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Synthesis and properties of organo-gels by thiol-benzoxazine chemistry
Accepted Manuscript Synthesis and Properties of Organo-gels by Thiol-Benzoxazine Chemistry Semiha Bektas, Baris Kiskan, Nermin Orakdogen, Yusuf Yagci PII:
S0032-3861(15)30165-8
DOI:
10.1016/j.polymer.2015.08.026
Reference:
JPOL 18046
To appear in:
Polymer
Received Date: 15 June 2015 Revised Date:
12 August 2015
Accepted Date: 14 August 2015
Please cite this article as: Bektas S, Kiskan B, Orakdogen N, Yagci Y, Synthesis and Properties of Organo-gels by Thiol-Benzoxazine Chemistry, Polymer (2015), doi: 10.1016/j.polymer.2015.08.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Synthesis and Properties of Organo-gels by Thiol-Benzoxazine Chemistry Semiha Bektas1, Baris Kiskan1*, Nermin Orakdogen1, Yusuf Yagci1 Istanbul Technical University, Department of Chemistry, 34469, Maslak, Istanbul, Turkey
RI PT
1
Abstract
SC
In the present work, a synthetic strategy based on oxazine-thiol ring-opening reaction for the synthesis of organogels is reported for the first time. For this purpose, main-chain
M AN U
polybenzoxazine (PBZ) was synthesized by the reaction of bisphenol A, 1,6-diaminohexane and paraformaldehyde in toluene:methanol mixture (2:1, v:v). The obtained reactive PBZ was then reacted at room temperature with dithiol compounds in CHCl3:CH3OH (1:1, v:v), namely 1,2-ethanedithiol and 1,6-hexanedithiol. The obtained PBZ based gels was characterized by
TE D
spectral, thermal analysis and uniaxial compression measurements. The influences of the gel preparation conditions on the swelling behavior and the mechanical properties of the resulting gels were investigated. Swelling ratios (J) up to 8.4 in CHCl3:CH3OH solution was obtained.
EP
Moreover, the curing of the gels was performed over remaining oxazine moieties in the gels and the thermally activated curing was found to be at lower temperatures than that
AC C
corresponding PBZ.
Keywords: Polybenzoxazine, Benzoxazine, Organo-gel, Ring-opening polymerization, Crosslinking Corresponding Author: Baris Kiskan e-mail: [email protected] Tel: +90 212 285 6825 Fax: +90 212 285 6386
1
ACCEPTED MANUSCRIPT INTRODUCTION Polybenzoxazines have gained an increasing interest in the field of thermosetting research due to the superior properties that overcome several short-comings of conventional novolac and resole-type phenolic resins. Thermally driven synthesis without any initiator or curing agent
RI PT
from the corresponding 1,3-benzoxazine monomers yields a cross-linked polymer composed of a phenolic and a tertiary amine bridge unit as the constitutional pattern (Scheme 1). Extensive intra- and inter-molecular hydrogen bonding formed between the phenolic –OH
SC
and tertiary amine groups augments excellent features to polybenzoxazine networks. These materials offer low water absorption, high char yield and resistance against flame, high
M AN U
modulus, high strength, high glass transition temperatures, chemical resistance, long shelf life and dimensional stability upon curing. [1-11] Thus, polybenzoxazines have become one of a rare few new polymers developed and commercialized in the past 20 years. Another reason for that is benzoxazine monomers have enormous molecular design flexibility. Basically, the
TE D
synthesis of a monomer is carried out by the reaction of any suitable phenol, formaldehyde and primary amine (aliphatic or aromatic) (Scheme 1).[12-34]
EP
In a recent publication, Gorodisher et al. reported that benzoxazines can undergo catalytic
AC C
ring opening reaction at ambient conditions by thiols. [35] The process is called as “Catalytic Opening of the Lateral Benzoxazine Rings by Thiols” (COLBERT) which proceeds over a two-step reaction. In the first step, the amine of benzoxazine is protonated by thiol to form an ionic intermediate. Subsequently, the formed thiolate attacks through the methylene carbon between N and O atoms in the oxazine causing ring-opening of the benzoxazine (Scheme 2). The overall process is similar to acid catalyzed nucleophilic addition and simultaneous ringopening reaction of benzoxazines.[36]
2
ACCEPTED MANUSCRIPT
COLBERT process was found to be a reversible reaction resulting in continuous regeneration of the thiol in the medium. Thus, only small amounts of thiols reduce the curing temperature
RI PT
of benzoxazines effectively. The reversible nature of COLBERT was investigated in detail by Endo and co-workers using model reactions and an apparent relationship was observed with the solvent system used. In the same study, the application of this reaction to a new
