RESEARCH ARTICLE
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Sunlight-Mediated Degradation of Polyethylene under the
Synergy of Photothermal C–H Activation and Modification
Shengnan Kong, Congze He, Jin Dong, Ning Li, Chaoran Xu,* and Xiangcheng Pan*
Heterogeneous catalysts such as amorphous silica-alumina, zeolites, and carbons
could also be used for the pyrolysis of
PE plastics at high temperatures.[12–15] Recently, Huang et al. reported the degradation of PE plastics at 150 °C for 3 days using
a tandem catalytic cross alkane metathesis
process to obtain oil and wax products.[16]
These degradation processes usually required high temperature, and the dagradation suffered from low energy efficiency.[17]
Since PE lacked functional groups, the
surface energies and adhesion with polar materials were poor.[18–20] A feasible
method was to introduce polar groups into
the side chains of the polymer by chemical
modifications.[21–23] The functionalization
of polyolefin using active carbon free radical
was commonly used at present, and chemical modification of
polyolefin had been successfully achieved in the industry by grafting maleimide to the polymer backbone using free radicals.[24–26]
In recent years, the photocatalytic hydrogen atom transfer (HAT)
process had attracted increasing attention.[27–29] The photocatalyst absorbed the light energy, and the catalyst in the excited
state might activate the C–H bond by the direct cleavage via
the HAT process to obtain an alkyl radical, which could form
the C–H functionalized product through subsequent free radical
reactions.[30,31] Therefore, we envisioned that the photocatalytic
HAT process would be applied to C–H activation and modification of PE. Once the polar group was introduced, the backbone
of PE could be feasibly degraded into low molecular weight PE
with tunable polarity under irradiation conditions.
In this paper, the photocatalytic reaction under sunlight was
used to graft the diisopropyl azodicarboxylate (DIAD) onto the PE
at 110 °C using tetrabutylammonium decatungstate (TBADT) as
photocatalysts (Figure 1). This C–H activation and modification
further induced and assisted the degradation of PE. Furthermore,
the reactions were optimized to fabricate PE with tunable polarity
by changing the amount of DIAD or TBADT. Subsequently, PE
plastics, including films, gloves, and bags, were successfully converted into low molecular weight PE with polar groups and then
used as blending compatibilizers for PE/starch and PP/starch.
The reuse of waste plastic has always been one of the challenges in the field
of macromolecular science. The authors report that the degradation of
polyethylene (PE) plastics can be achieved by a photothermal reaction using
tetrabutylammonium decatungstate (TBADT) as photocatalysts, and the
grafting of diisopropyl azodicarboxylate (DIAD) onto PE concurrently occurs
under sunlight irradiation. Through adjusting the amount of DIAD or TBADT
used in the reaction, low molecular weight PE waxes with tunable polarity can
be prepared. This method can be applied to different sorts of PE plastics
including films, gloves, and bags. Subsequently, synthesized low molecular
weight PE-graft-DIAD waxes are used as compatibilizers to successfully
realize the compatibilized blending of polyolefin plastics and starch, which
paved a viable way to reuse waste plastics to reduce environmental pollution.
1. Introduction
Since the 1950s, the industry of polyolefin has been rapidly
developed with the birth of Ziegler–Natta catalyst,[1,2] which
could polymerize olefin into stereo-regulated polymers, such
as high-density polyethylene (HDPE), low-density polyethylene
(LDPE), linear low-density polyethylene (LLDPE), and polypropylene (PP).[3] Polyethylene (PE) is by far the world’s most productive plastic, with annual production exceeding 100 million tons.[4]
PE plastics are inert and difficult to degrade under normal conditions, mainly because PE consists entirely of a strong C–C bond
and C–H bond. Since it is widely used and not easy to degrade,
waste PE plastic has caused environmental pollution.[5,6] Therefore, the disposal of waste plastic and the reuse of PE plastic has
become an inevitable problem.[7]
The degradation of polyolefin has been extensively studied,
mainly through catalytic cracking of polyolefin waste plastics at
high temperatures (>400 °C) to obtain hydrocarbon mixtures.[8]
Lewis acids were reported to be a homogeneous catalyst for PE
cracking,[9] and Ivanova et al. reported the PE cracking at 370 °C
using aluminum trichloride and metal aluminum tetrachloroaluminate melts as catalysts to obtain a high yield of gases.[10,11]
S. Kong, C. He, J. Dong, N. Li, C. Xu, X. Pan
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science
Fudan University
Shanghai 200438, China
E-mail: xcr@fudan.edu.cn; panxc@fudan.edu.cn
2. Results and Discussion
2.1. Photothermal Reaction for Linear Low-Density Polyethylene
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/macp.202100322
DOI: 10.1002/macp.202100322
Macromol. Chem. Phys. 2022, 223, 2100322
A commercial LLDPE (LLDPE-7042, Mn = 29.1 K, Ð = 3.63) was
used as a sample for the photothermal reaction in the presence
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Figure 1. A) Sunlight-mediated degradation of polyethylene under the synergy of C–H activation and modification in the presence of DIAD using TBADT
as photocatalyst. B) Illustrative scheme for reusing waste PE plastics as the blending compatibilizers.
