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Redox-initiated template polymerization of acrylic acid by means of Fenton's reagent: initiator complexation
Eur. Polym. J. Vol. 25, No. 7/8, pp. 731-735, 1989
0014-3057/89 $3.00+0.00 Maxwell Pergamon Macmillan pie
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REDOX-INITIATED TEMPLATE POLYMERIZATION OF ACRYLIC ACID BY MEA NS OF FENTON'S REAGENT: INITIATOR COMPLEXATION J. FERGUSON and A. EBOATU Fibre and Textile Research Unit, Department of Pure and Applied Chemistry, University of Strathclyde, Scotland
(Received 24 January 1989) Abstract--The polymerization of acrylic acid in the absence and in presence of polyvinylpyrrolidone (PVP) was carried out using a redox initiation system (Fenton's reagent). It was found that at 26° and 74° the rate of polymerization in the absence of template was very high, up to 80% conversion being achieved within 4 min. In the presence of the template, however, there was a pronounced decrease in rate which is directly related to the molar mass of the template--a "negative" chain effect. Rate measurements in the presence of N-methyl pyrrolidone showed that the overall fall in rate could be attributed to complexion of initiator and the pyrrolidone group. This was confirmed by spectroscopic data.
oxygen and pH and low temperature of reaction. The last condition was emphasized by Bandrup and Immergut [6] who reported that the activation energy for production of radicals by redox initiators is about 42 k J mol-1 as compared to 1 2 6 k j mol-1 or more for thermal dissociation of peroxygen compounds. Following the findings of these workers, there was a sudden burst of interest in the use of redox initiators for the polymerization of various unsaturated monomers. Though Evans and Tyrrall [7] reported the polymerization of acrylic acid by using Fenton's reagent in 1947, the application of reduction-activation in template polymerization does not seem to have been described.
INTRODUCTION
Typically, a redox catalyst is made o f a peroxygen compound, e.g. a persulphate, an organic hydroperoxide or hydrogen peroxide, a reducing agent and some water soluble metal salt whose cation has two stable oxidation states, one valence unit apart. The mechanism involved in redox initiation has been rather extensively discussed in the literature, but it is apparent from the propositions of Yorst [1] and Bacon [2] that it is primarily free-radical in nature. The mechanism of initiation using Fenton's reagent (H202/FeSO4/H 2SO 4) apparently involves the oxidation o f Fe 2+ to Fe 3+ by a peroxide, a one electron process which produces free radicals. When hydrogen peroxide is used, the reaction yields a hydroxyl radical, a species capable of interacting with many organic substrates. In the presence of unsaturated compounds, addition may result, thus making the iron (II)---hydrogen peroxide couple not only a hydroxylating reagent but also an effective free radical initiator for vinyl polymerization of water soluble monomers, thus: Fe 2÷ + H20: C H 2 = C H + "OH
I
X
~ Fe 3+ + O H - + "OH , HO--CH2--CH'.
I
X
The reactive nature of the hydroxyl radical as a hydrogen abstractor often makes the product resulting from reactions of Fenton's reagent with organic substrates complex. The first applications of redox catalysts to vinyl polymerization were those of Bacon [3] and Morgan [4] who coined the phrase "reduction-activation for like processes", These authors, together with Baxendale et aL [5] discussed widely the general properties of free radical redox initiation. They indicated that such polymerizations are characterized by high rate, comparatively low molar mass and narrow molar mass distributions of the product, high sensitivity to
EXPERIMENTAL PROCEDURES
Materials Acrylic acid (AA). A freshly distilled and twice crystallized sample was used throughout the experiments. Hydrogen peroxide. 100 volumes hydrogen peroxide (H202) was used directly without further purification. Potassium persulphate was crystallized twice from water and vacuum dried at 30°. Ferrous sulphate. Freshly prepared solutions were used for all experiments. Poly(vinyl pyrrolidone) (PVP). Three samples of different molecular weights--PVPKl5, PVPK30 and PVPK90 (viscosity average molecular weights, -~/v, in H 2 0 at 30° of 10,000, 41,000 and 360,000 respectively) were obtained from G.A.F. (Great Britain) Ltd. A fourth sample was prepared as follows: 250 ml of a 20% w/w aqueous solution of N-vinylpyrrolidone was placed in a reaction vessel, and thermostatted at 50°. The solution was vigorously flushed with dry nitrogen gas. The initiator comprising 2 ml of 30% H202 and 1 ml of concentrated aqueous ammonia, was introduced. The polymerization was continued for 4hr while being stirred. The highly viscous PVP solution which resulted was spread thinly on polythene sheets and dried in a vacuum oven for two days at 40° to constant weight. The dried PVP was stored under a dry atmosphere until required. Its molecular weight (h,7~in H20 at 30°) was found to be 8,0 x 104. 731
732
J. FERGUSON and A. EBOATU
N-methyl pyrrolidone (NMP) from BDH plc was distilled twice. The fraction boiling between 65 ° and 68 ° at 10 mm mercury pressure was collected.
