Towards biodegradable elastomers: Green synthesis of carbohydrate functionalized styrene-butadiene-styrene copolymer by click chemistry

Article abstract of DOI:10.1039/c1gc16146f

Random attachment of sugar molecules to synthetic polymers is an important strategy to induce biodegradability in these polymers. The present study successfully employs "click" chemistry to introduce low levels of sugar molecules onto styrene-butadiene-styrene (SBS) copolymer, a widely used commodity polymer which is not biodegradable. Spectral, morphological and thermal studies of the modified polymers were carried out to show the dramatic changes in the properties of these modified polymers. Thermal stability of glucose linked SBS had onset of degradation at 428 °C, down from 478 °C observed for SBS. Morphology studied by WAXRD and SEM showed destructuring of the polymer domains of SBS, which is beneficial for biodegradation of these polymers. Previous studies showed that sugars anchored by hydrolysable ester groups onto polystyrene were biodegradable; current studies show that sugars anchored by unhydrolyzable C-C bonds on the butadiene component of SBS copolymer are also significantly biodegradable.

Full text of DOI:10.1039/c1gc16146f

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PAPER
Towards biodegradable elastomers: green synthesis of carbohydrate
functionalized styrene–butadiene–styrene copolymer by click chemistry†
Rakesh Singh and A. J. Varma*
Received 14th September 2011, Accepted 20th October 2011
DOI: 10.1039/c1gc16146f
Random attachment of sugar molecules to synthetic polymers is an important strategy to induce
biodegradability in these polymers. The present study successfully employs “click” chemistry to
introduce low levels of sugar molecules onto styrene–butadiene–styrene (SBS) copolymer, a widely
used commodity polymer which is not biodegradable. Spectral, morphological and thermal
studies of the modified polymers were carried out to show the dramatic changes in the properties
of these modified polymers. Thermal stability of glucose linked SBS had onset of degradation at
428 ◦C, down from 478 ◦C observed for SBS. Morphology studied by WAXRD and SEM showed
destructuring of the polymer domains of SBS, which is beneficial for biodegradation of these
polymers. Previous studies showed that sugars anchored by hydrolysable ester groups onto
polystyrene were biodegradable; current studies show that sugars anchored by unhydrolyzable
C–C bonds on the butadiene component of SBS copolymer are also significantly biodegradable.
carboxyl, through conventional pre- or post-polymerization
Introduction
modification.12,13 However, the number of examples of click
Click reaction, that is Cu(I) catalyzed Huisgen 1,3-dipolar
chemistry reactions in polymer systems is still limited, but the
cycloaddition of azides and alkynes, has attracted more and
technique remains a potent tool for further exploitation for
more attention because of its high selectivity, mild reaction
developing new functional groups onto hydrophobic polymers.
conditions, quantitative yields and almost no by-products.
Cell surface saccharides of glycolipids, glycoproteins and
This has led to its application in many processes, including
glycans participate in numerous biological phenomena in which
the synthesis of therapeutics,1,2 protein-based biohybrids,3,4
there is mediation by carbohydrate–protein interactions. Carbo-
sugar arrays,5 dendrimers,6 functional polymers7,8 and nanotube
hydrate chains are involved in various recognition phenomena,
functionalization.9 In particular the “click” process has become
such as fertilization, cell adhesion, tissue formation, antigen–
an invaluable tool for the synthesis of carbohydrate-based
antibody reaction, cancer metastasis and infection of viruses
polymers. For example, Liebert et al. employed click chemistry in
and bacteria. Carbohydrate-carrying polymers have been used
as cell-specific culture substrata, in human vaccines, for tumor
diagnosis, as probes for receptors and in targeted drug delivery
the modification of polysaccharides10 and Sun et al. immobilized
carbohydrates and proteins onto solid surfaces.11 One advantage
of click chemistry is that the azide and alkyne groups can, in
systems.14
some cases, be introduced on the polymer backbone, and if
Therefore, greater attention has recently been paid towards
these compounds are stable in common organic reagents and
synthetic polymers with pendant carbohydrate chain moieties.
