1-1 Introduction of controlled/living polymerization
Free radical polymerization is extensively used in view of its industrial production and applications. This technique can be readily performed since the polymerization condition is not stringent. However, it possesses drawbacks of lack of control over the molecular weight, molecular weight distribution, and macromolecular architecture. Controlled/living free radical polymerization (CRP), which provides a versatile route to the synthesis of well-defined polymer architecture and promises facile control of polymer morphology as well as microstructure, has attracted much attention in the past two decades. Up to know, these techniques can be roughly categorized into three types of synthesis pathways as follows: Nitroxide-mediated polymerization (NMP),1-5reversible addition-fragmentation transfer (RAFT),6-10 and atom transfer radical polymerization (ATRP).11-15These three main CRP techniques are rather mature in academic research and are used widespread globally.
(1) Nitroxide-mediated polymerization (NMP)
In mid 1980s, Solomon et al. first introduced that introducing stable free radical such as nitroxides into polymerization system the free radical polymerization could be controlled. Afterward in the 1990s, George et al. utilized (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (2,2,6,6-TEMPO) as a stable free radical, using benzoyl peroxide (BPO) as an initiator to perform the styrene polymerization at 123oC. The molecular weight of polymer was found to increase with the monomer conversion in a linear fashion with acceptable polydispersity (PDI) lower than 1.3, presenting a controlled living character.1 The NMP mechanism is shown below.
Where Pn-X is the dormant species, Pn• is the active radical and X• is the stable free
radical, ka and kd are the dissociation and recombination reaction rate constant, respectively. These stable free radicals are not tended to undergo mutual terminations, but to deactivate active propagating centers of radical polymerization through a reversible termination reaction. The most typical stable free radical polymerization system used among the early research is 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) controlled styrene system. However, the restriction of the NMP is that monomer was limited to styrene derivatives under a high polymerization temperature (>120 oC). Therefore, modifying the chemical structure of TEMPO to adapt mild experimental conditions of CRP has been widely studied afterward.
Pn-X Pn
.
X.
+
(2) Atom transfer radical polymerization (ATRP)
Atom transfer radical polymerization (ATRP) or transition metal-mediated living radical polymerization is another commonly used controlled/living radical polymerization. In 1995, Matyjaszewski11 and Sawamoto14 independently performed a controlled/living polymerization by atom transfer radical polymerization (ATRP).
Similar to its counterpart, ATRA or atom transfer radical addition, it forms a carbon-carbon bond through a transition metal catalyst. The atom transfer step is the key step in the reaction which responsible for narrowly distributed polymer chain growth. 1-phenylethyl chloride was used as the initiator and CuCl and 2,2-bipyridine were used as catalyst for styrene CRP at 130oC. The monomer conversion reached
undergoes a one electron oxidation with concomitant abstraction of a (pseudo)halogen atom, X, from a dormant species, R-X. Polymer chains grow by the addition of the intermediate radicals to monomers similar to a conventional radical polymerization with the rate constant of propagation kp. The generally known mechanism for ATRP is shown below.
(3) Reversible addition-fragmentation transfer (RAFT)
Rizzardo et al. first reported addition–fragmentation chain transfer (RAFT) polymerization for CRP in 1998.16 The mechanism of RAFT polymerization is a sequence of addition–fragmentation equilibrium as shown in the scheme. Initiation and radical mutual termination occur as the same manner compared with conventional radical polymerization. In the early stages of the polymerization, addition of a propagating radical (P•n) to the thiocarbonylthio compound [RSC(Z)S], followed by fragmentation of the intermediate radical gives rise to a polymeric thiocarbonylthio compound [PnS(Z)CS,] and a free radical (R•). New propagating radicals (P•m) were formed by the primary radical (R•) and monomer. Rapid equilibrium between the active propagating radicals (P•n and P•m) and the dormant polymeric thiocarbonylthio compounds provides almost equal probability for chain growing, led to the formation of polymers with narrow polydispersity. When the polymerization is completed, most of chains retain the thiocarbonylthio end group and can be easily isolated.