Table I. Overall activation energies (Eo) for blank and template polymerizations using Fenton's reagent
Kinetic measurements
System
The concentrations of the reagents in aqueous solution were [AA] = [PVP] = [NMP] = 0.02 mol.l -I unless otherwise stated. For the initiators [H202] = 0.74 x 10 -2 m o l . l - L The concentration of Fe 2+ co-catalyst was 0.47 x 10 -4 mo1.1 -I and a pH of 1.5 was maintained. Experiments were conducted at 25 ° and 74 ° . A bromometric (iodiometric) technique [8] was employed to determine the extent of conversion of all blank polymerizations and for the polymerization of AA in presence of N-methyl pyrrolidone and PVP. Reaction rates were measured at 10% conversion.
Spectroscopic techniques u.v. Spectra of PVP and FeSO4 solutions were measured using a Perkin Elmer 402 Spectrophotometer. Because of the very strong absorbance of the PVP, concentrations of 0.001 mol.l -t were used, at 20 °. A modified Ubbelohde viscometer was used to measure the reduced viscosity of mixed solution of ferrous sulphate and PVP.
Elemental analysis Elemental analysis of the complex was carried out using the Dumas method. RESULTS AND DISCUSSION
When blank (in the absence of template) polymerization was performed in dilute aqueous solution at 25 ° by means of Fenton's reagent, one striking fact that became apparent was the very high rate of reaction, Fig. l, up to about 83% conversion, when the polymerization virtually ceased. It is interesting to note that Evans and Tyrrall [7] achieved about 84% conversion of AA to polymer in 4 min. Similarly, by using a diaryl iodonium salt/benzoin redox couple, Crivello and Lee [9] were able to obtain up to 80% polymerization of cyclohexane oxide at 25 ° in about 3 min. The very high rates of redox polymerizations generally arise from the fact that free radical production •
~ i l l ----------" i '
80
II •
•
•
70
•
~ 50 "~_ ~ 40 c
8 ~ 30 -~ zo 1() t 0
I 40
I
I 80
Time
I
I 120
I
I 160
~
1 200
(rain)
Fig. 1. Polymerization of AA by Eenton's reagent, i , Blank ([PVP] = 0); O, PVPKI5; - - A - - = PVPK30; I-q, PVPKx; O, PVPK90. [AA] =[PVP] = 0 . 0 5 m o l ' l - I ; [Fe 2+]= 0.47 x 10-4mol.1-1 [H202] = 0.74mo1'1-1; pH = 1.5; temperature = 74 °.
Blank Template Blank with NMP present
eo
kJ mol 0.25 9.6 3.8
[AA] ~ 0.05 mol. I- 1 [PVP] = [NMP] ffi 0.5 tool. I-~ where appropriate.
is as a result of simple electron transfer from the reducing component to the oxidizer partner (here Fe 2+ to H202). Such a transfer would require a comparatively low activation energy of initiation, El. Table 1 shows overall activation energies, Eo, values for both the blank and template reactions. These values are also very much lower than what would be expected from conventional free radical polymerizations. As the propagation and termination reactions are the same it can be concluded that the values for the activation energies of initiation for the redox polymerization are indeed low. The sudden cessation of polymerization before reaction was completed was not found when the redox initiator system K2S2Os/AgNO 3 was used. Similarly in template reactions where PVP was present and low Fe 2+ were employed, the reaction went to 100%. At high Fe concentration, the "inhibition" effect did occur in the latter reactions. The polymer cannot be the cause of the effect or it would have been observed when K2S2Os/AgNO3 was used. Turska and Matuszeuskia [ 1 0 ] measured a K2S2Os/Fe 2+ redox initiator and found a similar inhibition effect. It is known that Fe :+ can act as a free radical scavenger. It is, therefore likely that Fe 2+ was a contributor to, if not the sole cause of, the stoppage of the reaction in its later stages when free radical production had fallen from the high rate at the start of the reaction.
