Kyu Yong Choi

The finite molecular weight moments (FMWM) technique, originally developed for calculating the complete molecular weight distribution (MWD) in batch free radical polymerizations, is extended here to simulate transient changes in MWD in a series of continuous flow stirred‐tank solution polymerization reactors. Unlike the classical method of molecular weight moments, which only calculates molecular weight averages, the FMWM technique provides a simple and effective means to calculate the whole shape of the polymer molecular weight distribution curve, even during the transient period of reactor operations in continuous processes. In this work, the solution polymerization of methyl methacrylate is used as a model system to demonstrate that the FMWM technique can successfully simulate transient MWDs, particularly bimodal distributions of polymer molecular weight resulting from varying reactor operating conditions in a series of two consecutive continuous stirred‐tank reactors operating at different temperatures. The simulation results reveal several interesting aspects of how polymer MWD changes over time in each reactor.

The choice of the silica support for α‐olefin polymerization catalysts has overwhelming ramifications on the overall metal loading, catalyst performance, and properties of the resulting polymer. There are several physical properties of a silica support to consider but the pore volume and pore diameter are the two of the most important support properties in such considerations. During the catalyst immobilization process and the subsequent polymerization reaction, the change in the pore volume can either facilitate or inhibit the mass transfer of catalytic compounds and monomers within and throughout the catalyst particle. This work presents the experimental study on the effects of pore diameter and pore volume in porous silica particles used to support rac‐Et(ind)₂ZrCl₂ catalyst on ethylene polymerization. The amount of immobilized methylaluminoxane (MAO) is varied for three commercially available silica micro‐particles with similar surface areas and particle sizes. It has been observed that each type of silica exhibits different effects of the immobilized MAO on their pore characteristics, although a general trend is observed. Ethylene polymerization in slurry phase with the prepared supported catalysts also shows that the polymerization activities can be correlated with pore diameter and surface area for the three silica supports.

The preparation conditions of silica‐supported metallocene catalysts impact the catalytic performance and consistency in ethylene polymerization. In a commonly used catalyst impregnation technique, the concentrations of catalyst solutions and the contact time between silica microparticles and catalyst solutions are used to ensure uniform and stable immobilization of coactivator methyl aluminoxane (MAO) and metallocenes. The overall catalytic activities for olefin polymerization, catalyst particle fragmentation, and the resulting polymer particle morphology depend upon the distribution of active catalyst sites within the silica catalyst particle. This work presents both experimental and theoretical modeling studies on the intraparticle spatial distribution of MAO and metallocene in highly porous silica microparticles for ethylene polymerization in a liquid‐slurry phase. The silica‐catalyst solution contact times and the catalyst solution concentrations are used as two major variables to vary the intraparticle catalyst site distributions. The overall aluminum and zirconium contents in the catalyst and their radial distributions are observed to be strongly affected by the impregnation conditions. A mathematical model is also derived for the catalyst impregnation process and it provides adequate predictions of the impregnation process characteristics.

A mathematical model that describes a free radical solution terpolymerization is applied to styrene (St), methyl methacrylate (MMA), and maleic anhydride (MAh) polymerization system. This terpolymerization model allows for the use of either monofunctional or bifunctional peroxide initiators, and it is applied to batch and continuous reactions. The key model outputs are global monomer conversion, terpolymer composition, terpolymer average molecular weights, and terpolymer monomer sequence distributions. A few kinetic parameters are estimated such that global conversion is properly predicted by the model including initiator efficiency and two termination rate constants. Molecular weights are shown to be dominated by chain transfer reactions. Experimental trends of conversion, terpolymer molar masses (molecular weights), and composition are reasonably well predicted by the model.