Although these three techniques of controlled/living polymerization were relatively mature, some limitation and disadvantages hindered their application in industrial production. Drawbacks in three main techniques of CRP impede the mass production on an industrial scale. For example, the NMP system has to be carried out at high temperatures (>120C), due to its inherently slow reaction rate and it works better in styrene derivatives. The difficulties of catalyst removal from the polymer for ATRP and relatively complicated synthesis pathway as well as the unpleasant odor in RAFT are known defects. In 2001, Nuyken and co-workers developed a new additive for controlled radical polymerization.17 Most of the radical polymerizations were controlled if a small amount of 1,1-diphenylethene (DPE) was added, and the method was compatible with the choice of monomers and solvents. Although the reported polydispersity indexes of DPE-controlled systems were relatively large compared to the three well known CRP methods, facile synthesis of block copolymers by heating DPE-capped macroinitiator with the presence of second monomer was achievable on an industrial scale.18 Low PDI value was of minor concern in these cases. Moreover, the number of publications in DPE-mediated polymerization systems was very limited in the literatures. 19-25
1-2 Introduction of Monodisperse Latex
Monodisperse latexes (ML) are commercially and biologically important products in diagnostic analysis, medical treatment, electronics, and standard for calibration as well as standard for virus particles,26,27 which is widely used industrially and in laboratories. They can be prepared with desired characteristics such as particle size, degree of crosslinking, surface functional group, and porosity from various synthesis approaches. The first ML was discovered accidentally in 1947 at the Dow Chemical Co. A 10 micron ML was also the first commercial product made in space to prevent creaming and settling due to the absence of gravity.28 Due to the high profit margin, ML is also sometimes called “liquid gold”.
These monosize polymer beads are typically made from heterogeneous polymerization processes,29 including emulsion,30,31 membrane emulsification,32,33 emulsifier-free,34–36 dispersion,37–40 and precipitation polymerization.41–43 Nearly monosize beads with moderate polydispersity were also achieved by miniemulsion44–48 and microemulsion polymerization.49 Recent research has drawn attention to highly uniform irregularly-shaped latex nanoparticles with optical anisotropy for their potential applications in photonic materials.50,51 However, it turns out to be particularly difficult to prepare ML in micrometer to millimeter size by a single step process, therefore they are generally prepared by successive seeded emulsion polymerization and multistep suspension polymerization. Following are the introduction of commonly used stratagies for producing ML.
1-2.1 Emulsion polymerization
In classical emulsion polymerization, the monomer is not dissolved (or only to a negligible extent) in the polymerization medium, but emulsified in it with the aid of an emulsifier. Under there conditions, the monomer is present in the mixture partly in
the form of droplets (about 1-10 micron or even larger), and partly in the form of surfactant-coated micelles (ca. 50-100 Å), depending on the concentration of the surfactant. A small percentage of the monomer is also dissolved in the medium. For example, solubility of styrene in water at 70C is about 4g/L.
The volume ratio of the monomer phase to the medium in emulsion polymerization is usually within 10%-50%, and the polymerization is carried out at 40-80C. The state of the polymerization mixture in the early stages of emulsion polymerization is illustrated in Figure1-1. Since the initiator is present only in the medium, the initial locus of polymerization is in the medium, that is, outside the droplets or micelles. The oligo- radicals formed in the polymerization medium are either surrounded by the dissolved monomer and surfactant molecules, or the already present monomer micelles. In either case, the initially formed oligoradicals produce stabilized nuclei. Subsequently, surfactant-stabilized polymer nuclei become the main loci of polymerization by absorbing further oligoradicals and monomer from the medium, or from the monomer droplets. In this manner, the nuclei/particles grow gradually until the monomer is completely consumed. The size of the latexes thus produced is usually in the range of 50-300nm.
