Asymmetric Monomer Design Enables Structural Control of M(Salen)-Type Polymers

Conductive and electrochemically active polymers consisting of Salen-type metal complexes as building blocks are of interest for energy storage and conversion applications. Asymmetric monomer design is a powerful tool for fine-tuning the practical properties of conductive electrochemically active polymers but has never been employed for polymers of M(Salen)]. In this work, we synthesize a series of novel conducting polymers composed of a nonsymmetrical electropolymerizable copper Salen-type complex (Cu(3-MeOSal–Sal)en). We show that asymmetrical monomer design provides easy control of the coupling site via polymerization potential control. With in-situ electrochemical methods such as UV-vis-NIR (ultraviolet-visible-near infrared) spectroscopy, EQCM (electrochemical quartz crystal microbalance), and electrochemical conductivity measurements, we elucidate how the properties of these polymers are defined by chain length, order, and cross-linking. We found that the highest conductivity in the series has a polymer with the shortest chain length, which emphasizes the importance of intermolecular iterations in polymers of [M(Salen)].


Introduction
Sustainable energy storage and conversion, as well as a decrease in energy consumption, are pivotal challenges faced by modern society. To meet these challenges we need new materials that can store and convert green energy and assist in conserving energy in our everyday life. A promising class of materials that may address these challenges is conductive electrochemically active polymers [1]. Conventional examples of these materials are organic intrinsically conducting polymers with a conjugated backbone, such as polyaniline, [2], polypyrrole (PPy) [3], and poly(ethylenedioxythiophene) (PEDOT) [4]. These materials have been extensively studied and implemented as materials for electrochemical supercapacitors, batteries, electrochromic, and solar energy harvesting devices. Improving the functions and capabilities of such materials could be achieved by introducing a transitional metal center in the polymer backbone [5][6][7]. This enrichment of functions and properties arises from the properties of the metal center and from interactions between metal ions and a conjugated organic backbone.
The polymers of N 2 O 2 Salen-type Schiff bases complexes of transition metals, [M(Salen)], are conductive and electrochemically active materials that are of interest to researchers in the field due to their advantageous properties. Specifically, polymeric [Ni(Salen)] complexes have a wide electrochemical activity window and moderate [8,9] to high (up to 100 S cm −1 ) [10] conductivity. Specific redox capacitance of [Co(Salen)] is a record-high among poly-[M(Salen)]s, with up to 182 mAh g −1 observed in thin polymer films [11] and scaled up to 170 mAh g −1 in hybrid supercapacitor electrodes [12]. Successful implementation of advantageous poly-[M(Salen)] properties in hybrid supercapacitors [10,12,13], We chose the asymmetrical [Cu(3-MeOSal-Sal)en] complex (Scheme 1) with three potential redox centers: aromatic rings with and without methoxy-substituent, and a central metal atom, as a model system to probe the asymmetrical design of [M(Salen)]. We show that, due to drastic difference in redox potentials of pristine and methoxy-substituted aromatic rings, it is possible to control the C-C coupling site via simple potential control. We study three different polymeric structures using a combination of physicochemical and spectroelectrochemical methods. The dependence of the electrochemical, spectral, and structural features of polymers on the method of their formation is shown. We show that the efficiency of intermolecular charge transfer can be sufficient to impart conductivity and electroactivity to materials with an extremely low conjugated chain length, but a high degree of intermolecular interactions.
Polymers 2023, 14, x FOR PEER REVIEW 3 of 20 [49]. That improves the interchain interactions between polymer chains, increases conductivity, and reduces the oxidation/reduction potential of the films [50]. A few works have been devoted to the study of asymmetric copper complexes with Salen-type Schiff bases [51,52]. No polymers of asymmetrically designed [M(Salen)] are reported so far. We chose the asymmetrical [Cu(3-MeOSal-Sal)en] complex (Scheme 1) with three potential redox centers: aromatic rings with and without methoxy-substituent, and a central metal atom, as a model system to probe the asymmetrical design of [M(Salen)]. We show that, due to drastic difference in redox potentials of pristine and methoxy-substituted aromatic rings, it is possible to control the C-C coupling site via simple potential control. We study three different polymeric structures using a combination of physicochemical and spectroelectrochemical methods. The dependence of the electrochemical, spectral, and structural features of polymers on the method of their formation is shown. We show that the efficiency of intermolecular charge transfer can be sufficient to impart conductivity and electroactivity to materials with an extremely low conjugated chain length, but a high degree of intermolecular interactions.