This study presents the experimental study of semibatch emulsion and miniemulsion copolymerization of vinyl acetate (VAc) and ethylene to vinyl acetate‐ethylene (VAE) copolymer at 60°C and 80–300 psig. In the miniemulsion copolymerization, a water‐soluble initiator (K₂S₂O₈) is used and VAc miniemulsion is prepared in presence of surfactant and cosurfactant using a sonicator or a high‐shear homogenizer. Then, ethylene gas is supplied to the reactor at constant partial pressure. In a miniemulsion process, the mass transfer limitations of VAc from monomer droplets to the aqueous phase, and to micelles or polymer latex particles that are present in conventional macro‐emulsion polymerization can be eliminated and the transfer of ethylene dissolved in the aqueous phase to the miniemulsion droplets is the major ethylene transport process for the polymerization. The experimental data show that the amount of ethylene incorporation into the copolymer is higher in miniemulsion polymerization than in emulsion polymerization. The ethylene pressure has been found to have a strong impact on the ethylene incorporation into the copolymer phase in both emulsion and miniemulsion copolymerizations but the increase is more pronounced in miniemulsion case. The VAE copolymer latex particles prepared by miniemulsion polymerization exhibited higher storage stability than those prepared by macro‐emulsion polymerization.

Styrene has been polymerized to syndiotactic polystyrene (sPS) over a layer of heterogeneous Cp*Ti(OCH ₃ ) ₃ /MAO catalyst immobilized onto the surfaces of silica nanotube reactor (SNTR) arrays of 60–200 nm in diameter. The polymer produced in the SNTR arrays has been found to have the molecular weights much larger than the polymers synthesized by a liquid slurry polymerization over silica‐supported catalysts. A dynamic reactor model that consists of diffusion and reaction terms has been derived and solved to quantify the kinetics of styrene polymerization in a single nanotube reactor. The two‐site kinetic model applied to the silica nanotube reactor model shows that the experimentally observed high polymer molecular weight can be fitted if the chain transfer rate constants for monomer and β‐hydride elimination are reduced significantly. The simulation results suggest that the presence of dense crystalline sPS nanofibrils filling the nanotubes constrain the molecular movements of polymer chain ends in the proximity of catalyst sites to limit the chain transfer reactions. POLYM. ENG. SCI., 60:700–709, 2020. © 2020 Society of Plastics Engineers

The experimental study of polymer growth on spatially unconstrained flat silica surfaces is presented for propylene and ethylene polymerization in liquid and gas phases with high activity metallocene catalyst (rac‐Et(Indenyl)₂ZrCl₂/ methylaluminoxane at 30–70 °C and 30–50 psig. The study shows a surprisingly rich spectrum of nascent morphologies for each polymer synthesized in two different reaction phases. In propylene polymerization in a liquid phase, polypropylene nanofibrils are first formed at the catalyst sites and quickly develop into microglobular morphology, whereas in ethylene polymerization in a liquid phase, polyethylene nanofibrils formed at the catalyst sites grow to a ribbon‐like morphology. In gas phase polymerization where polymerization rate is much lower due to low monomer concentration, both polymers grow to compact aggregates of globules. Unlike in liquid phase polymerization, no ribbon‐like structure is formed in ethylene polymerization. The sequential polymerization of these olefin monomers has also been carried out and the experimental data show two distinct regions of corresponding polymers as observed in separate ethylene and propylene homopolymerization experiments.

The morphologies of sodium polyacrylate super absorbent polymer microparticles synthesized by inverse suspension polymerization at 65°C are presented. The spherical polymer microparticles of 30–100 μm in diameter were prepared in a hydrocarbon medium with water soluble initiators in presence of a cross‐linking agent and suspension stabilizer in a stirred batch reactor. A variety of microparticles morphology has been observed for different reaction conditions such as monomer/water ratio, reaction time, cross‐linker type and concentration, hydrocarbon medium properties, and postreaction solvent washing. The observed polymer particle morphologies include highly deformed particles, spherical particles with smooth surfaces, spherical particles with wrinkled surfaces, particles with interior multihollows, highly porous particles, particles of empty core‐shell structures, etc. Some of these particles with complex morphologies such as multihollow or porous interior can improve the water absorption efficiency of the polymer particles because water can penetrate the particle more effectively. The polymer particles with wrinkled particle surface, which is formed during the postreaction treatment process, can also enhance the water absorption rate because of its increased external surface area. POLYM. ENG. SCI., 58:1564–1574, 2018. © 2017 Society of Plastics Engineers