can coexist with other functional groups, such as hydroxyl,
We decided to use a click chemistry approach for the func-
amino and carboxyl, then the click reaction can be successfully
tionalization of polystyrene–polybutadiene–polystyrene (SBS)
accomplished. The triazole ring thus formed is generally stable,
copolymer with carbohydrates, as it eliminates the need to
besides the copper(I) catalyst system is easily available and
protect the sugar hydroxyl groups which otherwise becomes
is not sensitive to solvent and pH. Thus, the remarkable
necessary; thereby we also avoid use of several reagents and
functional group tolerance of click reactions has enabled the
work-up procedures leading to a greener process to make
facile introduction of reactive groups, such as hydroxyl and
a greener polymer molecule. Complete quantification of all
functional groups is not reported in this paper as this study
is just a model study to ascertain the synthesis methodology as
Polymer Science & Engineering Division, National Chemical laboratory,
well as preliminary studies on biodegradation and changes in
morphological properties and thermal properties which affect
the applications of the modified elastomer. SBS is an important
synthetic elastomer, and incorporation of carbohydrate moieties
Dr Homi Bhabha Road, Pune, 411008, India.
E-mail: aj.varma@ncl.res.in; Fax: 91-20-25902618;
Tel: +91-20-25902191
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1gc16146f
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along the SBS chain has the potential to render it biodegradable
as well as biocompatible. Our previous work has shown that
synthetic polymers (like polystyrene) with pendant sugar groups
have significant biodegradable properties.15 It is possible to
conceive that in bulk applications of SBS (such as certain types
of shoe soles), there will not be significant biodegradation whilst
in active use, since contact with soil will be occasional; loss of
physical properties would entail long term immersion in soil
bacteria environment for several days at a stretch. Needless
to say that more research will be needed to develop products
with appropriate degradation profiles. This can be achieved by
varying the type of sugar molecules as well as their content.
In a forthcoming publication we will present our results on the
effect of different sugar molecules (mannose, galactose, fructose,
xylose, lactose, etc.) on the rates of biodegradation. The current
study shows the application of click chemistry to achieve the
synthesis of such polymer molecules.
ter. The scanning speed was 4◦ min-1, with CuKa radiation. The
samples were scanned from 2q values of 3◦ to 30◦.
Synthesis of carbohydrate functionalized SBS by click chemistry
Synthesis of 22 mol% partially epoxidized SBS. We per-
formed the epoxidation of SBS with a peracid as a ho-
mogenous reactant. Partial epoxidation of polystyrene-block-
polybutadiene-block-polystyrene (SBS) was performed using 3-
chloroperoxybenzoic acid, mCPBA (amount used appropriate
for the desired percentage of epoxidation) in toluene at room
temperature.
In a 250 mL round-bottomed flask, 10 g SBS, Aldrich
(13 mmol g-1 unsaturation) was dissolved in 100 mL toluene
at room temperature. 6.25 g of mCPBA (75% Aldrich) dissolved
in 40 mL of toluene was added dropwise with a dropping
funnel over a time of 20 min. The reaction mixture was
maintained at room temperature and the reaction was continued
for 3.5 h after which it was precipitated in 1 L methanol. The
precipitated polymer was washed many times with methanol.
The polymer was then dried in a desiccator under vacuum at
room temperature.
Experimental
Materials
Polystyrene-block-polybutadiene-block-polystyrene (Aldrich,
Mw 140 000, 30% polystyrene, 70% polybutadiene) was used
as a starting material. 3-Chloroperoxybenzoic acid, 77% max.
(Aldrich Chemicals, USA), methanol LR (Ranbaxy, India),
toluene GR, pyridine GR, N,N-dimethylformamide GR
and 4-dimethylaminopyridine (DMAP) were purchased from
Merck and used as such without further purification. Dextrose
anhydrous GR was obtained from Merck. Ammonium chloride
AR and sodium ascorbate were obtained from Ranbaxy and
sodium azide GR was obtained from Merck. All the sugars
were used as received.
