Development of a Catalyst for Solution of Poly(Vinyl Alcohol) in Non-Aqueous Medium

DEVELOPMENT OF A CATALYST FOR SOLUTION OF POLY(VINYL ALCOHOL) IN NON-AQUEOUS MEDIUM

Prafulla Chetri, Narendra Nath Dass and Neelotpal Sen Sarma* Material Science Division, IASST, Paschim Boragaon, Guwahati-781 035, Assam, India

Abstract Chloro ethane dimethyl sulfoxide, C2H5Cl·DMSO (ECl·DMSO) was prepared by interaction of acrylic acid with conc. Hydrochloric acid in dimethyl sulfoxide (DMSO) and subsequent decarboxylation with H2O2 solution. The formation of the compound was confirmed by spectral and analytical methods; the molecular weight was determined by cryoscopic method. The solubility of poly(vinyl alcohol) (PVA) in different solvents or mixed solvents at 40°C, 50°C and 60°C temperature in the presence of 0.01% of ECl·DMSO was determined. It turned out that ECl·DMSO helps dissolution of PVA, and is very effective catalyst in the preparation of esters and acetals of PVA.

Keywords: Chloro ethane dimethyl sulfoxide; Poly(vinyl alcohol); Catalyst; Solubility.

INTRODUCTION Practically water is the only solvent in which poly(vinyl alcohol) (PVA) can be dissolved[1]. PVA is not soluble in non-aqueous medium due to strong intermolecular and intramolecular hydrogen bonding. The solubility of PVA in water depends on the degree of hydrolysis and molecular weight[2]. Fully hydrolyzed PVA are soluble only in hot to boiling water whereas partially hydrolyzed grades (88 mol%) are soluble at room temperature[3]. Due to the solubility problem some important derivatives of PVA like esters, ethers, acetals are difficult to prepare from PVA. Acetals are prepared from PVA in water by heterogeneous reaction process[4−7] and so the maximum extent of acetalization achieved is 86 mol%, although some patents claim 100 mol% acetalization[8]. Similarly, polyvinyl esters are prepared either by the polymerization of the vinyl esters or by the transvinylation method[9−11]. Due to the solubility problem of PVA in organic solvent the derivatives of PVA are synthesized in aqueous medium[12]. It is not possible to synthesize an ester by homogeneous esterification of PVA with the corresponding acid in aqueous medium. To solve this problem a catalyst C2H5Cl·DMSO (ECl·DMSO) is synthesized, which can help PVA to go into solution in an organic solvents or in a mixed solvents.

EXPERIMENTAL Materials and Methods Acrylic acid (Bayer’s Germany) was purified according to the procedure adopted by O’Neil[13]. Commercial-grade dimethyl sulfoxide (BDH) was purified by successive drying over a Linde Å molecular sieve and barium oxide, and finally by distillation under reduced pressure[14]. Hydrochloride acid (BDH) and hydrogen peroxide (E. Merck) 30 vol% were analytical grade and were used without further purification. PVA (BDH) having viscosity average molecular weight of 14000 was used (contained 0.1% acetate group) without further purification. The infrared spectrum of the catalyst was recorded in the 200−4000 cm−1 region with a Perkin-Elmer 883

* Corresponding author: Neelotpal Sen Sarma, E-mail: [email protected] Received May 28, 2007; Revised July 18, 2007; Accepted August 16, 2007 400 P. Chetri et al.

grating IR spectrophotometer. The proton NMR spectrum was recorded with a WH 60 NMR spectrometer. The spectrum was obtained on solution containing 100 g/L ECl·DMSO in CHCl3 (d) with tetramethyl silane as an internal reference. X-ray powder diffraction pattern of the catalyst was obtained using CuKα radiation. The measuring conditions were 30 kV, 10 mA with a scattering angle of 2°–60° and a step angle of 0.05°. The molecular weight of the catalyst was determined by depression of' freezing point method. For this experiment the solvent used was benzene. Molecular weights of the polymers were determined by GPC method with a Waters GPC-150C instrument using tetrahydrofuran (THF) as the solvent at 25oC. Intrinsic viscosity [η] was determined at 30oC by Ubbelohde viscometer using dimethyl formamide (DMF) as the solvent. Differential scanning calorimetric (DSC) thermogram of the polymers were traced by Perkin-Elmer DSC-7 kinetic software in air at a scanning rate of 10 K/min. Preparation of ECl·DMSO Acrylic acid (0.10 moL) and concentrated HCl (0.12 moL) were mixed with DMSO (5.0 mol). The mixture was o then heated at 50 C for 0.5 h. The solution was then treated with 30 vol% H2O2 for decarboxylation when brisk evolution of gases occurred. The gas evolved was identified to be CO2 by analytical and IR method. On freezing the solution, a white needle type crystal appeared. The product was filtered and washed with petroleum ether. The product was then recrystallized several times from acetone and stored over anhydrous calcium chloride. The yield of the product was 90%.

