Green Anionic polymerization of vinyl acetate using Maghnite-Na + (Algerian MMT): Synthesis characterization and reactional mechanism

In this work, the green polymerization of vinyl acetate is carried out by a new method which consists in the use of clay called Maghnite-Na + as an ecological catalyst, non-toxic, inexpensive and recyclable by simple ltration. X-ray diffraction (XRD) showed that Maghnite-Na + is successfully obtained after cationic treatment (sodium) on raw Maghnite. It is an effective alternative to replace toxic catalysts such as benzoyl peroxide (BPO) and Azobisisobutyronitrile (AIBN) which are mostly used during the synthesis of polyvinyl acetate (PVAc) making the polymerization reaction less problematic for the environment. The synthesis reaction is less energetic by the use of recycled polyurethane as container for the reaction mixture and which is considered as a renewable material and a good thermal insulator which maintains the temperature at 0°C for 6h. The reaction in bulk is also preferred to avoid the use of a solvent and therefore to stay in the context of green chemistry. In these conditions, the structure of obtained polymer is established by Nuclear Magnetic Resonance Spectroscopy 1H NMR and 13C NMR. Infrared spectroscopy (FT-IR) is also used to conrm the structure of PVAc. Thermogravimetric analysis (TGA) showed that it is thermally stable and it starts to degrade from 330°C while Differential Scanning calorimetry (DSC) shows that this polymer has a glass transition temperature (Tg = 50°C). The composition in PVAc/maghnite-Na + (7wt% of catalyst) is the most tensile resistant with a force of 182 N and a maximum stress of 73.16 MPa, the most exible (E = 955 MPa) and the most ductile (εr = 768 %)


Introduction
The chemistry is now moving towards the use of less polluting polymers and synthetic methods that involve low energy and less toxic reagents and therefore environmentally friendly [1][2].
Polyvinyl acetate (PVAc) meets these requirements, it is non-toxic biodegradable under speci c conditions and it received considerable attention in various elds such as the pharmaceutical industry as anti-tumor in cosmetics, in the food and food packaging as the cheese coating and gum base for chewing gum [3][4]. This polymer is also used in the building industry as paint and glue for wood, paper and in the textile industry [5][6]. PVAc can be polymerized using various catalysts. Indeed, polymerized vinyl acetate using Benzoyl Peroxide (PBO) as a catalyst at a temperature of 55°C, while another researchers used Azobisisobutyronitrile (AIBN) in benzene as a solvent at a temperature of 60°C [7][8]. Vinyl acetate was also polymerized in emulsion, the reaction was catalyzed by Potassium Persulfate (KPS) in the presence of surfactants at 70°C [9][10]. However, these catalysts are not recyclable and require treatment of waste, they are toxic and harmful to human health and can be dangerous if swallowed and cause skin and eye irritation [11][12]. In addition, the synthesis of this polymer in presence of these catalysts is carried out at high temperature requiring more energy which is less attractive economically and environmentally [13][14]. In this perspective, we developed in our laboratory a new method for the synthesis of PVAc with a sustainable way based on the principles of green chemistry [15][16]. For these reasons, we used rigid polyurethane as container for the reaction mixture that we recovered and recycled. Moreover, it is a material that remains undamaged for a very long time and considered today as the insulation that has the best thermal insulation maintaining a low temperature (0°C) for several hours [17]. The advantage of this new method of polymerization is also the use of a catalyst that promotes the synthesis reaction called Maghnite-Na+ [18][19]. It is easily available, green, non-toxic, in ammable and inexpensive because it can be separated from the system by simple ltration and reused in other reactions [20][21]. Unlike natural clay minerals maghnite-Na+ has high crystallinity, controllable composition and fewer impurities [22]. For these reasons, the use of such modi ed maghnite as host materials is expected to be more advantageous than the use of raw natural clay minerals [23]. In the past years, this catalyst has been used successfully as initiator for the anionic polymerization of several acrylamide and vinyl monomers as well as the synthesis of nanocomposites/clay [24][25]. The polymerization of vinyl acetate catalyzed by Maghnite-Na+ is carried out under mild conditions, in bulk without solvent and with the minimum of reagent, reducing the waste treatment [26][27].

