Solvent effects on the free-radical copolymerization of ...

Solvent Effects on the Free-Radical Copolymerization of Styrene with Butyl Acrylate. I. Monomer Reactivity Ratios ´ PEZ MADRUGA1, MARTA FERNAŃDEZ-GARCIÁ1, MARINA FERNAŃDEZ-SANZ1, ENRIQUE LO 2 2 ´ ROCIO CUERVO-RODRIGUEZ, VICENTE HERNANDEZ-GORDO, M. C. FERNAŃDEZ-MONREAL2 1

Instituto de Ciencia y Tecnologıá de Polı´meros (CSIC), Juan de la Cierva, 3. 28006, Madrid, Spain

2

Departamento de Quı´mica Orgańica I, Facultad de Ciencias Quı´micas, Universidad Complutense, Ciudad Universitaria, 28040, Madrid, Spain

Received 27 April 1999; accepted 24 September 1999

ABSTRACT: The free-radical copolymerization of styrene with butyl acrylate was carried out in benzene and benzonitrile at 50°C. Differences between the apparent reactivity ratios determined in this work and those previously reported in bulk indicated noticeable solvent effects. This is explained by a qualitative bootstrap effect. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 60 – 67, 2000

Keywords: styrene/butyl acrylate/benzene copolymerization; benzonitrile copolymerization; monomer reactivity ratios; solvent effect; bootstrap effect

INTRODUCTION Since Ito and Otsu1 described in 1969 the effect of benzene, benzonitrile, benzyl alcohol, and phenol on the free-radical copolymerization parameters of styrene (S) with methyl methacrylate (MMA), many reports about solvent effects on copolymerization have been published. In general, small effects of solvents on copolymerization reactivity ratios have been observed; their extents strongly have been dependent on the nature of the monomers and solvents involved. Although numerous explanations of the observed phenomenon have been given,2 the subject did not receive great attention until Harwood’s3 observation about the effect of different solvents on sequence distribution in copolymerization. In addition to Ito and Otsu’s work,1 solvent effects on the free-radical copolymerization of S/MMA have been reported by Bonta et al.4 and by our group.5,6 Our results subsequently were reanalyzed in publications by Davis7 and by Correspondence to: E. L. Madruga Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 38, 60 – 67 (2000) © 2000 John Wiley & Sons, Inc.

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Klumperman and O’Driscoll.8 In the first one,7 a bootstrap effect was claimed for the solvent effects on the free-radical copolymerization of S/MMA, and, in the second one,8 our results were taken to quantify the partition coefficient as described by Harwood.3 Recently, O’Driscoll, Monteiro, and Klumperman9 studied the homo- and copolymerization of S and MMA in the presence of several levels of benzyl alcohol, finding that the apparent propagation rate coefficient for the homopolymerization of MMA and S and the copolymerization of MMA : S (1 : 1) increased with increasing benzyl alcohol. They also showed that the reactivity ratios for S decreased somewhat, but those for MMA changed little with increasing benzyl alcohol. The results were interpreted on the assumption that radical–solvent complexes could describe the effect produced by the solvent. Although the copolymerization of S with acrylates is very important to industry for applications such as paints, adhesives, and coatings, these systems have not been studied extensively. In particular, only reactivity ratios of the styrene/ butyl acrylate (S/BA) system have been published.10 –14

SOLVENT EFFECTS ON THE FREE-RADICAL COPOLYMERIZATION OF STYRENE WITH BUTYL ACRYLATE

Recent papers have shown the copolymerization rate at a low conversion,13,14 and Dube´ et al.15,16 developed a mechanistic model to analyze the bulk copolymerization of this system in the full range of conversion. Kinetic analysis of S/BA copolymerization has been studied with the pulselaser polymerization technique,17 and, from studies of steady-state free-radical copolymerization, the monomer and radical reactivity ratios of the free-radical copolymerization of S/BA in bulk have been described.18 However, to our knowledge, solvent effects on this system have never been described. Thus, the aim of this article is to analyze the effect of two aromatic solvents, benzene and benzonitrile, on the monomer reactivity ratios of the S/BA system. Furthermore, to obtain the same insight into the solvent effect on copolymerization and on the basis of the structural similarity between MMA and BA, the obtained results are compared with those previously obtained for the S/MMA system.

