Hybrid organic – inorganic polymerci materials bearing triorganotin caroboxylate moieties and their applications

This work has been mainly devoted to the investigation of the most suitable structural modifications on polystyrenic resins bearing triorganotin carboxylate moieties in order to dispose of an effective heterogeneous catalyst useful for transesterification reaction. First results indicate that triphenyltin derivatives are more effective than the aliphatic analogues (Angiolini et. al., 2006). In order to obtain a higher catalytic activity, two different synthetic pathways have been used. The first one concerns the introduction on the aromatic ring of substituents having different electronic properties, in conjugated position with respect to the tin atom. The aim of this modification was to tune opportunely the Lewis acidity at the metal centre. The tested substituents were, in chronological order of synthesis, F, SCH3, CF3 and finally Cl. As expected, the resins deriving from the monomer bearing the fluorine atom gave a higher alcohol conversion in a transesterification model reaction than those having the SCH3 group as substituent. In fact, as known, the presence of an electron-withdrawing substituent, such as F, increases the Lewis acidity of tin, while the electron-donor SCH3 reduces it. This result has led to the idea of introducing a strong electron-withdrawing group as tin substituent. To this purpose the CF3 was chosen, but the catalytic performances of the resulting resins were the lowest of the series. It has been supposed, indeed, that a too high Lewis acidity degree actually affects the catalytic performances of the resins, probably due to the very strong interaction between the tin and the oxygen of carboxylic moiety, leading to an irreversible adduct formation resulting in a low catalytic activity. Thus, it has been tested a substituent having properties close to the fluorine atom, which displays, as reported, both electron-withdrawing and donor properties, owing to inductive and mesomeric effect, respectively. Accordingly, a monomer bearing a chlorine atom as ring substituent has been synthesized, as well as the related resins that, tested as catalyst in transesterification reactions, finally gave the best catalytic results of the overall series. At the same time, a further modification was introduced with the aim of increasing the catalytic centre mobility, in order to obtain an easier contact between substrate and catalyst, and therefore better catalytic performances. To address this, the metal centre was spaced from the polymeric backbone by introducing an aliphatic spacer of a certain length. In particular, a dimethylenic and a tetramethylenic spacer have been introduced and the corresponding resins tested as catalyst. As a result the sample having the dimethylenic spacer gave the highest alcohol conversions of the series and, moreover, better results than those achieved by the corresponding one bearing the fluorine. The resin having the tetramethylenic spacer instead, provides worse conversions, lower than those obtained by the basic catalyst. Finally, the above two modifications have been combined in a unique product, thus preparing monomers and the corresponding resins having both the aromatic substituent CF3, F, or Cl and the dimethylenic spacer. Unexpectedly, the obtained conversions, although they respected the conversion rank order observed for corresponding catalysts without the spacer, are lower than the ones obtained with the Cl substituent only. The testing model reaction was also performed in the absence of catalyst, with the aim of detecting the presence of additional contributions other than those given by the organotin carboxylate moiety, resulting in the lack of conversion of ethyl acetate at all, as expected. Moreover it can be safely said that any significant tin leaching from grafted materials into the reaction medium does not occur, meaning that the organometallic ester is stable under the trans-esterification conditions and the reaction takes place only at the solid-liquid inte...