2.1 Surfactant synthesis

2.2 Other materials

Most of the compounds used in the polymerization were commercial products (Aldrich) used as received. The monomers, styrene and butyl acrylate were distilled under vacuum and stored, before use, under nitrogen atmosphere. Non-reactive surfactants were either sodium dodecyl sulphate (SDS) or cetyl trimethyl ammonium bromide (CTAB). The initiators were, either potassium persulphate (KPS) or the cationic 2,2′-azobis (2-amidinopropane)dihydrochloride (AIBA).

2.3 Colloidal properties of the surfactants

The critical micellar concentrations (CMC) were determined from surface tension measurements by the Wilhelmy plate method, using a KRUSS K 12 processor tensiometer.

2.4 Styrene batch polymerization

In a typical experiment, the polymerization was performed under nitrogen atmosphere at 70 °C in a glass reactor equipped with a stirrer rotating at 315 rpm. The reactor was fed with water (160 g) and surfactant (0.4 g), and left for degassing for 30 min at 70 °C. Then styrene (40 g) was added, and the polymerization was started upon introduction of 0.15 g of K₂S₂O₈ in 5 g of water. Samples were taken at time intervals to determine the conversion by gravimetry and follow the changes in particle size. In most cases, the conversion of the monomer was completed after 5 h, and then the polymer was collected for further characterization.

2.5 Core-shell latexes preparation

A seed of polystyrene latex was prepared at 70 °C, under nitrogen atmosphere, in a 1-l reactor equipped with a stirrer rotating at 250 rpm. It was fed with 900 ml of deionized water containing 1.5 g of SDS and 1 g of NaHCO₃, and 100 g of distilled styrene. The polymerization was started upon the addition of 0.75 g of KPS dissolved in 5 g of water. The conversion was completed in 20 h. The seed latex was then washed by passing two times through ion-exchange resins. The particle size of that monodisperse seed latex was 98 nm.

A similar seed with a particle diameter of 87 nm was prepared using the cationic surfactant CTAB and the cationic initiator AIBA.

About 0.5 g of surfmer dissolved in 75 g of deionized water at 70°C were introduced in the reactor of 250 ml. 50 g of the seed latex was added. After degassing under nitrogen, 2 g of a mixture of styrene and butyl acrylate was added and the system was left overnight at room temperature. After that swelling step, a solution of surfmer and initiator was added, and the temperature allowed to reach 70 °C, and the polymerization was started by simultaneous addition of the monomer mixture (2.4 ml min^–1) and a solution of the remaining surfactant with 0.15 g of initiator. The addition was completed after 5 h, and then another portion of initiator was added and the polymerization was completed after 8 to 20 h. The amount of monomers added is calculated so that the targeted final size is 150 nm. Aliquot samples were taken from time to time in order to follow the process, and the conversion was determined by gravimetry.

At the end of the polymerization, the surface tension of the latex was measured, using the same equipment as that for the CMC analysis of the surfactants.

2.6 Polymer characterization

The amount of coagulum was determined upon filtration of the latex using a sintered glass filter, and after washing the reactor with tetrahydrofurane (THF); both the THF solution and the product collected on the filter were dried and the sum of the solids was taken as the amount of coagulum.

Particle-size measurements were carried out by dynamic light scattering with Autosizer LO-C (Malvern Instrument) giving the average size and the particle size distribution. In a few cases, TEM micrographs were also used.

Molecular weights were measured by Size Exclusion Chromatography (SEC), using a Waters instrument, Porapack gel filled columns, and polystyrene calibration standards.

2.7 Latex stability tests

The latexes were tested for their stability versus addition of electrolytes, addition of ethanol and versus freeze-thawing cycles.

To 1 g of latex, 1 g of 0.1 M NaCl solution was added. Flocculation was observed immediately if a higher concentration of salt was used.

The ethanol test consists in adding to a volume of latex an equal volume of ethanol, allowing desorbing the organic species not firmly adsorbed or anchored onto the surface of the particles. After this treatment, the latex is diluted and the particle size is measured, except if flocculation was observed.

For a freeze-thawing test, a small amount of latex (1–2 ml) was kept at –20 °C for one day, and after another day at room temperature the flocculation was observed, and, if not, the particle size was measured and compared with the initial particle size of the latex.

3.1 Surfactants synthesis

Maleic isoimides (MII, 1) are highly reactive intermediates in syntheses of various maleic acid derivatives.

