Alkaline Hydrolysis of Oligomers of Tartrate Esters: Effect of a Neighboring Carboxyl on the Reactivity of Ester Groups MANW L. MANIAR**', DEVENDRAs. k L O N I A * ,
AND
ANTHONYP. SIMONELLI*
Received February 12, 1991, from the 'School of Pharmacy and Institute of Material Sciences, University of Connecticut, 'Present address: Nova Pharmaceutical Corporation, Storrs, CT 06269. Accepted for ublication August 26, 1991. 6200 Freeport Center, Baltimore, ME 21224. Abstract 0 The importance and the effect of neighboring groups on the hydrolysis of polymeric esters were demonstrated. Several oligomers of
poly(butylenetartrate) were synthesized, and their kinetic behavior was studied under alkaline conditions. The oligomers and their degradation products were monitored by HPLC and identified by fast atom bombardment mass spectrometry. The rates of hydrolysis measured at pH 5-8 and at 75°C indicate that the degradation of the oligomers obeyed pseudo-first-orderkinetics. The rate of hydrolysis among the homologous series of oligomers increased as the molecular weight increased. However, the increase in hydrolysis rate was not proportional to the number of ester linkages in the oligomers. This deviation was explained on the basis of electrostatic repulsion of the neighboring carboxylate group toward the hydroxide ion. The calculations revealed that the electrostatic effect was so great that the ester linkage adjacent to the carboxylate anion did not contribute to the overall rate of hydrolysis.The technique presented here can be extended to any multifunctional-group compound that has repeat units and can undergo a specific reaction that can be accurately measured.
When macromolecules take part in organic reactions, the influence of polymeric structure not only on the course of the reaction but also on the final structure of the end products must be considered. With polymer reactions, when charged groups are involved, the reactivity of a functional group may not be entirely independent of the status of other groups in the molecule. The range over which the effects of neighboring groups are transmitted may depend on the locations of the neighboring groups and the polymer dimensions. However, these effects can significantly alter the reaction rates by many orders of magnitude.' One of the first studies reporting the influence of electrostatic effects on ester hydrolysis was done by Meyer.2 The acid-catalyzed hydrolysis of diethyl succinate conformed to the generalization that the reaction of a functional group in a molecule does not affect appreciably the reactivity of a neighboring group. However, the base-catalyzed hydrolysis presented quite a different situation. The rate constant for the base-catalyzed hydrolysis of diethyl succinate approximated the normal value for an aliphatic ester, but the rate constant for the monoester was 1 order of magnitude smaller.2 This observation was attributed to the electrostatic repulsion between the carboxylate ion of the monoester and the reacting hydroxide ion. The anomaly in polymer reactions was first pointed out by Freudenberg and co-workers.3 They postulated that, in the hydrolysis of cellulose, one or the other or both of the terminal linkages in cellulose and its derivatives are hydrolyzed more rapidly than the internal linkages. De Loecker and Smets4 showed the functional interaction of an ester group with the neighboring carboxylate groups. The hydrolysis of partially neutralized copolymers of methacrylic acid and methyl methacrylate of different compositions was interpreted on the basis of Bronsted catalysis theory; the 0022-3549/92/0700-0705$02.50/0 0 1992,American
Pharmaceutical Association
reaction was first order with respect to ester concentration.
TWO distinct hydrolysis rate constants, K H A and KA-, corresponding to the catalytic actions of undissociated and neutralized carboxyl groups, respectively, were evaluated. The methods currently used to determine kinetic parameters include measurements obtained by NMR6 and Raman spectroscopy.6 These methods assume that the individual microrate constants of the multigroup hydrolysis reactions are equal and therefore evaluate the rate of hydrolysis by use of an overall rate constant. Such approaches do not permit the development of specific mechanisms or the determination of individual rate constants, because the pathways of hydrolysis are complex. To overcome the problems associated with high-molecularweight compounds and to develop an accurate method to determine specific micro-rate constants (see ref 91, we initiated a study with diethyl tartrate, a model multifunctional compound.' After successfully determining the micro-rate constants for diethyl tartrate and monoethyl tartrate, we used a similar approach to investigate low-molecular-weight oligomers of poly(buty1ene tartrate). In our previous communication,s we showed that low-molecular-weight oligomers could be successfully synthesized, and we reported a technique that we developed to separate and identify them. Subsequently, we showed that specific rate constants for the hydrolysis of specific oligomers can be determined accurately from a reaction mixture in which various oligomers with different molecular weights undergo hydrolysis.9 In the present communication, we describe the second phase of our systematic study in an effort to understand the alkaline hydrolysis of oligomers of tartrate esters and to examine the influence of a carboxylate group on the hydrolysis of neighboring ester bonds.
