MONOESTERIFICATION OF STYRENE-MALEIC ANHYDRIDE
COPOLYMERS WITH ALIPHATIC ALCOHOLS

FRANCISCO MARTINEZ*, GLORIA NECULQUEO, MANUEL TORRES.
Dpto. Quimica, Facultad de Ciencias Físicas y Matemáticas. Universidad de Chile.
Casilla 2777-Santiago-CHILE. polimart@cec.uchile.cl
ANDRÉS OLEA
Dpto Fisico-Química, Facultad de Ciencias, Universidad de Chile.
Las Palmeras 3465-Santiago-CHILE. Olea@abello.uchile.cl
(Received: October 23, 2000 - Accepted: January 19, 2001)
*to whom all correspondence should be addressed.

SUMMARY

Maleic anhydride - styrene copolymers were synthesized in acetone-toluene solution at 60° C by azo-bisisobutironitrile. The copolymers were characterized by Gel Permeation Chromatography and Differential Scanning Calorimetry. The monoesterification of the maleic anhydride-styrene copolymers was carried out with methyl, n-propyl, n-butyl, n-hexyl, n-octyl and n-decyl alcohol in tetrahydrofuran solution at 65°C catalyzed by 4-dimethylaminopyridine. The esterification degree of the copolymers has been determined by FT-IR and ranged between 68-85%. An increasing reaction rate was observed with increasing alcohol chain length. The glass transition temperature of the monoesterified copolymers were also determined and ranged from 136-242°C for n-decanol and methanol copolymer respectively.

Keywords: maleic anhydride-styrene, copolymers, esterification.

RESUMEN

Se han sintetizado copolímeros de anhidrido maleico-estireno en solución acetona-tolueno a 60°C iniciado por azo-bisisobutironitrilo. Los copolímeros se han caracterizado por cromatografia de permeación de geles y calorimetría diferencial de barrido. Se llevó a cabo la monoesterificación de los copolímeros de anhídrido maleico-estireno con metano, etanol, n-propil, n-butil, n-hexil, n-octil alcohol y n-decil alcohol en tetrahidrofurano a 65°C, catalizada por 4-dimetilaminopiridina. El grado de esterificación de los copolímeros se determinó mediante FT-IR y fluctuó entre 68-85%. Un aumento de la velocidad de reacción fue observada con el aumento de la cadena del alcohol. Las temperaturas de transición vítrea de los copolímeros monoesterificados fueron también determinados y fluctuaron entre 242°C y 136°C para los copolímeros con metanol y n-decanol respectivamente.

Palabras claves: anhídro maleico-estireno, copolímeros, esterificación

INTRODUCTION

The synthesis of copolymers of styrene and maleic anhydride have been extensively studied [1-3]. Esterification of styrene-maleic anhydride copolymers with some pure and mixture of alcohols have also been reported [4,5].

In the present article we investigated the reactions of an alternating copolymer of styrene­maleic anhydride with alcohols: methyl, n-propyl, n-butyl, n-hexyl, n-octyl and n-decyl. We determined the degree of conversion of the esterification reaction in solution by using tetrahydrofuran as solvent, at 65°C and 4-dimethylaminopyridine as catalyst. The conversions are compared in terms of the esterification mechanism. We have also studied the thermal behavior of the esterified copolymers by Differential Scanning Calorimetry (DSC). The glass transition temperatures of the copolymers have been measured and are compared in terms of the length of the alcohol chain.

Materials

Maleic anhydride ( for synthesis) was supplied by Aldrich Chemical., Styrene and azo-bisisobutironitrile (AIBN) p.a. were obtained from Fluka A.G., 4-dimethyl-aminopyridine (DMAP) was obtained from Aldrich Co, aliphatic n-alcohols were purchased from Merck

Synthesis of copolymers

The polymerization of styrene-maleic anhydride was carried out in a mixture of toluene-acetone at 65°C with AIBN as initiator. 8.0 g (0.08 M) of maleic anhydride, 2.28 g (0.02M) of styrene were added to a 250 ml flask containing a mixture of 60 ml toluene and 12 ml acetone. AIBN, 0.18 g (1.09 x 10 ^­3 M) was added and the clear solution was degassed in a vacuum line for three periods of 20 minutes. Then, the reaction flask was placed into a thermoregulated silicon oil at 60 + 0.5°C during 40 minutes. The copolymer, a white milky suspension, was precipitated in n-hexane and dried under vacuum at 60°C for 24 h. The esterification reactions were carried out in tetrahydrofuran solution at 65°C. The alcohols studied were methanol, n-propanol, n-butanol, n-hexanol, n-octanol and n-decanol, and 4-dimethyl-aminopyridine (DMAP) was used as catalyst.

