Fabrication of hybrid polymers composed of functionalization boron cluster decahydro-closo-
decaborate derivative [B10H10L]x- and organic polymers was performed. In addition, a literature review that highlights previous work and explains the importance of our new work was presented. The
polymers resole, novolac, polymethyl methacrylate (PMMA) and epoxy were firstly synthsised.
Then Carbonyl closo-decaborate compounds [2-B10H9CO]- was synthesised. The prepared [2-
B10H9CO]- was then Grafted on the obtained polymers. Experimental design for the synthesis of hybrid polymers based on the functionalization of boron cluster decahydro-closo-decaborate type [B10H10]2-and organic polymers was performed. NMR (1H, 13C) and FT-IR spectral analysis for the prepared polymers and [2-B10H9CO]- were performed. It is expected that the Carbonyl Closo-Decaborates Derivatives will not react directly with PMMA and need specific functional groups like
hydroxyl or amine groups.
However, expected results via Chemdraw software and literature work were also presented. It
is expected that the Carbonyl Closo-Decaborates Derivatives may be reacted via carbonyl
group with the hydroxy group on the polymer. The experimental synthesis of hybrid polymers
and the deep investigation of their structures was unable to be performed due to the shutdown
of the country under the effect of COVID 19. Our future plain is to perform NMR (1H, 13C,
11B) analysis that will characterize the obtained hybrid polymers. The effect of cluster
decahydro-closo-decaborate type [2-B10H9CO]- on the physical properties of the polymers will
be studied through Thermogravimetric Analysis (TGA) and Differential Scanning Calorimeter
(DSC).
1.1. Historical Review of Boron-cage Containing Polymers
Since the discovery of polyhedral boranes in the early 1960s, new inorganic-organic, hybrid
boron cluster-containing polymers have been developed to combine, in a single polymeric
system, the desirable features of inorganic materials such as high-temperature stability,
hardness and chemical inertness with easy processability and high solubility characteristics of
organic materials [2]. Thus, polymer modification is feasible by introducing boron cage
compounds that will provide the polymer with desired properties such as enhanced thermal
stability, multiphase physical responses, flexibility, rigidity, compatibility or degradability,
impact response and biological resistance. Today, polymer modifications can be grouped into
two categories: the first group is physical modifications including entanglement, entrapment
and blending while the second group is chemical modifications including chemical and thermal
curing and derivatization of polymers.
Synthetic polymeric materials have remarkable properties which include low weight, high
strength and ease of preparing. However, their low chemical and thermal stability has limited
their use. This problem of stability can be solved by mixing with flame retardant additives such
as halogen compounds, phosphorus compounds, silicon compounds and ceramic powders (SiC
and B4C). For example, to improve the flame retardancy and thermo-oxidative resistance of
phenol resin, boron, phosphorus or silicon compounds has been added [3]. Boron compounds
such as zinc borate, boric acid and boron oxide are preferable to be used as flame retardants
over halogenated compounds since they are non-toxic and environmental-friendly. Also, the
use of ceramic powders as fillers in phenolic resins, such as SiC and B4C, improves the thermo-
oxidative stability but has disadvantages such as poor homogeneity, adhesion, and
manufacturing difficulties during composite molding [4]. Therefore, the addition of single
elements such as boron, silicon and phosphorous will enhance the thermo-oxidative stability
compared to traditional resins but avoiding the drawbacks of simple adding fillers [5, 6].
Concerning polysiloxanes, the degradation can be solved by incorporating borosiloxane and
metallosiloxane linkages into the polymer. Despite their high susceptibilty to hydrolysis,
polyborosiloxanes have been a research priority due to their thermodynamically stable B-O-Si
backbone linkage. Boron and silicon containing ceramic, when exposed to oxidizing
environment at high temperature, forms a protective borosilicate glassy layer on the surface,
which prevents further oxidation of the ceramic [7, 8].
Borane clusters represent unique kinds of boron-containing compounds not only because of
their special coordination and unusual 3-dimensional but also due to their broadapplications
ranging from the precursor of ceramics to BNCT agents. From the view of practical application,
on one hand, the clustered structure increases the concentration of boron and makes them
superior candidates for BNCT agents; on the other hand, such a compact structure also
improves the chemical and thermal stability, thus enabling their applications in aerospace
coatings. Therefore, borane clusters have been widely used as building blocks for the
construction of functional polymeric materials with tunable architectures, amphiphilicity, as
well as biocompatibility. With the recent progress of controlled polymerization techniques, a
number of borane cluster-containing polymers (BCCPs) have been developed during the last
two decades. The architectures of BCCPs are divided into the following kinds: linear,
dendrimer-like and dendron, macrocycles, and metal–organic frameworks (MOFs) [9] .