Chemicals
The monomer was synthesized as described in [51]. The identification of the complex was carried out using 1 H Nuclear magnetic resonance (NMR) (Jeol 400 MHz, Japan) in DMSO-d 6 and X-ray diffraction (XRD) [53]. Identification of polymer hydrolysis products were carried out using NMR ( 1 H, 13 C) in DMSO-d 6 .

Electrochemistry and Electrochemical Quartz Crystal Microbalance (EQCM)
All electrochemical measurements were performed in a three-electrode cell. Polymers were synthesized in the form of thin films on conducting surfaces by electropolymerization from a solution of 0.001 mol dm −3 [Cu(3-MeOSal-Sal)en] and tetraethylammonium tetrafluoroborate in acetonitrile. The reference electrode was a non-aqueous Ag/Ag + reference electrode (MW-1085, BASi, IN, USA) filled with a 0.005 mol dm −3 AgNO 3 solution in 0.1 mol dm −3 Et 4 NBF 4 /CH 3 CN. The potential of this electrode was −0.30 V versus the Ag/AgCl,sat'd NaCl reference electrode. All potentials reported in this paper use the Ag/AgCl,sat'd NaCl as the reference electrode. The supporting electrolyte was a 0.1 mol dm −3 Et 4 NBF 4 solution in CH 3 CN. The working electrode for cyclic voltammetry (CV) was a platinum disk (MF-2013, BASi, IN, USA, electrode area 0.02 cm 2 ), that was polished with an aqueous suspension of Al 2 O 3 (particle size 0.05 µm) prior to use.
Polymer 1 deposition was performed in a 0.001 mol dm −3 monomer/acetonitrile solution with 0.1 mol dm −3 Et 4 NBF 4 as the supporting electrolyte by cycling the potential of the working electrode between 0 and 1.4 V. Polymer 2 was deposited from the same solution at constant potential of 0.85 V until the specified charge passed. Polymer 3 was formed from polymer 2 by cycling the potential of the working electrode between 0 and 1.4 V in monomer free supporting electrolyte.
The installation for microgravimetric studies included the complex of a QCM100 Quartz Crystal Microbalance Analog Controller, a QCM25 Crystal Oscillator (Stanford Research Systems, USA), and Bio-Logic (Science Instruments, Seyssinet-Pariset, France). Variation of the oscillation frequency of the crystal (with the natural resonance frequency of 5 MHz) due to the variation of the polymer mass on the electrode was registered automatically using a Metex MXC 1600 frequency meter (Metex Co., Seoul, Republic of Korea). We used a silicon dioxide piezoelectric crystal sprayed with a layer of platinum as a working electrode (S = 1.37 cm 2 ). A counter electrode was glassy carbon plate (S = 12.5 cm 2 ).
The mass of the dried polymer was estimated by using the Sauerbrey equation [54], that relates the mass change per unit area at the electrode surface to the observed change in oscillation frequency of the crystal: where f 0 is oscillation frequency of the fundamental mode of the EQCM, A is crystal area (1.37 cm 2 ), ρ q is density of quartz (2.648 g cm −3 ), and µ q is the shear modulus of quartz (2.947 × 10 11 g cm −1 s −2 ). In the case of the device used in this work, the constant may be substituted with the sensitivity factor C f (56.6 Hz µg −1 cm 2 ), resulting in the Equation (2).
The value of C f was calibrated by the manufacturer. A change in this value is unlikely as deposition of the polymer film was conducted homogeneously and the electrochemically active area covered most of the crystal surface [55].
As the Sauerbrey equation is only applicable in this form to thin rigid films, i.e., when the product of immobilized film viscosity and density does not change significantly throughout the experiment, we monitored the viscoelastic phenomena via the motional resistance of the crystal to comply with the condition of a thin film [56].
The scan rates for deposition and the number of polymerization cycles and times were adjusted for each polymer to produce approximately 4.2 ± 0.1 µg of the dry polymer on the electrode surface. The electrode with the deposited polymeric film was rinsed with acetonitrile and dried in argon for 30 min. Then the electrode was transferred to the supporting electrolyte solution and electrochemical measurements were performed in potential range 0-1.4 V at different scan rates.
The number of electrons exchanged by each poly-[Cu(Schiff)] unit during redox conversion of the polymer, n, was calculated using the following equation [54]: where Q is the amount of charge passed in electrochemical reduction of a poly-[Co(Schiff)] film in C; M is a molar mass of a polymer fragment in g/mol; F is the Faraday constant (96,485 C mol −1 ); m is a mass of the poly-[Cu(Schiff)] film determined by QCM, g. The values of specific capacity, C mAh in mAh g −1 , were calculated for the investigated polymers using equation:

In Situ Conductance Measurements
For conductivity tests, the polymers were synthesized on an interdigitated platinum electrode (MicruX Technologies, Asturias, Spain) according to the methods mentioned above until the charge of synthesis reached 20 mC. The stability test, combined with in situ conductivity measurement, was carried out by potentiodynamic cycling from (0.0-0.8 V) 0.0 to 1.4 V with scan rate 0.01 V s −1 during 5 cycles.
The conductivity-potential profile of the polymers was determined by cyclic voltammetry on an interdigitated electrode (IDE) with a constant potential bias between grids of IDE with a bipotentiostate. As described in [8,57] in more detail, for conductivity measurements, bipotentiostat sets the potential profile of each grid in such a way that the potential difference between working electrode 1 WE(1) grid and working electrode 2 WE(2) grid to be a constant (we used 5, 10, and 20 mV differences). During the potential scanning, the current flowing through the working electrodes, WE(1) and WE (2), involves the faradaic current of the electrochemical process and the leakage ohmic current between grids. Assuming that the value of the Faraday currents is the same for both working electrodes as polymer thickness over the whole IDE electrode is uniform and a low potential bias between two electrodes applied, we can write that: I WE(2) = I F + I where I WE (1) and I WE (2) are currents passing through the first and second electrodes, I is the leakage current between the working electrodes, and I F is the faradaic current. Finally, conductivity σ and resistance R could be calculated according to the formula: where V is the potential difference between the working electrodes and ∆I is the difference between the currents on working electrodes. We checked that leakage currents were following the Ohm law within 5, 10, and 20 mV biases and that CV curve of the polymer on IDE electrode with shorted grids reasonably reassembles CV curve obtained with the (7) at bipotentiostate mode on the same polymer-coated IDE. Double-control results ensure that assumptions of faradaic currents equality for both grids of IDE are safe.

In Situ Spectroelectrochemistry
Studies of the electrochemical properties of the polymers were carried out using modular potentiostat/galvanostat Bio-Logic (BioLogic Science Instruments, Seyssinet-Pariset, France). Polymers were synthesized in a handmade three-electrode cell consisting of a working electrode, a counter electrode (glassy carbon plate S = 2 cm 2 ), and a reference electrode. We used an ITO coated glass slide (Sigma Aldrich) as a working electrode. Polymers were synthesized in the form of thin fims on a conducting surface by electropolymerization from a solution of 0.001 mol dm −3 [Cu(3-MeOSal-Sal)en] and 0.1 mol dm −3 Et 4 NBF 4 in acetonitrile. Electrochemical polymerization was performed under conditions identical to Section 2.2.
UV-vis absorption spectra of poly-[Cu(3-MeOSal-Sal)en] were registered by a spectrophotometer SF-2000 (Russia). UV-vis spectroscopy of the polymer films deposited on transparent electrodes was carried out in a photometric cell and in situ absorption spectra were registered during potentiostatic polarization measurements. This provided direct information about the polymer structure at any oxidized, reduction, or semi-oxidized state in visible UV or near infrared ranges of light. Film potential was measured against a silver chloride-coated silver wire as the pseudo-reference electrode (the potential of 0 V vs. Ag/AgCl,sat'd NaCl). The spectra were recorded after achieving a stationary state (until current became a constant for approximately 100-150 s).

X-ray Photoelectron Spectroscopy (XPS)
The X-ray Photoelectron Spectroscopy (XPS) of the materials was carried out on the Thermo Fischer Scientific Escalab 250Xi Spectrometer (Thermo Scientific, MA, USA) with nonmonochromatic Al Kα radiation (photon energy 1486.6 eV) at a research park of St. Petersburg State University Centre for Physical Methods of Surface Investigation (Russia). The total energy resolution of the experiment was about 0.2 eV. Spectra of the samples were recorded in the constant pass energy mode at 20 eV using a 650 mm diameter analysis area. Investigations were carried out at room temperature in an ultrahigh vacuum of the order of 10 −9 mbar.

Scanning Electron Microscopy (SEM)
Studies of the polymers' morphology were carried out using scanning electron microscopy. Polymers were synthesized on a silicon dioxide piezoelectric crystal sprayed with a layer of platinum in the same mode (see above). The cyclic voltammograms of polymers 1, 2, and 3 were registered in a supporting electrolyte by potential cycling in the range 0 ÷ 1.4 V, 0 ÷ 0.85 V, and 0 ÷ 1.4 V, respectively. Then the samples were washed in pure acetonitrile and dried.
The registration of SEM images was carried out at 45 • to the surface of the electrode. The scanning electron microscope (Zeiss Merlin, Jena, Germany) with a 5 kV accelerating voltage was used at a research park of St. Petersburg State University: Interdisciplinary Center for Nanotechnology.  [27,29,43] and has been reperformed here for comparison.