RESULTS AND DISCUSSION The following reaction may take place between acrylic acid and conc. HCl for the production of ECl·DMSO[15].

The first step of the reaction was electrophilic addition. Decarboxylation takes place in the second step, and [16, 17] this step can be accelerated by an electron-withdrawing chloro group at α carbon . H2O2 probably acts as decarboxylating catalyst. Microanalysis, IR and 1H-NMR spectral data supported the presence of one DMSO molecule as solvent of crystallization. The molecular weight of the compound, measured, by depression of the freezing point method with benzene as the solvent, was found to be 142.5. These findings support the proposed molecular structure. Interestingly no solid came out if DMF, N,N-dimethyl acetamide (DMA), dioxane etc. were used in place of DMSO, indicating that DMSO acts as molecule of crystallization. By microanalysis the elements present in ECl·DMSO were found as: Calculated: C = 33.68%, H = 7.72%, Cl = 24.91%, S = 22.45%; Found: C = 33.61%, H = 7.71%, Cl = 24.89%, S = 22.44%. The melting point of the compound was found to be 160oC. The IR spectrum of ECl·DMSO is shown in Fig. 1. The characteristic band for ECl·DMSO appeared at 1310 and 760 cm−1 due to C―Cl group[18]. The band at 950 cm−1 seemed to relate S＝O stretching vibration which indicated the presence of DMSO molecule. The bands at 1430, 1410, 1340 and 1130 cm−1 appeared due to the DMSO molecule[19]. Development of a Catalyst for Solution of Poly(vinyl alcohol) in Non-aqueous Medium 401

Fig. 1 IR spectrum of C2H5Cl·DMSO (in KBr)

A typical 1H-NMR spectrum for ECl·DMSO is shown in Fig. 2. The signals observed at δ = 2.0–2.7 and

3.3–4.2 were due to ―CH3 and ―CH2Cl protons, respectively. A signal at δ = 2.9 was due to the DMSO molecule. These signals confirmed the formation of the compound.

1 Fig. 2 H-NMR spectrum of C2H5Cl·DMSO (in CDCl3)

The powdered diffraction pattern of the catalyst showed three intense peaks corresponding to 2θ values

16.35°, 20.35° and 24.25°. Based on the XRD results an orthorhombic structure D2h can be predicted for the compound. The crystal may be a nearly orthorhombic structures with two interpenetrating faces having sides 5.424, 4.366 and 3.672 Å and the molecular formula ECl·DMSO. Solubility of PVA PVA is soluble in DMF, DMSO, glycol and glycerol at hot condition when the degree of hydrolysis of PVA is 402 P. Chetri et al.

less than 88 mol%[20, 21]. PVA used in this experiment is 98 mol% and only a trace amount of this is soluble in DMF, DMSO, glycol and glycerol at hot condition. The solubility of PVA in different solvent or mixed solvent was determined by flask method[22]. To measure the amount of solubility in different solvents a weighted amount of PVA was gradually added to 100 mL non-aqueous organic solvents or mixed solvents in presence of 0.01g ECl·DMSO at three different temperature namely 40°C, 50°C and 60°C just before a faint turbidity appeared. For solvent mixture like DMF/benzene, a measured volume of benzene was added drop wise to the solution of PVA in DMF just before a faint turbidity appeared. The experiment also carried out using non-aqueous organic solvents or mixed solvents without using any ECl·DMSO at the same three temperatures (Table 1) using the same procedure. The experiment was repeated thrice to get the accurate result.

