Ibrahem S. Altarawneh

RAFT with Bulk and Solution Polymerization: An Approach to Mathematical Modelling and Validation

Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization is a novel technique used to impart a living character in free radical polymerization. A mathematical model accounting for the concentrations of the propagating, intermediate, dormant and dead chains is developed based on their reaction pathways. The kinetic scheme used includes initiation, propagation, pre-equilibrium, core-equilibrium and termination of the propagating radicals along with termination reactions of the carbon-centered intermediate radical. This model is combined with chain-length dependant termination model in order to account for the decreased termination rate. The model has been validated against experimental data for solution polymerization of styrene with dithiobenzoate at 80°C. The fragmentation rate coefficient was used as a model parameter and a value equal to 6×104 sec−1 was found to provide a good agreement with the experimental data. The model predictions indicate that the observed retardation can be attributed to the cross termination of the intermediate radical and, to some extent, to the RAFT effect on increasing the average termination rate coefficient. The hypothesised irreversible self termination was found to have a negligible effect on the polymerization rate. While the linear growth of the number average molecular weight along with the low polydispersity, reveal the living nature of RAFT agent and the importance of the transfer constant in controlling these properties.

Inferential Conversion and Composition Monitoring via Microcalorimetric Measurements in Emulsion Terpolymerization

Online calorimetry was used to estimate the conversion and the cumulative terpolymer composition in free radical emulsion polymerization of styrene/methyl methacrylate (MMA)/methyl acrylate (MA) within batch and semi-batch stirred tank reactors. The model includes the mass and energy balance equations over the reactor and its peripherals. Energy balance equations include the heat of reaction, internal and external heat transfer effects, as well as external heat losses. Experiments were conducted in conjunction with a reactor instrumented with platinum resistance thermal transducers and gravimetric conversion measurement devices. The experiments were conducted at variable operational conditions of feed rate, temperature and monomer feed ratios. The estimated values of conversion and terpolymer composition obtained from calorimetric data were compared with measurements of conversion and cumulative terpolymer composition by gravimetry and NMR techniques. Good agreement was observed between the inferential calorimetric estimation and corresponding process measurements. We established that online calorimetry is suitable for use as a soft sensor for the prediction of the rate of reaction, conversion and terpolymer composition.

The influence of intermediate radical termination and fragmentation on controlled polymer synthesis via RAFT polymerization

Reversible addition-fragmentation chain transfer (RAFT) polymerization allows the design of tailored polymers by inducing addition and fragmentation reactions via a powerful kinetic mechanism; the process, however, is not well understood. A parametric analysis was conducted in conjunction with experimental data to provide insights into the RAFT mechanism, accounting for the key kinetics events in RAFT polymerization of styrene. The effects of overall, forward and backward fragmentations of intermediate radicals were analysed with experimental data and sensitivity investigations. Our results show that the intermediate radicals undergo relatively fast fragmentation and that the radical cleavage is symmetrical. These factors, critical in design and control, were found to profoundly influence the process kinetics and the final polymer product attributes.

Optimizing packing heterogeneity for sorption enhanced metathesis reaction

An equilibrium-limited heterogeneous catalytic reaction, propene metathesis is suitable for process intensification via sorption enhanced reaction. In this work, we investigate the effect of catalyst/adsorbent configuration for propene metathesis in conjunction with pressure swing reaction. The catalyst and adsorbent configuration variations were defined in terms of packing heterogeneity index (PHI) and their effects were investigated experimentally and theoretically. Model predictions were tested against experimental data with variable PHI and adsorption/reaction conditions, including the absence of heterogeneity and of separation process. The product 2-butene was strongly adsorbed and retained by the intermediate adsorbent layers, thereby increasing reactant concentration in the reaction zone and enhancing conversion and rate of reaction in the subsequent layer. Model predictions were found to agree reasonably with experimental data and were used to elucidate the mechanism and optimizing principle for such reactors.

Advanced Modelling for Investigating the Effects of Reactor Operation on Controlled Living Emulsion Polymerization

Accurate control of product properties through the manipulation of transfer agents can be of great benefit to industry in producing targeted polymeric materials. In this work we developed experimental protocols and mathematical models for understanding and characterising semi-batch emulsion polymerization in the presence of a xanthate-based transfer agent. Zero-one kinetics was employed with population balance equations to predict monomer conversion, molecular weight (MWD) and particle size (PSD) distributions in the presence of xanthate-based reversible addition-fragmentation chain transfer (RAFT) agents. The effects of the transfer agent (AR), surfactant, initiator (KPS) and temperature were investigated. Monomer feed rate was found to strongly affect conversion, MWD and PSD. The polymerization rate (Rp), number average molecular weight (Mn) and particle size () decreased with increasing AR. Rp increased with increase in SDS and KPS; while with increase in temperature, Mn decreased, Rp increased and increased. With semi-batch mode, Mn and increased with monomer flow rate.