ISSN 15600904, Polymer Science, Ser. B, 2010, Vol. 52, Nos. 7–8, pp. 465–472. © Pleiades Publishing, Ltd., 2010. Original Russian Text © E.V. Sivtsov, A.I. Gostev, 2010, published in Russian in Vysokomolekulyarnye Soedineniya, Ser. B, 2010, Vol. 52, No. 8, pp. 1506–1514.
POLYMERIZATION
FreeRadical Copolymerization of NVinylsuccinimide with Butyl Acrylate in the Presence of Zinc Chloride and Aluminum Chloride E. V. Sivtsov and A. I. Gostev St. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia email:
[email protected] Received October 6, 2009; Revised Manuscript Received November 6, 2009
Abstract—The freeradical copolymerization of Nvinylsuccinimide with butyl acrylate performed in dime thyl sulfoxide and benzyl alcohol in the presence of zinc chloride and aluminum chloride as complexing agents is studied. Under the given conditions, the reactivity ratios are determined. It is shown that zinc chlo ride influences the electrondensity distribution only in butyl acrylate molecules. It is found that benzyl alco hol retards the total rate of polymerization . The character of the monomerunit distribution in copolymer macromolecules is described. DOI: 10.1134/S1560090410070110
INTRODUCTION The copolymers of Nvinylsuccinimide (VSI) with Nbutyl acrylate (BA) are of practical interest for the production of film materials [1–3] and adhesives [4] for medical use. Medical applications require alkaline CH2 CH CH2 CH N C O O C C O O CH2 CH2 C4H9
NaOH
hydrolysis of the mentioned copolymers; in this case, the succinimide cycle is opened to form units of Nvinylamidosuccinic acid capable of reversible inter action with lowmolecularmass physiologically active substances of an alkaline character. The BA units function as an internal plasticizer.
CH2 CH CH2 NH C O CH2 CH2 COONa
The copolymerization of VSI with BA in DMSO [5, 6], acetic anhydride [7, 8], pyridine [9], and trieth ylamine and tributylamine [10] was investigated. The relative activities of the monomers and the parameters of the donor–acceptor capacity of the corresponding solvents are given in Table 1. The main problem encountered in the synthesis of the above copolymer is a significant difference in the activities of the mono mers, which leads to products of nonuniform compo sition. As can be seen from Table 1, variation in the nature of the reaction solutions affects the reactivity ratios, although it cannot substantially change them. It is well known [11] that complexing agents (as a rule, Lewis acids), which are specially added to the reaction system, can strongly influence the activity of mono mers in radical polymerization. Therefore, the aim of
CH C O O C4H9
NaOH –C4H9OH
CH2 CH CH2 CH NH COONa C O CH2 CH2 COONa
this study was to investigate the freeradical copoly merization of VSI and BA in the presence of zinc chlo ride and aluminum chloride. EXPERIMENTAL VSI was synthesized as described in [12] and recrystallized three times from solution in isopropyl alcohol (Tm = 48.5°С, and nD50 = 1.5020). BA was dis tilled under vacuum directly before use. DMSO and benzyl alcohol were distilled two times under vacuum. AIBN was recrystallized two times from ethanol at (50 ± 2)°C and vacuumdried at 20°C (Tm = 104°С). Zinc chloride and aluminum chloride (analytical grade) were used without further purification.
