Polymerization core-shell polymers

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

Core-shell polymers consist of particles with composite structure. The inner portion of particle (core) has a different composition than the outer portion (shell). Typically, they are prepared by introducing a comonomer (e.g., perfluoroalkylvinyl ether [PAVE]) during the polymerization under specific conditions [28]. An example is a composition with the core constituting 65% to 75% of the total weight of the particle. 

Typical particle sizes of the resulting lattices are between 50 nm and 500 pm. Generally, the size distribution of the latex particles is broad [252], Lattices with a very narrow size distribution can be achieved by a short nucleation period followed by a long growth period in the absence of coagulation [250]. Because the polymerization takes place within the outer periphery (shell) of the particle, latex polymers with a core-shell structure can be prepared, the core consisting of a cross-linked polymer, surrounded by a shell of tethered linear non-cross-linked polymer of different chemical composition. Recent reviews deal with the preparation and application of these core-shell polymers [212,253]. 

The implication of such stimuli-responsive particles as a solid polymer support of biomolecules in the biomedical field is probably due to various factors (1) easiest to prepare via precipitation polymerization (hydrogel particles) or a combination of emulsion and precipitation polymerizations (core-shell particles), (2) the colloidal properties are related to the temperature and to the medium composition (i.e., pH, salinity, surfactant etc.), (3) the adsorption and the desorption of antibodies and proteins are principally related to the incubation temperature, (4) the covalent binding of proteins onto such hydrophilic and stimuli-responsive particles can be controlled easily by temperature, and, finally, (5) the hydrophilic character of the microgel particles is an undeniably suitable environment for immobilized biomolecules. 

The acrylic core-shell polymers are considered to offer superior ultraviolet-light and thermal-oxidative aging properties than does the more conventional reactive liquid polymeric toughener, CTBN. Hence, there is current interest in the use of acrylic core-shell polymers as tougheners for adhesives and composite matrices that possess a relatively high glass-transition temperature. 

Spherical beads possess better hydrodynamic and diffusion properties than irregularly shaped particles. It is, hence, desirable to apply MIPs in a spherical bead format, especially for flow-through applications. Methods to synthesize spherical polymer beads are often classified according to the initial state of the polymerization mixture (i) homogeneous (i.e. precipitation polymerization and dispersion polymerization) or (ii) heterogeneous (i.e. emulsion polymerization and suspension polymerization). In addition, several other techniques have been applied for the preparation of spherical MIP beads. The techniques of two-step swelling polymerization, core-shell polymerization, and synthesis of composite beads will be detailed here. 

In the last few years, many efforts have been given to the preparation of magnetic latexes in dispersed media using suspension, precipitation, dispersion, emulsion, miniemulsion and microemulsion polymerizations. In this review chapter, the synthesis and functionalization of magnetic core-shell polymer particles in dispersed media have been reviewed with the main focus on emulsion polymerization. 

The segregated core-shell microstmctures, consisting of a hydrophobic core surrounded by a hydrophilic shell, are of great praaical interest as their mechanical properties are mainly influenced by the core polymer and the chemical properties and solubility mainly by the shell monomer units. These materials have attracted increased attention because they not only maintain the funaions of both the core and shell components but also exhibit additional excellent optical, electrical, and magnetic properties. Polymeric core-shell microstmctures can be obtained through phase separation, which is realized by inductive polymerization," solvent extraction and evaporation,"" self-assembly of amphiphilic block copolymers,"" or sequential predpitation,"" which is intrinsically a self-assembly and phase-separation process. However, these methodologies are usually complicated and tedious. 

M.D. Besteti, A.G. Cunha, D.M.G. Freire, J.C. Pinto, Core/shell polymer particles by semibatch combined suspension/emulsion polymerizations for enzyme immobilization, Macromolecular Materials and Engineering 299 (2014) 135-143. 

Y. Qu, J. Liu, K. Yang, Z. Liang, L. Zhang and Y. Zhang, Boronic Acid functionalized core-shell polymer nanoparticles prepared by distillation precipitation polymerization for glycopeptide enrichment, Chem-Eur J., 2012,18(29), 9056-9062. 

Very little has been reported about the use of spectroscopic methods for monitoring and control of other polymerization systems. Lenzi et al. [191] reported that the NIR spectra collected in a dispersive instrument with a transflectance probe may contain very useful information about the structure of core-shell polystyrene beads produced through simultaneous semibatch emulsion/suspension polymerizations. Lenzi et al. [192] developed a polymerization technique that combines recipes of typical emulsion and suspension polymerizations to produce core-shell polymer beads. More interesting, the appearance of the core-shell structure always led to qualitatively different NIR spectra that could not have been obtained with polymer suspensions, polymer emulsions, or mixtures of polymer suspensions and emulsions. As described by Lenzi et al. [191], different spectral peaks could be detected in the wavelength region constrained between 1700 and 1900nm when the core-shell structure developed. 

Membrane and microfiuidic devices have also been adopted for the precision manufacture of solids from double-emulsion templates. To date, several different types of particles have been successfully produced by incorporating use of various membrane and microfiuidic devices in processes of polymerization, gel formation, crystallization, and molecular or particle self-assembly. Membrane emulsification is more suited to the fabrication of less sophisticated particulates, such as solid lipid micro-Znanoparticles, gel microbeads, coherent polymeric microspheres, and inorganic particles such as silica microparticles. Microfiuidic devices allow more sophisticated particle designs to be created, such as colloidosomes, polymerosomes, 3D colloidal assemblies, asymmetric vesicles, core-shell polymer particles, and bichromal particles. 

When monomers of drastically different solubiUty (39) or hydrophobicity are used or when staged polymerizations (40,41) are carried out, core—shell morphologies are possible. A wide variety of core—shell latices have found appHcation ia paints, impact modifiers, and as carriers for biomolecules. In staged polymerizations, spherical core—shell particles are made when polymer made from the first monomer is more hydrophobic than polymer made from the second monomer (42). When the first polymer made is less hydrophobic then the second, complex morphologies are possible including voids and half-moons (43), although spherical particles stiU occur (44). 

Fig. 30 Types of nanocarriers for drug delivery, (a) Polymeric nanoparticles polymeric nanoparticles in which drugs are conjugated to or encapsulated in polymers, (b) Polymeric micelles amphiphilic block copolymers that form nanosized core-shell structures in aqueous solution. The hydrophobic core region serves as a reservoir for hydrophobic drugs, whereas hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble.

Nanocapsules are reservoir type systems comprising an oily liquid core surrounded by a polymeric shell [27]. The drug is usually dissolved in this liquid core but may be more closely associated with the shell polymer and the exposed surface, as illustrated in Figure 2b. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

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