Template polymerization When AA was polymerized with non-redox initiators the template reaction was found to be considerably faster than the blank reaction. However, when Fenton's reagent was used as initiator, Fig. 1, the rate of reaction was considerably reduced in the presence of template. Also, the rate fell as the molecular weight of the template PVP increased. In non-redox initiated template polymerizations the rate is known to increase with template molecular weight [11]. The unusual "chain effect" seems to be a characteristic of the system. It was observed over all temperatures between 25 ° and 74 ° It would, therefore appear to arise from some inherent property of the PVP template. Figure 2 shows the result of the polymerization carried out in the presence of N-methylpyrrolidone, a low molecular weight analogue of PVP. Once again a reduction in rate was found compared to the blank. This shows clearly that the fall in rate is due to the presence of the pyrrolidone ring rather than the presence of the polymer chain. A number of papers have dealt with association between polymers and small molecules. Studies from the point of view of charge transfer phenomena appear to be the most
Polymerization of AA with Fenton's reagent
733
10
100
A 80
P
o
0
%5
g
x 4O
/x
2O
I
I 40
t
} 80 Time
I
I Izo
I
[ 16o
I
I 200
J I
I
I
0.1
Q2
03
[PVP]
(min)
t
( m o L E 1)
Fig. 2. Influence of NMP on the polymerization of AA. O, Blank reaction; x, [NMP]=0.05moI.I-% [AA]= 0.5 mol.l-% [Fe2+]= 0.47 × 10-4mo1.1-% [H202]= 0.74 x 10-2 mol.l-~; pH = 1.5; temperature = 25°.
Fig. 4. The influence of PVPK30 concentration on the rate of polymerization of AA at 25°. Other experimental conditions as in Fig. 2.
numerous. A review of the subject and of the problems involved has been given by Pearson [12]. It is thought that there are generally two types of equilibria involved with such macromolecule/metal ion association, Either the functional group on the polymer interacts with various molecules in the same way as the isolated functional group on the low molecular weight analogue or interaction is exhibited only by the polymer. In the latter case, cooperative chain interaction is necessary while in the former the intermolecular forces are strong enough for this not to be a prerequisite. This would appear to be the case for the interaction of Fe 2÷ and the pyrrolidone ring. Nevertheless, there would seem to be some influence of the chain on the interaction. As Fig. 3 shows, the reduction in rate in the presence of methyl pyrrolidone is greater than that produced by PVP K15 and PVP K30 but less than that when PVP Kx and K90 were used as template. Additionally, Table 1, the Eo value for the polymerization in the presence
of NVP lies between those of the blank and the template reactions. It would appear from this that the presence of the template chain has a significant influence on the polymerization over and above that of the presence of the pyrrolidone ring. The role of the template PVP is illustrated in Fig. 4 which shows the variation of rate of polymerization of AA at constant AA concentration and varying amounts of template PVPK 15. The maximum in rate at equi-base-molar concentrations of AA and template corresponds closely to that found for the same system initiated by K2 $208. It indicates an interaction ratio between the two complexing species of 1: 1. The minimum in Fig. 4 presumably results from the rate of initiation increasing at low PVP concentrations as the amount of complexed Fe 2÷ falls. Nitrogen analysis on the PVP/PAA complex produced a ratio of PVP:PAA repeat units of 0.76:1, the discrepancy almost certainly being due to the presence of Fe in the complex. It was observed that the complex, even after repeated washings, turned brown on exposure to air indicating the presence of Fe 2+. If indeed Fe 2+ was adsorbed by the PVP, this would result in a depletion of the co-catalyst and hence a decrease in the effective initiator concentration (it should be noted that a low Fe 2+ concentration of 0.047 mol.l was used in these experiments). The observed fall in rate of polymerization would then be explained.
3.2 2.8 2.4 1 .0 >~ a , . z 1.6 "6 ~ 1.2
P V P / F e :+ interaction
u.v. Studies. Variation of absorption with wavelength was measuredr for aqueous PVP and Fe 2+ 0.4 solutions. These were then mixed to give the solute concentrations as in the original solutions, allowed to .I equilibrate for 10 rain and then measured also, Fig. 5. 3.5 4.0 4.5 5.0 5.5 6,0 The maximum values for the PVP, Fe 2+ and mixed Log M v Fig. 3. Influence of PVP molecular weight on the rate of solutions were 181,192 and 212 nm respectively, with template polymerization relative to the rate in the presence the Fe 2+ solution exhibiting a shoulder at 240 nm. If of NVP. Reaction conditions as in Fig. 2, [PVP]= no interaction had taken place the peaks for the 0.05 mol.l-~. original components would have persisted. 0,8,
f
o
734
J. FERGUSON and A. EBOATU
3O
90
80
~ 2o L Hixed sotn n,,
lo O
I
I
I
1
2
3
L 4
I 5
VoLume of ferrous suLphote solution (mL) 6O
Fig. 6. Reduced viscosity bchaviour of PVP/FeS04 solutions. A , The critical curves based on additivity; ©, experimental data; temperature= 30°.
