Direct extraction of carbonyl from waste polycarbonate with amines under environmentally friendly conditions: Scope of waste polycarbonate as a carbonylating agent in organic synthesis

Article abstract of DOI:10.1039/c4ra14319a

An efficient green method for converting waste polycarbonate into urea derivatives by reacting with primary amines has been developed. Simple treatment of polycarbonate plastic with primary amines in a closed vial at 80 °C without using any catalyst and toxic solvents made this process environmentally friendly. Digestion of the waste polycarbonate obtained from CDs and DVDs with amines affords functionalized urea and 4,4′-(propane-2,2-diyl)diphenol (BPA, bisphenol-A). The procedure is optimized to get maximum conversion of polymer to urea and its derivatives as a major product. The purification procedure to isolate the urea derivatives in the presence of bisphenol-A has been tuned to avoid chromatographic procedures. This environmentally friendly method provides (i) an alternative for recycling BPA from polycarbonate, (ii) a method of obtaining useful product like urea derivatives, (iii) scope for new carbonylating agents in organic synthesis, (iv) an amine functionalized polycarbonate surface. This journal is

Full text of DOI:10.1039/c4ra14319a

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PAPER
Direct extraction of carbonyl from waste
polycarbonate with amines under environmentally
friendly conditions: scope of waste polycarbonate
as a carbonylating agent in organic synthesis†
a
Cite this: RSC Adv., 2015, 5, 3454
Sukhdeep Singh, Yong Leib and Andreas Schobera
*
An efficient green method for converting waste polycarbonate into urea derivatives by reacting with primary
amines has been developed. Simple treatment of polycarbonate plastic with primary amines in a closed vial
at 80 ꢀC without using any catalyst and toxic solvents made this process environmentally friendly. Digestion
of the waste polycarbonate obtained from CDs and DVDs with amines affords functionalized urea and 4,40-
(propane-2,2-diyl)diphenol (BPA, bisphenol-A). The procedure is optimized to get maximum conversion of
polymer to urea and its derivatives as a major product. The purification procedure to isolate the urea
derivatives in the presence of bisphenol-A has been tuned to avoid chromatographic procedures. This
environmentally friendly method provides (i) an alternative for recycling BPA from polycarbonate, (ii) a
method of obtaining useful product like urea derivatives, (iii) scope for new carbonylating agents in
organic synthesis, (iv) an amine functionalized polycarbonate surface.
Received 11th November 2014
Accepted 4th December 2014
DOI: 10.1039/c4ra14319a
www.rsc.org/advances
that can be reused for the synthesis of fresh polycarbonate.3
Recently there are some chemical methods appeared in the
Introduction
literature that are suitable to recover BPA from waste PC. Most
of them are using alcoholysis and aminolysis as main chemical
processes for retro-polymerization of PC to BPA. In case of
alcoholysis methanol has proven to be the suitable solvent in
presence or absence of catalytic amount of alkali.3–5 The reac-
tion delivers BPA and dimethylcarbonate (DMC) as main
products. Though DMC is a useful by-product but it is a volatile
organic compound (VOC), which is difficult to handle under
high temperature conditions. On the other hand aminolysis of
PC produced urea derivatives as by-product that is not a volatile
organic compound but this method is known to be performed
in toxic organic solvents like DCM, toluene and dioxane.6,7 In
order to avoid toxic organic solvents in this process, super-
critical water8 and ethanol9 was used for depolymerization of
PC. However, these processes are requiring special reactors that
have to be operated at high temperature and pressures.
In this research article we are presenting an innovative
chemical transformation for waste polycarbonate. The
proposed process has two advantageous features: one is the
recovery of BPA, and the other is the utilization of carbonyl of
polycarbonate as carbonylating agent under green conditions.
