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Thiourea as a transfer agent in the radical polymerization of methyl methacrylate in aqueous solution at 42
lb..pLan V.l~mcl J.m'nul Vol 16. pp. 895 to ~99 © Pergamon Press Lid Igxo Prinl,'d in Grcal Britain
(X)I4-3057/XO/0901-0895502 0(|/0
T H I O U R E A AS A T R A N S F E R A G E N T I N T H E R A D I C A L POLYMERIZATION OF METHYL METHACRYLATE I N A Q U E O U S S O L U T I O N A T 42 °* D. PRAMANICKand A. K. CHATTER.lEE Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal, India
(Received 2 January 1980) Abstract Chain transfer involving thiourea in radical polymerization of methyl methacrylate in acidic aqueous media has been studied by polymer endgroup analysis using the dye-partition technique. Thiourea has feeble reactivity in chain transfer, the transfer constant with respect to poly (methyl methacrylate) radicals being 1.21 x 10 -4 at 42 °. This chain transfer study led to the development of a new method for studying the tautomeric equilibrium between the thione and thiol forms of thiourea. The equilibrium is pH dependent and the equilibrium constant at 42 ° is 232. The ratios of the equilibrium concentrations of the thiol to thione forms of thiourea at various pH's have been calculated. The thiol form is responsible for the chain transfer reactivity; it predominates in strongly acidic media and is almost absent above pH 3.5. This new method may be used for studying the thione-thiol tautomerism of other thiourea derivates.
INTRODUCTION
Thiourea [ l ] exists in a tautomeric equilibrium between the thione (SU) and thioi (HSU) forms and is considered as a potential thiol. Since thiols are potent transfer agents in radical polymerizations [2], thiourea is expected to participate in chain transfer during radical polymerization. Degradative chain transfer by thiourea in the photosensitized polymerization of acrylic monomers has been reported [3] from kinetic measurements. Recently we described [4] preliminary observations on ceric-thiourea initiated acid aqueous polymerization of methyl methacrylate in which transfer involving thiourea was indicated. An essential prerequisite to a full understanding of the mechanism of polymerization by this initiator system is a knowledge of the behaviour as a transfer agent of thioure& We have therefore undertaken a study of thiourea in a typical radical polymerization in acid aqueous media. Palit's dye-partition technique [5] has been employed for the determination of endgroups derived from the thiourea. The theoretical basis of this method has been discussed [6] and will be only briefly outlined here. If x is the concentration in g. l-1 of the test polymer solution for endgroup analysis and y is the concentration of end-group in tool l-a in the solution to be determined experimentally, then the chain transfer constant, Ctr, is given by [M] my [HSU] x
c,~ = - - . - -
(1)
where [ H S U ] is the equilibrium concentration of the thiol form of thiourea, [M] is the monomer concentration, and m is the molecular weight of the monomer. Thus simply determining .v experimentally and using the values of [M], [ H S U ] , m and x, C,r can be found out. As will be seen later, this treatment led to the development of a method for studying the tau* Presented at the International Symposium of Polymer Science, Madras, India (1980). 895
tomeric equilibrium of thiourea or more generally thione-thiol tautomerism. Methyl methacrylate (MMA) and azobisisobutyronitrile (AIBN) were chosen as monomer and initiator respectively, because they have negligible transfer constants and because poly(MMA) can be purified thoroughly. EXPERIMENTAL
Materials Monomeric MMA (BDH) was purified by the usual procedure and AIBN (Analytical reagent grade E. Merck product) was repeatedly crystallized from methanol. Thiourea (E. Merck; analytical reagent grade) was triply crystallized from distilled water to a m.p. of 180°. Since AIBN is sparingly soluble in water, a 1 M solution was prepared in a methanol-water mixture (3:1) and kept at low temperature.
Polymerization procedure Aqueous polymerization of freshly distilled MMA was carried out in stoppered pyrex flasks under Nz. The pH of the reaction medium was controlled by addition of H2SO4. Thiols are susceptible to oxidation by air and so, to avoid consumption of thiourea by 02, the reaction medium was thoroughly deaerated by flushing with purified N2 for about 30 min. The reaction flask was then placed in a water bath at 42 4- 0.05 °. Polymerization was stopped by hydroquinone before 10% conversion. The polymers were washed several times with distilled water and dried in a vacuum oven at 45 °. The dried samples were then rigorously purified by the usual procedure [7].