465
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SIVTSOV, GOSTEV
Table 1. Reactivity ratios of VSI (M1) copolymerization with BA (M2) in different solvents Reactivity ratios calculated by the method from
Electrondonor and acceptorproperties of solvents Solvent proton affinity, kJ/mol
[14]
acceptor capacity
124.7
19.3
DMSO + ZnCl2
–
–
–
–
0.006 3.47 0.004 3.47
Acetic anhydride [7]
43.9
–
10.0
–
0.05
0.94 0.04
0.97
Pyridine [9]
138.5
14.2
9.34
930.0
0.26
1.86 0.29
1.86
Triethylamine [10]
132.6
1.4
7.53
981.8
0.07
2.67 0.07
2.67
Tributylamine [10]
–
–
7.86
998.5
0.02
1.61 0.05
1.61
Benzyl alcohol
–
–
8.26
778.3
0.02
1.93 0.004 2.06
Benzyl alcohol + AlCl3
–
–
–
–
0.01
1.48 0.001 1.56
DMSO [6]
ionization energy, eV
[13]
donor capacity, kJ/mol
9.1
884.4
r1 0.07
r2
r1
r2
2.76 0.07
2.78
Table 2. Signal assignment in 1H NMR spectra of VSI–BA mixtures with ZnCl2 Composition of mixture, mol/mol
Chemical shifts of vinyl protons, ppm doublet H
doublet of doublets H
doublet H
VSI
5.98
5.94
6.66
6.64
6.62
6.60
5.00
4.98
VSI : ZnCl2 = 1 : 1
5.94
5.90
6.65
6.63
6.61
6.59
5.02
4.99
VSI : ZnCl2 = 1 : 2
5.94
5.90
6.65
6.62
6.61
6.58
5.01
4.99
VSI : ZnCl2 = 1 : 4
5.94
5.90
6.65
6.63
6.61
6.59
5.02
4.99
Samples for the VSI–BA copolymerization were prepared by dissolving calculated amounts of AIBN, VSI, BA, and a complexing agent (zinc chloride or aluminum chloride) in an appropriate solvent. The mixture was placed in an ampoule, which was filled with argon via repeated freeze–pump–thaw cycles and sealed. The reaction mixture was sampled with a syringe through a rubber stopper and was immediately frozen. 1H NMR spectra were recorded in deuterated chloroform at 25°С and a frequency of 500 MHz; tet ramethylsilane was used as an internal standard. In this study, the copolymerization of VSI and BA in DMSO in the presence of zinc chloride and in ben zyl alcohol in the presence of aluminum chloride was examined. The process conditions were as follows: a total concentration of monomers of 0.7 mol/l, [AIBN] = 1.65 × 10–2 mol/l, molar ratios of ZnCl2 : VSI = 4 : 1 and AlCl3 : VSI = 1 : 4 to 1 : 12; and a tem perature of 60°C. The concentration of complexing agents was close to the concentration of their saturated solutions under the conditions of synthesis. The solu bility of AlCl3 is significantly lower than that of ZnCl2;
however, under conditions of complexation resulting in redistribution of the electron density in monomer molecules, even at a complexingagenttomonomer ratio of 0.1 : 0.5 (mol/mol), the relative activities of monomers change markedly [11]. RESULTS AND DISCUSSION Effect of Complexing Agents on the ElectronDensity Distribution in Monomer Molecules 1 The H NMR spectra of monomer–ZnCl2 mix tures with molar ratios of 1 : 1, 1 : 2, and 1 : 4 were examined to discover the possible interactions between monomers and complexing agents. As can be seen from Table 2, the presence of zinc chloride has practically no effect on the positions of signals due to vinyl protons of VSI, but results in pronounced changes in the spectra of BA (Table 3). Thus, during further interpretation of the data on the relative activities of VSI and BA in copolymeriza tion conducted in the presence of ZnCl2, the effect of the complexing agent on the electrondensity distribu
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Table 3. Signal assignment in 1H NMR spectra of BA–ZnCl2 mixtures Composition of mixture, mol/mol
Chemical shifts of vinyl protons, ppm doublet H
doublet of doublets H
doublet H
BA
6.32
6.28
6.14
6.12
6.10
6.07
5.89
5.87
BA : ZnCl2 = 1 : 1
6.26
6.22
6.12
6.09
–
6.05
5.87
5.84
BA : ZnCl2 = 1 : 2
6.29
6.25
6.16
6.13
6.12
6.09
5.90
5.88
BA : ZnCl2 = 1 : 4
6.25
6.21
6.11
6.09
–
6.05
5.86
5.84
Table 4. Signal assignment in 1H NMR spectra of VSI–BA copolymers in the presence and in the absence of AlCl3 Composition of mixture, mol/mol
