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Copolymerization of 2-hydroxyethyl methacrylate with alkylacrylates. Temperature dependence of monomer reactivity ratios
| uropean Pobmcr Iournal Vo[ I~, pp 175 to 178 Pergamon Press 197"r Printed in (;real Britain
COPOLYMERIZATION OF 2-HYDROXYETHYL METHACRYLATE WITH ALKYLACRYLATES. TEMPERATURE DEPENDENCE OF MONOMER REACTIVITY RATIOS I. K. VARMA and S. PATNAIK Department of Chemistry, Indian Institute of Technology. New Delhi 110029, India (Receited 8 June 1976)
Abstract Copolymerizations of 2-hydroxyethyl methacrylate (HEMA) with methyl acrylate, ethyl acrylate, n-butyl acrylate and methyl methacrylate were carried out in bulk at temperatures between 5@ and 90 ° using 0.2°,i, benzoyl peroxide as initiator. The copolymer compositions were determined by estimation of the hydroxyl group of HEMA in the copolymers by acetylation. The monomer reactivity ratios were determined by the Joshi-Joshi method. The Arrhenius parameters were calculated. The activation energy (El l-El 2) decreases as the alkyl group becomes bulkier in the acrylate.
INTRODUCTION
The copolymerization of 2-hydroxyethyl methacrylate (HEMA) with various vinyl monomers has been investigated in the past and reactivity ratios have been reported [1-3]. However, the effect of temperature on the reactivity ratios has not been investigated for H E M A copolymers. Barson et al. [4-7] have reported the effect of temperature on the monomer reactivity ratios in copolymerizations of styrene with cinnamic acid, some of its esters and e-substituted cinnamic acids. They have found that the effect of substitution by CHa- or C6H~-group in the ~-position of cinnamic acid is much greater than that found with the methyl or phenyl esters. We have now investigated the effect of temperature on reactivity ratios of H E M A and alkyl acrylate copolymers. The size of the substituents in the acrylic acid esters (methyl acrylate (MA), ethyl acrylate (EA) and n-butyl acrylate (BA)) may influence the reactivity ratios. Similarly an ~.-methyl group, as in methyl methacrylate (MMA), may have a different effect. Therefore the copolymerizations of H E M A with MA, EA, BA and M M A were studied in the range 5 0 9 0 : .
A change in the alkyl component of the alkyl acrylate influences r 1. The value of rl increases with increase in temperature for all the copolymers of H E M A and alkyl acrylates. The values are highest for EA and lowest for M M A up to 80 c'. However, at 90 ° the M A and EA values are comparable. The reciprocal of rl can be used as a measure of the relative reactivity of the alkyl acrylate monomer towards the H E M A radical; the greater 1/rt, the greater will be the chance for the ester to add to the H E M A radical. The substitution of a methyl group for an e-hydrogen of M A increases l/rl. Thus values of 1/r I for M M A and M A at 60 ° are 0.98 and 0.14, and can be correlated with Taft polar constants value of 0 for methyl and 0.49 for hydrogen. This increased reactivity of M M A towards the attacking H E M A radical may be explained by the increased stability of the resulting polymeric radical. The methyl group may stabilise the radical by hyperconjugation. I.C
EXPERIMENTAL
06
C o p o l y m e r s w e r e p r e p a r e d at v a r i o u s t e m p e r a t u r e s as d e s c r i b e d earlier [3]. Five different m o n o m e r ratios w e r e
taken in the initial feed. The copolymer compositions were determined by acetylation; monomer reactivity ratios were determined by the Joshi Joshi [8] method. The standard deviations were also calculated by the same method.
mI O4
O2
RESULTS AND DISCUSSION 0
The dependence of mole fraction of H E M A in copolymer (m~) on mole fraction of H E M A in monomer feed (M1) at various temperatures was plotted; a typical plot is shown in Fig. 1. The reactivity ratios r~ for H E M A are given in Table 1. 175
I
O]
i
d
02
O 3
I
04
Mi
Fig. 1. Effect of m o l e fraction H E M A (M~) in m o n o m e r feed on the c o p o l y m e r c o m p o s i t i o n at v a r i o u s t e m p e r a tures: 50 (g]), 60 ° (x), 70° (&), 80 (O) and 90 ° (O), for
HEMA MA copolymers.
