Anomalous behaviour in copolymerizations in aqueous medium: copolymerization of acrolein with acrylic acid

Eur. Polym. J. Vol. 27, No. 8, pp. 803-806, 1991 Printed in Great Britain. All rights reserved

0014-3057/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc

ANOMALOUS BEHAVIOUR IN COPOLYMERIZATIONS IN AQUEOUS MEDIUM: COPOLYMERIZATION OF ACROLEIN WITH ACRYLIC ACID* J. M. GADGIL,t U. V. NAYAK, C. R. RAJAN, G. D. SHAHAPUREand S. PONRATHNAM Polymer Science & Engineering Group, Chemical Engineering Division, National Chemical Laboratory, Pune 4l 1008, India

(Received 22 February 1990; in revised form 16July 1990; received for publication 16 October 1990) Abstract--Copolymerization of acrolein with acrylic acid in aqueous solution is considered. The copolymerization becomes heterogeneous at acrolein mole fractions exceeding 0.45. The reactions were terminated at moderate conversions (6-38%) and the monomer reactivity ratios were computed using an integrated copolymer composition equation. Two different reactivity ratios were estimated for acrolein, each applicable for a particular range of compositions. The heterogeneity in reaction results in decrease in the relative rate of incorporation of acrolein into the copolymer chain. An explanation for the behaviour is presented.

INTRODUCTION

H o m o and copolymerizations of acrolein have been extensively investigated since the ease of modifying the aldehyde pendant groups offers considerable scope to make polymers with new properties [1-4]. The desired distribution of aldehyde group along the polymer chain may be achieved by the copolymerization of acrolein with vinyl comonomers. Functionalized hydrophilic polymers may be synthesized by aqueous solution copolymerization of acrolein with water-soluble comonomers and derivatizing them. The relevant variables are the m o n o m e r reactivity ratios for the comonomers and the self-cyclization reaction between adjacent aldehyde pendant groups. It is well documented that, in aqueous solution copolymerization of water-soluble monomers, the kinetic parameters (monomer reactivity ratios) are influenced by p H ionic strength of the medium [5], ionization [6, 7] and co-operative hydrophobic interactions [8, 9] of the monomers. These factors affect the copolymer composition and the distribution of the comonomers units along the copolymer chain and they control the properties of the resulting polymers. In the aqueous copolymerization of acrolein with acrylic acid, the copolymer immediately separates out as a coacervate from systems rich in acrolein ( > 45 tool%). This effect alters the apparent reactivity ratio of acrolein, while the reactivity of acrylic acid is unaltered. The effect of the nature of the reaction medium (homogeneous/heterogeneous) on the reactivity ratios is presented. EXPERIMENTAL PROCEDURES

Materials Acrylic acid was freed from inhibitors by distillation under reduced pressure in N2. Acrolein was distilled in N 2. Deionized water was used as the solvent. Acetone and

petroleum ether 60-80 were purified by standard procedures. Other reagents (analytical reagent grade) were used as received.

Synthesis Acrolein and acrylic acid in the required amounts were weighed into stoppered reaction vessels. Deionized water was added to give a volume of 46 ml. The pH of the reaction was noted but not controlled. The reaction vessels were flushed with purified N 2 for 15 min. Potassium persulphate and sodium sulphate solutions (2.0 ml, 0.125 M) were added to initiate the copolymerizations. The reaction vessels were stoppered and thermostated at 30.0 + 0.1 °. The combined monomer concentrations was 4 M and the redox initiator concentration was 5 × I0 3 M. The reaction times were adjusted to maintain the weight conversions below 40%; addition of 2 ml of 0.25 M hydroquinone solution in acetone was used to terminate the reactions. The copolymers were quantitatively precipitated with 11 of 7:1 v/v acetone/ petroleum ether 60-80 mixture, collected on sintered crucibles and dried to constant weight at 50°. The weight fraction conversions were determined. Analysis An exact weight of copolymer (~ 100 mg) was allowed to react for 16 hr with 20 ml 1.0 M aqueous KOH solution to convert it quantitatively into the potassium salt of the carboxylic acid. It was quantitatively precipitated with acetone/methanol mixture. The polymer was washed to remove the excess KOH. The potassium salt was converted quantitatively into potassium sulphate with conc. H2SO4 and analyzed by flame photometry to estimate the potassium and hence the acrylic acid content of the copolymer. The reliability of the procedure was estimated by analyzing poly(acrylic acid) and polyacrolein as well as synthetic mixtures of the two homopolymers. The results were reproducible and >98.5% accurate [10]. The copolymers were characterized in solid state by 75.5 mHz t3C CP/MAS NMR technique on a Bucker MSL 300 NMR spectrometer with contact time of 1 msec. RESULTS AND DISCUSSION

Copolymerizations were conducted for ratios of acrolein to acrylic acid such that the mole fraction of

*NCL Communication Number: 4859. ?To whom all correspondence should be addressed. 803

J . M . GADGIL et al.