Thus, various degrees of epoxidized SBS (7, 13, 22 and
46 mol% epoxidized) could be synthesized using the appropriate
amount of mCPBA with (SBS : toluene) ratio of 1 : 10 (w/v)
using the above procedure.
1H NMR [200 MHz, CDCl3, d ppm] showed signals of
different protons at d values of 7.05–6.57 (10H), 5.40 (2H), 4.99
(3H), 2.92 (2H), 2.68 (2H), 2.03(1H), 1.61, 1.58 and 1.55
13C NMR [400 MHz, CDCl3, d, ppm] showed values at 145.26,
142.6, 131.22–127.61, 114.20, 58.42, 56.73, 43.54, 40.28, 38.13,
33.90, 32.66, 27.35 and 24.34
FTIR: (KBr, cm-1) 3080, 3060, 3025, 3004, 2920, 2846, 1639,
1601, 1493, 1452, 1404, 1386, 1352, 1312, 1260, 1071, 1028, 967,
910, 841 and 804.
Analytical methods
1H and 13C NMR analysis. 1H and 13C spectra of all the
samples were recorded on a Bruker 200 MHz with CDCl3 as the
solvent and tetramethylsilane as external reference. The solid
state 13C NMR spectra were recorded at 300 MHz. All the
spectra are referenced at d = 77 ppm for 13C and d = 7.25 ppm
for 1H with respect to CDCl3.
Synthesis of SBS azide from partially epoxidized SBS. In a
250 mL round-bottomed flask maintained under nitrogen, 2 g
of epoxidized SBS (13 mol%) was dissolved in 20 mL pyridine.
After the epoxide dissolved completely, 40 mL of dry DMF
◦
was added slowly at 50 C to ensure that the epoxide does not
precipitate. The reaction mixture was stirred for some time and
the temperature was increased to 70◦ C. Sodium azide 1.5 g,
and ammonium chloride 1.5 g (1 : 1 wt%) was added to the
reaction mixture (Scheme 1). The reaction mixture was heated
FTIR analysis. Spectra were recorded with a Perkin Elmer
1 FTIR instrument with a resolution of 4 cm-1 in transmission
mode, in which the samples were cast onto a KBr disk from a
solution of the elastomer dissolved in chloroform.
◦
at 70 C for 3.5 h, after which the product was precipitated in
500 mL water and was washed until free from pyridine. The
final wash was carried out with methanol and the product was
dried in a desiccator under vacuum at room temperature. This
compound was characterized using FTIR spectroscopy (Fig.
1 and 2), which clearly showed the presence of azide peak at
2104 cm-1 (increasing azide peak with increasing epoxide content
of SBS).
Thermogravimetric analysis (TGA). The thermogravimetric
analysis (TGA) was performed on a Perkin Elmer TGA-7 in N2
atmosphere at heating rate of 10 ◦C min-1.
Scanning electron microscopy (SEM). The surface morphol-
ogy of SBS and functionalized SBS was investigated using a
Leica SEM stereoscan 440, Cambridge, UK. The polymeric
samples in the form of thin films, before analysis were coated
with gold in a sputter coater, in order to achieve conducting
surface and were analyzed at an accelerated voltage (potential)
of 10 kV.
Synthesis of sugar derivatives for click reaction
Synthesis of b-D-glucopyranose pentaacetate. This derivative
was synthesized following the procedure of Wolfrom et al.16 In
a 2 L flask containing 350 mL of acetic anhydride, dextrose
(50 g) was added and the reaction mixture was heated to 50 ◦C.
WAXRD. Wide Angle X-Ray Diffraction (WAXRD) analy-
sis was carried by using a powder XRD Xpert-1217 diffractome-
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Fig. 2 SBS azide with different degrees of azidation (increasing
intensity of N3 peak at 2104 cm-1).
Scheme 1 Synthesis of SBS azide from SBS.
stirred at room temperature for 2 h. After the completion of
the reaction, anhydrous K2CO3 (4.8 g) was added and stirring
was continued for an additional 30 min. The reaction mixture
was filtered and washed with dichloromethane. The filtrate
was washed with water (2 ¥ 150 mL), the aqueous phase was
separated and extracted with dichloromethane (2 ¥ 50 mL) and
the combined organic phase was dried over anhydrous sodium
sulphate (Na2SO4) and concentrated to yield a solid which was
crystallized in dichloromethane–hexane to obtain the derivative
2 (9 g, 90%) as a crystalline compound.