of DIAD and TBADT photocatalyst. The preliminary experiment
was conducted at 110 °C under sunlight for 24 h with a feed ratio
of [E]/[DIAD]/[TBADT] = 1/2/0.05, in which E represented the
methylene unit in the PE. After the reaction was cooled down,
the mixture was poured into methanol or acetonitrile to obtain
the precipitation. This step was repeated three times, and the
precipitate was dried under a vacuum. The obtained product was
then characterized by nuclear magnetic resonance (NMR) spectrometer, gel permeation chromatography (GPC), and Fourier
transform infrared spectrometer (FT-IR). NMR analysis was conducted at 110 °C using o-C6 D4 Cl2 as the solvent. By comparing the 1 H NMR spectra of PE and the obtained product (Figure 2A), three new peaks at 𝛿 4.12 ppm (b), 4.90 ppm (d) and
5.95 ppm (c) emerged in the spectrum of the product, which
are attributed from methine group in the backbone, methine
group in the DIAD, and secondary amine group, respectively.
As shown in 13 C NMR spectra (Figure 2B), three representative
peaks at 𝛿 58.2, 69.2, and 156.0 ppm emerged, which are attributed from DIAD. The NMR analysis demonstrated that DIAD
had been successfully grafted onto the LLDPE, and FT-IR spectra (Figure 2C) also confirmed that the carbonyl stretch C═O appeared at 1703 cm−1 in the spectrum of the product. GPC traces
(Figure 2D) clearly showed that the molecular weight of PE decreased after the photothermal reaction, probably because of the
presence of 𝛽-scission.[32,33] Overall, the photothermal reaction
grafted the DIDA onto the side chain and induced the polymer
chain to fracture, which might be a feasible method for reusing
waste PE plastics as blending additives.
To determine the optimal reaction conditions, we adjusted
the feed molar ratio of [E]/[DIDA], the amount of photocatalyst
TBADT, and reaction temperature in the photocatalytic reaction
of LLDPE (Table 1 and Table S1, Supporting Information). When
Macromol. Chem. Phys. 2022, 223, 2100322
Table 1. Optimization of the reaction conditions.
Entry
a)
b)
TBADT
Mn
c)
[g moL−1 ]
Mw /Mn
c)
g [w%]
d
nDIAD
e)
[E]/[DIAD]
1
1:0.1
5%
16 800
2.71
9.4%
1.30
2
1:0.2
5%
17 100
1.99
27.8%
3.83
3
1:0.5
5%
16 000
2.15
40.7%
5.63
4
1:1
5%
14 700
1.51
38.0%
5.26
5
1:2
5%
8700
2.47
49.0%
6.78
6
1:5
5%
18 100
1.89
44.2%
0.61
7
1:10
5%
20 700
2.44
4.7%
0.65
8
1:2
0.1%
15 400
2.38
18.7%
2.59
9
1:2
1%
6300
2.17
46.6%
6.45
10
1:2
2.5%
6100
2.25
51.6%
7.14
11
1:2
10%
8100
3.64
—
—
12
1:2
0
21 600
2.79
13.9%
1.93
13
1:0
2.5%
16 500
3.79
—
—
14
1:0
0
23 400
3.50
—
—
a)
LLDPE-7042 (Mn = 29.1 K, Ð = 3.63), solvent: 1,1,2,2-tetrachloroethane (0.4 m),
b)
110 °C, sunlight in the daytime, 2 days; Molar ratio of ethylene units to DIAD;
c)
Data measured with high-temperature GPC using 1,2,4-trichlorobenzene as elud
ent at 160 °C and polystyrene stands serving as calibration; ) Weight of the grafted
e)
DIAD relative to the PE; Number of the grafted DIAD group per 100 ethylene units
determined by NMR analysis.
the feed molar ratio of [E]/[DIDA] varied from 1/0.1 to 1/10 (Table 1, entries 1–7), the grafting efficiency of DIAD determined by
NMR analysis increased within certain ranges (entries 1–5), and
then it decreased significantly (entries 6–7). The degradation of
LLDPE showed a similar trend with the change of the feed molar
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Figure 2. Characterizations of the product after the photothermal reaction. A) 1 H NMR (o-C6 D4 Cl2 , 400 MHz, 110 °C) spectra of LLDPE (black line)
and the product (purple line). B) 13 C NMR (o-C6 D4 Cl2 , 400 MHz, 110 °C) spectra of LLDPE (black line) and the product (purple line). C) FT-IR spectra
of original PE (a), DIAD (b), and the product (c). D) GPC traces of original PE (a) and the product (b).
ratio. The amount of photocatalyst TBADT used in the reaction
was then changed with a feed molar ratio of [E]/[DIDA] at 1/2
(Table 1, entries 8–11), and we found that the amount of TBADT
at 2.5% was the optimal condition. In addition, the photocatalytic
reaction of LLDPE at different temperatures was shown in Table
S1, Supporting Information, which indicated that 110 °C was
the optimal temperature. These optimal conditions were used
for subsequent photothermal reactions.
