ABSTRACT
In this chapter an introductory overview of the different polymerization techniques that can lead to polymeric nanoparticles and nanocapsules is presented. We put an emphasis on the preparation of these particles in aqueous systems because for many applications, including controlled drug release, aqueous dispersions of the polymeric nanoparticles are required. Within the narrow definition of the IUPAC nanoparticles are particles with one dimension less than 100 nm. In practice in literature this definition is expanded to particles up to 1000 nm. This chapter (together with Chapter 3) should facilitate reading the more specialized chapters in Part II where polymerizations are used to produce polymer nanoparticles.
Most of the polymerization techniques are heterophase polymerizations (with the exception of solution polymerization, Section 5.1) and polymer chains can undergo transitions from one phase to another. Precipitation polymerization usually starts off with a solution polymerization but at some point the polymer precipitates (Section 5.2). A dispersion polymerization resembles a precipitation polymerization; the main difference is the presence of a colloidal stabilizer (Section 5.3). Emulsion polymerization, miniemulsion polymerization, and microemulsion polymerization all are producing polymers inside nanostructures (micelles, stabilized monomer droplets, thermodynamically unstable in miniemulsion and thermodynamically stable in microemulsion) (Sections 5.4-5.6, respectively). For the synthesis of nanocapsules one can resort to using the hydrophobic domain in vesicles instead of micelles for the locus of polymerization or one can use the vesicles as a template to form a polymer layer on the outside of the vesicle, resulting in a nanocapsule (Section 5.7). The latest approach to prepare nanoparticles is the polymer-induced self-assembly (PISA) method (Section 5.8). We try to stay as close as possible to the definitions of the IUPAC but we also realize that in the literature often the wrong polymerization technique is linked to the particular polymer nanoparticle synthesis at hand. 5.1 Solution PolymerizationAt first glance there seems to be a contradiction to produce polymer nanoparticles with solution polymerization. After all if the particles are produced by precipitation of the polymer out of the solution one deals with precipitation polymerization. However, the distinction between large dissolved polymer chains and small polymer nanoparticles becomes unclear in the region of several nanometers (1-20 nm). So in general, when we talk about polymeric nanoparticles prepared by solution polymerization we are in the gray area between dissolved polymer chains and nanoparticles. An overview of dendritic polymer architectures produced in solution polymerizations is shown in Fig. 5.1 (Gao, 2004a).