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Copolymer Analysis 1

H NMR spectroscopy was used to determine the copolymer compositions. Spectra were recorded at room temperature on about 8% (w/v) solutions in deuterochloroform with a Varian Geminis 200 spectrometer (Varian Associates Inc., Palo Alto, CA) operating at 200 MHz. The copolymer compositions were determined by the comparison of the integrated intensities of the resonance signals due to the phenyl protons of S and that of the methylene group closest to the oxygen in the ester portion of BA. 13 C NMR spectroscopy was used to determine the sequence distribution of the copolymers. Spectra were recorded with a Varian XLR-300 spectrometer (75 MHz) at 50°C on about 25% (w/v) solutions in deuterochloroform. The relative areas of the peaks were measured by an electronic integrator.

RESULTS AND DISCUSSION EXPERIMENTAL PART Materials The monomers S and BA (BA) were purified by conventional methods.19 2-29- Azoisobutyronitrile (AIBN) was purified by successive crystallizations from methanol. Benzene (Merck, Schuchardt 8011 Hohenbrunn bei Mu¨nchen) and benzonitrile (Fluka, Chemie AG CH-9470 Buchs) for analysis were used without further purification. All the others solvents used over the course of experiments and characterization (methanol, and chloroform-d) were employed as packaged without further purification.

Reactivity Ratios Free-radical copolymerization of S/BA was carried out at 50°C in a 3-mol/L benzene or benzonitrile solution with 5 z 1023 mol/L of AIBN as the initiator concentration. Monomer-feed compositions were planned according to the “approximate design scheme” proposed by McFarlane et al.20 As described by Tidwell and Mortimer,21 this scheme uses the optimally designed experiment, in which the value of the mole fractions in the feed, f19 and f10, are given by the following expressions: f91 5

2 2 1 r1

(1.a)

f 01 5

r2 2 1 r2

(1.b)

Copolymerization Copolymerization reactions were conducted with 5 z 1023 mol/L of AIBN as the initiator and benzene or benzonitrile as the solvent (3 mol/L) at 50 6 0.1°C in Pyrex-glass ampoules sealed under a high vacuum. For each feed composition, the calculated amount of the monomers and initiator were weighed and added to the required volume of benzene or benzonitrile. We isolated the resulting copolymers by pouring the contents of the glass ampoules into methanol. The precipitated materials were purified by reprecipitation from benzene-methanol and then filtered and dried until a constant weight was attained.

where r1 and r2 are the copolymer reactivity ratios of monomer 1 (S) and monomer 2 (BA), respectively. However, recognizing that the exact reactivity ratios cannot be known in advance, McFarlane et al.20 suggested assuming values for them that differ from r1 and r2 by 20%. To plan the feed composition, two preliminary pairs of reactivity ratios were used. When the copolymerizations were performed in benzene, r1 5 0.95 and r2

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´ NDEZ-GARCIÁ ET AL. FERNA

5 0.18 (reported by Dube´ et al.15) were used. When the copolymerization was carried out in benzonitrile, the preliminary reactivity ratios used were those obtained in benzene in this work. In addition to the optimal feed compositions, compositions as close as possible to those calculated with eq 1(a,b) for two intermediate monomer feeds in benzene (f1 5 0.450, f1 5 0.250) or in benzonitrile (f1 50.45, f1 5 0.245) were used. Each experiment, with a specific monomer feed, was conducted to various overall conversions (defined as the weight fraction of monomers converted to copolymer) but never higher than 11%. The monomer feed, overall conversion, and copolymer composition for each sample prepared in benzene or benzonitrile are shown in Tables I and II, respectively. For the intervals of the overall conversions studied, no significant changes in copolymer composition were observed. Consequently, the values of the monomer reactivity ratios were obtained by application of the nonlinear least-squares analysis, as suggested by Tidwell and Mortimer,21 to the data listed in Tables I and II. The values obtained for the S/BA/benzene and S/BA/benzonitrile systems along with those obtained previously in bulk18 are given in Table III. From these results, it seems that the solvent exerted an apparent effect on the monomer reactivity ratio values. To determine if the observed differences were significant, estimates of the accuracy of the values are represented in Figure 1 as the 95% joint confidence interval. In Figure 1, the confidence interval previously reported for the S/BA/bulk system18 also is reported. When the copolymerization was carried out in bulk or in benzene, the confidence intervals clearly overlapped, but the confidence interval obtained when the copolymerizations were performed in benzonitrile was separated from the other two. However, there was a remarkable difference between the surface areas of the confidence interval of the data obtained in bulk or benzene and those of the confidence interval of the data obtained in benzonitrile. This may have been a consequence of the copolymer conversions being slightly higher in benzonitrile, because in both solvents the numbers of the experiment were similar. Therefore, the apparent reactivity ratios obtained were influenced significantly when the solvent used was benzonitrile. The apparent reactivity ratios collected in Table III indicate that the reactivity ratio of S decreased whereas the reactivity ratio of BA increased with the raising of the solvent polarity. A

