Abstract
Controlled radical polymerization (CRP) is a rapidly developing area in polymer science. Its versatility and ability to produce novel polymer structures are the main reasons which attract both academic and industrial interests. In particular, Nitroxide mediated Radical Polymerization (NMRP) is currently one of the three popular approaches in CRP. Polymeric materials synthesized by NMRP can be utilized for coatings, adhesives, lubricants, gels, thermoplastic and also for biomedical applications. Open literature shows an academic controversy over the kinetic mechanisms of NMRP and also over the kinetic reaction rate parameters. In this study, a kinetic mechanism describing the bimolecular NMRP was thoroughly discussed, reviewed and improved. In fact, two side reactions have been added to the most updated NMRP reaction scheme. Therefore, a kinetic model for a NMRP polymer reactor operating in batch and CSTR modes was developed based on a detailed reaction mechanism for thermal polymerization of styrene and also for bimolecular NMRP of styrene using benzoyl peroxide (BPO) as initiator and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a radical controller. The kinetic model, consisting of a set of ordinary differential equations, was numerically integrated and validated with a set of experimental data obtained at temperature 120°C and [TEMPO]/[BPO] molar ratio 1.1. This model validation was done by means of a parameter estimation scheme to determine the "best" kinetic parameters. The model predictions were compared with data at 120 and 130°C for [TEMPO]/[BPO] molar ratios of 0.9, 1.1, 1.2, and 1.3. A good to very good agreement was obtained between the prediction and data. The non-linear behavior of the CSTR polymerization reactor was also analyzed using Matlab continuation program Matcont package. Typical hysteresis behavior, input and output multiplicities, as well as disjoint bifurcations were determined for this reactor. The bifurcation parameters selected are the coolant flow rate, feed stream temperature, residence time, initiator feed stream concentration and controller feed stream concentration. Bifurcation analyses reveal the stable and unstable operating regions of the reaction. Thus, the results obtained can be employed as a guide to develop a process control strategy for a better and safer operation of the NMRP polymerization reactors. Finally, a steady state optimization for the CSTR reactor was carried out in order to identify the optimal operating conditions of the NMRP process.
Highlights
Once a mathematical model has been developed for a polymerization system, a common approach is to compute the values of the model parameters so that the model can give an acceptable prediction of real data
In this case the monomer conversion, molecular weights as well as the reactor and the jacket temperatures demonstrate typical hysteresis behavior whereas the output multiplicity is the case for TEMPO conversion and just as the previous cases the polydispersity index (PDI) show self- intersecting curves as well as input and output multiplicities.Clearly, wider ranges for the residence time can be achieved when increasing the [TEMPO]/ [benzoyl peroxide (BPO)] molar ratio; the desired value for the monomer conversion can be achieved with considerably low residence times
Nitroxide Mediated Radical Polymerization (NMRP) of styrene provides a variety of special polymerization systems that are interesting from the viewpoint of the production of polymers with highly controlled structure, narrow molecular weight distribution and polydispersity index
Summary
In the last two decades, significant advances have been made in the field of controlled/ living free radical polymerization (CRP). There exist three techniques of CRP: atom transfer radical polymerization (ATRP), reversible addition fragmentation chain transfer (RAFT), and Nitroxide Mediated Radical Polymerization (NMRP). These techniques provide polymers with narrow molecular weight distributions and low polydispersities. The focus of this study was to investigate the kinetic mechanism and develop a kinetic model for bimolecular NMRP of styrene using benzoyl peroxide (BPO) as a conventional initiator and nitroxide stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as the controller. Detailed analysis of the steady state bifurcation behavior of the CSTR kinetic models of the bimolecular NMRP of styrene was performed using Matlab continuation program Matcont package.
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