bond. 'This would then give two bonds which are similar to the metal atom-olefin bonds of the platinum (or palladium) chloride complexes with ethylene, as suggested by Chatt and Duncanson,'O and subsequently confirmed by the X-ray analysis of Dempsey and Raenziger" and that of Holden and Haenziger.12 The bond angles between the carbonyl groups and those from the acetylene to the benzene rings differ appreciably froni the ideal values of either 01" the approximate models given above. Undoubtedly some of the distortion is due to steric interaction involving the benzene rings and also other molecules but on the other hand no attempt has been made to evaluate the contribution of the partial double bond character of the cobalt-carbony1 bonds to the configuration of the molecule. .I knowledge of the molecular dimensions of the dimethylacetylene derivatives and of the acetylene derivative would be valuable for comparison since they would have considerable less structural hin(10) J. Chatt and I,, A. Uuncanson, J . Chem. SOL.,2839 (1953). (11) J. S . nempsey and S . C Baenziger, THISJOURSAL, 77, 4984 (1855). (12) J. R. Holden and N . C . Raenziger, 7 h > d , ,77, 4087 (1955:.
drance. -1 discussion of the reldtivr merits oi' thc two alternative systems of bonding lor DHDPX, both of which lead to a closed krypton shell for the cobalt atoms, will be deferred until the qtriicture is completely refined. The structure which has been found for DHDP.1 is consistent with the absence of bridging carbonyl stretching frequencies in the infrared. the observed diamagnetism and the large observed dipole moment. Acknowledgments.-I wish to thank Dr. Heiw JV. Sternberg for supplying the material used in this investigation and for valuable discussion. 1 ani indebted to Dr. J. D. Britton for carrying out the preliminary X-ray investigation and the density measurements and to Prof. David P. Shoemaker for helpful criticism. The calculations reported here were carried out in part a t the lIIT Computation Center. This work was supported in part by the George Ellery Hale Fund of the California Institute of Technology, by the Kational Institutes of Health and by a National Science Foundation Fellowship. 2q
s3
CAMBRIDGF, MAS-
__ [COSTRIBKJTIONFROM
THE POLYMER
RESEARCH ISSTITUTE,
P o L Y T E C H N I C ISSTITUTE O F n R O O K L Y S I
Reactions of Polymers with Reagents Carrying Two Interacting Groups. I. The Quaternization of Poly-(4-~inylpyridine)with Bromoacetic Acid1,? BY H. LADENHEIM, E. 31.LOEBLAND H. MORAWETZ RECEIVED JULY
7, 19%
Initial rates of quaternization of poly-(4-vinylpyridinej and 4-methylpyridine with bromuacetate ion and a-bruxnoacrtamide were measured in water solution at 50". The quaternizations of 4-methylpyridine are subject to a small positive salt effect, The reaction rate of thc cationic polymer with bromoacetate increases sharply with an increase in the degree of ionization of the polymer and a decrease in the ionic strength of the solution due to the electrostatic attraction of the polycation with the anionic reagent. However, these variables have a smaller effect on the second-order rate constant for this reaction than on the apparent ionization constant of the pyridinium residues, I t is concluded that in the transition state of the quaternization reaction the carboxylate group of the bromoacetate is a t a lower electrostatic potential than the charge5 of the pyridinium residues.