Chemical shifts of vinyl protons, ppm doublet H
doublet of doublets H
doublet H
BA
6.41
6.38
6.15
6.12
6.11
6.09
5.82
5.80
BA : AlCl3 = 1 : 1
6.41
6.38
6.15
6.12
6.11
6.09
5.82
5.79
VSI
6.11
6.08
6.70
6.68
6.67
6.65
5.09
5.07
VSI : AlCl3 = 1 : 1
6.12
6.08
6.71
6.69
6.67
6.65
5.09
5.07
tion in BA must be considered, whereas its influence on VSI can be disregarded. A different situation is observed in the presence of aluminum chloride (Table 4). As is seen from Table 4, the complexing agent does not affect the positions of signals due to vinyl protons in the 1H NMR spectra of both monomers. Therefore, aluminum chloride will have no significant effect on copolymerization in the above system. The VSI–BA Copolymerization in DMSO in the Presence of ZnCl2 The copolymerization of VSI with BA in DMSO has been described in detail [5, 6]. The process occurs at a high rate under homogeneous conditions as well (Fig. 1a). In the presence of ZnCl2, the quality of the solvent with respect to the copolymer being formed worsens. Therefore, the polymer separates into an individual phase. Visually, this effect manifests itself as opalescence of the reaction mixture arising at certain conversions. Moreover, at monomer conversions of 25–35%, the reaction accelerates (Sshaped kinetic curves in Fig. 1b), a result that is more typical for bulk polymerization than polymerization in dilute mono mer solutions. This finding apparently can be POLYMER SCIENCE
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explained by the fact that growing radicals turn out to be in the forming phase with a relatively low molecular mobility. As a result, the rate of chain termination declines and, accordingly, the total rate of polymeriza tion increases. In both cases, the rate of polymeriza tion increases with an increase in the content of VSI in the initial monomer mixture. During polymerization, the reaction mixture was sampled, while the 1H NMR spectra of the samples were measured. A good resolution of 1H NMR signals due to vinyl protons of VSI and BA made it possible to determine the current concentrations of the mono mers and the composition of the resulting copolymers. Figure 2 shows variation in monomer concentrations with conversion during copolymerization of an equimolar monomer mixture conducted in the pres ence of zinc without zinc chloride. It is evident that the rate of incorporation of BA into the polymer chain is significantly higher than that of VSI. Note that, at a conversion of ~70%, a more active BA is fully exhausted (Fig. 2, curve 4), and, from this time, the homopolymerization of residual VSI occurs. As a result, an extremely nonuniform product must be obtained. With the data on the current concentrations of the monomers, the composition of the copolymers can be 2010
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SIVTSOV, GOSTEV [VSI], [BA], mol/l 0.4
Conversion, % (a)
90
2 1
60
0.2
1 3
30 4
2
(b)
90
2 0
15
45
1
Fig. 2. Concentration of (1, 3) VSI and (2, 4) BA mono mers vs. conversion for copolymerization of a VSI–BA equimolar mixture in the (1, 2) presence and (3, 4) absence of ZnCl2.
60
30
0
75 Conversion, %
40
80 Time, min
a decrease in the reactivity ratio r1 = k11/k12 by more than an order of magnitude can be rationalized by an increase in the rate constant of BA addition to a VSI endcapped macroradical (k12) due to a change in the polarization of the double bond of BA complexed with ZnCl2. The values of k11 are unaffected by the addition of ZnCl2. An insignificant gain in r2 is probably related to an increase in k22 at practically invariable constant k21.
120
Fig. 1. Conversion vs. time for VSI–BA copolymerization in DMSO in the (a) absence and (b) presence of ZnCl2. VSI content in the initial monomer mixture: (a) (1) 50 and (2) 60 mol %; (b) (1) 60 and (2) 70 mol %.