End-group estimation The thiol form of thiourea is expected to lead to aminebearing end-groups in poly(MMA) by chain-transfer. Hence analysis for amine-bearing end-groups was carried out following the modified dye partition (extrapolation [8]) method using disulphine blue VN 150 as the dye reagent. The principle of this method is that the amine-bearing endgroup in the polymer dissolved in an organic medium extracts an equivalent amount of the dye from the aqueous phase into the organic phase which becomes coloured. In
896
D. PRAMAN|CK and A. K. CHATTERJEE
the modified method, the polymer solution in chloroform is shaken with an equal volume of the aqueous dye reagent until dye-partition between the organic and aqueous phase is complete. For each polymer concentration, four or five dye-partition experiments are performed varying only the aqueous dye concentration. The reciprocal of the absorbance of the organic layer is then plotted against the reciprocal of the dye concentration in the aqueous phase at equilibrium. From the intercepts of such plots, the concentration of the amine or amine bearing end-groups in the polymer was determined. Absorbance of the test solutions was measured in a Hilger u.v, spectrophotometer.
solution thiourea is monoprotonated on the imminonitrogen, the true equilibrium which exists in acidic aqueous solution of thiourea is regarded as: +
NH2
/ \
K
S----C
+ H + ~-~ H ~ 4 2
// \
NH2
NH2 (3) NH2
where K is the tautomeric equilibrium constant, given by K -
[HSU]~ [SU]¢[H +],,
Determination of the number average degree of polymerization To compare the results of chain transfer obtained by the dye partition method with those obtained from the conventional degree of polymerization method, viscosity measurements for polymer solutions in benzene were carried out at 25 +__0.05° using an Ubbelohde dilution viscometer with large ett~ux times and negligible kinetic energy corrections. Intrinsic viscosities, [r/], were determined by extrapolation. The number average degrees of polymerization (P.) poly(MMA) samples were obtained using the equation I-9]: P. = 2.81 x 103 X l'~] 1"32
Here the subscript e refers to equilibrium concentration. According to our concept of chain transfer by thiols, only HSU in the equilibrium mixture may be effective as a transfer agent. Hence [ H S U ] in Eqns (1) and (2) is to be considered as [HSU]~. This is to be expressed in terms of total concentrations of thiourea, [SU], from eqns (4) and (5)
in benzene at 25 ± 0.05 °. Another relation [10]:
and is given by
[SU] = [HSU]~ + [SU]~
P , = 2.22 x 10311']] 1/0'76
1 P,
1 =
C [HSU]
Poo +
" ~
(6)
1
I + - K[H+], Combining Eqn (6) with Eqn (1) and (2), we obtain Eqns (7) and (8), m being replaced by 100 for MMA.
(2)
where P0 is the number average degree of polymerization of polymer obtained in the absence of thiourea and is a function of concentrations of monomer, initiator and any other component of the' system. The contribution to chain transfer by methanol, present in all cases at a fixed concentration for dissolution of AIBN in water, is allowed for in
(5)
[SU]
[HSU], =
in benzene at 30 + 0.05 ° was found to give nearly identical results (data not given). C,r values could be calculated from Mayo's equation,
(4)
[SU].
x 1 1 - 100y - K ' C , r [H+]~
[M]
+
1 -C,f
(7)
and 1
I
P.
-
Po
1
+
1
Po-
1
•
[su]
[M]
(8)
K.C,r. [H+],. + RESULTS AND DISCUSSION
Tautomeric equilibrium in thiourea between the thione (SU) and thiol (HSU) forms is represented as:
/
NH2
NH
K.C
~- H S - C ~
X"NH~
At fixed pH, [H+]¢ of the solution being fixed, the quantity
""NH~
It has been suggested [1] that the formation of H S U from SU is dependent on pH. Since in acid aqueous
.[H÷]o
+
of Eqns (7) and (8) should be a constant, because K and C,, are constants at a constant temperature. Results for chain transfer by thiourea in the aqueous polymerization of M M A by AIBN from dye partition tests are shown in Table 1. Values of P , for
Table 1. Chain transfer results for thiourea in the aqueous polymerization* of MMA by AIBN from dye partition and degree of polymerization measurements
I,SU] (tool- 1- i )
Conc of polymer soln (x) (g. 1-1 )
0.1 0.2 0.4 0.5 0.7 0.0
1.8 1.8 1.8 1.4 1.4 --
Conc of amine-bearing end-group (~) (~ 0,)(moll -l x 106) x 103 1.61 4.96 7.59 7.66 11.97 --
0.208 0.371 0.581 0.698 0.865 0.170
+
1 ¢)10_ 4 K.C,~-[H+] x 1.193 0.772 1.009 0.970 0.872 --
* Reaction condition: I,MMA] = 0.094 mol 1-1 ; I,AIBN] = 10 -2 mol 1-1 ; pH 1.5, temp. 42°; and I,CH3OH] = 1.01 mol 1- t.