Material
The chemicals and reagents used for the synthesis were obtained from commercial sources and were used as received without further puri cation. Vinyl acetate monomer (≥ 99%) and diethyl ether (≥ 99%) were obtained from Sigma Aldrich. NaCl was purchased from Sigma Aldrich. Raw-Maghnite (Algerian montmorillonite clay) was supplied by BENTAL (Algerian Society of Bentonite). Deionized water was used in the preparation of Maghnite-Na + .

Methods
Nuclear Magnetic Resonance (1H NMR and 13C NMR) spectra were recorded on a Bruker- Tensile tests were conducted using a LLoyd LR/10KN Universal Machine at room temperature and crosshead speed of 50mm min -1 for the determination of tensile modulus and yield strength, according to the standard ASTM D638.

2.3.Preparation of Maghnite-Na+
In an Erlenmeyer of 1 L, 15 g of raw Maghnite is put in 400 ml of distilled water and the mixture was stirred at room temperature for (2h) . Then 600 ml of a sodium chloride solution (1M) is added to the preceding mixture and left stirring for (48h) at room temperature. After this time, the product is ltered and washed with distilled water until complete removal of the Cl-ions. This is con rmed by the silver nitrate test. The last step is to crush the obtained Maghnite-Na+ after drying in the oven at 105°C .Structure of maghnite is established by FT-IR and XRD.

Synthesis of Poly (vinyl acetate )
The polymerization of vinyl acetate was carried out in bulk and in two steps: the rst step is the activation of the monomer by the catalyst. Amount of vinyl acetate (1 g, 0.012 mol) is mixed with Maghnite-Na+ (catalyst) at various weight percentages (1%, 2%, 3%, 5% and 7% ) in a sealed tube for one hour at a temperature of 0°C. The low temperature is maintained using a container which does not require energy to cool; it is made from polyurethane waste (it is a thermal insulator) which allowed the reaction to proceed smoothly under mild conditions that respect the principles of green chemistry. Then, in the second step, the remaining amount of the monomer (4 g, 0.046 mol) is added to the previous mixture and the polymerization is conducted at various times, still at 0°C. At the end of the reaction, solid Maghnite-Na+ is removed from the mixture by ltration then the polymer is obtained by precipitation of the ltrate in methanol as show in (Scheme1). The polymerization of vinyl acetate has been done by using different (solvents ) . In our work we will achieve a heterogeneous polymerization based on the principles of green chemistry, it is assumed that it proceeds according to an anionic mechanism. Scheme 1. Schematic representation of the synthesized polymer ( PVAc) catalyzed by maghnite-Na+ under optimal condition ( 0°C, 6 hours ).

Results And Discussion
In this work, we have prepared for the first time the PVAc by an anionic polymerization from VAc under effect of heterogeneous catalyst called maghnite-Na+ (Algerian MMT). The structure, morphology, mechanical and physical chemical properties of the synthesized PVAc was found to vary depending on the different amount of catalyst, the temperature and time.  The X-ray powder diffraction pro les ( Figure 2) show that there is an increase in basal spacing (d001) from 9.08 Å in the raw-maghnite after 12.70 Å this increase in the interlayer distance is explained by the adsorption of a water molecule on the surface of the sheets of the montmorillonite re ecting the changes in interlayer cation as a result of the basic treatment [29]. The other weak peaks are related to the structure of aluminum-oxygen octahedron and silicon-oxygen tetrahedron in the montmorillonite [30].

