## 2 Anodized Aluminum Oxide (AAO) Templates Geometry  
Several kinds of \"host\" hard geometries have been employed as nanocavities for the synthesis of functional polymers with controlled structures. These include alumina (AAO) templates [9; 61] as well as other geometries; microporous zeolites [62]; mesoporous silicates [63; 64; 65; 66]; controlled porous glasses [57]; metal-organic frameworks (MOFs) and porous polymers [67; 68; 69], or hybrid geometries [56; 70; 71; 72; 73]. These systems can provide internal spaces large enough to hold diverse monomers. Nevertheless, there are significant structural differences in the porosity of porous materials, not only in the pore size or pore surface functionality, but also in the interconnection of the pores. Pore geometries that have been reported include tubular, spherical and network-type morphologies, which can be disordered or assembled into ordered arrays. Moreover, the pore size is an important parameter, since smaller-sized pores produce materials with higher surface areas. In addition, the pore surface functionality can be designed by post-synthesis modification processes or through the use of functional monomers [74; 75; 8]. Therefore, the possibility of controlling the pore structure and functionality of the porous materials has boosted the study of polymerization reactions. Porous anodic aluminum oxide (AAO) templates have demonstrated tremendous versatility as nanomols to achieve sophisticated polymer nanostructures of various mor  
Figure 1: Scopus search for “polymerization in confinement”. phologies and sizes from confined polymer melts or solutions obtained by the so-called polymer infiltration process [8; 9; 13; 24; 76; 87]. More recently, AAO templates have also emerged as \"precise\" nanoreactors where polymerization reactions can be performed, thus extending the possibilities of this nanoprocessing approach to the entire library of polymers, including functional polymers that cannot be dissolved or melted [9]. The fabrication of AAO templates is based on a facile and economic electrochemical anodization of aluminum sheets. AAO templates and AAO membranes can be synthesized in the laboratory (lab-made) but are also commercially available. Currently, there are a number of different companies providing membranes with more or less irregularly ordered pores and a broad pore size distribution. These membranes were primarily designed for filter applications but have also been used in other processes in which the homogeneity of nanocavities is not so crucial. Figure 2 shows schemes of the nature of a typical AAO template and SEM images of a commercial AAO template and of lab-made templates. See figure caption for details. Comparing Figure 2B,C, it is easy to observe the difference between lab-made and commercial AAO samples. At the surface of the commercial one, some pores are connected. In addition, the lower section is very different from the upper side because pore homogeneity is not constant across the membrane and a heterogeneous alumina network is formed. For this reason and the experience gained in different groups with instrumental improvements, many laboratories now produce their own lab-made templates. In fact, since the first works of [PERSON] et al. [76], AAO templates have been the subject of research in a number of laboratories and companies world-wide as a relevant method in the rapid development of polymer nanotechnology providing polymer based nanomaterials with different shapes and morphologies [88]. More recently, commercial alumina templates have improved with respect to the regularity and homogeneity of pores and the range of pore diameters. Readers interested on the topic are directed to a recent review of [PERSON] et al. [8] on the Progress on nanoporous anodic aluminum oxide and to the advances in surface engineering and emerging applications. Subsequent developments have allowed for an increase in the level of complexity of AAO templates and for the preparation of \"branched\" AAO templates.