U. "~ 1.0 t.. b-. 0.8. 0.6. 0.4. 0.2. (A) t. ASA. Pa rt tl. =b. SSS. --. mO oQPOeo. A a8 ... a SAS. =Q ma .n o&. â¢. ~. â¢~° o AAS. 0 â¢. @0. /k. â¢. 0.2. 0.4. 0,6. 0.8. Ys. I. 1.0.
Eur. Polym. J. Vol. 28, No. 4, pp. 391-398, 1992
0014-3057/92 $5.00 + 0.00 Copyright © 1992PergamonPress plc
Printed in Great Britain.All rights reserved
THE EFFECT OF SOLVENT ON THE STYRENE-ACRYLONITRILE COPOLYMERIZATION D. J. T. HILL,* A. P. LANG, P. D. MUNRO and J. H. O'DONNIELL Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, Queensland 4072, Australia (Received 19 August 1991)
Abstract--The copolymerization of styrene and acrylonitrilehas been examined in acetonitrile and toluene and in bulk. All the systems display a significant penultimate unit effect. The comparison of the bulk system with these involving toluene and acetonitrile reveals a relatively small solvent effect. The bulk and toluene systems behave very similarly in the high styrene region (Xs > 0.3), while the bulk and acetonitrile systems behave very similarly in the high acrylonitrile region (Xs < 0.3). The solvents, therefore, behave as effective analogues of styrene and acrylonitrile, which are the "solvents" in the bulk copolymerization. The relationships between the copolymer compositions and triad fractions for the polymers prepared in each system have been assessed for evidence consistent with partitioning of monomer between polymer and solvent phases. Similar relationships were found for the triads independent of the solvent, a result which may be indicative of a "bootstrap" effect.
INTRODUCTION The copolymerization of styrene and acrylonitrile has been examined in the presence of various solvents, and a small solvent effect on the copolymerization has been observed. Previous workers have assigned this effect to a variety of factors. They include preferential solvation of one of the monomers about the macro-radical (in either homogeneous or heterogeneous systems) [1-4], solvent interference in the formation of electron donor-acceptor (EDA) comonomer complexes [5] or solvation of the monomers and macro-radicals with solvent molecules [6] altering their reactivities. The emulsion copolymerization of styrene and acrylonitrile, studied by Fordyce and Chapin [7], was re-examined by Smith [8]. Smith suggested that the reactivity ratios in the emulsion copolymerization were essentially the same as for the bulk copolymerization, but that the comonomer feed about the growing macro-radical, inside the emulsion particles, was different from the comonomer feed for the total system. Guyot and Guillot [9, 10] suggested that the copolymerization of styrene and acrylonitrile in toluene and D M F displayed penultimate and antepenultimate unit effects, which explained deviations from the terminal model. However, later, Guyot and coworkers [1-3] used gas-liquid chromatographic (GLC) techniques to monitor the initial rate of consumption of styrene and acrylonitrile, and Pichot et aL [3] examined the effect of adding pre-formed polymer to the comonomer solution. As a result of these studies, the authors concluded that preferential solvation of one of the monomers in the swollen polymer phase provided the best explanation for the observed results. *To whom all correspondence should be addressed.
Riess and Desvaiois [4] reported investigations of the styrene-acrylonitrile copolymerization in hydrocarbon solvents in which the polymerization is heterogeneous. They noted that copolymers formed under these conditions contained significantly greater quantities of acrylonitrile than when homogeneous conditions were used and suggested that preferential adsorption of acrylonitrile about the growing macroradical was responsible. Sandner and Loth [5, 11] have investigated the copolymerization in bulk [11] and in a variety of solvents [5]. In bulk, the authors suggested the presence and participation of an EDA complex as an explanation for the departure from a terminal model. Sandner and Loth [5] calculated the reactivity ratios for the copolymerization in each of the solvents; correlating the values with the dielectric constant and donor number of the solvent. The authors reported a correlation between the product r~. r2 and the dielectric constant (the value of the product r~. r 2 increased as dielectric constant increased), while r~ increased and r 2 decreased with increasing donor number of the solvent. The authors indicated that the alternating tendency (as indicated by the product r~.r2) was lowered because the EA exchange reactions between acrylonitrile and styrene were weakened by a polar solvent. Solvent effects on the styrene-acrylonitrile copolymerization were also examined by Asakura et al. [6]. These authors examined the copolymerization in benzene, DMSO, acetonitrile and ethanol at 60°C and 80°C. They suggested that the variation in the copolymerization with the nature of the solvent was due to interaction between acrylonitrile and the solvent. In 1981, Plochocka [12] reviewed the effect of the reaction medium for many copolymerization systems. Plochocka concluded that, for styrene-acrylonitrile copolymerization, the variation between solvents was probably the result of preferential solvation of one of the monomers about the growing macro-radical, 391
392
D . J . T . HILL et al.
