Comparative Kinetic Study on the Preparation of Poly(Vinyl Acetate) Activated by Microwave and Conventional Heating

摘要

Background: Microwaves (MW) activation often leads to higher yields and more selective products are obtained, compared to the same reaction activated by conventional heating (CH). In polymer science, the use of MW irradiation has been widely investigated; nevertheless few articles are dedicated to controlled radical polymerization reaction (CRP). Considering the relative difficulty of vinyl acetate (VAc) polymerization, we decided to concentrate our attention on the kinetic aspect of the process. In this work MW versus CH activation reactions are discussed. Solvent effects in the presence of Ru~(II) complexes were considered and MW power effect studied. Methods: In this work, we report the effect of solvent media on the polymerization reaction of VAc monomer, under controlled conditions of temperature and molar ratio [VAc]_0/[Ru~(II)]_0/[Al]_0/[CCl_4]_0 =200/1/1/1. The reactions were activated by MW and CH. The conversion vs. time and molecular weight vs. time are important characteristics of this system. Four different solvents were tested with low and high dielectric constants DMSO, DMF, anisole, MEK; three Ru~(II) complexes were probed mediating the polymerization reaction. Results: In experiments with MW activation, the effect of solvent media was clear, since the DMSO solvent with the highest value of dielectric constant (ε), produced the best yield and molecular weight (M_n, GPC), under controlled temperature of 70°C and power conditions, 100, 500 W. For reactions activated by CH, the solvent effect was important too; values of % conversion and M_n, GPC in DMSO did follow a clear trend; good molecular weight values and high conversion were observed. There is a clear and interesting difference in the reaction time between both methods of activation, CH and MW. For the same % conversions, the reaction time is ten times shorter when activated by MW, the conversion with MW reaches 42% in 45 min, and 46% conversion in 600 min was observed with CH. Ru~(II)-1 complex was effective for the preparation of polymers under MW in DMSO. This complex is cationic and presents an activated structure more quickly under MW than CH. It was found that polymerization reaction rate decreases in the following order MW > CH. Conclusion: The kinetic data were analyzed using a first-order model. The reaction kinetics are very much dependent on the activation method, as an example k_(p,MW)= 1x10~(-2) y k_(p,CH) = 6×10~(-4)， so， these values indicate that the reaction activated by MW proceeds 16.4 times faster than the reaction activated by CH. The character of the VAc polymerization reaction under MW, in the presence of DMSO as a solvent, goes on by a living polymerization mechanism, the molecular weight increases with conversion and it is very similar in value to the theoretical molecular weight. The living character was probed and confirmed through the formation of the copolymer p(VAc-co-MMA).