The inhibited polymerization of lithium methacrylate and copolymerization with acrylonitrile

The inhibited polymerization of lithium methacrylate and copolymerization with acrylonitrile

European Polymer Journal 36 (2000) 635±642 The inhibited polymerization of lithium methacrylate and copolymerization with acrylonitrile Tadeusz Czern...

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European Polymer Journal 36 (2000) 635±642

The inhibited polymerization of lithium methacrylate and copolymerization with acrylonitrile Tadeusz Czerniawski Faculty of Chemistry, N. Copernicus University, 87-100 TorunÂ, Poland Received 14 April 1997; received in revised form 10 February 1999; accepted 11 March 1999

Abstract Results for the determination of initiation rate of the polymerization reaction of lithium methacrylate and its copolymerization with acrylonitrile in dimethylformamide at various temperatures are reported. The inhibition method involving the stable radical N,N-diphenyl-N'-picrylhydrazyl has been used. The initiation rate constant 2kd f and the activation energy of initiation E2kd f have been determined. An in¯uence of the initial monomer mixture concentration on the measured quantities has been revealed. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction In our previous articles, the results for inhibited polymerization of zinc, cobalt and nickel acrylates [1] and copolymerization of zinc acrylate with acrylonitrile (AN) [2] have been reported. The transition metal salts of acrylates displayed some problems in interpretation of the results. Therefore, an investigation of the inhibited polymerization of lithium methacrylate (LiMA) and its copolymerization with AN was performed.

centration was 100:0520:07% relative to the theoretical value. Acrylonitrile (Koch-Light Laboratories, England) and dimethylformamide (DMF) (Reachim, USSR), both commercial grade, were puri®ed according to standard procedures. Azo-bis-isobutyronitrile (AIBN) (International Enzymes, Windsor, England) was puri®ed by crystallization from methanol. The stable radical DPPH was synthesized as described in the literature [3]. 2.2. Apparatus and polymerization

2. Experimental 2.1. Materials LiMA was obtained by reaction of methacrylic acid and lithium hydroxide. Freshly distilled methacrylic acid (Reachim, USSR) was added in parts to a methanolic solution of recrystallized lithium hydroxide (Chemapol, Prague, Czechoslovakia). A 10% excess of methacrylic acid was used. The monomer was precipitated by a large excess of cool acetone and dried in vacuum at room temperature. The double bond con-

For polymerization kinetic investigation, a Pulfrich nephelometer was used. The initial, clear reaction mixture containing monomer, initiator and inhibitor in DMF solution was put into the nephelometer at a ®xed temperature. The reaction mixture was stirred by blowing of pure nitrogen. The polymer of LiMA is insoluble in DMF. Therefore, the ®rst turbidity of the solution may be taken as the true start of the polymerization. From the plot of turbidity versus time, the length of inhibition period could be determined with good accuracy. The rate of copolymerization of LiMA with AN was

0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 0 5 - 6

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Table 1 The inhibited polymerization of lithium methacrylate in DMF solution (initiator I = AIBN, inhibitor Z = DPPH) Runa

‰I Š0  102 (mol/l)

‰Z Š0 =‰I Š0  102

tinh  10ÿ2 (s)

2kd f  105 (sÿ1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

2.28 1.84 1.84 2.24 1.92 1.89 1.87 1.89 1.90 1.84 1.86 1.87 1.84 1.95

2.16 2.28 3.09 3.39 5.14 2.01 3.05 5.03 6.00 1.04 1.84 3.25 5.16 6.82

6.60 6.90 9.30 10.20 15.66 3.42 4.86 7.98 9.48 0.72 1.20 2.34 3.66 4.92

3.27 3.30 3.32 3.32 3.29 5.88 6.27 6.30 6.33 14.44 15.30 13.88 14.09 13.86

a

Run 1±5: 333 K, 6±9: 338 K, 10±14: 343 K.

determined by the dilatometric method. A two-capillary dilatometer was used. The bulb volume was vd ˆ 11:1212 cm3, the capillary diameter d ˆ 2:119 mm. The meniscus was observed with accuracy of 20.005 mm, using a cathetometer of type KM-8 (USSR). The temperature was measured to better than 20.01 K. The changes of height in the capillaries were converted into relative change of volume …DV=V0 † and plotted as DV=V0 ˆ f (time). From the plot the inhibition period (tinh) was determined.

