Influence of solvents on apparent reactivity ratios in the free radical copolymerization of polar monomers

Fur. Polym. J. Vol. 25, No. 7/8, pp. 713-717, 1989

0014-3057/89 $3.00+0.00 Copyright © 1989MaxwellPergamonMacmillanplc

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I N F L U E N C E OF SOLVENTS ON A P P A R E N T REACTIVITY RATIOS IN THE FREE R A D I C A L C O P O L Y M E R I Z A T I O N OF P O L A R M O N O M E R S ADOLPHE CHAPIRO CNRS, 2 rue Henri Dunant, 94320 Thiais, France

(Received 27 January 1989)

Abstract--The influence of solvents on the composition of the resulting copolymers is investigated using the following monomer pairs: acrylic acid-methyl acrylate; acrylic acid-acrylonitrile; acrylic acid-acrylamide; acrylonitrile-acrylamide and acrylic acid-methacrylic acid. It is found that copolymer compositions may either vary drastically with the nature of the solvent (copolymerization of acrylamide with acrylic acid or with acrylonitrile) or be slightly affected only. It is shown, in the case of acrylic acid, that a solvent mixture which enhances the "matrix effect" in the homopolymerization of this monomer also favors its introduction in the copolymer (with methyl acrylate). The apparent reactivity ratios are particularly sensitive to solvents in copolymerizations involving acrylamide, but no simple correlation could be established between the values of the reactivity ratios and the manner in which the solvents may affect the equilibria between the various molecular associations present in the system. If the copolymer is insoluble in the reaction medium, the resulting precipitation may or may not influence the apparent reactivity ratios depending on the system under investigation.

INTRODUCTION

It is generally accepted that solvents only exert a limited influence, if any, on the reactivity ratios in free radical copolymerization. Only small variations in the compositions of copolymers were indeed observed for such systems as styrene-methylmethacrylate [l] or styrene-methacrylonitrile [2] when the reaction was conducted in various solvents. However, if at least one of the two monomers may become involved in hydrogen bonded associations, much larger variations in copolymer compositions take place. This is particularly true in copolymerizations involving acrylic or methacrylic acids, acrylamide and methacrylamide (the pertinent literature sources on the subject are reviewed in Ref. [3]. The present communication is concerned with a summary of work along these lines carried out in these laboratories over the past years. An interpretation of the data is presented based on the most recent results. BACKGROUND

The influence of molecular associations on the kinetics of homopolymerization and on the structure of the resulting polymers is best illustrated by the behaviour of acrylic acid [4]. This monomer, like all carboxylic acids, forms hydrogen bonded associations which arise as "cyclodimers" or as "linear oligomers'. Free monomer is only present in dilute solutions of the acid in non-polar solvents such as hydrocarbons, carbon tetrachloride and the like [4]. These various species are in equilibrium. Depending on the nature of the medium (solvent, concentration) and on temperature, the equilibrium is shifted one way or the other. The nature of the associated species can be characterized by i.r. analysis, viscosity measurements and the like. Based on a detailed study of the polymerization of acrylic acid in numerous sol713

vents a good correlation was established between kinetic "anomalies" such as auto-acceleration and the formation of a syndiotactic poly(acrylic acid), on the one hand, and the presence of "linear oligomers" on the other [4], Two groups of solvents were defined: (a) non-polar compounds, such as hydrocarbons and chlorinated derivatives, which shift the equilibrium from oligomers towards cyclodimers; (b) polar compounds, which may become hydrogen-bonded to acrylic acid, such as water, methanol and dioxane; these solvents "stabilize" the oligomers down to fairly high dilutions. However, the oligomers being present in the system from the beginning, they cannot account for the observed auto-acceleration. It was therefore assumed that these oligomers may associate with the polymer formed in the early stages of the reaction to build an oriented structure in which chain propagation occurs by a fast "zip" mechanism. This assumption refered to as the "matrix effect" was used to explain most experimental data [4]. Very similar results were obtained in a study of the polymerization of acrylonitrile. Here the matrix effect was assumed to result from similar association complexes in which hydrogen bonding is replaced by strong dipole-dipole coupling involving the CN groups [5]. The polymerization of acrylamide is also affected by various solvents. This monomer forms numerous hydrogen bonded association complexes but the system appeared to be more complicated and no clear correlation could be established between the expected modifications of these complexes by solvents and the observed kinetic anomalies [3]. The copolymerization of polar monomers of this type should in principle exhibit the same anomalies as