Thiourea as a transfer agent t.0
I
I
I
I
]
897 ~
I
/
0.9
0.8
0.7
0.6
x
0.5
0.4
0.5 0.2
./
J
0.1
0.0
[
I
I
I
~
I
I
o.,
o.z
0.3
0.4
o.s
0.6
0.7
0.8
[s u] M
Fig. 1. Variation of Pn with thiourea concentration in the radical polymerization of MMA in acid aqueous medium. the polymers as obtained by viscometry are also included in the table. Figure 1 shows the same results according to the Mayo plot. Both end-group and degree of polymerization methods necessarily gave 1
values as obtained according to Eqns (7) and (8). Dyepartition results in Table I shows that this quantity is approximately constant, as supported by the linearity of the Mayo plot (Fig. 1). These results indicate that equilibrium (4) really exists. The average value of K.C,~[H+], + obtained from dye partition tests is 0.963 x 104 and the same value obtained from the slope of the Mayo plot is 1.018 x 104; the agreement is fairly satisfactory. This shows again that the dye partition method of determining the chain transfer property is as efficient as the standard degree of polymerization method with the added advantage of avoiding viscosity measurements which may be m error for low molecular weight polymers. As discussed above, chain transfer involving thiourea will produce a polymer molecule with the end structure + NH2
//
In the calculation of end-group concentration (y), it has been assumed that only one dye molecule binds with this end-group. This assumption is justified in view of the fact that in acid solutions of thiourea only the immino-nitrogen is protonated [11], and so only the immino-nitrogen is sufficiently basic to bind with the dye. That only one dye molecule binds with the
//
NH
\ NH2 end-group has been found for polymers obtained by polymerization of MMA initiated by Ce4+-thiourea system [12]. To discover to what extent thiourea takes part in chain transfer during the radical polymerization of MMA in acid aqueous solution, it is necessary to know the separate values of C,r and K. Since the concentration of HSU in the reaction medium depends upon [H ÷ ]c, the extent of chain transfer by thiourea will depend upon the pH of the medium. Hence radical polymerization of MMA in the presence of thiourea has been carried out at various pH's keeping all other conditions unaltered. Since the end-group method and the degree of polymerization method give almost identical results on chain transfer, only the end-group method has been used here and the results are shown in Table 2. It is seen that, for pH between 0.5 and 2.5,
• ," S----C
\ NH2
[SU] x 1 11 [M] l O O y - K . C , - [ H + ] , . + C'~
898
D. PRAMANICK and A. K. CHATTERJEE 2.5
I
I
I
I
I
I
[
2.0
_o ,5
1.0
0.6
L 50
I 150
I I00
I 7'00
I 2.50
I 300
I 3,50
380
I
I-.+] Fig. 2. Variation of \[M]([SU]"100yX)
with
1
___-==-z=-_ of the medium in the radical polymerization of
MMA in the presence of thiourea in acid aqueous solution. values and P, decrease progressively with increasing [H ÷], of the medium. Since Ct, and K are constants at a constant temperature, decrease in . . . . . . . 1.
