Prepolymerization Step Research Articles

New optically active poly(amide–imide–urethane) thermoplastic elastomers derived from poly(ethylene glycol diols), 4,4′‐methylene‐bis(4‐phenylisocyanate), and a diacid based on an amino acid by a two‐step method under microwave irradiation

AbstractA new class of optically active poly(amide–imide–urethane)s (PAIUs) was synthesized via a two‐step diisocyanate route under microwave irradiation. In these reactions, 4,4′‐methylene‐bis(4‐phenylisocyanate) was reacted with bis(p‐amido benzoic acid)‐N‐trimellitylimido‐L‐leucine and poly(ethylene glycol diol)s (PEGs), such as PEG‐400, PEG‐600, PEG‐1000, and PEG‐2000, to furnish a series of new PAIUs. The effects of different reaction conditions, such as the method of preparation (polyol or acid chain extension), the prepolymerization step (NCO‐terminated oligoamide or NCO‐terminated polyether polyol), the irradiation time and power, the reaction solvent, the soft‐segment length, and the presence or absence of reaction catalysts (e.g., triethylamine, pyridine, and dibutyltin dilaurate), on the properties of the copolymers, including the solubility, viscosity, and thermal behavior, were investigated. The resulting multiblock copolymers had inherent viscosities of 0.15–0.53 dL/g. These multiblock copolymers were optically active, thermally stable, and soluble in amide‐type solvents. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1781–1792, 2005

Recycling of methylaluminoxane (MAO) cocatalyst in ethylene polymerization with supported metallocene catalyst

The economy of the metallocene catalyst system in olefin polymerization depends more on the cost of methylaluminoxane (MAO) cocatalyst rather than on the catalyst cost since high ratio of cocatalyst to catalyst is required to have sufficient activity. The conditions to minimize the consumption of MAO have been studied for the ethylene polymerization with supported metallocene catalyst. By introducing the prepolymerization step, in which the supported metallocene catalyst is activated at high MAO concentration before polymerization, the MAO could be recovered after the prepolymerization and recycled repeatedly for the subsequent activation with marginal decrease in activity. No extra MAO was needed during the main polymerization. The addition of small amount of MAO or less expensive alkylaluminum at each recycle step kept the catalyst activity to the initial level. It compensates the MAO losses occurring both by the incomplete decantation of MAO solution and by the reaction with metallocene complex or impurities. As a result, the actual consumption ratio of Al/Zr in moles in commercial applications could be reduced to about 30 without sacrificing the activity. This value is significantly low considering that conventionally an Al/Zr ratio of 1,000 is required for sufficient activity.

Modeling of Particle Fragmentation in Heterogeneous Olefin Polymerization Reactions

A model‐based methodology is proposed to describe fragmentation of polymer particles during the very early stages of heterogeneous olefin polymerization reactions. The method is based on the analysis of the particle capacity to release the amount of energy that is accumulated in its interior during the polymerization, due to the fast polymer production. The balance between the rates of energy accumulation and dissipation inside the polymer particle is calculated with the help of a dynamic mathematical model of the prepolymerization reaction. The combination of the presented fragmentation criteria and of the dynamic model may allow one to design the prepolymerization step more precisely and to predict the morphology of the final polymer particles as a function of the initial catalyst particle diameter, reactor temperature, reactor pressure and concentration of active sites. Numerical examples are presented for gas and slurry propylene polymerizations. #An earlier version of this paper was presented at ECOR...

Propene bulk polymerization kinetics: Role of prepolymerization and hydrogen

AbstractAn experimental setup for the polymerization of liquid propylene was used to carry out main polymerizations with and without a prepolymerization step. Two types of prepolymerization are introduced: at a constant temperature and at rapidly increasing reactor temperatures. With the present catalyst system, at high polymerization temperatures, a prepolymerization step will increase polymerization rate. Hydrogen was used to control the molecular weight of the product. The polymerization reactor was used in combination with a gas chromatograph to determine the vapor‐liquid equilibrium (VLE) of the propylene‐hydrogen system. Measurements are compared to predictions of different equations of state. The Peng‐Robinson equation of state is best able to describe the VLE for this binary system. A temperature‐dependent interaction parameter was derived from the fits to the measured data.

Polymerization of liquid propylene with a fourth‐generation Ziegler–Natta catalyst: Influence of temperature, hydrogen, monomer concentration, and prepolymerization method on powder morphology

AbstractLiquid propylene was polymerized in a 5‐L autoclave batch reactor using a commercially available TiCl4/MgCl2/Al(ethyl)3/DCPDMS Ziegler–Natta catalyst, with a phthalate ester as internal electron donor. The powders from these polymerizations were characterized using laser diffraction particle size distribution (PSD) analysis, scanning electron microscopy (SEM), and bulk density measurements. These characteristics were analyzed as a function of the process conditions, including hydrogen and monomer concentration, polymerization temperature, and the prepolymerization method. It was shown that polymerization temperature influences the powder morphology to a large extent. At low temperatures, high‐density particles were obtained, showing regular shaped particle surfaces and low porosities. With increasing temperature, the morphology gradually was transferred into a more open structure, with irregular surfaces and poor replication of the shape of the catalyst particle. When using a prepolymerization step at a relatively low temperature, the morphology obtained was determined by this prepolymerization step and was independent from conditions in main polymerization. The morphology obtained was the same as that observed after a full polymerization at temperature. Even when using a short polymerization at an increasing temperature, the morphology was strongly influenced by the initial conditions. The effect of variation in hydrogen concentration supported the conclusion that the initial polymerization rate determines the powder morphology. In the absence of hydrogen, high bulk densities, and regularly shaped particles were obtained, even at high temperatures. With increasing hydrogen concentration, the reaction rates increased rapidly, and with that changed the morphology. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1421–1435, 2003

Modelagem do Processo de Fragmentação de Catalisadores Suportados Durante a Pré-polimerização de Olefinas

Uma metodologia é proposta para descrever a morfologia das partículas de polímero que são obtidas durante os momentos iniciais da polimerização de olefinas via catálise heterogênea. O método é baseado na análise matemática da capacidade da partícula em liberar a energia mecânica acumulada no seu interior devido à rápida produção de polímero. O balanço entre as quantidades de energia acumulada e liberada é calculado com o auxílio de um modelo dinâmico da reação de pré-polimerização. A combinação da metodologia proposta com o modelo dinâmico permitiu a análise dos mecanismos de fragmentação, indicando a morfologia da partícula de polímero produzida em função do tamanho da partícula e da temperatura do reator.

