# Naphthalene-Based Polymers as Catalytic Supports for Suzuki Cross-Coupling
## 3 Discussion  
In this work, for the first time, a series of NA-based polymers was synthesized at variations of the amount of crosslinking agent (methylal). It was shown that methylal not only participates in the formation of Ar-CH\({}_{2}\)-Ar bridges between neighboring NA molecules but also contributes in the formation of oxygen-containing moieties. Moreover, functionalization of the NA-based polymers with -OH, -SO\({}_{3}\)R, or -NO\({}_{2}\) groups was carried out in order to alter their porosity and relative hydrophilicity.  
Using different types of polymers (fully aromatic: NA60 and NA120), containing hydroxyl groups (NL60), sulfonate groups (SNA120), and nitro-groups (NNA120), palladium-containing catalysts were synthesized (1 wt.% of Pd) with Pd acetate as a precursor. It was found that NA-based HAPs can serve as a support for Pd NPs. In our previous work [21; 39], we showed that small Pd NPs at 2-4 nm in diameter can be formed after reduction in a hydrogen flow at 300 \({}^{\circ}\)C of Pd-precursor on commercial hyper-crosslinked polystyrene. In this study, to demonstrate the effect of functional groups of the synthesized novel NA-based polymers on the formation of the metal NPs, we chose liquid-phase reduction under mild conditions (0-5 \({}^{\circ}\)C, NaBH\({}_{4}\)) to protect polymers' functionalities (-OH, -SO\({}_{3}\)R, or -NO\({}_{2}\) groups) from destruction. As a result, it was shown that the higher hydrophilicity of the polymeric support allows the formation of smaller Pd NPs with a narrower particle size distribution. The best results were obtained for the nitrated polymer (NNA120, where 120 refers to methylal content in mmoles): the diameter of Pd NPs was about 3 nm, and the initial micro-mesoporous polymer became the meso-macroporous one. Thus, it was concluded that in a liquid phase where Pd salt precursor has a tendency to migrate, the porosity is a minor factor influencing the NPs' sizes as compared to the presence of surface functional groups.  
Both the initial (unreduced) catalysts, containing Pd(II) species, and the nanoparticulate ones were tested in a model reaction of the Suzuki cross-coupling between BrAN and PBA under mild conditions (EtOH-H\({}_{2}\)O mixture (5:1) as a solvent, 60 \({}^{\circ}\)C, 0.2 mol.% of Pd). For the preliminarily reduced samples, no correlation between the sizes of Pd NPs and catalytic efficiency was found. This may be partially due to the known homogeneous/heterogeneous mechanism of the Suzuki reaction, but it may also indicate the influence of the support nature. Afterwards, each initial (unreduced) and reduced sample was tested in a second run of the Suzuki reaction. It was found that Pd catalysts based on functionalized (hydrophilic) polymers are more stable as compared with the relatively hydrophobic polymer (NA120) used as a support.  
Earlier [40], we showed that the loss of catalytic activity can be due to numerous factors: leaching of Pd followed by its precipitation leading to the growth of Pd NPs, adsorption of the reaction products on Pd surface and on the support, etc. Therefore, the hydrophilicity of the polymeric support is also very important for the catalyst stability. The results presented in this work demonstrated that the introduction of hydrophilic substituents in HAPs may improve catalyst stability during cross-coupling reactions.