176
I.K. VARMAand S. PATNAIK Table l. Variation of reactivity ratios r~ with temperature Temp. (°C) 50 60 70 80 90
M2 = Methyl acrylate 5.314 7.148 10.123 14.147 18.231
+ 0.008 + 0.001 _____0.000 + 0.004 ___+0.004
M2 = Ethyl acrylate 10.129+ 13.530+ 14.104 + 16.279+ 17.511+
0.059 0.073 0.045 0.041 0.032
The reactivity of alkyl acrylates in copolymerization may be influenced by the polar nature of the alkyl group. An electron releasing alkyl substituent may reduce the possibility of conjugation between the carbonyl and vinyl bonds.
M2 = n-butyl acrylate
M2 = Methylmethacrylate
3.936 5.404 6.278 7.954 8.911
0.840 +__0.014 1.016 + 0.036 1.267 + 0.019 1.404 + 0.018 t.575 + 0.019
+ 0.035 + 0.039 ___+0.023 + 0.026 + 0.025
A study of the temperature dependence of the monomer reactivity ratios may provide an alternative way for assessing the steric and polar effects of the substituents. The rate constants may be expressed thus:
k t t = All e -E,1/gT k~2 = A12e-E12/nr CH2~------CH-M2--O--R ~ CH2==CH--C-------O + R
;_
where A = frequency factor, E = activation energy. Therefore,
Conjugation between carbonyl and alkoxy group C H 2~'-HN----C--4)---R' ~ C + H2--CH~------C--OR'
;
Conjugation between carbonyl and vinyl bonds The steric effects of substituents have also to be considered. Therefore, a correlation between relative reactivities of the ester and polar (a*) and steric (E,) substitution constants values using the modified Taft [9] equation was attempted. log (i/rl) = p* ~* + 6Es where p* is the reaction constant. In Fig. 2, log 1/rl at 60 ° is plotted against a*; from the slope of the line p* was found to be -0.7. p* being negative indicates that the H E M A radical reacts readily with monomers having electron donor groups. Taking this value of p*, log l/r1 - (-0.7a*) was plotted vs Es (Fig. 3); since the value of 6 came out to be positive, it clearly seems that the size of substituents also is important.
In r, = I n
k,~ = I n All --(El, -- Elz)/RT. k,2 A12
According to transition state theory
lnr I = (AS~, -
aS~,2)/e
- ( A H ~ , - AH~2)/RT
where AS = entropy of activation, AH = heat of activation. Hence a plot of In rl vs 1/T gives a straight line with slope (ElrE12)/R and intercept All~A12. Figure 4 shows such a plot. The Arrhenius parameters for various systems are given in Table 2. The differences in entropies of activation are also shown in Table 2. F r o m the activation energies, it is clear that crosspropagation is favoured in all these systems. For MA there is maximum value of (El l-El2) indicating crosspropagation is less dependent on temperature. This result can be explained on the basis of the steric factor. In all these acrylates, the activation energy (Ell-E12) variation is due to the size of the alkyl group. As the alkyl group becomes smaller, the H E M A radical can easily attack the acrylate forming
2.0
2.0
b-
~o,
I.O
~_1 .O ,,,..,,-o-
O
I
-0.3
I
-0.2
I
-0.1
I
0
0"*
Fig. 2. A plot of log (l/r1) at 60 ° against the Taft polar constant ~r*.
o
J -o.3
I -0.2
I -0. I Es
I 0
Fig. 3. A plot of [log 1/ri + 0.7a*] at 60 ° against the Taft steric constant E~.