Among these clusters: [B9H10]-, [B10H10]2, [B11H12]-, and [B12H12]2- where the physical or
chemical modification of polymers with these clusters is supposed to enhance their mechanical
and physical properties. Moreover, polymer containing boron cage compounds (C2B10H12
(carboranes), [B10H10]2-, [B12H12]2-) have significant interest due to their stability towards
acids/bases and their very high thermal stability [10].
Teshome et al synthesized new form of boron-cage-containing polymers prepared from a
dimethyl sulfide derivative of the closo-[B12H12]2- cage. They prepared one new closo-
[B12H12]2- cage containing methacrylate ([(1- MeSCH2CH2OC(O)C(Me)=CH2)-7-(Me2S)
B12H10]) and two new styrene ([(1-(MeS-CH2-Ph-CH=CH2)-7-(Me2S) B12H10] and [NBu4][1-
(MeS-CH2-Ph- CH=CH2)B12H11]) derivatives of the closo-[B12H12]2- cage. they used these
monomers to synthesize various homopolymers and copolymers [11].
Fatima Abi-Ghaida et al [12] synthesized triethoxysilylated closo-decaborate clusters and
immobilized them into the pores of mesoporous silica. They grafted these clusters on
mesoporous silica SBA-15 with a molar percentage between 2.6 and 5.2% in which the
hexagonal symmetry and mesoporosity of SBA-15 were retained after graftings.
This research project will fabricate, for the first time, a new form of boron-cage-containing
hybrid polymers prepared from decahydro-closo-decaborate derivatives of type [B10H10]2- and
organic polymers (polyacrylates, phenolic resin and epoxy) due to its significant properties
using two routes. Route 1 involves the synthesis of closo-decborate monomers having an
alkene functional group which then will be polymerized to prepare the hybrid polymers via
free-radical initiated bulk and solution polymerization methods. While route 2 involves the
synthesis of organic polymers which will be reacted with functionalization [B10H10L]X- to
prepare the hybrid polymers.
1.2. Boranes
1.2.1. General Review of Boranes
The chemistry of the boron hydrides or boranes began with the pi
oneering and fundamental work by Alfred Stock in 1912 where this boom in boron hydride chemistry allows the use of these compounds as powerful rocket fuels superior to the available hydrocarbon fuels [13].
Currently, there are over 25 neutral boron hydrides and many more boron hydride anions where in this project we are interested in studying [B10H10]2- anion that was first reported by
hawthorne and pitochelli in 1959.
Polyhedral boron hydrides are formed with up to twelve boron atoms and divided into three
principle classes called closo-, nido-, and arachno- possessing the general formulas [BnHn]2-
, [BnHn]4+and [BnHn]6+, respectively [14].
Closo- refers to a closed polyhedron, the most symmetrical possible arrangement. For example,
6 Boron atoms form an octahedron, [B6H6]2- . nido-Boranes can be examplified by B5 H9 with
a nest-like open structure in which a vertex of a closo species is removed. And finally arachno-
refers to more open web-like structure derived by removing two neighboring vertices from a
closo structure for example B4H10. The most prominent
anion among [BnHn]2- is
closo decahydro decaborate because of its extreme thermal, hydrolytic, and oxidative stability
coupled with its wide scope of derivative chemistry [15].
The most difficult aspect of developing [B10H10]2- chemistry is its functionalization, that is the
activation of the B-H bond to the B-L bond. As a result of fifty years of extensive research into
boranes and their derivatives, it has been discovered that this class of compounds is highly
diverse and that boron has a high capacity for forming self-bonded complex molecular
networks. Furthermore, the closo-decaborate anion and its derivatives have a high stability and
low toxicity, making them ideal candidates for further research.
1.2.2. Chemistry of Closo-Decarborate Anion
1.2.2.1. Structure of [B10H10]2-
The non-icosahedral structure of the closo-decaborate anion [B10H10]2- is a bicapped square
antiprism (D4d)[16]. It consists of two staggered 4-membered rings of boron
atoms (B2 to B9, six-coordinated equatorial) and two capping boron vertices (B1 and B10,
five-coordinated apical) with each boron vertex being connected to a terminal hydrogen atom
by a single bond [17]. The 11B NMR of the anion reflects the presence of two distinct
environments, one peak of intensity two appears at around -1ppm representing the two apical
boron atoms (B1, B10) and the second peak of intensity eight appears at around -31 ppm
representing the equatorial boron atoms (B2-9) [18].Furthermore, the electronic structure was described as a three dimensional delocalization of
charge (3-dimensional aromaticity) and the aromatic character of these electron-rich anions
makes them susceptible to electrophilic attacks, and their derivatives have been extensively
studied [1, 18].