Table 1. Solubilities of PVA in gram per liter at different organic solvents and mixed solvents in the presence and absence of 0.01% ECl·DMSO at different temperature Solubility (g/L) Sample Solvent PVA with ECl·DMSO PVA without ECl·DMSO no. 40°C 50°C 60°C 40°C 50°C 60°C 1 DMF 30.0 33.3 36.5 Sw 1.17 2.02 2 DMSO 31.0 34.1 37.2 Sw 1.37 2.32 3 N,N-Dimethyl acetamide 27.7 29.8 33.1 − − − 4 CH3COOH 9.2 12.1 15.0 − − − 5 C2H5OH 2.1 3.1 5.8 − − − 6 Glycol 8.1 10.2 12.3 − Sw 1.70 7 Glycerol 10.2 13.0 15.2 − Sw 1.82 8 DMF + C2H5OH (5:4 V/V) 14.9 17.0 19.0 − − − 9 DMSO + C2H5OH (5:3 V/V) 20.4 22.3 24.0 − − − 10 CH3COOH + C2H5OH (5:2 V/V) 3.8 5.6 7.3 − − − 11 DMF + Toluene (5:2 V/V) 10.9 22.3 27.0 − − − 12 DMSO + Toluene (5:1 V/V) 24.7 27.8 30.9 − − − 13 DMF + Benzene (5:3 V/V) 18.8 20.1 22.8 − − − 14 DMSO + Benzene (5:2 V/V) 25.3 27.8 30.0 − − − 15 CH3COOH + THF (2:1 V/V) 2.1 2.9 5.5 − − − 16 C2 H5OH + THF (5:1 V/V) 1.9 2.3 4.5 − − − 17 DMF + Dioxane (5:4 V/V) 20.9 24.1 26.0 − − − 18 DMSO + Dioxane (5:3 V/V) 24.2 26.0 28.5 − − − 19 DMA+Toluene(5:3 V/V) 8.7 11.9 13.6 − − − 20 DMA + Benzene (5:l V/V) 8.9 10.9 12.0 − − − 21 Glycol + Benzene (2:1 V/V) 0.32 0.74 0.98 − − − 22 Glycerol + Benzene (2:1 V/V) 0.73 1.02 1.19 − − − Sw: Swelling; −: Insoluble

Mechanism for dissolution of PVA in the presence of ECl·DMSO can be described in the following way. PVA cannot be soluble in non-aqueous medium due to its strong hydrogen bonding. When PVA dissolves in water, PVA go into solution due to the minimization of this hydrogen boding as follows:

PVA in water Development of a Catalyst for Solution of Poly(vinyl alcohol) in Non-aqueous Medium 403

The same type of hydrogen bonding is also formed when ECl·DMSO is added either to DMSO or to DMF because chlorine in ECl·DMSO pulls electron towards itself.

PVA in DMSO PVA in DMF

So, this way ECl·DMSO helps PVA to go into solution in DMF and DMSO. From the Table 1 it is seen that ECl·DMSO is the most efficient catalyst for dissolving PVA in DMSO at 60°C. In the case of mixed solvent the ratio of the solvent also plays a role and mixture of DMSO and Toluene (5:1 V/V) is found to be optimum. Again it is also observed that the solubility of PVA in DMF at 40°C is near about the same as that in DMSO/benzene (5:2 V/V) mixture. Preparation of Esters and Acetals of PVA

PVA, 4.4 g, (0.10 mol, based on ―CH2―CH―OH as the repeat unit) was dissolved in 150 mL of a solvent mixture of DMF and benzene (4:1, V/V) in presence of ECl·DMSO at 60°C in a round-bottom flask. The molar ratio of PVA to ECl·DMSO was maintained at 1:(1.4 × 10−3). Corresponding organic acid or aldehyde (0.11 mol) in 100 mL DMF was then added slowly to the PVA solution. Homogeneous esterification or acetalization of PVA was carried out by heating the reaction mixture, at (90 ± 1)oC. The water produced during the reaction was removed from the reaction medium as it was formed using the Dean and Stark principle. After completion of the reaction the solvent was removed by distilling under vacuum. The ester or the acetal as the case might be was precipitated by pouring the reaction mixture into a mixed solvent of acetone and petroleum ether (1:2 V/V) four times in volume of the ester or the acetal solution with constant stirring. Reprecipitation was done twice to ensure the complete removal of unreacted PVA and organic acid or aldehyde. To remove PTSA, ECl·DMSO and the last traces of other impurities the product was washed with benzene, dried at 40°C and stored over anhydrous calcium chloride. The percentage of unconverted hydroxyl groups in the ester or acetal was estimated by acetylation process. The acetylation was done with a mixture richer in acetic anhydride (1 vol. of acetic anhydride, 3 vol. of pyridine) for ten hours at 60°C. As a result, the unreacted acetic anhydride was hydrolyzed to acetic acid and was titrated with standard sodium hydroxide. Thus the percentage of unconverted hydroxyl groups can be estimated. The esters and acetals synthesized are noted in Table 2 along with their physical properties. The esters and acetals were soluble in a number of organic solvents like DMF, DMSO, dioxane, THF, chloroform, acetone, ethanol, cyclohexanol, methanol, acetic acid, methylene dichloride, etc.[23].