From the general scheme for inhibited radical polymerization [1], the value of 2kd f may be determined:

2kd f ˆ

‰Z Š0 ‰I Š0 tinh

…1†

where Z is inhibitor, I is initiator, tinh is the time of inhibition, f is the eciency of initiation.

Fig. 1. Polymerization of lithium methacrylate. R Ð 333 K, * Ð 338 K, Q Ð 343 K.

T. Czerniawski / European Polymer Journal 36 (2000) 635±642

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Table 2 Copolymerization of acrylonitrile with lithium methacrylate. [AN]0 = [LiMA]0=1 mol/l Runa

‰I Š0  102 (mol/l)

‰Z Š0 =‰I Š0  102

tinh  10ÿ2

…‰Z Š0 =‰I Š0 tinh †3=2  107

1 2 3 4 5 6 7 8 9 10 11 12 13 14

9.04 10.06 10.96 9.74 8.99 9.38 9.08 9.04 9.06 9.67 9.23 9.23 9.18 9.52

2.69 4.23 6.10 10.14 14.56 1.77 2.84 6.90 10.41 2.04 3.30 6.60 10.44 13.57

1.58 2.24 2.40 3.04 3.48 0.65 0.75 1.01 1.25 0.60 0.75 0.90 1.17 1.25

0.70 0.82 1.28 1.93 2.70 1.42 2.33 5.65 7.59 1.98 2.92 6.28 8.43 11.30

a

Run 1±5: 333 K, 6±9: 338 K, 10±14: 343 K.

3. Results and discussion 3.1. Polymerization of LiMA The data on the polymerization of LiMA, the inhibition period and values of 2kd f are shown in Table 1. A clear-cut inhibition period was observed in all cases. Polymerizations were carried out at 333, 338 and 343 K with various concentration ratios of inhibitor to initiator. Inhibition period tinh vs. ‰Z Š0 =‰I Š0 was plotted (Fig. 1).

At all temperatures, linear dependence is observed. The results may be taken as proof that inhibitor molecules do not participate in reactions other than the basic reaction with the initiator radicals. The average values of 2kd f  105 at 333, 338 and 343 K are equal to 3:3020:02, 6:1920:21 and 14:3120:60 sÿ1, respectively. These values are used to estimate the activation energy of the initiation of polymerization of LiMA. The range of temperatures was only 10 K, but the conditions were those used previously [1,4,5]. From the

Fig. 2. Copolymerization of acrylonitrile with lithium methacrylate. Determination of the inhibition time.

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Fig. 3. Copolymerization of acrylonitrile with lithium methacrylate (before linearization).

Arrhenius plot, the linear correlation coecient is close to unity …r ˆ 0:9918), con®rming the correctness of initiation reaction. The calculated value of E2kd f is 134.2 kJ/mol and the logarithm of its pre-exponential factor A from Eq. (2):

  EA k ˆ A  exp ÿ RT

…2†

is 16.54. The value of E2kd f for LiMA is higher than those for

Fig. 4. Copolymerization of acrylonitrile with lithium methacrylate at di€erent temperatures. R Ð 333 K, * Ð 338 K, Q Ð 343 K.

T. Czerniawski / European Polymer Journal 36 (2000) 635±642 Table 3 Copolymerization of acrylonitrile with lithium acrylate. The values of initiation rate constants and the observed activation energy 2kd f  105 (sÿ1) 0.70 0.93 1.28

T (K) 333 338 343

E2kd f (kj/mol)

log A

57.20

3.82

itiator concentrations and the inhibition time are shown in Fig. 3. It is evident that these relationships are not linear (contrary to LiMA homopolymerization). Similar dependencies were observed for copolymerization of AN with p-bromophenyl acrylate in DMF solution [6]. The lack of linear relationships may indicate that there are additional reactions between inhibitor and monomer. In that case, Eq. (4) transforms into (3) [7]: 

zinc, cobalt and nickel acrylates which are 128.2, 116.9 and 129.8 kJ/mol, respectively [1]. 3.2. Copolymerization of LiMA with AN

‰Z Š0 ‰I Š0 tinh

3=2

  ‰Z Š0 ˆ …2kd f†3=2 1 ‡ 0:74a ‰I Š0

…3†

where a is a coecient describing the shape of relation (4). 