Copolymerization of 2-hydroxyethyl methacrylate
177
Table 2. Arrhenius parameters and difference in entropies of activation
Eli-El2
M2 = Methyl acrylate
M2 = Ethyl acrylate
M2 = n-butyl acrylate
M2 = Methylmethacrylate
30.2 _+ 2.5
14.6 +_ 3.1
14.3 + 1.0
16.0 + 2.6
1317 + 3.0 59.6 _ 0.2
1035 + 2.0 57.7 + 0.1
175.6 +_ 0.2 43.0 + 0.01
3.85 + 0.01 11.2 + 0.01
(kJ/mole)
All~A12 AS~-AS~z (JK - 1/mole)
2£
1.0
27
28
3.(3"~; I
2.9
I/Txl03
- 1.0
Fig. 4. Arrhenius plots of the reactivity ratio ra for HEMA MA (x), HEMA EA (O), HEMA BA (@) and HEMA MMA (z~) copolymers.
due to the electron releasing methyl group in the :(-position of the ester group. A plot of (Ett-EI2) vs a* (Fig. 5) clearly indicates that the cross-propagation is reduced by the presence of electron releasing ethyl and n-butyl groups in the acrylates. The ratios of the frequency factors A ~ / A 12, however. indicate a different order. As the substituent becomes bulkier, one would expect a decrease in A12 and a consequent increase in this ratio. However, our results indicate a higher value for MA than BA. Figure6 is a plot of AII/AI2 against E, for the various alkyl acrylates. From the frequency factor point of view, M M A has more tendency to form copolymers than MA and the order is just reversed from that of activation energy. From the frequency factor, it is clear that M M A as comonomer is more reactive than the others. Thus (E t 1-E12) values Is,your cross-propagation in the order MA > M M A > EA > BA, while frequency factor indicates the following order for cross-propagation: M M A > BA > EA > MA. However, if one looks at the rt values then the order of cross propagation is M M A > BA > MA > EA. These results thus indicate that the frequency factor predominates in these copolymerizations. It is also difficult to predict the effect of alkyl groups on the copolymerization of H E M A and alkyl acrylates. It has been shown [10] that in vinyl polymerization a substituent can increase the reactivity of the monomer towards an attacking radical if the resulting polymeric radical is stabilised by the substituent. In the alkyl acrylate CHz~------CHCOOR, the reactivity of the morlomer may
30
Y
20
Lit I_
/
1500
1000
I
-0.2
,I
-o.i
Fig. 5. A plot of
1
o
I
o.i
(Eli-El2) against a*.
a copolymer rather than undergoing self-propagation. Another interesting observation is that (EI~-E,2) for the H E M A / M M A system is between those for the H E M A / M A and H E M A / E A systems. This may be
500
-0.3
-02
-0
I
0
Es
Fig. 6. A plot of A~1'.,t~ ~gain~t i:,.
178
I.K. VARMAand S. PATNAIK
be affected by (a) the polar nature of R, (b) steric requirements of R. If one looks at the electron releasing tendency of the alkyl groups, then n-butyl is a better electron donor than methyl whereas ethyl is intermediate. However, the observed reactivity for copolymerization is greater for methyl than ethyl. This implies that both the polar effect of R and its size influence rl.
REFERENCES
1. F. Mikes, P. Strop, Q. Seycek, J. Roda and J. Kalal, Europ. Polym. J. 10, 1029 (1974). 2. F. Mikes, P. Strop and J. Kalal, Chem. Ind. 1164 (1973).
3. I. K. Varma and S. Patnaik, Europ. Polym. J. 12, 259 (1976). 4. C. A. Barson and M. S. Rizvi, Europ. Polym. J. 6, 241 (1970). 5. C. A. Barson and M. J. Turner, Europ. Polym. J. 9, 789 (1973). 6. C. A. Barson and M. J. Turner, Europ. Polym. J. 10, 917 (1974). 7. C. A. Barson and M. J. Turner, Europ. Polym. J. 10, 1053 (1974). 8. M. R. Joshi and S. G. Joshi, J. Macromol. Sci. Aft(8), 1329 (1971). 9. R. W. Taft, Jr., Steric Effects in Organic Chemistry (edited by M. S. Newman), p. 556. John Wiley, New York (1956). 10. F. R. Mayo and C. Walling, Chem. Rev. 46, 191 (1950).
