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Effect of 1,1-diphenylethylene on the radical polymerization of di-n-butyl itaconate in benzene
European Polymer Journal 37 (2001) 2055±2061
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Eect of 1,1-diphenylethylene on the radical polymerization of di-n-butyl itaconate in benzene T. Sato *, N. Morita, M. Seno Faculty of Engineering, Department of Chemical Science and Technology, Tokushima University, Minamijosanjima 2-1, Tokushima 770-8506, Japan Received 17 July 2000; received in revised form 8 December 2000; accepted 13 March 2001
Abstract Eect of 1,1-diphenylethylene (DPE) on the polymerization of di-n-butyl itaconate (DBI) with dimethyl 2,20 azobisisobutyrate (MAIB) was studied at 50°C and 60°C in benzene kinetically and by means of ESR. DPE in the low concentration range (up to 0.03 mol/l) was found to serve as a retarder in the polymerization at 50°C and 60°C. As a result, the polymerization rate and the molecular weight of resulting polymer decreased with increasing DPE concentration. The rate constant of the addition of poly(DBI) radical to DPE was estimated to be 74 l/mol s at 50°C and 97 l/mol s at 60°C, respectively, compared to 9.5 l/mol s at 50°C and 11 l/mol s at 60°C as propagation rate constant of DBI. ESR spectrum of the propagating polymer radicals was observed for the actual polymerization mixtures containing DBI, DPE and MAIB in benzene. ESR-determined apparent kp value decreases with increasing DPE concentration, while the apparent kt value increases with the DPE concentration. This is because the relative concentration of DBI-ended polymer radical to DPE-ended one decreases with increasing DPE concentration. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radical polymerization; Di-n-butyl itaconate; Diphenyl ethylene; Retarder; ESR spectrum; Rate constants of propagation and termination
1. Introduction Dialkyl itaconates, 1,1-disubstituted vinyl monomer, are readily polymerized by radical initiators, yielding high molecular weight polymers, in spite of the severe steric requirements of their bulky a-substituents [1±9]. This is explicable on the basis that termination processes are much more suppressed by the substituents than propagation processes. In conformity with this view, the polymerization systems of itaconate esters involve ESRobservable propagating polymer radicals under the actual polymerization conditions [3±9]. So, such polymerization systems have been successfully used for ESR
*
Corresponding author. Tel.: +81-88-656-7402; fax: +81-88655-7025. E-mail address: [email protected] (T. Sato).
elucidation of solvent eect [10,11], Lewis acid eect [12± 14], substituent eect [5,6,15], penultimate eect [16,17] and so on in the radical polymerization. On the other hand, 1,1-diarylethylene such as 1,1-diphenylethylene (DPE) and 1,1-di(p-tolyl) ethylene (DTE) are known to show a high reactivity toward radical species [18] and to serve as retarder in the radical polymerizations of some monomers [19]. DPE and DTE were satisfactorily used as radical scavenger in studies of initiation mechanisms [19,20]. Recently, we have observed that the radical polymerization of di-n-butyl itaconate (DBI) is also retarded by the presence of DPE and studied the eect of DPE kinetically and by means of ESR. The present paper describes the eect of DPE on the polymerization of DBI with dimethyl 2,20 -azobisisobutyrate (MAIB) in benzene on the basis of kinetic and ESR results.
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 7 5 - 1
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T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
2. Experimental 2.1. Materials MAIB was recrystallized from methanol. DBI, DPE as monomer were used after distillation. Benzene as solvent was puri®ed by usual method. 2.2. Polymerization Polymerization of DBI in benzene in the presence of DPE was initiated with MAIB in a degassed and sealed tube at a given temperature under shaking. The resulting polymer was isolated by pouring the polymerization mixture into a large excess of methanol containing a small amount of water. The polymer was ®ltered, dried under vacuum and weighed.
Fig. 1. Time±conversion curves in the polymerization of DBI with MAIB in the presence of DPE at 50°C: DBI 1:99 mol/ 1, MAIB 5:00 10 2 mol/1, DPE 1: 0, 2: 1:42 10 3 , 3: 2:84 10 3 , 4: 5:68 10 3 , 5: 11:4 10 3 , 6: 17:0 10 3 , 7: 28:4 10 3 , 8: 42:6 10 3 mol/1.
2.3. Measurements Gel permeation chromatograms (GPC) were recorded on a Toso HLC-802A chromatograph at 38°C using tetrahydrofuran as eluent. From the GPC results, the number-average (M n ) and weight-average (M w ) molecular weights were determined and calibrated with poly(styrene (St)) standards. ESR spectra of the polymerization mixtures in degassed and sealed ESR tube were obtained using a JEOL JES-FE2XG spectrometer operating at X-band (9.5 GHz) with a TE mode cavity. The concentration of polymer radicals was determined by computer double integration of the ®rst derivative ESR spectrum, where 2,2,6,6-tetramethylpiperidin1-oxyl radical (TEMPO) in the same polymerization mixture without MAIB was used for calibration. 3. Results and discussion 3.1. Eect of DPE on the polymerization of DBI with MAIB In order to examine the eect of DPE on the radical polymerization of DBI, DBI was polymerized with MAIB as initiator at 50°C and 60°C in benzene in the presence of DPE. The concentrations of DBI and MAIB were kept constant at 1.99 and 5:00 10 2 mol/l, re-
Fig. 2. Time±conversion curves in the polymerization of DBI with MAIB in the presence of DPE at 60°C: DBI 1:99 mol/ 1, MAIB 5:00 10 2 mol/l, DPE 1: 0, 2: 1:42 10 3 , 3: 2:84 10 3 , 4: 5:68 10 3 , 5: 11:4 10 3 , 6: 17:0 10 3 , 7: 22:7 10 3 , 8: 28:4 10 3 mol/l.
