Acrylonitrile Copolymerization - Vii - Solvents Effects in Styrene Copolymerization

8/13/2019 Acrylonitrile Copolymerization - Vii - Solvents Effects in Styrene Copolymerization

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J . POLYMER SCI.: Symposium No. 5 2 . 5 5 - 6 6 ( 1 9 7 5)

ACRY LONITRILE COPOLYMERIZATION. VIISOLVENTS EFFECTS IN STYRENECOPOLYMERIZATION

C . PlCHOT, E. ZAGANIARIS,* and A . GUYOTCNRS- Ciiigtique Chimique Macroiiiol;c.ulaire,69626 Villeurbanne France

SYNOPSISThe styrene-acrylonitrile radical copolymerization kinetics have been studied forpolymerizations carried out in two different solvents, toluene and dimethylformamide (DMF), inthe whole range of monomer mixture compositions and at various dilutions. The copolymerprepared in toluene solutions are poorer in acrylonitrile than those prepared in bulk. The reversesituation is observed in DMF solution, where the polymerization rate is the highest. In the lattercase, also for acrylonitrile rich mixtures, the relative polymerization rate of styrene tends toincrease with conversion; this effect is attributed to preferential solvation of styrene. Possible

explanation of these facts are briefly discussed.

INTRODUCTIONIn the previous paper of this series [ l ] , i t has been shown that kinetic

deviations from the accepted theory of radical copolyme rization was the rule in thecase of acrylonitrile copolymerization with many comonomers. A possible cause ofsuch a behavior, namely the cyclization reaction involving the attack of a CN groupon the penultimate unit by the chain-end radical, previously suggested for thecopolymerization with vinyl chloride [ 2 ] , is not a satisfactory explanation in theother cases, especially with a nonpolar, pure hydrocarbon monomer such asbutadiene or styrene. The apparent correlation between the difference in polarityand the ex ten t of the kinetic deviation led us now to turn toward a physicalexplanat ion.

On the other hand, and contrary to the generally accepted view, it has beenshown recently that the reactivity ratios in radical copoly merizatio n involving polarmonomers a re dependent on the nature of the solvents. This is important chiefly inthc case of acrylamide copolymerization [3-61 Some results have bee n published alsofor acrylonitri le-methyl-methacrylate 171 and styrene-methacrylonitr i le [8]

The present paper deals with the system styrene@)-acrylonitrile(AN). A previousstudy has been carried out using two different solvents, toluene and dimethyl-formaniide for the two different parts of the composition range in order to be sure

*Present address: Physical Chernistry Laboratory, University of Patras, Greece.55

0 1975 by John Wiley Sons, Inc.


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56 PICHOT, ZAGANIARIS, AND GUYOT

d A5 m i n ,

.0.3960 .320

0.267

0.205

0.126

0 . 1 0 10 . 1 4 0

0.106

0 .0950 . 0 9 3

that the copolymers formed were fully soluble in the reaction medium. The resultswere interpreted in terms of penultimate effect [9]. This study was extended inboth solvents in the whole range of composition and also used the dilution as aparameter. The results point out a strong dependence of the reactivity ratios uponthe nature, as well as the amount, of the solvents. Other effects are shown.

d SS min.

0.118

0 . 1 4 3

0 .1 30

1 . 6 50.133

0 .1840.345

3 .373

0.520

0 56

RESULTS AND DISCUSSIONReactivity Ratios

The copolymerization reactions have been carried out at 60C usingazobisisobutyronitrile (AIBN) as initiator in three series: two in DMF with dilutionaround 2 mole/liter and 0.6 mole/liter, respectively, and the third one in toluene atabout 2 mole/liter. The charge of the reaction, the composition ratio A/S in themonomer mixture ( X A ) (or the reverse xs = S/A) and in the copolymer initiallyformed (nA ), (ns), the initial individual con sump tion rate of each monom er (dpAand dps) corrected from the influence of the initiator concentration I by r , andfinally some results of intrinsic viscosity data in DMF at 25 C, are reported inTables I, 11, and 111 for each series of copolymerization runs.