COPOLYMERIZATION OF 2-HYDROXYETHYL METHACRYLATE WITH ALKYLACRYLATES. TEMPERATURE DEPENDENCE OF MONOMER REACTIVITY RATIOS I. K. VARMA and S. PATNAIK Department of Chemistry, Indian Institute of Technology. New Delhi 110029, India (Receited 8 June 1976)
Abstract Copolymerizations of 2-hydroxyethyl methacrylate (HEMA) with methyl acrylate, ethyl acrylate, n-butyl acrylate and methyl methacrylate were carried out in bulk at temperatures between 5@ and 90 ° using 0.2°,i, benzoyl peroxide as initiator. The copolymer compositions were determined by estimation of the hydroxyl group of HEMA in the copolymers by acetylation. The monomer reactivity ratios were determined by the Joshi-Joshi method. The Arrhenius parameters were calculated. The activation energy (El l-El 2) decreases as the alkyl group becomes bulkier in the acrylate.
INTRODUCTION
The copolymerization of 2-hydroxyethyl methacrylate (HEMA) with various vinyl monomers has been investigated in the past and reactivity ratios have been reported [1-3]. However, the effect of temperature on the reactivity ratios has not been investigated for H E M A copolymers. Barson et al. [4-7] have reported the effect of temperature on the monomer reactivity ratios in copolymerizations of styrene with cinnamic acid, some of its esters and e-substituted cinnamic acids. They have found that the effect of substitution by CHa- or C6H~-group in the ~-position of cinnamic acid is much greater than that found with the methyl or phenyl esters. We have now investigated the effect of temperature on reactivity ratios of H E M A and alkyl acrylate copolymers. The size of the substituents in the acrylic acid esters (methyl acrylate (MA), ethyl acrylate (EA) and n-butyl acrylate (BA)) may influence the reactivity ratios. Similarly an ~.-methyl group, as in methyl methacrylate (MMA), may have a different effect. Therefore the copolymerizations of H E M A with MA, EA, BA and M M A were studied in the range 5 0 9 0 : .
A change in the alkyl component of the alkyl acrylate influences r 1. The value of rl increases with increase in temperature for all the copolymers of H E M A and alkyl acrylates. The values are highest for EA and lowest for M M A up to 80 c'. However, at 90 ° the M A and EA values are comparable. The reciprocal of rl can be used as a measure of the relative reactivity of the alkyl acrylate monomer towards the H E M A radical; the greater 1/rt, the greater will be the chance for the ester to add to the H E M A radical. The substitution of a methyl group for an e-hydrogen of M A increases l/rl. Thus values of 1/r I for M M A and M A at 60 ° are 0.98 and 0.14, and can be correlated with Taft polar constants value of 0 for methyl and 0.49 for hydrogen. This increased reactivity of M M A towards the attacking H E M A radical may be explained by the increased stability of the resulting polymeric radical. The methyl group may stabilise the radical by hyperconjugation. I.C
EXPERIMENTAL
06
C o p o l y m e r s w e r e p r e p a r e d at v a r i o u s t e m p e r a t u r e s as d e s c r i b e d earlier [3]. Five different m o n o m e r ratios w e r e
taken in the initial feed. The copolymer compositions were determined by acetylation; monomer reactivity ratios were determined by the Joshi Joshi [8] method. The standard deviations were also calculated by the same method.
mI O4
O2
RESULTS AND DISCUSSION 0
The dependence of mole fraction of H E M A in copolymer (m~) on mole fraction of H E M A in monomer feed (M1) at various temperatures was plotted; a typical plot is shown in Fig. 1. The reactivity ratios r~ for H E M A are given in Table 1. 175
I
O]
i
d
02
O 3
I
04
Mi
Fig. 1. Effect of m o l e fraction H E M A (M~) in m o n o m e r feed on the c o p o l y m e r c o m p o s i t i o n at v a r i o u s t e m p e r a tures: 50 (g]), 60 ° (x), 70° (&), 80 (O) and 90 ° (O), for
HEMA MA copolymers.
