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New aspects of the aqueous polymerization of methyl methacrylate in the presence of some iron salts and complexes
Eur. Polyrn.J. Vol. 27, No. 2, pp. 209-211, 1991 Printed in Great Britain. All rights reserved
0014-3057/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc
NEW ASPECTS OF THE AQUEOUS POLYMERIZATION OF METHYL METHACRYLATE IN THE PRESENCE OF SOME IRON SALTS A N D COMPLEXES A. B. MOUSTAVA, M. M. H. AVOUB, A. A. ABD EL-HAKIM and A. S. BADRAN Laboratory of Polymers and Pigments, National Research Centre, Dokki, Cairo, Egypt
(Received 10 October 1988; in revisedform 4 April 1990) Abstract--The aqueous polymerization of methyl methacrylate was carried out at 40 and 50° in the absence and in the presence of some iron salts and complexes. It was found that the polymerization is catalysed by the iron salts but inhibited by the iron complexes as the iron is chelated in the latter cases. The average molecular weights of the polymers were found to be highest in the case of iron complexes and least in the case of iron salts. The mechanism of the polymerization is proved to be radical.
INTRODUCTION
0.8 #m. The GPC chromatograms were obtained using a Waters Associates Liquid Chromatograph ALC/GPC 502. The instrument was equipped with a column 16 ft long by 3/8 in diameter packed with styragel having a linear poresize distribution up to 105 A,. The eluent was monitored by a differential refractometer (Waters Associates, Model 401) and an i.r. Analyser (Techmation Ltd). The refractometer was used at an attenuation setting of four. The i.r. detector gave a linear absorbance scale (0.25 absorbance units for full-scale deflection). The wave length was set at 5.8 #m in order to monitor the carbonyl band. A slit width of 2 mm was used. The instrument was operated with CHCI 3 as solvent at a flow rate of 1 ml min-~.
The polymerization of methyl methacrylate ( M M A ) has been carried out in aqueous media by many scientists [1-5] but the mechanism of the reaction has not been exactly settled as some scientists [6] proposed an ionic mechanism and others a radical mechanism. In a previous publication [7], we confirmed the occurrence of a radical mechanism but observed two large broad peaks in the Gel Permeation Chromatograms. This result prompted us to study further the mechanism of the polymerization. EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
Materials Methyl methacrylate (MMA) (Merck Schuchardt) stabilized with 100 ppm hydroquinone was purified by washing with NaOH solution, dried over Na2SO4 and then fractionated using a column of ca 15 theoretical plates; the fraction boiling at 100-100.5 ° was used. Ferrous sulphate, ammonium ferrous sulphate, ferric nitrate, potassium ferricyanide, potassium ferrocyanide and sodium nitroprusside were A.R. products, (BDH). Polymer preparation The polymerization of MMA (4.7 g) was carried out in oxygen-free water (95 ml) in flasks with well-fitting stoppers, at 40 and 50°, in the presence of air or in N 2 using (0.2 g mol/1 sodium bisulphite) as initiator. The polymerization was stopped by addition of a few drops of a 2% hydroquinone solution. The reaction mixture was filtered through a sintered glass funnel (G3), washed several times with distilled water and finally dried at 105°. Viscosity-average molecular weights The intrinsic viscosity It/] for each polymer was obtained by the usual method of extrapolation. The viscosity-average molecular weights were determined in thiophene-free benzene at 25 ° using the equation [8]: [r/] = 0.94 x 10-4/~v 0 " 7 6 . Preparation of polymer solutions for gel permeation chromatography A solution of the polymer in chloroform (AR)(0.5%) was prepared, filtered under pressure using a Sartorius membrane filter (Gm bH 34 Gottingen, F.R.G.) having a pore size of
The present experimental set-up is essentially a precipitation polymerization system in aqueous medium. The nature of such a heterogeneous system was found to depend greatly on the extent of polymerization in such a way that the obtained average molecular weights increased with increase of conversion. The presence of oxygen in the reaction medium gives rise to O H radicals resulting in acceleration of the polymerization relative to that in a N2 atmosphere. Although it has been stated that O H radicals are weak initiators yet the average molecular weights in air are lower than in N2 because of the larger number of radicals. The polymerization was carried out in the presence of various concentrations of ionizable salts of iron (ferrous sulphate, ammonium ferrous sulphate and ferric nitrate) and also chelated iron salt (potassium ferrocyanide, potassium ferricyanide and sodium nitroprusside). It was found that the salts with free iron ions catalyse the polymerization whereas the salts with chelated iron either had very little effect, as in the case of sodium nitroprusside, or a completely inhibiting effect as in the case of potassium ferricyanide. Potassium ferrocyanide was found to increase the rate slightly at the beginning but after 4 hr the conversion was nearly the same as in the case of the absence of any salt; we can explain this effect by postulating that the potassium ferrocyanide shortens the induction period during which the bisulphite radicals are formed. Higher salt concentrations result
209
210
A.B. MOUSTAFAet al.
