CHROMOPHORF OF POLYACRYLONITRILF 59
Ir and nmr spectra agree with the structure. The uv spectrum is seen in Figure 4. 2-Cyano-2-butene was prepared from the cyanohydrin of butanone. Preparation of 2-Butanone Cyanohydrin. 2-Butanone (144 g, 2 mol) and 132 g (2 mol) of KCN were stirred at low temperature. Sulfuric acid (200 ml, 50%) was slowly added. The temperature was not allowed to rise above 20‘. It should be noted that a satisfactory reaction is obtained if (a) the temperature is not too low (below - 10’) and (b) the acid is not added too fast. If the acid is added too fast, the reaction medium becomes acidic and the reaction stops. After the reaction was completed, the two layers were separated. acidified, extracted with ether, dried with CaC12. and distilled, bp 65’ (4 mm). Preparation of 2-Cyano-2-butene. SOCl? (60 ml) was added slowly with cooling to 90 ml of 2-butanone cyano-
hydrin in 270 ml of pyridine and 440 ml of dry ether. The mixture was refluxed for 3 hr. Water was added carefully and the ether was washed free from pyridine with dilute sulfuric acid. The ether was washed until neutral, dried with CaCl,, and distilled, bp llG140‘. The original product contained some ketone which was removed by reaction with Girard’s T reagent. Ana/. Calcd for C3HgN: C, 73.17; H, 9.76; N, 17.07. Found: C, 72.89; H, 9.40: N, 16.89. Acknowledgment.
We wish to thank Professor
M. J. S. Dewar for his contribution concerning the nature of the chromophoric weak acid group and Mr. R. H. Salmon and Mrs. G. Yount for experimental assistance.
On the Chromophore of Polyacrylonitrile. 111. The Mechanism of Ketone Formation in Polyacrylonitrile J. Brandrup,la J. R. Kirby, and L. H. Peebles,
Jr.1b
Contribution No. 480 .from the Chenistrand Research Center, Inc., Durham, North Carolina. Receiced Octciber 27, I967
ABSTRACT: Polyacrylonitrile (PAN) made in organic solvents possesses an intense absorption at 265 mp (absorption I). Short-time contact of the polymer with acid removes this peak, but produces another absorption at 275 mw whose intensity is pH dependent (absorption 11). Addition of base causes an intensity increase of the latter. This spectral behavior is caused by groups introduced by polymerization of the growing polymer radical through a pendant nitrile group on preformed polymer. The nitrile polymerization reaction which influences reaction kinetics in a manner comparable to a chain-transfer reaction naturally does not involve the tertiary hydrogen that is usually assumed to be the site of the chain-transfer-to-polymer reaction. In the first step of the nitrile polymerization reaction, an acid-sensitive enamine is produced (absorption I). This enamine is hydrolyzed to a ketonitrile which can be titrated by base photometrically (absorption 11). Thus the determination of the change in absorption intensity at 275 mp upon addition of base is a direct measure of the number of branch points in the polymer. The number of branch points determined this way (5-10 X 10-4) is in excellent agreement with constants determined by conventional kinetic methods (4.7 X 10-4) by assuming chain transfer to polymer.
P
olyacrylonitrile prepared by free-radical catalysis possesses an absorption in the ultraviolet region at 265 mp (absorption I). This was discussed by BeeversZa and by Schurz, Zah, and Ullrich2b and is known as the “characteristic” absorption of PAN. We found that the intensity of this band becomes more and more p H dependent (intensity increase by base addition) with decreasing initial absorption intensity shifting at the same time to 275 m11.1.~ This was taken as evidence for the presence of two different defects in the polymer chain where the intensity of one group varies with p H (pH always greater than 5 ) (absorption 11) and where the other does not (absorption I). These groulps must be present in the polymer in different amounts depending o n the condition of polymerization. (1) (a) Farb&erk.e Hoechst, Kunststoff-Forschung, Frankfurt am Main, German,”.; (b) to whom all correspondence should be addressed. ( 2 ) (a) R. B. Beevers, J . Phj’s. Chem., 66, 1271 (1962); (b) J. Schurz, H. Zah, arid A. Ullrich, Z . Phj’s. Chem. (Frankfurt am Main), 21, 185 (1959). (3) J. R. Kirby, J. Brandrup, a n d L. H. Peebles, Jr., Macromolecules, 1, 53 (1968), paper I1 in this series.