Electrochemical Behavior of Monomer [Cu
The CV curves of these symmetrical complexes reveal two distinct peaks corresponding to two redox processes. The first process can be attributed to the one-electron oxidation of the monomeric complexes with the phenoxyl radical formation, oxidation of [Cu(Salen)] and [Cu(3-MeOSalen)] complexes is well studied in the literature [27,29,43] and has been reperformed here for comparison.
The CV curves of these symmetrical complexes reveal two distinct peaks corresponding to two redox processes. The first process can be attributed to the one-electron oxidation of the monomeric complexes with the phenoxyl radical formation, [Cu(Salen)] •+ and [Cu(3-MeOSalen)] •+ , respectively. The peak potential of the [Cu(3-MeOSalen)] oxidation (0.79 V) is in a less positive region compared to the [Cu(Salen)] complex (0.94 V). The presence of electron-donor substituents in the ligand of the [Cu(3-MeOSalen)] complex increases the electron density in the phenyl rings, thereby facilitating the process of electron transfer [27].

Polymerization of [Cu(3-MeOSal-Sal)en]
The phenoxyls of asymmetric monomeric complex [Cu(3-MeOSal-Sal)en] can be selectively oxidatively activated at different potentials. At mild potentials, only the methoxylated phenoxyl radical formation occurs, while at higher potentials, dication diradical species are presumable intermediates of the polymerization process. (Scheme 1). Two approaches are used for the electrochemically induced polymerization of [Cu(3-MeOSal-Sal)en]: (i) deposition under potentiodynamic control. The films were obtained by scanning the potential in a wide potential range of 0-1.4 V (scan rate 0.05 Vs −1 ). At this type of film deposition, any combination of crosslinkable fragments is possible: MeOPh-MeOPh, Ph-Ph, or MeOPh-Ph, since the applied potential is sufficient to oxidize both phenyl rings. The polymer obtained using this method will have a random structure and is denoted as polymer 1.
(ii) deposition under potentiostatic control. The films were obtained by slow deposition at a potential of 0.85 V; that value corresponds to the beginning of the oxidative polymerization process. In this case, the formation of C-C bonds occurs only via methoxysubstituted phenyl ring activation (Scheme 1). The polymer film obtained under potentiostatic control is denoted as polymer 2. It is expected to have a uniform and ordered structure and predominantly consist of the dimeric units. NMR analysis of polymer 2 acid hydrolysis products (Figures S1-S3) revealed a 2.5-to-1 ratio of di-methoxylated biphenyl to mono-methoxylated biphenyl. No unmethoxylated biphenyl was found using NMR. Thus, polymer 2 has 71.5% MeOPh-MeOPh, 28.5% MeOPh-Ph, and 0% Ph-Ph linkages.
In addition, after the formation of the polymer 2 film in the monomer solution under potentiostatic control, the film was transferred to a supporting electrolyte solution where it was further oxidized by cyclic potential scan up to 1.4 V. The polymer film obtained by cross-linking of the polymer 2 is denoted as polymer 3.