in the swollen p o l y m e r phase. This suggestion was similar to t h a t p r o p o s e d by P l o c h o c k a a n d H a r w o o d [13] for the styrene--methacrylic acid copolymerization a n d m o r e recently by H a r w o o d [14] for a variety o f o t h e r systems. In 1982, Hill et al. [15] reported that, after examining the c o p o l y m e r c o m p o s i t i o n s a n d triad fractions for the styrene-acrylonitrile copolymerization in bulk, the c o p o l y m e r i z a t i o n was best described when the p e n u l t i m a t e unit effect was included in the analysis. This model was f o u n d to provide a superior prediction o f the triad fractions c o m p a r e d with the terminal, complex p a r t i c i p a t i o n a n d complex dissociation models. The p e n u l t i m a t e reactivity ratios were reported as rss = 0.22, rAA = 0.03, rAS = 0.63 a n d rsA = 0.09. M o r e recently, Tirrell a n d coworkers [16-19] have used small molecule models o f the macro-radicals to d e m o n s t r a t e t h a t the relative rates o f a d d i t i o n o f styrene a n d acrylonitril¢ are consistent with a significant p e n u l t i m a t e unit effect, a n d t h a t these rates o f a d d i t i o n are in close agreement with the reactivity ratios r e p o r t e d by Hill et al. [15] for the penultimate model. J o n e s et al. [16] d e m o n s t r a t e d t h a t the n a t u r e o f the y-substituent o n the macro-radical a n a l o g u e could significantly affect the relative rates o f addition o f styrene a n d acrylonitrile. This effect was particularly evident w h e n the ~-substituent was a nitrile group, leading to a depression in the rate o f addition o f acrylonitrile in c o m p a r i s o n to t h a t for styrene by a factor o f 3.5. This result c o m p a r e s f a v o u r a b l y with those r e p o r t e d by Hill et al. [15] (rss/rAs a n d rSA/rAA ) o f ca 2.5. EXPERIMENTAL PROCEDURES
Materials The monomers, styrene and acrylonitrile, were purified by distilling under N 2 at reduced pressure at a temperature of 60°C. Benzoyl peroxide was used as the initiator following purification by dissolving in chloroform and reprecipitating by addition of methanol. The procedure of dissolving and reprecipitating the benzoyl peroxide was then repeated. Pyrex glass ampoules were used as reaction vessels, with a 2 M concentration of the comonomer where solvents were required.The solvents used were AR toluene and distilled LR acetonitrile. The purities of both monomers and solvents were checked using ~H-NMR and gas chromatography (GC). The polymerizations were taken to no more than 4% conversion (by weight) in the high acrylonitrile regions and no more than 7.5% conversion throughout the remainder of the comonomer feed range. The conversion was determined using a gravimetric method. The copolymers were precipitated from solution using acidified methanol in 10-fold excess and, in the cases of the polymerizations in bulk and toluene, purified by dissolving in chloroform or in a mixture of chloroform and DMSO for the high acrylonitrile content copolymers. These solutions were filtered and the copolymers re-precipitated by addition to acidified methanol. For the polymerizations in acetonitrile, the precipitated copolymers were dissolved in acetonitrile, filtered and re-precipitated in acidified methanol. The copolymers were thoroughly dried in vacuo at ambient temperature. Characterization The compositions of the copolymers were determined using IH- and 13C-NMR from either:
(i) the relative intensities of the aromatic (S units) and the methylene plus methine (A + S units) resonances in IH-NMR spectra (these spectra were obtained on a BRUKER CXP-300 or a JEOL GX-400 NMR spectrometer at 55°C or room temperature, with CDC13 or Me2 SO-dr as solvents); (ii) the relative intensities of the five aromatic carbon resonances (S units, 125-130 ppm) and the nitrile carbon resonance (A units, l18-123ppm) in the ~3C-NMR spectra; (iii) the relative intensities of the five aromatic carbon resonances and the methine carbon resonance (A units, 25-30 ppm) in the 13C-NMR spectra. The ~3C-NMR spectra were recorded on a BRUKER CXP-300 (75.46MHz) or a JEOL GX-400 (100 MHz) spectrometer at 80°C or ambient temperatures in 10 mm o.d. sample tubes. Solutions were made to 10 wt/vol % in Me2SO-d 6, CD3NO z or CDCI~. These spectra were run with a 15 sec pulse cycle time and 90 ° pulse angle using gated decoupling to suppress NOE. The recycle time of 15 sec is longer than 5 T~ for the slowest relaxing carbon, as obtained by relaxation time measurements. The copolymer triad fractions were obtained from ~3CNMR spectra using the conditions described in (iii) above.