804

Table I. Data of monomer feed copolymer composition, in moles Moles In feed

Moles In polymer

Expt

Acrolein x 10

Acrylic acid × 10

Initial pH

1 2 3 4 5 6 7 8 9 10 11 12 13

0.425 0.600 0.700 0.852 0.900 1.065 1.100 1.200 1.299 1.401 1.500 1.703 1.916

1.605 1.381 1.282 1.200 1.085 1.000 0.888 0.789 0.690 0.592 0.493 0.401 0.200

2.26 2.30 2.33 2.37 2.46 2.46 2.51 2.54 2.57 2.66 2.74 2.84 2.93

Differential YtlR method

Acrolein x 10

Acrylic acid × 10

Final pH

0.277 0.410 0.306 0.135 0.482 0.116 0.525 0.286 0.220 0.264 0.228 0.684 0.301

0.115 0.130 0.085 0.039 0.113 0.026 0.077 0.063 0.045 0.056 0.052 0.129 0.051

2.33 2.40 2.43 2.45 2.54 2.47 2.52 2.57 2.66 2.65 2.72 2.76 2.79

acrolein ranged between 0.20 and 0.90. The polymerizations were allowed to proceed to moderate conversions (6-38%) to allow more accurate estimates of copolymer composition. The experimental data are presented in Table 1. The pH of the medium was found prior to the initiation as well as at the termination of the reaction. It was in the range 2.26-2.93 at the start and between 2.33-2.76 at the end; these changes are small. The pK~ of acrylic acid and poly(acrylic acid) are 4.2 and 6.4 respectively. The fractional degree of ionization in the experiments is between 0.01 and 0.04. The carboxyl groups are essentially unionized in the system. The reactions were completely homogeneous in experiments 1-5, i.e. for acrolein mol fractions up to 0.45. At higher proportions of acrolein (experiments 6-13), the copolymer immediately separated as a coacervate. The monomer reactivity ratios were estimated by an integrated form of the copolymer composition equation which gives a better estimate of the ratios [11], especially for systems with greatly differing r~ and r2. In this method, the symmetrical linearization procedure of Yezrielev et al. (YBR) [12] is modified [13] to compute the reactivity ratios. The weight of acrolein, its mole fraction in the feed and in the formed copolymer were used, as shown in Table 2, to obtain the reactivity ratios with the integrated copolymer composition equation [12]. The reactivity ratios determined by the differential YBR and the integrated methods are presented in Table 3a for the homogeneous and in Table 3b for the Table 2. Copolymerization experimental data Expt No.

MI

m~

WFC

1 2 3 4 5 6 7 8 9 10 11 12 13

0.209 0.303 0.353 0.415 0.454 0.516 0.553 0.603 0.653 0.703 0.753 0.810 0.905

0.707 0.783 0.782 0.777 0.810 0.819 0.873 0.814 0.831 0.825 0.815 0.841 0.856

0.170 0.243 0.177 0.077 0.274 0.064 0.279 0.167 0.127 0.155 0.138 0.383 0.169

M~, mole fraction of acrolein in feed; m~, mole fraction of acrolein in polymer; WFC, weight fraction conversion.

Table 3. Calculation of reactivity ratio by YBR, JJ method

Exptn. Input*

Acrolein R~

Integral JJ method

Acrylic acid R2

Acrolein R~

Acrylic acid R2

-0.08 -0.09 -0.06 -0.08 -0.06 --0.14

3.80 3.56 4.61 3.70 3.89 3.17

-0.07 -0.07 -0.05 -0.07 -0.05 -0.12

-0.81 -0.68 -0.81 -0.80 -0.79 -0.79 -0.81 - 0.88 -0.86

0.53 0.88 0.49 0.52 0.52 0.53 0.53 0.52 0.52

-0.83 -0.67 -0.83 -0.82 -0.81 -0.80 -0.82 - 0.88 -0.89

(a) Homogeneous phase 10,500 10,505 10,504 10,503 10,502 10,501

3.80 3.45 4.84 3.69 3.98 3.11

(b) Heterogeneous phase 61,300 61,313 61,312 61,311 61,310 61,309 6t ,308 61,307 61,306

0.54 0.88 0.50 0.52 0.52 0.53 0.54 0.52 0.53

*The first digit represents starting experiment number. The next two digits represent experiment number upto which the data was input. The last two digits indicate the experiment number omitted from the series. Thus, EXPTN. INPUT 10,504 means experiments I-5 excluding experiment 4.