spectively, and the concentration of DPE was varied from 0 to 4:26 10 2 mol/l. Figs. 1 and 2 show time± conversion curves observed at 50°C and 60°C, respectively. Thus, DPE was found to retard eectively the radical polymerization of DBI. The polymer yield in the early stage at each DPE concentration increased pro-
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
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Table 1 Eect of DPE on the polymerization of DBI with MAIB in benzenea T (°C)
DPE 103 (mol/l)
Rp 106 (1/mol s)
M n =103
M w =103
DP
vb
50 50 50 50 50 50 50 50
0 1.42 2.84 5.68 11.4 17.0 28.6 42.6
20.2 9.46 5.80 3.70 2.20 1.67 1.08 0.70
26 23 17 13 11 8 ± ±
50 40 28 20 14 10 ± ±
107 95 70 54 45 33 ± ±
166 78 48 30 18 14 ± ±
60 60 60 60 60 60 60 60
0 1.42 2.84 5.68 11.4 17.0 22.7 28.4
38.3 26.1 19.7 14.1 9.19 6.90 5.06 4.18
14 12 11 11 9 7 ± ±
27 22 19 17 12 9 ± ±
58 50 45 45 37 29 ± ±
64 44 32 24 15 12 ± ±
a b
DBI 1:99 mol/l, MAIB 5:00 10 2 mol/1. Estimated from v Rp =Ri 2kd f MAIB.
portionally with time, although a short induction period was observed in the higher concentration range of DPE where the primary radicals from MAIB may be trapped by DPE. The polymerization rate (Rp ) at each DPE concentration was determined from the time±conversion curves observed in Figs. 1 and 2. The results obtained are summarized in Table 1. Table 1 also lists the molecular weights of resulting polymers and the degrees of polymerization (DP). Thus, the molecular weights of resulting polymers also decreased with increasing DPE concentration. As described above, DPE in the range of concentration used serves as a retarder in the polymerization of DBI with MAIB. The reactivity of DPE toward propagating poly(DBI) radical was estimated according to the method of Kar et al. The present polymerization is considered to be composed of the following elementary reactions: I ! 2R
Rd 2kd I
P M ! P
Rp kp P M
3
Rx kx P X
4
PX M ! P
Ro ko PX M
5
Rt1 kt1 P 2
6
P P ! Polymer PX P ! Polymer
Rt2 kt2 PX P
where I: MAIB, R : primary radical from MAIB, M: DBI, X: DPE, P : propagating poly(DBI) radical, and PX : DPE-ended polymer radical. Rd , Ri , Rp , Rx , Ro , Rt1 , Rt2 , Rt3 , kd , ki , kp , kx , ko , kt1 , kt2 , and kt3 are the rates and rate constants of elementary reactions, respectively. Ri equals 2kd f I, where f initiator eciency.
1=Rp kx X= Ri kp M kt10:5 = R0:5 i kp M
2
8
1
Ri ki R M
Rt3 kt3 PX 2
Assuming stationary concentrations of P and PX , the Kar equation (Eq. (9)) is obtained when Ro 0 and 2 kt2 kt1 kt3 are presumed. Ro 0 is in conformity with the fact that no polymerization proceeded at high DPE concentrations.
R M ! P
P X ! PX
PX PX ! Polymer
7
9
The third term in Eq. (9) is the inverse of the unretarded rate of polymerization in the absence of DPE and can be expressed as 1=R0p . Thus Eq. (9) is reduced to Eq. (10). 1=Rp kx =Ri kp X=M 1=R0p
10
Fig. 3 shows the Kar plots of 1=Rp against the DPE concentration in the polymerization of DBI with MAIB in the presence of DPE as retarder at 50°C and 60°C, respectively, where the concentrations of DBI and MAIB were 1.99 and 5:00 10 2 mol/l. From the slopes
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Fig. 3. Kar plots for the polymerization of DBI with MAIB in the presence of DPE at 50°C (a) and 60°C (b).
of straight lines observed, the value of kx =Ri kp was estimated to be 6:40 107 at 50°C and 1:48 107 at 60°C, respectively. In order to obtain the Ri value (Ri 2kd f MAIB), kd f was determined under the polymerization conditions without DPE by means of radical trapping with TEMPO in the same manner as described in a previous paper [7]. MAIB decomposes into 1-methyl-1-methoxycarbonylethyl radicals as primary radical nitrogen. Some of the primary radicals are deactivated through cage reactions. The others diuse through the solvent cage to react with BDI, leading to polymerization. When MAIB is allowed to decompose in the presence of TEMPO, the primary radicals escaping from the solvent cage are trapped with TEMPO to yield a coupling product (I). Even if the primary radicals react with DBI before being trapped with TEMPO, the resulting oligomer radicals can also be trapped with TEMO. So, the kd f value is able to be evaluated by following the decrease in the TEMPO concentration by ESR.