in an increase of the average molecular weights because of the reduced ionization of the sodium bisulphate and consequent reduced formation of bisulphate radicals. Although the radical nature of bisulphite initiated polymerization has been established and sulphite endgroups have been detected in the resulting polymer, there is no direct evidence for the initiation of MMA polymerization by HSO; (bisulphite radicals) directly derived from NariS03. Moreover, the existence of other end-groups in the polymer, particularly sulphate, was not detected in the freshly prepared polymers. Mechanism o f polymerization
The occurrence of a radical mechanism has been formerly proved by electron spin resonance by (o)
Moustafa et al. [7]. Bimodal distributions have been found in the gel permeation chromatograms [Fig. l (a), (b) and (c)]. The average molecular weights were found to increase with increase of the reaction time. The increase of the average molecular weights with time is not quite linear (Fig. 2). The increase of molecular weight with time may be due to an occlusion phenomenon which can be explained by considering the participation of radicals. The participation of the resulting ferric ions or the direct involvement of ferric salt in the transfer termination reaction during polymerization may well affect the average molecular weights of the resulting polymers and account for the nonlinearity of the increase of molecular weight with reaction time. The viscosity average molecular weights of polyMMAs obtained in the absence and in the presence of ammonium ferrous sulphate, and with ferric nitrate were found to increase with temperature in air (Table 1); this effect is attributed to the lower concentration of the initiator under the reaction condition because of the partial oxidation of the sodium bisulphite by oxygen. In nitrogen atmosphere, the average molecular weights were higher than in air, a result which could be attributed to the greater number of .OH radicals formed in air, few of which are formed in an atmosphere of N2 [6]; in the case of a N 2 atmosphere, the bisulphate radicals are believed to be form thus: CH 3
(b)
CH2 = (~
+ 2H20 + 2HSO~(1)
~OOCH 3 CH 3
I
CH 3 - C H
+ 2OH- + 2HSO 3.
~OOCH 3
K4 [ Fe(CN)6], 40*C, N2 160
/ ^
v/x
X
120
(c)
NO odditiv: 50*C air
'o I~ 80
40
~ I 60
f Fig. 1. Refractive index (RI) and infrared (IR) Absorbance traces of polyMMA.
, L 120
40~C I 180
/~v vs time ( rain )
Fig. 2. git vs time.
I 240
Aqueous polymerization of MMA
211
Table I. Viscosity-averagemolecularweightsof polyMMAsprepared in the absenceand in the presenceof some free and chelated iron ions in air and in N 2 atmospheres Air Time (rain)
60 Inorganic substance None None FeSO4(NH4)2.SO4
FeSO4 Fe(NO3)3
K4(Fe(CN)6)
Convn (mol/I)
Temp. (°C)
--0.01 0.01" 0.02 0.03 0.01 0.01 0.02 0.03 0.01 0.01" 0.02 0.01 0.01 0.01"
40 50 40 40 40 40 50 40 40 40 40 40 40 50 40 40
120
Concn (%) .~'v x 10-4 2.5 3 25 28 33 35 34 27.5 35.7 47.8 21 18 43 22 7.6 4.6
3.0 26.6 4.4 7.7 8.8 11.3 22.5 14.0 19.2 44.0 14.6 14.0 21.4 36.4 10.2 141.5
Convn (%)
180
/14"v x 10-4
Convn (%)
240
/t4v x 10-4
22.5
70.0
31.7
102.5
55.5
31.4
74.0
33.1
65.7
41.3
82.3
43.5
8.8
151.6
12.7
159.0
Convn (%)
/14v x 10 4
40
84.4
81 85 82.5 85 82.5 85.3 87.4 89.2
13.1 39.5 23.9 39.4 25.3 27.2 34.7 82.7
75
2"7.6
90 33
48.2 35.1
*Polymer prepared in nitrogen atmosphere. The reaction o f ferrous salt in aqueous solution with dissolved oxygen is well-known and forms the basis o f some quantitative analyses. In the presence o f a d d e d NaHSO3, a series o f complicated redox processes may be envisaged: Fe +2 + H S O f ---*Fe +3 +
HS(33
H S 0 3 + M ~ / ¢ I ---. H S 0 ; I ~ I - - H S 0 3 + n M ---, I ( , I - - M n - - H S O ; . There are m a n y ideas explaining the existence o f b i m o d a l distribution in G P C , such as the case o f M M A autoacceleration in b o t h heterogeneous and h o m o g e n e o u s radical processes, also in polymers with molecular weights close to the exclusion limit o f the G P C , as well as in the heterogeneous precipitation polymerization. It is also possible that the 105/~ styragel used does n o t provide adequate separation o f all the polymer species o f the average molecular weights u n d e r consideration.
