Ionic polymerization in aqueous emulsion

A kinetic study of the anionic polymerization of octamethylcyclotetrasiloxane (D₄) in aqueous emulsion has been carried out in the presence of ionic additives. The rate of polymerization of several cyclosiloxanes has been compared, leading to additional evidence for an interfacial mechanism of polymerization. The emulsion process has been applied to the cationic polymerization of D₄ and of tetramethylcyclotetrasiloxane (D^H₄) initiated by dodecylbenzenesulfonic acid. Very efficient for the synthesis of linear polymethylhydrogenosiloxanes (PMHS), these conditions did not seem suitable for the polymerization of D₄. The extension of the process to other heterocyclic monomers is discussed through the anionic polymerization of phenylglycidylether.     

Kinetics of emulsion polymerization

The quantitative theory of the free-radical mechanism in emulsion polymerization is reexamined. A mechanism involving desorption and reabsorption of radicals is discussed. The average number of radicals per particle has been calculated as a function of three parameters. A simplified, approximate solution for the average number of radicals per particle is given for cases where this number is low.     

Radical desorption in emulsion polymerization

The derivation of a radical desorption coefficient in emulsion polymerization is presented, which takes into account the possible reactions of the desorbed radical in the aqueous phase as well as the competition between desorption and termination in polymer particles containing more than one radical. This model overcomes some important limitations of the previous models and is able to explain previously reported experimental results that the other models were unable to explain.     

All-acrylic film-forming colloidal polymer/silica nanocomposite particles prepared by aqueous emulsion polymerization

The efficient synthesis of all-acrylic, film-forming, core-shell colloidal nanocomposite particles via in situ aqueous emulsion copolymerization of methyl methacrylate with n-butyl acrylate in the presence of a glycerol-functionalized ultrafine silica sol using a cationic azo initiator at 60 °C is reported. It is shown that relatively monodisperse nanocomposite particles can be produced with typical mean weight-average diameters of 140-330 nm and silica contents of up to 39 wt %. The importance of surface functionalization of the silica sol is highlighted, and it is demonstrated that systematic variation of parameters such as the initial silica sol concentration and initiator concentration affect both the mean particle diameter and the silica aggregation efficiency. The nanocomposite morphology comprises a copolymer core and a particulate silica shell, as determined by aqueous electrophoresis, X-ray photoelectron spectroscopy, and electron microscopy. Moreover, it is shown that films cast from n-butyl acrylate-rich copolymer/silica nanocomposite dispersions are significantly more transparent than those prepared from the poly(styrene-co-n-butyl acrylate)/silica nanocomposite particles reported previously. In the case of the aqueous emulsion homopolymerization of methyl methacrylate in the presence of ultrafine silica, a particle formation mechanism is proposed to account for the various experimental observations made when periodically sampling such nanocomposite syntheses at intermediate comonomer conversions.     

Non-seeded semi-continuous emulsion polymerization

Semi-continuous emulsion polymerization of acrylic monomers was investigated from the point of view of the particle growth. In the process no seed latex was used and the monomers were fed in an emulsion form. Oscillations of surface tension and particle number indicated a periodical generation and flocculation of particles during the polymerization. Investigation of particle size and molecular weight changes in the first tens of seconds of polymerization has shown that the coagulation process proceeds even in the earliest stage of the polymerization. Initial emulsifier concentration in the reactor strongly affects the particle growth and the final particle number.     

Stereochemistry of photoinitiated emulsion polymerization

The tacticity of poly(methyl methacrylate) (PMMA) samples made by emulsion polymerization (EP) has been determined by NMR spectroscopy at higher resolution than any previous study. When photoinitiation is employed at room temperature, the polymer obtained is highly syndiotactic and more stereoregular than that obtained in homogeneous radical polymerization under the same conditions. The percentage of racemic dyads and the length of racemic sequences vary significantly with the degree of conversion and the temperature, but not with variation of the hydrophobic or the hydrophilic part of the detergent molecules. A magnetic field of 5 kG or less accelerates the polymerization reaction and increases the polymer molecular weight when dibenzyl ketone is employed as oil-soluble photoinitiator, and has an indirect influence on the polymer tacticity. From measurements of the temperature dependence of the polymer steric composition, the difference of activation enthalphy and entropy of the meso and racemic additions have been calculated. The values do not follow the correlations from precedent literature for radical polymerization in solution if the reaction is considered a first-order Markov process. The persistence ratio p depends on the detergent and the temperature of the EP; in some cases its difference from unity exceeds the experimental error. These results are interpreted as a conformational effect of the locus of the polymerization in the first stage of the EP (when micelles are present) that favors the racemic addition more than is expected in a homologous solvent such as an alkane, e.g., at the same temperature.     