+
1
with increasing [H ÷ ]e is obvious. Again, according to the equilibrium (4), an increase in [H ÷]e will increase [HSU]e which in turn will enhance the frequency of transfer reactions and P. will decrease progressively. At pH 3.0 and 3.5, the coiour developed in the organic phase in the dye test is rather faint and the dye test becomes insensitive. Beyond pH 3.5, no coiouration in the organic phase is found indicating complete absence of chain transfer by thiourea. According to Eqn (7), a plot of . . . . [SU] x 1 - VS [M] 100y [H*], should be linear and this is found to be approximately true (Fig. 2), again confirming the existence of the equilibrium (4). From the intercept and slope of this plot, Ct, and K have been found to be 1.21 x 10 -4 and 232 respectively at 42 °. This small C , value indicates that transfer of thiourea with poly(MMA) radicals is very weak compared with that for thiols, e.g. ethyl mercaptan, butyl mercaptan. Using the observed value of K, the ratios of the equilibrium concentrations of the thiol to thione forms of thiourea have been calculated and included in Table 2. These values indicate that at low pH thiourea exists almost entirely
as thiol. The thioi form is progressively transformed into the thione form with increasing pH. Beyond pH 3.5, thiourea exists almost entirely as a thione. This conclusion agrees with the findings that P, for polymer obtained at pH 3.5 is nearly equal to that of polymer obtained in the absence of thiourea. Our observation is supported by the earlier report [13] that, in the potentiometric titration of aqueous solution of thiourea against aqueous alkali, only 2% was titratable. Other workers have shown [14] that, in neutral aqueous solution of thiourea, the thione structure predominates. That the thiol form of thiourea is almost absent near or at neutral pH is reflected by the fact that, in the bulk polymerization of MMA initiated by AIBN, the addition of diphenyl thiourea to the reaction system affected neither the rate of polymerization nor the degree of polymerization of the resulting polymer [15]. CONCLUSION
Our observations suggest that the small concentration of thiourea, used as an initiator component in the aqueous polymerization of MMA for the usual range of acidity used in such polymerizations, will not appreciably affect the polymerization mechanism by chain transfer. Hence in such polymerizations, chain transfer by thiourea can be disregarded. However, our observations lead to a new purely chemical method for determining the equilibrium constant of tautomeric equilibria in thiourea and various substituted thioureas.
899
Thiourea as a transfer agent Table 2. Effect of pH on the chain-transfer property of thiourea and on its tautomeric equilibrium in the aqueous polymerization* of MMA by AIBN from dye partition measurements
pH
P, x 10 -3
Conc of polymer soln (x) (g'l -t)
0.5 1,0 1.5 2.0 2.2 2.5 3.0 3.5 7.0
0.883 1,067 1.432 1.820 2.183 2.732 3.554 4.630 --
1.87 1.73 1.20 1.63 1.69 2. lO ----
Conc of amine-bearing end-groups [SU] x (y)(moll -~ × 1061 [M] 100y 11.97 10.70 6.60 7.44 6.38 5.47 ---
0,831 0.869 0,966 1.180 1.427 2.042 ----
xlO
4
[HSU]~ [SU]¢ 73.35 23.20 7.34 2.32 1.46 0.73 0.23 0.07 2.32 × 10 -5
* Reaction condition: [ M M A ] = 0,094 mol 1- ~ ; [ A I B N ] = 10 -2 mol 1- ', [thiourea] = 0.5 mol I- ~ : temp. 425 and [ C H 3 O H ] = 1.01 mol 1- 1.
REFERENCES
1. H. Gilman, Orffanic Chemistry, Vol. I, p. 940, 2nd Edn. Wiley, New York. 2. P. J. FIory, Principles of Polymer Chemistry, p. 146. Cornell University Press, New York (19533. 3. G. Delzenne, W. Dewinter S. Toppet, and G. Smets, J. Polym. ScL A2 1079 (19643. 4. D Pramanick, A. K. Chatterjee and S. K. Sarkar, Makromolek. Chem. 180(4), 1085 (19793. 5. S. R. Palit, Makromolek. Chem. 38, 96 0960). 6. K. K. Roy, D. Pramanick and S. R. Palit, Makromolek. Chem. 153, 71 (19723. 7. S. R. Palit and P. Ghosh, J. Polym. Sci. 58, 1225 (19623.
J.i'J
16 9
{;
8. B. M. Mandal and S. R. Palit, d. Polymer Sci., AI 9, 3301 (19713. 9. J. H. Baxendale, S. Bywater and M. G. Evans, d. Polym. ScL I, 237 (19463. 10. T. G. Fox, J. B. Kinsinger, H. F. Mason and E. M. Schuele, Polymer 3, 71 (1962). I1. L. Bauer and T. L Welsh, J. org. Chem. 26, 1443 0961). 12. D. Pramanick and A. K. Chatterjee, J. Polym. Sri. In press (1980). 13. R. D. Gupta, J. Indian chem. $oc. 31, 179 (1954). 14. H. Rivier and J. Borel, Heir. chim. Acta I ! , 1219 (1928). 15. Y. Minoura and T. Sugimura. J. Polym Sci. A2, 2721 (1966).