Experimental Section Reagents-L-Tartaric acid, 1,4-butanediol,p-toluenesulfonic acid, monobasic and dibasic sodium phosphate, and acetonitrile (Aldrich Chemical Company, Milwaukee, WI)were of reagent grade and were used without further purification. Synthesis of PolymereEquimolar quantities of tartaric acid and 1,4-butanediol were added to a sidearm flask. p-Toluenesulfonic acid (equivalent to 5% of the total weight of tartaric acid and 1,4butanediol) was added to the mixture. All three components were mixed thoroughly with a spatula. The resultant mixture was purged with nitrogen for 1 h at room temperature. Meanwhile, an oil bath was equilibrated to 120 "C. After 1 h, the sidearm flask was transferred to the oil bath and heated for 30 min, after which the reaction mixture was subjected to vacuum for 15 min to remove the liberated water. The reaction mixture was poured into 250 ml of acetonitrile to precipitate the ionic oligomers. The initial precipitate, which was light brown in color, was washed with several portions of acetonitrile until a white precipitate was obtained. Several washes with acetonitrile reduced the amount of p-toluenesulfonic acid in the precipitated polymer. Equipment-A liquid chromatograph (SP8100;Autolab Division,
Journal of Pharmaceutical Sciences I 705 Vol. 81, No. 7, July 1992
Spectra Physics, San Jose,CA) comprising gradient-forming components and three solvent reservoirs was used to deliver the solvent mixture to the column. The liquid chromatograph was equipped with an SP8110 autoeampler. A strong-anionsxchange column (Zorbax Bioeeries; Biomedical Products Department, Du Pont Company, Wilmington, DE)was used to separate the components. The eluant was monitored at 220 nm with a variable-wavelength spectrophotometer (SP8400; Spectra Physics). A computing integrator (SP4200; Spectra Physics) operating at 1mV was used to record the signals and measure peak areas. Kinetic StudiegHydrolysis studies were carried out at pH 5-8.0 at 75°C. Phosphate buffer (0.1 M) was used to control the pH. Arrhenius parametera were determined by conducting studies at pH 7.0 and with a temperature range of 50-85 "C. The hydrolysis studies were conducted with 2 g of a mixture of oligomers. The mixture was suspended in 60 ml of water, stirred, and filtered through a 0.45-pm-pore-size membrane filter. Appropriate amounts of monobasic and d i b d c sodium phosphate were added to the filtrate to achieve a buffer concentration of 0.1 M and the required pH. For the pH 7.0 study, 341.93 mg of NaHzP04 and 500.12 mg of NazHP04 were added to 60 ml of the filtrate. The final pH was adjuated with 0.1 M phosphoric acid or sodium hydroxide solution. The aolution was transferred to a 100-ml glass-stoppered flask,which was then placed in a n oil bath equilibrated to 75 "C. Samples were withdrawn at appropriate times, and the pHs of those samples were immediately adjusted to 4.5 by adding 0.5 M phosphate buffer. The samples were frozen until the study was completed. All the samples from one particular study were analyzed at the same time by a previously described assay.*
Results and Dlscusslon
180-
r
I
150h
E
I:
I
E
v
$1
..
120'
I-
t
-1
L?