Copolymerization of LiMA with AN was carried out at various temperatures and various concentration ratio of comonomers. Total concentration of comonomers was 2 mol/dm3 in all runs. The conditions are shown in Table 2. The relative changes in the volume of the reaction mixture with reaction time are shown in Fig. 2. The numbers near the straight line correspond to those of runs in Table 2. The increase of inhibition period with inhibitor concentration is evident. From the start of copolymerization, the dependence of relative volume variation upon the time is in all cases identical. It enabled the inhibition period to be found with high accuracy. Similar dependencies were obtained for all the other systems. The relations between the ratio of inhibitor and in-

639

tinh ˆ f

‰Z Š0 ‰I Š0

 …4†

As a result of this transformation, the relation (3) may be represented as the linear function (5): 

‰Z Š0 ‰I Š0 tinh



3=2 ˆf

‰Z Š0 ‰I Š0

 …5†

It is shown in the last column of Table 2 and Fig. 4. Table 3 gives the values of initiation rate constants and the derived values of activation energy for the system. The values of 2kd f may be compared to those obtained for copolymerization of AN with zinc [2] and cobalt, nickel or copper acrylates. Cobalt(II) acrylate (CoA2), nickel acrylate (NiA2) and copper(II) acrylate

Table 4 Copolymerization of AN with CoA2, NiA2 and CoA2 in DMF solution No.

‰Z0 Š  104 (mol/dm3)

T (K)

1 2 3 4

4.172 4.172 4.155 4.177

333.2 338.2 343.2 348.2

5 6 7 8 9 10

6.00 4.00 2.00 4.00 2.00 4.00

333.2 333.2 343.2 343.2 348.2 348.2

11 12 13 14 15

4.173 4.173 4.173 4.173 4.173

333.2 338.2 343.2 343.2 348.2

tinh (sÿ1) AN + CoA2 4031 1444 884 437 AN + NiA2 5554 3719 587.3 1098 294.4 486.5 AN + CuA2 4542 2353 959.0 957.1 476.5

‰Z0 Š=‰I0 Š  102

r

2kd f  105 (sÿ1)

3.4767 3.4767 3.4626 3.4808

ÿ0.9995 ÿ0.9992 ÿ0.9994 ÿ0.9994

0.862 2.407 3.917 7.965

5.0000 3.3333 1.6667 3.3333 1.6667 3.3333

ÿ0.9981 ÿ0.9943 ÿ0.9919 ÿ0.9948 ÿ0.9998 ÿ0.9993

0.902 0.896 2.837 3.037 5.661 6.852

3.4775 3.4775 3.4775 3.4775 3.4775

ÿ0.9968 ÿ0.9978 ÿ0.9993 ÿ0.9986 ÿ0.9997

0.766 1.478 3.626 3.633 7.298

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Table 5 Copolymerization of acrylonitrile with some transition metal acrylates System: AN + ZnA2 AN + CoA2 AN + NiA2 AN + CuA2 2kd f  105 (sÿ1)

T (K) 333 338 343

1.56 3.07 5.92

0.987 2.09 4.08

0.876 1.69 3.19

0.734 1.62 3.51

(CuA2) were obtained as described previously [8]. The data concerning copolymerizations of AN with CoA2, NiA2 and CuA2 in DMF solution are shown in Table 4. In all cases, concentrations of AN and monomeric salts in the initial system were 3.9 and 0.1 mol/l, respectively. Concentration of AIBN was 1:2  10ÿ2 mol/l. A clear-cut inhibition period was observed in all cases. After inhibitor consumption, the relations of monomer conversion versus time were linear. The values of linear correlation coecient r were close to unity. The values of 2Kd f in the last column of Table 4 suggest some e€ect of the metal in monomeric salt on the course of initiation of copolymerization. Taking