(X)I4-3057/XO/0901-0895502 0(|/0
T H I O U R E A AS A T R A N S F E R A G E N T I N T H E R A D I C A L POLYMERIZATION OF METHYL METHACRYLATE I N A Q U E O U S S O L U T I O N A T 42 °* D. PRAMANICKand A. K. CHATTER.lEE Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal, India
(Received 2 January 1980) Abstract Chain transfer involving thiourea in radical polymerization of methyl methacrylate in acidic aqueous media has been studied by polymer endgroup analysis using the dye-partition technique. Thiourea has feeble reactivity in chain transfer, the transfer constant with respect to poly (methyl methacrylate) radicals being 1.21 x 10 -4 at 42 °. This chain transfer study led to the development of a new method for studying the tautomeric equilibrium between the thione and thiol forms of thiourea. The equilibrium is pH dependent and the equilibrium constant at 42 ° is 232. The ratios of the equilibrium concentrations of the thiol to thione forms of thiourea at various pH's have been calculated. The thiol form is responsible for the chain transfer reactivity; it predominates in strongly acidic media and is almost absent above pH 3.5. This new method may be used for studying the thione-thiol tautomerism of other thiourea derivates.
INTRODUCTION
Thiourea [ l ] exists in a tautomeric equilibrium between the thione (SU) and thioi (HSU) forms and is considered as a potential thiol. Since thiols are potent transfer agents in radical polymerizations [2], thiourea is expected to participate in chain transfer during radical polymerization. Degradative chain transfer by thiourea in the photosensitized polymerization of acrylic monomers has been reported [3] from kinetic measurements. Recently we described [4] preliminary observations on ceric-thiourea initiated acid aqueous polymerization of methyl methacrylate in which transfer involving thiourea was indicated. An essential prerequisite to a full understanding of the mechanism of polymerization by this initiator system is a knowledge of the behaviour as a transfer agent of thioure& We have therefore undertaken a study of thiourea in a typical radical polymerization in acid aqueous media. Palit's dye-partition technique [5] has been employed for the determination of endgroups derived from the thiourea. The theoretical basis of this method has been discussed [6] and will be only briefly outlined here. If x is the concentration in g. l-1 of the test polymer solution for endgroup analysis and y is the concentration of end-group in tool l-a in the solution to be determined experimentally, then the chain transfer constant, Ctr, is given by [M] my [HSU] x
c,~ = - - . - -
(1)
where [ H S U ] is the equilibrium concentration of the thiol form of thiourea, [M] is the monomer concentration, and m is the molecular weight of the monomer. Thus simply determining .v experimentally and using the values of [M], [ H S U ] , m and x, C,r can be found out. As will be seen later, this treatment led to the development of a method for studying the tau* Presented at the International Symposium of Polymer Science, Madras, India (1980). 895
tomeric equilibrium of thiourea or more generally thione-thiol tautomerism. Methyl methacrylate (MMA) and azobisisobutyronitrile (AIBN) were chosen as monomer and initiator respectively, because they have negligible transfer constants and because poly(MMA) can be purified thoroughly. EXPERIMENTAL
Materials Monomeric MMA (BDH) was purified by the usual procedure and AIBN (Analytical reagent grade E. Merck product) was repeatedly crystallized from methanol. Thiourea (E. Merck; analytical reagent grade) was triply crystallized from distilled water to a m.p. of 180°. Since AIBN is sparingly soluble in water, a 1 M solution was prepared in a methanol-water mixture (3:1) and kept at low temperature.
Polymerization procedure Aqueous polymerization of freshly distilled MMA was carried out in stoppered pyrex flasks under Nz. The pH of the reaction medium was controlled by addition of H2SO4. Thiols are susceptible to oxidation by air and so, to avoid consumption of thiourea by 02, the reaction medium was thoroughly deaerated by flushing with purified N2 for about 30 min. The reaction flask was then placed in a water bath at 42 4- 0.05 °. Polymerization was stopped by hydroquinone before 10% conversion. The polymers were washed several times with distilled water and dried in a vacuum oven at 45 °. The dried samples were then rigorously purified by the usual procedure [7].