Introduction poly-(4vinylpyridine) with bromoacetate ions, since i t might have been expected that the disThe generally accepted theories of enzyme placement of the bromine by an un-ionized pyriaction assume that a number of distinct sites of the enzyme cooperate in attaching themselves to dif- dine residue would be aided by the interaction of carboxylate with a neighboring cationic pyridiferent portions of the substrate molecule. A typical the nium group. The reaction of the anionic bromoexample of this principle is provided by the action acetate was compared with that of the uncharged of acetylcholine esterase, where an "esteratic site" bromoacetamide both in the quaternization of of the enzyme is involved in the attack on the ester poly-(4-vinylpyridine) and its monofunctional anafunction, while an "anionic site" serves to stabilize log 4-methylpyridine. the transition state by partly electrostatic interaction of a negatively charged site on the enzyme Experimental with the cationic end of the substrate m ~ l e c u l e . ~ Materials.-Bromoacetic acid of highest purity was In this and a following investigation, we shall melted, distilled under nitrogen, b.p. 118" (15 mm.) and explore the possibilities of producing similar co- kept frozen in a desiccator over sulfuric acid. The a-brooperative effects in reactions of high polymers with moacetamide was prepared by the procedure of Papendieck' bifunctional small molecules. In this study we (m .p. 91"). Highest purity 4-methyl-pyridine was distilled dry nitrogen from barium oxide, b.p. 64" (30 mm.), selected the quaternization of partially ionized nunder z 5 1.5034. ~ Poly-(4-vinylpyridine) (PVPy) was prepared (I) Taken in p a r t from a thesis submitted b y H. Ladenheim in partial fulfillment of t h e requirements for a Ph.D. degree, June, 1958. (2) Financial assistance of this study by t h e Office of Ordnance Ke-
from freshly distilled 4-vinylpyridine by emulsion polymerization,6with Tergitol P-28 (Carbide and Carbon Chem. Co.) as the emulsifier. The intrinsic viqcosity of the polymer
search, U. S. Army, and b y a grant of the Monsanto Chemical Company are gratefully acknowledged. (R) I . R. Wilson, D . Bergmann and n.Nachmansohn, J . B i d . C h e m . , 186. 761 (1950).
(4) A . Papendieck, Be?., 25, 1100 (1892). ( 5 ) E. B. Fitzgerald and K. 37. Iiuoss, I d . En?. C h e v f , , 4%, 1603 (1950).
Jan. 5 , 1939
$&ATERNIZATIOK
OF
POLY(~-VINYI,PYRIDINE)
WITH
BROMOACETIC .IUD
21
([TI = 0.78 in 92% ethanol) corresponded to a molecular constants over a range of temperatures.'0 Poweight of 168,000.* tentiometric titration of 4-methylpyridine a t 50' Silver nitrate was standardized potentiometrically against gave PKMP = 5.23. The mean ionic activity highest purity potassium bromide. coefficients y f were calculated from the DebyeSpectrophotometric Titration of Poly-(4-vinylpyridine) The variation of the state of ionization of PVPy with gH at Hiickel equation assuming an effective ionic various ionic strengths was determined by spectrophotometric titration using a stirred 10 cm. cell thermostated a t radius of 4 The state of ionization of PVPy was determined 50" and a Beckman DU spectrophotometer. For titrations a t ionic strengths 0.0375 and 0.1515 the solutions contained by spectrophotometric titration as described in the initially 0.0375 M monosodium citrate, 0.0005 M citric acid experimental section. The results are shown in and about 1.7 X base molar PVPy in a 50-ml. volume; for the higher ionic strength the requisite amount of sodium Fig. 1. For any given degree of neutralization, chloride was added. At the lowest ionic strength (0.0065) no buffer was used. The solutions were titrated with 1.325 N HCl, observing changes in the optical density D = log ( l o / I )a t 254 mfi. The exact base molar polymer concentration C, was obtained from the optical density observed when 0.44 ml. of 12 N HC1 were added a t the end of the titration to ensure complete conversion of the polymer to the cationic form with an extinction coefficient of 3688 1-base mole-Ic m . 3 . The extinction coefficient of PVPy in its basic form was determined by measurements in absolute methanol as 1403 1.