We f o u n d that the average molecular weights increased with time and, as this increase is not quite linear, we p r o p o s e that it could be due to the reaction o f two radicals in the termination process. REFERENCES
1. O. Y. Mansour and A. B. Moustafa. J. Polym. Sci.; Polym. Chem. Edn 13, 27905 (1975). 2. J. V. Breitenbach and H. Preussler. J. Polym. Sci. 4, 754 (1949). 3. A. B. Moustafa. Agnew. Makromolek. Chem. 39, 1-6 (1974). 4. A. B. Moustafa, M. H. Nosseir and N. E. Nashed. Angew. Makromolek. Chem. 52, 71 (1976). 5, T. Yamaguchi, H. Tanaka, T. Ono, M. Endo, H. Ito and O. Itabashi. Kobushi Ronbunsbu 32, 120 (1975). 6. A. R. Mukhorjee, P. Ghosh, S. C. Chadhen and S. R. PAIR. Makromolek. Chem. 80, 208 (1967). 7. A. B. Moustafa, J. R. Ebdon and B. J. Hunt. J. appl. Polym. Sci. 22, 2471 (1978). 8. A. I. Goldberg, W. P. Hohenstein and H. Mark. J. Polym. Sci. 2, 502 (1947).
0014-3057/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc
NEW ASPECTS OF THE AQUEOUS POLYMERIZATION OF METHYL METHACRYLATE IN THE PRESENCE OF SOME IRON SALTS A N D COMPLEXES A. B. MOUSTAVA, M. M. H. AVOUB, A. A. ABD EL-HAKIM and A. S. BADRAN Laboratory of Polymers and Pigments, National Research Centre, Dokki, Cairo, Egypt
(Received 10 October 1988; in revisedform 4 April 1990) Abstract--The aqueous polymerization of methyl methacrylate was carried out at 40 and 50° in the absence and in the presence of some iron salts and complexes. It was found that the polymerization is catalysed by the iron salts but inhibited by the iron complexes as the iron is chelated in the latter cases. The average molecular weights of the polymers were found to be highest in the case of iron complexes and least in the case of iron salts. The mechanism of the polymerization is proved to be radical.
INTRODUCTION
0.8 #m. The GPC chromatograms were obtained using a Waters Associates Liquid Chromatograph ALC/GPC 502. The instrument was equipped with a column 16 ft long by 3/8 in diameter packed with styragel having a linear poresize distribution up to 105 A,. The eluent was monitored by a differential refractometer (Waters Associates, Model 401) and an i.r. Analyser (Techmation Ltd). The refractometer was used at an attenuation setting of four. The i.r. detector gave a linear absorbance scale (0.25 absorbance units for full-scale deflection). The wave length was set at 5.8 #m in order to monitor the carbonyl band. A slit width of 2 mm was used. The instrument was operated with CHCI 3 as solvent at a flow rate of 1 ml min-~.
