3, 5-Dihydroxybenzyl alcohol based dendrimers and hyperbranched polymers : synthesis, characterization and applications in catalysis

A new divergent methodology to prepare a variety of dendritic polymers containing dimethylsilyl-linked-3,5-dihydroxybenzyl alcohol units in the backbone is reported. This simple and versatile synthetic route is based on the acid-base hydrolytic chemistry of 3,5-dihydroxybenzyl alcohol (DHBA) and dimethyldiaminosilane Me2Si(NMe2)2. Controlled sequential addition of reactants was used to prepare dendrimer generations 1--5. The dendritic growth was found to be dictated by a preferential attack of the benzylic hydroxyl group of DHBA on the aminosilane, as no traces of residual benzylic OH groups were observed using 1H NMR spectroscopy. By a continuation of this divergent route, these dendrimers were functionalized with a variety of terminal units, including trimethylsilyl, n-octanoate, organic dye disperse red 1, and hydroxypropyldiphenylphosphine. Uncontrolled reaction protocols using the same reacting partners were also investigated to prepare the corresponding hyperbranched polymers. Resulting macromolecules showed structural fragments ranging from that of dendrimer generation 1 to that of dendrimer generation 2 in their MALDI-TOF mass spectra. Addition of more reactants did not seem to increase the size of these hyperbranched polymers.Determination of some of the structure-property relationships in these dendrimers was carried out to obtain a better understanding of their potential applications. Hydrolytic stability of the dendrimers was examined using dendrimer generations 1--3 solvated in wet DMSO. Their resistance towards hydrolysis was found to increase with an increase in generation number. Intrinsic viscosities were determined for dendrimer generations 1--5. A non-linear relationship was found between the generation number and the intrinsic viscosity, as it reached a maximum for dendrimer generation 4, followed by a decrease. 3,5-dihydroxybenzyl alcohol based dendrimers were found to self-assemble via hydrogen bonding between terminally situated hydroxyl groups. This phenomenon was found to be dependent on their concentration in solution, and a critical aggregation concentration was determined at 3.7 mg mL-1 for dendrimer generations 1--3. Higher generation dendrimers showed aggregate formation at low concentrations as the number of peripheral OH groups was significantly increased.By functionalizing dendrimers and hyperbranched polymers with hydroxypropyldiphenylphosphine ligands, followed by coordination with [Rh(COD)Cl], we were able to synthesize metallodendrimers and organometallic hyperbranched polymers, in which active organometallic fragments are present at the periphery. Their catalytic activity was assessed in hydrogenation of 1-decene. Conversion rates of decene into decane for metallodendrimers were found to be dependent on the time of reaction and the generation number. Maximum conversion rates were found after 5 h reaction time in each case