The kinetic data have been used to draw the Fineman-Ross [lo] plot for theextreme ends of the composition range, illustrated in Figures 1 and 2, respectively.

TABLE I(AN-Styrene) Copolymerization in DMF Solutiona;Charges and Kinetic Results

R U NONF

moles

4 .454 .81

5.335.55

5.505.10

5.504 . 8 0

5 .006 00

Nmoles

0 . 0 1 30 . 1 0 4

0 .1 45

0 36

0.530

0 . 6 4 80 . 8 8 0

1 .0 71 . 1 7 11.23

STYRmolws

1 . 0 20 95

0.66

0 . 6 7

0 .487

0 . 3 7 00 .236

0 .117

0 . 0 5 90 .037

AIBN I[moles

4 . 8 94 . 8 6

4 .894 .87

4 .47

4 . 9 14 .57

4 .98

4 .8 84 .92

X

0 . 0 1 20 .1 10

0.22

0 535

1.08

1 . 7 6

3.73

9 .17

20 .253 3 . 3 4

n

0 . 0 40.25

0 . 4 50 66

0.87

0 96

1 . 4 0

2 . 6 5

3.70

5 60

aDilution: 2 mole/liter.


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SOLVENT EFFECTS IN STYRENE-AN 5 7

RUN

TABLE I1(AN-Sty rene) Copolym erization in DMF Solu tiona; Charges and Kinetic Results

OMF AN STYR AIEN 10moles moles moles moles

0.00940.0470.113

0 200

0 226

0.29840.408

4.8565.45210

21 1212

6.06

5.745.2636 . 5p286.829

0.185 4.020.201 3.960.212 4.22

0.095 4.760.050 4.570.0134 4.880.0080 4.88

v i sco s i ty

9.16

19.6217.7213.7514.45

aDilution: 0.6 mole/liter.

TABLE 111(AN-Styrene) Copolymerization in Toluen e Solutiona; Charges and Kinetic Results

RUNTOLUENEmoles

5

5

5

4.424.353.0

4.602 . 5 0

2.60

Nmoles

1.020.980.1970.804

0.6450.6650 .5250.3720.2620.03E

0.018

STYRmoles

0.02680.06750.026

0.0920.1000.3270.500

0.5751 ,2600.8300.980

I BN 10moles

4.864.894.94.94.894.884.892.124.551.782.18

~

dpAmin

0.051

0.038

0,0270 . 0 5 0

0.066

0 063

0.090.0890.160

dPSm i n .

0.364

0.338

0.1860.1270.126

0.0910.029

0.031

0.043

X

38.10

14.607 . 6

8.746 . 4 0

2.031.07

0 65

0.2080.04350.0185

n

5.341.644 . 6

6

0.960 80

0.56

0.605

0.3560.126E0.0685

86.7

36.0

32.7

aDilution: 2 mole/liter.


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5 8 PICHOT, ZAGANIARIS, AN D GUYOT

FIG. 1. Fineman Ross plot for copolymerization with high acrylonitrile contents: (X)polymerization in bulk (ref. 11); 0) n toluene solution (2 mole/liter); 0) n DMF solution (0.6rnole/liter); A) in DMF solution (2 mole/liter).

FIG. 2. Fineman Ross plot for copolymerization with high styrene contents: (X) polymerizationin bulk (ref. 1 1 ; 0) n toluene solution 2 mofe/liter); 0 ) in DMF solution 0.6 molefliter); A)in DMF solution (2 mole/liter).


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SOLVENT EFFECTS IN STYRENE-AN 59

0.3

0.2

0.I

4A

xA //

I/I I

0 0.1 0.2FIG. 3. Derivation of IA: (X) bulk; 0 ) toluene; 0) DMF 0.6 mole/liter); a) DMF (2mole/liter).

The data from bulk copolymerization by Thomson and Raines [ l 11 have beenincluded for comparison.Obviously, it is not possible to draw straight lines for each series of data, so thatthe deviations from the Lewis and Mayo theory are confirmed. Furt he r, the resultsare strongly dependent upon the nature of the solvent and even the dilution.In order to derive the reactivity ratios, the plots of n/x versus x or l / x (Figures3 and 4) were used and extrapolation to X A = 0 and I / XA = 0, respectively, for rsand rA. The results are reported in Table IV, together with the previous results and

XA

0 0.2 X 0.6FIG. 4. Derivation of IS: (X) bulk; 0 ) toluene; 0) DMF (0.6 molelliter); (a) DMF 2mole/liter).