RESULTS AND DISCUSSION
(i) C o p o l y m e r compositions The copolymerization o f styrene a n d acrylonitrile has been c o n d u c t e d in a n u m b e r o f systems viz. in bulk [16,20] a n d in toluene a n d acetonitrile. The purpose in choosing toluene a n d acetonitrile was t h a t they are analogues o f styrene a n d acrylonitrile respectively. Table 1. Comparison between the copolymer compositions, Ys, obtained at comonomer feeds. Xs, for the copolymerizations of styrene and acrylonitrile in bulka, toluene and acetonitrile at 60°C ( Ys is the mole fraction of styrene in the copolymer, and X s is the mole fraction of styrene in the comonomer feed) r?.c Xs Bulk Toluene Acetonitrile 0.021 0.234 0.023 0.248 0.035 0.135 0.270 0.047 0.323 0.300 0.053 0.333 0.072 0.360 0.275 0.365 0.104 0.406 0.125 0.375 0.419 0.212 0.440 0.475 0.221 0.476 0.276 0.485 0.500 0.314 0.510 0.364 0.520 0.542 0.416 0.542 0.462 0.550 0.577 0.530 0.582 0.596 0.600 0.629 0.631 0.627 0.696 0.649 0.645 0.674 0.802 0.705 0.808 0.700 0.740 0.889 0.772 0.900 0.780 0.840 0.939 0.829 aRefs [16] and [20]. bObtainedfrom IH- and 13C-NMR, and elemental analysis (% nitrogen); average result. + 0.005.
Styrene-acrylonitfile copolymerization 1.0
have very similar compositions while the copolymers produced in acetonitrile contain considerably higher proportions of styrene. Therefore, there apears to be a cross-over point at Xs ~ 0.3 (or Ys ~ 0.5). The bulk system behaves like the copolymerization in acetonitrile at high acrylonitrile comonomer feeds but at higher styrene feeds, the bulk system behaves like the copolymerization in toluene. This result suggests that the molecular environment about the macro-radical and/or about the comonomers is altering their reactivity [5, 6] or, as Harwood [14] suggested, the comonomer feed in the environment about the growing macro-radical is different from that of the bulk solution.
B
0.8 0.6 YS
m/o i/o
0.4
/ 0.2
0
393
! J
I
I
I
I
0.2
0.4
0.6
0.8
1.0
(ii) Triad sequence distribution The acrylonitrile-centred and styrene-centred triads have been calculated from the nitrile region (118-123 ppm) and the quaternary aromatic region (139-145 ppm) respectively of the s3C-NMR spectrum. The acrylonitrile-centred and styrene-centred triad fractions for each of the three systems have been outlined against their comonomer feeds in Tables 2-4.
Xs
Fig. 1. Copolymer composition curves for the copolymerizations of styrene and acrylonitrile in bulk, toluene and acetonitrile at 60°C. Ys is the mole fraction of styrene in the copolymer, Xs is the mole fraction of styrene in the comonomer f e e d : . , Acetonitrile; O, toluene; - - - , bulk.