-
IC
L
1
5000 m i nut e s
flgure 1-Typical degradation behavior of oligomers A (O), B (A), and
C (0)at pH 6.03and 75 "C. 180-
120 1
I
c
I
_I
'I
2
60
The same oligomers as those described earlier9 (oligomerA: " J P T-A-T-A-T oligomer B: T-A-T-A-T-A-T; and oligomer C: T-A-T-A-T-A-T-A-T,in which , ( O C C H ( o ~ H ( o H ~ ( O ~ C H ~ ) ~ ~ ) T A were studied to investigate further the use of the technique discussed in previous publications8.9for determining specific 0 20 40 60 00 100 rate constants and micro-rate eonstants. However, in the i n 1n u t e s present study, the technique was used for obtaining specific Flgure 2-Typical degradation behavior of oligomers A (O), B (A), and rate constants for individual species in the alkaline pH C (0)at pH 7.53 and 75 "C. region. Considering the PK,(K,is acid dissociation constant) values for tartaric acid (2.93 and 4.231'0 and expecting similar 2 400 r values for the oligomers, we conducted the hydrolysis studies at pH 5-8. In this way, the effect of the negative charge of the carboxylate group on the hydrolysis rate could be studied. As previously described,@the reaction mixture contained a range of components with molecular weights higher or lower than those of oligomers A, B, and C. Therefore, a degradation schematic similar to that used previously (see Scheme I in ref 9) is applicable in the present study@to the reaction mixture containing oligomers A, B, and C. The heights of the individual peaks in the chromatogram were used for quantitative measurements. At moderate pHs, at which the reaction was slow, an initial phase for the formation of oligomers A, B, and C can be seen in Figure 1. \ 04001-, - ,-\ ' -__ This result was expected, because the reaction mixture 0 10 20 5),e the terms k, and kHIH'l can be omitted from eq 5. Hence, when the reaction rate is determined primarily by hydroxide ion catalysis, eq 5 is reduced to eq 6
m. I m
-
.. -
This equation can be written in the following form:
.4
This equation, in which kw is the ion product of water, predicts that a plot of log ko, versus pH should be linear, with a slope equal to 1. In Figure 5,the profile for log kob versus pH is linear, with a slope equal to 1, indicating that specific base catalysis occurred. The slope of 1 provides further evidence that the specificrate constants determined from the terminal linear portion were correct. The rate constant for specific base catalysis, kOH, for each oligomer was calculated from the slope of the plot of ko, versus the hydroxide ion concentration (Figure 6). Values of KO, are given in Table II. pH Profiles for Oligomers-The pH profiles for the experimentally determined rate constants for oligomers A, B, and C are shown in Figure 7. The pH profiles are divided into three pH regions to simplify the discussion. The first region is the profile between pHa 1.0 and 3.0.This region exhibits a linear relationship between log ko, and pH, with a slope of -1, indicating that the hydrolysis undergoes specific acid catalysis.9 Rate constants for specific acid catalysis were obtained from the slopes of the plot of ,k versus hydrogen ion concentration. The average micro-rate constant for hydrogen ion catalysis, obtained from the rate constants for specific acid catalysis, was 0.05306 M-' * min-' linkage-'. The second region is the profle between pHs 5.0 and 8.0. This region exhibits a linear relationship between log ,k and pH, with a slope of 1, indicating that the hydrolysis undergoes specific base catalysis. Rate constants for specific base catalysis were obtained fromthe slopes of the plot of,k versus hydroxide ion concentration.The average micro-rate constant for hydroxide ion catalysis, obtained from the rate constanta for specific base catalysis, was 2433.12 M-' min-' linkage-'.
e
am.
I..
OH - ion concentratton X 10
7
Flgure +Plot of ks versus hydroxide ion concentration at 75 "C for oligomers A (O), B (A), and C (0). Table Il-Rate Constant8 for SpecMc Base Catalysis of HydrOly8l8 of Ollgomem A, 6, and C No. of Ester
Rate Constant, M-1 .mi"-'
A B
4
C
8
4708.67 9804.03 14 724.25
Ollgomer
Linkages 6
-
-
-
M
3
5 1
Flgure 7-pH
6.m
6.m
~
.
m
b.6
r.m
r.m
m.m
PH Flgure bPlot of log &.versus pH at 75 "Cfor oligomers A (O), B (A), and C (0). 700 I Journal of Phannaceutkal Sciences Vol. 81, No. 7, July 1992
2
4
I
I
I
3
4
5
6
~
7
._.. T
0
9
PH profiles for oligomers A (O), B (A), and C (0)at 75 "C.
