Microemulsion polymerization of styrene in the presence of a cationic emulsifier

The principal subject discussed in the current paper is the radical polymerization of styrene in the three- and four component microemulsions stabilized by a cationic emulsifier. Polymerization in the o/w microemulsion is a new polymerization technique which allows to prepare the polymer latexes with the very high particle interface area and narrow particle size distribution. Polymers formed are very large with a very broad molecular weight distribution. In emulsion and microemulsion polymerizations, the reaction takes place in a large number of isolated loci dispersed in the continuous aqueous phase. However, in spite of the similarities between emulsion and microemulsion polymerization, there are large differences caused by the much larger amount of emulsifier in the latter process. In the emulsion polymerization there are three rate intervals. In the microemulsion polymerization only two reaction rate intervals are commonly detected: first, the polymerization rate increases rapidly with the reaction time and then decreases steadily. Essential features of microemulsion polymerization are as follows: (1) polymerization proceeds under non-stationary state conditions; (2) size and particle concentration increases throughout the course of polymerization; (3) chain-transfer to monomer/exit of transferred monomeric radical/radical re-entry events are operative; and (4) molecular weight is independent of conversion and distribution of resulting polymer is very broad. The number of microdroplets or monomer-starved micelles at higher conversion is high and they persist throughout the reaction. The high emulsifier/water ratio ensures that the emulsifier is undissociated and can penetrate into the microdroplets. The presence of a large amount of emulsifier strongly influences the reaction kinetics and the particle nucleation. The mixed mode particle nucleation is assumed to govern the polymerization process. At low emulsifier concentration the micellar nucleation is dominant while at a high emulsifier concentration the interaction-like homogeneous nucleation is operative. Furthermore, the paper is focused on the initiation and nucleation mechanisms, location of initiation locus, and growth and deactivation of latex particles. Furthermore, the relationship between kinetic and molecular weight parameters of the microemulsion polymerization process and colloidal (water/particle interface) parameters is discussed. In particular, we follow the effect of initiator and emulsifier type and concentration on the polymerization process. Besides, the effects of monomer concentration and additives are also evaluated.     

Microemulsion polymerization of styrene using a polymerizable nonionic surfactant and a cationic surfactant

High polymer/surfactant weight ratios (up to about 15:1) of polystyrene microlatexes have been successfully produced by microemulsion polymerization using a small amount of polymerizable surfactant, ω-methoxypoly(ethylene oxide)₄₀ undecyl α-methacrylate macromonomer (PEO-R-MA-40), and cetyltrimethylammonium bromide (CTAB). After generating “seeding particles” in a ternary microemulsion containing only 0.2 wt% CTAB and 0.1 wt% styrene, the additional styrene containing less than 1 wt% PEO-R-MA-40 was added dropwise to the polymerized microemulsion for a period of about 4 h at room temperature. PEO-R-MA-40 copolymerized readily with styrene. The stable microlatexes were bluish-transparent at a lower polymer content and became bluish-opaque at a higher polymer content. Nearly monodisperse latex particles with diameters ranging from 50 to 80 nm and their molar masses ranging from 0.6 to 1.6 × 10⁶ g/mol could be obtained by varying the polymerization conditions. The dependence of the number of particles per milliliter of microlatex, the latex particle size and the copolymer molar mass on the polymerization time is discussed in conjunction with the effect of the macromonomer concentration. Received: 25 October/2000 Accepted: 2 February 2001     

Microemulsion polymerization of acrylamide and styrene: Effect of the structures of reaction media

