Gaseous monomers

Polymerization in Hquid monomer was pioneered by RexaH Dmg and Chemical and Phillips Petroleum (United States). In the RexaH process, Hquid propylene is polymerized in a stirred reactor to form a polymer slurry. This suspension is transferred to a cyclone to separate the polymer from gaseous monomer under atmospheric pressure. The gaseous monomer is then compressed, condensed, and recycled to the polymerizer (123). In the Phillips process, polymerization occurs in loop reactors, increasing the ratio of available heat-transfer surface to reactor volume (124). In both of these processes, high catalyst residues necessitate post-reactor treatment of the polymer. 

Although these polymers have iaadequate stabiUty and processibiUty for most plastics appHcations, the abiUty to undergo scission back to the gaseous monomers has afforded some utility ia fabrication of electron-beam resists for photoUthography. Polybutene sulfone (251) and polyhexene sulfone (252) have been developed for this small-volume but high value appHcation. 

Gaseous monomers may also be trapped within the processing equipment and accidents have occurred as a consequence of the resulting pressure buildup. In the case of the polyacetals and poly(vinyl chloride) it is reported that at elevated temperatures these materials form a more or less explosive combination so that it is important to separate these materials rigorously at the processing stage. 

Of these polymers, poly(ethylene) is produced from a gaseous monomer under pressure, either high or low, and thus some of the disadvantages... 

Gaseous monomers can polymerize in the gas phase in the presence of a fluidized catalyst bed. As polymer forms, hot gas forces the newly made material out of the reactor to a collector. Figure 2.15 shows a simplified schematic diagram of a generic polymerization reactor. 

Recent thermochemical data for SiCl2 have been reported by Schafer and co-workers91 a and by Teichmann and Wolf 91b from transpiration studies of the SiCl4/Si system. The heat of formation of the gaseous monomer now seems well established ... 

The polymerization of the gaseous monomer actually takes place in the polymer droplets, not in the gas phase ... 

Gaseous monomer of ethane was purchased from the Matheson Gas Co.. The plasma polymerized ethane (PPE) was deposited on aluminum foil set on the discharge electrode throughout this work and the... 

The effect of pressure on polymerization, although not extensively studied, is important from the practical viewpoint since several monomers are polymerized at pressures above atmospheric. Pressure affects polymerization through changes in concentrations, rate constants, and equilibrium constants [Ogo, 1984 Weale, 1974 Zutty and Burkhart, 1962], The commercial polymerizations of most gaseous monomers (e.g., vinyl chloride, vinylidene chloride, tetrafluoroethylene, vinyl fluoride) are carried out at very moderate pressures of about 5-10 MPa (1 MPa = 145 psi), where the primary effect is one of increased... 

The usual procedures of fractional, azeotropic, or extractive distillation under inert gases, crystallization, sublimation, and column chromatography, must be carried out very carefully. For liquid, water-insoluble monomers (e.g., styrene, Example 3-1), it is recommended that phenols or amines which may be present as stabilizers, should first be removed by shaking with dilute alkali or acid, respectively the relatively high volatility of many of these kinds of stabilizers often makes it difficult to achieve their complete removal by distillation. Gaseous monomers (e.g., lower olefins, butadiene, ethylene oxide) can be purified and stored over molecular sieves in order to remove, for example, water or CO2. 

For reactions in which only a slight pressure rise is expected (e.g., emulsion polymerization of gaseous monomers), thick-walled pressure bottles which are carefully placed behind a protective shield, can be used. For higher pressures or large-scale experiments, autoclaves are always to be preferred. 

Compounds that arc formally analogous to carbon compounds arc found to have quite different structures. Thus carbon dioxide is a gaseous monomer, hot silicon dioxide is an infinite single bonded polymer. In a similar manner, /ym-diok arc unstable relative to ketones ... 

---