Fig. 4 shows the plots of the TEMPO concentration against time at 50°C and 60°C in the polymerization mixture in the presence of TEMPO, where the concentrations of DBI, MAIB, and TEMPO were 1.99, 5:00 10 3 , and 2:07 10 4 mol/l, respectively. From the
Fig. 4. Relationship between time and the TEMPO concentration in the polymerization of DBI with MAIB in benzene at 50°C and 60°C: DBI 1:99 mol/l, MAIB 5:00 10 3 mol/l.
slopes of the plots, the disappearance rate (namely 2kd f MAIB) of TEMPO and then the kd f value were estimated. The kd f values thus determined are 1:22 10 6 s 1 at 50°C and 5:93 10 6 s 1 at 60°C. Using kx =Ri kp and Ri obtained above, and kp values [9.5 l/mol s (50°C), 11 l/mol s(60°C)] of DBI determined below by ESR, kx was estimated as the rate constant of the reaction of poly (DBI) radical with DPE (Eq. (11)). The kx values thus obtained are 74 l/mol s at 50°C and 97 l/mol s at 60°C, respectively. A large value of kx =kp (8) may be ascribable to opposite electrical characters of DPE (e 1:35) [21] and DBI (e 0:77) [10]. kx
DBI DPE!
DBI
DPE
11
The kinetic chain length (v) of the present polymerization was calculated from Rp =Ri and is also shown in Table 1. Thus, the obtained v values are comparable with the degrees of polymerization of poly(DBI)s but the former are smaller than the latter except for the case in the absence of DPE. This is probably because the latter values were based on poly(St) standards. The ratio of DP to v tends to increase with the DPE concentration, suggesting that the chain transfer to the monomer less contributes to the DP determining step at higher DPE concentration. 3.2. ESR study on the polymerization of DBI with MAIB in the presence of DPE Fig. 5 presents ESR spectra observed at 50°C, 60°C, and 70°C in the polymerization of DBI with MAIB in
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
Fig. 5. ESR spectra of the DBI/DPE/MAIB system: DBI 0:794 mol/l, DPE 1:14 mol/l, MAIB 5:00 10 2 mol/l.
the presence of DPE at a high concentration, where the concentrations of DBI, DPE and MAIB were 0.794, 1.14 and 5:00 10 2 mol/l, respectively. These spectra are apparently dierent from the ESR spectrum of propagating poly(DBI) radical (P ) and are considered to be arised from DPE. A broad doublet spectrum was reported for a polymer radical with DPE-ended radical center [22]. The hyper®ne structures of the observed spectra indicates that they are assignable to a low molecular radical, namely, primary propagating radical (II) which is formed by the reaction of primary radical with one DPE monomer (Eq. (12)). Observation of radical II alone in the polymerization system containing DPE of a high concentration is in agreement with the above assumption that it is dicult for the DPE radical to further propagate.
12
Next, we have examined the polymerization systems containing DPE at low concentrations used earlier in the kinetic study. Fig. 6 shows ESR spectra of the poly-
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Fig. 6. ESR spectra observed in the polymerization of DBI with MAIB at 60°C in benzene in the presence of DPE: DBI 1:99 mol/l, MAIB 5:00 10 2 mol/l.
merization systems observed at 60°C when the DPE concentration was varied keeping the concentrations of DBI and MAIB constant at 1.99 and 5:00 10 2 mol/l, respectively. The ®ve-line spectrum observed in the absence of DPE is due to propagating poly(DBI) radical (P ). With increasing DPE concentration, the intensity of ESR spectrum decreased and the spectrum shape became broader. Broadening of the spectrum indicates coexistence of another propagating radical center (PX ) in the polymerization system in the presence of DPE. Thus, the total concentration of propagating radicals decreases with DPE concentration. The relative concentration polymer radical with the end of DBI also decreases with increasing DPE concentration. The observed ESR spectra were unchanged in shape and in intensity during ESR measurement for several hours, indicating that a stationary state is reached for the propagating polymer radicals. In order to estimate the apparent rate constants of propagation (kp ) and termination (kt ), the stationary total concentration of polymer radicals ([T ]) was determined at 50°C and 60°C by ESR. Table 2 summarizes the results obtained at various DPE concentrations.
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T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
Table 2 Eect of DPE on kp , kt , the total polymer radical concentration (T ), and the DBI-ended radical (P ) and DPE-ended radical (PX ) concentrations in the polymerization of DBI with MAIB in benzenea DPE 103 (mol/l)
T (°C)
T 106 (mol/l)
kp (l/mol s)
kt 105 (l/mol s)
P 106 (mol/l)
PX 106 (mol/l)
0 1.42 2.84 5.68 11.4 0 1.42 2.84 5.68 11.4 17.0
50 50 50 50 50 60 60 60 60 60 60
1.1 0.72 0.51 0.41 0.35 1.8 1.4 1.1 0.88 0.77 0.68
9.5 6.6 5.7 4.5 3.2 11 9.4 9.0 8.1 6.0 5.1
1.0 2.3 4.7 7.2 10.2 1.8 3.0 4.9 7.7 10.1 12.9
1.1 0.50 0.31 0.19 0.12 1.8 1.20 0.90 0.65 0.42 0.32
0 0.22 0.20 0.21 0.23 0 0.20 0.20 0.23 0.35 0.36
a
DBI 1:99 mol/l, MAIB 5:00 10
2
mol/l.