Polymer characterization
The polymerization of PVAc is also established by the Infrared spectroscopy (FT-IR). Indeed, the characteristic bands of the vinyl group at 1646 cm-1, stretching band at 949 cm -1 at 873 cm -1 corresponding to C=C and the stretching vibration at 1132.99 cm -1 corresponding to the group =C-O-C observed in FT-IR spectrum of vinyl acetate (Figure 3) that disappear in FT-IR spectrum of PVAc (Figure 4) con rming that polymerization of the monomer catalyzed by Maghnite-Na + (7% wt) has been successfully completed [31]. In addition, the stretching vibration C=O and the band corresponding to the ester group of vinyl acetate which appear, respectively at 1728.97cm -1 and at 1206.74 cm -1 shift to 1755.89 cm -1 and 1224.83 cm -1 for PVAc [32]. The stretching vibrations of CH (2923.82 cm -1 ), CH 2 (2923.82 cm -1 ), CH 3 (3017.66 cm -1 ) and COCH 3 (1018.77 cm -1 ) are observed in the spectra of PVAc [33].
The structure of PVAc is analyzed and con rmed by 1H NMR and 13 C NMR spectroscopy. The peaks at 21.04 ppm, 39.16 ppm, 66.98 ppm and 170.38 ppm observed in the 13C NMR spectrum of ( Figure 6) are assigned, respectively, to the groups CH3, CH2, CH and C=O of the polymer [34]. The 1H NMR spectrum ( Figure 5) also con rms the PVAc structure by the presence of two peaks centered at 1.52 and 1.78 ppm corresponding to the methylene group (CH 2 ) corroborated by the disappearance of the peaks (two doublets) at 4.56 ppm and at 4.87 ppm corresponding to the double bond H 2 C=CH 2 [35]. The peak situated at 2.01-2.05 ppm is assigned to the methyl group (CH 3 ) and the broad peak located at 4.88 ppm for the CH 2 group. These results are similar to the literature [36].

.Thermal properties of polymer
The Thermogravimetric analysis is used in order to study the thermal stability of PVAc. The obtained thermogravimetric curve which is the representation of the relative weight loss depending on the heating temperature is given in (Figure 7 ) and which shows that there are two distinct weight loss stages. The rst and intense weight loss between 300 and 400°C (69% wt) corresponds to the deacetylation step which consists in smaller fragments corresponding to the acetic acid mother molecule [37].
During the deacetylation process, (CH)n fragments evaporate from the polymeric material indicating scission of the polymer main chain at the end [38]. At the temperature of 400°C, PVAc decomposes into a highly regular unsaturated material or polyene [39]. The second weight loss observed at high temperatures, in the range of 400 to 500°C (19% wt) can be attributed to the complete degradation of the formed polyene through chain scission reactions. These results are in agreement with literature [40] Analysis by Differential scanning calorimetry (DSC) determines the thermal properties of PVAc and in particular its glass transition temperature, Tg, which corresponds to the passage of the polymer from the vitreous state to the rubbery state [41]. The hermetic capsule containing a mass of the polymer is introduced into the oven at room temperature and then the temperature is lowered to -20°C by a ow of (N 2 gas). After that, a temperature gradient of -20°C to 200°C is achieved with a rate of 10°C/min [42]. At 200°C, the temperature is again lowered to -20°C, again with 10°C/min through the N 2 gas ows, before a second temperature rise in the same conditions. The DSC thermogram thus obtained allows the determination of Tg. The rst passage in temperature gives an enthalpy relaxation peak which depends on the internal tensions, resulting from the synthesis process and the thermal history of the polymer cannot be taken into consideration [43]. To eliminate the «thermal history» of PVAc, Tg is determined from the second pass. The obtained curve (Figure 8) shows that Tg is about 50°C which is in agreement with the data from the literature [44].  The tensile test was carried out to evaluate the tensile properties of the various samples compositions in order to determine the in uence of the addition of the clay on the tensile properties of the virgin matrix. Young modulus, tensile strength and elongation at break were evaluated as a function of the mass fraction of clay in all series of samples. Thus Young's modulus increased in compositions with the highest clay contents, (1-7%).The composition in PVAc/maghnite-Na + (7w% of catalyst) has the highest tensile values [45]. A Signi cant increase in the elongation at the break to PVAc/maghnite-Na + was observed. This was attributed to the aggregate formation of maghnite-Na+ within the polymer matrix and the matrix with 7wt% maghnite-Na + exhibited an maxima elongation at break of 86.65%.These dramatic improvements were attributed to the ne dispersion of maghnite-Na+ in the polymer matrix [46].This is attributed to the interactions between the polymer chains and the nanometric layers of the clay with a decrease in the value of the young's modulus [47]. This composition is the most tensile resistant with a force of 182 N and a maximum stress of 73.16 MPa, the most exible (E = 955 MPa) and the most ductile (εr = 768 %). From these results, it can be deduced that the incorporation of the clay into the PVAc matrix, with different percentages, has signi cantly improved all of its tensile properties [48].