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DIPOLAR MOMENT DIEL ECT RIC CONST.CoMPoUNo p [Oebye 20' C1 E ( 2 5 ' C1

DMF 3.82 35ACRYLONITRILE 3 .88 38

STYRENE 0 37 2 . 4 3TOLUENE 0.39 2 . 8 8

TABLE IVReactivity Ratios

~~~~~~~~~

6 1 2 5 ' C l

1 2 . 110 4

8 . 68 . 9

Temp.

60

Oilutlormole/lo l v e n t

T o l u e n e

DMF 0.6

Plethad

Re x t r

FR

extr.e x t r .e x t r .

Ref .

I 1r om11

9

t h i s w o r kthis w o r kt h i s w o r k

aFR = (Fineman Ross plot): best fit with straight line forbextr. = extrapolation using plots n/x v e m s x or UX.

a large composition range.

those derived from bulk polymerization. In the latter case, there is practically nodifference between the values obtained using the best straight line of aFineman-Ross plot for the whole range of monomer mixture composition, or fromextrapolation of the data to xA = 0 or l / X A = 0. In solution, the differences arelarge, especially for the acrylonitrile radical; its relative reactivity versus acrylonitrileis larger than in bulk and is enhanced by the use of DMF and upon dilution in thissolvent. It is well known that the nitriles are strongly associated through dipolarbonding [I21 hus, possibly, the acrylonitrile monomer may react in two ways: asa free monomer and in associated form; upon dilution the first one is favored.Further, it has been shown [I31 that acrylonitrile may be associated with DMF,which is a solvent with about the same polarity and dielectric constant (see TableV). So a third kind of acrylonitrile monomer species may be expected in thisstudy. The large change in acrylonitrile radical reactivity in DMF upon dilution may

TABLE VSelected Physical Properties of Monomers and Solvents


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reflect the com petition betw een t he three diff erent species. Possibly also one has toconsider solvation and association effects concerning the radicals, as suggested byCameron [8] for the similar system styrene-methacrylonitrile.

Initial CompositionWhen a copolymer system does not follow the classical theory of Lewis andMayo, a set of two reactivity ratios is not sufficient to describe the copolymercompositions in the whole range. The deviation from ideality may be estimatedfrom An g/n gc where A ns is the difference between the experim ental value o f nS

and the nsc calculated from reactivity ratios. From Table IV it may be seen thatfor bulk copolymerization, the two sets of possible reactivity ratios are very closetogether. Using the extrapolated rA = 0.06 and rS = 0.34, the calculation of An/ngives the results illustrated in Figure 5. In toluene solution, the copolymers arericher in styrene than expected, chiefly for acrylonitrile rich monomer mixtures;admitting the explanation given above, and because styrene and toluene have aboutthe same electrical properties, the change in composition might be caused by anenhanced participation of free acrylonitrile monomer. On the other hand, upondilution in DMF the reverse composition change is observed: the copolymerbecomes richer in acrylonitrile.We may exp ect t ha t this effect is caused by th e participation of DMF-associatedacrylonitrile monomer.

Initial RateIn Figures 6 and 7 the initial rates are plotted, expressed as individual yield perminute for each monomer, and corrected from the minor changes in initiator

concentrations upon multiplying by I-. The corresponding data in bulk are not

- 0.20.4

-0.6

100

FIG. 5. Copolymer com position deviations in: 1, DMF 2 mole/liter); 2, DMF (0.6 molelliter);3, toluene; 4, bulk. From the calculated values assuming rs = 0.34 and A = 0.06.


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62 PICHOT, AGANIARIS, ND GUYOT

50 1FIG.6. Corrected initial rate (yield in /min.mole.liter-' ) for styrene conversion versus percentstyrene in monomer mixture for copolymerization in 0 ) toluene (2 mole/liter); 0) DMF (2mole/liter); (A)DMF 0.6 mole/liter).

I CRYLONITRIL~ v.0 5 0 1