714

ADOLPHECHAPIRO 100

Table 1. "Apparent" reactivityratios in the copolymerization of acrylic acid (AA) with methyl acrylate(MA) [7, 81 r~(AA) r2(MA)

"~ 50 "~ 50

50

100

htole % AA in tnonomer feed

Fig. 1. Composition diagram for the monomer pair acrylic acid-methyl acrylate [7, 8]. l, in a mixture of n-hexane (71%) and methanol (5.7%; 2, in bulk and 3, in toluene (50%) and in DMF (33%). those observed in their homopolymerization. But, in addition, the competition between the four propagation steps involving both monomers and the corresponding growing chains terminated by these monomers may become further influenced by changes in local concentrations of the monomers. These may originate from molecular associations and/or selective adsorption of the monomers on the resulting copolymer with or without some orientation (see also Ref. [6]). All these effects are particularly sensitive to the nature of the solvent and to its concentration and are expected to interfere with the various parameters of copolymerization. In such systems copolymer compositions are not only determined by true "reactivities" of both monomers but also by various physical interactions. Therefore, only "apparent" reactivity ratios can be derived using the classical copolymerization equations. It should be noted that in some of the systems investigated the copolymers are insoluble in the reacting medium and precipitate as they are formed. An interesting conclusion derived from these studies is that this precipitation may or may not affect the compositions of the ¢opolymers (i.e. the "apparent" reactivity ratios) (see below). RESULTS

The cepolymerization of the following monomer pairs was investigated in many solvents: acrylic acid-methyl acrylate, acrylic acid-acrylonitrile, acrylic acid-acrylamide, acrylonitrile-acrylamide, acrylic acid-methacrylic acid. The corresponding composition diagrams are shown in Figs 1-5 respectively.

1. Acrylic acid-methyl acrylate The eopolymerization of this monomer pair in bulk generates polymers with compositions close to those

In bulk

1.1

0.95

In DMD In the mixture: n-Hexane(71%) Methanol(5.7%)

0.42

0.98

2.2

0.1

of the monomer feed (curve 2, Fig. I) [7, 8]. In toluene and in D M F solutions the acrylic acid content in the copolymer is somewhat lower (curve 3, Fig. 1) but the differences are small. The corresponding (apparent) reactivity ratios are shown in Table 1. A remarkable effect, leading to copolymers with much higher acrylic acid contents, arises in a mixture of n-hexane and methanol. In this mixture a strong enhancement of the matrix effect was observed for the homopolymerization of acrylic acid [9]. The reaction exhibited an explosive character with "auto-acceleration indexes" >10. A molecular association complex of two molecules of acrylic acid with one molecule of methanol was characterized in the mixture and this complex could be the cause of the enhanced matrix effect. In order to explain the high acrylic acid content in the resulting copolymers one could assume that the complex associates with growing chains and that more than one acrylic acid unit builds into the polymeric chain at each propagation step. The mechanism of such a process remains to be explained. The apparent reactivity ratios calculated from the compositions are also included in Table 1. The very high "reactivity" of acrylic acid in this system clearly appears from the figures. It is noteworthy, however, that the same binary solvent system has no significant effect on the composition of the copolymer formed in

100

.! 50

S I 50 Mole % AN in

i

i 100

monomer feed

Fig. 2. Composition diagram for the monomer pair acrylic acid-acrylonitri|e [10]. I, in bulk; 2, in a mixture of n-hexane (71%) and methanol (5.?%); 3, in methanol (20%); 4, in methanol (50%) and 5, in toluene (61%).