Traditionally, carbonylation is a metal catalyzed process that
involves the use of toxic carbon monoxide (CO), e.g., for the
formation of amide bonds and ureas.10 Other chemical reagents
that are used for carbonylation are phosgene,11 triphosgene,12
dimethyl carbonate13 and carbon dioxide.14 Recently commer-
cial devices for carbonylation through ex situ generation of CO
Since the early 1900s, synthetic polymers have been introduced
as man-made materials that have been used for many purposes
in our daily life. In addition to the rst examples of polymer like
bakelite, polyethylene, polystyrene etc., polycarbonate (PC) has
received a great success in the plastic industry. Due to its high-
performance properties like transparency, temperature resis-
tance and toughness the utility of this polymer has spread to
medical, construction, electronic and multimedia industries.
This way a huge amount of synthetic polycarbonate has been
introduced to our ecosystem. The major challenge is the
management of the waste polycarbonate. Therefore, polymer
recycling methods are of high importance in the present time
where environment protection is at priority. One way is the
mechanical recycling or blending of PC that provides material
which is not suitable for high-performance applications.1
Another way is the thermal degradation or pyrolysis that is
limited by its low selectivity and large amount of side products.2
Still another way is the chemical recycling that deals with the
de-polymerization or digestion of PC to obtain BPA as monomer
aDepartment of Nanobiosystem Technology, Institute of Micro- and Nanotechnologies
MacroNano®, Institute of Chemistry and Biotechnology, Ilmenau University of
Technology, Prof.-Schmidt-Str. 26/Heliosbau, 98693 Ilmenau, Germany. E-mail:
sukhdeep.singh@tu-ilmenau.de; Fax: +49 3677 69-3379
bDepartment of 3D Nanostructuring, Institute for Physics & IMN MacroNano® (ZIK),
Ilmenau University of Technology, Prof. Schmidt Str. 26, 98693 Ilmenau, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14319a
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has also been introduced.15 Recently, Yalfani et al. has devel- we have used little excess of benzylamine (2.5 equivalents) so
oped a green protocol for the carbonylation by using base that the reaction occurrence can be monitored simply by
catalyzed in situ generation of CO from methyl formate as disappearance of PC solid (Table 1). For 100 mg of poly-
reagent of choice.16 These reagents have very well served the carbonate 1.0 mmol of benzylamine was used, which is about
purpose of carbonylation in organic synthesis but they are 2.5 equivalent as compared to preserved carbonyl or BPA in the
suffering from their extreme toxicity, complex reaction condi- polymer matrix.
tions and use of other co-reagents.
At 40 and 60 ꢀC in all the reactions no conversion can be seen
In this study, we are introducing an environmentally and over 72 hours (Table 1, entry 1, 2, 5, 6, 9, 10). However at 80 ꢀC
user friendly protocol for harvesting carbonyl from waste poly- both EtOH and i-PrOH have shown disappearance of the poly-
carbonate that is suitable to be used under normal laboratory mer by reaction with benzylamine over 12 hour heating. Aer
ꢀ
cooling at 0 C 58% and 55% N,N0-dibenzylurea (3a) has been
conditions. The procedure is optimized to achieving
a
minimum use of toxic solvents, high temperature and pressure isolated from EtOH and i-PrOH respectively (Table 1, entry 3
conditions and high efficiency in terms of processing time. In and 11). In case of treatment of PC with benzylamine at 100 ꢀC
addition to recover BPA as raw material for PC synthesis, the in EtOH and i-PrOH the polymer gets completely dissolved only