Table I. Experimental Results of S and BA Copolymerization in Benzene fs Feed

Conversion (%)

FS Copolymer

0.723 0.723 0.723 0.723 0.723 0.723 0.678 0.678 0.678 0.678 0.678 0.678 0.637 0.637 0.637 0.637 0.637 0.637 0.450 0.450 0.450 0.450 0.450 0.450 0.250 0.250 0.250 0.250 0.250 0.097 0.097 0.097 0.097 0.097 0.097 0.083 0.083 0.083 0.083 0.083 0.083 0.067 0.067 0.067 0.067

1.59 2.42 3.22 4.97 5.43 5.92 1.34 1.72 2.67 3.43 3.98 4.86 1.28 1.73 2.43 3.44 4.36 5.19 1.65 2.58 3.15 3.56 4.19 4.97 1.84 2.29 2.56 3.61 4.13 1.30 2.21 3.45 3.89 5.06 4.82 1.00 2.17 4.11 5.35 6.26 8.42 1.45 3.69 4.69 8.71

0.767 0.742 0.763 0.657 0.756 0.760 0.732 0.691 0.708 0.764 0.721 0.717 0.721 0.692 0.727 0.682 0.695 0.695 0.561 0.573 0.490 0.601 0.585 0.580 0.472 0.453 0.472 0.483 0.475 0.276 0.295 0.293 0.302 0.302 0.268 0.281 0.264 0.292 0.262 0.228 0.255 0.244 0.239 0.256 0.216

similar behavior also was observed in the S/MMA copolymerization system5 when the copolymerization was performed in benzene, chlorobenzene, and benzonitrile; that is, the apparent reactivity ratio of S diminished and the MMA reactivity


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Table III. Apparent Monomer Reactivity Ratios for the Copolymerization S/BA System In Various Solvents at 50°C

Table II. Experimental Results of S and BA Copolymerization in Benzonitrile fs Feed

Conversion (%)

FS Copolymer

0.066 0.066 0.066 0.066 0.066 0.081 0.081 0.081 0.081 0.081 0.096 0.096 0.096 0.096 0.096 0.096 0.245 0.245 0.245 0.245 0.245 0.450 0.450 0.450 0.450 0.450 0.664 0.664 0.664 0.664 0.664 0.704 0.704 0.704 0.704 0.704 0.748 0.748 0.748 0.748 0.748

0.50 4.32 6.24 8.33 10.84 2.39 3.92 5.85 8.94 9.21 2.16 3.79 4.98 6.40 7.93 9.62 2.89 4.50 6.46 7.78 9.32 2.74 4.29 5.60 6.61 8.39 2.44 6.10 5.16 6.10 8.35 1.23 2.50 3.70 4.97 7.00 2.39 4.29 6.07 8.29 10.83

0.194 0.194 0.217 0.241 0.123 0.151 0.116 0.251 0.173 0.266 0.171 0.183 0.193 0.144 0.206 0.199 0.357 0.368 0.363 0.373 0.421 0.547 0.546 0.537 0.528 0.547 0.661 0.685 0.672 0.681 0.684 0.709 0.708 0.714 0.712 0.709 0.733 0.740 0.745 0.745 0.748

ratio increased with the increase of the solvent polarity.5 This behavior apparently indicated that the solvent polarity had an important effect on the reactivities of the two radicals. The solvent polarity first was considered by Ito and Otsu,1 who stated that the observed variations of the reactivity ratios in the S/MMA copolymerization were due to the polarized structure of MMA in the

Solvent

r1

r2

Bulka Benzene Benzonitrile

0.865 0.842 0.734

0.189 0.176 0.330

a

See ref. 18.