Influence of solvents on apparent reactivity ratios

715

matrix effect is also destroyed by the strong dipole of acrylonitrile.

o.4"Xd /2"

3. Acrylic acid-acrylamide

Y.W//

L " V " / i v . z"-

-*

I/3~._(6 / ~I

.lligi;" / ip II .~/

I

i

50 Mole

100

% AM in monor~er feed

Fig. 3. Composition diagram for the monomer pair acrylic acid-acrylamide [10]. 1, in benzene; 2, in dioxane; 3, in bulk; 4, in methanol; 5, in acetic acid; 6, in DMF; and 7, in water. Monomer concentration: 2.5 mol/l.

the reaction of acrylic acid with acrylonitrile (see following section).

2. Acrylic acid-acrylonitrile Solvents only appear to exert a mild influence on the composition of the copolymers formed from this monomer pair as can be seen from Fig. 2 [10]. The curves only deviate moderately from the broken line which represents copolymers having the same composition as the monomer feed. An investigation of the molecular associations of these two monomers showed that the addition of acrylonitrile to acrylic acid efficiently destroys the "linear oligomers" present in concentrated solutions of this second monomer. The formation of a one to one molecular association complex between acrylonitrile and acrylic acid was detected [10]. The relatively small influence of solvents on this system may result from the high stability of this complex which would control the reactivity of both monomers. The n-hexane-methanol mixture, which enhances the matrix effect in acrylic acid, generates the copolymer with the lowest content of acrylic acid. This would indicate that the species responsible of the enhancement of the

Both monomers form hydrogen bonded association complexes and the compositions of the resulting copolymers very strongly depend on the solvent used. Figure 3 [10] shows that the highest content of acrylamide occurs in copolymers arising in benzene solutions (curve 1), while the lowest content is observed in aqueous solutions (curve 7). The equimolecular mixture of the monomers generates copolymers containing 63 and 36% acrylamide respectively in these two solvents. The apparent reactivity ratios derived from these data are presented in Table 2 which also includes the dielectric constants of the corresponding solvents. No simple correlation can be established on the basis of these data between copolymer compositions and dielectric constants of the solvents or any expected influence the solvents may exert on the complicated equilibria between the various associated species present in the system [10].

4. Acrylonitrile-acrylamide In this system a very strong variation in copolymer composition is observed depending on the solvent used (Fig. 4) [3]. The equimolecular mixture of monomers leads to copolymers containing 79% acrylamide if the solvent is a hydrocarbon, whereas in D M F solution, the copolymer only contains 31% acrylamide. No significant differences in the compositions of the copolymers could be ascribed to reactions occurring either in homogeneous or under precipitating conditions [3]. The apparent reactivity ratios for this system are shown in Table 2 and compared with the dielectric constants of the solvents. Here again no clear correlation can be established between the reactivity ratios and dielectric constants. However, the table leads to an interesting conclusion. In both systems described here the solvents influence in a similar manner the content of acrylamide in the copolymer. This content is highest if the reaction occurs in hydrocarbons, lowest in water and in D F M and intermediate in dioxane, acetic acid and methanol. It appears that the solvents determine the apparent reactivity of acrylamide irrespective of the partner monomer. It would be interesting to find out whether this observation also holds in the copolymerization of acrylamide with other monomers.