main attention has been given to trap carbonyl monomer in the aer 4 hours and upon cooling furnished 3a in 44 and 42%
form of urea derivatives. These useful by-products can serve as isolated yields (Table 1, entry 4 and 12). In order to recover BPA,
starting material for the synthesis of new drug-like precursors. from such reaction mixtures where polymer gets completely
consumed, the dried ltrate was dissolved in 1 : 1 mixture of
EtOH in H2O and slow evaporation furnished BPA 2 (yields are
Result and discussion
given in Table 1). The water based reactions have shown no
ꢀ
ꢀ
disappearance of polymer at 40 and 60 C. At 80 and 100 C a
melting effect on the polymer has been seen. Moreover upon
cooling the reaction mixture furnished a solid product which
appears to be complex mixture of polymers that is why water
was not considered as optimal reaction solvent under these
conditions (Table 1, entry 7 and 8). From the optimization
experiment it is clear that both ethanol and 2-propanol can be
used as reaction medium at 80 ꢀC and 100 ꢀC. Among these two
alcoholic solvents we opted for ethanol as reaction medium for
further investigations. The temperature 80 ꢀC has been
Looking at the chemical composition of the polycarbonate (PC),
it consists of two monomers one is 4,40-(propane-2,2-diyl)
diphenol (BPA) and other is carbonyl group (Scheme 1). Most
oen the green methods for chemical recycling of the PC are
emphasizing on the recovery of BPA, but the carbonyl group is
either wasted or converted to simple chemicals like dimethyl
carbonate (DMC).5 Few other, non-green chemical recycling
methods of PC are showing utilization of the carbonyl unit by
reacting with mono and bi-dentate amine, alcohols and
thiols.6,17 Our aim in this study is to develop a green method
that can be used for harvesting carbonyl from polycarbonate for
making useful derivatives, and to keep this method as green as
possible by obeying the 12 principles of green chemistry.18
Therefore, during optimization experiments we have limited
our self to safe solvents that don't possess high toxicity e.g.
ethanol, i-propanol and water. Initially, we have chosen simple
benzylamine 4a as reactant and reaction temperatures of 40 ꢀC,
60 ꢀC, 80 ꢀC and 100 ꢀC. Because all the reactions were per-
formed in closed vial we limited the reaction screening
ꢀ
considered as optimal instead of 100 C due to safety issues of
using ethanol in close vial at 100 ꢀC. Aer screening the reaction
conditions in terms of temperature and solvent, we reduced the
Table 1 Reaction conditions optimization for converting PC to N,N0-
dibenzylurea
ꢀ
temperature to 100 C only. Technically, minimum 2.0 equiva-
lents of 4a are required to react with the carbonate functional
group in order to make N,N0-dibenzylurea 3a. In order to screen
the best solvent and temperature conditions for this reaction, Entry
Solvent
T (ꢀC)
% Yielda,b (2)
% Yielda (3a)
c
1
2
3
4
5
6
7
8
EtOH
EtOH
EtOH
EtOH
H2O
H2O
H2O
H2O
i-PrOH
i-PrOH
i-PrOH
i-PrOH
40 ꢀC
60 ꢀC
80 ꢀC
100 ꢀC
40 ꢀC
60 ꢀC
80 ꢀC
100 ꢀC
40 ꢀC
60 ꢀC
80 ꢀC
100 ꢀC
—
—
62
60
—
—
—
—
—
—
63
60
—
c
—
58
44
c
—
c
—
d
—
d
—
c
9
—
c
10
11
12
—
55
42
Combined isolated yield. b Slow evaporation of residual mixture from
1 : 1 mixture of EtOH : H2O. c Undigested polymer recovered. d Complex
polymer melt is obtained.
a
Scheme 1 Polycarbonate as source of BPA and carbonyl equivalent.
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amine concentration to nd the required amount. It has been further chemical transformations. This reaction went smoothly
noticed that minimum 2 equivalent of benzylamine are neces- and furnished desired product 3j in 72% yield.
sary to consume whole polymer under above stated conditions.