The third region is the profile between pHs 3.0and 5.0.This region shows that the rate of hydrolysis was much higher than that predicted if the only contributions to the overall rate were the hydrogen ion and hydroxide ion catalytic pathways. Effect of C h a rg e -O n the basis of the structures of oligomers A, B, and C and in the absence of any extraneous effect, the hydroxyide ion has an equal chance of hydrolyzing all the ester linkages in a n oligomer. In this case, all micro-rate constants would be equal, and k, would equal the sum of the individual micro-rate constants. If this assumption is true, statistical analysis predicts that the ratio of ko, for oligomer A to Itobs for oligomer B to kobe for oligomer C should be 1:1.5:2.0. Instead, the experimental ratio was 1:2.05:3.07. These data show that alkaline hydrolysis of oligomers of tartrate esters is not controlled solely by a statistical fador of all ester linkages. Oligomer A contains four ester linkages, oligomer B contains six, and oligomer C contains eight. At pH > 6.0,these
compounds are expected to be completely ionized. If so, then the terminal ester linkage would be only two carbons away from the ionized carboxylate group. Subsequent ester linkages would be seven or more carbons away. Because alkaline hydrolysis of a charged tartrate ester is catalyzed by hydroxide ions, the hydrolysis involves a reaction between two ions that are of the same charge. The structures of the oligomerscanjustify disregarding the effect of intermolecular polar transmission on the intermediate ester linkages, because any inductive effect of the carboxylate anion would decrease exponentially along a series of bonds. Also, the mesomeric or electromeric effect of the carboxylate anion can be ruled out because of the lack of resonance or conjugation in the oligomers. Only a field effect of the carboxylate anion, which propagates through the solvent molecules, remains. Compared with the inductive effect, the field effect decreases less rapidly with distance. Because the hydrolysis reactions were carried out in solution, the carboxylate anion would be solvated and its electric charge would be effectively dispersed over a shell of solvent molecules.This polarizationof surrounding solvent molecules would reduce the field effect of the carboxylate anion on the intermediate ester linkages. Because of the spread of this negative charge, the hydroxide ions would be repelled, such repulsion leading to a decrease in the rate of hydrolysis. Apparently, the experimental results indicate that the effect of the negative charge is significantat the terminal ester linkages and that the rate of hydrolysis of the terminal ester linkages can be neglected relative to that of the rest of the ester bonds in the molecule. If this assumption is true, then the effective number of ester bonds accessible for hydrolysis would be two, four, and six in oligomersA, B,and C,respectively, indicating that the statistical factors for the hydrolysis of these compounds would be two, four, and six and that the ratios of specific rate constants should be 1:2:3, in agreement with the experimentally determined ratios of rate constants.
Conclusions The results of this investigation showed that the determination of the specificrate constant of a given polyester species from the terminal slope of a h t s r d e r plot is a useful technique, yielding results not only for uncharged species in
the acidic region but also for charged species in the alkaline region. From the rates of hydrolysis measured, it was shown that the degradation of oligomers obeys pseudo-first-order kinetics. The hydrolysis was catalyzed by hydroxide ion, and the catalytic rate constant increased predictably with the number of ester linkages. However, the rate constant was not proportional to the number of ester linkages in the oligomers. This deviation was explained on the basis of a field effect causing an electrostatic repulsion between the neighboring carboxylate anion and the attacking hydroxide ion. The calculations revealed that the field effect was so great that the ester linkage adjacent to the carboxylate anion did not contribute to the overall rate of hydrolysis. For the polyesters studied, the micro-rate constant for hydroxide ion catalysis for an individual ester linkage not located next to a charged carboxylate anion was 2433.12 M-' min-' * linkage-'. This approach can be used to study the effecta of many variables on polyester hydrolysis, including structural effecta of large polyester compounds. This approach may also be applied to the degradation of other types of polymers containing a repeating labile group located next to another functional group.
-
References and Notes 1. Morawetz, H.Polym.Prepr. (Am. Chem. Soc.,Div.Polym. Chem.) 1986,27,62-63. 2. Meyer, J. 2.Phys. Chem. 1909,67,257. 3. Freudenberg, K.; Kuhn, W.; Durr, W.; Steinbrunn, G. Ber. 1930, 63,1510. 4. De Loecker, W.; Smeta, G. J. Polym. Sci. 1959,26,203-216. 5. Barth, V.;Kleeper, E. Polymer 1976,17,787. 6. Daviee, M.C.;Tudor, A.; Melia, C. D.; Hendra, P.; Domb, A. J.; Langer, R. 17th International S p i u m on ControlledReleaee of Bioactive Materials, 1990; 2 c 7. Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1989,78, 78-84. 8. Maniar, M.; Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1989,78, 858-862. 9. Maniar, M.;Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1991,80, 77~82. 10. TheMerckZndex, 11th ed.; Budavari, S.,Ed.; Merck: Rahway, NJ, 1989;p 1433. 11. Remi ton's Pharmaceutical Sciences; Gennaro, A. R., Ed.; Mack%arrton, PA, 1985;p 252. 12. Martin, A.; Swarbrick, J.; Cammarap, A. PhysicalPharmucy, 3rd ed.; Lea and Febiger: Philadelpha, 1983;p 379.
