Thermal Polymerization of Styrene,Part 1 – Bulk Polymerization

The thermal bulk polymerization of styrene is critically reviewed. There is still no generally accepted kinetic model for the thermal radical formation process, but ideal second‐ or third‐order models are widely used for modeling bulk systems. Since initiation and chain transfer reactions cannot be treated independently from one another as long as the same species is considered to be involved, it is concluded that non‐ideal kinetics, possibly in form of a (micro‐)viscosity dependency of the Mayo mechanism, are likely to be present. A mathematical model is presented that keeps the predictive capabilities of the Hui‐Hamielec model, but allows facile implementation of reaction specific modifications. Part 2 of this paper will focus on the effect of compartmentalization on the thermal polymerization of styrene.

     

Bulk Copolymerization of Styrene and Methyl Methacrylate at Elevated Temperatures

Reactivity ratios and full conversion range data (conversion, copolymer composition, molecular weight averages, glass transition temperature) are presented for styrene (STY)/methyl methacrylate (MMA)/copolymerization at elevated temperatures (130 and 150°C), a region where polymerization studies are scarce for the system, both for purely thermal polymerization and for peroxide‐initiated ones.     

A Simplified Comprehensive Kinetic Scheme for Modeling of Ethylene/1‐butene Copolymerization Using Ziegler‐Natta Catalysts

A simplified kinetic scheme of eythylene/α‐olefin copolymerization has been developed by adding reactions responsible for the unusual kinetic behavior to a general mechanism. The estimation of rate constants has been simplified by making physically meaningful initial guesses. Rate constants affecting yield, MWD and comonomer content have been estimated separately. Experiments were designed to investigate the effects of each rate constant independently. The obtained rate constants show that the sites which are responsible for formation of short chains with higher 1‐butene content are more active at the beginning of polymerization, while the sites which are responsible for formation of longer chains with lower 1‐butene units are more active at the final stages of polymerization.

     

苯乙烯本体聚合反应动力学实验的改造与拓展

对苯乙烯本体聚合反应动力学基础实验进行了改造与拓展,利用直接沉淀称重法代替膨胀计法,在有限的课时安排中完成对聚合反应速率和聚合物相对分子质量的测定,增加了实验的直观性和综合性,加深了学生对聚合反应动力学的认识。     

Cashew Nut Shell Liquid (CNSL) as a Source of Drugs for Alzheimer’s Disease

Alzheimer’s disease (AD) is a complex neurodegenerative disorder with a multifaceted pathogenesis. This fact has long halted the development of effective anti-AD drugs. Recently, a therapeutic strategy based on the exploitation of Brazilian biodiversity was set with the aim of discovering new disease-modifying and safe drugs for AD. In this review, we will illustrate our efforts in developing new molecules derived from Brazilian cashew nut shell liquid (CNSL), a natural oil and a byproduct of cashew nut food processing, with a high content of phenolic lipids. The rational modification of their structures has emerged as a successful medicinal chemistry approach to the development of novel anti-AD lead candidates. The biological profile of the newly developed CNSL derivatives towards validated AD targets will be discussed together with the role of these molecular targets in the context of AD pathogenesis.     

Bulk Copolymerization of Conjugated Linoleic Acid With Styrene and Butyl Acrylate: Reactivity Ratio Estimation

Free radical bulk copolymerizations of conjugated linoleic acid (CLA)/styrene (Sty) and CLA/butyl acrylate (BA) were performed at 80°C. Copolymers were characterized for composition, conversion, molecular weights and glass transition temperature (T_g). A pseudo-kinetic model was developed and validated with experimental data. Reactivity ratios estimations were performed and one impurity commonly found in CLA, oleic acid, influenced the reaction kinetics significantly. The T_g of CLA homopolymer was predicted to be 5°C.     