Additionally, the amount of DIAD grafted onto the polymer
could be determined by thermal gravimetric analysis (TGA), as
shown in Figure S3, Supporting Information. In comparison
with the LLDPE-7042, the TGA curves of the product were divided into two parts. While the temperature ranging from 50 to
325 °C corresponded to the decomposition of DIAD, the temperature ranging from 325 to 500 °C was the decomposition of PE.
Moreover, the loss of weight ratio for the first part would increase
with the improvement of the grafting efficiency.
Some comparative experiments were performed to verify further the role of DIAD and TBADT in the photothermal reaction (Table 1, entries 12–14). When the feed ratio of [E]/[DIDA]
was 1/2 without the addition of photocatalyst (entry 12), a small
amount of DIAD would be grafted onto the polymer with no significant change in molecular weight, which might be the reason
Macromol. Chem. Phys. 2022, 223, 2100322
that DIAD could generate free radicals at 110 °C for modification of the polymer. Compared with the reaction in the absence of
DIAD and TBADT (entry 14), the addition of photocatalyst would
decrease molecular weight (entry 13), which might illustrate the
generation of the alkyl radical in the presence of photocatalyst.
The possible mechanism for the photocatalytic reaction of PE
was illustrated in Figure 3. The excited polyoxodecatungstate anion ([W10 O32 ]4−* ) abstracted a hydrogen atom from the C–H bond
of PE to form a long-chain alkyl radical, which underwent consecutive addition of DIAD to form aminyl radical. Back-HAT from
the reduced form of the decatungstate anion to aminyl radical
provided the grafted polymer, restoring the starting photocatalyst TBADT.[34,35] Meanwhile, 𝛽-scission and chain transfer might
happen during the formation of the long-chain radical, and the
short-chain radical would react with DIAD, which led to the decrease of molecular weight.
2.2. Photothermal Reaction for Different Types of Polyethylene
To demonstrate the scope of this photocatalytic degradation and
modification reaction, we investigated a variety of commercial PE
and PE plastics, including LDPE, HDPE, preservative film, white
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Figure 3. The possible mechanism for the photothermal reaction.
Table 2. The photothermal reaction of other kinds of PE.
Entry
a)
Mn
PE type
b)
[g moL−1 ]
Mw /Mn
b)
g [w%]
c)
nDIAD
d)
1
LDPE-Q281 (Mn = 16.6 K, Ð = 5.25)
3000
3.09
85.0%
11.8
2
HDPE-T5070 (Mn = 18.6 K, Ð = 4.68)
1400
3.52
80.2%
11.1
3
Preservative film (Mn = 22.2 K, Ð = 3.71)
2100
3.47
144.4%
20
4
White trash bag (Mn = 20.4 K, Ð = 6.76)
2500
2.66
131.3%
18.2
5
Glove (Mn = 24.6 K, Ð = 3.81)
1400
3.98
131.3%
18.2
a)
1,1,2,2-tetrachloroethane: 0.4 m, [E]/[DIAD]/[TBADT] = 1.0/2.0/0.025 (E represented ethylene units), 110 °C, sunlight in the daytime, 2 days, photothermal reaction for three
b)
c)
times; Data measured with high-temperature GPC using 1,2,4-trichlorobenzene as eluent at 160 °C and polystyrene stands serving as calibration; Weight of grafted units
d)
relative to the PE; Number of the grafted DIAD group per 100 ethylene units determined by NMR analysis.
trash bag, and PE gloves, and the results are summarized in Table 2. The photothermal reaction was conducted at the optimal
conditions and repeated three times, and we found that PE plastic could degrade into the wax products with a molecular weight
below 3000 g moL−1 , and the number of the grafted DIAD group
per 100 ethylene units would reach more than 10. Such experimental results were beneficial for the subsequent recycling of
waste PE plastics as blending compatibilizers.