Table 2. "Apparent" reactivity ratios in the copolymerization of acrylamide (AM) with acrylic acid (AA) [10] and acrylonitrile (AN) [31 Solvent Dioxane Benzene Toluene Acetic acid Acetone Methanol DMF Acetonitrile Water No solvent (in bulk)

Dielectric constant 2.2 2.3 2.4 6.15 20.7 32.6 36.7 39.0 78.5

rl(AM )

r2(AA )

rt(AM )

r2(AN )

1.02 1.0

0.35 0.30

0.44

0.08

0.55

0.85

0.84 0.52

0.75 1.00

2.22 0.47 0.44 0.33 0.21 0.50 0.55

0.06 0.50 0.08 1.13 1.90 0.10 1.91

0.47

1.30

0.60

0.57

716

ADOLPHECHAPIRO 100

100

80

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.¢ 50

2O I

0

,0

'~0

~0

~0

100

2S

50

75

100

Mole % AM in monomer feed Mole % HAA in monomer feed

Fig. 4. Composition diagram for the monomer pair acrylamide-acrylonitrile [3]. 1, in benzene toluene and n-hexane; 2, in bulk, dioxane, acetonitrile and acetone, 3, in acetic acid; 4, in methanol; 5, in water; and 6, in DMF. Monomer concentration: 2.5 mol/I.

5. Acryfic acid-methacrylic acid This system involves two carboxylic monomers which both form hydrogen bonded association complexes in numerous solvents. However, whereas these complexes dominate the homopolymerization kinetics of acrylic acid [4], they have no obvious influence on the polymerization of methacrylic acid [11]. The conversion curves are auto-catalytic in the case of acrylic acid and the resulting polymer is syndiotactic (matrix effect), whereas methacrylic acid polymerizes at a constant rate in bulk and in solution and the resulting polymer is atactic. Upon addition of methacrylic acid to acrylic acid the reaction gradually loses its auto-catalytic character and for methacrylic acid concentrations exceeding 25% the conversion curves become linear [7,12]. The monomer feed containing 25% methacrylic acid generates a copolymer containing 50% of each monomer unit and this structure no longer acts as an efficient matrix, perhaps due to steric hindrance of the methyl groups in the polymer which prevent the formation of a regularly aligned association complex with the monomers. Copolymers with similar compositions are formed either in bulk or in 60% molar n-hexane and in 75 or 50% molar methanol solutions (curves 2 and 3, Fig. 5). Methacrylic acid has a higher reactivity in copolymerization than acrylic acid as was observed earlier for other systems [13]. The lack of influence of solvents on the compositions of copolymers of two monomers each of which form hydrogen bonded aggregates can be understood by a similar action of solvents on both aggregates [4, 10] and by the fact that mixed aggregates undoubtedly arise in the mixtures in which both monomers are more or less randomly distributed. In these various systems the reaction occurs under precipitating conditions and since similar compositions are observed in bulk, in n:hexane or in concentrated methanol solutions, in which the precipitate has different textures, one could conclude that, here again, precipitation does not affect the apparent

Fig. 5. Composition diagram for the monomer pair acrylic acid-methacrylic acid [7, 12]. I, 25% in methanol; 2, 50% in methanol; and 3, 75% in methanol.

reactivity ratios. This is not true, however, in more dilute methanol solutions (25% molar in monomers), when the reaction proceeds in a homogeneous medium. Here the resulting copolymers contain a much larger fraction of methacrylic acid as shown on curve l, Fig. 5. These results are further illustrated in Fig. 6 which is a plot of the methacrylic acid content in the copolymers obtained with three different monomer feeds in methanol solutions of various concentrations. A discontinuity clearly appears at the methanol concentration at which the reaction conditions change from precipitating to homogeneous. The reason for this effect is not clear. In order to account for the results we must assume that the composition of the monomers adsorbed on the polymer are significantly different when the polymer is either in solution or precipitated. This point requires further clarification.

100

? ]

l

1 3

~

l~

I

II

~~'" /~

2

75

!

1

50

25

PRECIPITANT

i

2s

HOMOfiENEOUS

~o

;~

,00

Volume % HeOH

Fig. 6. Changes in the composition of acrylic acid-methacrylicacid copolymers as a function of monomer concentration in methanol, for three different monomer feeds [12]. 1, 25% methacrylic acid; 2, 50% methacrylic acid; and 3, 75% methacrylic acid.