Isothermal phase diagrams of the system cetyltrimethylammonium bromide (CTAB)/n‐butanol/n‐octane/water were constructed, and the effect of the oil (n‐octane) contents on the microemulsions was studied at 40 °C. We determined the microemulsion structures of two systems, CTAB/n‐butanol/10% n‐octane/water and sodium dodecyl sulfonate (As)/n‐butanol/20% styrene/water, by conductivity measurements to investigate the polymerization of acrylamide and styrene in the two microemulsion systems. The polymerization kinetics of the water‐soluble monomer acrylamide in CTAB micelles and the different CTAB/n‐butanol/10% n‐octane/water microemulsion media [water‐in‐oil (W/O), bicontinuous (BC), and oil‐in‐water (O/W)] were studied with water‐soluble sodium bisulfite as the initiator. The maximum polymerization rate in CTAB micelles was found at the second critical micelle concentration. A mechanism of polyacrylamide formation and growth was proposed. A connection between the structures of the microemulsions and the polymerization rates was observed; the maximum polymerization rate occurred at two transition points, from W/O to BC and from BC to O/W, and the polyacrylamide molecular weights, which depended on the structures of the microemulsions, were also found. A square‐root dependence of the polymerization rates on the initiator concentrations was obtained in CTAB micelles and O/W microemulsion media. The polymerization of the oil‐soluble monomer styrene in different As/n‐butanol/20% styrene/water microemulsion media (W/O, BC, and O/W) was also investigated with different initiators: water‐soluble potassium persulfate and oil‐soluble azobisisobutyronitrile. A similar connection between the structures of the microemulsions and the conversions of styrene in CTAB/n‐butanol/10% n‐octane/water for the polymerization of acrylamide was observed again. The structures of the microemulsions had an important role in the molecular weights and sizes of polystyrene. The polystyrene particles were 10–20 nm in diameter in BC microemulsion media and 30–60 nm in diameter in O/W microemulsion media according to transmission electron microscopy. We determined the solubilization site of styrene in O/W microemulsion drops by ¹H NMR spectra to analyze the results of the microemulsion polymerization of styrene. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3320–3334, 2001     

Microemulsion polymerization of styrene stabilized by sodium dodecyl sulfate and diethylene glycol monoalkyl ether

The effects of the cosurfactants diethylene glycol monoalkyl ether [C _i H_{2 i +1}O(CH₂CH₂O) _j OH (C _i E _j ; i=4, 6 and j=1, 2)] on the formation of an oil-in-water styrene (ST) microemulsion and the subsequent free radical polymerization were studied. For comparison, the data for the C _i H_{2 i +1}OH (C _i OH; i=4, 6) systems obtained from the literature were also included in this work. Sodium dodecyl sulfate was used as the surfactant. The pseudo three-component phase diagram (macroemulsion, microemulsion and lamellar gel phases) was constructed for each cosurfactant. The primary parameters selected for the polymerization study are the concentrations of cosurfactant and styrene. The number of latex particles nucleated is much smaller than that of the microemulsion droplets initially present in the reaction system. Limited flocculation of the latex particles occurs to some extent during polymerization. Among the cosurfactants investigated, the C₄OH-containing polymerization system is the least stable. By contrast, the diethylene glycol monoalkyl ether group of C _i E _j tends to enhance the latex stability. C _i E _j is more effective in stabilizing the ST microemulsion and the subsequent polymerization in comparison with the C _i OH counterpart. Received: 24 December 1999 Accepted: 9 February 2000     

Microemulsion polymerization of styrene stabilized by sodium dodecyl sulfate and short‐chain alcohols

Styrene microemulsion polymerizations with different short‐chain alcohols [n‐C_iH_2i+1OH (C_iOH), where i = 4, 5, or 6] as the cosurfactant were investigated. Sodium dodecyl sulfate and sodium persulfate (SPS) were used as the surfactant and initiator, respectively. The desorption of free radicals out of latex particles played an important role in the polymerization kinetics. An Arrhenius expression for the radical desorption rate coefficient was obtained from the polymerizations at temperatures of 50–70 °C. The polymerization kinetics were not very sensitive to the alkyl chain length of alcohols compared with the temperature effect. The maximal polymerization rate in decreasing order was C₆OH > C₄OH > C₅OH. This was related to the differences in the water solubility of C_iOH and the structure of the oil–water interface. The feasibility of using a water‐insoluble dye to study the particle nucleation mechanisms was also evaluated. The parameters chosen for the study of the particle nucleation mechanisms include the cosurfactant type (C_iOH), the SPS concentration, and the initiator type (oil‐soluble 2,2′‐azobisisobutyronitrile versus water‐soluble SPS). © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3199–3210, 2001     

Gold-promoted styrene polymerization

Styrene can be polymerized at room temperature in the presence of equimolar mixtures of the gold(III) complexes (NHC)AuBr3 (NHC = N-heterocyclic carbene ligand) and NaBAr'4, in the first example of a gold-induced olefin polymerization reaction.     

