(21a) Kinetic Radical Polymerization Solution of Methacrylate Alkyls By Using a Reactor Calorimeter

The radical polymerization is different from other process of chain polymerization by its ease of implementation. It can be carried out in bulk, solution, emulsion or suspension [1]. Its range of application is very wide as it applies to most of the vinyl monomers for the synthesis of polystyrene, poly (vinyl chloride) and poly (alkyl methacrylate). Statistically, these reactions such as nitration, sulfonation and hydrolysis reactions are responsible of the greatest number of thermal runaway [2]. The most of these reactions are also well known to be highly exothermic or likely to release significant amounts of gaseous products. In the case of the polymerization, in addition of the safety of polymerization processes, the knowledge of kinetic parameters can provide meaningful information on the ways in which we can modify the molecular weight of the polymer to obtain products with particular desired properties.

There are a wide variety of techniques that allow the monitoring of the free radical polymerization kinetics. This monitoring can be performed by the evolution of one or more physical or chemical properties during the reaction: spectroscopic techniques, rotating sector method, pulsed laser polymerization coupled with size exclusion chromatography PLP-SEC, and thermal analysis and calorimetry technical. Most of the kinetic studies of the radical polymerization reactions, found in the literature based on thermal analysis technical, use the differential scanning calorimeter DSC [3]. This device contrary to calorimetric reactor uses very small amounts of reactants (mg) and is not equipped with stirring system, or adding system to represent the operating conditions close to those of a semi-industrial reactor.

In this work, the calorimeter reactor RC1-RTCal of Mettler Toledo with capacity of 500mL was used to study the kinetics of the radical solution homopolymerization in isothermal conditions, of Dodecyl MethAcrylate (DDMA) and Methyl MethAcrylate (MMA). The polymerization is initiated by a bifunctional peroxide, 1-di (tert-butylperoxy) cyclohexane (BPCH) in an organic solvent: anisole. The superposition of the thermal power profiles shows that the global rate of polymerization of DMAA is faster than that of MAA.

A simplified model of the chemical polymerization based on three main steps (initiation, propagation and termination) was used. The estimation of the kinetic parameters of each step (pre-exponential factor and activation energy) was reached by comparison of the thermal power profiles given by the model to that obtained experimentally.

The kinetics of decomposition of the BPCH in an organic solvent (anisole) was investigated separately without monomer to avoid the polymerization reaction. A Differential Scanning Calorimeter (DSC) was used for thermal analysis to obtain thermokinetics data of the decomposition step [4].  The result of this study has been used to initialize the kinetic parameters estimation of the initiation step of the polymerization reaction.

The parameter estimation of the three steps was achieved by using a mixed estimation method where a genetic algorithm is combined with a locally convergent method: Rosenbrock method [5]. The values of the estimated kinetic parameters (are the same order of magnitude as those met in the literature [1]. The values of the propagation constant k_p of DMAA is significantly higher than that of MMA. This result is consistent with the measurements of the molecular weight of polymer carried out using size exclusion chromatography SEC.

References:

[1] Matyjaszewski, K., Davis, T.P., Wiley, J., 2002. Handbook of radical polymerization. Wiley Online Library.

[2] Nigel MADDISON, Richard L. ROGERS, 1994. Chemical Runaways: Incidents and their causes. Chemical Technology Europe.

[3] Jaso V., R. Radivcević et D. Stoiljković, 2010. J. Therm. Anal. Calorim. 101, 1059.

[4] Ben talouba I., Medjkoune H., Bensahla N., Balland L., Mouhab N., Abdelghani-idrissi, 2014. Thermal decomposition kinetic of 1,1-di(tert-butylperoxy)cyclohexane in organic solvents, J. Thermochimica Acta, Vol. 589 pp.270–277.

[5] Balland, L., Estel, L., Cosmao, J.-M., Mouhab, N., 2000. A genetic algorithm with decimal coding for the estimation of kinetic and energetic parameters. Chemom. Intell. Lab. Syst. 50, 121–135.