PVPso|n
,~
50
water in all cases. However, it can be argued that the joining by bridge formation of two independent parts of the chain is tantamount to extending the chromophore. This, akin to conjugation, would lead to the electronic transitions occurring at longer wavelengths.
Viscometry studies = Fe2+sotn
150
200 250 WAVELEN6TH(nm)
300
Fig. 5. u.v. Spectra of solutions.
The shift of the maximum value to a longer wavelength, i.e. a red or bathochromic shift, is usually produced by a change in the medium or by the presence of an auxochrome [12]. The medium was (o)
The reduced viscosities of solutions of PVP, Fe 2+ and mixtures of the solutions were measured over a range of concentration at 30 °. A hypothetical curve was then constructed assuming that no interaction between the solute molecules in the mixture would mean that their reduced viscosities would be additive. When compared with experimentally determined reduced viscosities for the mixture, Fig. 6, the two curves are seen to depart substantially from each other, indicating a definite interaction. The fact that the experimental values of reduced viscosity were lower than the additive leads to the conclusion that the interaction results in a coiling of the PVP chain so that hydrodynamic volume is decreased. A bridging structure with the Fe 2÷ drawing the PVP chains closer together would produce such an effect. The chemistry of the Fe 2÷ ion and the structure of PVP would suggest the possibility of the formation of (b)
!
I
. . . .
'.m..f--
L.J
Possible structure of Po|y(vinyL pyrrolidone)-Fe2+ complex
r
'
Poty(8-hydroxyquinoline diy1-7,5-efhytene)-rnel'nl complex Fig. 7. P o l y m e r - m e t a l ion coordination structures.
Polymerization of AA with Fenton's reagent a coordination complex between the two. Fe 2÷ has vacant d-orbitals characteristic of the first transition metal series. They are available for electron donation by suitable ligands. The pyrrolidone ring is a bidentate ligand, a chelating species, and there is every likelihood that in an aqueous environment a coordination complex is feasible to convert the hexaquo or sulphate ferrate (II) ions to either a square planar, or more likely, an octahedral iron-pyrrolidone complex. Such adducts are not uncommon. The chelating properties of poly(8-hydroxyquinol-7,5-diylethylene), (PQDE) have been studied, Fig. 7. This polymer is very similar to PVP. Patel and Patel [13] have given a list of metals from copper to zinc with which the quinoline ring complexes. It is possible to construct a possible complex between PVP and Fe 2÷, Fig. 7, based on the close resemblance between PVP and PQDE. Such a one-to-two metal to pyrrolidone ring structure has been found by Bekturov et al. [14] between bivalent cupric ions and PVP and NVP copolymers. In conclusion it would seem that the polymerization of AA, initiated by Fenton's reagent, in the presence of PVP, is a template reaction rather similar to the same reaction using K2S20 8 as initiator. How-
735
ever, the reaction is complicated by the tendency for the PVP to complex the Fe 2+. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
D. M. Yost. J. Am. chem. Soc. 48, 152 (1926). R. G. R. Bacon. Quarterly Rev. (London)9, 287 (1955). R. G. R. Bacon. Trans. Faraday Soc. 42, 140 (1946). L. B. Morgan. Trans. Faraday Soc. 42, 169 (1946). J. H. Baxendale, M. G. Evans and G. S. Park. Trans. Faraday Soc. 42, 155 (1946). J. Bandrup and H. Immergut. Polymer Handbook, Vol. II, p. 70. Interscience (1967). A. G. Evans and E. Tyrrall. J. Polym. Sci. 2, 387 (1947). J. Ferguson and S. A. O. Shah. Eur. Polym. J. 4, 343 (1968). J. V. Crivello and J. L. Lee. J. Polym. Sci.; Polym. Chem. Edn 21, 1109 (1983). E. Turska and J. Matuszeuskia. Makromolek. Chem. 104, 167 (1967). J. Ferguson, S. Al-Alawi and R. Granmeyah. Eur. Polym. J. 19, 475 (1983). D. W. Williams and I. Fleming. Spectroscopic Methods in Organic Chemistry, p. 10. McGraw-Hill (1980). B. S. Patel and S. R. Patel. Macromolek. Chem. 180, 1159 (1979). E. A. Bekturov, S. E. Kudaibergenov, G. S. Kanapyanova and A. A. Kurmanbaeva. Polym. Comm. 25, 220 (1984).