For oil in water emulsion polymerization (e.g., those of styrene or methyl methacrylate in water), potassium peroxydisulfate (KPS) and sodiumdodecylsulfate (SDS) are commonly used as initiator and surfactant, respectively. Combinations of ionic and nonionic surfactants may also be used. An interesting example is the use of SDS and Triton X-100, as reported by Woods et al.30 for the preparation of monodisperse polystyrene latexes around 250nm.
Figure 1-1. Schematic presentation of early stages of emulsion polymerization (adapted from B. Vollmert, Polymer Chemistry, Springer Verlag, New York 1973).
1-2.2 Emulsifier-free emulsion polymerization
In emulsifier-free emulsion polymerization there exist some similarities to the classical emulsion polymerization. The monomer is present in the mixture in the form of large droplets (1-10 micron in diameter). These droplets act as reservoirs of monomer for particle growth. In addition, a small amount of the monomer is present in the polymerization medium (usually water) where the polymerization reaction starts. The polymerization mechanism depends on the reactivity and solubility of the monomer in the medium. For slightly water-soluble monomers, such as methyl methacrylate, oligomer radicals are no longer soluble in water and they precipitate in small primary particles (nuclei) which become the main loci of polymerization (some
claimed homogeneous nucleation). On the other hand, oligomer radicals of highly water-insoluble monomers, such as styrene, terminate first with the formation of surface-active oligomers and then precipitate (some claimed micelle nucleation). The nuclei and surface-active oligomers collide and form larger latex particles until the electric charge on their surface reaches a value that resists further aggregation. Latex particles are stabilized by orientation of their own polymer chain ends originating from initiator molecules. For example, in the case of potassium peroxydisulfate initiator (KPS), the chains end with sulfate groups. Subsequently, the polymerization occurs in the monomer-swollen latex particles, in which free-radical oligomers enter from the polymerization aqueous medium. As a result, the original latex particle is transformed into a particle of much greater size. As the monomer in a particle is consumed, additional monomer diffuses from the droplets through the aqueous phase to the site of polymerization. Latex particles grow until the supply of monomer or radicals is exhausted. Since the number of latex particles becomes constant within a short time after the start of the polymerization, the resulting beads are uniform in size.
This is the main difference from classical emulsion polymerization, in which latex particles are formed over rather a long period. The reason for the absence of polymerization in the monomer droplets is that the droplets are too large, and hence the total droplet surface too small, compared to that of the latex particles, to be able to compete for the radicals formed in the aqueous medium. The degree of monodispersity of the latex particles increases during the growth as they adsorb the newly formed nuclei, which at the same time stabilize them. The average number of growing free radicals within the particle is not constant but increases with increasing particle size. The molecular weight of the polymer in the latexes formed in surfactant-free systems is generally lower than that found in latexes prepared in the presence of emulsifier. The control of particle size is affected by polymerization
temperature. By raising the temperature, smaller particles were obtained. A wide range of monomers can be emulsion-polymerized in the absence of emulsifier, e.g., styrene, methyl methacrylate, divinylbenzene, and vinyl acetate. In most cases, the concentration of monomer dissolved in aqueous medium must be less than 5 wt% in order to produce good quality latexes without agglomeration. Emulsifier-free emulsion polymerization typically yields monosized beads with diameter in the range 0.5-1 micron. These beads can be used for the multi-step swelling and polymerization method.
1-2.3 Dispersion polymerization
Dispersion polymerization is an attractive and promising alternative to other polymerization methods that affords micron-size monodisperse particles in a single batch process. Dispersion polymerization may be defined as a type of precipitation polymerization in which one carries out the polymerization of a monomer in the presence of a suitable polymeric stabilizer soluble in the reaction medium. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer polymers, but a non-solvent for the polymer being formed.