176
I.K. VARMAand S. PATNAIK Table l. Variation of reactivity ratios r~ with temperature Temp. (°C) 50 60 70 80 90
M2 = Methyl acrylate 5.314 7.148 10.123 14.147 18.231
+ 0.008 + 0.001 _____0.000 + 0.004 ___+0.004
M2 = Ethyl acrylate 10.129+ 13.530+ 14.104 + 16.279+ 17.511+
0.059 0.073 0.045 0.041 0.032
The reactivity of alkyl acrylates in copolymerization may be influenced by the polar nature of the alkyl group. An electron releasing alkyl substituent may reduce the possibility of conjugation between the carbonyl and vinyl bonds.
M2 = n-butyl acrylate
M2 = Methylmethacrylate
3.936 5.404 6.278 7.954 8.911
0.840 +__0.014 1.016 + 0.036 1.267 + 0.019 1.404 + 0.018 t.575 + 0.019
+ 0.035 + 0.039 ___+0.023 + 0.026 + 0.025
A study of the temperature dependence of the monomer reactivity ratios may provide an alternative way for assessing the steric and polar effects of the substituents. The rate constants may be expressed thus:
k t t = All e -E,1/gT k~2 = A12e-E12/nr CH2~------CH-M2--O--R ~ CH2==CH--C-------O + R
;_
where A = frequency factor, E = activation energy. Therefore,
Conjugation between carbonyl and alkoxy group C H 2~'-HN----C--4)---R' ~ C + H2--CH~------C--OR'
;
Conjugation between carbonyl and vinyl bonds The steric effects of substituents have also to be considered. Therefore, a correlation between relative reactivities of the ester and polar (a*) and steric (E,) substitution constants values using the modified Taft [9] equation was attempted. log (i/rl) = p* ~* + 6Es where p* is the reaction constant. In Fig. 2, log 1/rl at 60 ° is plotted against a*; from the slope of the line p* was found to be -0.7. p* being negative indicates that the H E M A radical reacts readily with monomers having electron donor groups. Taking this value of p*, log l/r1 - (-0.7a*) was plotted vs Es (Fig. 3); since the value of 6 came out to be positive, it clearly seems that the size of substituents also is important.
In r, = I n
k,~ = I n All --(El, -- Elz)/RT. k,2 A12
According to transition state theory
lnr I = (AS~, -
aS~,2)/e
- ( A H ~ , - AH~2)/RT
where AS = entropy of activation, AH = heat of activation. Hence a plot of In rl vs 1/T gives a straight line with slope (ElrE12)/R and intercept All~A12. Figure 4 shows such a plot. The Arrhenius parameters for various systems are given in Table 2. The differences in entropies of activation are also shown in Table 2. F r o m the activation energies, it is clear that crosspropagation is favoured in all these systems. For MA there is maximum value of (El l-El2) indicating crosspropagation is less dependent on temperature. This result can be explained on the basis of the steric factor. In all these acrylates, the activation energy (Ell-E12) variation is due to the size of the alkyl group. As the alkyl group becomes smaller, the H E M A radical can easily attack the acrylate forming
2.0
2.0
b-
~o,
I.O
~_1 .O ,,,..,,-o-
O
I
-0.3
I
-0.2
I
-0.1
I
0
0"*
Fig. 2. A plot of log (l/r1) at 60 ° against the Taft polar constant ~r*.
o
J -o.3
I -0.2
I -0. I Es
I 0
Fig. 3. A plot of [log 1/ri + 0.7a*] at 60 ° against the Taft steric constant E~.