calculated. Figure 3 shows the overall and instanta neous compositions of copolymers as a function of conversion during copolymerization of an equimolar mixture of the monomers. The instantaneous compo sition implies the composition of the copolymer frac tion formed between successive samplings of the reac tion mixture. When copolymerization is performed in the presence of a complexing agent, the copolymer composition changes more rapidly (Fig. 3, curves 1, 2) owing to a greater difference in the activities of the monomers. The reactivity ratios of VSI–BA copolymerization in DMSO in the presence of ZnCl2 were calculated by the Ezrielev–Brokhina–Roskin [13] and Kelen– Tüdõs [14] methods (Table 1). The introduction of the electron acceptor ZnCl2 resulted in a much higher difference in the relative activities of the monomers. As was noted above, the presence of ZnCl2 affects only the electrondensity distribution in BA molecules. On the basis of this fact,
VSI–BA Copolymerization in Benzyl Alcohol in the Presence of AlCl3 The copolymerization of VSI with BA in benzyl alcohol was studied for the first time. On the basis of the data on the composition of the copolymers iso lated at different conversions (Fig. 4), the reactivity ratios were calculated for an equimolar monomer mix ture (Table 1). These values proved to be very close to those obtained previously for copolymerization con ducted in tributylamine as a solvent, in accordance with the electrondonor characters of both solvents. In benzyl alcohol, the copolymerization proceeds at a lower rate than that in DMSO (Fig. 5), but the retar dation effect of benzyl alcohol is more weakly pro nounced than that of tributylamine. The presence of aluminum chloride has practically no effect on the rate of reaction. Retardation should be related to for mation of sufficiently stable πcomplexes of growing macroradicals with benzyl alcohol. This phenomenon is typical of polymerization in aromatic solvents [11]. In addition, the rate of polymerization is affected by the viscosity of solvents. However, in this case, the
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1
60 3 2 4 30
0
30
60
90 Conversion, %
Fig. 3. (1, 3) Overall m and (2, 4) instantaneous dm composition of copolymer vs. conversion for copolymerization of a VSI–BA equimolar mixture in DMSO in the (1, 2) presence and (3, 4) absence of ZnCl2.
m, dm, mol/l 90
8 4
60
5 1 6 2
3 30
7
0
20
40 Conversion, %
Fig. 4. (1, 2, 5, 6) Instantaneous dm and (3, 4, 7, 8) overall m composition of copolymer vs. conversion for copolymerization of a VSI–BA equimolar mixture in benzyl alcohol in the (1–4) presence and (5–8) absence of AlCl3.
reverse effect might be expected (at 25°С, the viscosi ties of DMSO and benzyl alcohol are 2.0 and 5.05 mPa s, respectively). The formation of π complexes of radicals with a solvent is difficult to verify experimentally. However, a phenomenon providing support for this assumption was discovered. In all earlier studied VSI–BA systems, the total polymerization rate increased with the amount of VSI in the initial monomer mixture (Fig. 1, POLYMER SCIENCE
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curves 1, 2), a result that was explained by a high reac tivity of VSI radicals. As was shown in [11], it is pre cisely these active radicals that are prone to complex ation with aromatic solvents. There are indications that the mentioned complexes do not participate in the chainpropagation reaction. The kinetic curves shown in Fig. 5 (curves 3, 5, 6) demonstrate that the rate of the VSI–BA copolymerization decreases as the content of VSI in the initial monomer mixture increases. This result can be explained by an increase 2010
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SIVTSOV, GOSTEV Conversion, % 1 60
5 3 30
4
2
6
0
200
400 Time, min
Fig. 5. Conversion vs. time for copolymerization of a VSI–BA mixture in (1) DMSO, (2) tributylamine, and (3–6) benzyl alcohol in the (1–3, 5, 6) absence and (4) presence of AlCl3: VSI : BA = (1–4) 50 : 50, (5) 25 : 75, and (6) 75 : 25, mol/mol .