End-group estimation The thiol form of thiourea is expected to lead to aminebearing end-groups in poly(MMA) by chain-transfer. Hence analysis for amine-bearing end-groups was carried out following the modified dye partition (extrapolation [8]) method using disulphine blue VN 150 as the dye reagent. The principle of this method is that the amine-bearing endgroup in the polymer dissolved in an organic medium extracts an equivalent amount of the dye from the aqueous phase into the organic phase which becomes coloured. In
896
D. PRAMAN|CK and A. K. CHATTERJEE
the modified method, the polymer solution in chloroform is shaken with an equal volume of the aqueous dye reagent until dye-partition between the organic and aqueous phase is complete. For each polymer concentration, four or five dye-partition experiments are performed varying only the aqueous dye concentration. The reciprocal of the absorbance of the organic layer is then plotted against the reciprocal of the dye concentration in the aqueous phase at equilibrium. From the intercepts of such plots, the concentration of the amine or amine bearing end-groups in the polymer was determined. Absorbance of the test solutions was measured in a Hilger u.v, spectrophotometer.
solution thiourea is monoprotonated on the imminonitrogen, the true equilibrium which exists in acidic aqueous solution of thiourea is regarded as: +
NH2
/ \
K
S----C
+ H + ~-~ H ~ 4 2
// \
NH2
NH2 (3) NH2
where K is the tautomeric equilibrium constant, given by K -
[HSU]~ [SU]¢[H +],,
Determination of the number average degree of polymerization To compare the results of chain transfer obtained by the dye partition method with those obtained from the conventional degree of polymerization method, viscosity measurements for polymer solutions in benzene were carried out at 25 +__0.05° using an Ubbelohde dilution viscometer with large ett~ux times and negligible kinetic energy corrections. Intrinsic viscosities, [r/], were determined by extrapolation. The number average degrees of polymerization (P.) poly(MMA) samples were obtained using the equation I-9]: P. = 2.81 x 103 X l'~] 1"32
Here the subscript e refers to equilibrium concentration. According to our concept of chain transfer by thiols, only HSU in the equilibrium mixture may be effective as a transfer agent. Hence [ H S U ] in Eqns (1) and (2) is to be considered as [HSU]~. This is to be expressed in terms of total concentrations of thiourea, [SU], from eqns (4) and (5)
in benzene at 25 ± 0.05 °. Another relation [10]:
and is given by
[SU] = [HSU]~ + [SU]~
P , = 2.22 x 10311']] 1/0'76
1 P,
1 =
C [HSU]
Poo +
" ~
(6)
1
I + - K[H+], Combining Eqn (6) with Eqn (1) and (2), we obtain Eqns (7) and (8), m being replaced by 100 for MMA.
(2)
where P0 is the number average degree of polymerization of polymer obtained in the absence of thiourea and is a function of concentrations of monomer, initiator and any other component of the' system. The contribution to chain transfer by methanol, present in all cases at a fixed concentration for dissolution of AIBN in water, is allowed for in
(5)
[SU]
[HSU], =
in benzene at 30 + 0.05 ° was found to give nearly identical results (data not given). C,r values could be calculated from Mayo's equation,
(4)
[SU].
x 1 1 - 100y - K ' C , r [H+]~
[M]
+
1 -C,f
(7)
and 1
I
P.
-
Po
1
+
1
Po-
1
•
[su]
[M]
(8)
K.C,r. [H+],. + RESULTS AND DISCUSSION
Tautomeric equilibrium in thiourea between the thione (SU) and thiol (HSU) forms is represented as:
/
NH2
NH
K.C
~- H S - C ~
X"NH~
At fixed pH, [H+]¢ of the solution being fixed, the quantity
""NH~
It has been suggested [1] that the formation of H S U from SU is dependent on pH. Since in acid aqueous
.[H÷]o
+
of Eqns (7) and (8) should be a constant, because K and C,, are constants at a constant temperature. Results for chain transfer by thiourea in the aqueous polymerization of M M A by AIBN from dye partition tests are shown in Table 1. Values of P , for
Table 1. Chain transfer results for thiourea in the aqueous polymerization* of MMA by AIBN from dye partition and degree of polymerization measurements
I,SU] (tool- 1- i )
Conc of polymer soln (x) (g. 1-1 )
0.1 0.2 0.4 0.5 0.7 0.0
1.8 1.8 1.8 1.4 1.4 --
Conc of amine-bearing end-group (~) (~ 0,)(moll -l x 106) x 103 1.61 4.96 7.59 7.66 11.97 --
0.208 0.371 0.581 0.698 0.865 0.170
+
1 ¢)10_ 4 K.C,~-[H+] x 1.193 0.772 1.009 0.970 0.872 --
* Reaction condition: I,MMA] = 0.094 mol 1-1 ; I,AIBN] = 10 -2 mol 1-1 ; pH 1.5, temp. 42°; and I,CH3OH] = 1.01 mol 1- t.