Dispersion polymerization, therefore, involves a homogeneous solution of (co)monomer with initiator and dispersant, in which sterically stabilized polymer particles are formed by the precipitation of the resulting polymers. As a continuous medium, the properties of the solvent also change with increasing monomer conversion. Under favorable circumstances, the polymerization can yield, in a batch step, polymer particles of 0.1–15 micron in diameter, often of excellent monodispersity. This dispersant polymer can be formed as a reactive, polymerizable macromonomer. It can be a block copolymer in which one block has an affinity for the surface of the precipitated polymer, or it can be a soluble polymer (a “stabilizer
precursor”) to which grafting is thought to occur during the polymerization reaction.
In all instances, this soluble dispersant polymer – a hairy layer – plays a crucial role in the dispersion polymerization process. By adsorbing or becoming incorporated onto the surface of the newly-formed precipitated polymers, it acts as a steric stabilizer, directing the particle size and colloidal stability of the system. This feature of dispersion polymerization is widely appreciated and well understood (Fig. 1-2).
Figure 1-2. Schematic illustration of dispersion polymerization. (Kawaguchi, S.; Ito, K. Adv. Polym. Sci. 2005, 175, 299–328.)
1-2.4 Precipitation polymerization
In precipitation polymerization, the initial state of the reaction mixture is essentially the same as that in dispersion polymerization, i.e., a homogeneous solution.
Therefore the particle formation and growth mechanism resemble those in dispersion polymerization. The only difference, which is to advantage, is that it does not require a stabilizer which would remain in the product as a contaminant. Beads resulting from precipitation polymerization are thus pure. The absence of stabilizer, on the other hand, restricts the number of monomers from which monosized beads can be formed.
Monosized poly (divinylbenzene) beads with diameters between 2 and 5 micron are
prepared by precipitation polymerization in acetonitrile initiated with azobisisobutyronitrile or dibenzoyl peroxide. On the other hand, the initiator with a high decomposition rate (2,2’-azobis(2,4-dimethylvaleronitrile)) form beads with broad size distribution. Depending on the selection of a suitable solvent (ethyl propionate, butyl acetate or methyl butyl ketone), monosized poly (glycidyl methacrylate-co-2- hydroxyethyl methacrylate-co-triethylene glycol dimethacrylate) beads of the size ranging from 0.5 to 5 micron were prepared. Precipitation copolymerization of acrylamide with methylenebisacrylamide in ethanol and isopropyl alcohol affords coarse and bulky particles having a diameter of about 100 micron. Methacrylic acid added to the polymerizing mixture contributes to the stabilization of the particles formed at the initial stage of polymerization and to the enhancement of swelling of the particles by monomers and alcohols resulting in fine monosized beads having a diameter around 1 micron.
1-2.5 Other techniques
Monosized polyamide beads were prepared by rapid cooling of nylon solution in a theta solvent above the theta temperature;52 the phase separation mechanism was proposed for their formation.53 Monosized polymer beads were also prepared by chemical reactions in aerosols.54,55
1-2.6 Applications of monodisperse latexes
Monosized polymer latexes are finding an ever-increasing number of applications. Non-porous MLs have found widespread use in medicine as a means for clinical diagnosis and detection of antigens and antibodies.56 Besides for applications in various immunoassays, they are used as support materials for cell separation (sorting, labelling, attachment) and cultivating, as spacers in large liquid crystal
displays, fillers in advanced composites, materials for adhesives, paints, electrostratographic toners, calibration standards of various instruments (e.g., electron microscopes, light scattering devices, ultracentrifuges, aerosol and particle size counters), standards for determination of pore size and efficiency in membranes and filters. On the other hand, porous monosized beads are used either underivatized or derivatized. The former are applied in controlled drug release vehicles and in size exclusion chromatography, the latter as ion exchangers (in particular for water treatment, i.e., softening and demineralization chromatographic packings (in high-performance liquid and ion chromatography), carriers for reagents, enzymes and catalysts. Due to uniform column packing, uniform flow velocity profile and high resolution, monosized polymer beads make a great contribution to the improved separation efficiency, fast kinetics, and high flow rates and capacity in chromatography. Advantages of monosized macroporous polymer packing materials compared with silica consist in stability in alkaline conditions and in broader pore size distribution, which enhances the access of protein molecules to the active site and renders polymers with higher capacity. In ion-exchange resins for industrial water treatment, monosized beads have a higher operating capacity and efficiency and lower pressure drop than conventional beads, resulting in significant savings in salt usage and regenerant as well as wastewater consumption.