Apparent values of kp and kt were estimated according to the following Eqs. (13) and (14) for the stationary state polymerization involving bimolecular termination. Rp kp T DBI
13
2kd f MAIB kt T 2
14
The results obtained are also presented in Table 2. The apparent kp value shows a tendency to decrease with increasing DPE concentration. This is caused by the poor reactivity of the DPE-ended polymer radical (PX ). It was above assumed that the DPE-ended radical is dicult to further propagate. Such assumption allows us to estimate each concentration of the DBI-ended radical (P ) and the DPE-ended one (PX ) by using the total polymer radical concentration (T ) and the apparent kp value as follows: P T kp =kp0
15
PX T
16
P
where kp0 is the kp value in the absence of DPE. These results are likewise listed in Table 2. Thus, the concentration of DBI-ended radical (P ) drastically decreases with increasing DPE concentration, while that of DPE-ended radical (PX ) is not so sensitive to change in the DPE concentration. As a result, the total concentration of polymer radicals decreases with the DPE concentration. The apparent kt value increases with increasing DPE concentration, indicating that termination involving DPE-ended radical, PX , is more rapid than that involving DBI-ended radical, P . Such decrease in kp and increase in kt with increasing DPE concentration are responsible for the decrease in Rp observed earlier with increasing DPE concentration.
The termination reactions in the present polymerization are expressed by Eqs. (6)±(8) as described above. Eq. (17) is obtained on the basis of a stationary state shown earlier in the present polymerization. Ri kt T 2 ktl P 2 kt2 P PX kt3 PX 2
17
where kt1 corresponds to the kt value in the absence of DPE. In order to determine the values of kt2 and kt3 , the known values (Table 2) were substituted for Eq. (17) at dierent DPE concentrations. From the crossing points of plots of kt2 against kt3 at each DPE concentration, kt2 and kt3 were estimated as shown in Table 3. These values approximately satisfy the Kar's assumption described 2 above, that is, kt2 kt1 kt3 . As shown in Table 3, the termination involving DPE-ended polymer radical (PX ) is more rapid than that involving DBI-ended polymer radical (P ). This might come from radical combination reaction via the para-positions of the phenyl rings of DPE-ended radical (Eq. (18)) [23]. Since the presence of DPE causes a decrease in the molecular weight of resulting polymer, it is dicult to rule out the possibility that the apparent kt increases with decrease in the molecular weight of polymer as a result of increase in the DPE concentration.
Table 3 Termination rate constants in the polymerization of DBI with MAIB in the presence of DPE T (°C)
kt1 105 (l/mol s)
kt2 105 (l/mol s)
kt3 105 (l/mol s)
50 60
1.0 1.8
4 11
23 53
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
18
4. Conclusions DPE in the low concentration range (up to 0.03 mol/l) serves as a retarder in the polymerization of DBI in benzene at 50°C and 60°C. As a result, the polymerization rate and the molecular weight of poly(DBI) decreases with the DPE concentration. The rate constant (kx ) of the addition of poly(DBI) radical to DPE is 74 l/mol s at 50°C and 97 l/mol s at 60°C, compared to 9.5 l/mol s at 50°C and 11 l/mol s at 60°C as the propagation rate constant of DBI. The polymerization system involves ESR-observable polymer radicals under the actual polymerization conditions. The ESR-determined apparent kp value (3.2±9.5 l/mol s at 50°C) decreases with increasing DPE concentration, while the apparent kt value (1.0±10.2 105 l/mol s at 50°C) increases with the DPE concentration. This is because the relative concentration of DBI-ended polymer radical to DPEended one decreases with increasing DPE concentration. References [1] Nagai S, Yoshida K. Kobunshi Kagaku 1960;17:79. [2] Cowie JMG, Pedram MY, Ferguson R. Eur Polym J 1985;21:227. [3] Sato T, Inui S, Tanaka H, Ota T, Kamachi M, Tanaka K. J Polym Sci Polym Chem Ed 1987;25:637.
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[4] Sato T, Morino K, Tanaka H, Ota T. Makromol Chem 1987;188:2951. [5] Otsu T, Yamagishi K, Matsumoto A, Yoshioka M, Watanabe H. Macromolecules 1993;26:3026. [6] Otsu T, Yamagishi K, Yoshioka M. Macromolecules 1992;25:2713. [7] Sato T, Hirose Y, Seno M, Tanaka H, Uchiumi N, Matsumoto M. Eur Polym J 1994;30:347. [8] Sato T, Hirose Y, Seno M, Tanaka H. J Polym Sci A: Polym Chem 1995;33:717. [9] Nakamura H, Seno M, Tanaka H, Sato T. Colloid Polym Sci 1995;273:122. [10] Sato T, Morita N, Tanaka H, Ota T. J Polym Sci A: Polym Chem 1989;27:2497. [11] Sato T, Shimizu T, Seno M, Tanaka H, Ota T. Makromol Chem 1992;193:1439. [12] Sato T, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2659. [13] Nakayama H, Seno M, Tanaka H, Sato T. Makromol Chem 1993;194:1773. [14] Nakamura H, Seno M, Sato T. J Polym Sci A: Polym Chem 1997;35:153. [15] Sato T, Takahashi Y, Seno M, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2909. [16] Sato T, Takahashi K, Tanaka H, Ota T. Macromolecules 1991;24:2330. [17] Sato T, Kawasaki S, Seno M, Tanaka H, Kato K. Makromol Chem 1993;194:2247. [18] Leavitt F, Levy M, Szwarc M, Stannet V. J Am Chem Soc 1954;77:5493. [19] Hageman HJ. Eur Polym J 1999;35:991. [20] Groenenboom CJ, Hageman HJ, Overeem T, Weber AJM. Makromol Chem 1982;183:281. [21] Young LJ. Polymer handbook. 2nd ed. New York: Wiley; 1975. p. II-387. [22] Tanaka H, Sato T, Otsu T. Makromol Chem 1980;181:2421. [23] Gaur HA, Groenen CJ, Hageman HJ, Hakvoort GTM, Oosterho P, Overeem T, Polman RJ, Werf S. Makromol Chem 1984;185:1795.