SC
polyaddition–depolymerization system was also performed yielding linear polymers. [37] Another useful application concerns construction of a highly efficient curing system. Cross–
M AN U
linked but additionally curable soft benzoxazine films were prepared by simultaneous photoinduced thiol-ene and COLBERT reactions using difunctional thiol and diallybenzoxazine. [38] Similar approach was also used to form dual-cure hybrid polymer networks. [39] In a recent report from the authors’ laboratory, it was demonstrated that
TE D
benzoxazines can be used as a linker between two different polymers to form a block copolymer by using COLBERT as a ligation process in a manner of Thiol-X chemistry. [4046] Likewise, main-chain polybenzoxazine precursor from bisphenol A and diaminohexane
EP
was reacted with various thiols at ambient temperature, namely thiophenol, 2-ethanethiol and 1-butanthiol to yield side-chain functional polybenzoxazines. [47] These initial studies clearly
AC C
present the potential use of thiol-oxazine reaction in polymer and material chemistry. It appears that the ease of the reaction and ambient conditions provide an apparent advantage for the fabrication of new polymeric materials or improve the properties by modifications. Typically, organogels, one of the important materials applied in various areas, can be prepared by such coupling process. Although there exist numerous approaches in gelling processes and various gelators are used, extensive research is devoted to the synthesis of organogels. [48, 49] As part of our continuous interest in developing new applications of
3
ACCEPTED MANUSCRIPT benzoxazine based systems, herein we report a simple and efficient method for the preparation of organogels by using thiol-benzoxazine chemistry main-chain polybenzoxazine precursor and dithiols as scaffold and crosslinker, respectively. The swelling capacity and network characteristics of the resulting PBZ gels were also determined through the
RI PT
observation of swelling behavior in solvent mixtures and compression modulus determination after their equilibrium swollen state.
SC
EXPERIMENTAL
M AN U
Materials
Aniline (Aldrich, 99%), paraformaldehyde (Acros, 96%), bisphenol A (Alfa Aesar, 98%), 1,6diaminohexane (Acros Organics, 99.5%), were used as received for the synthesis of the polybenzoxazine precursor. Difunctional thiol compounds, 1,2-ethanedithiol (Aldrich, 99%) and 1,6-hexanedithiol (Aldrich, 99%), methanol (MeOH, Aldrich, 99%), chloroform (CDCl3,
Characterization
H NMR spectrum of starting polymer was recorded in CDCl3 with Si(CH3)4 as an internal
EP
1
TE D
Aldrich, 99%) were used as received.
AC C
standard, using a Bruker AC250 (250.133 MHz) instrument. Fourier Transform Infrared (FTIR) spectra of the precursor polybenzoxazine and corresponding gels were recorded using Perkin Elmer Spectrum One. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TA/TGA instrument from 30 to 900 °C at a heating rate of 20 °C/min under nitrogen atmosphere. Thermal properties of the gels were determined by differential scanning calorimetry (DSC) using a Perkin–Elmer Diamond DSC instrument with scanning rate of 20 °C/min. between 30 °C to 330 °C. Molecular weight of polybenzoxazine precursor was determined by gel permeation chromatography (GPC) instrument, Viscotek GPCmax
4
ACCEPTED MANUSCRIPT Autosampler system consisting of a pump, three ViscoGEL GPC columns (G2000HHR, G3000HHR, and G4000HHR), a Viscotek UV detector, and a Viscotek differential refractive index detector with a tetrahydrofuron at a flow rate of 1.0 mL min-1. Both detectors were
evaluated by using Viscotek OmniSEC Omni–01 software. Synthesis of main-chain polybenzoxazine precursor
RI PT
calibrated against PS standards, having narrow molecular weight distribution. Data were
SC
In a 250 mL round bottomed flask, 2,7 g (88 mmol) of paraformaldehyde, 5,3 g (22 mmol) bisphenol A and 3.24 g (22 mmol) 1,6-hexanediamine were dissolved in 150 mL toluene and
M AN U
ethanol mixture of (2:1, v:v) and the reaction mixture was refluxed for 12h. The solution was concentrated using a rotary evaporator and the resulting polymer was precipitated in 250 mL methanol with added 1 mL of brine solution. Then the mixture was kept at 4 °C for 6 h. The polymer was collected by filtration and washed with excess amount of methanol. Thereafter,
TE D
the polymer placed in a tube and 10 mL 1,4-dioxane was added to polymer. The mixture was stood overnight to swell the polymer. Finally, freeze drying was applied for 48h. Light yellow powder was obtained as pure product (yield: 64%).