Acrylonitrile–divinylbenzene copolymer beads: influence of pre-polymerization step, stirring conditions and polymerization initiator type on the polymer particle characteristics

Cross-linked acrylonitrile–divinylbenzene copolymer beads have been synthesized by suspension polymerization in presence of an inert diluent (toluene). In this study a set of experiments has been performed to evaluate the physical features of the beads. The influence of the pre-polymerization step, process stirring conditions and type of polymerization initiator on the polymer particle size distribution, apparent density and visual appearance by optical microscopy was investigated.

Heat and mass transfer phenomena during the early growth of a catalyst particle in gas-phase olefin polymerization : the effect of prepolymerization temperature and time

A comprehensive mathematical particle growth model, accounting for both external and internal mass and heat transfer limitations appearing during the early growth of highly active Ziegler–Natta catalysts in gas-phase olefin polymerizations, is developed. Based on the well-known “Polymeric Flow Model” approximation for particle growth [see Schmeal and Street (A.I.Ch.E. Journal 17 (1971) 1188) and Galvan and Tirrell (Chemical Engineering Science 41 (1986) 2385)], dynamic mass and energy balances, including both diffusional and convective mass and heat fluxes, are derived to describe the spatial-time evolution of monomer concentration and temperature in a growing polymer particle. To calculate the equilibrium monomer concentration in the amorphous polymer phase, the Sanchez–Lacombe (Macromolecules 11 (1978) 1145) equation of state is employed. The effect of effective monomer diffusivity, initial catalyst size and catalyst activity on the overall particle growth rate and particle overheating are analyzed in detail. It is shown that, during the early growth of highly active catalyst particles (e.g., polymer production rates >15,000 g/ g cat h ), internal and external mass and heat transfer resistances can become significant, leading to catastrophic particle burn. In an attempt to reduce overheating of highly active catalyst particles, a prepolymerization step is introduced to control catalyst activity and, thus, particle morphology during the early growth of catalyst particles. The effect of prepolymerization time and temperature on the overall polymerization rate and particle overheating is investigated. It is demonstrated that by proper selection of prepolymerization temperature and time, particle overheating can significantly be reduced while the polymerization rate is enhanced.

Development of a refined poly(propylene) growth model for silica supported metallocene catalyst systems

The poly(propylene) growth process on silica supported metallocene catalysts has been examined by means of electron microscopy and kinetic investigations. It was possible to extend the polymerization process and to identify different polymerization phases with different characteristic kinetic data by lowering the reaction temperature in the slurry phase. EDX analysis of microtome sections of a polymerized model system led to the development of a refined polymer growth model. It was found that the addition of activating hydrogen reduces the initial diffusion limitations as well as a prepolymerization step with 1-octene in the bulk phase.

Optimization of the pre-polymerization step of polyethylene terephthalate (PET) production in a semi-batch reactor

The time minimization (free-end-time problem) of the transesterification reaction in a semi-batch reactor was performed using successive quadratic programming (SQP) and the cubic spline interpolation method, and the numerical optimization result was verified by experiment. The process constraints of the semi-batch reactor are the rate of temperature increase, the amount of ethylene glycol (EG) in the vapour phase (as an indicator of the flooding of dimethyl terephthalate (DMT) and the concentration of side products (especially diethylene glycol (DEG)). The proposed optimal temperature trajectory was applied to the industrial process and reduced the reaction time of the transesterification from 150 to 110 min (about a 25% reduction), while satisfying the process constraints, such as the upper limit of the by-product concentration, the rate of temperature increase and the flooding.

New coating materials prepared by radiation‐induced polymerization. I. Mar‐resistant coating composition and properties

AbstractA mar‐resistant coating system was obtained by screening combinations of silicone and vinyl compounds. It was found that binary systems, such as the γ‐glycidoxypropyltrimethoxysilane–glycidyl methacrylate system, were excellent not only in mar resistance but also in adhesion to base resin polymers. Haze values of these coatings, after the sand‐falling test, reached less than 10%. The coating process consisted of three steps: the preparation of prepolymer, the coating to plastic surface, and the curing of the coating by heating. It was found that the prepolymerization step could be performed by irradiation very conveniently; the control of viscosity for coating application and thickness control could be possible without any gel formation. These coatings were also good in weather resistance due perhaps to the good adhesion to base resin and could be applicable to plastic spectacles, glazing materials, and a variety of other products.

The production of rubber‐modified polystyrene. Some criteria influencing the selection and design of the agitator system

AbstractThe importance of selecting the most suitable form of agitation for the prepolymerization step in the production of rubber‐modified polystyrene by a bulk polymerization procedure is related to the non‐Newtonian behavior of the polymerizing system and to the existence of a minimum shear rate requirement. The procedure for the design and calibration of a small‐scale reactor system is described which is shown to produce an adequate phase inversion of the prepolymer and from which data can be evaluated for any change in scale of the operation. Structural patterns in the final polymer are linked with average shear levels in the reactor.