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Eect of 1,1-diphenylethylene on the radical polymerization of di-n-butyl itaconate in benzene T. Sato *, N. Morita, M. Seno Faculty of Engineering, Department of Chemical Science and Technology, Tokushima University, Minamijosanjima 2-1, Tokushima 770-8506, Japan Received 17 July 2000; received in revised form 8 December 2000; accepted 13 March 2001
Abstract Eect of 1,1-diphenylethylene (DPE) on the polymerization of di-n-butyl itaconate (DBI) with dimethyl 2,20 azobisisobutyrate (MAIB) was studied at 50°C and 60°C in benzene kinetically and by means of ESR. DPE in the low concentration range (up to 0.03 mol/l) was found to serve as a retarder in the polymerization at 50°C and 60°C. As a result, the polymerization rate and the molecular weight of resulting polymer decreased with increasing DPE concentration. The rate constant of the addition of poly(DBI) radical to DPE was estimated to be 74 l/mol s at 50°C and 97 l/mol s at 60°C, respectively, compared to 9.5 l/mol s at 50°C and 11 l/mol s at 60°C as propagation rate constant of DBI. ESR spectrum of the propagating polymer radicals was observed for the actual polymerization mixtures containing DBI, DPE and MAIB in benzene. ESR-determined apparent kp value decreases with increasing DPE concentration, while the apparent kt value increases with the DPE concentration. This is because the relative concentration of DBI-ended polymer radical to DPE-ended one decreases with increasing DPE concentration. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radical polymerization; Di-n-butyl itaconate; Diphenyl ethylene; Retarder; ESR spectrum; Rate constants of propagation and termination
1. Introduction Dialkyl itaconates, 1,1-disubstituted vinyl monomer, are readily polymerized by radical initiators, yielding high molecular weight polymers, in spite of the severe steric requirements of their bulky a-substituents [1±9]. This is explicable on the basis that termination processes are much more suppressed by the substituents than propagation processes. In conformity with this view, the polymerization systems of itaconate esters involve ESRobservable propagating polymer radicals under the actual polymerization conditions [3±9]. So, such polymerization systems have been successfully used for ESR
*
Corresponding author. Tel.: +81-88-656-7402; fax: +81-88655-7025. E-mail address: [email protected] (T. Sato).
elucidation of solvent eect [10,11], Lewis acid eect [12± 14], substituent eect [5,6,15], penultimate eect [16,17] and so on in the radical polymerization. On the other hand, 1,1-diarylethylene such as 1,1-diphenylethylene (DPE) and 1,1-di(p-tolyl) ethylene (DTE) are known to show a high reactivity toward radical species [18] and to serve as retarder in the radical polymerizations of some monomers [19]. DPE and DTE were satisfactorily used as radical scavenger in studies of initiation mechanisms [19,20]. Recently, we have observed that the radical polymerization of di-n-butyl itaconate (DBI) is also retarded by the presence of DPE and studied the eect of DPE kinetically and by means of ESR. The present paper describes the eect of DPE on the polymerization of DBI with dimethyl 2,20 -azobisisobutyrate (MAIB) in benzene on the basis of kinetic and ESR results.
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 1 ) 0 0 0 7 5 - 1
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T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
2. Experimental 2.1. Materials MAIB was recrystallized from methanol. DBI, DPE as monomer were used after distillation. Benzene as solvent was puri®ed by usual method. 2.2. Polymerization Polymerization of DBI in benzene in the presence of DPE was initiated with MAIB in a degassed and sealed tube at a given temperature under shaking. The resulting polymer was isolated by pouring the polymerization mixture into a large excess of methanol containing a small amount of water. The polymer was ®ltered, dried under vacuum and weighed.
Fig. 1. Time±conversion curves in the polymerization of DBI with MAIB in the presence of DPE at 50°C: DBI 1:99 mol/ 1, MAIB 5:00 10 2 mol/1, DPE 1: 0, 2: 1:42 10 3 , 3: 2:84 10 3 , 4: 5:68 10 3 , 5: 11:4 10 3 , 6: 17:0 10 3 , 7: 28:4 10 3 , 8: 42:6 10 3 mol/1.
2.3. Measurements Gel permeation chromatograms (GPC) were recorded on a Toso HLC-802A chromatograph at 38°C using tetrahydrofuran as eluent. From the GPC results, the number-average (M n ) and weight-average (M w ) molecular weights were determined and calibrated with poly(styrene (St)) standards. ESR spectra of the polymerization mixtures in degassed and sealed ESR tube were obtained using a JEOL JES-FE2XG spectrometer operating at X-band (9.5 GHz) with a TE mode cavity. The concentration of polymer radicals was determined by computer double integration of the ®rst derivative ESR spectrum, where 2,2,6,6-tetramethylpiperidin1-oxyl radical (TEMPO) in the same polymerization mixture without MAIB was used for calibration. 3. Results and discussion 3.1. Eect of DPE on the polymerization of DBI with MAIB In order to examine the eect of DPE on the radical polymerization of DBI, DBI was polymerized with MAIB as initiator at 50°C and 60°C in benzene in the presence of DPE. The concentrations of DBI and MAIB were kept constant at 1.99 and 5:00 10 2 mol/l, re-
Fig. 2. Time±conversion curves in the polymerization of DBI with MAIB in the presence of DPE at 60°C: DBI 1:99 mol/ 1, MAIB 5:00 10 2 mol/l, DPE 1: 0, 2: 1:42 10 3 , 3: 2:84 10 3 , 4: 5:68 10 3 , 5: 11:4 10 3 , 6: 17:0 10 3 , 7: 22:7 10 3 , 8: 28:4 10 3 mol/l.