0014-3057/89 $3.00+0.00 Maxwell Pergamon Macmillan pie
Printed in Great Britain
REDOX-INITIATED TEMPLATE POLYMERIZATION OF ACRYLIC ACID BY MEA NS OF FENTON'S REAGENT: INITIATOR COMPLEXATION J. FERGUSON and A. EBOATU Fibre and Textile Research Unit, Department of Pure and Applied Chemistry, University of Strathclyde, Scotland
(Received 24 January 1989) Abstract--The polymerization of acrylic acid in the absence and in presence of polyvinylpyrrolidone (PVP) was carried out using a redox initiation system (Fenton's reagent). It was found that at 26° and 74° the rate of polymerization in the absence of template was very high, up to 80% conversion being achieved within 4 min. In the presence of the template, however, there was a pronounced decrease in rate which is directly related to the molar mass of the template--a "negative" chain effect. Rate measurements in the presence of N-methyl pyrrolidone showed that the overall fall in rate could be attributed to complexion of initiator and the pyrrolidone group. This was confirmed by spectroscopic data.
oxygen and pH and low temperature of reaction. The last condition was emphasized by Bandrup and Immergut [6] who reported that the activation energy for production of radicals by redox initiators is about 42 k J mol-1 as compared to 1 2 6 k j mol-1 or more for thermal dissociation of peroxygen compounds. Following the findings of these workers, there was a sudden burst of interest in the use of redox initiators for the polymerization of various unsaturated monomers. Though Evans and Tyrrall [7] reported the polymerization of acrylic acid by using Fenton's reagent in 1947, the application of reduction-activation in template polymerization does not seem to have been described.
INTRODUCTION
Typically, a redox catalyst is made o f a peroxygen compound, e.g. a persulphate, an organic hydroperoxide or hydrogen peroxide, a reducing agent and some water soluble metal salt whose cation has two stable oxidation states, one valence unit apart. The mechanism involved in redox initiation has been rather extensively discussed in the literature, but it is apparent from the propositions of Yorst [1] and Bacon [2] that it is primarily free-radical in nature. The mechanism of initiation using Fenton's reagent (H202/FeSO4/H 2SO 4) apparently involves the oxidation o f Fe 2+ to Fe 3+ by a peroxide, a one electron process which produces free radicals. When hydrogen peroxide is used, the reaction yields a hydroxyl radical, a species capable of interacting with many organic substrates. In the presence of unsaturated compounds, addition may result, thus making the iron (II)---hydrogen peroxide couple not only a hydroxylating reagent but also an effective free radical initiator for vinyl polymerization of water soluble monomers, thus: Fe 2÷ + H20: C H 2 = C H + "OH
I
X
~ Fe 3+ + O H - + "OH , HO--CH2--CH'.
I
X
The reactive nature of the hydroxyl radical as a hydrogen abstractor often makes the product resulting from reactions of Fenton's reagent with organic substrates complex. The first applications of redox catalysts to vinyl polymerization were those of Bacon [3] and Morgan [4] who coined the phrase "reduction-activation for like processes", These authors, together with Baxendale et aL [5] discussed widely the general properties of free radical redox initiation. They indicated that such polymerizations are characterized by high rate, comparatively low molar mass and narrow molar mass distributions of the product, high sensitivity to
EXPERIMENTAL PROCEDURES
Materials Acrylic acid (AA). A freshly distilled and twice crystallized sample was used throughout the experiments. Hydrogen peroxide. 100 volumes hydrogen peroxide (H202) was used directly without further purification. Potassium persulphate was crystallized twice from water and vacuum dried at 30°. Ferrous sulphate. Freshly prepared solutions were used for all experiments. Poly(vinyl pyrrolidone) (PVP). Three samples of different molecular weights--PVPKl5, PVPK30 and PVPK90 (viscosity average molecular weights, -~/v, in H 2 0 at 30° of 10,000, 41,000 and 360,000 respectively) were obtained from G.A.F. (Great Britain) Ltd. A fourth sample was prepared as follows: 250 ml of a 20% w/w aqueous solution of N-vinylpyrrolidone was placed in a reaction vessel, and thermostatted at 50°. The solution was vigorously flushed with dry nitrogen gas. The initiator comprising 2 ml of 30% H202 and 1 ml of concentrated aqueous ammonia, was introduced. The polymerization was continued for 4hr while being stirred. The highly viscous PVP solution which resulted was spread thinly on polythene sheets and dried in a vacuum oven for two days at 40° to constant weight. The dried PVP was stored under a dry atmosphere until required. Its molecular weight (h,7~in H20 at 30°) was found to be 8,0 x 104. 731
732
J. FERGUSON and A. EBOATU
N-methyl pyrrolidone (NMP) from BDH plc was distilled twice. The fraction boiling between 65 ° and 68 ° at 10 mm mercury pressure was collected.