Figure 1-3. General kinetic features and particle size ranges of heterogeneous polymerization processes.
1-3 Introduction of Pickering (solid-stabilized emulsion) emulsion
From the beginning of the early 20th centry, Ramsden proposed that colloidal particles were able to stabilize emulsion. Afterward, the phenomena had been investigated thoroughly by Pickering, therefore, so-called Pickering emulsion as known to date. The generally well accepted mechanism of Pickering emulsion is the adsorption of solid particles at the oil/water interface, forming solid mono/multi-layer structures. Figure1-4 illustrated the mechanism of solid stabilized emulsions.
As compared to the conventional surfactant stabilized emulsion, Pickering emulsions possess several advantages such as (1) Reducing the usage of molecular surfactant, which is known to be high cost also detrimental to the environment. (2) Low toxcitity to human bodies. (3) Environmental friendly. (4) The emulsion stability is neglegibly affected by the pH, ionic strength, temperature, and the oil components.
Hence, Pickering emulsions have been widely used in the field of food, cosmetics, and medicine. Over last several decades, people fabricated various kinds also shapes of nanoparticles along with the progress in nanotechnology. Deep insights into the solid stabilized emulsions have attracted some attentions. Recently, Pickering emulsions were used as templets for the preparation of colloidosome, core/shell structures. The migration, aggregation as well as self-assembling of colloidal particles at the curved oil/water interfaces were also studied. Up to now, the nanoparticles involved into the studies focused on monodisperse particulate particles such as silica, iron oxide, titania, and organic latexes.
There are still several topics remained unsolved in the Pickering emulsion. For instance, (1) Whether the interfacial tension reduce or not by the adsorption of colloidal particles on the oil/water interface. (2) The mechanism of emulsion co-stabilized by surfactants and colloidal particles. (3) Grafting environmental-stumuli chemical bondings on the nanoparticles brings wide development space in this area. (4) Thermodynamically stable Pickering emulsion.
Figure 1-4. Mechanisms of Solid stabilized emulsions. (a) Barrier from full coverage of monolayer particles. (b) Sparsely covered droplets through bridging stabilization.
1-4 Flow chart of this work
References
(5) Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001, 101, 3661-3688.
(6) Hawthorne, D. G.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1999, 32, 5457-5459.
(7) Mayadunne, R. T. A.; Rizzardo, E.; Chiefari, J.; Chong, Y. K.; Moad, G.; Thang, S. H.; Macromolecules 1999, 32, 6977-6980.
(8) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.;
Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H.
Macromolecules 1998, 31, 5559-5562.
(9) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379-410.
(10) Barner-Kowollik, C.; Perrier, S. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 5715-5723.
(11) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614-5615.
(12) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921-2990.
(13) Matyjaszewski, K.; Patten, T. E.; Xia, J. H. J. Am. Chem. Soc. 1997, 119, 674-680.
(14) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721-1723.
(15) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. Rev. 2001, 101, 3689-3745.
(16) Krstina, J.; Moad, C. L.; Moad, G.; Rizzardo, E.; Berge, C. T. In A new form of
controlled growth free radical polymerization, 2nd International Symposium on Free Radical Polymerization - Kinetics and Mechanisms, Genoa, Italy, May 26-31, 1996;
controlled growth free radical polymerization, 2nd International Symposium on Free Radical Polymerization - Kinetics and Mechanisms, Genoa, Italy, May 26-31, 1996;