Thiourea as a transfer agent t.0
I
I
I
I
]
897 ~
I
/
0.9
0.8
0.7
0.6
x
0.5
0.4
0.5 0.2
./
J
0.1
0.0
[
I
I
I
~
I
I
o.,
o.z
0.3
0.4
o.s
0.6
0.7
0.8
[s u] M
Fig. 1. Variation of Pn with thiourea concentration in the radical polymerization of MMA in acid aqueous medium. the polymers as obtained by viscometry are also included in the table. Figure 1 shows the same results according to the Mayo plot. Both end-group and degree of polymerization methods necessarily gave 1
values as obtained according to Eqns (7) and (8). Dyepartition results in Table I shows that this quantity is approximately constant, as supported by the linearity of the Mayo plot (Fig. 1). These results indicate that equilibrium (4) really exists. The average value of K.C,~[H+], + obtained from dye partition tests is 0.963 x 104 and the same value obtained from the slope of the Mayo plot is 1.018 x 104; the agreement is fairly satisfactory. This shows again that the dye partition method of determining the chain transfer property is as efficient as the standard degree of polymerization method with the added advantage of avoiding viscosity measurements which may be m error for low molecular weight polymers. As discussed above, chain transfer involving thiourea will produce a polymer molecule with the end structure + NH2
//
In the calculation of end-group concentration (y), it has been assumed that only one dye molecule binds with this end-group. This assumption is justified in view of the fact that in acid solutions of thiourea only the immino-nitrogen is protonated [11], and so only the immino-nitrogen is sufficiently basic to bind with the dye. That only one dye molecule binds with the
//
NH
\ NH2 end-group has been found for polymers obtained by polymerization of MMA initiated by Ce4+-thiourea system [12]. To discover to what extent thiourea takes part in chain transfer during the radical polymerization of MMA in acid aqueous solution, it is necessary to know the separate values of C,r and K. Since the concentration of HSU in the reaction medium depends upon [H ÷ ]c, the extent of chain transfer by thiourea will depend upon the pH of the medium. Hence radical polymerization of MMA in the presence of thiourea has been carried out at various pH's keeping all other conditions unaltered. Since the end-group method and the degree of polymerization method give almost identical results on chain transfer, only the end-group method has been used here and the results are shown in Table 2. It is seen that, for pH between 0.5 and 2.5,
• ," S----C
\ NH2
[SU] x 1 11 [M] l O O y - K . C , - [ H + ] , . + C'~
898
D. PRAMANICK and A. K. CHATTERJEE 2.5
I
I
I
I
I
I
[
2.0
_o ,5
1.0
0.6
L 50
I 150
I I00
I 7'00
I 2.50
I 300
I 3,50
380
I
I-.+] Fig. 2. Variation of \[M]([SU]"100yX)
with
1
___-==-z=-_ of the medium in the radical polymerization of
MMA in the presence of thiourea in acid aqueous solution. values and P, decrease progressively with increasing [H ÷], of the medium. Since Ct, and K are constants at a constant temperature, decrease in . . . . . . . 1.