Copolymerization of 2-hydroxyethyl methacrylate
177
Table 2. Arrhenius parameters and difference in entropies of activation
Eli-El2
M2 = Methyl acrylate
M2 = Ethyl acrylate
M2 = n-butyl acrylate
M2 = Methylmethacrylate
30.2 _+ 2.5
14.6 +_ 3.1
14.3 + 1.0
16.0 + 2.6
1317 + 3.0 59.6 _ 0.2
1035 + 2.0 57.7 + 0.1
175.6 +_ 0.2 43.0 + 0.01
3.85 + 0.01 11.2 + 0.01
(kJ/mole)
All~A12 AS~-AS~z (JK - 1/mole)
2£
1.0
27
28
3.(3"~; I
2.9
I/Txl03
- 1.0
Fig. 4. Arrhenius plots of the reactivity ratio ra for HEMA MA (x), HEMA EA (O), HEMA BA (@) and HEMA MMA (z~) copolymers.
due to the electron releasing methyl group in the :(-position of the ester group. A plot of (Ett-EI2) vs a* (Fig. 5) clearly indicates that the cross-propagation is reduced by the presence of electron releasing ethyl and n-butyl groups in the acrylates. The ratios of the frequency factors A ~ / A 12, however. indicate a different order. As the substituent becomes bulkier, one would expect a decrease in A12 and a consequent increase in this ratio. However, our results indicate a higher value for MA than BA. Figure6 is a plot of AII/AI2 against E, for the various alkyl acrylates. From the frequency factor point of view, M M A has more tendency to form copolymers than MA and the order is just reversed from that of activation energy. From the frequency factor, it is clear that M M A as comonomer is more reactive than the others. Thus (E t 1-E12) values Is,your cross-propagation in the order MA > M M A > EA > BA, while frequency factor indicates the following order for cross-propagation: M M A > BA > EA > MA. However, if one looks at the rt values then the order of cross propagation is M M A > BA > MA > EA. These results thus indicate that the frequency factor predominates in these copolymerizations. It is also difficult to predict the effect of alkyl groups on the copolymerization of H E M A and alkyl acrylates. It has been shown [10] that in vinyl polymerization a substituent can increase the reactivity of the monomer towards an attacking radical if the resulting polymeric radical is stabilised by the substituent. In the alkyl acrylate CHz~------CHCOOR, the reactivity of the morlomer may
30
Y
20
Lit I_
/
1500
1000
I
-0.2
,I
-o.i
Fig. 5. A plot of
1
o
I
o.i
(Eli-El2) against a*.
a copolymer rather than undergoing self-propagation. Another interesting observation is that (EI~-E,2) for the H E M A / M M A system is between those for the H E M A / M A and H E M A / E A systems. This may be
500
-0.3
-02
-0
I
0
Es
Fig. 6. A plot of A~1'.,t~ ~gain~t i:,.
178
I.K. VARMAand S. PATNAIK
be affected by (a) the polar nature of R, (b) steric requirements of R. If one looks at the electron releasing tendency of the alkyl groups, then n-butyl is a better electron donor than methyl whereas ethyl is intermediate. However, the observed reactivity for copolymerization is greater for methyl than ethyl. This implies that both the polar effect of R and its size influence rl.
REFERENCES
1. F. Mikes, P. Strop, Q. Seycek, J. Roda and J. Kalal, Europ. Polym. J. 10, 1029 (1974). 2. F. Mikes, P. Strop and J. Kalal, Chem. Ind. 1164 (1973).
3. I. K. Varma and S. Patnaik, Europ. Polym. J. 12, 259 (1976). 4. C. A. Barson and M. S. Rizvi, Europ. Polym. J. 6, 241 (1970). 5. C. A. Barson and M. J. Turner, Europ. Polym. J. 9, 789 (1973). 6. C. A. Barson and M. J. Turner, Europ. Polym. J. 10, 917 (1974). 7. C. A. Barson and M. J. Turner, Europ. Polym. J. 10, 1053 (1974). 8. M. R. Joshi and S. G. Joshi, J. Macromol. Sci. Aft(8), 1329 (1971). 9. R. W. Taft, Jr., Steric Effects in Organic Chemistry (edited by M. S. Newman), p. 556. John Wiley, New York (1956). 10. F. R. Mayo and C. Walling, Chem. Rev. 46, 191 (1950).