EP
General procedure for synthesis of polybenzoxazine gel
AC C
The gelation reactions were carried out in chloroform at ambient temperature in the presence of 1,2-ethanedithiol or 1,6-hexanedithiol as the cross-linkers. Typically, main-chain polybenzoxazine (100 mg) was mixed with 0.8 ml chloroform:methanol solvent (0.5:0.3, v:v) and 1,2-ethanedithiol (4 mg, 0.04 mmol) was added to this solution. The mixture was then stirred for 15 min. vigorously to provide homogeneity. Then the solution was withdrawn into a plastic syringe having a 10 mm diameter and 2.5 mL total volume. The gelation took place at ambient temperature for a specific time (see table 1). After gelation the syringe was cut into
5
ACCEPTED MANUSCRIPT pieces; the gel was taken out and washed with the same solvent system. Finally, the gel was obtained as light yellow material. Uniaxial compression measurements
RI PT
Measurements were conducted on the gel samples after their swelling in solvent mixture. Typically, a cylindrical gel sample of 5 mm in diameter and 7 mm in length was placed on the digital electronic balance. A load was transmitted vertically to the gel through a rod fitted
SC
with a PTFE (Teflon) end-plate. The force acting on the gel F was calculated from the reading of balance m as F = mg, where g is gravitational acceleration. The resulting deformation
M AN U
∆ = − where and are the initial un-deformed and deformed lengths, respectively. The deformation ratio α (deformed length/initial length) was calculated using the equation, = 1 − ∆/ . The force and the resulting deformation were recorded after 20 sec. of relaxation and all the measurements were conducted up to about 20% compression. The
[50-53]
(Eq.1)
EP
f = F / A = G (α − α − 2 )
TE D
elastic modulus of gels at equilibrium G was calculated according to the following equation 2.
where f is the compressive stress applied that is the force acting per unit cross-sectional area
AC C
(A) of the gel sample.
RESULTS AND DISCUSSIONS
As mentioned in the introduction section, our organogel synthesis approach is the crosslinking of polybenzoxazine precursor based on oxazine-thiol reactions. Initially, main chain polybenzoxazine precursor was synthesized according to the classical procedure used for
6
ACCEPTED MANUSCRIPT monomer synthesis (Scheme 3) with a slight modification to prevent gelation stemming from triazine formation. Thus, toluene:methanol (2:1, v:v) mixture was used as solvent for the purpose. [54] The precursor (PBZ) was obtained as Mn: 3150 Da. The structure of the polymer
RI PT
obtained was characterized by spectral analysis (Supplementary Data Figures S1).
According to Kawaguchi et al., the addition reaction of a model benzoxazine and thiol
SC
proceeds smoothly in CHCl3 up to 50% conversion and thereafter leveled off, probably owing to the equilibrium conversion. [55] Higher conversions could be obtained when a mixture of
M AN U
CHCl3 and CH3OH (1:1, v:v) was used as the reaction solvents. Interestingly, in DMSO, the reaction did not proceed at all and no product was formed. In the light of these findings, for the synthesis of organogel, PBZ and dithiols were reacted in chloroform:methanol solution. The overall reaction is depicted in Scheme 4. Apart from the used solvent system, also
TE D
solution concentration was found to be an important factor in the synthesis as diluted solutions did not yield a stable gel. In a typical experiment only 0.8 mL solvent was used for 100 mg PBZ. Moreover, the ratio of co-solvent methanol as should not exceed certain limit
EP
0.5:0.3 (CHCl3:CH3OH, 0.5:0.3, v:v) since PBZ tends to precipitate at higher ratios of methanol under the applied conditions. The amounts of the components, solvent systems and
AC C
abbreviations for the gels are tabulated in Table 1.