FIG.7. Corrected initial rate (yield in /min.mole/liter) for acrylonitrile conversion versuspercent acrylonitrile in monomer mixture for copolymerization in 0 ) toluene (2 mole/liter); 0)DMF (2 mole/liter); A) DMF 0.6 molelliter).


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given in reference 11 and others papers in the literature so that we may onlycompare the effect of solvents and dilution.In the case of styrene (Fig. 6 ) and in styrene-rich mixtures the polymerization isnot dependent on the dilution in DMF and this fact shows that the order of thereaction versus monomer concentration is first-order as expected. The rate is lowerin toluene. The reason for that might be a change in initiator efficiency which maybe larger in DMF because the transfer reaction in DMF may allow the primaryradicals to escape from their initial cage. The transfer reaction in DMF of polymerradicals is shown here by the lower values of molecular weight although thepolymerization rates are higher in DMF. Upon increasing the acrylonitrile contents,the sty rene polymerization rate increases; that may be assumed to be acopolymerization effect because the acrylonitrile radicals are more reactive versusstyrene than styrene radicals. For acrylonitrile (Fig. 7) a t high acrylonitrile contents,the rate is again higher in DMF and increases a little upon dilution; that might meanthat the DMF-associated acrylonitrile monomer is more reactive. The rate tends toincrease with styrene content, after a plateau value or even a slight minimum, ifcopolymerization is carried out in toluene solution. The situation is very differenthere from that of vinyl chloride-acrylonitrile copolymerization described in aprevious paper [2]. We have suggested to explain the depression in the rate by theformation of imine radicals upon a cyclization reaction involving the attack of a C5 N bond by the growing radicals. This reaction causes a marked coloration of thevinyl chloride-acrylonitrile copolymers. However, in the case of styrene-acrylonitrilecopolymers, the optical density at 290 nm remains very low and d oes not increasenotably upon dilution.

Conversion EffectIn a previous work [9] it has been shown that the composition of the

copolymer of styrene and methyl methacrylate with acrylon itrile, at highacrylonitrile contents do not follow, upon increasing conversion, the law expectedfrom the initial composition for various monomer mixture composition; theacrylonitrile content is lower than expected and the discrepancy increases withincreasing conversion. This effect has been interpreted at first in terms ofacrylonitrile monomer trapped by association with nitrile groups of the copolymerand , thus , no t yet available for polymerization. However it appeared that this effectwas not general: for instance it has not been observed in the case of vinyl chlorideor vinyl acetate copolymerization with acrylonitrile [14]. So another kind ofexpla nation is necessary.In this work, the effect has been confirmed for copolymerization in DMFsolution with acrylonitrile-rich monomer mixtures. But it is not observed in thesame solvent with styrene-rich monomer mixtures, and it is not observed forcopolymerization in toluene in the whole range of composition. The results areillustrated in Figures 8 and 9. The deviation observed in DMF solution increasesobviously with the value of X A , i.e., with the amount of copolymer formed. It is


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2 5

S

- //520

S

-/////

FIG. 8. Copolymer composition ns versus xs for styrene-rich monomer mixture (2 mole/liter)composition for acrylonitrile-styrene copolymerization in: 0 ) toluene; 0 ) DMF.

A

lo i b-

X A0 50 1

FIG. 9. Copolymer composition nA versus acrylonitrile-rich mon omer mixtu re composition(-- ) initial composition curve, ( composition at increasing conversion. Copolymerizationin: 0 ) toluene (2 mole/liter); 0 ) DMF (2 mole/liter); A) DMF 0.6 mole/liter). The data forbulk copolymerization (x) ref. 11) are given for com parison.


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SOLVE NT EFFECTS IN STYRENE-AN 65

A,O- A , t

STYRENE ,CONVERSION Y50 1