Journal of Pharmaceutical sdeoces/ 709 Vol. 81, No. 7, July 1992
AND
ANTHONYP. SIMONELLI*
Received February 12, 1991, from the 'School of Pharmacy and Institute of Material Sciences, University of Connecticut, 'Present address: Nova Pharmaceutical Corporation, Storrs, CT 06269. Accepted for ublication August 26, 1991. 6200 Freeport Center, Baltimore, ME 21224. Abstract 0 The importance and the effect of neighboring groups on the hydrolysis of polymeric esters were demonstrated. Several oligomers of
poly(butylenetartrate) were synthesized, and their kinetic behavior was studied under alkaline conditions. The oligomers and their degradation products were monitored by HPLC and identified by fast atom bombardment mass spectrometry. The rates of hydrolysis measured at pH 5-8 and at 75°C indicate that the degradation of the oligomers obeyed pseudo-first-orderkinetics. The rate of hydrolysis among the homologous series of oligomers increased as the molecular weight increased. However, the increase in hydrolysis rate was not proportional to the number of ester linkages in the oligomers. This deviation was explained on the basis of electrostatic repulsion of the neighboring carboxylate group toward the hydroxide ion. The calculations revealed that the electrostatic effect was so great that the ester linkage adjacent to the carboxylate anion did not contribute to the overall rate of hydrolysis.The technique presented here can be extended to any multifunctional-group compound that has repeat units and can undergo a specific reaction that can be accurately measured.
When macromolecules take part in organic reactions, the influence of polymeric structure not only on the course of the reaction but also on the final structure of the end products must be considered. With polymer reactions, when charged groups are involved, the reactivity of a functional group may not be entirely independent of the status of other groups in the molecule. The range over which the effects of neighboring groups are transmitted may depend on the locations of the neighboring groups and the polymer dimensions. However, these effects can significantly alter the reaction rates by many orders of magnitude.' One of the first studies reporting the influence of electrostatic effects on ester hydrolysis was done by Meyer.2 The acid-catalyzed hydrolysis of diethyl succinate conformed to the generalization that the reaction of a functional group in a molecule does not affect appreciably the reactivity of a neighboring group. However, the base-catalyzed hydrolysis presented quite a different situation. The rate constant for the base-catalyzed hydrolysis of diethyl succinate approximated the normal value for an aliphatic ester, but the rate constant for the monoester was 1 order of magnitude smaller.2 This observation was attributed to the electrostatic repulsion between the carboxylate ion of the monoester and the reacting hydroxide ion. The anomaly in polymer reactions was first pointed out by Freudenberg and co-workers.3 They postulated that, in the hydrolysis of cellulose, one or the other or both of the terminal linkages in cellulose and its derivatives are hydrolyzed more rapidly than the internal linkages. De Loecker and Smets4 showed the functional interaction of an ester group with the neighboring carboxylate groups. The hydrolysis of partially neutralized copolymers of methacrylic acid and methyl methacrylate of different compositions was interpreted on the basis of Bronsted catalysis theory; the 0022-3549/92/0700-0705$02.50/0 0 1992,American
Pharmaceutical Association
reaction was first order with respect to ester concentration.
TWO distinct hydrolysis rate constants, K H A and KA-, corresponding to the catalytic actions of undissociated and neutralized carboxyl groups, respectively, were evaluated. The methods currently used to determine kinetic parameters include measurements obtained by NMR6 and Raman spectroscopy.6 These methods assume that the individual microrate constants of the multigroup hydrolysis reactions are equal and therefore evaluate the rate of hydrolysis by use of an overall rate constant. Such approaches do not permit the development of specific mechanisms or the determination of individual rate constants, because the pathways of hydrolysis are complex. To overcome the problems associated with high-molecularweight compounds and to develop an accurate method to determine specific micro-rate constants (see ref 91, we initiated a study with diethyl tartrate, a model multifunctional compound.' After successfully determining the micro-rate constants for diethyl tartrate and monoethyl tartrate, we used a similar approach to investigate low-molecular-weight oligomers of poly(buty1ene tartrate). In our previous communication,s we showed that low-molecular-weight oligomers could be successfully synthesized, and we reported a technique that we developed to separate and identify them. Subsequently, we showed that specific rate constants for the hydrolysis of specific oligomers can be determined accurately from a reaction mixture in which various oligomers with different molecular weights undergo hydrolysis.9 In the present communication, we describe the second phase of our systematic study in an effort to understand the alkaline hydrolysis of oligomers of tartrate esters and to examine the influence of a carboxylate group on the hydrolysis of neighboring ester bonds.