in the fraction of “inactive” radicals complexed with benzyl alcohol. (Such complexation is not typical for acrylates.) The relative activities of the monomers are affected only slightly by the complexation of growing radicals with benzyl alcohol in comparison to that with some other electrondonor solvents (Table 1). This circum stance is easily understood under the assumption that the rate of attaining equilibrium in the complexation reaction is lower than the rate of radical addition to the monomer. In this case, the complexation results in a rapid transformation of a fraction of radicals into a state in which they cannot participate in polymeriza tion, while the uncomplexed radicals become involved in chain propagation. Thus, the effect of complexation is probably reduced to a decrease in the number of the active centers of propagation, while the relative activ ities of monomers remain unchanged. In fact, the addition of aluminum chloride to the copolymerization of VSI with BA in benzyl alcohol does not change the rate of monomer consumption during the process (Fig. 4); i.e., their relative activities remain unchanged. Effect of Complexing Agent on the Microstructure of VSI–BA Copolymers Information on the structure of polymer chains can be collected with the use of 13C NMR spectroscopy. Figure 6 shows the 13C NMR spectra of the VSI–BA copolymers with similar compositions prepared in the
absence and in the presence of zinc chloride. The spectrum displays signals at 177.9–177.5, 45.5–44.2, 34.6–30.8, and 27.6 ppm due to carbonyl carbon atoms, methine carbons of the backbone, methylene carbons, and methylene carbons of the VSI cycle, respectively. The accurate assignment of the signals to the unit sequences of different types calls for addi tional investigation with some model compounds. However, even at our level of interpretation, it is inferred that the simplified structure of the signals in the spectrum of the copolymer synthesized in the pres ence of ZnCl2 is indicative of a narrower set of unit sequences of different types and is explained by the predominance of individual VSI units located between long sequences of BA units. This suggestion correlates with the predicted character of the monomerunit dis tribution in macromolecules of VSI–BA copolymers synthesized in DMSO in the presence and in the absence of ZnCl2, which was calculated from reactiv ity ratios r1 and r2 by known methods [15] with the use of the Kinetika program developed at the Department of Chemical Technology of Plastics, St. Petersburg Technological Institute (Technical University) [16] (Table 5). The addition of zinc chloride leads to a decrease in the content of alternating structures in polymer chains f12 and f21 and to an increase in the length of sequences composed of several BA units (Z2). A marked differ ence in the activities of the monomers favors wide range changes in the monomer mixture in the course of copolymerization, and this circumstance, in turn, determines the type of distribution over the length of
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CH2 (a)
CH CH3
C=O
CH2 CH
200
150
100
50
(b)
CH
0 CH3 δ, ppm CH2
CH2 C=O CH 180
120
60
0 δ, ppm
(c) C=O
CH
174
180
CH2
45
40
35
δ, ppm
(d) C=O CH
180
174
CH2
45
40
35 δ, ppm
Fig. 6. 13C NMR spectra of VSI–BA copolymers obtained in the (a, c) absence of ZnCl2 (38 mol % VSI in copolymer) and (b, d) presence of ZnCl2 (32 mol % VSI in copolymer)
BA unit sequences in copolymers obtained at high conversions. The probability of formation of short BA blocks increases with the VSI content in the monomer mixture, and the tendency toward unit alternation in a macromolecule becomes more pronounced.