NMR: 1H (CDCl3) d 5.28–4.9 (m, 3H, H-2,3,4), 4.75 (d, 1H,
H-1), 4.35 (d, 2H, H-1), 4.4–4.09 (m, 4H, H-1,6), 3.76–3.67 (m,
1H, H-5), 2.46 (t, 1H, H-3), 2.09, 2.05, 2.01 and 2.0 (4 s, 12 H,
OCOCH3). FTIR: 3278 ( CH), 2118 (C C) and 1749 cm-1
(C O). The clean NMR spectrum along with matching proton
integrations proved the purity of the compound (Fig. 3).
Synthesis of 2-propynyl-b-D-glucopyranoside. These sugar
derivatives were synthesized following the procedure of Roy et
al.17 D-Glucose (7.2 g, 40 mmol) was suspended in propargyl
alcohol (11.6 mL, 200 mmol) and stirred at 65 ◦C. H2SO4–silica
was prepared as follows: to a slurry of silica gel (10 g, 200–
400 mesh) in dry ethyl ether (50 mL) was added commercially
available conc. H2SO4 (3 mL) with shaking for 5 min. The solvent
was evaporated under reduced pressure resulting in free flowing
H2SO4–silica which was then dried at 110 ◦C for 3 h. 200 mg
of this catalyst was added to the propargyl alcohol and stirring
was continued for 2.5 h until all the solids had dissolved. The
reaction mixture was then allowed to stir at room temperature
for another 12 h. TLC (CH2Cl2–MeOH; 5 : 1) showed complete
conversion of D-glucose. The reaction mixtures were transferred
to a silica gel column and the excess propargyl alcohol was
eluted with CH2Cl2 followed by elution of the product glycoside
with CH2Cl2–MeOH (15 : 1) to afford the propargyl derivative
in 80% yield (Scheme 3). This product was used as such for the
succeeding click chemistry step.
Fig. 1 FTIR spectra of unmodified SBS.
Anhydrous sodium acetate (25 g) was added and the mixture was
stirred until a clear solution was obtained. The temperature was
increased to 90 ◦C and the reaction was continued for 2.5 h at this
temperature. It was then cooled and poured with stirring onto 2
L of ice water. After 3 h the crystalline material was filtered and
crystallized from hot methanol. The product is further purified
by recrystallization (1H NMR spectra of the purified product is
presented in the ESI, Fig. (i),† the clean spectrum confirming
the purity of the product).
Synthesis of 2-propynyl-2,3,4,6-tetra-O-acetyl-b-D-glucopy-
ranoside from b-D-glucopyranose pentaacetate. 2-Propynyl-
2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside (2) as shown in
Scheme 2, was synthesized following the method reported by
Mereyala et al.19 To a solution of b-D-glucopyranose pentaac-
etate (10 g, 25.6 mmol) in dichloromethane (200 mL) was
added propargyl alcohol (1.8 mL, 30.7 mmol) and BF3·Et2O
(4.8 mL, 38.4 mmol) at 0◦ C and the reaction mixture was
Synthesis of 2-propynyl-2,3,4,6-tetra-O-acetyl-b-D-glucopy-
ranoside functionalized SBS. SBS azide (2 g) was dissolved
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Scheme 2 Synthesis of 2-propynyl-2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside.
Fig. 3 1H NMR of 2-propynyl-2,3,4,6-tetra-O-acetyl-b-D-glucopyranoside.