Experimental Section Reagents-L-Tartaric acid, 1,4-butanediol,p-toluenesulfonic acid, monobasic and dibasic sodium phosphate, and acetonitrile (Aldrich Chemical Company, Milwaukee, WI)were of reagent grade and were used without further purification. Synthesis of PolymereEquimolar quantities of tartaric acid and 1,4-butanediol were added to a sidearm flask. p-Toluenesulfonic acid (equivalent to 5% of the total weight of tartaric acid and 1,4butanediol) was added to the mixture. All three components were mixed thoroughly with a spatula. The resultant mixture was purged with nitrogen for 1 h at room temperature. Meanwhile, an oil bath was equilibrated to 120 "C. After 1 h, the sidearm flask was transferred to the oil bath and heated for 30 min, after which the reaction mixture was subjected to vacuum for 15 min to remove the liberated water. The reaction mixture was poured into 250 ml of acetonitrile to precipitate the ionic oligomers. The initial precipitate, which was light brown in color, was washed with several portions of acetonitrile until a white precipitate was obtained. Several washes with acetonitrile reduced the amount of p-toluenesulfonic acid in the precipitated polymer. Equipment-A liquid chromatograph (SP8100;Autolab Division,
Journal of Pharmaceutical Sciences I 705 Vol. 81, No. 7, July 1992
Spectra Physics, San Jose,CA) comprising gradient-forming components and three solvent reservoirs was used to deliver the solvent mixture to the column. The liquid chromatograph was equipped with an SP8110 autoeampler. A strong-anionsxchange column (Zorbax Bioeeries; Biomedical Products Department, Du Pont Company, Wilmington, DE)was used to separate the components. The eluant was monitored at 220 nm with a variable-wavelength spectrophotometer (SP8400; Spectra Physics). A computing integrator (SP4200; Spectra Physics) operating at 1mV was used to record the signals and measure peak areas. Kinetic StudiegHydrolysis studies were carried out at pH 5-8.0 at 75°C. Phosphate buffer (0.1 M) was used to control the pH. Arrhenius parametera were determined by conducting studies at pH 7.0 and with a temperature range of 50-85 "C. The hydrolysis studies were conducted with 2 g of a mixture of oligomers. The mixture was suspended in 60 ml of water, stirred, and filtered through a 0.45-pm-pore-size membrane filter. Appropriate amounts of monobasic and d i b d c sodium phosphate were added to the filtrate to achieve a buffer concentration of 0.1 M and the required pH. For the pH 7.0 study, 341.93 mg of NaHzP04 and 500.12 mg of NazHP04 were added to 60 ml of the filtrate. The final pH was adjuated with 0.1 M phosphoric acid or sodium hydroxide solution. The aolution was transferred to a 100-ml glass-stoppered flask,which was then placed in a n oil bath equilibrated to 75 "C. Samples were withdrawn at appropriate times, and the pHs of those samples were immediately adjusted to 4.5 by adding 0.5 M phosphate buffer. The samples were frozen until the study was completed. All the samples from one particular study were analyzed at the same time by a previously described assay.*
Results and Dlscusslon
180-
r
I
150h
E
I:
I
E
v
$1
..
120'
I-
t
-1
L?