Electrochemical Behavior Poly-[Cu(3-MeOSal-Sal)en]
CV curves of symmetrical copper polymers and polymer 1 are shown in Figure 2. All polymers were obtained in the potentiodynamic polymerization mode under the same conditions. One can observe that the introduction of electron-donor methoxysubstituents into the ligand shifts the electroactivity region of the complexes towards less positive potentials. The introduction of one methoxy group into the ligand structure   The CV curve of polymer 2 obtained by the potentiostatic mode has a pair of sharp redox peaks (blue curve, Figure 3). They correspond to the redox processes in the film and are located in the same potentials as the initial stage of oxidation of the polymer of symmetrical [Cu(3-MeOSalen)]. The CV curve of polymer 2 obtained by the potentiostatic mode has a pair of sharp redox peaks (blue curve, Figure 3). They correspond to the redox processes in the film and are located in the same potentials as the initial stage of oxidation of the polymer of symmetrical [Cu(3-MeOSalen)].  Excursion of polymer 2 to higher potentials leads to the formation of polymer 3 (Figure 3, red curve). Oxidation of polymer 2 causes an additional irreversible peak at a potential of 1.2 V, that is the same with the potential for the formation of biphenoxyl radicals during film formation in the monomer solution ( Figure 3, magenta curve). More than 30% of the total oxidation charge is spent on the irreversible oxidation of the unsubstituted phenyl rings of the polymer film. Further cycling of polymer 3 gives a reversible CV curve that mimics the shape of the CV curve of polymer 1, except that polymer 3 exhibits a wider electroactivity range than polymer 1.
We conducted a series of cyclic voltammetry measurements at different scan rates. Figure S5a,b illustrates a thin layer/surface-type regime for charge transport.
To evaluate the electrochemical capacity and gain insights into mass transport during electrochemical doping of the new polymers, they were additionally studied by the EQCM method in a monomer-free electrolyte. Electrochemical oxidation of the polymers is accompanied by an increase in their weight due to injection of charge-compensating ions and solvent molecules into the film; this reduction is accompanied by corresponding species ejection the film ( Figure S4).
For all the studied polymers, one linear region was observed on ∆m(Q) curves ( Figure S4). For polymers 1 and 3, the same masses of charge carriers of approx. 50 g mol −1 was found. This corresponds to the entry of BF 4 anions into the film with the simultaneous ejection of solvent molecules CH 3 CN (Table 1). Thus, during the oxidation/reduction of polymers in the full range of potentials, there are no significant differences in the charge-transferring species. When polymer 2 is oxidized in the range from 0 V to 0.85 V, anions BF 4 (molar mass 87 g mol −1 ) inject into the film without the ejection of solvent molecules.
We combined the results of the electrochemical studies with the data on the polymer mass obtained by QCM for dried dedoped films to determine the number of electrons per a monomer unit exchanged by investigated poly-[Cu(3-MeOSal-Sal)en] films during their oxidation/reduction (n). Calculated values of n are shown in Table 1. As can be seen, all poly-[Cu(3-MeOSal-Sal)en] films investigated in the present study exchange more than one electron per monomer unit in the oxidation/reduction processes. Polymer 3 demonstrates the highest number of electrons reversibly exchanged per monomeric unit of the polymer. Table 1. Molar mass of charge-transferring species participating in the polymer oxidation/reduction process and the number of electrons per monomer unit exchanged during redox processes in the polymer films found by the EQCM. The multielectronic nature of the oxidation/reduction processes in all polymers makes them viable candidates for energy storage applications. Table 1 lists the values of specific capacity (C mAh in mAh g −1 ) calculated for the studied polymers using Equation (4).

Poly-[Cu(3-MeOSal-Sal)en
The calculated capacity for polymer 3 is 25% higher than for polymer 1. The specific capacity for polymer 3 is on par with the numbers reported for conductive polymers for use in electrochemical energy storage [11,59].

In Situ Conductance Studies of Polymers Films
The dependence of poly-[Cu(3-MeOSal-Sal)en] polymer conductivity versus potential was studied using IDE electrodes. Figure 4 shows the conductivity of the polymer complexes in line with their CV curves.

In Situ Сonductance Studies of Polymers Films.
The dependence of poly-[Cu(3-MeOSal-Sal)en] polymer conductivity versus potential was studied using IDE electrodes. Figure 4 shows the conductivity of the polymer complexes in line with their CV curves. A linear increase in the conductivity of polymers 1 and 3 is observed at the onset of film oxidation. The maximum of conductivity coincides with the maximum of the main oxidation peak of the voltammograms. In this state, the polymer is in half of the maximum doping state. Further increase of the potential leads to a decrease in conductivity. The conductivity value approaches zero when the polymer is fully oxidized. In this state, mobile charge carriers are almost completely absent. The reverse scanning of the potential is  A linear increase in the conductivity of polymers 1 and 3 is observed at the onset of film oxidation. The maximum of conductivity coincides with the maximum of the main oxidation peak of the voltammograms. In this state, the polymer is in half of the maximum doping state. Further increase of the potential leads to a decrease in conductivity. The conductivity value approaches zero when the polymer is fully oxidized. In this state, mobile charge carriers are almost completely absent. The reverse scanning of the potential is accompanied by an increase in conductivity, with the maximum being reached at the potential of the reverse peak. With a further decrease in the potential, the conductivity falls to zero; since the film is in a completely reduced state, there are no mobile charge carriers.
The conductivity profiles of polymers 1 and 3 show several peaks that pointed to several types of charge carriers. The conductivity profile of polymer 1 contains one weakly resolved conductivity maximum at 0.6 V and a pronounced maximum at 0.8 V. The conductivity profile of polymer 3 contains at least two peaks located at the same potentials as the first and second CV waves.
The conductivity profile of polymer 2 shows one peak at a potential corresponding to a weakly resolved voltammogram peak at 0.6 V. The conductivity of the film begins to grow from the beginning of its oxidation. However, the maximum conductivity was attained at more positive potentials than the potential of the main peak (0.35 V). Polymer 3 exhibits a higher conductivity than polymer 1 and a lower conductivity than polymer 2.