Table I. Overall activation energies (Eo) for blank and template polymerizations using Fenton's reagent
Kinetic measurements
System
The concentrations of the reagents in aqueous solution were [AA] = [PVP] = [NMP] = 0.02 mol.l -I unless otherwise stated. For the initiators [H202] = 0.74 x 10 -2 m o l . l - L The concentration of Fe 2+ co-catalyst was 0.47 x 10 -4 mo1.1 -I and a pH of 1.5 was maintained. Experiments were conducted at 25 ° and 74 ° . A bromometric (iodiometric) technique [8] was employed to determine the extent of conversion of all blank polymerizations and for the polymerization of AA in presence of N-methyl pyrrolidone and PVP. Reaction rates were measured at 10% conversion.
Spectroscopic techniques u.v. Spectra of PVP and FeSO4 solutions were measured using a Perkin Elmer 402 Spectrophotometer. Because of the very strong absorbance of the PVP, concentrations of 0.001 mol.l -t were used, at 20 °. A modified Ubbelohde viscometer was used to measure the reduced viscosity of mixed solution of ferrous sulphate and PVP.
Elemental analysis Elemental analysis of the complex was carried out using the Dumas method. RESULTS AND DISCUSSION
When blank (in the absence of template) polymerization was performed in dilute aqueous solution at 25 ° by means of Fenton's reagent, one striking fact that became apparent was the very high rate of reaction, Fig. l, up to about 83% conversion, when the polymerization virtually ceased. It is interesting to note that Evans and Tyrrall [7] achieved about 84% conversion of AA to polymer in 4 min. Similarly, by using a diaryl iodonium salt/benzoin redox couple, Crivello and Lee [9] were able to obtain up to 80% polymerization of cyclohexane oxide at 25 ° in about 3 min. The very high rates of redox polymerizations generally arise from the fact that free radical production •
~ i l l ----------" i '
80
II •
•
•
70
•
~ 50 "~_ ~ 40 c
8 ~ 30 -~ zo 1() t 0
I 40
I
I 80
Time
I
I 120
I
I 160
~
1 200
(rain)
Fig. 1. Polymerization of AA by Eenton's reagent, i , Blank ([PVP] = 0); O, PVPKI5; - - A - - = PVPK30; I-q, PVPKx; O, PVPK90. [AA] =[PVP] = 0 . 0 5 m o l ' l - I ; [Fe 2+]= 0.47 x 10-4mol.1-1 [H202] = 0.74mo1'1-1; pH = 1.5; temperature = 74 °.
Blank Template Blank with NMP present
eo
kJ mol 0.25 9.6 3.8
[AA] ~ 0.05 mol. I- 1 [PVP] = [NMP] ffi 0.5 tool. I-~ where appropriate.
is as a result of simple electron transfer from the reducing component to the oxidizer partner (here Fe 2+ to H202). Such a transfer would require a comparatively low activation energy of initiation, El. Table 1 shows overall activation energies, Eo, values for both the blank and template reactions. These values are also very much lower than what would be expected from conventional free radical polymerizations. As the propagation and termination reactions are the same it can be concluded that the values for the activation energies of initiation for the redox polymerization are indeed low. The sudden cessation of polymerization before reaction was completed was not found when the redox initiator system K2S2Os/AgNO 3 was used. Similarly in template reactions where PVP was present and low Fe 2+ were employed, the reaction went to 100%. At high Fe concentration, the "inhibition" effect did occur in the latter reactions. The polymer cannot be the cause of the effect or it would have been observed when K2S2Os/AgNO3 was used. Turska and Matuszeuskia [ 1 0 ] measured a K2S2Os/Fe 2+ redox initiator and found a similar inhibition effect. It is known that Fe :+ can act as a free radical scavenger. It is, therefore likely that Fe 2+ was a contributor to, if not the sole cause of, the stoppage of the reaction in its later stages when free radical production had fallen from the high rate at the start of the reaction.