Reactions of MII (1) with nucleophilic reagents - amines or amino alcohols (2) would allow preparing a set of corresponding maleic diamides (MD, 3) with highly hydrophilic substituents at one of the amide groups. MII (1) with hydrophobic substituents at nitrogen atom were used (Scheme 1 ) in order to introduce hydrophobic part at the second amide group in the planned surfmers – MD (3). These MD (3) represent a new group of potential surfmers with variable hydrophobic/hydrophilic balance caused by different substituents at both nitrogen atoms. They are very much more resistant against hydrolysis by comparison with earlier described surfmers – maleic acid diesters in alkaline media –, which is essential for copolymerization reactions.

Reactions of MII with amines were carried out in polar solvents, best of all DMFA or dichloromethane. Low reaction temperatures (~0 –5 °C) are necessary to avoid undesirable side reactions (addition to the C=C double bond, isomerization into fumaric acid derivatives, etc.). Usually amines are added to maleisoimides (1) solutions, but amines also can be generated in situ from their salts, as it is demonstrated in case of preparation of an amino sugar derivative (3f) from its salt. The obtained MD (3) have particular structure – both their hydrophilic NH groups are placed next to polymerizable C=C double bond. It means that the C=C double bond might be placed on phase surface or even in water phase during the exploitation in polymerization of these surfmers. Albeit yields of MD (3) are variable (30–90 %), the method seems to be among the best ones for large-scale preparation of maleic surfmers, mainly due to higher stability of starting materials (MII) by comparison with other activated forms of HA. Conditions of synthesis and characteristics of the obtained MD are collected in Table 1.

Structures of synthesized MD (3) were conformed by their NMR and IR spectra. Signals of protons at the C=C double bond (δ ~ 6.0…6.2), amide NH protons (δ ~ 8.3…9.5), and those of CH₂–N groups (δ ~ 3.1…3.4 ppm) differ substantially from the starting materials (isoimides, 1). Coupling constants of protons at the C=C double bond (J = 8…12 Hz) confirm the fact that isomerization to the corresponding fumaric derivatives has not taken place during their synthesis, provided that reactions (Scheme 2 ) were performed on sufficient cooling. In ¹³C NMR spectra of MD (3), the most characteristic resonance signals are those of carbon atoms in C=C double bond (131–134 ppm) and C=O groups (164–168 ppm). IR spectra of diamides (3) have the most characteristic intensive absorption bands at 1615–1670 (amide I), 1540–1560 (amide II), 3250–3310 (ν_N–H) and 3060–3080 (ν_=C–H) cm^–1. Spectral characteristics of the MD are summarized in Tables 2 and 3.

Investigated MD (3) contain short-chain alkyl groups (R¹ in 3) with increased hydrophilicity, due to the presence of extra hydroxyl-, ether- or amino-groups in R¹. Selection of the right group allows obtaining predictable balance between hydrophobic and hydrophilic part of the surfmer molecule, which, in turn, influences total surface activity of the surfmer.

Further increase of the hydrophilicity of some MD (3) was achieved by N-alkylation of MD, provided that they contain external amino group (3e, 3l). Alkylation with alkyl halides or sulphonates (4) leads to the corresponding ammonium salts (5) – cationic surfmers –, while alkylation with 1,3-propane sultone (6) allows preparing aminosulfonic acids – zwitterionic surfmers (7). All alkylation reactions proceed in polar aprotic solvents at 20…80 °C, and yields of the mentioned ammonium salts (5, 7) are high enough for preparation of these surfactants in preparative scale (Table 4).

Once more, prepared ionic surfactants (5, 7) were characterized by NMR and IR spectra. Apart from resonance signals of parent molecule protons (at C=C double bond, of the group NH–CH₂–, etc.), ¹H NMR spectra of the ammonium salts (5) contain characteristic triplet of CH₂–CH₂–NR₃ group at ~3.4 ppm, but zwitterionic derivatives (7) – signals at ~ 3.2 and ~ 2.9 ppm of protons next to N and S correspondingly in ≡N⁺–(CH₂)₃–SO₃^– group.¹³C NMR spectra of 5 and 7 apart from resonance signals of C atoms in double bonds have other characteristic signals of C atoms next to quaternary ammonium groups (~ 65 ppm). IR spectra of the compounds (5) and (7) have strong characteristic absorption bands of amide groups at 1620–1670 (amide I), 1545–1560 (amide II) cm^–1, as well as those of N–H bonds at 3220–3270 and =C–H bonds at 3030–3040 cm^–1. Unfortunately, it is not easy to find something particular for the introduced hydrophilic groups of 5 and 7 in these spectra. The most characteristic spectral data of these ionic surfmers are collected in Tables 5 and 6.