(ii0 Calculated reactivity ratios from triad fractions
The copolymer compositions, expressed as the mole fraction of styrene, for the low conversion studies in bulk, toluene and acetonitrile are given in Table 1. These results represent averages of the compositions obtained from ~H- and ~3C-NMR, and elemental (% nitrogen) analysis. The three techniques employed in calculating compositions agreed to within _+0.005, i.e. comparable with the expected error in each technique based on repeated experiments. The comparison between the copolymerization systems is displayed more clearly in Fig. 1, where the copolymer composition vs comonomer feed curves are presented. It is evident that, at high acrylonitrile comonomer feeds (Xs < 0.3), the compositions of the copolymers produced in bulk and in acetonitrile agree very closely but those produced in toluene contain considerably higher proportions of acrylonitrile. This effect is altered at higher styrene feeds (Xs > 0.3) where the copolymers produced in bulk and in toluene
The fit, using non-linear least squares (NLLS), of the penultimate model to the triad sequence distribution data for the acetonitrile solvent system is given in Fig. 2. The penultimate model provides an excellent fit to the data. The calculated values for the terminal model and penultimate model from the NLLS fit to the triad fraction data are given for each of the systems in Table 5. In each of the three copolymerization systems (bulk, toluene and acetonitrile), the penultimate model provides a significant improvement ( > 99.5% probability by the F-test) in the fit to the triad sequence data over the terminal model. The similarity of the bulk and toluene systems in the high styrene comonomer feed regions is reflected in the similarity of the values for both rss and rAS in the two solvents. Conversely, the similarity of the bulk and acetonitrile systems in the high acrylonitrile
Table 2. Experimentaltriad fractionsfor the styrene-acrylonitrilecopolymerization in bulka at 60°C Triad fractionsb Cony. Xs wt % Fsss FSSA+ASS FASA FAAA F~s +SAA FSAS 0.021 0.023 0.047 0.053 0.072 0.104 0.221 0.314 0.416 0.530 0.631 0.696 0.802 0.889 0.939
0.2 3.0 2.1 0.4 1.4 1.2 1.8 3.0 2.2 2.3 0.8 1.2 0.7 0.5 0.3
. 0.00 0.00 . 0.00 0.00 0.00 0.02 0.06 0.07 0.12 0.16 0.30 0.44 0.61
.
. 0.07 0.06
.
. 0.06 0,15 0.27 0.34 0.42 0.51 0.56 0.59 0.55 0.50 0.37
. 0.93 0.94 . 0.94 0.85 0.73 0.64 0.52 0.42 0.32 0.25 0.15 0.06 0.02
. 0.41 0.24 . 0.12 0.11 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00
. 0.50 0.55
0.09 0.21
0.57 0.55 0.37 0.29 0.22 0.17 0.12 0.08 0.06 0.00 0.00
0.31 0.34 0.63 0.69 0.78 0.83 0.88 0.92 0.94 1.00 1.00
.
~Refs[16,20]. b+ 0.03. The styrene-centredtriad fractionscalculatedfrom the quarternaryaromaticcarbon resonance and the acrylonitrile-centredtriad fractionscalculatedfrom the nitrile carbon resonancein the I~C-NMRspectraof the copolymers.
D. J. T. HILL et al.
394
Table 3. Experimentaltriad fractions for the styrene-acrylonitrilecopolymerization in tolueneat 60°C Triad fractionsa Conv. Xs wt % Fsss Fss^ + Ass FASA FAA^ FAAS + SAA FSAS 0.035 0.8 . . . . . . 0.035 2.3 0.000 0.034 0.966 0.665 0.283 0.052 0.035 3.4 0.000 0.050 0.950 0.707 0.261 0.033 0.072 1.2 0.000 0.080 0.920 0.364 0.536 0.100 0.125 0.5 . . . . . . 0.125 0.212
3.4 3.6
0.000 0.015
0.II0 0.225
0.890 0.760
0.144 0.080
0.534 0.420
0.322 0.500
0.212 0.276 0.364 0.364 0.364 0.463 0.596 0.596 0.696 0.808 0.808 0.900 0.900
5.1 3.9 1.3 3.8 3.7 3.1 1.3 6.9 5.2 1.2 7.2 0.7 5.6
0.030 0.022 0.029 0.024 . 0.057 0.110 0.114 0.193 0.303 0.297 0.496 0.473
0.240 0.300 0.385 0.385
0.370 0.678 0.586 0.591 . 0.480 0.337 0.335 0.233 0.139 0.138 0.065 0.050
0.070 0.034 0.038 0.021 . 0.009 0.000 0.011 0.000 -0.000 0.000 0.000
0.430 0.358 0.258 0.284
0.500 0.608 0.703 0.695
0.201 0.141 0.133 0.086 -0.052 0.000 0.033
0.790 0.859 0.856 0.914 -0.948 1.000 0.967
.
