
KOH was ground before use. Liquid monomers were distilled
over calcium hydride in vacuum before polymerization.
Instrumentation. 1H and 13C NMR spectra were recorded
using a Bruker Avance 400 NMR spectrometer. The 1H and 13
C
NMR chemical shifts were reported in ppm and were deter-
mined by referencing the peaks to the residual solvent peaks.
Conclusions
In this work, the polymers based on natural compounds and The data were processed using MestReNova (version 6.0.2)
calcium carbide were obtained. Both starting substrates can be desktop NMR data processing software.
2492 | Green Chem., 2021, 23, 2487–2495
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Molar masses and polydispersity indices of the polymers General procedure for the pyrolysis in a quartz tube
were measured with gel permeation chromatography (GPC) in
Polyvinyl menthol (P1): 35 mg of the polymer was placed in a
quartz tube under argon atmosphere and heated to 450 °C for
tetrahydrofuran (THF) at 40 °C (flow rate = 1.0 mL min−1) vs.
polystyrene standards. GPC was carried out using a Shimadzu
30 minutes. A yellow liquid appeared immediately on the walls
LC-20 modular system equipped with a GPC Column MZ-Gel
of the cooled part of the tube. After the pyrolysis, the tube
cooled to room temperature. No more than 1 mg of the solid
SDplus 10E3A (8 mm × 300 mm) and an RID-20A differential
refractive index detector. The unit calibration was conducted
insoluble residue remained inside the tube. The liquid was
using commercially available narrow molecular-weight-distri-
washed off with methanol, the solvent was evaporated, and
23 mg of the mixture was isolated (76% isolated yield). Then, a
bution polystyrene standards (0.5–1000 kDa, Polymer
Laboratories). The chromatograms were processed using
sample was taken for GC-MS (in methanol). The mixture con-
Shimadzu LC solution software. The polymer samples were
sisted of menthol and menthone only.
initially filtered through a polyvinylidene difluoride (PVDF)
filter (0.45 μm, 13 mm) and dissolved in THF (2 mg mL−1).
Differential Scanning Calorimetry (DSC) measurements
were carried out using a NETZSCH STA 449 F3 Jupiter system.
Conflicts of interest
DSC scans were performed in argon atmosphere from −80 to
300 °C (the second heating). A rate of 10 K min−1 was used
during heating scans. The glass transition temperature (Tg)
was taken at half-height of the glass transition heat capacity
step.
There are no conflicts to declare.
Acknowledgements
Thermal and decomposition characteristics of the materials
were determined using thermogravimetric analysis (TGA), con-
ducted on a NETZSCH STA 449 F3 Jupiter apparatus, in the
temperature range of 40–530 °C with a heating rate of 10 K
The authors thank Dr Evgueni Kirillov for a helpful discussion.
The authors thank the Resource Centers of Saint Petersburg
State University: Magnetic Resonance Research Centre,
Chemical Analysis and Materials Research Centre, and
Thermogravimetric and Calorimetric Research Centre.
min−1 under a flow of argon at 50 mL min−1
.
GC-MS measurements (and GC-MS coupled with a pyroly-
zer) were carried out using a QP2010 Ultra Shimadzu system,
multi-shot pyrolyzer EGA/PY-3030D (Frontier Lab). Capillary
columns were Rtx, 5 ms (30 m × 0.25 mm, 0.25 μm; Restek)
and HP, 5 ms (60 m × 0.32 mm, 0.25 μm; Agilent); the carrier
gas was He (high purity, grade 6.0).
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