transition state when the solvent polarity increased. Other works indicate linear ratios between copolymerization parameters and the solvent dielectric constant in the copolymerization of S with methacrylonitrile22 or in the copolymerization of S with MMA.4 The observed results were explained on the basis of the growing radical polarized forms by the electronic donating or electron-withdrawing character of its functional groups. However, this effect may have been only apparent because, besides the polarity effect, other mechanisms also have been proposed to describe the effect of solvents. Monomer–monomer, monomer–solvent, or radical–solvent (which may be not only a solvent but also one or both comonomers) complexes and the so-called boot-strap model also could explain the apparent variation of the copolymerization parameters in the different solvents. To date, there is experimental evidence7,8,23–25 that the bootstrap model is the most

Figure 1. Monomer reactivity ratios and 95% joint confidence interval for the reactivity ratios of the S/BA system as given in Table III.

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Figure 2. Expanded spectra of a carbonyl carbon resonance region of a S/BA copolymer.

appealing as a general model. Harwood,3 who observed that copolymers with the same compositions have the same microstructure, proposed this model independently of the solvent used during their preparation. Therefore, the conditional probabilities governing sequence distribution are unaffected by the polymerization medium and, consequently, the monomer reactivity ratios are not affected either. Taking this fact into account, Harwood3 suggested that solvent effects in copolymerization are not manifested in the chainend reactivity but rather in a solvent partitioning. Thereby, an equilibrium may exist in which monomers are distributed between the solvent and the domains of growing polymer radicals. However, bootstrap effects may arise when the effective free-monomer concentration, radical concentration, or both differ from those calculated through the monomer-feed ratios, leading to discrepancies between the predicted and actual propagation rates.23 In this way, when radical– solvent or monomer–solvent complexes do not propagate, the effect of complexation is to alter the radical or monomer concentration, causing a bootstrap effect.23 Then, to know if the bootstrap holds in the S/BA system, we had to consider the sequence distributions of the copolymers. The monomer sequence distributions in the S/BA copolymers were calculated from 13C NMR spectra according to the

assignment of Llauro-Darricates et al.26 Thus, Figure 2 shows experimentally observed peak areas of the carbonyl resonance of BA units (174 – 175.50 ppm) divided, from high field to low field, into three resonance regions (175.20, 174,80, and 174.45 ppm) that were assigned to AAA, AAS 1 SAA, and SAS triads, respectively, being A, butylacrylate and S, styrene. The different arrangements of S-centered triads were determined from the quaternary carbon atom resonance region of S between 142 and 147 ppm. As indicated in Figure 3, Llauro-Darricates et al.26 assigned the peak from 146.5 to 144.8 ppm to SSS triads, the peak from 144.8 to 143.5 to SSA or ASS triads, and the peak from 143.5 to 142.0 ppm to ASA triads. The data for the monomer sequence distribution of the copolymers performed in bulk, benzene, and benzonitrile are depicted in Figure 4 as the triad fractions of both S-centered triads and BA-centered triads versus copolymer composition. Although there is some scatter, data from the different solvents appear to be described by one set of curves that indicate that the bootstrap effect was important for the copolymerization of S/BA in bulk, benzene, and benzonitrile. Then, the variation in the monomer reactivity ratios, in a general sense, may have been due to differences between the local monomer concentration at the polymerization site, which differed from that in the bulk phase. To determine if the monomers were distributed between the solvent and the domains of growing polymer radicals and on the basis of Harwood’s3 assumption, an attempt to

Figure 3. Expanded spectra of a quaternary carbon resonance region of a S/BA copolymer.


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Figure 4. Comparison of the triad fractions for S/BA copolymers prepared in bulk (E), benzene (‚), and benzonitrile (h). The BA-centered triads (top) and the S-centered triads (bottom) are represented as a function against the copolymer composition, FA (top) or FS (bottom).