In Situ UV-vis Spectroelectrochemistry
To probe the nature of species induced in polymers by electrochemical oxidation, spectroelectrochemical analysis of the polymers at different doping levels was performed. Figure S6 shows the UV-vis spectra of poly-[Cu(3-MeOSal-Sal)en] films on an ITO-coated glass electrode acquired during redox switching. Figure S7 shows the characteristic absorption spectra in a differential form, obtained by subtracting the spectrum of the reduced polymer (at the potential 0 V) from the spectra recorded at all other potentials.  Figure 6). These bands are attributed to the formation of dications in the film [28,[60][61][62]. Polymers 1 and 3 contain a pair of overlapping bands in the same wavelength region (502 nm and 610 nm). These bands are also attributed to the formation of dications in the polymer films. The decrease in intensity of the band at 380 nm in the completely doped polymeric film indicates the conversion of cation radicals into dications. However, the intensity of the bands at low doping levels for polymer 3 is somewhat higher than for polymer 1. This may indicate a higher concentration of cation radicals in the film. wavelength region (502 nm and 610 nm). These bands are also attributed to the formation of dications in the polymer films. The decrease in intensity of the band at 380 nm in the completely doped polymeric film indicates the conversion of cation radicals into dications. However, the intensity of the bands at low doping levels for polymer 3 is somewhat higher than for polymer 1. This may indicate a higher concentration of cation radicals in the film. Some works devoted to elucidation of the localization of positive charge in oxidized protected [Cu(Salen)] found evidence of Cu(III)-phenolates formation [32,33,35,63]. The band corresponding to the LMCT transition in Cu(III)-phenolate species is located in the region of 550 nm [32,35]. In the spectra of asymmetric polymer complexes poly-[Cu(3-MeOSal-Sal)en] •+/2+ , the absorption is observed in this wavelength range without any defined band. This absorption is likely due to the superposition of broad neighboring bands; as such, it is not possible to unambiguously interpret this region of wavelengths. Some works devoted to elucidation of the localization of positive charge in oxidized protected [Cu(Salen)] found evidence of Cu(ΙΙΙ)-phenolates formation [32,33,35,63]. The band corresponding to the LMCT transition in Cu(ΙΙΙ)-phenolate species is located in the region of 550 nm [32,35]. In the spectra of asymmetric polymer complexes poly-[Cu(3-MeOSal-Sal)en] •+/2+ , the absorption is observed in this wavelength range without any defined band. This absorption is likely due to the superposition of broad neighboring bands; as such, it is not possible to unambiguously interpret this region of wavelengths. The results of spectroelectrochemical studies revealed that the redox processes in poly-[Cu(3-MeOSal-Sal)en] films are complex and that participation of a metal center in the redox processes in the polymers could not be excluded.

XPS.
The XPS method can be used to prove the participation of the copper of the polymer complexes in redox processes.
For polymer 1, XPS data were used to estimate the oxidation state of the central copper atom ( Figure S8). A comparative analysis of the binding energy of 2p Cu electrons for the solid monomer and the polymer in the oxidized state showed a difference towards  The results of spectroelectrochemical studies revealed that the redox processes in poly-[Cu(3-MeOSal-Sal)en] films are complex and that participation of a metal center in the redox processes in the polymers could not be excluded.

XPS
The XPS method can be used to prove the participation of the copper of the polymer complexes in redox processes.
For polymer 1, XPS data were used to estimate the oxidation state of the central copper atom ( Figure S8). A comparative analysis of the binding energy of 2p Cu electrons for the solid monomer and the polymer in the oxidized state showed a difference towards higher energies for the polymer compound. A shift of 2.3 eV in the binding energies of Cu 2p 3/2 (from 932.3 to 934.6 eV) and Cu 2p 1/2 (from 952.0 to 954.4 eV) is consistent with the change in the oxidation state of Cu(II) to Cu(III) [32,33,35]. The broadened satellite peak in the form of a weakly resolved triplet, the shape of which is determined by the multiplet splitting of the 3D states of copper, is likely due to the tetragonal distortion of the structure of the complex. Thus the copper atom acts as a separate redox center in the poly-[Cu(3-MeOSal-Sal)en]. A more detailed electronic structure of 3D orbitals of the copper atom and spin-allowed transitions is shown in the Scheme S1.  (Figure 7a). Polymer 2 has a uniform surface with very low visible porosity (Figure 7b). It can be assumed that the polymer consists of tight stacks of rigid monomers and has a homogeneous structure. One-level relief is primarily observed.