1.2.2.2. Synthesis of Closo-Decaborates
The synthesis of the decahydro-closo-decaborate anion can be done in many routes. The most
common route is based on 6,9-bis-adducts of decaborane (B10H14) which is a toxic compound
obtained from diborane (B2H6) pyrolysis [1]. The preparative synthesis include reactions of
decaborane or 6,9-bis(acetonitrile)decaborane with triethylamine in refluxing benzene giving
triethylammonium closo-decaborate (Et3NH)2[B10H10] in 92% yield (figure 3a) or the reaction
of 6,9-bis(dimethylsulfane)decaborane with liquid ammonia giving ammonium closo-
decaborate (NH4)2[B10H10] in 84% yield [1].But due to the toxicity of decaborane, an alternative method has been established based on solid state pyrolysis (185 °C) of easily available tetraethylammonium tetrahydroborate [NEt4][BH4]resulting in tetraethyl- ammonium closo-decaborate (Et4N)2[B10H10] with high yield. Despitethe high yield, various parameters such as the amount of starting material, the size and geometry of the reaction vessel, the heating rate and conditions,and others made this reaction difficult to replicate. As shown in the reaction below, part of the problem was handled by employing high-boiling hydrocarbons as reaction media [1].
2.1. Experimental Procedures
2.1.1. Polymers Synthesis
2.1.1.1. Procedure of Preparation of Resole
In a 100 ml two-neck round bottom flask equipped with a magnetic bar , dissolve 9.425g of
phenol at temperature 40-42 ̊C. Drop by drop, add 5g of 40% NaOH aqueous solution over 10
minutes with stirring. The 40% NaOH aqueous solution was prepared by dissolving 10.042g
of NaOH pellets in 15 ml of deionized water. This step is followed by a drop wise addition of
9.740g of 37% formalin solution at 45-50 ̊C. Reflux the mixture for 1-1-1:15 hour at 70 ̊C and
then cool it to about 40-45 ̊C. In order to remove water, place the flask in rotary evaporator
under vacuum for about 9 hours at a temperature < 50 ̊C until complete extraction of water. For
purification, the resole is dissolved in 5 ml of deionized water until it becomes viscous. After
that, we add 10 ml chloroform. Two layers will appear where the resole being the upper layer
is separated from water being the lower layer by decantation. After this step, we place the
sample in rotary evaporator under vacuum for about 5 hours to remove water again.
2.1.1.2. Procedure of Preparation of Novolac
In a 100 ml two-neck round bottom flask, equipped with a magnetic bar, dissolve 9.411g of
phenol at a temperature 40-42 ˚C. Drop by drop, add 8.8g of 37% formalin solution at a
temperature lower than 50 ˚C. Add dropwise 0.299g of oxalic acid dissolved in 2-3 ml of
deionized water and then add 0.102g of calcium acetate (we use calcium acetate instead of zinc
acetate since its not available in the lab). Reflux the mixture at 110 ˚C for 3 hours. Stop heating and then add 5 ml of deionized water where two layers will appear. Remove the upper layer by decantation and distill the lower layer under vacuum by using the rotary evaporator at 90 ˚C
for 2 hours to remove water. For purification, dissolve the polymer in 10 ml of ethanol followed
by precipitation in 10 ml of deionized water. Repeat this step 3 times: during the first time, we
use for purification 10 ml of ethanol and 10 ml of deionized water. During the second time, we
use 7.5 ml of ethanol and 10 ml of deionized water. While during the last time, we use 5 ml of
ethanol and 10 ml of deionized water. After purification, the polymer is dried by vacuum
distillation at 80 ˚C for 3 hours by using rotary evaporator until total extraction of water.
2.1.2. Closo-decaborate Anion Derivatives Synthesis
Boranes were supported from LCIO laboratory that was done under subervision of prof. Daoud
Naoufal and his coworkers. [2-B10H9CO]- was prepared according to the literature [1].