quantify the partition coefficient was made by Klumperman and O’Driscoll,8 who considered it to be independent of the copolymer composition. Later, Klumperman and Kraeger24 modified the proposed method, thinking that the partition coefficient may be linearly dependent on copolymer composition. Maxwell et al.27 considered a “template” or “solvent–polymer complex” effect as a second way in which the local monomer concentration at the polymerization site may differ from that in the bulk phase, suggesting an empirical formulation. However, neither of these proposed models have a universal application. If the bootstrap model holds and the cause of the solvent effect on the monomer reactivity ratios was mainly due to the differences between the local and analytical monomer ratios, a certain mole fraction in the copolymer chain would be obtained with the same local monomer fraction in the feed.28 Then, if the bulk copolymerization is considered a thermodynamic reference state in which the partition coefficient has a value of unity,8,24 which monomer preferentially was sorbed could be estimated qualitatively. To achieve this, the mole fraction of FBA in the copolymer chain was considered, and, from the copoly-

mer composition equation and the apparent reacb tivity ratios obtained, the analytical bulk (fBA ) s and the analytical solution (fBA) monomer-feed compositions were determined. Differences beb s tween fBA and fBA as a function of copolymer chain composition gave us an indication of the monomer preferentially sorbed. b s Differences between fBA and fBA are shown in Figure 5. From that figure, it is clear that the enrichment or the depletion of a determined monomer around the growing radical was dependent on both the copolymer composition and the solvent in which the copolymer was performed. The effect of the solvent polarity was indicated by a comparison of the analytical-solution monomerfeed compositions in benzene and those in benzonitrile. When benzonitrile (dipole moment: 3.57 s b D) was used, fBA was lower than fBA , which means that in the S/BA/benzonitrile system, an BA enrichment around the growing radical was operative, being the amount of BA surrounding the growing radical depends on copolymer composition. However, when the copolymers were produced in a solvent with a relatively low polarity, in this case benzene (dipole moment: 0.3 D), a slight BA enrichment around the main growing

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Figure 5. Predicted differences between the bulk b s (fBA ) and solution (fBA ) BA mole fraction in the feed versus BA mole fraction in copolymer chain (FBA) for S/BA copolymers performed in benzene (solid line) and benzonitrile (dashed line) solutions.

radical took place for the BA mole fraction in the copolymer chain lower than approximately 0.4, and a slight S enrichment around the growing radical was operative for higher BA proportions. However, the variations in local monomer concentration in benzene with respect to those in bulk were small, and because the joint confidence limits found for the S/BA copolymerization performed in bulk and in benzene overlapped, the results should be taken with caution. As has been previously stated, in the copolymerization of S/MMA, a decrease of the apparent reactivity ratio of S and an increase of the apparent reactivity ratio of MMA with the increase of the solvent polarity were observed.5 Furthermore, Davis7 reanalyzed the microstructure data previously published6 for this system and claimed that the bootstrap model was operative in the copolymerization of S/MMA in benzene, chlorobenzene, and benzonitrile. Considering that the bootstrap model holds in both systems (S/BA and S/MMA) and also considering the structural similarity between BA and MMA and the quite similar dipole moment of both monomers (MMA 5 1.67 D, BA 5 1.93 D), we compared the different influences of the solvents in their behavior. The analytical-bulk and analytical-solution monomer-feed composition for the S/MMA system in benzene and in benzonitrile were calculated with the apparent reactivity ratios reported for this system.25 Figure 6 shows the differences between fAb and fAs as a function of FA, A being the meth(acrylic) monomer, that is, MMA or BA. This figure clearly indicates two different trends.

When benzonitrile was used as the solvent, fAs was lower than fAb, whereas when the copolymerization was performed in benzene, the opposite happened. This means that in benzonitrile (the most polar solvent), the environment of the growing radicals became relatively rich in MMA or BA, whereas for the S/BA system, a higher amount of acrylic monomer surrounded the growing radicals. This can be explained by consideration of the polarity of the reaction medium. If the reaction medium polarity was a function of each component polarity and its corresponding molar fraction, S/BA would have had a slightly higher polarity than S/MMA. When benzene was used as the solvent, there was not a significance influence for the S/BA system, but in the case of the S/MMA copolymerization, S was the monomer preferentially sorbed around the growing radical. The enrichment or depletion of MMA in the vicinity of growing radicals for the S/MMA system when the copolymerization was performed in benzonitrile

Figure 6. Predicted differences between the bulk (fab) and solution (fas) meth(acrylic) mole fraction in the feed versus the meth(acrylic) mole fraction in a copolymer chain (Fa) for S/BA copolymers performed in benzene (solid line) and benzonitrile (dashed line) solutions.