spectively, and the concentration of DPE was varied from 0 to 4:26 10 2 mol/l. Figs. 1 and 2 show time± conversion curves observed at 50°C and 60°C, respectively. Thus, DPE was found to retard eectively the radical polymerization of DBI. The polymer yield in the early stage at each DPE concentration increased pro-
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
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Table 1 Eect of DPE on the polymerization of DBI with MAIB in benzenea T (°C)
DPE 103 (mol/l)
Rp 106 (1/mol s)
M n =103
M w =103
DP
vb
50 50 50 50 50 50 50 50
0 1.42 2.84 5.68 11.4 17.0 28.6 42.6
20.2 9.46 5.80 3.70 2.20 1.67 1.08 0.70
26 23 17 13 11 8 ± ±
50 40 28 20 14 10 ± ±
107 95 70 54 45 33 ± ±
166 78 48 30 18 14 ± ±
60 60 60 60 60 60 60 60
0 1.42 2.84 5.68 11.4 17.0 22.7 28.4
38.3 26.1 19.7 14.1 9.19 6.90 5.06 4.18
14 12 11 11 9 7 ± ±
27 22 19 17 12 9 ± ±
58 50 45 45 37 29 ± ±
64 44 32 24 15 12 ± ±
a b
DBI 1:99 mol/l, MAIB 5:00 10 2 mol/1. Estimated from v Rp =Ri 2kd f MAIB.
portionally with time, although a short induction period was observed in the higher concentration range of DPE where the primary radicals from MAIB may be trapped by DPE. The polymerization rate (Rp ) at each DPE concentration was determined from the time±conversion curves observed in Figs. 1 and 2. The results obtained are summarized in Table 1. Table 1 also lists the molecular weights of resulting polymers and the degrees of polymerization (DP). Thus, the molecular weights of resulting polymers also decreased with increasing DPE concentration. As described above, DPE in the range of concentration used serves as a retarder in the polymerization of DBI with MAIB. The reactivity of DPE toward propagating poly(DBI) radical was estimated according to the method of Kar et al. The present polymerization is considered to be composed of the following elementary reactions: I ! 2R
Rd 2kd I
P M ! P
Rp kp P M
3
Rx kx P X
4
PX M ! P
Ro ko PX M
5
Rt1 kt1 P 2
6
P P ! Polymer PX P ! Polymer
Rt2 kt2 PX P
where I: MAIB, R : primary radical from MAIB, M: DBI, X: DPE, P : propagating poly(DBI) radical, and PX : DPE-ended polymer radical. Rd , Ri , Rp , Rx , Ro , Rt1 , Rt2 , Rt3 , kd , ki , kp , kx , ko , kt1 , kt2 , and kt3 are the rates and rate constants of elementary reactions, respectively. Ri equals 2kd f I, where f initiator eciency.
1=Rp kx X= Ri kp M kt10:5 = R0:5 i kp M
2
8
1
Ri ki R M
Rt3 kt3 PX 2
Assuming stationary concentrations of P and PX , the Kar equation (Eq. (9)) is obtained when Ro 0 and 2 kt2 kt1 kt3 are presumed. Ro 0 is in conformity with the fact that no polymerization proceeded at high DPE concentrations.
R M ! P
P X ! PX
PX PX ! Polymer
7
9
The third term in Eq. (9) is the inverse of the unretarded rate of polymerization in the absence of DPE and can be expressed as 1=R0p . Thus Eq. (9) is reduced to Eq. (10). 1=Rp kx =Ri kp X=M 1=R0p
10
Fig. 3 shows the Kar plots of 1=Rp against the DPE concentration in the polymerization of DBI with MAIB in the presence of DPE as retarder at 50°C and 60°C, respectively, where the concentrations of DBI and MAIB were 1.99 and 5:00 10 2 mol/l. From the slopes
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T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
Fig. 3. Kar plots for the polymerization of DBI with MAIB in the presence of DPE at 50°C (a) and 60°C (b).
of straight lines observed, the value of kx =Ri kp was estimated to be 6:40 107 at 50°C and 1:48 107 at 60°C, respectively. In order to obtain the Ri value (Ri 2kd f MAIB), kd f was determined under the polymerization conditions without DPE by means of radical trapping with TEMPO in the same manner as described in a previous paper [7]. MAIB decomposes into 1-methyl-1-methoxycarbonylethyl radicals as primary radical nitrogen. Some of the primary radicals are deactivated through cage reactions. The others diuse through the solvent cage to react with BDI, leading to polymerization. When MAIB is allowed to decompose in the presence of TEMPO, the primary radicals escaping from the solvent cage are trapped with TEMPO to yield a coupling product (I). Even if the primary radicals react with DBI before being trapped with TEMPO, the resulting oligomer radicals can also be trapped with TEMO. So, the kd f value is able to be evaluated by following the decrease in the TEMPO concentration by ESR.