SEM Studies of Polymers
Polymer 3 also has a homogeneous structure (Figure 7). However, a small number of globules is observed on the surface, as in the case of polymer 1. More SEM-images can be seen in Figure S9. Polymer 2 has a uniform surface with very low visible porosity (Figure 7b). It can be assumed that the polymer consists of tight stacks of rigid monomers and has a homogeneous structure. One-level relief is primarily observed.
Polymer 3 also has a homogeneous structure (Figure 7). However, a small number of globules is observed on the surface, as in the case of polymer 1. More SEM-images can be seen in Figure S9.

Discussion
An asymmetrical monomer design provides new possibilities for fine-tuning the properties of conductive electroactive polymers composed of these building blocks. The asymmetrical copper complex poly-[Cu(3-MeOSal-Sal)en] structure has two fragments with different electron saturation: a fragment of electron-unsaturated phenyl ring and a fragment of a methoxy-substituted electron-saturated phenyl ring. By varying the electropolymerization potential, polymer films with different combinations of crosslinked fragments have been obtained.
Under potentiodynamic polymerization with a wide potentials range, polymer 1 with randomly crosslinked phenyl rings was obtained. Polymer 2 was obtained under potentiostatic deposition at a low potential. Under these conditions, only activation of the methoxy-substituted phenyl sites is possible. This polymerization pathway is confirmed by the NMR analysis of polymer 2 hydrolysis products. Moreover, the presence of Me-OPh-Ph biphenyls in polymer 2's structure allows us to propose the significance of the radical-substrate coupling mechanism of C-C bond formation for this system under the conditions studied. Asymmetrical ligand design allows the C-C coupling radical-radical dimerization and radical-substrate mechanisms to be distinguished in a way that, to the best of our knowledge, has never been conducted for these complexes. More precise control over oxidative coupling conditions may favor dimer formation and further improve structural control under polymerization pathway. Polymer 3 is characterized by a more regular alternation of monomeric fragments