-base rnole-'-cm. -l, and these two extinction coefficients were used to calculate spy, the fraction of the pyridine residues present in their basic form. All the titrations were duplicated to correlate HC1 addition with pH. Potentiometric Measurements.-Measurements of pH were made a t 50' on a Cambridge Research Model pH meter using external shielded electrodes and applying corrections for errors of the glass electrode a t that temperature from information supplied by the p H meter manufacturer. In titrations involving PVPy a stream of nitrogen was used to stir the solution and protect i t from atmospheric carbon dioxide and 3-4 minutes were allowed after each titrant addi2.0 tion for attainment of equilibrium. For bromide determina0.2 0.4 0.6 tions, the acidified test solution was titrated potentiometricaPY* ally with standardized silver nitrate using either a pure silver or a silver-silver bromide electrode' with a reference Fig. 1.-Spectrophotometric titration of poly-(4-vinylglass electrode. Kinetic Runs.-Kinetic runs were performed a t 50 2~0.05" pyridine), ionic strength: 0,0.0065; 0 , 0.0375; 0, 0.1615. in three neck flasks to allow the insertion of electrodes for pH measurement. The pH remained constant within 0.1 the ionization of the pyridine residues is strongly dependent on the concentration of simple electrolytes unit in any one run. A 50-ml. solution containing 9.6 X lo-* base molar PVPy or 4-methylpyridine partially neu- as would be expected, since the electrostatic free tralized with nitric acid and sodium nitrate to produce the desired ionic strength was allowed to come to temperature equi- energy of ionization is very sensitive to the ionic librium and 0.5 ml. of a freshly prepared about 0.2 M solu- atmosphere.l1P1* A complication was introduced tion of bromoacetic acid or a-bromoacetamide was injected. by the fact that the spectrophotometric titrations The progress of the reaction was followed by quenching the had to be carried out, because of the high exreaction in 5-ml. aliquots with 15 ml. of cooled 0.08 N nitric acid and determining bromide potentiometrically as de- tinction coefficient of PVPy, a t a polymer conscribed above. Since a previous investigation of the quat- centration two orders of magnitude below that ernization of poly-(vinylpyridine) demonstrated kinetic used in the kinetic runs. ' To compensate for this complications a t high polymer conversions,s only initial difference, 0.5 mole of NaCl was substituted for rates a t less than 10% quaternization were determined. each base mole of ionized polymer, following the
.-
k.
suggestion of Katchalsky and Lifson,I2 that in polyelectrolyte solutions only the small ions contribute to the "effective ionic strength." Correction for Side Reactions.-Under the experimental conditions used (pH 2.52-7.34) the hydrolysis of the amide linkage of a-bromoacetamide was not significant. For the hydroxyl ion pH = ~ K B A A log [(A-)/(H-i)] log (1) displacement of bromine in a-bromoacetamide and PH = PKNP log [(B)/(BH+)l - log y f ( 2 ) a-bromoacetate a t SOo, rate constants of 0.0274 where (A4-), (HA), (B) and (BH)+ stand for con- and 0.0245 1. mole-' mix-', respectively, were decentrations of bromoacetate, un-ionized bromo- termined; both of these rate constants are too low acetic acid, 4-methylpyridine and 4-methylpy- to contribute significantly to the observed rates ridinium ion, respectively. The value of ~ K B A Aof bromide formation. The only side reaction = 2.95 a t 50' was chosen since this is the $K of which had to be corrected for was the uncatalyzed chloroacetic acid a t 50°9 and since the two acids hydrolysis of the C-Br bond; this reaction was have been shown to have identical dissociation characterized by rate constants of 3.7 X 10-5, (6) A. G. Boyes a n d U. P. S t r a w s , J . Polymer Sci., 22, 483 (1956). 4.1 X and 9.7 X 10-5 min.-l for a-bromo( 7 ) F. Halls, Z. Eleklrochem., 17, 179 (1911). acetamide, un-ionized bromoacetic acid and bromo( 8 ) B. D. Coleman and R. M. Fuoss, THISJOURNAL, 77, 5472 Treatment of Experimental Data The State of Ionization of the Reagents.-Before the experimental results can be interpreted, the state of ionization of the reagents has to be known. For bromoacetic acid and 4-methylpyridine
+ +
+
(1955).
(9) H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions," 2nd Ed., Reinhold Puhl. Corp., Xew York, N. Y . , 1950, p. 580, 583.