Syndiospecific polymerization of styrene

The current development of the metallocene-based catalysts for syndiotactic polystyrene (SPS) has been reviewed. SPS is a new semi crystalline engineering thermoplastic with a crystalline melting point of 270°C. Because of its crystalline nature, SPS has high heat resistance, excellent chemical resistance and water/steam resistance. In this review, some mechanistic models for polymerization and stereoregulation, as well as the factors which affect the activity and stereospecificity of the catalysts, are discussed.     

Electroinitiated cationic polymerization of styrene

A constant controlled current was passed through a solution of styrene in methylene chloride containing a tetraalkylammonium salt as supporting electrolyte. Reproducible rates of polymerization were initiated by the electrochemical techniques employed and the kinetics of the reaction were investigated. Sigmoidal curves of conversion versus time were observed. A kinetic relationship of the form In ([M₀]/[M]) = ½ Kt² was derived on the basis of simple assumptions regarding the mechanism and fitted the data accurately. The rate constants obtained were compared to others reported, and the influences of ion association on the values of the rate constants obtained are discussed. The reactions were decreased in rate by a reversal of polarity of the electrodes. However, the stoichiometry of the production of active centers and of their destruction was not ideal, in that each electron did not result in the initiation of a polymer chain.     

Effect of styrene oligomers on syndiospecific polymerization of styrene

Styrene oligomers, preferentially consisting of styrene dimers and trimers, are formed by a free radical mechanism at the thermal polymerization of stabilizer-free styrene during storage and at higher polymerization temperatures. The identity of several dimer and trimer fractions formed in such a free radical polymerization, their influence on a coordinative polymerization reaction, the syndiospecific polymerization of styrene, as well as their effect on the properties of the resulting polymers has been investigated.Styrene dimers and styrene trimers reduce the polymerization activity of the transition metal catalyst significantly, especially at low amounts of oligomers added to the styrene. This behavior is discussed with respect to a proposed mechanism involving complexation of the active transition metal species with the specific oligomer instead of the styrene monomer, resulting in increased steric hindrance towards insertion of a styrene molecule to the active site.Both oligomers reduce the molecular weight of the syndiotactic polystyrene, by acting as chain-transfer agents. The constancy of the polydispersity over the whole concentration range of added dimer or trimer indicates that the uniformity of the active sites of the coordinative polymerization is not significantly influenced by the presence of the oligomers.The thermal properties of the polymers demonstrate that the oligomers do not affect the high syndiospecificity of the active catalytic sites, whereas the increase in crystallization temperature with increasing amounts of styrene dimer or trimer is comparable to effects observed by the addition of crystallization nucleators to semicrystalline polymers.     

Mechanism of syndiotactic-specific polymerization of styrene

Perusal of literature data and some new results concerning syndiotactic-specific polymerization of styrene suggest a reaction mechanism accounting for the steric control. Key features of the proposed mechanism are stereorigid ηⁿ coordination of the growing chain end and diastereoselective coordination of the monomer imposed by direct interactions with the ancillary ligand of the metal complex, a pseudotetrahedral chiral Ti(III) or Zr(III) cation, which inverts its configuration after every syndiospecific insertion step.     

Admicellar polymerization of styrene on cotton

Thin-film coating on cotton by the admicellar polymerization process was investigated. In this work, styrene was used as the monomer to coat styrene on cotton. The effects of surfactant, styrene, initiator, and electrolyte concentrations on the polymerization process were determined. The polystyrene film formed was characterized by SEM, FTIR, and GPC. The increase in the hydrophobicity of the treated cotton surface was determined by the drop test. Results show that polystyrene thin film was successfully formed on cotton, resulting in cotton that can resist wetting by a water droplet for longer than 30 min.     