into account the great excess of AN in the initial mixture of comonomers, the results are not unexpected. They are generally lower than for copolymerization of AN with zinc acrylate (for example, 2kd f ˆ 1:56  10ÿ5 sÿ1 at 333 K [2]). Changes of 2kd f at temperatures below 340 K form a sequence: ZnA2 > CoA2 > NiA2 > CuA2. The order change at temperatures above 340 K: ZnA2 > CoA2 > CuA2 > NiA2. The di€erence is due to di€erences between the activation energies E2kd f . In Table 5, the values of 2kd f for AN with (ZnA2), (CoA2), (NiA2) and (CuA2) systems are compared. By an analytical method, from Arrhenius plots, the following values of the activation energies E2kd f were obtained. AN±CoA2: E2kd f ˆ 135213 kJ/mol and A ˆ …4:5220:59†  1016 ; AN±NiA2: E2kd f ˆ 12326 kJ/ mol and A ˆ …1:4820:09†  1014 ; AN±CuA2: E2kd f ˆ 14826 kJ/mol and A ˆ …1:3720:06†  1018 , respectively. The higher value of E2kd f for AN±CuA2 scheme is notable. It correlates with the generally lower reactivity of CuA2 in copolymerization with AN [9]. A more complete view of the e€ect of metal type in the acrylate on the initiation of copolymerization can be obtained by changing over a wide range the ratio of monomer concentration in the original reaction mix-

Fig. 5. Copolymerization of acrylonitrile with lithium methacrylate. [AN]0 : [LiMA]0 * Ð 3 : 1, w Ð 1 : 3.

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Table 6 Copolymerization of acrylonitrile with lithium acrylate. The values of initiation rate constants No.

‰I Š0  103

‰Z Š0 =‰I Š0  102

15 16 17 18 19

9.70 9.51 9.18 9.60 10.20

2.50 4.08 6.63 9.97 11.04

20 21 22 23

9.16 9.06 9.18 9.08

1.99 3.36 7.12 13.90

tinh  10ÿ3 [AN] : [LiMA] = 3 : 1 1.26 1.62 2.01 2.28 2.49 [AN] : [LiMA] = 1 : 3 1.17 1.80 2.52 3.27

ture. However, the diculties are poor solubility of the transition metal salts in DMF and insolubility of the copolymers. The values of 2kd f for the studied system are lower than for the copolymerization of AN with the transition metal acrylates. The activation energy …E2kd f ˆ 57:25 kJ/mol) is also signi®cantly lower. This result can be interpreted as indicating a complex LiMA± AN  AIBN [11,12]; such a complex could dissociate more rapidly than the initiator itself. This hypothesis,

0 … ‰IŠ‰ZŠ †3=2  107 0 tinh

2kd f  105 (sÿ1)

0.88 1.26 1.89 2.89 2.95

0.82

0.70 0.81 1.50 2.77

0.89

however, has to be con®rmed by additional spectroscopic studies in UV±vis range. The e€ect of feed composition on 2kd f was studied. Three series experiments at 333 K were performed. Total concentration of monomers was always 2 mol/l (see Table 6. The dependencies of inhibition time on ‰Z Š0 =‰I Š0 for all temperatures were not linear (Fig. 5). Based on Eq. (5), linearization of experimental date was made, as shown in Fig. 6.

Fig. 6. Copolymerization of acrylonitrile with lithium methacrylate. Lines obtained by Eq. (5).[AN]0 : [LiMA]0 * Ð 3 : 1, w Ð 1 : 3.

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Fig. 7. Copolymerization of acrylonitrile with lithium methacrylate. Dependence of 2kd f versus mole fraction of AN in initial mixture.

The values of 2kd f were derived. For [AN] : [LiMA] = 3 : 1, 2kd f ˆ 0:82  10ÿ5 sÿ1; for [AN] : [LiMA] = 1 : 3, 2kd f ˆ 0:89  10ÿ5 sÿ1; for AN 2kd f ˆ 1:55  10ÿ5 sÿ1 [2]. The variation of 2kd f with mixture composition is shown in Fig. 7. Similar results were obtained during studies of AN± p-bromophenyl acrylate in DMF [6] and styrene±ethyl maleate in DMF [10]. On the other hand, for styrene±ethyl acrylate in benzene [13] a linear dependence of Vinh upon the comonomer mixture composition was found. It can be concluded that the nature of the solvent and the comonomer mixture composition a€ects the rate of initiation. The in¯uence of the type of metal on the overall rate of the polymerization and copolymerization of acrylic and methacrylic salts, as investigated by a quantum chemistry method, will be the subject of another paper.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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