heterogeneous copolymerizations. The reactivity ratios for the two monomers differ widely. However, the two methods give similar estimates of reactivity ratios over a wide range of feed compositions. Thus, the YBR method can be used to make fairly accurate estimates of reactivity ratios for systems, with differing relative reactivities, taken to moderately high conversions. It is also observed from Table 3a and b that the reactivity ratio of acrolein is altered by the solubility characteristics of the formed polymer in the reaction medium, while the reactivity ratio of acrylic acid is negative i.e. effectively zero [14]. Within each range (homogeneous/heterogeneous) the reactivity ratios are independent of the sets of experiments taken to compute them. Thus, the existence of two observed reactivity ratios for acrolein is an intrinsic feature of the system. It is probable that different reactivity ratios of acrolein are not due to change in the rate of self addition (k~) but is reflective of change in the solubility of acrolein in the precipitated but still growing copolymer chain. In the homogeneous range, the reactivity ratio of acrolein is 3.80 and in the precipitation range it is 0.53. The primary objective of our study is to generate hydrophilic copolymers with free aldehyde groups spaced between long but definite sequence lengths of the hydrophilic acrylic acid segments. The acroleinacrylic acid system has been investigated by D'Alelio et al. [15], at pH 3, 5 and 7. The copolymerizations were conducted at 54 ° in sealed ampoules and terminated at very low conversions. The monomer reactivity ratios were estimated by means of the differential form of the copolymer composition equation. The investigators did not report precipitation at pH 3. Precipitations were observed at pH 5 and 7 but no dependence of reactivity ratios on precipitation was recorded. The estimates of r~ (acrolein) and r 2 (acrylic acid) were 0.50 __ 0.30 and 1.15 _+ 0.20 respectively at pH 3. These reactivity ratios are consistent with the generation of copolymers with single aldehyde group separating acrylic acid sequences. However, on the more exact investigation as presented here, this

Copolymerization of acrolein with acrylic acid

805

erization decrease dramatically in relatively nonpolar solvents [9]. This effect is attributed to the formation of linear aggregates of acrylic acid in bulk and in polar solvents shifting to a dimer in nonpolar solvents. This trend is reflected in the copolymerization of acrylic acid with comonomers such as N-vinyl pyrrolidone and acrylamide in water [6, 8, 14] at low pH (pH 1-3) where acrylic acid does not ionize. These comonomers do not hydrogen bond effectively with acrylic acid and the aggregates of acrylic acid are not destroyed. The copolymerization results in long sequences of acrylic acid units in the polymer chain and the reactivity ratio of acrylic acid is high. Acrolein, however, is more reactive than acrylic acid. It also disturbs this aggregation by effective hydrogen bonding with acrylic acid, so decreasing the reactivity ratio of acrylic acid. The anomaly between the reactivity ratios for acrolein in the homogeneous and the precipitating range is not due to changed reactivity. The growing copolymer chain phase-separates but continues to grow. The relative concentration of acrolein in the precipitated phase relative to acrylic acid decreases so reducing the rate of self-addition. Homopolymerization of acrolein in water is heterogeneous. In

system does not lead to "widowed" aldehyde groups between long acrylic acid sequences. However, the system undergoes cooperative cyclization which in turn generates "widowed" aldehydes, as observed by solid state t3C-NMR. Cross Polarized (CP)/Magic Angle Spinning (MAS) ~3C-NMR spectra of poly(acrylic acid), polyacrolein and acrolein (acrylic acid) copolymers in the solid state (Fig. 1) show signals for carboxyl (180 ppm), aldehyde (203 ppm) and hemiacetal-acetal (99 ppm). They reveal that the relative abundance of free aldehyde groups in the copolymer decreases with increase in acrolein content. In water insoluble polyacrolein, the aldehyde pendant groups are present predominantly as cyclized ether linkages which eliminate water. It is the relative importance of this co-operative cyclization in copolymer rich in acrolein which results in the phase separation during copolymerization from feeds rich in acrolein. In the presence of a comonomer, the co-operative cyclization is partially prevented resulting in relatively higher concentrations of the free aldehyde groups. Homopolymerization of acrylic acid in bulk and in polar solvents is extremely fast, resulting in very high molecular weights [16]. The rate and degree of polym-

I

I l l

I

300

*

l i l i l l

250

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l i l

200

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l

t

I

150

llk

100

i I I l

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Fig. l. Solid state ~3C-NMR of acrylic acid (AA) homopolymer, acrolein acrylic acid copolymer (AC 70) and acrolein homopolymer (AC 100).

806

J.M. GADGILet al.

copolymerizations at acrolein mole fraction exceeding 0.45, a similar phenomenon is observed. In experiments with the total m o n o m e r concentrations set at

Council of Scientific and Industrial Research (CSIR, New Delhi) for a Junior Research Fellowship (JRF).

1 M, the phase separation was apparent at conversions as low as 1.0%.

REFERENCES

CONCLUSION The reactivity ratios for acrolein with acrylic acid have been examined in water at 30 ° without pH control. The copolymerization changes from a homogeneous to a precipitating reaction when the acrolein mole fraction in the medium exceeds 0.45. The reactivity ratio of acrolein is altered by the precipitation, while the reactivity ratio of acrylic acid remains unaltered. The different reactivity ratios for the homogeneous and heterogeneous (solution and pseudosuspension) copolymerization of the acrolein-(acrylic acid) system is related to the solubilities of acrolein and acrylic acid in the polymer phase. Acrylic acid is preferentially adsorbed by precipitated polymer (experiments 6-13). This effect results in a decrease in the reactivity ratio of acrolein from 3.80 to 0.54. This decrease is not due to change in the actual reactivities of the two monomers but to changes in local concentration. The use of global concentrations in the calculations of reactivity ratios is not valid. Acknowledgements--Authors thank Dr S. Ganpathy and Mr P. R. Rajmohanan for characterization by solid state ~3C-NMR spectra. One of us, J. M. Gadgil thanks the

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