#### 2.1.2 Polymers Prepared by Electropolymerization of [M(Salen)] Monomeric Molecules  
The first studies of polymers based on transition metal complexes with salen-type Schiff ligands were largely focused on characterizing electrochemical methods for obtaining conducting polymer layers on an electrode from monomers and studying the conductive properties of the resulting polymers using electrometric methods depending on the solvent and type of ligand. It was demonstrated in [19; 20] that these complexes are reversibly oxidized in strong-donor solvents and oxidatively polymerized in solvents with weak donors. The monomers' oxidation mechanisms were analyzed using cyclic voltammetry and scanning electron microscopy. It has been shown that the polymer layer's thickness on the electrode increases linearly over several voltammetric scans. Also, the initial growth rate of the polymer layer increases linearly with rising monomer concentration in the electrolyte solution. Despite a significant amount of research into the structure and properties of electrically conductive redox polymers poly-[M(Salen)], and in particular poly-[Ni(Salen)], there is no consensus on the mechanism of polymerization of monomer molecules [1; 2; 35; 36; 37; 38; 39; 40; 41; 42; 43; 40; 43; 1]. The first model assumes chemical linkage between the phenyl groups of the ligands of  
Figure 1: Schematic illustration of the H\({}_{2}\)(Salen) salen ligand and [Ni(Salen)] molecular complex [46]. neighboring molecules (chainlike structure). In this instance, the polymer forms through the formation of covalent C-C bonds between carbon atoms in the para positions of the phenyl moieties (Figure 2a) and is inherently purely liganded. [19; 24]. The second model of the polymerization mechanism is based on processes associated with the metal atom, which we will consider using the example of nickel [21; 60]. In this case, the complexing nickel cation Ni(II) is first additionally oxidized Ni(II) \(\rightarrow\) Ni(III) + e\({}^{-}\). Subsequently, it can form a stacklike structure which is stabilized by chemical bonding (and charge transfer) between the nickel cation and the phenyl moieties of neighboring monomer molecules (as shown in Figure 2b). Finally, the third model combines the two previous ones: the electro-oxidation of monomers leads to the formation of stacklike structures. These structures are then cross-linked through the formation of covalent C-C bonds, leading to the appearance of \(\pi\)-conjugated chains involving the phenyl rings of the ligand (Figure 2c) [21; 60]. Over the past 20 years, two contrasting viewpoints regarding the electrochemical activity of conducting polymers have been significantly discussed, in which two options are considered as redox positions (places where redox reactions occur): cations of metal atoms [20; 21] or ligand atoms [22; 23]. Over these two decades, different groups of researchers have studied salient complexes and polymers based on them using voltammetry and chronoamperometry [11; 12], _in situ_ UV-vis and IR spectroscopy [11; 13], _in situ_ ellipsometry [12], electrochemical quartz microbalances [12; 14; 15], and impedance spectroscopy [16]. Both mechanisms are characterized by different charge transfer in [Ni(Salen)] and, as a consequence, different charge states of Ni, N, O, and C atoms in the reduced (neutral) and oxidized states of poly-[Ni(Salen)]. To directly identify nickel atoms in a highly oxidized state within redox polymers, the electron paramagnetic resonance (EPR) technique was used [12; 14; 23; 24; 40]. The conducted measurements did not detect highly oxidized Ni(III) atoms within the polymer. This led the authors of these studies to choose the first model, based on the processes of oxidation and reduction of ligands, for the mechanism of polymerization and electrochemical activity of polymers.