(10) F. L. Kortright, A n i . Chem. J . , 18, 3b5 (1890); W. Ostwald. Z . p h y s i k . C h e m . , 3 , 170 (1889); K. Drucker, i b i d . . 40, 563 (1904). (11) J. T. G. Overbeek, Bull. SOL. chim. Belpes., 57, 252 (1948). (12) A. Katchalsky and S. Lifson, J . Polymer Sci., 11, 409 (1953).
22
H. LADENHEIM, E. M. LOEBLAND H. MORAWETZ
1-d.s1
acetate ion, respectively. None of these rates was significantly affected by electrolyte concentrations up to an ionic strength of 0.15. WBROMOACETAMIDE Calculation of Rate Constants.-For quaternizakn lO~K$ P P €1 OLA -PY tion by a-bromoacetamide, second-order rate conBromoacetate stants were calculated by deducting from the ob1.01 1.0 0.148 3.54 0.84 0.244 served initial rate of bromide evolution the expected 1.0 1.1 .145 3.34 .77 ,193 rate of a-bromoacetamide hydrolysis and dividing .14s 3.2s .-iD .172 1.0 1.1 by the concentration of a-bromoacetamide and the .I45 2.6s .43 .092 1.3 1,8 concentration of un-ionized pyridine residues. 2.3 1.8 3.31 .73 ,034 .280 With reactions involving bromoacetic acid, the cor2.7 2 .5 ,034 3.07 .01 ,225 rection for hydrolysis contained two terms, cor,034 4.4 3.9 2.64 .G7 .145 responding to the hydrolysis of the ionized and the 4 .5 4.5 084 2 . 5 2 .31 ,135 un-ionized acid. The corrected quaternization 3.26 .6S .512 ,003 4.5 rate was again the sum of rates due to uii-ion d 5.8 .00:3 .17 .Mi 5.1 I(1 2.80 bromoacetic acid and to bromoacetate ion, rc,003 5.8 .41 .404 2.70 13 spectively. It was assumed that the quaterniza.003 6.7 tion rate constant for un-ionized bromoacetic acid 2.55 .30 .364 16 was equal to that determined for bronioaceta~nide~~ Bromoacetamide and after allowing for this reaction, the residual ,143 3.60 ,246 .37 rate was divided by the concentrations of bromo3.32 .284 .031 .34 acetate and the un-ionized pyridine residues to ,003 3.22 ,506 .23 obtain the second-order rate constant of the bromo3.01 ,455 ,003 .23 acetate reaction. All second-order rate constants ,003 2.60 .373 .21 are given in 1. mole-l-min-l. has previously been shown to be subject to a positive Results and Discussion salt effect. Both these phenomena may be exThe second-order t ate constant for the quater- plained qualitatively by the mutual electrostatic nization of 4methylpyridine with a-bromoacet- attraction of PVPy and bromoacetate. This ninide a t .TOo wLis found to be 0.39, 0.48 and 0.57 attraction increases with increasing polymer charge 1. nick-' tnin a t ionic strengths of 0.003, 0.0% and decreases with the concentration of countcrand 0.145, respectively. Under the same con- ions, which shield the electrical field of the polyditions, the quaternization with bromoacetate cation. Unfortunately, only a comparatively narion was characterized by rate constants of 0.40, row range of upy could be investigated, since PVPv 0.41 and 0.43, respectively. I t may be con- is insoluble in aqueous media a t low degrees of cluded that the reactivity of the two species is very ionization, while a t high acidities the side reactions similar and subject to a positive salt effect which is account for a large fraction of the bromide evolusignificantly larger for the or-bromoacetamide. tion, introducing a prohibitive uncertainty in the The results obtained with PVPy are listed in calculations of kz. Table I. Here p is the ionic strength, (YA the degree The first kinetic investigation of a reaction inof ionization of bromoacetic acid, a p Y the fraction volving a polyion and a small charged species was of pyridine residues in the basic form, kz the second- carried outi4on the hydroxyl ion catalyzed hydrolyorder rate constant for the quaternization reaction sis of ester groups carried by the anionic chains of and I