Mathematical Modeling of Atom‐Transfer Radical Copolymerization

A comprehensive mathematical model for atom transfer radical copolymerization in a batch reactor is presented using the concept of pseudo‐kinetic rate constants and the method of moments. The model describes molecular weight, monomer conversion, polydispersity index, and copolymer composition as a function of polymerization time. Model predictions were compared with experimental data for styrene and butyl acrylate copolymerization and excellent agreement was obtained. We have also tested the model with styrene‐acrylonitrile copolymerization data obtained in our laboratory. Finally, we used the model to study the effect of comonomer reactivity ratio, feed composition, activation and deactivation rate constants on the copolymer composition.

     

Semibatch Atom Transfer Radical Copolymerization of Styrene and Butyl Acrylate

Summary: Batch and semibatch butyl acrylate (BA) polymerizations are carried out using a heterogeneous atom transfer radical polymerization (ATRP) catalyst system, with excellent molecular weight (MW) control maintained at temperatures below 80 °C. A kinetic model, using rate coefficients from literature and catalyst solubility data from this study, provides a good representation of the experimental results, after modifying the model to account for the decrease in rate caused by intramolecular chain transfer. It is also demonstrated experimentally that well-defined random, gradient, and block styrene/BA copolymers can be synthesized by manipulating monomer feed profiles in the ATRP semibatch process.     

Effects of Operating Conditions on the Copolymerization of Castor Oil Maleate–Styrene by Suspension Polymerization

This work studies the synthesis of copolymers (MACO‐St) of castor oil maleate (MACO) and styrene (St) initiated using benzoyl peroxide (BPO) as free radical initiator through suspension polymerization. The study investigates the effects of temperature (100–140 °C), the molar ratio between styrene and MACO (2:1–4:1), BPO concentration (0.10–0.20 wt%), and water concentration (50–100 wt%) on the molecular weight distribution, thermal stability, viscosity, and biodegradability of the copolymers. Suspension polymerization allows the production of a broad range of number average molecular weight (3465–18 995 g mol^?1) and molecular weight distributions with dispersions (?) ranging from 1.8 to 4.4. The reaction presents high yields of castor oil into copolymers (>90%), which displays thermal stability up to 200 °C and are highly biodegradable according to the International Organization of Standardization reference.     

Thermal Polymerization of Styrene,Part 2 – (Mini)emulsion Polymerization

An experimental and theoretical study on the thermal (spontaneous) polymerization of styrene in compartmentalized systems is presented. Experimental data on thermally polymerized miniemulsions varying temperature and droplet size is provided. This data is used to test a new model approach to describe the thermal polymerization process of a disperse phase. The mathematical model is based on a chemical master equation balancing radical species that are capable of desorption (monomeric radicals) and ones that cannot leave a particle. Reasonable agreement between theory and experiment for the data provided here and for literature data can be achieved. Part 1 of this series is a review on the thermal bulk polymerization of styrene.

     

Modeling Insights on the TEMPO Mediated Radical Polymerization of Styrene

Despite a great deal of research on nitroxide mediated radical polymerization (NMRP), its kinetic mechanism is not fully known yet. The focus of this work was to contribute further to the understanding of the kinetics of NMRP processes through refinements of a comprehensive mathematical model developed by our group for the bimolecular system. This work considered important secondary reactions that can occur during the NMRP process. It also analyzed important kinetic aspects via a sensitivity study on some key parameters of the system. It was observed that an irreversible reaction between the TEMPO controller and the chemical initiator is very important and must be considered in the NMRP mechanism, in order to be able to describe properly both conversion and average molecular weight data.     

Simulation of Styrene Polymerization by Monomolecular and Bimolecular Nitroxide‐Mediated Radical Processes over a Range of Reaction Conditions

Simulations of polymerization rate, molecular weight development and evolution of the concentrations of species participating in the reaction mechanism over a range of operating conditions, and a parameter sensitivity analysis showing the effects of temperature, activation/deactivation equilibrium constant and initial concentrations of controller and initiator (if present) on these variables are presented for the nitroxide‐mediated radical polymerization of styrene. The simulations were performed with a computer program based on a detailed reaction mechanism. The simulated profiles of conversion, number average molecular weight ( ), and polydispersity agree well with experimental data. Previously unknown activation energies for reactions involved in the mechanism are estimated. The temperature dependence of the kinetic rate constants obtained in this study will be useful for future modeling and optimization studies.