S0032-3861(15)30165-8
DOI:
10.1016/j.polymer.2015.08.026
Reference:
JPOL 18046
To appear in:
Polymer
Received Date: 15 June 2015 Revised Date:
12 August 2015
Accepted Date: 14 August 2015
Please cite this article as: Bektas S, Kiskan B, Orakdogen N, Yagci Y, Synthesis and Properties of Organo-gels by Thiol-Benzoxazine Chemistry, Polymer (2015), doi: 10.1016/j.polymer.2015.08.026. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Synthesis and Properties of Organo-gels by Thiol-Benzoxazine Chemistry Semiha Bektas1, Baris Kiskan1*, Nermin Orakdogen1, Yusuf Yagci1 Istanbul Technical University, Department of Chemistry, 34469, Maslak, Istanbul, Turkey
RI PT
1
Abstract
SC
In the present work, a synthetic strategy based on oxazine-thiol ring-opening reaction for the synthesis of organogels is reported for the first time. For this purpose, main-chain
M AN U
polybenzoxazine (PBZ) was synthesized by the reaction of bisphenol A, 1,6-diaminohexane and paraformaldehyde in toluene:methanol mixture (2:1, v:v). The obtained reactive PBZ was then reacted at room temperature with dithiol compounds in CHCl3:CH3OH (1:1, v:v), namely 1,2-ethanedithiol and 1,6-hexanedithiol. The obtained PBZ based gels was characterized by
TE D
spectral, thermal analysis and uniaxial compression measurements. The influences of the gel preparation conditions on the swelling behavior and the mechanical properties of the resulting gels were investigated. Swelling ratios (J) up to 8.4 in CHCl3:CH3OH solution was obtained.
EP
Moreover, the curing of the gels was performed over remaining oxazine moieties in the gels and the thermally activated curing was found to be at lower temperatures than that
AC C
corresponding PBZ.
Keywords: Polybenzoxazine, Benzoxazine, Organo-gel, Ring-opening polymerization, Crosslinking Corresponding Author: Baris Kiskan e-mail: [email protected] Tel: +90 212 285 6825 Fax: +90 212 285 6386
1
ACCEPTED MANUSCRIPT INTRODUCTION Polybenzoxazines have gained an increasing interest in the field of thermosetting research due to the superior properties that overcome several short-comings of conventional novolac and resole-type phenolic resins. Thermally driven synthesis without any initiator or curing agent
RI PT
from the corresponding 1,3-benzoxazine monomers yields a cross-linked polymer composed of a phenolic and a tertiary amine bridge unit as the constitutional pattern (Scheme 1). Extensive intra- and inter-molecular hydrogen bonding formed between the phenolic –OH
SC
and tertiary amine groups augments excellent features to polybenzoxazine networks. These materials offer low water absorption, high char yield and resistance against flame, high
M AN U
modulus, high strength, high glass transition temperatures, chemical resistance, long shelf life and dimensional stability upon curing. [1-11] Thus, polybenzoxazines have become one of a rare few new polymers developed and commercialized in the past 20 years. Another reason for that is benzoxazine monomers have enormous molecular design flexibility. Basically, the
TE D
synthesis of a monomer is carried out by the reaction of any suitable phenol, formaldehyde and primary amine (aliphatic or aromatic) (Scheme 1).[12-34]
EP
In a recent publication, Gorodisher et al. reported that benzoxazines can undergo catalytic
AC C
ring opening reaction at ambient conditions by thiols. [35] The process is called as “Catalytic Opening of the Lateral Benzoxazine Rings by Thiols” (COLBERT) which proceeds over a two-step reaction. In the first step, the amine of benzoxazine is protonated by thiol to form an ionic intermediate. Subsequently, the formed thiolate attacks through the methylene carbon between N and O atoms in the oxazine causing ring-opening of the benzoxazine (Scheme 2). The overall process is similar to acid catalyzed nucleophilic addition and simultaneous ringopening reaction of benzoxazines.[36]
2
ACCEPTED MANUSCRIPT
COLBERT process was found to be a reversible reaction resulting in continuous regeneration of the thiol in the medium. Thus, only small amounts of thiols reduce the curing temperature
RI PT
of benzoxazines effectively. The reversible nature of COLBERT was investigated in detail by Endo and co-workers using model reactions and an apparent relationship was observed with the solvent system used. In the same study, the application of this reaction to a new
SC
polyaddition–depolymerization system was also performed yielding linear polymers. [37] Another useful application concerns construction of a highly efficient curing system. Cross–