All examined maleic diamides (3, 5, 7) are efficient surfactants indeed, and they also contain activated (by two amide groups) polymerizable C=C double bonds.

3.2 Colloidal properties

The CMC of the three classes of surfmers are reported in Table 7, together with the values of the surface tension at the CMC. The values of these CMC are rather small, in between 1 and 10 × 10^–5 mol l^–1. Such values are in the range of common non-ionic surfactants, but they indicate a rather poor hydrophilicity of the molecules, possibly because the hydrophilic moieties are rather short. However, the surface tension values at the CMC, in between 25 and 50 mN m^–1 indicate rather good surface activity.

An example of determination of the CMC is shown in Fig. 1 .

3.3 Batch emulsion polymerization of styrene

When applied in batch emulsion polymerization of styrene, the non-ionic surfmers used alone without co-surfactant are unable to cause high conversion of the monomer in small polymer particles. In fact, they are not able to provide a successful nucleation step in the polymerization process. So it is needed to associate with them an anionic surfactant such as SDS. As soon as the surfactant system does include SDS, very good results can be obtained: high conversion in a rather limited time, with a narrow population of small particles and very little amount of coagulum. Most of the surfactants remain attached or at least strongly adsorbed onto the particle surface, as shown by the measurement of the surface tension of the final latex, close to the value of pure water. The replacement of SDS with the reactive anionic surfactant HA12 gives the same result. High molecular weight was obtained with a broad distribution, as usual in emulsion polymerization. These data are reported in Table 8, while a few conversion curves are shown in Fig. 2 .

Further data about the cationic surfactants (5) and the zwitterionic surfmers (7) are also reported in Table 8. In these cases, the ionic nature of the surfactants allows to obtain an efficient nucleation, so that there is no need for a co-surfactant. However, with the compounds 5e and 7b, poor polymerization yields are obtained. The larger particle sizes are an indication that the nucleation is less efficient, but it seems that the main cause of these poor results has to be found in the benzylic structure of the hydrophobic part. It is well known that the benzylic groups are very sensitive, in radical polymerization to transfer reactions, leading to delocalized radicals (in the benzene ring) with low reactivity [13]. Otherwise, when the hydrophobic part is an alkyl group, as in the case of the previous non-ionic surfmers, the results of the polymerization are very good, chiefly in the case of the cationic surfmers, leading to even smaller particle sizes. However the molecular weights are very small in the case of the cationic surfmers, most probably due to the counterions. The zwitterionic compound 7a leads to somewhat slower polymerization with larger particle size, but again with high molecular weight and usual polydispersity.

3.4 Core-shell semi-batch seeded polymerization

A smaller number of surfmers with only alkyl hydrophobic moiety were engaged in a seeded semi-batch copolymerization. The corresponding results are reported in Table 9.

In all cases, monodisperse particles with convenient sizes have been obtained. The conversion is almost complete, with very small amounts of flock. At the end of the polymerization, it remains a significant amount of surface active compounds. This feature may be surprising, because the conversions reach high values in all cases. Most probably, because the reactivity of styrene is somewhat higher than that of butyl acrylate, the very surface of the particles is composed chiefly of butyl acrylate units, which have adsorption properties quite different from polystyrene. This remark involves idea that the contribution of the grafting of the surfmer units should be limited as compared to the adsorption of these surfmers. Such behaviour was already observed in the case of surfmers derived from maleic hemiesters [7]. Again, the molecular weights measured in the case of the cationic surfmers are much lower and polydisperse than of other polymers.

Fig. 3 shows an example of conversion curves (both instantaneous and global) in the semi-batch copolymerization process.

3.5 Latex stability

All the latexes remain stable upon addition of 0.1 N solutions of NaCl, but floculate if higher concentrations of electrolytes are used. They also resist to the addition of ethanol. However freeze-thawing tests cause floculation in all cases. In our previous work [9], using derivatives of hemiesters, similar results were obtained, except in one case in which a zwitterionic surfmer was shown to pass successfully the freeze-thawing test.