     

Kinetic Modeling as a Tool to Understand and Improve the Nitroxide Mediated Polymerization of Styrene

Kinetic modeling is used to obtain insight in the complex interplay between reaction rates and obtained polymer properties in the SG1 and the TEMPO mediated bulk polymerization of styrene at 396 K. The increase of the viscosity during NMP is accounted for. At higher targeted chain lengths, chain transfer to dimer and transfer from nitroxide to dimer are shown to cause the experimentally observed reduced control over the average polymer properties and to result in a clear fronting of the polymer chain length distribution. The potential of kinetic modeling to design tailor‐made synthesis strategies is illustrated. Simulations indicate that careful control of the polymerization conditions allows to obtain an important improvement of the polymer properties. The approach is also applicable for NMP mediated by other alkoxyamines/nitroxides and allows to expand the application range of NMP for styrene polymerization in particular to synthesize complex polymer architectures by assembly of functionalized polymers.

     

Model Studies of Nitroxide‐Mediated Styrene Miniemulsion Polymerization – Opportunities for Process Improvement

Modeling studies were performed to investigate how persulfate‐initiated nitroxide‐mediated styrene miniemulsion polymerizations are influenced by changes to the polymerization recipe. By manipulating the initial concentrations of potassium persulfate and nitroxide, and the aqueous phase volume, trends in the predicted polymerization time, number average molecular weight, polydispersity and degree of polymer livingness were identified that indicate operating conditions for improved process performance. Specifically, our model predicts the existence of experimental conditions that simultaneously minimize polymer polydispersity and maximize the livingness of the polymer. The mechanisms responsible for the predicted trends were identified from the predicted molecular weight distributions of the living and dead polymer chains.

Predicted number MWDs at 20% monomer conversion for styrene NMMP systems employing various levels of [KPS]_aq,0. Dormant KPS‐initiated polymer radicals.     

The Influence of Xanthate‐Based Transfer Agents on Styrene Emulsion Polymerization: Mathematical Modeling and Model Validation

A mathematical model was developed to account for the evolution of polymer product attributes in the emulsion polymerization of styrene. The effects of transfer agent, surfactant, initiator and temperature were investigated. Polymerization rate, and particle size decreased with increasing concentration of the transfer agent. The polymerization rate increased with increasing surfactant and initiator concentrations, while an increase in temperature led to a decrease of molecular weight but an increase of polymerization rate and particle size. Chain extension was successfully achieved in the presence of our RAFT agent. The model predictions compared well with our experimental results.

     

Atom‐Transfer Radical Batch and Semibatch Polymerization of Styrene

Batch and semibatch styrene polymerizations are carried out using a heterogeneous ATRP catalyst system that provides excellent molecular‐weight control. The observed initiator efficiency is lower for semibatch operation due to the high initiator concentrations required to make a low‐MW polymer. Experiments verified that the insoluble metal complex does not participate in the polymerization and that Cu(I) solubility is an order of magnitude higher than that of Cu(II). A mechanistic model, using kinetic coefficients from literature and the solubility data from this study, provides a good representation of the experimental results.

     

Styrene polymerization in the presence of the CpTiCl3/Al2O3–SiO2/MAO catalytic system

Syndiotactic polymerization of styrene in the presence of heterogenized hemititanocene catalysts CpTiCl₃/Al₂O₃–SiO₂/MAO (Cp = cyclopentadienyl; MAO = methylaluminoxane) showed that the yield and selectivity of this reaction depend on the support composition, i.e. on the Al₂O₃ content in the support. The most active catalysts contained Al₂O₃ in a quantity of 50 to 70 wt%. Despite a relatively lower selectivity of 75–59%, the amount of syndiotactic polystyrene in the presence of those catalysts was the greatest. Copyright © 2002 John Wiley & Sons, Ltd.