Fig. 4 shows the plots of the TEMPO concentration against time at 50°C and 60°C in the polymerization mixture in the presence of TEMPO, where the concentrations of DBI, MAIB, and TEMPO were 1.99, 5:00 10 3 , and 2:07 10 4 mol/l, respectively. From the
Fig. 4. Relationship between time and the TEMPO concentration in the polymerization of DBI with MAIB in benzene at 50°C and 60°C: DBI 1:99 mol/l, MAIB 5:00 10 3 mol/l.
slopes of the plots, the disappearance rate (namely 2kd f MAIB) of TEMPO and then the kd f value were estimated. The kd f values thus determined are 1:22 10 6 s 1 at 50°C and 5:93 10 6 s 1 at 60°C. Using kx =Ri kp and Ri obtained above, and kp values [9.5 l/mol s (50°C), 11 l/mol s(60°C)] of DBI determined below by ESR, kx was estimated as the rate constant of the reaction of poly (DBI) radical with DPE (Eq. (11)). The kx values thus obtained are 74 l/mol s at 50°C and 97 l/mol s at 60°C, respectively. A large value of kx =kp (8) may be ascribable to opposite electrical characters of DPE (e 1:35) [21] and DBI (e 0:77) [10]. kx
DBI DPE!
DBI
DPE
11
The kinetic chain length (v) of the present polymerization was calculated from Rp =Ri and is also shown in Table 1. Thus, the obtained v values are comparable with the degrees of polymerization of poly(DBI)s but the former are smaller than the latter except for the case in the absence of DPE. This is probably because the latter values were based on poly(St) standards. The ratio of DP to v tends to increase with the DPE concentration, suggesting that the chain transfer to the monomer less contributes to the DP determining step at higher DPE concentration. 3.2. ESR study on the polymerization of DBI with MAIB in the presence of DPE Fig. 5 presents ESR spectra observed at 50°C, 60°C, and 70°C in the polymerization of DBI with MAIB in
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
Fig. 5. ESR spectra of the DBI/DPE/MAIB system: DBI 0:794 mol/l, DPE 1:14 mol/l, MAIB 5:00 10 2 mol/l.
the presence of DPE at a high concentration, where the concentrations of DBI, DPE and MAIB were 0.794, 1.14 and 5:00 10 2 mol/l, respectively. These spectra are apparently dierent from the ESR spectrum of propagating poly(DBI) radical (P ) and are considered to be arised from DPE. A broad doublet spectrum was reported for a polymer radical with DPE-ended radical center [22]. The hyper®ne structures of the observed spectra indicates that they are assignable to a low molecular radical, namely, primary propagating radical (II) which is formed by the reaction of primary radical with one DPE monomer (Eq. (12)). Observation of radical II alone in the polymerization system containing DPE of a high concentration is in agreement with the above assumption that it is dicult for the DPE radical to further propagate.
12
Next, we have examined the polymerization systems containing DPE at low concentrations used earlier in the kinetic study. Fig. 6 shows ESR spectra of the poly-
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Fig. 6. ESR spectra observed in the polymerization of DBI with MAIB at 60°C in benzene in the presence of DPE: DBI 1:99 mol/l, MAIB 5:00 10 2 mol/l.
merization systems observed at 60°C when the DPE concentration was varied keeping the concentrations of DBI and MAIB constant at 1.99 and 5:00 10 2 mol/l, respectively. The ®ve-line spectrum observed in the absence of DPE is due to propagating poly(DBI) radical (P ). With increasing DPE concentration, the intensity of ESR spectrum decreased and the spectrum shape became broader. Broadening of the spectrum indicates coexistence of another propagating radical center (PX ) in the polymerization system in the presence of DPE. Thus, the total concentration of propagating radicals decreases with DPE concentration. The relative concentration polymer radical with the end of DBI also decreases with increasing DPE concentration. The observed ESR spectra were unchanged in shape and in intensity during ESR measurement for several hours, indicating that a stationary state is reached for the propagating polymer radicals. In order to estimate the apparent rate constants of propagation (kp ) and termination (kt ), the stationary total concentration of polymer radicals ([T ]) was determined at 50°C and 60°C by ESR. Table 2 summarizes the results obtained at various DPE concentrations.
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T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
Table 2 Eect of DPE on kp , kt , the total polymer radical concentration (T ), and the DBI-ended radical (P ) and DPE-ended radical (PX ) concentrations in the polymerization of DBI with MAIB in benzenea DPE 103 (mol/l)
T (°C)
T 106 (mol/l)
kp (l/mol s)
kt 105 (l/mol s)
P 106 (mol/l)
PX 106 (mol/l)
0 1.42 2.84 5.68 11.4 0 1.42 2.84 5.68 11.4 17.0
50 50 50 50 50 60 60 60 60 60 60
1.1 0.72 0.51 0.41 0.35 1.8 1.4 1.1 0.88 0.77 0.68
9.5 6.6 5.7 4.5 3.2 11 9.4 9.0 8.1 6.0 5.1
1.0 2.3 4.7 7.2 10.2 1.8 3.0 4.9 7.7 10.1 12.9
1.1 0.50 0.31 0.19 0.12 1.8 1.20 0.90 0.65 0.42 0.32
0 0.22 0.20 0.21 0.23 0 0.20 0.20 0.23 0.35 0.36
a
DBI 1:99 mol/l, MAIB 5:00 10
2
mol/l.
Apparent values of kp and kt were estimated according to the following Eqs. (13) and (14) for the stationary state polymerization involving bimolecular termination. Rp kp T DBI
13
2kd f MAIB kt T 2
14
The results obtained are also presented in Table 2. The apparent kp value shows a tendency to decrease with increasing DPE concentration. This is caused by the poor reactivity of the DPE-ended polymer radical (PX ). It was above assumed that the DPE-ended radical is dicult to further propagate. Such assumption allows us to estimate each concentration of the DBI-ended radical (P ) and the DPE-ended one (PX ) by using the total polymer radical concentration (T ) and the apparent kp value as follows: P T kp =kp0
15
PX T
16
P
where kp0 is the kp value in the absence of DPE. These results are likewise listed in Table 2. Thus, the concentration of DBI-ended radical (P ) drastically decreases with increasing DPE concentration, while that of DPE-ended radical (PX ) is not so sensitive to change in the DPE concentration. As a result, the total concentration of polymer radicals decreases with the DPE concentration. The apparent kt value increases with increasing DPE concentration, indicating that termination involving DPE-ended radical, PX , is more rapid than that involving DBI-ended radical, P . Such decrease in kp and increase in kt with increasing DPE concentration are responsible for the decrease in Rp observed earlier with increasing DPE concentration.