In an independent experiment we have checked the utility of
Which is logical, as the synthesis of urea derivative requires this reaction when a mixed polymer waste is considered. We
reaction of one carbonyl equivalent with 2 amine molecules. have deliberately mixed the pieces of 100 mg each of poly-
When lower concentrations of benzylamine were reacted with styrene, polypropylene, polyethylene and polycarbonate in the
polycarbonate the reaction solution turns turbid and partially same vessel and treated with 200 mL of benzylamine in 3 ml
ꢀ
broken polymer was also recovered from these reactions.
ethanol at 80 C. As expected only PC pieces disappeared from
Aer optimizing the reaction conditions we explored the the reaction, while the other polymers remain unreacted and
scope of this reaction by using differently substituted benzyl- can be ltered away. Upon cooling the mixture N,N0-dibenzy-
amines (4, Table 2). In case of the reaction of polycarbonate with lurea 3a is precipitated (NMR) and BPA remains in the ltrate.
p-methoxy (4b) and m-methoxy benzylamines (4c) the required We consider this reaction as a valuable method to selectively
products 3b and 3c were isolated in 62% and 60% isolated treat and separate waste PC from mixed plastic waste, which can
yields. To keep this method green we avoid chromatography be a very valuable method where type of polymer is not dened.
with toxic solvents, therefore, the reported yields are from the
For extending the scope of this reaction we have chosen few
sum of directly precipitated products from crude ethanol reac- alkyl amines in order to see if the reactivity varies by the way
tion mixture at 0 ꢀC. It's important to mention that in the cases (Table 3). The reaction of the commercially available 40%
where substituted dibenzylurea is having similar solubility in solution of methyl amine (4k) in methanol with PC went very
ethanol like BPA, the pure product isolation by precipitation smoothly. The polymer chip disappeared completely within 5
method became difficult.
minutes of heating at 80 ꢀC. We have performed the reaction in
In order to see the inuence of electron withdrawing groups excess amount of methyl amine, which can be easily removed
on this reaction we have chosen uoride and triuoromethyl from reaction by simple evaporation. However, both BPA and
groups on benzylamine. Reaction of p-uoro benzylamine with N,N0-dimethylurea (3k) are very well soluble in ethanol, which
PC furnished required product 3d in 52% isolated yield. The was a suitable green solvent in the optimized cases discussed
second component bisphenol-A (BPA) has been precipitated by before. Therefore, we opted water as another friendly solvent for
slow evaporation from ca. 1 : 1 ethanol–water mixture of crude precipitating BPA (low solubility in water) instead of N,N0-
ltrate.
methylurea (relatively high solubility in water). In this case urea
In case of ortho substituted triuoromethyl benzylamine derivative has been puried by rst removing BPA from water
prolong heating at 80 ꢀC did not helped in digesting the PC for solution by ltration and then concentrating ltrate water.
yielding 3e under optimized reaction conditions. Unreacted but However, aer multiple crystallization steps a trace amount of
damaged PC pieces were recovered from this reaction. The bulk BPA was always present in the puried sample of 3k. Though
and electron withdrawing property of the o-triuoromethyl the sample of N,N0-methylurea (ꢁ50% yield) thus obtained is
group hinders the nucleophile addition of the primary amine sufficiently pure for being further use as reactant in organic
on the carbonate functional group of polymer. In order to synthesis (for NMR purity see ESI†). Similarly, direct reaction of
understand this particular reaction, we investigated the effect of excess of 30% solution of ammonia (4l) in water with PC pieces,
triuoromethyl group on meta and para positions of the under these conditions furnished clear solution at 80 ꢀC. Upon
benzene ring. It has been noticed that in case of m-tri- cooling the reaction mixture at room temperature BPA gets rst
uoromethyl benzylamine and p-triuorobenylamine the precipitated and isolated by ltration. Aer extracting three
required products 3f (28%) and 3g (48%) were formed under crops of BPA from ltrate solution pure urea (4l) has been le in
these conditions. This concluded that it's not the high electro- the ltrate, which is isolated in 68% yield. In case of long chain
withdrawing capacity of triuoromethyl group that hinders alkyl amines like decylamine (4m) the reaction was performed