M AN U
linked but additionally curable soft benzoxazine films were prepared by simultaneous photoinduced thiol-ene and COLBERT reactions using difunctional thiol and diallybenzoxazine. [38] Similar approach was also used to form dual-cure hybrid polymer networks. [39] In a recent report from the authors’ laboratory, it was demonstrated that
TE D
benzoxazines can be used as a linker between two different polymers to form a block copolymer by using COLBERT as a ligation process in a manner of Thiol-X chemistry. [4046] Likewise, main-chain polybenzoxazine precursor from bisphenol A and diaminohexane
EP
was reacted with various thiols at ambient temperature, namely thiophenol, 2-ethanethiol and 1-butanthiol to yield side-chain functional polybenzoxazines. [47] These initial studies clearly
AC C
present the potential use of thiol-oxazine reaction in polymer and material chemistry. It appears that the ease of the reaction and ambient conditions provide an apparent advantage for the fabrication of new polymeric materials or improve the properties by modifications. Typically, organogels, one of the important materials applied in various areas, can be prepared by such coupling process. Although there exist numerous approaches in gelling processes and various gelators are used, extensive research is devoted to the synthesis of organogels. [48, 49] As part of our continuous interest in developing new applications of
3
ACCEPTED MANUSCRIPT benzoxazine based systems, herein we report a simple and efficient method for the preparation of organogels by using thiol-benzoxazine chemistry main-chain polybenzoxazine precursor and dithiols as scaffold and crosslinker, respectively. The swelling capacity and network characteristics of the resulting PBZ gels were also determined through the
RI PT
observation of swelling behavior in solvent mixtures and compression modulus determination after their equilibrium swollen state.
SC
EXPERIMENTAL
M AN U
Materials
Aniline (Aldrich, 99%), paraformaldehyde (Acros, 96%), bisphenol A (Alfa Aesar, 98%), 1,6diaminohexane (Acros Organics, 99.5%), were used as received for the synthesis of the polybenzoxazine precursor. Difunctional thiol compounds, 1,2-ethanedithiol (Aldrich, 99%) and 1,6-hexanedithiol (Aldrich, 99%), methanol (MeOH, Aldrich, 99%), chloroform (CDCl3,
Characterization
H NMR spectrum of starting polymer was recorded in CDCl3 with Si(CH3)4 as an internal
EP
1
TE D
Aldrich, 99%) were used as received.
AC C
standard, using a Bruker AC250 (250.133 MHz) instrument. Fourier Transform Infrared (FTIR) spectra of the precursor polybenzoxazine and corresponding gels were recorded using Perkin Elmer Spectrum One. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TA/TGA instrument from 30 to 900 °C at a heating rate of 20 °C/min under nitrogen atmosphere. Thermal properties of the gels were determined by differential scanning calorimetry (DSC) using a Perkin–Elmer Diamond DSC instrument with scanning rate of 20 °C/min. between 30 °C to 330 °C. Molecular weight of polybenzoxazine precursor was determined by gel permeation chromatography (GPC) instrument, Viscotek GPCmax
4
ACCEPTED MANUSCRIPT Autosampler system consisting of a pump, three ViscoGEL GPC columns (G2000HHR, G3000HHR, and G4000HHR), a Viscotek UV detector, and a Viscotek differential refractive index detector with a tetrahydrofuron at a flow rate of 1.0 mL min-1. Both detectors were
evaluated by using Viscotek OmniSEC Omni–01 software. Synthesis of main-chain polybenzoxazine precursor
RI PT
calibrated against PS standards, having narrow molecular weight distribution. Data were
SC
In a 250 mL round bottomed flask, 2,7 g (88 mmol) of paraformaldehyde, 5,3 g (22 mmol) bisphenol A and 3.24 g (22 mmol) 1,6-hexanediamine were dissolved in 150 mL toluene and
M AN U
ethanol mixture of (2:1, v:v) and the reaction mixture was refluxed for 12h. The solution was concentrated using a rotary evaporator and the resulting polymer was precipitated in 250 mL methanol with added 1 mL of brine solution. Then the mixture was kept at 4 °C for 6 h. The polymer was collected by filtration and washed with excess amount of methanol. Thereafter,
TE D
the polymer placed in a tube and 10 mL 1,4-dioxane was added to polymer. The mixture was stood overnight to swell the polymer. Finally, freeze drying was applied for 48h. Light yellow powder was obtained as pure product (yield: 64%).