Diagnostic analysis of styrene plasma polymerization

For studying plasma polymerization of styrene, two in situ diagnostic methods, optical spectroscopy and mass spectroscopy, were used to measure chemical components formed in the discharge volume and their concentrations in plasma column and two sheaths. The synergetic influence of power (W), pressure (p), and monomer flow rate (F) on plasma polymerization was expressed with a composite parameter, W/pF, which is proportional to the energy transferred to styrene monomer molecule. In a certain range of W/pF, the population of C₂H₂ and H₂ produced in the discharge decreased with W/pF, while the concentration of C₈ and C₆ fragments increased, which indicates that different chemical reactions may occur in different intervals of W/pF value. The similarity in change tendency between the deposition rate, the emission intensity of CH and C₄H and mass peak vs. W/pF implies that the polymerization is controlled by the reaction in the gas phase plasma, and supports the view that initial reactive species are produced in plasma, and polymerization is performed on the substrate surface. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 325–330, 1999     

Electroinitiated polymerization of styrene in dimethylsulphate

The electroinitiated polymerization of styrene in tetrabutylammonium perchlorate-dimethylsulphate solutions has been investigated. Polystyrene of low molecular weight is formed cationically through a direct monomer oxidation to the radical cation. Kinetic as well as mechanistic considerations for this system are made difficult by a number of factors: (a) the polymer is to a large extent insoluble in dimethylsulphate; (b) side reactions of both monomer and polymer tend to decrease the yields and therefore the calculated k_p values: (c) the supporting electrolyte concentration tends to affect the ionic equilibria in solution, so again influencing the k_p values.

A polymer degradation, found for polystyrene in LiClO₄-propylene carbonate solutions, was again noted and attributed to the effect of the current on the polymer covering the anode. The polymerization has no true termination, the only chain-stopping reactions being those giving rise to chain transfer.     

Microwave-assisted nitroxide-mediated radical polymerization of styrene

The first single-mode microwave-assisted nitroxide-mediated radical polymerizations (NMRP) of styrene in bulk were successfully performed. The results showed that the polymerization proceeded in a controlled way. The power of microwave irradiation has considerable effect on the polymerization rate. The polymerization rates under appropriate power of microwave irradiation were faster than that under conventional heating (CH) at the same polymerization temperature. The living nature of the polymer was proved by successful chain extension polymerization and ¹H NMR spectrum analysis.     

Microwave-assisted nitroxide-mediated miniemulsion polymerization of styrene

Nitroxide-mediated free-radical miniemulsion polymerizations (NMRPs) of styrene were successfully performed under microwave irradiation at 135 °C. The polymerizations proceeded in a controlled manner, yielding polymers that showed an incremental increase in molecular weight with conversion and had narrow molecular weight distributions. The resulting latexes were colloidally stable. The polymerization behavior, molecular weights of polymers and Z-average size of latex particles were also investigated under two different heating methods, microwave irradiation and conventional heating.     

The kinetics of styrene emulsion polymerization

A study has been carried out on the kinetics of persulfate-initiated emulsion polymerization of styrene in the presence of an anionic (oleate) or mixed anionic-nonionic emulsifier. In both cases it appears that Smith-Ewart kinetics are obeyed, i.e., there is a constant-rate period up to 40–50% conversion, during which there is a concomitant constant molecular weight development. The sharp increases in molecular weight with conversion reported by Grancio and Williams appear to be an artifact resulting from the use of an impure emulsifier (Triton X-100), which acts as a chain transfer agent to reduce the molecular weight by approximately an order of magnitude. Hence there does not appear to be any kinetic justification for assuming an inhomogeneous swollen latex particle (“core-shell” morphology), and normal thermodynamic considerations should still apply to this swelling phenomemon.     

Microemulsion polymerization for producing fluorinated structured materials

When perfluoropolyether microemulsions are used during polymerization of fluoropolymers, the structure of the reaction environment can be strictly controlled. In particular, the number and size of polymer particles in latexes can be set freely, yielding a number of advantages. First, as a result of radical segregation, terminations can be decreased without reducing polymerization rate: in this way high molecular weights are easily obtained also with poorly reactive monomers. Moreover, in combination with a reversible chain transfer mechanism based on iodine, particle segregation allows establishing pseudo-living polymerization conditions. In this situation formation of long branches in the polymer can be controlled by using bifunctional molecules that are able to link two different polymer chains to each other during polymerization. This is the so-called “branching and pseudo-living” technology. Finally, by co-coagulating latexes of different polymers prepared by microemulsion polymerization, very small particles and, thus, high interface areas are generated. In this way properties of different polymers, such as fluoroelastomers and fluorinated semicrystalline polymers, are matched effectively, generating new nanocomposite materials that exhibit outstanding properties. In this paper these results are reported and an overview of some novel sophisticated fluoropolymers obtained in microemulsion is given.