the reaction but the steric hindrance of bulky group at ortho in ethanol and the product 3m get separated immediately upon
position. The other component BPA was recovered from crude cooling at room temperature. However the product is highly
ltrate as described before. For investing further the effect of insoluble in most organic solvents. The reaction of cyclohexyl-
bulk near the amine functional group, we explored the use of amine (4n) under similar conditions furnishes N,N0-bis
diphenylmethylamine 4h and tritylamine 4i under optimized (cyclohexyl)urea 3n in 81% isolated yield. For providing further
set of conditions. In both the cases the polymer gets completely diversity to this method we have chosen propagylamine (4o) as
digested and le clear reaction mixture behind. Upon cooling nucleophile in ethanol. The terminal alkyne in this case can act
the crude ethanolic reaction mixture, where 4h was used as as a very good functional group to react further chemical
reactant, the required product 3h has been obtained in 81% moieties through click chemistry. This reaction has furnished
yield. However, in second case where tritylamine 4i was reacted desired product 3o in 72% yield. Overall, we have noticed that
with PC, though the polymer get completely digested, no pure the reactions of alkylamines with PC are relatively faster than
N,N0-ditritylurea (DTU) can be isolated using green solvents.19 that of benzylamines (2). With an exception of aqueous NH3
Lastly, we used p-amino benzylamine 4j as reactants for this (Table 3, entry 2) with PC, where the reaction time was relatively
transformation because it provides aromatic amine on the longer. The probable reaction for this could be the hydropho-
benzene ring which could be used as functional group for bicity of the PC which repels the water from the surface and
could delay the digestion process.
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Table 2 Derivatives of dibenzylurea 3 synthesized from reaction of benzylamines 4 with PC 1 in EtOH after 12 h at 80 ꢀ
C
Entry
Benzylamine (4)
Urea derivatives (3)
Isolated yield
58
a
b
c
62
60
d
e
52
—
—
f
28
48
g
h
81
i
j
—
72
Aer checking the feasibility of the reaction for benzyl withdrawing and releasing substitutions. Unfortunately, none
(Table 2) and alkyl (Table 3) amines, we tested the same reaction of them seems to be reacting with the PC even when treated for
ꢀ
conditions for its performance when aryl amines are used. We long period of time. Even raising the temperature until 100 C
have examined various derivatives of aniline with electron does not pushed the reaction and undigested PC was recovered
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Table 3 Derivatives of N,N0-dialkylurea 3 synthesized from reaction of
alkylamine 4 with PC 1 in EtOH after 12 h at 80 ꢀC
solution and form urea derivatives D (Scheme 2). Other possible
route of this reaction is the direct addition of amine as nucle-
ophile on the carbonate group of the polymer to form urethane
linkage (B0) as rst step. In the second step another molecule of
amine (C) react with urethane functional group and release urea
derivative D from polymer into solution. Aer carefully exam-
ining the reaction we have found no evidence of the formation
of DEC. It has also been reported in the literature that in case of
digestion of PC with supercritical ethanol DEC is formed only
Entry Alkylamine (4)
Urea derivatives (3)
Isolated yield
k
l
MeNH2
NH3
ꢁ50
68
78
ꢀ
when reaction is performed at 290 C by using a high-pressure
batch autoclave reactor.9 Therefore, looking at the reaction
conditions of presented method that uses 80 ꢀC, and no signs of
DEC in analysis rules out the possibility of rst route.
m
N
CH3(CH2)9NH2
For proving the second reaction route as a probable
sequence, we have done an independent experiment where PC
(A) is reacted with bidentate 1,5-pentyldiamine (E) just below
the concentration where polymer get started cracking. In order
to see if 1,5-pentyldiamine is incorporated to the PC surface a
washed sample is treated with dansyl chloride (G) in ethanol at
room temperature to covalently bind dye molecule to free
terminal amines (F). As expected the dye gets very well integrate
to the polymer surface (Scheme 3, image H). However, when
monodentate pentylamine (I) reacted with polymer surface and
81
72
O
as it is. The probable reason for this reaction failure is low
nucleophilicity of the aryl amines, which can be increased using
some catalyst or base. To keep present method green and
catalyst free this does not come under the scope of present
study.