EP
General procedure for synthesis of polybenzoxazine gel
AC C
The gelation reactions were carried out in chloroform at ambient temperature in the presence of 1,2-ethanedithiol or 1,6-hexanedithiol as the cross-linkers. Typically, main-chain polybenzoxazine (100 mg) was mixed with 0.8 ml chloroform:methanol solvent (0.5:0.3, v:v) and 1,2-ethanedithiol (4 mg, 0.04 mmol) was added to this solution. The mixture was then stirred for 15 min. vigorously to provide homogeneity. Then the solution was withdrawn into a plastic syringe having a 10 mm diameter and 2.5 mL total volume. The gelation took place at ambient temperature for a specific time (see table 1). After gelation the syringe was cut into
5
ACCEPTED MANUSCRIPT pieces; the gel was taken out and washed with the same solvent system. Finally, the gel was obtained as light yellow material. Uniaxial compression measurements
RI PT
Measurements were conducted on the gel samples after their swelling in solvent mixture. Typically, a cylindrical gel sample of 5 mm in diameter and 7 mm in length was placed on the digital electronic balance. A load was transmitted vertically to the gel through a rod fitted
SC
with a PTFE (Teflon) end-plate. The force acting on the gel F was calculated from the reading of balance m as F = mg, where g is gravitational acceleration. The resulting deformation
M AN U
∆ = − where and are the initial un-deformed and deformed lengths, respectively. The deformation ratio α (deformed length/initial length) was calculated using the equation, = 1 − ∆/ . The force and the resulting deformation were recorded after 20 sec. of relaxation and all the measurements were conducted up to about 20% compression. The
[50-53]
(Eq.1)
EP
f = F / A = G (α − α − 2 )
TE D
elastic modulus of gels at equilibrium G was calculated according to the following equation 2.
where f is the compressive stress applied that is the force acting per unit cross-sectional area
AC C
(A) of the gel sample.
RESULTS AND DISCUSSIONS
As mentioned in the introduction section, our organogel synthesis approach is the crosslinking of polybenzoxazine precursor based on oxazine-thiol reactions. Initially, main chain polybenzoxazine precursor was synthesized according to the classical procedure used for
6
ACCEPTED MANUSCRIPT monomer synthesis (Scheme 3) with a slight modification to prevent gelation stemming from triazine formation. Thus, toluene:methanol (2:1, v:v) mixture was used as solvent for the purpose. [54] The precursor (PBZ) was obtained as Mn: 3150 Da. The structure of the polymer
RI PT
obtained was characterized by spectral analysis (Supplementary Data Figures S1).
According to Kawaguchi et al., the addition reaction of a model benzoxazine and thiol
SC
proceeds smoothly in CHCl3 up to 50% conversion and thereafter leveled off, probably owing to the equilibrium conversion. [55] Higher conversions could be obtained when a mixture of
M AN U
CHCl3 and CH3OH (1:1, v:v) was used as the reaction solvents. Interestingly, in DMSO, the reaction did not proceed at all and no product was formed. In the light of these findings, for the synthesis of organogel, PBZ and dithiols were reacted in chloroform:methanol solution. The overall reaction is depicted in Scheme 4. Apart from the used solvent system, also
TE D
solution concentration was found to be an important factor in the synthesis as diluted solutions did not yield a stable gel. In a typical experiment only 0.8 mL solvent was used for 100 mg PBZ. Moreover, the ratio of co-solvent methanol as should not exceed certain limit
EP
0.5:0.3 (CHCl3:CH3OH, 0.5:0.3, v:v) since PBZ tends to precipitate at higher ratios of methanol under the applied conditions. The amounts of the components, solvent systems and
AC C
abbreviations for the gels are tabulated in Table 1.