+
1
with increasing [H ÷ ]e is obvious. Again, according to the equilibrium (4), an increase in [H ÷]e will increase [HSU]e which in turn will enhance the frequency of transfer reactions and P. will decrease progressively. At pH 3.0 and 3.5, the coiour developed in the organic phase in the dye test is rather faint and the dye test becomes insensitive. Beyond pH 3.5, no coiouration in the organic phase is found indicating complete absence of chain transfer by thiourea. According to Eqn (7), a plot of . . . . [SU] x 1 - VS [M] 100y [H*], should be linear and this is found to be approximately true (Fig. 2), again confirming the existence of the equilibrium (4). From the intercept and slope of this plot, Ct, and K have been found to be 1.21 x 10 -4 and 232 respectively at 42 °. This small C , value indicates that transfer of thiourea with poly(MMA) radicals is very weak compared with that for thiols, e.g. ethyl mercaptan, butyl mercaptan. Using the observed value of K, the ratios of the equilibrium concentrations of the thiol to thione forms of thiourea have been calculated and included in Table 2. These values indicate that at low pH thiourea exists almost entirely
as thiol. The thioi form is progressively transformed into the thione form with increasing pH. Beyond pH 3.5, thiourea exists almost entirely as a thione. This conclusion agrees with the findings that P, for polymer obtained at pH 3.5 is nearly equal to that of polymer obtained in the absence of thiourea. Our observation is supported by the earlier report [13] that, in the potentiometric titration of aqueous solution of thiourea against aqueous alkali, only 2% was titratable. Other workers have shown [14] that, in neutral aqueous solution of thiourea, the thione structure predominates. That the thiol form of thiourea is almost absent near or at neutral pH is reflected by the fact that, in the bulk polymerization of MMA initiated by AIBN, the addition of diphenyl thiourea to the reaction system affected neither the rate of polymerization nor the degree of polymerization of the resulting polymer [15]. CONCLUSION
Our observations suggest that the small concentration of thiourea, used as an initiator component in the aqueous polymerization of MMA for the usual range of acidity used in such polymerizations, will not appreciably affect the polymerization mechanism by chain transfer. Hence in such polymerizations, chain transfer by thiourea can be disregarded. However, our observations lead to a new purely chemical method for determining the equilibrium constant of tautomeric equilibria in thiourea and various substituted thioureas.
899
Thiourea as a transfer agent Table 2. Effect of pH on the chain-transfer property of thiourea and on its tautomeric equilibrium in the aqueous polymerization* of MMA by AIBN from dye partition measurements
pH
P, x 10 -3
Conc of polymer soln (x) (g'l -t)
0.5 1,0 1.5 2.0 2.2 2.5 3.0 3.5 7.0
0.883 1,067 1.432 1.820 2.183 2.732 3.554 4.630 --
1.87 1.73 1.20 1.63 1.69 2. lO ----
Conc of amine-bearing end-groups [SU] x (y)(moll -~ × 1061 [M] 100y 11.97 10.70 6.60 7.44 6.38 5.47 ---
0,831 0.869 0,966 1.180 1.427 2.042 ----
xlO
4
[HSU]~ [SU]¢ 73.35 23.20 7.34 2.32 1.46 0.73 0.23 0.07 2.32 × 10 -5
* Reaction condition: [ M M A ] = 0,094 mol 1- ~ ; [ A I B N ] = 10 -2 mol 1- ', [thiourea] = 0.5 mol I- ~ : temp. 425 and [ C H 3 O H ] = 1.01 mol 1- 1.
REFERENCES
1. H. Gilman, Orffanic Chemistry, Vol. I, p. 940, 2nd Edn. Wiley, New York. 2. P. J. FIory, Principles of Polymer Chemistry, p. 146. Cornell University Press, New York (19533. 3. G. Delzenne, W. Dewinter S. Toppet, and G. Smets, J. Polym. ScL A2 1079 (19643. 4. D Pramanick, A. K. Chatterjee and S. K. Sarkar, Makromolek. Chem. 180(4), 1085 (19793. 5. S. R. Palit, Makromolek. Chem. 38, 96 0960). 6. K. K. Roy, D. Pramanick and S. R. Palit, Makromolek. Chem. 153, 71 (19723. 7. S. R. Palit and P. Ghosh, J. Polym. Sci. 58, 1225 (19623.
J.i'J
16 9
{;
8. B. M. Mandal and S. R. Palit, d. Polymer Sci., AI 9, 3301 (19713. 9. J. H. Baxendale, S. Bywater and M. G. Evans, d. Polym. ScL I, 237 (19463. 10. T. G. Fox, J. B. Kinsinger, H. F. Mason and E. M. Schuele, Polymer 3, 71 (1962). I1. L. Bauer and T. L Welsh, J. org. Chem. 26, 1443 0961). 12. D. Pramanick and A. K. Chatterjee, J. Polym. Sri. In press (1980). 13. R. D. Gupta, J. Indian chem. $oc. 31, 179 (1954). 14. H. Rivier and J. Borel, Heir. chim. Acta I ! , 1219 (1928). 15. Y. Minoura and T. Sugimura. J. Polym Sci. A2, 2721 (1966).