## 3 Polymerization Reactions within AAO Templates
### Step-Growth Polymerization  
[PERSON] et al. [132] studied the fabrication of high-aspect-ratio uniformly bent polymeric nanopillars of polyurethanes that could be used as a physical adhesive. To do so, they carried out the reaction of a functionalized prepolymer with an acrylate group acting as crosslinker on surface-treated reusable AAO molds. They also studied the collapse and sticking properties of polymer nanopillars as a function of the dimensions of the nanocavity. Related to stepwise polymerization in confinement, [PERSON] et al. reported a theoretical study on the effects of confinement in diffusion-controlled stepwise polymerization by Monte Carlo simulation [133]. They found that confinement modified both the spatial pair distribution function and the diffusive properties of the polymers and, therefore, that the system can experience either faster or slower reaction kinetics when compared to bulk reactions, depending on the strength of intermolecular interactions. This model has been used in experimental works of step polymerization in AAO templates, as will be described later.  
The first kinetic study of the step-growth polymerization reactions in AAO membranes was carried out by [PERSON] et al. and [PERSON] et al. using epoxy based systems [134; 135]. The authors studied the step-growth polymerization of bisphenol-A diglycidyl ether (DGEBA) with aniline both in anodic aluminum oxide (AAO) membranes and in bulk. The authors found [102] that polymerization was faster under confinement compared to the analogous reaction carried out in the bulk system at the same temperature conditions. They also found that the reaction accelerated with the degree of confinement and that the initial step of the polymerization was  
Figure 9: **(A)** Schematic copolymerization procedure of BMA and HEA monomers under confinement. **(B)** Infrared spectroscopy (IR) spectra of the copolymers obtained under confinement and in bulk. **(C)** SEM image of BMA-HEA copolymer nanostructures [130]. Copyright 2019. Reproduced with permission from the publisher: MDPI.  
significantly reduced or even suppressed in nanochannels, i.e., the kinetic curves did not follow the sigmoidal shape characteristic for this kind of autocatalytic chemical reaction.  
Very recently, [PERSON] et al. studied step-growth polymerization within AAO templates [136] and concomitant confinement effects on the reaction kinetics of triethylene glycol (TEG) and hexamethylenediisocyanate (HDI) for three different pore diameters: 140, 60 and 35 nm. The authors found that the kinetics of the reaction in AAO nanoreactors were faster than in bulk, while the molecular weight and the dispersity of extracted polymer from AAO template were reduced. In that work, a mathematical model was developed taking into account the effects of confinement and chemical interactions of OH with the pore walls (Figure 10). Briefly, it was assumed that diisocianate also reacted with the OH of the AAO walls, and the polyaddition in nanoconfinement was catalyzed by hydroxyl groups on the pore wall, resulting in an increase in the rate of polymerization during the early stages of the reaction. The model explained the differences found for both the kinetics of step-growth polymerization and the molecular weights of the obtained polymers in confinement in comparison to bulk.