2.1.3. Decahydro-closo-decaborate Polymers Synthesis
2.1.3.1. Synthesis Procedure for Carbonyl Closo-Decaborates-Polymer
In a 100 ml two-neck round botton flask, equipped with a magnetic bar, dissolve 1 g of the
polymer in 160 ml dry THF under inert atmosphere connected into the schlenk tube. Stir the
mixture rapidly at room temperature for 10 minutes. This step is followed by the addition of 2
ml triethyl amine and then perform vacuum-inert exchange one time. Add 0.2g of the
monocarbonyl derivative [2-B10H9CO]- to the mixture. After that, reflux the mixture at 120°C
for 6 hours under inert atmosphere. When the reflux ends, reduce the solvent to 7 ml under the
hood and then wash the viscoussolution by 15 ml of dichloromethane. Dry the product obtained
under the hood for the next day.
Note 1: while performing a reaction of monocarbonyl derivative with novolac, the solvent used
is dry THf with a total volume of 160 ml since THF is highly volatile with a boiling
point around 66°C and we perform the reaction at 120°C that’s why we use THF in a
big volume.
Note 2: while performing a reaction of monocarbonyl with these polymers: epoxy, PMMA and
resole, the solvent used is DMSO. DMSO has a boiling point around 189°C so it is
heated on a heater to around 110°C in order to evaporate water. After this step, the
above mentioned polymers is dissolved in hot DMSO at a temperature below 100°C with stirring under inert atmosphere. Then the reaction proceeds with the same steps
for the three mentioned polymers.
2.1.3.2. Synthesis Procedure for Dimethylsulfane and Diazonium Closo-Decaborates-
Polymer
Due to the hard and sensitive economic and health situation of our country, we were unable to
perform this part in this research project. This part will be completed in future work.
Results and Discussions
3.1. Elucidation of Polymers Structure
3.1.1. Resole
The resole obtained had a brown-reddish color, viscous texture and it is a thermoset that
hardens upon cooling. Confirmation and structure elucidation of Resole are performed via IR and NMR analysis based on previous work of our colleague [40].
1H NMR Analysis:
Figure 3. 2 illustrates the 1H NMR spectrum of resole. As shown in figure 3. 2, the wide
resonance lines seen as multiplets at about 6.74-7.45 ppm correspond to the aromatic
hydrogens. However, the resonance lines appeared at about 3.36-4.93 ppm correspond to
methylene bridge and that of 4.91-5.29 ppm correspond to ether linkage.
13C NMR Analysis:
Figure 3. 3 illustrates the 13C NMR spectrum of resole and novolac. As shown in figure 3. 3,
the signals attributed to methylol groups (h) at about 58-63 ppm and dimethylene ether bridges
(g) at about 65-75 ppm are considered the key differences between the 13C NMR of resole and
novolac. Furthermore, the resonance peaks appeared between 115-155 ppm (a,b,c,d,e) were
attributed to the aromatic hydrogens. The methylene bridges and the hydroxyl group OH next
to carbon atom appears at 30-40 ppm and 91ppm(f), 53-58 ppm (i) respectively.
Conclusion and Perspective
In this research project, we highlight the synthesis of hybrid polymers based on the
functionilization of boron cage clusters [B10H10]2-. Among the organic polymers that we
synthesise in this project are: resole resin, novolac resin, epoxy resin, and PMMA was already
synthesised.
To synthesise resole and novolac resins, formalin method was used. The method was based on
reacting phenol with formalin under the effect of a basic catalyst to give a resole resin or under the effect of an acidic catalyst to give a novolac resin. The synthesis of epoxy was performedby allowing BPA to react with ECH under the effect of a basic catalyst, and the elucidation of its structure was done using FT-IR.
Furthermore, the grafting of the closo-decaborate derivative [2-B10H9CO]- with the four
prepared organic polymers was done by reacting the polymer with the derivative in the presence
of trimethylamine under inert atmosphere.
Following the synthesis, FT-IR was performed for the 4 prepared polymers and for the grafting
of the polymer with the boron cage derivative especially with [2-B10H9CO]-
. Both 1H-NMR and 13C-NMR of the polymers were taken from previous work due to technical problems in the NMR machine at the Lebanese University.
Theoretical NMR (13C) spectra was estimated via Chemdraw software and literature work were
used to analysis the expected results. It is expected that the Carbonyl Closo-Decaborates
Derivatives will not react directly with PMMA and need specific functional groups like
hydroxyl or amine groups.
In future work, the boron modified resins will undergo further studies using NMR, TGA, and
DSC. Moreover, the effect of the closo-decaborate type [B10H10]2- on the physical properties
of the hybrid polymers will be investigated. The synthesis of such hybrid polymers are of great
interest for future work due to their wide use in many applications.
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