Identificador: TDX:790
Autors: Andreu Pujol, Robert
Resum:
Nowadays, in the field of polymeric materials a new type of phenolic resins are being developed from 3,4-dihydro-2H-1,3-benzoxazines. The synthesis of these compounds takes place by reaction of a phenol, a primary amine and formaldehyde. These monomers give polybenzoxazines by means of temperature or catalysts by a ring-opening process.The resulting polymers exhibit good thermal stability, interesting mechanical properties, and excellent resistance to chemicals and radiation UV. The low change of volume during polymerization together with their low viscosity facilitates its processing in industrial applications. Their low dielectric constant and low water absorption make them useful in electronic applications. Finally, the presence of nitrogen and aromatic rings contributes to confer them a flame retardant character.Usually, the benzoxazine polymerization is carried out at high temperatures, generally exceeding 200ºC. Nevertheless, is typical to use acidic catalyst to accelerate the process. The propagation mechanism of benzoxazine polimerization described in the literature is cationic and proceeds through the attack of the nucleophilic positions to the oxonium activated heterocyclic ring.Most benzoxazines used in the industry are difunctional like the Bisphenol-A derivate, and lead to crosslinked materials. Consequently there is not much information concerning the mechanism of polymerization and the structure of these polymers. For this reason this work is focused on the study of the polymer formation by means the use of monofunctional benzoxazines that have been synthesised for this purpose.The main aim of this work is to establish a feasible polymerization mechanism. For this reason a family of benzoxazines containing functional groups with different electron-withdrawing o electron-donating capacity has been prepared.First, the thermal polymerization of these monomers has been studied by differential scanning calorimetry, analysing the influence of the electronic character of the substituents in the polymerization temperature. Moreover, the thermal polymerization of highly reactive monomers that containing acid and phenol groups has been studied. In this case a relationship between the acidity of these groups and the activating effect has been established.Based in the thermal polymerization results a new initiator system consisting in a combination of BF3 and alcohols has been proposed. This initiator system results especially effective in the polymerization in solution of this type of monomers allowing to obtain the corresponding polymers with high conversions in mild polymerization conditions. It has been demonstrated that with this initiator the propagation mechanism is different to the described in the literature. A new polymerization mechanism has been proposed in basis of kinetic studies, NMR microstructural characterization of the polymers and deuterium isotopic labelling experiments.Finally, the thermal polymerization of benzoxazinic monomers that contain glycidyl groups in their structure has been studied. It has been possible to separate the two polymerization processes with the use of an appropriate catalyst. This result allows to prepare linear polyethers with pendant benzoxazine groups that can be further crosslinked at higher temperatures.
Repositori URV