. 0.463 0.552 0.551 0.574 0.557 0.564 0.439 0.477
.
a__.+0.03. The styrene-centredtriad fractions calculated from the quaternary aromatic carbon resonance,and the acrylonitrile-centredtriad fractionscalculated from the nitrile carbon resonancein the ~3C-NMRspectra of the copolymers.
c o m o n o m e r feed regions is reflected in the values for rAA. Little can be ascertained from the value for rsA , which does not display m u c h variation between the systems. The terminal model reactivity ratios display a similar trend with the r s value being similar for the bulk and toluene systems, and r A being similar for the bulk and acetonitrile systems.
(iv) Analysis o f triad fractions using Chujo 's equations A n o t h e r m e a n s o f assessing the above i n f o r m a t i o n is t h r o u g h C h u j o ' s equations [21]. A n advantage o f this procedure is that it provides a knowledge o f the consistency (or otherwise) o f the reactivity ratios over the c o m o n o m e r feed range. In this instance, the reactivity ratios have been calculated from the triad fractions o f each copolymer. The results are illustrated in Figs 3-5. Within experimental error (which varies as examples across the range indicate), the values for m o s t o f the indi-
vidual reactivity ratios in each system are consistent across the c o m o n o m e r feed range. In some cases, however, such as the values o f rAA in toluene, some question exists as to whether the reactivity ratio displays a systematic deviation as the c o m o n o m e r feed varies. Pichot et al. [3] have previously suggested that, when toluene is used as the solvent, acrylonitrile preferentially solvates the polymer. As a consequence, the growing macro-radical in the polymer phase was suggested as growing in a local e n v i r o n m e n t with an acrylonitrile c o m o n o m e r content which is greater t h a n in bulk. Hence, longer than expected sequences o f acrylonitrile units may be formed, resulting in an increase in the calculated value o f rAA. However, if this were the case, the values o f the other copolymerization reactivity ratios should be affected, and this effect is not observed. In all cases (except for rsA and rAA in the toluene system), a significant penultimate effect is illustrated
Table 4. Experimentaltriad fractions for the styrene-acrylonitrilecopolymerization in acetonitrileat 60°C Triad fractionsa Conv. Xs
wt %
Fsss
bass + Ss^
FASA
FAAA
FSAA + AAS
FSAS
0.035 2.3 0.0 0.05 0.95 0.37 0.52 O.11 0.047 2.4 0.0 0.07 0.28 0.28 0.57 0.15 0.720 2.4 0.0 0.09 0.91 0.23 0.56 0.21 0.125 2.5 0.0 0.17 0.83 0.09 0.50 0.41 0.212 2.5 0.0 0.25 0.75 0.09 0.42 0.48 0.276 4.4 0.02 0.32 0.66 0.06 0.33 0.61 0.364 2.2 0.03 0.37 0.60 0.05 0.27 0.68 0.462 2.6 0.06 0.48 0.46 0.01 0.22 0.78 0.596 2.4 0.12 0.58 0.30 0.01 0.12 0.87 0.696 2.1 0.21 0.59 0.20 0.0 0.08 0.92 0.808 1.7 0.35 0.54 0.12 0.0 0.04 0.97 0.900 1.0 0.60 0.38 0.02 0.0 0.03 0.97 The styrene-centredtriad fractions calculated from the quaternary aromatic carbon resonance, and the acrylonitrile-centredtriad fractionscalculated from the nitrile carbon resonancein the ~3C-NMRspectra of the copolymers.
Styrene-acrylonitrile copolymerization
(A)
395
(A)
1.0 F
1.0
/''"#~'~"
-
0.8
s 0.6
0.6
0.4
0.4
0.2
0.2--
I1
0
o ..~ o
0
"o
10
I---
0.8
B)
~-"
i.
"E'. •
&]
I
I
I.
_. I _ . .
• I
I
I
_._. I
0.8
1.0
>
/
(B)
0
0.3 r~
",.,, /
N ASA
\ , /
,,,/
0. I o
,A 0.2
0.4
0.2
0.6
o.s
0.1
1 .o
Xs
0.2
0.4
0.6
Xs
Fig. 2. Non-linear least squares fit of the penultimate model to the triad fractions for the copolymerization of styrene and acrylonitrile in acetonitrile at 60° across the comonomer feed range. Xs is the mole fraction of styrene in the comonomer feed. (A) Acrylonitrile-centred triads; (B) styrenecentred triads. - - , Fitted curve for the penultimate model.