Template polymerization When AA was polymerized with non-redox initiators the template reaction was found to be considerably faster than the blank reaction. However, when Fenton's reagent was used as initiator, Fig. 1, the rate of reaction was considerably reduced in the presence of template. Also, the rate fell as the molecular weight of the template PVP increased. In non-redox initiated template polymerizations the rate is known to increase with template molecular weight [11]. The unusual "chain effect" seems to be a characteristic of the system. It was observed over all temperatures between 25 ° and 74 ° It would, therefore appear to arise from some inherent property of the PVP template. Figure 2 shows the result of the polymerization carried out in the presence of N-methylpyrrolidone, a low molecular weight analogue of PVP. Once again a reduction in rate was found compared to the blank. This shows clearly that the fall in rate is due to the presence of the pyrrolidone ring rather than the presence of the polymer chain. A number of papers have dealt with association between polymers and small molecules. Studies from the point of view of charge transfer phenomena appear to be the most
Polymerization of AA with Fenton's reagent
733
10
100
A 80
P
o
0
%5
g
x 4O
/x
2O
I
I 40
t
} 80 Time
I
I Izo
I
[ 16o
I
I 200
J I
I
I
0.1
Q2
03
[PVP]
(min)
t
( m o L E 1)
Fig. 2. Influence of NMP on the polymerization of AA. O, Blank reaction; x, [NMP]=0.05moI.I-% [AA]= 0.5 mol.l-% [Fe2+]= 0.47 × 10-4mo1.1-% [H202]= 0.74 x 10-2 mol.l-~; pH = 1.5; temperature = 25°.
Fig. 4. The influence of PVPK30 concentration on the rate of polymerization of AA at 25°. Other experimental conditions as in Fig. 2.
numerous. A review of the subject and of the problems involved has been given by Pearson [12]. It is thought that there are generally two types of equilibria involved with such macromolecule/metal ion association, Either the functional group on the polymer interacts with various molecules in the same way as the isolated functional group on the low molecular weight analogue or interaction is exhibited only by the polymer. In the latter case, cooperative chain interaction is necessary while in the former the intermolecular forces are strong enough for this not to be a prerequisite. This would appear to be the case for the interaction of Fe 2÷ and the pyrrolidone ring. Nevertheless, there would seem to be some influence of the chain on the interaction. As Fig. 3 shows, the reduction in rate in the presence of methyl pyrrolidone is greater than that produced by PVP K15 and PVP K30 but less than that when PVP Kx and K90 were used as template. Additionally, Table 1, the Eo value for the polymerization in the presence
of NVP lies between those of the blank and the template reactions. It would appear from this that the presence of the template chain has a significant influence on the polymerization over and above that of the presence of the pyrrolidone ring. The role of the template PVP is illustrated in Fig. 4 which shows the variation of rate of polymerization of AA at constant AA concentration and varying amounts of template PVPK 15. The maximum in rate at equi-base-molar concentrations of AA and template corresponds closely to that found for the same system initiated by K2 $208. It indicates an interaction ratio between the two complexing species of 1: 1. The minimum in Fig. 4 presumably results from the rate of initiation increasing at low PVP concentrations as the amount of complexed Fe 2÷ falls. Nitrogen analysis on the PVP/PAA complex produced a ratio of PVP:PAA repeat units of 0.76:1, the discrepancy almost certainly being due to the presence of Fe in the complex. It was observed that the complex, even after repeated washings, turned brown on exposure to air indicating the presence of Fe 2+. If indeed Fe 2+ was adsorbed by the PVP, this would result in a depletion of the co-catalyst and hence a decrease in the effective initiator concentration (it should be noted that a low Fe 2+ concentration of 0.047 mol.l was used in these experiments). The observed fall in rate of polymerization would then be explained.
3.2 2.8 2.4 1 .0 >~ a , . z 1.6 "6 ~ 1.2
P V P / F e :+ interaction
u.v. Studies. Variation of absorption with wavelength was measuredr for aqueous PVP and Fe 2+ 0.4 solutions. These were then mixed to give the solute concentrations as in the original solutions, allowed to .I equilibrate for 10 rain and then measured also, Fig. 5. 3.5 4.0 4.5 5.0 5.5 6,0 The maximum values for the PVP, Fe 2+ and mixed Log M v Fig. 3. Influence of PVP molecular weight on the rate of solutions were 181,192 and 212 nm respectively, with template polymerization relative to the rate in the presence the Fe 2+ solution exhibiting a shoulder at 240 nm. If of NVP. Reaction conditions as in Fig. 2, [PVP]= no interaction had taken place the peaks for the 0.05 mol.l-~. original components would have persisted. 0,8,
f
o
734
J. FERGUSON and A. EBOATU
3O
90
80
~ 2o L Hixed sotn n,,
lo O
I
I
I
1
2
3
L 4
I 5
VoLume of ferrous suLphote solution (mL) 6O
Fig. 6. Reduced viscosity bchaviour of PVP/FeS04 solutions. A , The critical curves based on additivity; ©, experimental data; temperature= 30°.
