Prepolymerized catalysts

A technique called "prepolymerization" is practiced with selected psd controlled catalysts used in slurry and gas phase processes. The catalyst is suspended in a suitable solvent (usually a C -C alkane) and exposed to cocatalyst, ethylene, and, optionally, comonomer and hydrogen under very mild conditions in a separate, smaller reactor (11). Prepolymerization is allowed to proceed until the original catalyst comprises 5-30% of the total weight of the composition. Prepolymerized catalysts have limited storage stability and are ordinarily introduced without delay to the large-scale reactor. Prepolymerization provides several advantages ... 

A prepolymerization with a small portion of butadiene monomer in the presence of only trace amounts of water is required. A hydrocarbon solution of this prepolymerized catalyst mixture is subsequently dispersed in water, together with further butadiene, and polymerization proceeds. It has been speculated that the catalytically active species is shielded from water by the polymer formed during the prepolymerization [88]. 

Figure 10. Propene polymerization profiles of a silica-supported metallocene/MAO catalyst prepared by suspension impregnation (a) depending on polymerization time and polymerization temperature and (b) depending on particle size and polymerization time, (c) Comparison of the activity profiles between an 1-octene prepolymerized catalyst and an untreated system, (d) Comparison of the activity profiles between a catalyst system employing 2 vol % hydrogen and the not activated system.

By far the most commonly used PP-process is Montell s Spheripol Process. The first reaction stage consists of one or two tubular loop reactors where bulk polymerization of homopolymers, random and terpol5nners is carried out in liquid propylene. The prepolymerized catalyst, liquid propylene, hydrogen for controlling molecular weight and eventually comonomers are continuously fed into the reactor in which polymerization takes place at temperatures of 60-80°C and pressures of 35-40 bar. The tubular configuration enables a perfect heat transfer and control of the reaction temperature. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Process flow for a typical batch-mass polystyrene process(1) is shown in Figure 1. Styrene monomer is charged to the low conversion prepolymerization reactor with catalyst and other additives, and the temperature is increased stepwise until the desired conversion is reached. It is then transferred into the press. Polycycles are 6 to 14 hours in the low conversion reactor, and 16 to 24 hours in the press. At completion, the cakes are then cooled with water and removed from the press to be ground and then (usually) extruded into pellets. 

The prepolymerization reactor for HIPS is similar ( ). A solution of rubber and styrene monomer is charged to the reactor along with catalysts, antioxidants, and other additives, and the temperature program is carried out until the desired conversion is reached. This is usually close to the point where increasing viscosity seriously limits mixing and temperature control. 

Description Catalyst components are mixed and fed directly to prepolymerization (1) with a light inert hydrocarbon, where a first bulk polymerization occurs under mild controlled conditions. This step exploits the catalyst system potential in terms of morphology, mileage and complete reliability in the following gas-phase reaction sections. 

The catalyst is mixed with propane diluent and fed into the compact prepolymerization reactor. Cocatalyst, ethylene, comonomer and hydrogen are also fed into this reactor. The pre-polymerized slurry is then fed into a second larger slurry loop reactor, which is designed for supercritical conditions and is typically operated at 75°C—100°C... 

Borealis Borstar HDPE, MDPE, LLDPE slurry loop and gas phase in series Ziegler-Natta Catalyst prepolymerized... 

The Borstar process employs a small loop prepolymerization reactor (see section 3.6 for a discussion of the advantages of prepolymerization). Ziegler-Natta catalysts and triethylaluminum cocatalyst are commonly used but the process is capable of using single site catalysts (15). 

Very recently, an aqueous olefin polymerization using an early transition metal catalyst has also been reported [84]. A toluene solution of styrene is prepolymerized briefly by a catalyst prepared by combination of [(CsMesjTifOMe),] with a borate and an aluminum-alkyl as activators. The reaction mixture is then emulsified in water, where further polymerization occurs to form syndiotactic polystyrene stereoselectively. It is assumed that the catalyst is contained in emulsified droplets and is thus protected from water, with the formation of crystalline polymer enhancing this effect. Cationic or neutral surfactants were found to be suitable, whereas anionic surfactants deactivated the catalyst. The crystalline polystyrene formed was reported to precipitate from the reaction mixture as relatively large particles (500 pm). 

In this polyurethane-prepolymerized isocyanate reaction, the fastest catalyst are the acetylacetonate complexes. Looking at the molecular structures involved, an octoate is a 2-ethylhexoate, having the structure... 

Alkene polymerization in an aqueous system using an early transition metal catalyst has also been reported [27]. A toluene solution of styrene is prepolymerized briefly by a catalyst prepared by combination of [(C5Me5)Ti(OMe)3] with a borate and an aluminum alkyl as activators. The reaction mixture is then emulsified in... 

Increasing the polymerization temperature from 40 to 48 or 57 °C, respectively, leads to a rise of the total activity of the catalyst system and a shift of the maximum activity to shorter reaction times (Figure 10a). At the same time the induction period shortens, nevertheless retaining the prepolymerization maximum. Therefore, an increase in temperature yields a more active catalysts system for propene polymerization. This gain in activity leads to an increase of the velocity with which the individual stages of polymerization are passed. 

As an alternative to prepolymerizing with a gaseous monomer such as ethylene, Brant added hydrocarbon-soluble poly(4-methyl)styrene or low molecular weight waxes to the supported catalyst to fill the pores of the support.Using a pore-filling... 

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