2.3. Low Molecular Weight Polyethylene with Polar Groups for
Blending Compatibilizer
Starch-based degradable polymeric materials are receiving increasing attention, and many efforts are made to blend polyolefin
and starch to reduce the environmental pollution of plastics.[36]
However, polyolefin and starch are incompatible due to the absence of polar groups in the polyolefin. Therefore, the blending of starch and polyolefin requires compatibilizers to reduce
the interfacial tension and enhance the adhesion between the
phases.[37] Waste PE gloves were used as raw materials to produce low molecular weight PE-graft-DIAD (PE-g-DIAD) polymer
through abovementioned photothermal reaction, which could be
utilized as the blending compatibilizers.
As shown in Table S2, Supporting Information, waste PE
gloves were converted into PE-g-DIAD polymer with a molecular weight of 6.6 K after 72 h of photothermal reaction. This
grafted polymer was then used as a compatibilizer to blend polyolefin (PE and PP) and starch with a feed weight ratio of poly-
Macromol. Chem. Phys. 2022, 223, 2100322
olefin/starch/compatibilizer at 4/1/0.05. The blends were extruded and granulated at 180 °C with a speed of 60 rpm for 6 min
in a twin-screw extruder. The blended pellets were then extruded
into films, which could be observed by scanning electron microscopy (SEM) to judge the effect of blending. The representative SEM images (Figure 4A,C) of the uncompatibilized blend
clearly showed the poor adhesion between the polyolefin and the
starch phases. However, with the addition of PE-g-DIAD polymer
(Figure 4B,D), the adhesions improved significantly, indicating
an excellent compatibilization effect.
3. Conclusion
The low molecular weight PE with tunable polarity was fabricated
from waste plastics with photothermal reaction, which could be
used as compatibilizers to blend polyolefin plastics and starch.
Using TBADT as photocatalysts, DIAD was successfully grafted
onto the PE under sunlight at 110 °C. The grafting number
of DIAD was controlled by adjusting the amount of DIAD or
TBADT used in the reaction. At the same time, PE could decompose in the reaction due to the presence of 𝛽-scission, which was
used to degrade PE plastics. Waste PE films, gloves and bags were
degraded into low molecular weight PE waxes with polar groups,
and we used them for the blending compatibilizers. According to
the SEM images of blending films, the adhesion improved significantly upon the addition of the PE-g-DIAD polymer. Therefore,
it was expected that this method would be used to recycle waste
polyolefin plastics to reduce the pollution on the environment.
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Figure 4. SEM images of the surfaces of blending films: A) PE/starch; B) PE/starch/PE-g-DIAD; C) PP/starch; D) PP/starch/PE-g-DIAD.
4. Experimental Section
Engineering of Polymers, Department of Macromolecular Science, and Fudan University.
Materials: TBADT was synthesized according to the reported
procedure.[38] LLDPE (LLDPE-7042, Mn = 29.1 K, Ð = 3.63) was
from Sinopec Qilu petrochemical company, China. LDPE (LDPE-Q281,
Mn = 16.6 K, Ð = 5.25) was from Sinopec Shanghai petrochemical
company, China. HDPE (HDPE-T5070, Mn = 18.5 K, Ð = 4.68) was from
Sinopec Panjin petrochemical company, China. DIAD (98%, Adamas) and
1,1,2,2-tetrachloraethan (99%, Adamas) were purchased from Shanghai
Titan Technology company. Methanol and acetonitrile were analytical
grade from Sinopharm chemical reagent company and used directly.
Characterizations: 1 H and 13 C NMR spectra were recorded on Bruker
AVANCE III HD 400 MHz and DMX 300 MHz instruments. NMR tests of
polymers were conducted at 110 °C using o-C6 D4 Cl2 as the solvent; FTIR spectra were acquired using a Thermo Fisher Nicolet 6700 FT-IR spectrometer. The spectra were collected at 64 scans in an ATR reflection mode
with background deduction; molecular weights and molecular weight distributions of polymers were performed on Agilent 1260 Infinity II high temperature GPC system with a refractive index detector. Analysis was carried
out at 160 °C in 1,2,4-trichlorobenzene with a flow rate of 1.0 mL min−1 .
Polystyrene standards were used for calibration. The thermal decomposition behaviors of polymers were recorded by using a Mettler Toledo TGA
1 equipment from 40 to 600 °C with a heating rate of 10 °C min−1 under a
nitrogen flow at a rate of 20 mL min−1 . The compatibility of different blends
of polymers was performed on a Zeiss Ultra 55 SEM with an accelerating
voltage of 10 kV. The pressed films were fractured in liquid nitrogen, and
the fracture surfaces were sprayed with gold to enhance the electron conductivity on the surface of the samples.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors thank the support from the National Natural Science Foundation of China (21871056 and 91956122), State Key Laboratory of Molecular
Macromol. Chem. Phys. 2022, 223, 2100322
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support this work are available from the corresponding author upon reasonable request.
Keywords
degradation, modification, photothermal, polyethylene
Received: August 30, 2021
Revised: October 3, 2021
Published online: November 8, 2021
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