Polymeric microspheres

Fig. 9 SEM images of (a) magnetic PGMA microspheres. Polymerization conditions 68 g ethanol, 12 g GMA, 2 g Fe203, 4.5 wt% PVP (based on polymerization mixture), and 2 wt% AIBN (based on monomer), (b, c) crosslinked magnetic PGMA nuciospheres. Polymerization conditions (b) 66 g ethanol, 2g water (c) 62 g ethanol, 6g water 11.04g GMA, 0.96 g EDMA, Ig Fe203, 2.25 wt% PVP (relative to the polymerization mixture), and 2 wt% AIBN (relative to the monomers). Reprinted from [125] with permission...

When a stimuli-responsive gel is used for various devices, the response time is an essential property. The response time of gels is proportional to the square of the gel size [50]. Hence, the possibility of manufacturing as small a gel as possible is an important consideration. A monodisperse microsphere is such an example. The PNIPAAm, which is polymerized by precipitation polymerization, has a 0.94-pm diameter [51 ]. The PNIPAAm microsphere polymerized inside a micelle is 80-150 nm in diameter and its response time is excellent [52]. 

Various novel applications in biotechnology, biomedical engineering, information industry, and microelectronics involve the use of polymeric microspheres with controlled size and surface properties [1-31. Traditionally, the polymer microspheres larger than 100 /urn with a certain size distribution have been produced by the suspension polymerization process, where the monomer droplets are broken into micron-size in the existence of a stabilizer and are subsequently polymerized within a continuous medium by using an oil-soluble initiator. Suspension polymerization is usually preferred for the production of polymeric particles in the size range of 50-1000 /Ltm. But, there is a wide size distribution in the product due to the inherent size distribution of the mechanical homogenization and due to the coalescence problem. The size distribution is measured with the standard deviation or the coefficient of variation (CV) and the suspension polymerization provides polymeric microspheres with CVs varying from 15-30%. 

Uniform polymeric microspheres of micron size have been prepared by dispersion polymerization. This process is usually utilized for the production of uniform polystyrene and polymethylmethacrylate microspheres in the size range of 0-1-10.0 /Am. 

Stable particles in sufficient number, all the oligo-radi-cals and nuclei generated in the continuous phase are captured by the mature particles, no more particles form, and the particle formation stage is completed. The primary particles formed by the nucleation process are swollen by the unconverted monomer and/or polymerization medium. The polymerization taking place within the individual particles leads to resultant uniform microspheres in the size range of 0.1-10 jjLvn. Various dispersion polymerization systems are summarized in Table 4. 

Uniform polymeric microspheres in the micron size range have been prepared in a wide variety of solvent combinations by dispersion polymerization. The polarity of the dispersion medium is one of the most important... 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

The soapless seeded emulsion copolymerization method was used for producing uniform microspheres prepared by the copolymerization of styrene with polar, functional monomers [115-117]. In this series, polysty-rene-polymethacrylic acid (PS/PMAAc), poly sty rene-polymethylmethacrylate-polymethacrylic acid (PS/ PMMA/PMAAc), polystyrene-polyhydroxyethylmeth-acrylate (PS/PHEMA), and polystyrene-polyacrylic acid (PS/PAAc) uniform copolymer microspheres were synthesized by applying a multistage soapless emulsion polymerization process. The composition and the average size of the uniform copolymer latices prepared by multistage soapless emulsion copolymerization are given in Table 11. 

The uniform polymeric microspheres in submicron-or micron-size range can also be prepared as seed particles by the soapless emulsion or dispersion polymerization of a hydrophobic monomer like styrene. The uniform seed particles are swollen with the organic phase including functional comonomer, monomer, and oil-soluble initiator at a low temperature in an aqueous... 

As previously described, all microspheres discussed in this chapter were synthesized from AB type diblock copolymers. Precursor block copolymers, poly(styrene-b-4-vinyl pyridine) (P[S-b-4VP]) diblock copolymers, were synthesized using the additional anionic polymerization technique [13]. The basic properties of the block copolymers were determined elsewhere [24,25] and are listed... 

As these block copolymers were synthesized using the anionic polymerization technique, their molecular weight distributions were narrow. The microspheres with narrower size distribution are better for well-ordered self-organization. Actually, all block copolymers synthesized for these works formed poly(4-vinyl pyridine) (P4VP) spheres in the PS matrices with narrow size distributions. 

The poly(styrene-b-isoprene) (P(S-b-IP)) and poly(-styrene-b-2-vinyl pyridine) (P(S-b-2VP)) block copolymers with narrow molecular weight distributions for blending with the microspheres were also synthesized using the additional anionic polymerization technique. The number-average molecular weights (Mns) and PS contents are also shown in Table 1. 

A large variety of drug delivery systems are described in the literature, such as liposomes (Torchilin, 2006), micro and nanoparticles (Kumar, 2000), polymeric micelles (Torchilin, 2006), nanocrystals (Muller et al., 2011), among others. Microparticles are usually classified as microcapsules or microspheres (Figure 8). Microspheres are matrix spherical microparticles where the drug may be located on the surface or dissolved into the matrix. Microcapsules are characterized as spherical particles more than Ipm containing a core substance (aqueous or lipid), normally lipid, and are used to deliver poor soluble molecules... 

A unique method of formulating delivery systems based on starch/ PLA systems was studied (138). In that approach, the goal was to provide a better matrix for delivery of high molecular weight hydrophilic molecules. A hydrophilic material, starch, was combined through graft polymerization to PLA. The carbolactic polymers were then used to entrap bovine serum albumin in microspheres. 

Kanke, M., Geissler, R. G., Powell, D., Kaplan, A., and De-Luca, P. P., Interaction of microspheres with blood constituents. III. Macrophage phagocytosis of various types of polymeric drug carriers, J. Parent. Sci. Technol., 42, 157, 1988. 

Schroder, U., Lager, C., and Norrlow, O., Carbo-lactic microspheres Graft polymerization of PLA to starch microspheres, Proc. Int. Control. Rel. Bioact. Mater.. 14, 238,... 

Starch is usually derivatized by the introduction of acrylic groups, prior to polymerization and manufacture into microspheres. Poly(acryl) starch microspheres, as they are referred to, are an example of a semisynthetic polymer system. Their extensive use as... 

Core-shell nanocomposite of Mg(OH)2/PMMA with an average particle size of ca. 500nm where Mg(OH)2 is the core and PMMA is the shell was successfidly prepared by the emulsion polymerization of MMA in the presence of surface modified Mj OH)2. The grapelike ( re-shell microspheres with PMMA nodules could he obtained as stable latex. 

The preparation by dispersion polymerization of the microsphere sample employed in this study was previously described [8]. The microsphere sample utilized in this study has a monomodal diameter distribution with mean diameter value d= 3.09 pm and standard deviation dsdev= 0-74 pm. The microsphere surface is covered by a poly(methacrylic acid-co-ethylacrylate) whose percent by weight is 1.1... 

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