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CONCLUSIONS The effect of the complexing agents zinc chloride and aluminum chloride on the freeradical copoly merization of Nvinylsuccinimide with butyl acrylate has been studied for the first time. With the use of 1H NMR spectroscopy, it has been shown that zinc chloride affects the electrondensity distribution only in BA molecules, whereas chemical shifts in the spec 2010
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Table 5. Probability of diad formation f and statistical length Z of blocks of VSI (M1) and BA (M2) units in VSI–BA copolymers obtained in DMSO in the presence and in the absence of ZnCl2 Z1
Z2
0
0
0.925
0.940
0.037
0.030
1.0
1.0
25.8
32.2
20
0.001
0
0.846
0.874
0.077
0.063
1.0
1.0
12.0
14.9
30
0.004
0
0.760
0.802
0.118
0.099
1.0
1.0
7.4
9.1
40
0.007
0.001
0.669
0.722
0.162
0.139
1.0
1.0
5.1
6.2
50
0.014
0.001
0.572
0.634
0.207
0.183
1.0
1.0
3.8
4.5
60
0.026
0.002
0.467
0.535
0.254
0.231
1.1
1.0
2.8
3.3
70
0.048
0.004
0.354
0.425
0.299
0.286
1.2
1.0
2.2
2.5
80
0.093
0.009
0.233
0.300
0.337
0.346
1.3
1.0
1.7
1.9
90
0.212
0.024
0.105
0.158
0.342
0.409
1.6
1.1
1.3
1.4
BA
tra of both monomers are insensitive to aluminum chloride. A significant drop in the rate of polymeriza tion is found when benzyl alcohol is used as a solvent. This phenomenon is explained by the formation of πcomplexes of VSI radicals with benzyl alcohol that are inactive in polymerization, as evidenced by an increase in the total rate of reaction with a decrease in the content of VSI in the monomer mixture. The reac tivity ratios have been determined for the VSI–BA copolymerization in DMSO in the presence of zinc chloride and in benzyl alcohol in the presence and in the absence of aluminum chloride. 13C NMR spec troscopy studies have shown that, in the presence of zinc chloride, the structure of polymer chains in which individual VSI units are surrounded by sequences composed of several BA units is predominant. REFERENCES 1. L. I. Shal’nova, V. M. Chudnova, and E. A. Trofimova, Plast. Massy, No. 11, 29 (1989). 2. N. A. Lavrov, L. I. Shal’nova, and A. F. Nikolaev, Zh. Prikl. Khim. (S.Peterburg) 70, 1356 (1997). 3. N. A. Lavrov and L. I. Shal’nova, Klei Germet. Tekh nol., No. 9, 2 (2008). 4. E. V. Sivtsov and N. A. Lavrov, Klei Germet. Tekhnol., No. 3, 13 (2007).
BA
BA–ZnCl2
10
VSI
VSI–ZnCl2
VSI
VSI content in the initial monomer mixture, mol %
VSI–ZnCl2
VSI–BA–ZnCl2
f12 = f21
VSI–BА
f22
BA–ZnCl2
f11
5. N. A. Lavrov, E. V. Sivtsov, and A. F. Nikolaev, Zh. Prikl. Khim. (S.Peterburg) 71, 2055 (1998). 6. E. V. Sivtsov, A. I. Gostev, and N. A. Lavrov, Zh. Prikl. Khim. (S.Peterburg) 80, 1679 (2007). 7. N. A. Lavrov, E. V. Sivtsov, and A. F. Nikolaev, Zh. Prikl. Khim. (S.Peterburg) 73, 683 (2000). 8. N. A. Lavrov, E. V. Sivtsov, and A. F. Nikolaev, Plast. Massy, No. 10, 43 (2001). 9. N. A. Lavrov, E. V. Sivtsov, and A. G. Pisarev, Zh. Prikl. Khim. (S.Peterburg) 76, 1154 (2003). 10. E. V. Sivtsov, A. I. Gostev, and N. A. Lavrov, Zh. Prikl. Khim. (S.Peterburg) 82, 1186 (2009). 11. V. A. Kabanov, V. P. Zubov, and Yu. D. Semchikov, ComplexRadical Polymerization (Khimiya, Moscow, 1987) [in Russian]. 12. V. M. Chudnova, N. A. Lavrov, and L. I. Shal’nova, Synthesis of Homopolymers and Copolymers of NVinyla mides of Succinic and Phthalic Acids: Methodical Instructions (LTI, Leningrad, 1989) [in Russian]. 13. A. I. Ezrielev, E. L. Brokhina, and E. S. Roskin, Vysokomol. Soedin., Ser. A 11, 1670 (1969). 14. F. Tüdõs, T. Kelen, T. FöldesBerezhnykh, and B. Turc sányi, React. Kinet. Catal. Lett. 2, 439 (1975). 15. N. A. Lavrov, Calculation of Unit Alternation in Copoly mers: Methodical Instructions (LTI, Leningrad, 1988) [in Russian]. 16. E. V. Sivtsov, N. A. Lavrov, and R. A. Kulinskii, Reac tivity Ratios and Microstructure of Copolymers: A Manual (S.Peterb. Gos. Tekhnol. Inst. (Tekh. Univ.), St. Petersburg, 2007) [in Russian].
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