The polymerization of methyl methacrylate ( M M A ) has been carried out in aqueous media by many scientists [1-5] but the mechanism of the reaction has not been exactly settled as some scientists [6] proposed an ionic mechanism and others a radical mechanism. In a previous publication [7], we confirmed the occurrence of a radical mechanism but observed two large broad peaks in the Gel Permeation Chromatograms. This result prompted us to study further the mechanism of the polymerization. EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
Materials Methyl methacrylate (MMA) (Merck Schuchardt) stabilized with 100 ppm hydroquinone was purified by washing with NaOH solution, dried over Na2SO4 and then fractionated using a column of ca 15 theoretical plates; the fraction boiling at 100-100.5 ° was used. Ferrous sulphate, ammonium ferrous sulphate, ferric nitrate, potassium ferricyanide, potassium ferrocyanide and sodium nitroprusside were A.R. products, (BDH). Polymer preparation The polymerization of MMA (4.7 g) was carried out in oxygen-free water (95 ml) in flasks with well-fitting stoppers, at 40 and 50°, in the presence of air or in N 2 using (0.2 g mol/1 sodium bisulphite) as initiator. The polymerization was stopped by addition of a few drops of a 2% hydroquinone solution. The reaction mixture was filtered through a sintered glass funnel (G3), washed several times with distilled water and finally dried at 105°. Viscosity-average molecular weights The intrinsic viscosity It/] for each polymer was obtained by the usual method of extrapolation. The viscosity-average molecular weights were determined in thiophene-free benzene at 25 ° using the equation [8]: [r/] = 0.94 x 10-4/~v 0 " 7 6 . Preparation of polymer solutions for gel permeation chromatography A solution of the polymer in chloroform (AR)(0.5%) was prepared, filtered under pressure using a Sartorius membrane filter (Gm bH 34 Gottingen, F.R.G.) having a pore size of
The present experimental set-up is essentially a precipitation polymerization system in aqueous medium. The nature of such a heterogeneous system was found to depend greatly on the extent of polymerization in such a way that the obtained average molecular weights increased with increase of conversion. The presence of oxygen in the reaction medium gives rise to O H radicals resulting in acceleration of the polymerization relative to that in a N2 atmosphere. Although it has been stated that O H radicals are weak initiators yet the average molecular weights in air are lower than in N2 because of the larger number of radicals. The polymerization was carried out in the presence of various concentrations of ionizable salts of iron (ferrous sulphate, ammonium ferrous sulphate and ferric nitrate) and also chelated iron salt (potassium ferrocyanide, potassium ferricyanide and sodium nitroprusside). It was found that the salts with free iron ions catalyse the polymerization whereas the salts with chelated iron either had very little effect, as in the case of sodium nitroprusside, or a completely inhibiting effect as in the case of potassium ferricyanide. Potassium ferrocyanide was found to increase the rate slightly at the beginning but after 4 hr the conversion was nearly the same as in the case of the absence of any salt; we can explain this effect by postulating that the potassium ferrocyanide shortens the induction period during which the bisulphite radicals are formed. Higher salt concentrations result
209
210
A.B. MOUSTAFAet al.
in an increase of the average molecular weights because of the reduced ionization of the sodium bisulphate and consequent reduced formation of bisulphate radicals. Although the radical nature of bisulphite initiated polymerization has been established and sulphite endgroups have been detected in the resulting polymer, there is no direct evidence for the initiation of MMA polymerization by HSO; (bisulphite radicals) directly derived from NariS03. Moreover, the existence of other end-groups in the polymer, particularly sulphate, was not detected in the freshly prepared polymers. Mechanism o f polymerization
The occurrence of a radical mechanism has been formerly proved by electron spin resonance by (o)
Moustafa et al. [7]. Bimodal distributions have been found in the gel permeation chromatograms [Fig. l (a), (b) and (c)]. The average molecular weights were found to increase with increase of the reaction time. The increase of the average molecular weights with time is not quite linear (Fig. 2). The increase of molecular weight with time may be due to an occlusion phenomenon which can be explained by considering the participation of radicals. The participation of the resulting ferric ions or the direct involvement of ferric salt in the transfer termination reaction during polymerization may well affect the average molecular weights of the resulting polymers and account for the nonlinearity of the increase of molecular weight with reaction time. The viscosity average molecular weights of polyMMAs obtained in the absence and in the presence of ammonium ferrous sulphate, and with ferric nitrate were found to increase with temperature in air (Table 1); this effect is attributed to the lower concentration of the initiator under the reaction condition because of the partial oxidation of the sodium bisulphite by oxygen. In nitrogen atmosphere, the average molecular weights were higher than in air, a result which could be attributed to the greater number of .OH radicals formed in air, few of which are formed in an atmosphere of N2 [6]; in the case of a N 2 atmosphere, the bisulphate radicals are believed to be form thus: CH 3
(b)
CH2 = (~
+ 2H20 + 2HSO~(1)
~OOCH 3 CH 3
I
CH 3 - C H
+ 2OH- + 2HSO 3.