The termination reactions in the present polymerization are expressed by Eqs. (6)±(8) as described above. Eq. (17) is obtained on the basis of a stationary state shown earlier in the present polymerization. Ri kt T 2 ktl P 2 kt2 P PX kt3 PX 2
17
where kt1 corresponds to the kt value in the absence of DPE. In order to determine the values of kt2 and kt3 , the known values (Table 2) were substituted for Eq. (17) at dierent DPE concentrations. From the crossing points of plots of kt2 against kt3 at each DPE concentration, kt2 and kt3 were estimated as shown in Table 3. These values approximately satisfy the Kar's assumption described 2 above, that is, kt2 kt1 kt3 . As shown in Table 3, the termination involving DPE-ended polymer radical (PX ) is more rapid than that involving DBI-ended polymer radical (P ). This might come from radical combination reaction via the para-positions of the phenyl rings of DPE-ended radical (Eq. (18)) [23]. Since the presence of DPE causes a decrease in the molecular weight of resulting polymer, it is dicult to rule out the possibility that the apparent kt increases with decrease in the molecular weight of polymer as a result of increase in the DPE concentration.
Table 3 Termination rate constants in the polymerization of DBI with MAIB in the presence of DPE T (°C)
kt1 105 (l/mol s)
kt2 105 (l/mol s)
kt3 105 (l/mol s)
50 60
1.0 1.8
4 11
23 53
T. Sato et al. / European Polymer Journal 37 (2001) 2055±2061
18
4. Conclusions DPE in the low concentration range (up to 0.03 mol/l) serves as a retarder in the polymerization of DBI in benzene at 50°C and 60°C. As a result, the polymerization rate and the molecular weight of poly(DBI) decreases with the DPE concentration. The rate constant (kx ) of the addition of poly(DBI) radical to DPE is 74 l/mol s at 50°C and 97 l/mol s at 60°C, compared to 9.5 l/mol s at 50°C and 11 l/mol s at 60°C as the propagation rate constant of DBI. The polymerization system involves ESR-observable polymer radicals under the actual polymerization conditions. The ESR-determined apparent kp value (3.2±9.5 l/mol s at 50°C) decreases with increasing DPE concentration, while the apparent kt value (1.0±10.2 105 l/mol s at 50°C) increases with the DPE concentration. This is because the relative concentration of DBI-ended polymer radical to DPEended one decreases with increasing DPE concentration. References [1] Nagai S, Yoshida K. Kobunshi Kagaku 1960;17:79. [2] Cowie JMG, Pedram MY, Ferguson R. Eur Polym J 1985;21:227. [3] Sato T, Inui S, Tanaka H, Ota T, Kamachi M, Tanaka K. J Polym Sci Polym Chem Ed 1987;25:637.
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[4] Sato T, Morino K, Tanaka H, Ota T. Makromol Chem 1987;188:2951. [5] Otsu T, Yamagishi K, Matsumoto A, Yoshioka M, Watanabe H. Macromolecules 1993;26:3026. [6] Otsu T, Yamagishi K, Yoshioka M. Macromolecules 1992;25:2713. [7] Sato T, Hirose Y, Seno M, Tanaka H, Uchiumi N, Matsumoto M. Eur Polym J 1994;30:347. [8] Sato T, Hirose Y, Seno M, Tanaka H. J Polym Sci A: Polym Chem 1995;33:717. [9] Nakamura H, Seno M, Tanaka H, Sato T. Colloid Polym Sci 1995;273:122. [10] Sato T, Morita N, Tanaka H, Ota T. J Polym Sci A: Polym Chem 1989;27:2497. [11] Sato T, Shimizu T, Seno M, Tanaka H, Ota T. Makromol Chem 1992;193:1439. [12] Sato T, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2659. [13] Nakayama H, Seno M, Tanaka H, Sato T. Makromol Chem 1993;194:1773. [14] Nakamura H, Seno M, Sato T. J Polym Sci A: Polym Chem 1997;35:153. [15] Sato T, Takahashi Y, Seno M, Nakamura H, Tanaka H, Ota T. Makromol Chem 1991;192:2909. [16] Sato T, Takahashi K, Tanaka H, Ota T. Macromolecules 1991;24:2330. [17] Sato T, Kawasaki S, Seno M, Tanaka H, Kato K. Makromol Chem 1993;194:2247. [18] Leavitt F, Levy M, Szwarc M, Stannet V. J Am Chem Soc 1954;77:5493. [19] Hageman HJ. Eur Polym J 1999;35:991. [20] Groenenboom CJ, Hageman HJ, Overeem T, Weber AJM. Makromol Chem 1982;183:281. [21] Young LJ. Polymer handbook. 2nd ed. New York: Wiley; 1975. p. II-387. [22] Tanaka H, Sato T, Otsu T. Makromol Chem 1980;181:2421. [23] Gaur HA, Groenen CJ, Hageman HJ, Hakvoort GTM, Oosterho P, Overeem T, Polman RJ, Werf S. Makromol Chem 1984;185:1795.