-
IC
L
1
5000 m i nut e s
flgure 1-Typical degradation behavior of oligomers A (O), B (A), and
C (0)at pH 6.03and 75 "C. 180-
120 1
I
c
I
_I
'I
2
60
The same oligomers as those described earlier9 (oligomerA: " J P T-A-T-A-T oligomer B: T-A-T-A-T-A-T; and oligomer C: T-A-T-A-T-A-T-A-T,in which , ( O C C H ( o ~ H ( o H ~ ( O ~ C H ~ ) ~ ~ ) T A were studied to investigate further the use of the technique discussed in previous publications8.9for determining specific 0 20 40 60 00 100 rate constants and micro-rate eonstants. However, in the i n 1n u t e s present study, the technique was used for obtaining specific Flgure 2-Typical degradation behavior of oligomers A (O), B (A), and rate constants for individual species in the alkaline pH C (0)at pH 7.53 and 75 "C. region. Considering the PK,(K,is acid dissociation constant) values for tartaric acid (2.93 and 4.231'0 and expecting similar 2 400 r values for the oligomers, we conducted the hydrolysis studies at pH 5-8. In this way, the effect of the negative charge of the carboxylate group on the hydrolysis rate could be studied. As previously described,@the reaction mixture contained a range of components with molecular weights higher or lower than those of oligomers A, B, and C. Therefore, a degradation schematic similar to that used previously (see Scheme I in ref 9) is applicable in the present study@to the reaction mixture containing oligomers A, B, and C. The heights of the individual peaks in the chromatogram were used for quantitative measurements. At moderate pHs, at which the reaction was slow, an initial phase for the formation of oligomers A, B, and C can be seen in Figure 1. \ 04001-, - ,-\ ' -__ This result was expected, because the reaction mixture 0 10 20 5),e the terms k, and kHIH'l can be omitted from eq 5. Hence, when the reaction rate is determined primarily by hydroxide ion catalysis, eq 5 is reduced to eq 6
m. I m
-
.. -
This equation can be written in the following form:
.4
This equation, in which kw is the ion product of water, predicts that a plot of log ko, versus pH should be linear, with a slope equal to 1. In Figure 5,the profile for log kob versus pH is linear, with a slope equal to 1, indicating that specific base catalysis occurred. The slope of 1 provides further evidence that the specificrate constants determined from the terminal linear portion were correct. The rate constant for specific base catalysis, kOH, for each oligomer was calculated from the slope of the plot of ko, versus the hydroxide ion concentration (Figure 6). Values of KO, are given in Table II. pH Profiles for Oligomers-The pH profiles for the experimentally determined rate constants for oligomers A, B, and C are shown in Figure 7. The pH profiles are divided into three pH regions to simplify the discussion. The first region is the profile between pHa 1.0 and 3.0.This region exhibits a linear relationship between log ko, and pH, with a slope of -1, indicating that the hydrolysis undergoes specific acid catalysis.9 Rate constants for specific acid catalysis were obtained from the slopes of the plot of ,k versus hydrogen ion concentration. The average micro-rate constant for hydrogen ion catalysis, obtained from the rate constants for specific acid catalysis, was 0.05306 M-' * min-' linkage-'. The second region is the profle between pHs 5.0 and 8.0. This region exhibits a linear relationship between log ,k and pH, with a slope of 1, indicating that the hydrolysis undergoes specific base catalysis. Rate constants for specific base catalysis were obtained fromthe slopes of the plot of,k versus hydroxide ion concentration.The average micro-rate constant for hydroxide ion catalysis, obtained from the rate constanta for specific base catalysis, was 2433.12 M-' min-' linkage-'.
e
am.
I..
OH - ion concentratton X 10
7
Flgure +Plot of ks versus hydroxide ion concentration at 75 "C for oligomers A (O), B (A), and C (0). Table Il-Rate Constant8 for SpecMc Base Catalysis of HydrOly8l8 of Ollgomem A, 6, and C No. of Ester
Rate Constant, M-1 .mi"-'
A B
4
C
8
4708.67 9804.03 14 724.25
Ollgomer
Linkages 6
-
-
-
M
3
5 1
Flgure 7-pH
6.m
6.m
~
.
m
b.6
r.m
r.m
m.m
PH Flgure bPlot of log &.versus pH at 75 "Cfor oligomers A (O), B (A), and C (0). 700 I Journal of Phannaceutkal Sciences Vol. 81, No. 7, July 1992
2
4
I
I
I
3
4
5
6
~
7
._.. T
0
9
PH profiles for oligomers A (O), B (A), and C (0)at 75 "C.