PVPso|n
,~
50
water in all cases. However, it can be argued that the joining by bridge formation of two independent parts of the chain is tantamount to extending the chromophore. This, akin to conjugation, would lead to the electronic transitions occurring at longer wavelengths.
Viscometry studies = Fe2+sotn
150
200 250 WAVELEN6TH(nm)
300
Fig. 5. u.v. Spectra of solutions.
The shift of the maximum value to a longer wavelength, i.e. a red or bathochromic shift, is usually produced by a change in the medium or by the presence of an auxochrome [12]. The medium was (o)
The reduced viscosities of solutions of PVP, Fe 2+ and mixtures of the solutions were measured over a range of concentration at 30 °. A hypothetical curve was then constructed assuming that no interaction between the solute molecules in the mixture would mean that their reduced viscosities would be additive. When compared with experimentally determined reduced viscosities for the mixture, Fig. 6, the two curves are seen to depart substantially from each other, indicating a definite interaction. The fact that the experimental values of reduced viscosity were lower than the additive leads to the conclusion that the interaction results in a coiling of the PVP chain so that hydrodynamic volume is decreased. A bridging structure with the Fe 2÷ drawing the PVP chains closer together would produce such an effect. The chemistry of the Fe 2÷ ion and the structure of PVP would suggest the possibility of the formation of (b)
!
I
. . . .
'.m..f--
L.J
Possible structure of Po|y(vinyL pyrrolidone)-Fe2+ complex
r
'
Poty(8-hydroxyquinoline diy1-7,5-efhytene)-rnel'nl complex Fig. 7. P o l y m e r - m e t a l ion coordination structures.
Polymerization of AA with Fenton's reagent a coordination complex between the two. Fe 2÷ has vacant d-orbitals characteristic of the first transition metal series. They are available for electron donation by suitable ligands. The pyrrolidone ring is a bidentate ligand, a chelating species, and there is every likelihood that in an aqueous environment a coordination complex is feasible to convert the hexaquo or sulphate ferrate (II) ions to either a square planar, or more likely, an octahedral iron-pyrrolidone complex. Such adducts are not uncommon. The chelating properties of poly(8-hydroxyquinol-7,5-diylethylene), (PQDE) have been studied, Fig. 7. This polymer is very similar to PVP. Patel and Patel [13] have given a list of metals from copper to zinc with which the quinoline ring complexes. It is possible to construct a possible complex between PVP and Fe 2÷, Fig. 7, based on the close resemblance between PVP and PQDE. Such a one-to-two metal to pyrrolidone ring structure has been found by Bekturov et al. [14] between bivalent cupric ions and PVP and NVP copolymers. In conclusion it would seem that the polymerization of AA, initiated by Fenton's reagent, in the presence of PVP, is a template reaction rather similar to the same reaction using K2S20 8 as initiator. How-
735
ever, the reaction is complicated by the tendency for the PVP to complex the Fe 2+. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
D. M. Yost. J. Am. chem. Soc. 48, 152 (1926). R. G. R. Bacon. Quarterly Rev. (London)9, 287 (1955). R. G. R. Bacon. Trans. Faraday Soc. 42, 140 (1946). L. B. Morgan. Trans. Faraday Soc. 42, 169 (1946). J. H. Baxendale, M. G. Evans and G. S. Park. Trans. Faraday Soc. 42, 155 (1946). J. Bandrup and H. Immergut. Polymer Handbook, Vol. II, p. 70. Interscience (1967). A. G. Evans and E. Tyrrall. J. Polym. Sci. 2, 387 (1947). J. Ferguson and S. A. O. Shah. Eur. Polym. J. 4, 343 (1968). J. V. Crivello and J. L. Lee. J. Polym. Sci.; Polym. Chem. Edn 21, 1109 (1983). E. Turska and J. Matuszeuskia. Makromolek. Chem. 104, 167 (1967). J. Ferguson, S. Al-Alawi and R. Granmeyah. Eur. Polym. J. 19, 475 (1983). D. W. Williams and I. Fleming. Spectroscopic Methods in Organic Chemistry, p. 10. McGraw-Hill (1980). B. S. Patel and S. R. Patel. Macromolek. Chem. 180, 1159 (1979). E. A. Bekturov, S. E. Kudaibergenov, G. S. Kanapyanova and A. A. Kurmanbaeva. Polym. Comm. 25, 220 (1984).