Investigação experimental e computacional da polimerização via radical livre controlada em presença de radicais nitroxido (NMRP) utilizando-se iniciadores difuncionais / Experimental and computational investigation of nitroxide mediated radical polymerization (NMRP) using di-functional initiators

Rafael Silva Dias

Controlled/Living Free Radical Polymerization (LFRP) is one of the most important techniques for production of polymeric materials with good control of the microstructure (e.g., narrow molecular weight distributions). Such properties can also be achieved via ionic polymerizations, but being a free radical polymerization process, LFRP is much less sensitive to impurities and solvents. The Nitroxide-Mediated Radical Polymerization (NMRP), as well as other LFRP processes, provides end functional polymers, and recently it has shown advantages related to polymerization in aqueous media, synthesis of hybrid materials, block copolymers, and environmental issues. However, one challenge that still must be overcome in LFRP, as well as in NMRP process, is to increase slow polymerization rates experienced due to the establishment of an intrinsic equilibrium. In order to achieve higher productivities, higher reaction temperatures or higher (monofunctional) initiator concentrations are usually used, but as a result, polymers with lower molecular weight are produced. It is well known that the use of di-functional initiators provides an enhancement of the reaction rate of conventional free radical polymerization with either no change or even an increase in molecular weight averages. Therefore, in this thesis, it was taken an experimental (polymerization in ampoules) and computational (deterministic and empirical methods) investigation of the effect of di-functional initiators on nitroxide-mediated radical polymerization with the purpose of enhancing the reaction rate. Particularly, the molecular weight development (number average molecular weight and polydispersity) of polystyrene synthesized by using TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) as controller was evaluated by making a comparison with polymerizations using mono-functional initiators (BPO and TBEC). From the use of a design of experiments, the influence of the L531 initiator concentration, the TEMPO/initiator ratio, and the temperature on the reaction rate, molecular weights and polidispersity were determined, and an empirical model was generated. A kinetic mechanism has been proposed, from which mass balance and moment equations were derived for a batch reactor. The model was well adjusted to the experimental data with the estimation of some parameters. Lastly, a dynamic sensitivity analysis of the parameters was developed. It was shown both in experimental and simulation results that in the case of the bi-functional initiator L531, the reaction rate and molecular weights were increased when comparing to the performance of the mono-functional initiators keeping the livingness of the system. To the best of our knowledge, this is the first report of TEMPO-mediated polymerization of styrene using difunctional initiators

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