Fig. 3. The calculated reactivity ratios at the individual comonomer feeds for the copolymerization of styrene and acrylonitrile in bulk at 60°C, using Chujo's equations. Examples of the estimated experimental error in the reactivity ratios are indicated. (A) rss, A; ms, II. (B) ran, A;
through the consistent difference between the values of rss and rAs , and between those of rsA and rAA
across the comonomer feed range in each of the solvents. The average values for the reactivity ratios, calculated according to Chujo's equations [21] and weighted on the basis of the experimental error in the individual values, are given in Table 6.
Table 5. Reactivity ratios calculated for the terminal and penultimate models from the NLLS fit to the triad monomer sequence distribution data for the copolymerization of styrene and acrylonitrile in bulk a, toluene and acetonitrile at 60°C Penultimate model
Bulk
Toluene
Acetonitrile
rss rAS rSA rAA NO. of data points Standard error Estimated error
0.232 0.566 0.087 0.036 78 0.022 0.03
0.242 0.566 0.109 0.133 96 0.023 0.03
0.322 0.621 0.105 0.052 72 0.020 0.03
0.394 0.063 78 0.063 0.03
0.423 0.118 96 0.049 0.03
0.485 0.081 72 0.051 0.03
Terminal Model rs rA No. of data points Standard error Estimated error 'Refs [16, 20].
rSA, ~ .
(v) The effect of solvent on the reactivity ratios The values of rss for the copolymerizations in bulk, toluene and acetonitrile (from analysis of the triad fractions) are given in Table 5. The bulk (0.232) and toluene (0.242) results compare favourably with the value for the relative rate of addition of styrene and acrylonitrile to the l-phenylethyl radical [17], ks/kA = 0.20. However, the value of rss for the copolymerization in acetonitrile is 0.322, which is ~ 60% greater than the value found by Cywar and Tirrell. The nitrile y-substituent, as shown by rss and rAs for the copolymerizations in each of the systems, causes a depression in the addition of acrylonitrile in comparison to styrene by a factor of 1.9 to 2.5.
396
D.J.T. HILLet al. (A) 1.0
A)
-
1.0 0.8
-
0.8 Ill
0.6 i f
m
0.6
E"
:..1•
0.4
~
m
0.4 •
0 . 2 --
--
0.2
o t_
o
•r.
---"
--
I
I
I
I
I
o
_.._
L
I
I
I
I
(B)
(B)
V o.3-
(0 n-
o
0.3
--
Q:
0.2 0.2
0.1
E 0.1~~[ : .
:iif 0.2
0.4
--
0.6
0.8
1.0
0
•
I
I
I
0.2
0.4
0.6
Xs
_.
t
•
0.8
I
1.0
Xs
Fig. 4. The calculated reactivity ratios at the individual comonomer feeds for the copolymerization of styrene and acrylonitrile in toluene at 60°C, using Chujo's equations. Examples of the estimated experimental error in the reactivity ratios are indicated (A) rss, &; rAs, m. (.) rAh, A, rsA, m.
Fig. 5. The calculated reactivity ratios at the individual comonomer feeds for the copolymerization of styrene and acrylonitrilein acetonitrile at 60°C, using Chujo's equations. Examples of the estimated experimental error in the reactivity ratios are indicated. (A) rss, &; rAs, 1 . (B) r ~ , &;
This result compares favourably with values reported by Jones et al. [16] for the small molecule radical analogues, which yielded a depression factor of 3.5. Similarly, the relative rates of addition of styrene and acrylonitrile to the 1-cyanoethyl radical [18], kA/ks = 0.12, compare very favourably with the value of rsA in each of the solvents; in bulk, rSA = 0.087, in toluene 0.109, and in acetonitrile 0.105. The nitrile y-substituent, as shown by comparing rs^ and ran, causes a depression in the rate of addition of acrylonitrile in comparison to that of styrene by a factor of 2.0 in acetonitrile and 2.5 in the bulk copolymerization. This compares favourably with values reported by Jones et al. [16] who found a depression in the rate of addition of acrylonitrile in the small radical analogues by a factor of 3.5. In toluene, the copolymerization departs significantly from the observations of Jones et al. with an enhancement in the addition of acrylonitrile. This effect could be a consequence of the preferential
solvation of acrylonitrile as proposed by Pichot et al. [3]. However, the reason for the difference has not been firmly established.
rSA, 1 .