Discussion
An asymmetrical monomer design provides new possibilities for fine-tuning the properties of conductive electroactive polymers composed of these building blocks. The asymmetrical copper complex poly-[Cu(3-MeOSal-Sal)en] structure has two fragments with different electron saturation: a fragment of electron-unsaturated phenyl ring and a fragment of a methoxy-substituted electron-saturated phenyl ring. By varying the electropolymerization potential, polymer films with different combinations of crosslinked fragments have been obtained.
Under potentiodynamic polymerization with a wide potentials range, polymer 1 with randomly crosslinked phenyl rings was obtained. Polymer 2 was obtained under potentiostatic deposition at a low potential. Under these conditions, only activation of the methoxy-substituted phenyl sites is possible. This polymerization pathway is confirmed by the NMR analysis of polymer 2 hydrolysis products. Moreover, the presence of MeOPh-Ph biphenyls in polymer 2's structure allows us to propose the significance of the radicalsubstrate coupling mechanism of C-C bond formation for this system under the conditions studied. Asymmetrical ligand design allows the C-C coupling radical-radical dimerization and radical-substrate mechanisms to be distinguished in a way that, to the best of our knowledge, has never been conducted for these complexes. More precise control over oxidative coupling conditions may favor dimer formation and further improve structural control under polymerization pathway. Polymer 3 is characterized by a more regular alternation of monomeric fragments when compared with polymer 1, since it was obtained by further crosslinking of uncoupled unsubstituted phenyl rings of polymer 2.
Oxidation of polymer 2 is accompanied by the formation of poly-[Cu(3-MeOSal-Sal)en] •+ cation radicals with a charge localized in the methoxy-substituted phenyl ring, as evidenced by the UV-spectrum data (the growth of the bands at 380 nm and 720 nm). The concentration of cation radicals, as well as the conductivity value, are the highest among all the studied polymers, indicating a high degree of structural order. It is known that the electrical conductivity of metallopolymers decreases with increased distance between polymer chains [7]. Interchain interactions play an important role in the processes of charge transport in polymeric metal complexes. When two polymer chains interact, a weak π-bond is formed between the aromatic rings of the monomeric fragments. The formation of π-aggregates is facilitated by the segmentation of the polymer structure, i.e., the presence of fragments oxidized at different potentials [64]. The presence of an efficient system of interchain interactions provides bulk conductivity and is a necessary step for achieving high values of specific conductivity. The structure of polymer 2 provides the most efficient charge transport via π-stacking among all three structures studied.
The structure of polymer 2 tends to a long-range pathway of charge transport only via π-stacking of methoxy-substituted biphenyl moieties. Each polymer fragment has one electroactive center at potential range of reversible redox processes. However, the number of electrons exchanged by a polymer fragment in redox processes exceeds one.
Poly-[Cu(Schiff)] polymer complexes in the uncharged state are paramagnetic due to Cu(II) ions, that are in the low-spin 3d 9 conformation (a detailed discussion of orbitals is given in SI). The one-electron oxidation of polymers results in the formation of cation radicals located on the ligand. In situ UV-vis spectra show the characteristic features of the phenoxyl radical. Since there is an orthogonality between the metal 3d x 2 −y 2 orbital and the phenolate-based orbital, the primary oxidation can lead to the transformation of monomer fragments Cu(II)-phenolate into Cu(II)-phenoxyl radicals. However, the formation of diamagnetic 3d 8 Cu(III)-phenolate particles in poly-[Cu(Salen)] in addition to Cu(II)-phenoxy radicals cannot be completely ruled out, since an equilibrium can exist in the one-electron oxidation of the polymer (Cu(II) -phenoxyl radicals ↔ Cu(III)-phenolates). Both types of particle have very similar spectroscopic characteristics in the UV-vis region.
The existence of Cu(III) in the polymer 2 structure of the oxidized film is indirectly indicated by the presence of a shoulder at a potential of 0.6 V in CV ( Figure 4) and the number of electrons exchanged by the polymer fragment in redox processes is greater than 1. The nature of the second peak in the voltammogram can be attributed to the formation Cu(III)-phenolates, since the equilibrium takes place during one-electron oxidation of the film (phenoxyl radicals ↔ Cu(III)-phenolates).
The properties of polymers 1 and 3 are similar, despite polymer 1 being obtained by rapid deposition in the full potential range. Polymer 3 initially had the highly ordered structure of polymer 2 and then was further oxidatively cross-linked. According to the spectra, both polymers have the same set of bands and, consequently, the same nature of charge carriers. Oxidation of polymers 1 and 3 results in the formation of cation radicals that are converted into dications with increasing potential. However, for polymer 3, the intensity of the bands corresponding to the formation of cation radicals is higher than for polymer 1, and hence the concentration of cation radicals in polymer 3 is higher. This explains the higher specific capacity of polymer 3 when compared to polymer 1, since a greater number of cation radicals can be converted into dications.
The number of electrons exchanged by a polymer fragment during oxidation is highest in polymer 3. Each fragment of the poly-[Cu(3-MeOSal-Sal)en] has two types of potential redox centers: biphenyl rings and a metal center. During the oxidation of biphenyl fragments, the maximum number exchanged electrons is two. However, the number of electrons per polymer fragment for polymer 3 exceeds 2 (2.3 electrons), which indicates the participation of copper atom in redox processes, as in the case of polymer 2.
Polymer 3 shows a higher conductivity, a larger number of electrons exchanged in redox processes, and has a wider area of electrical activity compared to polymer 1. All this indicates a more uniform and ordered structure of polymer 3. Polymer 3 initially had the highly ordered structure of polymer 2, consisting of methoxy-biphenyls organized in π-stacks. As a result of further oxidation, the π-stack structures were partially destroyed through simultaneous cross-linking of the Sal-Sal fragments. The film no longer has the highly ordered structure of polymer 2 but is significantly more ordered than polymer 1, which was obtained by rapid deposition and contains random combinations of fragments. Rapid deposition leads to polymer films with a disordered structure, numerous defects, and frequent chain terminations.
Differences in the structure of the films are also noticeable at the macrolevel. According to SEM photographs, films of polymers 2 and 3 have a dense and uniform structure with a small amount of small globular formations on the surface. Polymer 1 has an uneven, porous structure; many large loose structures are present on the film surface.

Conclusions
By varying the deposition conditions and the composition of the ligand environment of monomeric metal complexes with an asymmetric design, polymer films with desired properties can be obtained. The choice of potential and rate of deposition of unsymmetrical metal complexes poly-[Cu(3-MeOSal-Sal)en] enables control of the coupling site of an asymmetrical monomer. At a low potential, polymer films were obtained with a highly ordered stack structure and high conductivity. Further oxidation leads to a partial disruption of the order and the simultaneous formation of chain structures. A stepwise formation of a polymer film, in which methoxy fragments are cross-linked at the first step, and then unsubstituted fragments crosslinked at the second one, affords significantly improved polymer electrochemical characteristics, achieving more complete oxidation of the film and an increase in its specific capacity by 25%. With this model system, we have shown that asymmetrical monomer design may be successfully implemented to fine-tune polymerization site, conductivity, and capacitive properties for [M(Salen)]-type polymers. Further investigations in electrocatalytic and sensory systems as well as an understanding of the advantages of this approach in polymeric systems with other metal centers are promising and desirable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.