**4. Conclusions**

Basically, poly(phenylene methylene) (PPM) is a valuable polymer for studies in the area of corrosion prevention coatings due to its thermal stability, hydrophobicity, and fluorescence. The latter facilitates optical detection of inhomogeneities, cracks and other defects caused by pit attacks upon observation under UV-light. However, PPM has to be used with additives, such as plasticizers or polysiloxanes, in order to enable processing into crack-free coating surfaces. Therefore, we aimed at the synthesis of copolymers on the basis of PPM which can be processed to coatings without addition of additives. For this purpose, benzyl chloride and 4-octyloxybenzyl chloride were copolymerized, using tin (IV) chloride as catalyst. Thus, copolymers with 6.1% mol/mol and 13.4% mol/mol octyloxy units were obtained. Notably, fluorescence of PPM was preserved in presence of side-chains on the polymer backbone.

In addition, it was shown that the copolymers are materials with high thermal stability. Further, the alkoxy chains lead to a decrease of the glass transition temperature of the copolymers, confirming the plasticizing effect of the substituent. This finding is line with the viscosity drop at higher fraction of alkyloxy units (13.4% mol/mol).

Gratifyingly, powders of the copolymers can be processed into coatings, providing homogeneous -crack-free surfaces on pretreated AA2024 substrates. The presence of alkoxy side chains along the polymer chains enhances the elasticity and the cohesion of the system, and hence the coating can adapt better to deformation which occurs upon cooling of the coated system prepared by hot pressing.

The isolation of copolymers based on PPM and their corrosion protection ability opens the way for studies on the application of such materials in the area of corrosion protection and monitoring.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3417/9/17/3551/s1. Figure S1: SEM image of cross section of AA2024 coated CO-PPM containing 13.4% mol/mol of 4-octyloxy side chains. Figure S2: SEM image of cross section of AA2024 coated CO-PPM containing 6.1% mol/mol of 4-octyloxy side chains. Figure S3: Behavior of uncoated AA2024, in naturally aerated near-neutral 0.6 M NaCl solution.

*Appl. Sci.* **2019**, *9*, 3551

Figure S4: Reproducibility tests (anodic polarization) for sample AA2024 coated with PPM\_6.1%-OcOx. Figure S5: Reproducibility tests (anodic polarization) for sample AA2024 coated with PPM\_13.4%-OcOx. Figure S6: SEM image of AA2024 coated CO-PPM containing 6.1% mol/mol of 4-octyloxy side chains. Figure S7: SEM image of cross section of a defect on AA2024 coated CO-PPM containing 6.1% mol/mol of 4-octyloxy side chains after polarization. Figure S8: SEM image of AA2024 coated CO-PPM containing 13.4% mol/mol of 4-octyloxy side chains which shows a uniform and homogenous surface even after polarization (top view). Figure S9: SEM image of cross section of AA2024 coated CO-PPM containing 13.4% mol/mol of 4-octyloxy side chains after polarization. Figure S10: Anodic polarization curves for AA2024 coated with PPM\_13.4%-OcOx at 35 ◦C. Figure S11: optical microscope pictures of PPM\_13.4%-OcOx under 24h polarization at 0 V vs. SCE.

**Author Contributions:** M.F.D. performed the synthesis of the monomers and copolymers. The manufacture of the coatings was performed under supervision of W.C. and M.N.; T.B.S. contributed to the rheology measurements which were performed and designed together with M.F.D.; the corrosion tests were performed by M.M. under supervision of S.P.M.T., and the manuscript was written by M.F.D., W.C. and M.M.

**Funding:** This research received no external founding.

**Acknowledgments:** We thank Elena Tervoort for taking SEM images.

**Conflicts of Interest:** The authors declare no conflict of interest.
