## 3 Polymerization Reactions within AAO Templates  
The polymerization reaction determines the main characteristics of the resulting polymer molecules, including the chemical structure, the molecular weight and molecular weight distribution, the sequence distribution, etc. As with any chemical reaction, polymerization reactions are strongly dependent on the nature of the reactive species and/or specific reaction conditions, solvents, molecular diffusions/interactions, etc. [89; 90; 91; 92]. Therefore, if a polymerization reaction is conducted in spatially restricted geometries, spatial confinement effects may impact any of the fundamental steps of the polymerization reaction is based on, i.e., initiation, propagation and termination [6; 93; 94; 95; 96]. This in turn can result in significant differences in the polymer structure and properties. For instance, in free radical polymerization (half of the industrial production of common polymers), the propagation process is the most relevant step in the growth of chains and, as a consequence, is largely responsible for the final molecular weight of polymers. Therefore, nanoconfinement is expected to substantially impact the propagation step where the diffusion of reactive species is hindered by the nanoscale dimensions of the reactor. Indeed, it has been noted in many free radical systems that strong modifications of the viscosity can be observed in confinement [97; 98], along with changes to the nature of the vitrification process [99], the cage effect, and the gel effect, and these changes in the polymerization reactions are coupled with changes to the molecular weight of the chains produced [56; 100; 101]. In the step-growth polymerization method, bi- or multifunctional monomers react to form first dimers, then trimers, etc., eventually leading to the generation of long-chain polymers, depending on the final conversion. Therefore, any strong reduction of the initial part of the step-growth polymerization or T\({}_{\text{g}}\) depression, as a consequence of the degree of confinement, would modify the reaction kinetics [102].  
The first studies on polymerization reactions in confinement were reported by [PERSON] et al. [58] and dealt with the fabrication of conducting polypyrrole nanotubes using AAO templates [4; 5; 7] and [PERSON] et al. on the polymerization of methyl methacrylate (MMA) in controlled pore glasses (CPG) [57]. Some years later, the polymerization of styrene in porous coordination polymers (PCP) was reported by [PERSON] et al. in 2004 [59] and the ATRP polymerization by [PERSON] et al. and [PERSON] et al. in 2008 [60; 103] etc. These works only represent a few examples of those reported from the early 1990s until around 2010. Nevertheless, in these papers, neither the influence of confinement on polymerization kinetics nor the rigorous chemical characterization of the obtained polymers was reported. Since the first work on the electrochemically initiated synthesis of divinylbenzene (DVB) and ethylvinylbenzene (EVB) copolymers in porous alumina (AAO) membranes in 1990 [1], a relatively broad range of different polymerization techniques have been reported. These include the radical polymerization of several vinyl monomers, step-growth polymerizations, ATRP and others. With a few exceptions, the polymerization mechanism was assumed to be the same as that of bulk polymerization or, simply, was not taken into consideration.  
In this section, we report and discuss the first electrochemical polymerization reaction and the most-relevant polymerization reactions within porous AAO templates classified by the polymerization mechanism. In addition, this section includes some of the previous studies on nanoporous polycarbonate membranes (both sides open) of similar diameter to those of AAO templates.