Morphological properties of polymer
Scanning electron microscopy SEM was used to see the distribution of the maghnite clay in the polymer synthesized [49]. The morphology of the surfaces was observed in (Fig 9A, B, C and D) and the representative scanning electron micrographs of raw maghnite, maghnite-Na + and PVAc/Maghnite-Na+ from the anionic polymerization, respectively. The particle diameter of raw maghnite ( Fig 9A) and maghnite-Na+ (Fig 9B) ranged from 50 to 100 nm, while PVAc/Maghnite-Na+( Fig 9C) and pure PVAc ( Fig  9D) ranged from 150 to 200 nm, the smaller MMT particles were adsorbed around the larger PVAc particles to form the tube structure [50]. SEM images of PVAc/MMT (Fig. 9C) and pure PVAc (Fig 9D) showed that when the maghnite was incorporated in the PVAc, the average particles size of the composite was increased to 200 nm. The obtained results indicate that the polymerizing chains were aggregated into the clay interlayer regions and consequently the exfoliation of maghnite-Na + (MMT-Na + ) was almost completed [51].

Kinetic's Studies
The effect of the amount of maghnite on the yield of polymerization expressed by using various weight ratios Mag-Na+/monomer, on the polymerization yield of vinyl acetate is showed in (Figure10).
The polymerization is carried out in bulk at 0°C for (6h) using various amounts of Mag-Na+ (3%, 4%, 6%, 7%, 10% and 12 % wt/wt). It can be noted that the yield of PVAc increases with increasing the amount of the catalyst in which the effect of Mag-Na + as a catalyst is clearly shown [52]. However, the maximum yield is obtained at 7% by weight of Mag-Na + and from this rate it remains unchanged [53]. The phenomenon (increasing of the yield) can be attributed to the active sites available in the catalyst which are responsible for the initiation and acceleration of the polymerization reaction until the saturation of these sites (stabilization of the yield) [54].
( Figure 11) shows the effect of time on the yield of PVAc. The polymerization of vinyl acetate is carried out at different times in bulk at 0°C using (7%) by weight of Maghnite-Na+. The obtained results show that the yield increases slightly during four hours of reaction and this can be considered as an induction period [55]. From that time, the yield begins to increase rapidly until it stabilizes and reaching a maximum value of 72.85 % after 6 hours of reaction [56].

Proposed Mechanism
This new anionic polymerization consists of three main reactions: (a) initiation, (b) propagation, and (c) termination, as described in (Scheme 3). However, the initiation reaction is generally fast and is not re ected in the overall rate of the polymerization [57]. The interaction of propagating ion pairs with functional groups of the monomer or the polymer chain can affect the propagation rate and in some cases induces side reactions that can cease the polymerization [58]. The termination is brought about intentionally using a suitable electrophile, which can be useful for end group modi cation [59].

Conclusion
In this work, poly (vinyl acetate) is synthesized and structurally characterized. The synthesis of this polymer is carried out successfully by highlighting a completely ecological process by integrating the principles of green chemistry. This new approach allows developing an energy-e cient process by using a recycled polyurethane-based as container for the reaction mixture which is an excellent thermal insulator and thus let it possible to work at low temperature for a long time. In addition, PVAc is synthesized for the rst time in bulk using an e cient green catalyst called Maghnite-Na + under heterogeneous conditions. The polymerization proceeds via an anionic mechanism due to the presence of intercalated sodium ion (Na+) in the lamellar structure of the montmorillonite. FT-IR spectroscopy as well as 1H NMR and 13C NMR spectroscopy con rm the structure of PVAc. TGA shows that the obtained polymer is thermally stable with a degradation temperature higher than 300°C and its glass transition temperature is 50°C which is obtained by DSC analysis. Consequently, Maghnite-Na+ showed that it is an attractive eco-catalyst with many advantages such as being cheap, safe and reusable. Indeed, it can be easily separated from the polymer products and regenerated by heating to a temperature above 100 °C.   Effect of time on the yield of PVAc (Reaction temperature = 0°C, Mag-Na+ = 7%wt, in bulk)