For the mechanism of this reaction there are two possibili-
ties how urea can be formed by harvesting carbonyl from PC.
One possible way is the base (amine) catalyzed reaction of
ethanol with PC forming diethylcarbonate (DEC, B) as inter-
mediate. Thus formed DEC can react with amines (C) present in
Scheme 2 Proposed reaction mechanism.
3458 | RSC Adv., 2015, 5, 3454–3460
Scheme 3 Polycarbonate surface functionalization.
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Separation of N,N0-disubstituted ureas and bisphenol-A
similarly treated with dansyl chloride (G) have not shown
incorporation of dye as indicated by LSM imaging (Scheme 3,
image K). These experiments hint towards the validity of the
second proposal of the reaction mechanism (Scheme 2). Addi-
tionally, this experiment provides another method that can be
used to functionalize the surface of the PC with primary amine
groups. This indicates that in addition to be a green method for
urea derivatives, BPA recovery from waste PC and harvesting
carbonyl from PC this method is useful also for functionalizing
PC surface with various amines. Therefore, by controlling the
concentration of stock solution all of the amines used in this
study (Table 2 and 3) are capable for being graed on the
polymer surface without causing any serious damage to the
surface (SEM images of samples reacted with propargyl amine
under different concentrations are given in ESI†). It makes this
method further useful for surface modication of poly-
carbonate with applications in biomaterials and biotechnology.
Aer complete digestion of the polymer under above described
conditions, the reaction has been transferred at 0 ꢀC and stored
until the crystallization occurs. The precipitates are washed
with 2 ml cold ethanol to obtain pure crystals of N,N0-disubsti-
tuted ureas. Thus obtained ltrate has been treated same way to
obtain further crop of the urea derivatives.
Aer complete extraction of urea derivatives (usually in three
crops) the rest of ethanoic mixture is diluted with equal amount
of distilled water. A slow evaporation of the mixture at room
furnished amorphous bisphenol-A as pure product which has
been checked with NMR and elemental analysis.
Acknowledgements
The authors gratefully thank Federal Ministry of Education and
Research (BMBF) for providing nancial support within the
Initiative “Centre for Innovation Competence” Meta-ZIK (BioL-
ithoMorphie: FKZ 03Z1M511). Also the nancial support from
Optimi, FKZ: 16SV3701, FKZ 16SV5473, Carl Zeiss FKZ 0563-2.8/
399/1 and by the Thuringian Ministry of Culture (Nano-
zellkulturen, FKZ: B714-09064) is gratefully acknowledged.
Additionally authors thank Prof. Uwe Ritter and Dr Yixin Zhang
for helpful scientic discussions and Katrin Risch, Carmen
Siegmund for technical support. Sukhdeep Singh would like to
sincerely thank Jagdeep Kaur for the inspiration to pursue
polymer recycling and environmental issues.
Conclusion
In summary, we have described an unprecedented green and
user friendly method for the treatment of polycarbonate plastic
for recovering useful monomer BPA. Apart from the polymer
recycling, in the reported method a proof of the concept to
extract carbonyl group from polycarbonate has been success-
fully demonstrated. To the best of our knowledge, it is the rst
green method where carbonate group of polycarbonate has
been extracted to make useful urea derivatives under environ-
mentally friendly conditions. Further, this methodology
provides a scope to remove PC from the mixed plastic waste,
which could be helpful for the waste management of unspeci-
ed plastic material. Using this chemical approach, poly-
carbonate can be explored as source of carbonyl equivalent that
can be used under green set of conditions to make chemically
and biologically interesting molecules.
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