Influence of solvents on apparent reactivity ratios

717

CONCLUSION

REFERENCES

The results presented above convincingly demonstrate that molecular association complexes affect the apparent reactivity of polar monomers in copolymerization. Solvents, which modify association equilibria affect the compositions of the resulting copolymers, but these effects are more or less pronounced depending on the system under investigation. The role of solvents can be, at least qualitatively, understood in some systems, such as with the acrylic acid-methyl acrylate pair, but in many other systems, where we have no clear knowledge of the numerous associations which are present and involve m o n o m e r and monomer, monomers and solvent, monomers and polymer, the results cannot be interpreted satisfactorily at present time. The data presented here also shed some light on the conflicting reports in the literature concerning the influence of copolymer precipitation during the reaction on its composition. This precipitation effect, which presumably results from "selective adsorption" of the monomers on the polymer, i.e. from m o n o m e r - p o l y m e r associations, may or may not have a detectable influence on the apparent reactivity ratios depending on the system.

1. T. Ito and T. Otsu. J. Macromolec. Sci. A-3, 197 (1969). 2. G. G. Cameron and G. F. Esslemont. Polymer 13, 435 (1972). 3. A. Chapiro and L. Perec-Spritzer. Eur. Polym. J. 11, 59 (1975). 4. A. Chapiro and J. Dulieu. Fur. Polym. J. 13, 563 (1977). 5. G. Burillo, A. Chapiro and Z. Mankowski. J. Polym. Sci. 18, 327 (1980). 6. L. H. Peebles. In High Polymers Vol. XVIII (Edited by G. E. Ham), pp. 525-586. Interscience, New York (1964). 7. S. Ali-Miraftab, M. Ansarian A. Chapiro and Z. Mankowski. Fur. Polym. J. 17, 947 (1981). 8. A. V. Ryabov, Yu. D. Semchikov and N. N. Slavnitskaya. Vysokomolek. Soedin. 12, 623 (I970). 9. M. Ansarian, A. Chapiro and Z. Mankowski. Fur. Polym. J. 17, 823 (1981), 10. A Chapiro, J. Dulieu, Z. Mankowski and N. Schmitt. Eur. Polym. J. Ms. 912. I I. A. Chapiro and R. Gouloubandi. Fur. Polym. J. 10, 1159 (1974). 12. A. Chapiro, Z. Mankowski and N. Renauld. Fur. Polym. J. 13, 401 0977). 13. H. Mark, B. Immergut, E. H. Immergut, L. J. Young and K. I. Beynon. In High Polymers, Vol. XVIII (Edited by G. E. Ham), Appendix A, Reactivity ratios. Interscience, New York (1964).

R6sum6---On a 6tudi6 l'influence des solvants sur la composition des copolym6res form6s 5. partir des couples suivants: acide acrylique-acrylate de m6thyle; acide acrylique-acrylonitrile; acide acrylique-acrylamide; acrylonitrile-acrylamide et acide acrylique-acide methacrylique. On a trouv6 que la composition des copolym6res peut soit varier tr6s fortement selon la nature du solvant (copolym6risation de l'acrylamide avec l'acide acrylique ou l'acrylonitrile) soit ~tre peu sensible ~. l'action des solvants. Dans le cas de l'acide acrylique on montre qu'un m61ange de solvants qui exalte 'Teffet de matrice" dans l'homopolym6risation de ce monom6re peut favoriser son incorporation clans le copolym6re (avec l'acrylate de m6thyle). Dans la copolym6risation de l'acrylamide les rapports de r6activit6 apparents sont particuli6rement sensibles aux solvants mais on ne peut pas d6finir de corr61ation simple entre les valeurs de ces rapports et l'action que les solvants exercent sur les 6quilibres entre les diff6rentes associations mol6culaires pr6sentes dans le syst6me. Enfin, on montre que lorsque le copolym6re qui se forme est insoluble darts le milieu r6actionnel sa precipitation peut influencer ou non les rapports de r6activit6 apparents selon le syst6me consider&