or benzene, may have been due to differences in the polarity of the reaction medium. The influence of the polarity of the reaction medium has been put forward since the first papers on the effect of solvents on copolymerization parameters were published.1,4,5,22 Thus, two different and opposite trends, as a function of the solvent polarity, also have been observed for MMA/BA copolymerizations performed in benzene or benzonitrile.29 Furthermore, the effect of solvent polarity on the local monomer concentration has been noted when 2-hydroxyethyl methacrylate is copolymerized with S in solvents with a low dipole moment (toluene) or with a high dipole moment (N,N9-dimethylformamide).30 On the basis of the quite similar values for the dipole moment of MMA and BA and the large differences in polarity between benzonitrile and benzene, the polarity of the reaction medium for both S/BA and S/MMA is between five and six times greater in benzonitrile than in benzene throughout the interval of the monomer molar fraction. Then, the enrichment or depletion of a determined monomer in the vicinity of growing radicals as a function of copolymer composition might be the same for both systems and would be related tentatively with changes in the medium polarity. However, differences between local and analytical monomer-feed composition would be determined by the polarity of copolymer chain, which is dependent of copolymer composition. Financial support from the Comisioń Interministerial de Ciencia y Tecnologıá (CICYT; Mat 97-682) is acknowledged gratefully.

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6. San Romań, J.; Madruga, E. L.; del Puerto, M. A. Angew Makromol Chem 1980, 86, 1. 7. Davis, T. P. Polym Commun 1990, 31, 442. 8. Klumperman, B.; O’Driscoll, K. F. Polymer 1993, 34, 1032. 9. O’Driscoll, K. F.; Monteiro, M. J.; Klumperman, B. J Polym Sci Polym Chem Ed 1997, 35, 515. 10. Arlman, E. J.; Melville, H. W. Proc R Soc London Ser A 1950, 203, 301. 11. Bradbury, J. H.; Melville, H. W. Proc R Soc London Ser A, 1954, 222, 456. 12. Gruber, E.; Knell, W. L. Makromol Chem 1978, 179, 733. 13. Kasza´s, G.; Fo¨ldes-Berezsnich, T.; Tu¨dos, F. Eur Polym J 1984, 20, 395. 14. Kasza´s, G.; Fo¨ldes-Berezsnich, T.; Tu¨dos, F. J Macromol Sci Chem 1985, 22, 1571. 15. Dube´, M. A.; Penlidis, A.; O’Driscoll, K. F. Can J Chem Eng 1990, 68, 974. 16. Dube´, M. A.; Penlidis, A.; O’Driscoll, K. F. Chem Eng Sci 1990, 45, 2785. 17. Davis, T. P.; O’Driscoll, K. F.; Piton, M. C.; Winik, M. A. Polym Int 1991, 24, 65. 18. Fernańdez-Garcıá, M.; Fernańdez-Sanz, M.; Madruga, E. L.; Fernańdez-Monreal, M. C. Macromol Chem Phys 1999, 200, 199. 19. Stickler, M. Makromol Chem Macromol Symp 1987, 10/11, 17. 20. McFarlane, R. C.; Reilly, P. M.; O’Driscoll, K. F. J Polym Sci Polym Chem Ed 1980, 18, 251. 21. Tidwell, P. W.; Mortimer, G. A. J Polym Sci Part A 1965, 3, 369. 22. Cameron, G. C.; Esslemont, G. F. Polymer 1972, 13, 435. 23. Coote, M. L.; Davis, T.; Klumperman, B.; Monteiro, M. J Macromol Sci Rev Macromol Chem Phys 1998, C38, 567. 24. Klumperman, B.; Kraeger, I. R. Macromolecules 1994, 27, 1529. 25. Madruga, E. L. Makromol Chem Rapid Commun 1993, 14, 581. 26. Llauro-Darricates, M. F.; Pichot, C.; Guillot, J.; Rios, L.; Cruz, M. A.; Guzman, C. Polymer 1986, 27, 889. 27. Maxwell, I. A.; Aerdts, A. M.; German, A. L. Macromolecules 1993, 26, 1956. 28. Madruga, E. L.; Fernańdez-Garcıá, M. Macromol Chem Phys 1996, 197, 3743. 29. de la Fuente, J. L.; Madruga, E. L. Macromol Chem Phys 1999, 200, 1639. 30. Sanchez-Chaves, M.; Martinez, G.; Madruga, E. L. J Polym Sci Part A: Polym Chem 1999, 37, 2941.