Paper I1 in this series shows that a photometrical titratable weak acid group is responsible for absorption II.3 In its un-ionized form it is a P-ketonitrile group present as defect in the polymer; its enol anion form is responsible for the strong increase in absorption upon addition of base. This paper deals with the nature of the nontitratable, p H independent absorption (absorption I) and with the origin of both absorbing groups in the polymer. Results Acrylonitrile was polymerized in various solvents with various catalysts and over a range of temperatures (details below). The polymers were recovered, extracted with methanol, and dried. The ultraviolet spectrum of the polymers obtained was measured in ethylene carbonate-propylene carbonate (EC-PC) a t room temperature. All of these polymers contained either an intense untitratable uv absorption at 265 m p (absorption I, Figure 1) or a less intense uv absorption at 275 m p whose intensity greatly increased upon addition of base (absorption 11, Figure 2). Apparently, all samples of polyacrylonitrile made in
60 J. BRANDRUP,J. R. KIRBY,AND L. H. PEEBLES, JR.
Mucroriiolecules
I
-CH2-$H-C= CN
N-R
0 AIBN - E C / P C 0
13 A I B N - 0 M F
- EC/PC Redox - DMF
A Redox X
I
220
I
I
I
I
I
I
I
230
240
250
260
270
280
290
P
3
mP
Figure 1. Uv absorption of polyacrylonitrile prepared in EC-PC initiated by AIBN at 50". Spectra taken in ECPC ( I ) before addition of base, ( 2 ) after addition of base.
I before]
1.5
I
20
I
40
1
I
60
Weight % Solvent
I
I
80
I
I
I00
Figure 3. Amount of absorption I cs. weight per cent solvent in the solvent water polymerization medium. Absorption I was measured as the background absorption at 265 mp before addition of base.
base addition
/%\ 0 AIBN 0 AlBN
- EC/FC - DMF
X Redox
1.c -
- DMF
-CH,-
CH-
P 2 ,
FH C=O
CN
' I I
0.5
I
I
I
233
250
275
I
m
-v
-
Weqhl % SDlvent
Figure 2. Uv absorption of polyacrylonitrile prepared in water initiated by K2S2Oa-NaHSO3at 50". Spectra taken in EC-PC ( 1 ) before addition of base, (2) after addition of base.
Figure 4. Amount of absorption I1 cs. weight per cent solvent in the solvent water polymerization medium. Absorption I1 was measured as the difference in absorption at 275 mu before and after addition of base.
organic solvents with various free-radical catalysts contain the intense untitratable absorption I while all polymers made in water contain absorption I1 (Table I). This relationship was confirmed by a series of polymerizations in mixed media of varying solventto-water ratios. Acrylonitrile was polymerized at 50" in different mixtures of dimethylformamide-water or ethylene carbonate-water with azobisisobutyronitrile (AIBN) o r benzoyl peroxide (BPO) or a redox catalyst (potassium persulfate-sodium bisulfite). The polymers were either precipitated in water (high solvent content in polymerization mixture) and filtered or directly filtered (high water content), washed, and extracted with methanol to remove any solvent or other impurities. The polymers were dried a t 50" and the ultraviolet absorptions were determined in EC-PC solution. Absorption I was measured as the
background absorption a t 265 mp before addition of base while absorption I1 was measured as the difference in absorption intensity at 275 mp before and after addition of base. Again, all polymers prepared by free-radical catalysts contained either absorption I or absorption II. As seen in Figures 3 and 4, absorption I increases with increasing solvent content in the polymerization medium while absorption I1 decreases. This result is independent of the particular solvent or catalyst used and solely dependent on the solvent-towater ratio. Thus these absorptions are most likely connected with each other and one must be the precursor of the other. Acrylonitrile was photopolymerized in bulk with incident North Carolina sunlight. The polymer again showed a strong absorption I as seen in Table I. Since here only the monomer, the polymer, and free
CHROMOPHORE OF POLYACRYLONITRILE 61
Vof. I , No. I , Jcinunry-February 1968
C
/Lo
+
mCHZCH
HZNCH2CHw I CN
1
CN d radicals are present, this is clear evidence that the formation of the (entities responsible for this absorption must be an inherent property of the polymerization system itself. Discussion All polyacrylonitriles obtained by free-radical catalysis show either an intense untitratable absorption at 265 mp (absorption I) o r a less intense absorption at 275 mp whose intensity increases tremendously upon addition of base (absorption 11). This last absorption was identified as arising from a P-ketonitrile group present in the polymer whose enol anion form absorbs ~ nature of the other untitratstrongly at 275 r n ~ .The able absorption will be explained below. Since even bulk polymerization by light catalysis produces a polymer which contains absorption I , these defects cannot be introduced in the polymer by reaction with solvents or catalysts o r other impurities present; therefore, they must be an inherent part of the polymerization system itself. Growing free radicals are known to undergo a number of reactions. Besides monomer addition (that is, chain propagation), they will react with other compounds present in the system (monomer, polymer, or sohent) in transfer reactions. I n many systems, the transfer reaction with the polymer is known t o occur primarily at the tertiary hydrogen atom and to produce branches in the chain. A similar reaction was suggested for P A N by U l b r i ~ h t . ~A place of attack of equal probability would be the pendant nitrile group of the chain. This group is known t o polymerize ionicallyj and by irradiation.6 Peebles and Brandrup have used the polymerization of nitrile groups t o provide model compounds of the P A N chromophc~re.~The sequence of reactions given in Scheme I describes a postulated reaction through the nitrile group reaction. Attack of the free radical (4) J. Ulbricht, Z . Phj,s. Chem. (Leipzig), 221, 346 (1962). (5) M. L. Miller, J . Po!,,mer Sci., 56, 203 (1962). (6) Y.Tabata, I