(vi) "Bootstrap" approach Harwood [14] has shown that copolymer sequence information obtained in different solvents may be plotted against the copolymer composition to yield Table 6. Weighted average values of the reactivity ratios for the penultimate model for the copolymerization of styrene and acrylonitrile in bulk', toluene and aeetonitrile at 60°C Penultimate model
Bulk
Toluene
Acetonitrile
rss rAS rsA tax
0.253 0.585 0.090 0.040
0.263 0.549 0.125 0.123
0.31 I 0.637 0.104 0.073
"Refs [16, 20]. The reactivity ratios calculated from the triad fractions according to Chujo's equations.
Styrene--acrylonitrile copolymerization
systems and particularly the apparent variation in the value of rAA with feed composition in the toluene system. Any solvation effect, however, does not disguise the consistent, significant penultimate unit effect. Thus, while not the major influence, the bootstrap effect may be superimposed on a penultimate unit effect, causing the observed, small variation between the solvent systems. However, these variations could also arise through other phenomena which are influenced by the character of the solvent.
(A) 1.0
t ASA
Pa rt
0.8
tl =b
SSS
0.6
--
mO o Q P O e o
0.4
-
eO
A
a8 O
qP 0.2 ({3 eO
•=,
• []
A
•
CONCLUSIONS
#
SSA
-
O
80
• 0
8
=.A =a
A
~'1
U
(B) "~
1.0
.a~
t..
[]
b-
,a SAS
0.8
=Q
AAA
ma .n
0.6 o&
0.4
0.2
•
•~°o A A S
~ 0
•
@0
/k 0.2
0.4
397
0,6
•
I 0.8
1.0
Ys Fig. 6. Comparison between the triad fractions for the copolymerization of styrene and acrylonitrile in toluene and acetronitrile at 60°C. The styrene-centred and acrylonitrilecentred triad fractions have been plotted against the copolymer composition, Ys. (A) Styrene-centred triad fractions, (B) acrylonitrile-centred triad fractions. • • n , Triad fractions for the copolymerization in toluene;/X © I-q,triad fractions for the copolymerization in acetonitrile. co-existent curves. This feature has been demonstrated by Harwood [14] to apply for several copolymerizations, and he has interpreted it as providing evidence for preferential solvation of the macroradicals by one of the monomers. This approach has also been applied here to test for co-existent curves for the styrene-acrylonitrile system. The toluene and acetonitrile styrene-centred triad fraction data have been plotted in Fig. 6(A). Similarly, the acrylonitrilecentred triad fraction data for the polymerizations in the two solvents is given in Fig. 6(B). The copolymerization data in the different solvents fit the same curves when plotted in this manner. However, this observation is not definitive for this copolymerization, because the effect of solvent is considerably smaller than for the copolymerizations investigated by Harwood [14]. Nevertheless, the explanation provided by the bootstrap effect may play some role in the variations witnessed in the reactivity ratios between the various
Copolymerizations of styrene with acrylonitrile in bulk and in toluene and acetonitrile display significant penultimate unit effects, as indicated from statistical analysis of the triad fraction data. The penultimate unit effect is also evident in plots obtained using Chujo's equations [21]. The only system which displays a significant deviation in the reactivity ratios across the comonomer feed range is the copolymerization in toluene. Here the deviation is limited to a variation in the reactivity ratio rAA. This result is consistent with the findings of Pichot et al. [3], who also found that the copolymerization of styrene and acrylonitrile in toluene exhibited unusual behaviour, particularly at high acrylonitrile compositions. The comparison of bulk, toluene and acetonitrile solvents reveals that the bulk and toluene systems behave very similarly in the high styrene region (Xs > 0.3), while the bulk and acetonitrile systems behave very similarly in the high acrylonitrile region (Xs < 0.3). This finding is possibly not unexpected, as the bulk system in the high styrene region principally has styrene as "solvent". In this region, the bulk copolymerization might therefore be expected to resemble the polymerization in toluene whereas, in the high acrylonitrile region, the bulk copolymerization might be expected to resemble that in acetonitrile. The copolymerizations in the various solvents conform to the co-existent curves obtained by plotting the triad fractions against copolymer composition. However, it has not been possible, on the basis of this work alone, to identify with certainty the reason for the solvent effect observed for this copolymerization. Acknowledgements--We thank the Australian Research Council, the Australian Institute of Nuclear Science and Engineering, and the Brisbane NMR Centre for supporting our research. REFERENCES
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