~OOCH 3
K4 [ Fe(CN)6], 40*C, N2 160
/ ^
v/x
X
120
(c)
NO odditiv: 50*C air
'o I~ 80
40
~ I 60
f Fig. 1. Refractive index (RI) and infrared (IR) Absorbance traces of polyMMA.
, L 120
40~C I 180
/~v vs time ( rain )
Fig. 2. git vs time.
I 240
Aqueous polymerization of MMA
211
Table I. Viscosity-averagemolecularweightsof polyMMAsprepared in the absenceand in the presenceof some free and chelated iron ions in air and in N 2 atmospheres Air Time (rain)
60 Inorganic substance None None FeSO4(NH4)2.SO4
FeSO4 Fe(NO3)3
K4(Fe(CN)6)
Convn (mol/I)
Temp. (°C)
--0.01 0.01" 0.02 0.03 0.01 0.01 0.02 0.03 0.01 0.01" 0.02 0.01 0.01 0.01"
40 50 40 40 40 40 50 40 40 40 40 40 40 50 40 40
120
Concn (%) .~'v x 10-4 2.5 3 25 28 33 35 34 27.5 35.7 47.8 21 18 43 22 7.6 4.6
3.0 26.6 4.4 7.7 8.8 11.3 22.5 14.0 19.2 44.0 14.6 14.0 21.4 36.4 10.2 141.5
Convn (%)
180
/14"v x 10-4
Convn (%)
240
/t4v x 10-4
22.5
70.0
31.7
102.5
55.5
31.4
74.0
33.1
65.7
41.3
82.3
43.5
8.8
151.6
12.7
159.0
Convn (%)
/14v x 10 4
40
84.4
81 85 82.5 85 82.5 85.3 87.4 89.2
13.1 39.5 23.9 39.4 25.3 27.2 34.7 82.7
75
2"7.6
90 33
48.2 35.1
*Polymer prepared in nitrogen atmosphere. The reaction o f ferrous salt in aqueous solution with dissolved oxygen is well-known and forms the basis o f some quantitative analyses. In the presence o f a d d e d NaHSO3, a series o f complicated redox processes may be envisaged: Fe +2 + H S O f ---*Fe +3 +
HS(33
H S 0 3 + M ~ / ¢ I ---. H S 0 ; I ~ I - - H S 0 3 + n M ---, I ( , I - - M n - - H S O ; . There are m a n y ideas explaining the existence o f b i m o d a l distribution in G P C , such as the case o f M M A autoacceleration in b o t h heterogeneous and h o m o g e n e o u s radical processes, also in polymers with molecular weights close to the exclusion limit o f the G P C , as well as in the heterogeneous precipitation polymerization. It is also possible that the 105/~ styragel used does n o t provide adequate separation o f all the polymer species o f the average molecular weights u n d e r consideration.
We f o u n d that the average molecular weights increased with time and, as this increase is not quite linear, we p r o p o s e that it could be due to the reaction o f two radicals in the termination process. REFERENCES
1. O. Y. Mansour and A. B. Moustafa. J. Polym. Sci.; Polym. Chem. Edn 13, 27905 (1975). 2. J. V. Breitenbach and H. Preussler. J. Polym. Sci. 4, 754 (1949). 3. A. B. Moustafa. Agnew. Makromolek. Chem. 39, 1-6 (1974). 4. A. B. Moustafa, M. H. Nosseir and N. E. Nashed. Angew. Makromolek. Chem. 52, 71 (1976). 5, T. Yamaguchi, H. Tanaka, T. Ono, M. Endo, H. Ito and O. Itabashi. Kobushi Ronbunsbu 32, 120 (1975). 6. A. R. Mukhorjee, P. Ghosh, S. C. Chadhen and S. R. PAIR. Makromolek. Chem. 80, 208 (1967). 7. A. B. Moustafa, J. R. Ebdon and B. J. Hunt. J. appl. Polym. Sci. 22, 2471 (1978). 8. A. I. Goldberg, W. P. Hohenstein and H. Mark. J. Polym. Sci. 2, 502 (1947).