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Scheme 3 Synthesis of propargyl glucoside from D-glucose.
in 30 mL dichloromethane. Propynyl-2,3,4,6-tetra-O-acetyl-b-
D-glucopyranoside (500 mg) dissolved in 20 mL of DMSO
was slowly added to the SBS azide polymer solution to avoid
precipitation. CuSO4·5H2O (8 mg) dissolved in 1 mL of water
and sodium ascorbate (13 mg) dissolved in 1 mL of water were
added to the reaction mixture at room temperature (Scheme
4). The reaction was continued for 3 h at the end of which it
was precipitated in methanol. The product was washed with
methanol and finally with water. The FTIR spectrum (Fig. 4)
showed the disappearance of the azide peak at 2104 cm-1, and
appearance of a carbonyl peak at 1759 cm-1.
Fig. 4 Overlapped FTIR spectra of (a) SBS azide and (b) 2,3,4,6-tetra-
O-acetyl-b-D-glucopyranoside linked SBS by click chemistry.
Fig. 5 Overlapped FTIR spectra of (a) SBS azide and (b) glucose linked
SBS by click chemistry.
Results and discussion
Scheme 4 Synthesis of carbohydrate functionalized SBS using click
chemistry approach.
Synthesis
The overall synthesis strategy, as presented in the reaction
schemes above, is based on the azidation of epoxidized SBS
followed by coupling with carbohydrate alkyne partner using
click chemistry to yield carbohydrate functionalized SBS.
Click reaction is the Cu(I) catalyzed cycloaddition of azides
and alkynes. This reaction can be performed in a number of
solvents including water or other polar solvents and is not
affected by presence of functional groups. In addition, the
copper(I) catalyst system is easily available and insensitive to
solvent and pH.11 In our experiments, the copper(I) catalyst
system was introduced to the reaction system by reducing
copper(II) sulphate pentahydrate with sodium ascorbate. The
reaction was performed in CH2Cl2–DMSO solution at room
temperature for 3 h. We do not perceive any difficulty in scale up
of this reaction since the reaction proceeded smoothly at room
temperature.
Synthesis of 2-propynyl glucose functionalized SBS. SBS
azide (2 g) was dissolved in 30 mL dichloromethane. Sugar
derivative (2-propynyl glucose, 230 mg) dissolved in 15 mL of
DMSO was slowly added to the SBS azide polymer solution
to avoid precipitation. CuSO4·5H2O (8 mg) dissolved in 1 mL
of water and sodium ascorbate (13 mg) dissolved in 1 mL of
water were added to the reaction mixture at room temperature.
The reaction was continued for 3 h at the end of which it
was precipitated in methanol. The product was washed with
methanol and finally with water. The product was characterized
by FTIR spectroscopy (Fig. 5), the disappearance of the azide
peak signifying completion of the reaction. This reaction was
very facile in our hands and appears to be easy to scale up. Scale-
up studies will be taken up as we prepare more such modified
polymers for developing applications.
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Fig. 2 shows facile formation of azide groups on epoxidized
SBS. Fig. 4 shows the overlapped FTIR spectra of SBS azide and
the click reaction product of SBS azide with propargyl acetyl
protected glucose. It can be observed from the above spectra
that after click reaction the peak at 2104 cm-1, corresponding
to azide, completely disappears. The acetyl protected glucose
shows a new peak at 1759 cm-1 due to the carbonyl stretching
of acetyl group. The simultaneous disappearance of the azide
peak and appearance of carbonyl peak in the FTIR spectra of
the product clearly proves that functionalization of SBS with
protected glucose has been successful. We might add here that
had we carried out the other possible synthetic strategy – i.e.
sugar azides to react with propargyl SBS – then the sugar azide
may have become an explosive molecule. It is well known that six
carbon atoms per azide groups is approximately the threshold for
safety of azides. However, by attaching azide on the SBS polymer
chain, and also in very low concentration, we make the process
very safe. We selected the sample with 13% epoxy content of the
butadiene component of SBS to prepare the azide, and only a
fraction of this epoxy was converted to the azide. We estimate
less than 5% of the SBS chain to have azide linkages, which
makes for a large interval between azides, and correspondingly
they are very safe. Our synthesis procedures lead us to believe
that the synthesis of the azide as well as the click reaction were
very safe, and amenable to scale up.