Table 2. Physical properties of esters and acetals of PVA a −5 −5 b −2 Sample no. Esters/Acetals Yield mp (°C) Tg (°C) [η] (dL/g) Mn × 10 Mw × 10 DPn × 10 1 N1 Poly(vinyl propionate) (%)85 185 46 0.42 0.14 0.30 3.00 2 Poly(vinylbenzoate) 84 215 49 1.54 0.87 1.41 5.90 3 Poly(vinyl borate) 80 245 56 0.78 0.32 0.60 5.26 4 Poly(vinyl cinnamate) 82 207 50 1.38 0.77 1.28 7.35 5 Poly(vinyl butyral) 90 258 38 0.72 0.15 0.55 3.87 6 Poly(vinyl crotonal) 88 205 40 0.64 0.13 0.50 3.57 7 Poly(vinyl acetal) 85 225 54 1.12 0.42 0.72 6.32 a Obtained from DSC analysis; b Calculated from GPC trace

404 P. Chetri et al.

It was observed that the degrees of polymerization were different for different products. This might be due to the variation caused by the fraction of hydroxyl groups of' PVA converted into derivatives with different acids and aldehydes. Again, the degree of polymerization might change due to the oxidative or catalytic degradation[24] of the esters or acetals by ECl·DMSO. The degradation might result in the cleavage of the main molecular chain of the polymer resulting in a decrease in molecular mass of the polymer constitution.

ACKNOWLEDGEMENT Authors are thankful to DST, GoI for financial help.

REFERENCES

1 Billmeyer, F.W.Jr., “Text Book of Polymer Science”, Wiley Interscience, New York, 1984, p.393 2 Urbanski, J., Czerwinski, W., Janicka, K., Majewska, F. and Zowall, H., “Hand Book of Analysis of Synthetic Polymers and Plastics”, John Wiley and Sons. Inc, Aberdeen, 1977, p.392 3 Lindemann, M.K, “Encycl. Polym. Sci. Technol.,” ed. by Bikales, N.M., John Wiley and sons, Inc., New York, 1971, Vol.14, p.169 4 Shibatani, K. and Fujii, K., J. Polym. Sci. A-1, 1970, 8: 1647 5 Minamino, H., 1994, Jpn. Pat., 06,239,929 6 Asahima, K. and Sakashita, K., 1989, Jpn. Pat., 01,318,009 7 Flory, P.J., J. Am. Chem. Soc., 1950, 72: 5052 8 Harrison, S.A. and Wheeler, D.N., J. Am. Chem. Soc., 1951, 73: 839 9 Rosen, I., McCain, G.H., Endrey, A.L. and Sturm, C.I., J. Polym. Sci., A-1, 1963, 1: 951 10 Gamichon, C. and Herery, P., J. Polym. Sci. Polym. Chem. Ed., 1982, 20: 3261 11 Terada, K., Myazaki, H. and Maruyama, H., 1992, Jpn. Pat., 264,387,02 12 McNally, J.G. and van Dyke, R.H., 1942, U.S. Pat., 2,269,216 13 O'Neil, T., J. Polym. Sci., A-l, 1972, 10: 569 14 Drago, R.S., Hart, D.M. and Carlson, R.L., J. Am. Chem. Soc., 1965, 87: 1900 15 Chetri, P., Islam, N. and Dass, N.N., J. Polym. Sci. A, 1996, 34: 1613 16 Gould, E.S., “Mechanism and Structure in Organic Chemistry”, Holt, Rinehart, and Winston, New York, 1959, p.346 17 Fraenkel, G., Belford, R.L. and Yonkwich, P.E., J. Am. Chem. Soc., 1954, 76: 15 18 Bellamy, L.J., “Advances in Infrared Group Frequencies”, Methven, London, 1968, p.233 19 Silverstein, R.M., Bassler, G.C. and Morrill, T.C., “Spectrometric Identi Fication of Organic Compounds”, Wiley, New York, 1981, p.164 20 Saunders, K., “J. Organic Polymer Chemistry’’, 2nd ed., Chapman and Hall, New York, 1988, p.121 21 Brandrup, J., Immergut, E.H. and Grulke, E.A., “Polymer Handbook”, John Wiley & Sons, New York, 1999, p.504 22 Maiti, S., J. Macromol. Sci. Chem., 1982, A18: 955 23 Chiklis, C.K. and Grasshoff, J.M., J. Polym. Sci. A-2, 1970, 8: 1617 24 Sobbolev, D., “A First Course in Polymer Chemistry”, Mir Publishers, Moscow, 1971, p.304