A typical esterification reaction is as follows: 2.00 g ( 0.01M) of the maleic anhydride copolymer were added to 3.24 ml of methanol (0.08M) and 0.05 g of 4DMAP ( 4.1x 10^-4 M) in 60 ml tetrahydrofuran (THF). The reaction mixture was stirred and a transparent and homogeneous solution was obtained. The flask equipped with a condenser was immersed in a thermoregulated bath at 65 + 0.5°C and samples were collected, precipitated in hexane, filtered, redissolved in THF and reprecipitated in hexane. These samples were used to determine conversion using potassium bromide disk pellets in a FTIR spectrometer.

Methods

-Infrared spectroscopy
-The FT-IR spectra of the copolymers were recorded using the KBr pellet technique by means of a Bruker -IFS-55 Vector 22 infrared spectrophotometer.
-The DSC thermograms were recorded using a Perkin Elmer DSC-7 instrument
-GPC-chromatograms were measured by a Bruker LC-31B Chromatograph

Determination of esterification degree

The conversion of reaction was calculated according to Lindt [6].Two band absorptions were chosen, the peak at 1602 cm^-1, characteristic of the styrene residue, which does not vary with the reaction and the peak at 1779 cm^-1 corresponding to the maleic anhydride residue. The conversion p, was defined as: p = (1- A _t/ A ₀ )x100

A₀ = initial ratio between absorbance at 1779 cm^-1 and 1602 cm^-1

A_t = ratio between absorbance at 1779 cm^-1 and 1602 cm^-1 at reaction time t.

RESULTS AND DISCUSSION

The esterification of the copolymers was carried out by using the DMAP, reported by Lindt [6] as the best nucleophilic catalyst among others like pyridine, tributylamine and 2-dimethylaminopyridine. The starting styrene-maleic anhydride copolymers were synthesized and characterized by us.

The intrinsic viscosity and molecular weights of maleic anhydride-styrene copolymers obtained under the experimental conditions are shown in Table 1.

Table 2 gives the conversion of esterification of the copolymer with methanol versus reaction time. This conversion was obtained under the experimental conditions described in the experimental section. The table is given as example and the same procedure was done for the other alcohols.

The esterification reaction was followed by the 1779 cm^-1 and 1602 cm^-1 peaks which correspond to C-O-C stretching absorbance of maleic anhydride and C=C vinyl stretching of styrene respectively.

 

Fig.1 shows clearly the decreasing of the 1779 cm^-1 band, from the initial copolymer esterified with n-butanol up to the 72 h of reaction time. Fig.2 compares the different conversion obtained for the copolymers esterified with methanol, propanol, butanol, octanol and decanol, the alcohols studied in this work. Clearly, it is observed that reaction rate increases with the different chain length of the alcohol.

By considering the kinetic of esterification reaction, Fig. 2, for example, the methanol and n-octanol have big difference in conversion after 4h. of reaction time. In the case of methanol, actually there is no conversion to the ester at this time, while with n-octanol a 39% of the ester has been already formed. A similar situation occurs at 8 h, we have with methanol only 22% of formed ester and with n-octanol a 54% of the ester has been reached. These extreme cases demonstrates the influence of the aliphatic length of the alcohol towards the esterification, which seems to be favored by the longer aliphatic alcohol.

By considering the esterification mechanism proposed by Lindt [6], the action of the nucleophilic catalyst DMAP is to attack the carbonylic group producing the C-O-C rupture. The next step would be the shifting of the pyridinic rest by the attack of the alcohol. This attack would be facilitated by longer chain alcohol. The longer the aliphatic alcohol chain, the greater the mobility of the chain and the more easily formed ester. If we compare alkyl groups like methyl, octyl or decyl, we see from Fig 2., that decyl produces higher ester conversion than octyl and methanol.

The esterified copolymers were also thermally characterized through DSC. The glass transition temperature, Tg, was determined for each copolymer, Fig. 3 , shows a comparison of the Tg for the copolymer esterified with the different alcohols. The Tg ranged from 241°C to 136°C, for methanol and decanol respectively. As expected, the Tg are influenced by the aliphatic side chain of the alcohol, which provides a higher freedom degree and movement to the polymer structure. For this reason, we found the lowest Tg with the longer alcohol chain, decanol in our case, at 136°C, while the shorter one, methanol, would give a more rigid structure and its Tg should be the highest, as it was found, at 241°C. The other Tg values, are shown in Fig. 3 and are comprised between the Tg values of methanol and decanol.

CONCLUSIONS

The copolymerization system of styrene-maleic anhydride esterified with different aliphatic alcohol and catalyzed with 4DAMP gave ester conversion ranged between 61 and 85% . The esterification rate was dependant of the length of alcohol chain. Thus, the octanol and decanol esterified copolymers were formed faster than with methanol or propanol.

The glass transition temperatures of the different esterified copolymers agreed with the expected values. The obtained values ranged between 241°C and 136°C, for methanol. and decanol respectively. It is important to take into account that the initial styrene-maleic anhydride copolymer is alternating one and the Tg are closely related to this particular structure.

ACKNOWLEDGEMENTS

The authors wish to thank to FONDECYT, Project N 1990968 for financial support.

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