Fig. 5 shows the overlapped FTIR spectra of SBS azide and
the click reaction product of SBS azide with propargyl glucose. It
can be observed from the above spectra that after click reaction
the peak at 2104 cm-1, characteristic of azide (N3), disappears
completely due to the cycloaddition of azide and alkyne leading
to the formation of a 1,2,3-triazole linkage. Thus FTIR clearly
shows the successful anchoring of glucose units to SBS by click
chemistry. In this reaction it is not necessary to protect the
hydroxyl groups of the glucose because hydroxyl groups do not
interfere with the formation of the triazole ring.
Fig. 6 (a) Overlapped TGA curves of SBS, SBS azide and glucose
linked SBS by click reaction. (b) Overlapped TGA curves of SBS, SBS
azide and tetraacetyl glucose linked SBS by click reaction.
Thermogravimetric analysis (TGA)
These results are also borne out from the differential ther-
mogravimetry (DTG) studies (Fig. 7). The DTG curve of SBS
is clearly differentiated from those of chemically modified SBS
TG curves of various sugar linked SBS were ◦studied under
pure nitrogen atmosphere at a heating rate of 10 C min-1 from
50–600 ◦C.
Fig. 6(a) shows the comparative thermal stabilities of SBS,
SBS azide and glucose linked SBS synthesized by click reaction.
It is clear that the thermal stability of the three polymers is
in the order of SBS > SBS azide > glucose linked SBS. SBS
is stable up to 400 ◦C, after which it starts degrading rapidly.
The rate of degradation reached a maximum at 525 ◦C and
at 600 ◦C it degrades almost completely leaving behind only
~0.63 wt% residue. It is seen from Fig. 6 that sugar linked
SBS samples shows higher residue at 600 ◦C, as compared to
unmodified SBS, with SBS azide being in between the two.
Clearly, chemical modification affects the thermal stability of the
SBS. The tetraacetyl glucose linked SBS is even more thermally
unstable than the (unprotected) glucose linked SBS (Fig. 6(b)).
Acetyl groups may be the cause of lower thermal stability,
since these groups dissociate at low temperatures. The onset of
degradation of SBS was ~478 ◦C, while it was ~450 ◦C for SBS
azide, 428 ◦C for glucose linked SBS and 426 ◦C for tetraacetyl
glucose linked SBS (Fig. 6).
Fig. 7 Overlapped DTG curves of SBS by click reaction.
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samples by its main exothermic peak at a significantly higher
temperature than the modified SBS samples.
Thus we can say that functionalization of SBS with sugar
leads to a decrease in thermal stability.
Morphology by wide-angle X-ray diffraction studies
SBS is an amorphous thermoplastic elastomer, hence broad
peaks corresponding to polybutadiene and polystyrene are
observed in the wide-angle X-ray diffraction of unmodified
SBS.18 All the samples synthesized were scanned from 2q =
3–30◦ at room temperature. Fig. 8(a) shows the wide-angle X-
ray diffraction plots of unmodified SBS and azidated SBS. The
more intense peak is centered at 2q = 19.7◦ and corresponds
to amorphous polybutadiene, the weaker one is centred at
2q = 9.5◦ and corresponds to polystyrene. It can be observed
from Fig. 8 that after azidation of SBS there is significant
decrease in intensity of the peak at 2q = 19.7◦ associated with
the polybutadiene domain. The peaks at 2q = 9.5◦ and 28.7◦
associated with polystyrene disappear.
Fig. 8(b, c) shows the wide-angle X-ray diffraction plots of
tetraacetyl glucose and glucose linked SBS by click reaction. It
can be observed that after functionalization of SBS with sugar
there is a significant decrease in intensity of the peak at 2q = 19.7◦
associated with the polybutadiene domain. This decrease in
intensity is more pronounced in tetraacetyl glucose linked SBS.
Thus, there is significant disruption of the polybutadiene domain
upon attaching sugar moieties. These could be important sites
for biodegradation of the SBS polymer chain.