The third region is the profile between pHs 3.0and 5.0.This region shows that the rate of hydrolysis was much higher than that predicted if the only contributions to the overall rate were the hydrogen ion and hydroxide ion catalytic pathways. Effect of C h a rg e -O n the basis of the structures of oligomers A, B, and C and in the absence of any extraneous effect, the hydroxyide ion has an equal chance of hydrolyzing all the ester linkages in a n oligomer. In this case, all micro-rate constants would be equal, and k, would equal the sum of the individual micro-rate constants. If this assumption is true, statistical analysis predicts that the ratio of ko, for oligomer A to Itobs for oligomer B to kobe for oligomer C should be 1:1.5:2.0. Instead, the experimental ratio was 1:2.05:3.07. These data show that alkaline hydrolysis of oligomers of tartrate esters is not controlled solely by a statistical fador of all ester linkages. Oligomer A contains four ester linkages, oligomer B contains six, and oligomer C contains eight. At pH > 6.0,these
compounds are expected to be completely ionized. If so, then the terminal ester linkage would be only two carbons away from the ionized carboxylate group. Subsequent ester linkages would be seven or more carbons away. Because alkaline hydrolysis of a charged tartrate ester is catalyzed by hydroxide ions, the hydrolysis involves a reaction between two ions that are of the same charge. The structures of the oligomerscanjustify disregarding the effect of intermolecular polar transmission on the intermediate ester linkages, because any inductive effect of the carboxylate anion would decrease exponentially along a series of bonds. Also, the mesomeric or electromeric effect of the carboxylate anion can be ruled out because of the lack of resonance or conjugation in the oligomers. Only a field effect of the carboxylate anion, which propagates through the solvent molecules, remains. Compared with the inductive effect, the field effect decreases less rapidly with distance. Because the hydrolysis reactions were carried out in solution, the carboxylate anion would be solvated and its electric charge would be effectively dispersed over a shell of solvent molecules.This polarizationof surrounding solvent molecules would reduce the field effect of the carboxylate anion on the intermediate ester linkages. Because of the spread of this negative charge, the hydroxide ions would be repelled, such repulsion leading to a decrease in the rate of hydrolysis. Apparently, the experimental results indicate that the effect of the negative charge is significantat the terminal ester linkages and that the rate of hydrolysis of the terminal ester linkages can be neglected relative to that of the rest of the ester bonds in the molecule. If this assumption is true, then the effective number of ester bonds accessible for hydrolysis would be two, four, and six in oligomersA, B,and C,respectively, indicating that the statistical factors for the hydrolysis of these compounds would be two, four, and six and that the ratios of specific rate constants should be 1:2:3, in agreement with the experimentally determined ratios of rate constants.
Conclusions The results of this investigation showed that the determination of the specificrate constant of a given polyester species from the terminal slope of a h t s r d e r plot is a useful technique, yielding results not only for uncharged species in
the acidic region but also for charged species in the alkaline region. From the rates of hydrolysis measured, it was shown that the degradation of oligomers obeys pseudo-first-order kinetics. The hydrolysis was catalyzed by hydroxide ion, and the catalytic rate constant increased predictably with the number of ester linkages. However, the rate constant was not proportional to the number of ester linkages in the oligomers. This deviation was explained on the basis of a field effect causing an electrostatic repulsion between the neighboring carboxylate anion and the attacking hydroxide ion. The calculations revealed that the field effect was so great that the ester linkage adjacent to the carboxylate anion did not contribute to the overall rate of hydrolysis. For the polyesters studied, the micro-rate constant for hydroxide ion catalysis for an individual ester linkage not located next to a charged carboxylate anion was 2433.12 M-' min-' * linkage-'. This approach can be used to study the effecta of many variables on polyester hydrolysis, including structural effecta of large polyester compounds. This approach may also be applied to the degradation of other types of polymers containing a repeating labile group located next to another functional group.
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References and Notes 1. Morawetz, H.Polym.Prepr. (Am. Chem. Soc.,Div.Polym. Chem.) 1986,27,62-63. 2. Meyer, J. 2.Phys. Chem. 1909,67,257. 3. Freudenberg, K.; Kuhn, W.; Durr, W.; Steinbrunn, G. Ber. 1930, 63,1510. 4. De Loecker, W.; Smeta, G. J. Polym. Sci. 1959,26,203-216. 5. Barth, V.;Kleeper, E. Polymer 1976,17,787. 6. Daviee, M.C.;Tudor, A.; Melia, C. D.; Hendra, P.; Domb, A. J.; Langer, R. 17th International S p i u m on ControlledReleaee of Bioactive Materials, 1990; 2 c 7. Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1989,78, 78-84. 8. Maniar, M.; Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1989,78, 858-862. 9. Maniar, M.;Kalonia, D.; Simonelli, A. J . Pharm. Sci. 1991,80, 77~82. 10. TheMerckZndex, 11th ed.; Budavari, S.,Ed.; Merck: Rahway, NJ, 1989;p 1433. 11. Remi ton's Pharmaceutical Sciences; Gennaro, A. R., Ed.; Mack%arrton, PA, 1985;p 252. 12. Martin, A.; Swarbrick, J.; Cammarap, A. PhysicalPharmucy, 3rd ed.; Lea and Febiger: Philadelpha, 1983;p 379.
Journal of Pharmaceutical sdeoces/ 709 Vol. 81, No. 7, July 1992
