Most of the plastic recycling is thermal recycling by incineration. As a matter of course, thermal recycling is not a sustainable recycling method for plastic material. Material recycling is being carried out in which some used plastics are molded and reused.[1] However, material recycling has problems that cannot recover the monomer and separate the pigments and dyes. These problems degrade the quality of recycling plastics. Therefore, we focused on the depolymerization and re-polymerization between polymers and monomers by electrolysis in order to develop a new recycling method without degrading the quality of the plastics.

However, since polymers are generally insulators, even if they are electrolyzed, the number of reports is small, and the decomposition behavior is unknown. [2, 3] In this study, to obtain basic information for the electrolysis of polymers, a monovalent alcohol molecule bonded to the terminal or side chain and a divalent alcohol molecule bonded to both ends are used as the model molecule, and their electrochemical behavior was investigated. In addition, electrolysis of PVA and PEG, which have similar structures to alcohol molecules, was conducted as the first step of depolymerization into monomers.

The electrochemical cell in this study was composed of Pt working and counter electrodes, Ag/AgCl reference electrode, and 0.1 M HClO₄. Cyclic voltammetry (CV) was conducted at room temperature in an inert atmosphere, and the scanning speed was 10 mV s^-1. The decomposition behavior of model molecules was studied by adding 1 mL each of 1 to 10 carbon atoms (linear) of primary alcohol and 3, 5, 7, 9 carbon atoms (linear) of a secondary alcohol, ethylene glycol, PVA, and PEG into the electrolyte.

In the case of primary alcohols, the oxidation currents decreased with the longer chain length. The relatively short carbon chain alcohols composed of 1 to 4 carbon atoms showed first-step oxidation between 1.0 V and 1.2 V on the anodic scan and second-step oxidation near 0.8 V on the cathodic scan. In the alcohol with 5 to 8 carbon atoms, the first-step oxidation peak was shifted to the high potential region with the increasing length of the carbon chain. Here, the potential of the second-step oxidation peak was the same as the short carbon chain alcohols. In contrast, the alcohol with 9 or 10 carbon atoms hardly oxidizes and may be adsorbed on the Pt electrode surface. In Addition, the oxidation behavior of ethylene glycol was similar to that of primary alcohols with short chain lengths such as methanol and ethanol.

The trend of oxidation behavior in the secondary alcohols was almost agreed with the primary alcohols. In the cases of 3-pentanol and 4-heptanol, the oxidation current at the high potential region and the reduction current at the low potential region were observed at the boundary of 0.8V. Currently, this electrochemical behavior is thought that these alcohols may change reversibly. The oxidation current of 3-pentanol and 4-heptanol was slightly higher than the reduction current. Thus, the oxidation reaction of secondary alcohols appeared to be slightly predominant.

The above results indicated that the longer the chain length of primary and secondary alcohols was hard to oxidize. The changing position of the functional group influenced oxidation. In particular, those attached to the ends were more easily oxidized.

Both PVA and PEG were oxidized, but PEG was found to be more oxidized. Based on the results for the alcohol molecule, the position of the functional group on the main chain is thought to be a factor.

Electrochemical measurements were performed on linear primary alcohols with 1 to 10 carbon atoms, secondary alcohols with 3, 5, 7, and 9 carbon atoms, and ethylene glycol. The trend of oxidation behavior was that the oxidation currents decreased with the longer chain length. Obtained results indicated that oxidation reaction in the case of the functional group attaching to the end of the carbon chain progressed easier than others. PVA and PEG, similar in structure to the alcohol molecule, showed that PEG and PVA were oxidized, but PEG was more oxidized.

[1] T. Kamo, Science of the Field, 1, (2021) 28-44.

[2] O. R. Luca et al. Molecules, 25 (2020) 1-9.

[3] T. Jiang et al. Solar Energy Materials & Solar Cells, 204 (2020) 1-10.