Branching and crosslinking in emulsion polymerization

The seeded semibatch emulsion polymerization of butyl acrylate (BA) with allyl methacrylate (AMA) and butanediol diacrylate (BDA) was used to study the influence of the crosslinkers on the kinetics, branching and crosslinking density, gel fraction and sol MWD produced during the experiments carried out at 80°C using potassium persulfate as initiator. Surprisingly, the most reactive crosslinker, BDA, produced the less crosslinked, branched and gel containing polymer. These results were explained with the help of a mathematical model in terms of cyclization reactions and diffusion controlled propagation and termination reactions.     

Radical capture efficiencies in emulsion polymerization

A theoretical procedure is developed that allows the importance of bimolecular termination in the aqueous phase of an emulsion polymerization to be determined. This shows that with sparinglysoluble and slowly propagating monomers like styrene, significant termination occurs in the aqueous phase at high initiator concentrations, as found experimentally. With more water soluble monomers, aqueous phase termination is likely to be small.     

Kinetics of long-chain branching in emulsion polymerization

A kinetic model suitable to deal with the case of branched polymers produced in emulsion both in the case of chain transfer to polymer and propagation to terminal double bond is briefly presented and numerically solved through the method of the moments. Thanks to the “numerical fractionation” approach, the whole molecular weight distribution of the polymer is evaluated while accounting for the compartmentalized nature of the system. The results of some illustrative calculations concerning the effects upon the molecular properties of the final polymer of starved semibatch monomer feed policies, addition of a chain transfer agent and propagation to terminal double bond are discussed.     

The control of structure in emulsion polymerization

Free-radical addition polymerization can be carried out using four different processes: mass or bulk, solution, suspension, and emulsion polymerization. Of these four processes, emulsion polymerization is unique because it is a heterogeneous process, in which the polymerization reactions can take place in three different sites: in the continuous aqueous phase, on the surface of growing particles, and within the growing particles. This unique feature of emulsion polymerization offers many possibilities for designing different polymers and latexes: e.g., high-molecular-weight polymers, uniform copolymers, copolymers of difficult-to-copolymerize monomers, functionalized (surface-modified) latexes, uniform size latex particles, grafted latexes, and structured latexes having core-shell, microdomain structures, interpenetrating polymer networks, etc. This paper will describe several aspects of the control of structure in emulsion polymerization.     

Modeling composite emulsion polymerization kinetics

The conversion profiles of a number of factorial designed experiments used to study composite emulsion polymerization were modeled using a deterministic mathematical construct as well as an empirical neural network approach. In the deterministic modeling approach, existing mechanistic models for emulsion polymerization were employed for which estimates of rate constants were obtained from established literature sources as well as experiments. Fitting of the kinetic data was done using nonlinear fitting algorithms to adjust the estimated rate constants to provide the best fit of the conversion profiles. In the case of the empirical modeling using neural networks, the neural net inputs were in the form of the factor levels of the various experimental designs. Several nonrelated experimental designs could be combined in this way to serve as the input, whereas the conversion profiles were targeted as outputs. Following the successful implementation of both modeling strategies, a hybrid modeling approach was tested by combining the neural network predictive power to estimate values for rate constants while retaining the aforementioned mechanistic models to fit the data. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 101–117, 2013     

Preparation of PEG-modified urethane acrylate emulsion and its emulsion polymerization

In order to improve stability and reduce droplet size, the PEG-modified urethane acrylates were synthesized by the reaction of polyethylene glycol (PEG) with residual isocyanate groups of urethane acrylate to incorporate hydrophilic groups into the molecular ends. The droplet sizes of the PEG-modified urethane acrylate emulsions were much smaller than those of unmodified urethane acrylate emulsions at the same surfactant composition, and the droplet sizes of these emulsions were significantly effected not by surfactant compositions and types, but by the reaction molar ratio of PEG, because the urethane acrylate containing polyoxyethylene groups as terminal groups aided the interfacial activity of surfactant molecules and acted as a polymeric surfactant. The actions of PEG-modified urethane acrylate were confirmed by the investigation of adsorption of urethane acrylate in a water/benzene interface.For polymerization of emulsions, the stability of emulsion in the process of emulsion polymerization was changed by the type of surfactant or initiator. In the case of emulsion polymerization with a water soluble initiator (K₂S₂O₈), the emulsions prepared using TWEEN 60 were broken in the process of polymerization. However, polymerization of these emulsions could be carried out using an oil soluble initiator (AIBN). The conversion of emulsion polymerization changed with the type of urethane acrylates, that is, the reaction molar ratio of PEG to 2-HEMA.