**1. Introduction**

Poly(phenylene methylene) (PPM) is a hydrocarbon polymer with the general formula (C6H4[CH2])n. It is structurally located between polyethylene and polyphenylene, consisting of an alternating sequence of phenylene and methylene units (Figure 1a). Remarkably, it exhibits a rather unique combination of material properties. Besides high hydrophobicity [1], it is highly thermally stable (onset of decomposition temperature at 450–470 ◦C) [2–5] and fluorescent [6]. This optical property, unusual for a non-conjugated polymer such as PPM, was attributed to homoconjugation [6]. The rare phenomenon of homoconjugation can arise under particular geometric conditions when conjugated π-orbital systems interact with each other, even though they are electronically separated by an insulating methylene group [7,8], as illustrated in Figure 1b.

**Figure 1.** (**a**) Repeating units of poly(phenylene methylene) (PPM), (**b**) schematic representation of homoconjugation in PPM: p-orbitals of phenylene rings overlap even if they are electronically separated by methylene group, (**c**) scheme of the synthesis of random copolymers based on PPM obtained by mixing different fractions (*n* and *m*, *m*/(*m*+*n*) = 5.3% mol/mol or 11.2% mol/mol) of benzyl chloride and its derivative (4-octyloxybenzyl chloride), and structure of a sequence of the resulting random copolymers.

Moreover, importantly, PPM has also been shown to be effective in corrosion protection, however, only when blended with rheological additives, such as polysiloxanes and benzylbutyl phthalate, as an external plasticizer in order to prevent cracking of the surface due to the stiffness of the polymer [1]. Notably, the principal aim of plasticizers is to ameliorate the elasticity and processability of polymers by lowering the second order transition temperature (glass transition temperature), thus decreasing the tendency of coatings to formation of cracks [1,9]. More specifically, external plasticizers are low molar mass compounds which are dispersed in the polymeric matrix spreading the polymer chains apart [10]. Thus, plasticizers depress the polymer–polymer secondary interactions into the polymeric matrix enhancing the mobility of the polymer chains, resulting in a softer material which can be easily deformed. However, due to the weak interactions between polymer chains and external plasticizers, the plasticizer can leave the material matrix by evaporation, migration or extraction, and upon exposure to UV light the plasticizer can degrade and subsequently also initiate degradation of the polymer [11,12]. Further, external plasticizers can increase the erodibility of the coating, thus reducing the material's lifetime [10,12].

However, migration of external plasticizers and the film-forming properties of polymers can be managed by application of polymer-bound molecules which can quasi be regarded as internal plasticizers [13]. In particular, the presence of bulky side chains along the polymer backbone lowers the internal forces between the polymer main chains [13,14].

In this way, we sought to design a copolymer based on phenylene methylene units for corrosion protection, by inserting different fractions of *n*-octyloxy side chains into the PPM backbone. The poly(phenylene methylene) derivative was synthesized by copolymerization of a mixture of benzyl chloride and variable fractions of 4-octylbenzyl chloride in presence of tin tetrachloride as catalyst, analogously to the synthesis of PPM itself [2].

Hence, in this presented work, the preparation of corrosion-resistant PPM-related coatings on pretreated aluminum alloy AA2024 containing different molar fractions of *n*-octyloxy side chains (Figure 1c) were prepared to explore the efficacy of long alkoxy side-chains as a reliable alternative to rheological additives. Accordingly, the ability of PPM derivatives in anti-corrosion protection was examined by coating pretreated aluminum alloy samples (AA2024) and studying their behavior in a naturally aerated near-neutral 0.6 M sodium chloride solution anodic polarization and potentiostatic polarization techniques. Additionally, thermal stability and glass transition temperature of the copolymers were investigated by thermogravimetric analysis (TGA) and di fferential scanning calorimetry (DSC).

#### **2. Materials and Methods**

#### *2.1. Reagents and Solvents*

Benzyl chloride stabilized by propylene oxide (99%), tin(IV) chloride, phosphoric acid (85%), thionyl chloride (99%), sodium sulfate (99%) and chloroform were purchased from Sigma Aldrich (Buchs, Switzerland), benzyltriethoxysilane from Fluorochem (Hadfield, UK), 4-hydroxybenzaldehyde (98%) and 1-bromooctane (98%) from abcr (Karlsruhe, Germany), sodium borohydride (98%) and tetrahydrofuran (CROMANORM) from VWR Chemicals BDH (Leuven, Belgium), acetonitrile (99.9%), dichloromethane (99.8%), sodium hydroxide (98.66%) and potassium hydroxide (86%) from Fischer Chemicals (Loughborough, UK), and methanol (98%) from Merck (Darmstadt, Germany).