FIG. 10. Relative enrichment of the copolymer in styrene as a function of the s tyreneconversion for copolymerization in DMF solution. The init ia l conditions are: 0 XA,O = 22.2(dilution 0.6 mole/l i ter) ; 0 , XA, O = 20.2 (dilution 2 mole/l i ter) ; A , XA,O = 9.15 (dilution 2mole/liter).also larger upon dilution. It is interesting to consider the deviation expressed by (non)/no, as shown in Figure 9.

I t appears that the deviation increases a great deal with styrene conversion (Fig.10) and is slightly dependent on factors such as initial composition and dilution.The cause o f this behavior might be a preferential solvation of styr ene by t hepolymer formed. Obviously, as shown in Table V, styrene is very different from

DMF and acrylonitrile and has physical properties close to that of toluene. One mayconsider that the reaction medium is not totally homogeneous as soon as thepolymer is formed. There are polymer-rich regions where the polym erization rate isthe highest, owing to the effect of diffusion on the termination rate, and if styreneis preferentially adsorbed in these regions, the relative polymerization of styrenebecomes more and more rapid. Such a situation is obviously not valid if the solventis toluene. Preferential solvation has been shown recently to occur in thestyrene-acrylonitrile polymerization in the presence of polybutadiene [ 151 .

CONCLUSIONSIt is clear that in the present system, the nature of the solvent has a large

influence on the behavior of the solution copo lymeriza tion, although the effect isnot so large as in the case of acrylonitrile-acrylamide copolymerization [ 6 ] .Secondary reactions su ch as the cyclization reaction involving th e inter nalcopolymerization of the nitrile groups, are probably negligible in the present case. Areasonable explanation of the deviations from the Lewis and Mayo theory seems to


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be the various reactivity of acrylonitrile in the various possible forms: free monomer(upon dilution in nonpolar solvent) dimer associated by dipolar nitrile-nitrile bonds,or polar-solvent-associated monomer. Possibly also the association may involve notthe monomer but the radicals. However, another explanation is not excluded, basedon physical factors: owing to their large difference in solubility parameter ordipolar mom ent, sty rene and acrylonitrile might be segregated in solution. Thepresence of polymer may increase this tendency through a phenomenon ofpreferential solvation. If such a situation actually occurs, it might cause sometendency to blockiness in the copolymer and might be detectable through a studyof sequence distribution. Our future work will be directed in this way.

REFERENCES[ 11 A. Guyot, M. Dumont, Ch. GraiUat, J . Guillot, and C. Pichot, J. Macromol. Sci. C h em , inpress.121 Ch. Graillat, J. Guillot and A. Guyot, J. Macromol. Sci Chem A 8 1099 (1974).[3] G . Saini, A. Leoni, and S. Franco, Macromol. Chem., 144, 235 (1971); 146, 165 (1971).[4] L. M Minsk, C. Kotlarchik, and R. S . Darlack, J. Polym. Sci. Polym Chem. Ed., 11 353(1973).[ 5 ] L. M. Minsk, C. Kotlarchik, and G. N. Meyer, J. Polym. Sci. Polym. Chem. Ed., 11 3037(1973).[6] L. Perec, Polym Let t . 11 267 (1973); A. Chapiro and L. Perec-Spritzer, Eur. Polym. J.

11. 59 (1975).[7] M. M. Zafar, R. Mahmud and A. M . Syed, Makromol. Chem, 175, 1531 (1974).[8] G. C. Cameron and G. F. Esslemont, Polymer /G. B.), 13 435 (1972).[9] A. Guyot and J. Guillot, J. Macromol. Sci Chem), A I 793 (1967); A 2, 889 (1968).[ l o ] M. Fineman and S. D. Ross, J. P o l y m S c i , 5 , 259 (1950).[ l l ] B. R. Thomson and R. M. Raines, J. Polym Sci . , 4 1 , 265 (1959).[12] A.M. Saum, J. P o l y m S c i , 42 , 57 (1960).[13] C. Spritzer, J. C h i m P h ys , 68 , 340 (1971).[ 141 J. Guillot, unpublished results.[151 J. L. Locatelli and G. R i m , Makromol. Chem., 27, 201 (1974).

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Acrylonitrile Copolymerization - Vii - Solvents Effects in Styrene Copolymerization

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