Morphology by scanning electron microscopy (SEM)
Thin films of the polymer samples were prepared from chlo-
roform solutions for examining their surface morphology by
SEM. The SEM images are shown below in Fig. 9(a–d), which
shows the surface morphology of SBS, SBS azide, glucose linked
SBS and tetraacetyl glucose linked SBS, respectively, by click
reaction. SBS is considered a two-phase thermoplastic block
copolymer in which spherical polystyrene (PS) domains are dis-
persed in a continuous polybutadiene (PB) phase. It is observed
from Fig. 9 that there is not much change in morphology after
azidation of SBS, but after functionalizing with sugar by click
reaction the morphology changes. The glucose linked SBS shows
significant destructuring of the two polymer domains, and this is
even more accentuated in the tetraacetyl glucose linked SBS, due
to the bulky acetyl groups. These morphological studies show
that the destructured polymers are more likely to biodegrade
than the parent unmodified polymer, since they have greater
surface area and more functional groups for the soil microbes
to latch on to.
Fig. 8 (a) WAXRD diffraction plots of curve a (unmodified SBS) and
curve b (SBS azide) (b) WAXRD diffraction plots of curve a (SBS
azide) and curve b (tetraacetyl glucose linked SBS by click reaction) (c)
WAXRD diffraction plots of curve a (SBS azide) and curve b (glucose
linked SBS by click reaction).
Biodegradation studies
Preliminary studies were carried out on quantification of
the sugar units attached to SBS, using the phenol–sulfuric
acid method, and the same protocols were followed for as-
sessing biodegradability as in our studies with sugar-linked
polystyrene.15 It was found that incorporation of 0.09% glucose
onto SBS resulted in a weight loss of 3.3%, i.e. 36.6 times
more than the weight of sugar incorporated. These studies are
being supplemented by studies with several bacterial and fungal
cultures, and also by incorporation of a variety of sugars such
as xylose, lactose, mannose, etc. The detailed studies will be
published independently.20 Suffice to say, incorporation of even
minute quantities of sugar units onto hydrophobic polymers
like SBS significantly affects its thermal, morphological and
biodegradation properties. While we have not presented any
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Fig. 9 (a) SEM micrographs of SBS. (b) SEM micrographs of SBS azide. (c) SEM micrographs of glucose linked SBS. (d) SEM micrographs of
tetraacetyl glucose linked SBS.
spectral evidence to show biodegradation occurs on the polymer
chain, we feel that this can easily be speculated to be so, since
a 36.6 fold weight loss (based on sugar content) of the sugar
containing SBS polymer cannot be explained simply on the
basis of sugar content; some part of the polymer has also to
be lost. Our detailed forthcoming paper on spectral analysis of
biodegraded samples will shed more light on this aspect. This
work is currently ongoing.
unit weight of sugar incorporated on SBS) is much higher than
expected. Thus, it can be considered to be a general result that
random attachment of sugar molecules on to synthetic polymers
is an important strategy to induce biodegradability in these
polymers. The present study successfully shows that low levels
of sugar molecules anchored onto styrene–butadiene–styrene
copolymer, a widely used commodity polymer which is not
biodegradable, biodegrades to a larger than expected extent.
Spectral, morphological and thermal studies of the modified
polymers were carried out to show the dramatic changes in
the properties of these modified polymers. Thermal stability
of glucose linked SBS had onset of degradation at 428 ◦C,
down from 478 ◦C observed for SBS. Morphology studied
by WAXRD and SEM showed destructuring of the polymer
domains of SBS, which is beneficial for biodegradation of
these polymers. These results are in tandem with those earlier
reported for polystyrene-linked sugars, which were found to be
biodegradable.
Conclusions
In our previous study,15 we had shown the effect of attach-
ment of a few sugar molecules onto styrene–maleic anhydride
copolymer by simple esterification reaction and proved the
dramatic improvement in its rate and extent of biodegradation.
In this study, we have used click chemistry to incorporate non-
hydrolysable sugar units (C–C bonds), and shown that even
here the degree of biodegradation (in terms of weight loss per
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Acknowledgements
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This work was supported by a Senior Research Fellowship grant
by CSIR, New Delhi, for working towards a Ph.D. degree, to
Mr. Rakesh Singh.
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