Process monomer/polymer concentration

This task is a standard part of the manufacturing process with polymer concentrations between 10 and 85%. Depending on the manufacturing process, the solvent (in the case of solution polymerization) or monomer (in the case of bulk polymerization) must be removed from the polymer, which may not contain more than the legally admissible or market-dictated residual components at the end of the process. 

To maintain a high polymerization rate at high conversions, reduce the residual amount of the monomer, and eliminate the adverse process of polyacrylamide structurization, polymerization is carried out in the adiabatic mode. An increase in temperature in the reaction mixture due to the heat evolved in the process of polymerization is conductive to a reduction of the system viscosity even though the polymer concentration in it rises. In this case, the increase in flexibility and mobility of macromolecules shifts the start of the oncoming gel effect into the range of deep transformation or eliminates it completely. 

Lack of termination in a polymerization process has another important consequence. Propagation is represented by the reaction Pn+M -> Pn+1 and the principle of microscopic reversibility demands that the reverse reaction should also proceed, i.e., Pn+1 -> Pn+M. Since there is no termination, the system must eventually attain an equilibrium state in which the equilibrium concentration of the monomer is given by the equation Pn- -M Pn+1 Hence the equilibrium constant, and all other thermodynamic functions characterizing the system monomer-polymer, are determined by simple measurements of the equilibrium concentration of monomer at various temperatures. 

Most small olefins produced in the chemical industry are used to make polymers, with a global production of the order of 100 million tons per year. Polymers are macromolecules with molecular weights of typically lO" to 10 and consist of linear or branched chains, or networks built up from small monomers such as ethylene, propylene, vinyl chloride, styrene, etc. The vast majority of polymers are made in catalytic processes. Here we concentrate on ethylene polymerization over chromium catalysts as an example [M.P. McDaniel, Adv. Catal. 33 (1985) 47]. 

Because the onset of monomer-polymer equilibrium can occur before the filaments achieve their own equilibrium concentration behavior, these filaments will undergo polymer length redistribution. This is a slow process in vitro that in many respects resembles crystallization (See Ostwald Ripening). 

For negative AHP and ASP, (most commonly the case encountered in addition polymerization) AFP becomes positive above a certain critical temperature, Te = A HpjA Sp, known as the ceiling temperature" of the system. The value of Te depends on the concentrations of the monomer and of the polymer as well as on the nature of the solvent, if the latter is present in the system. Of course, the high polymer cannot be formed above the ceiling temperature. For any monomer-polymer system, the process which converts pure liquid monomer into a crystalline polymer has the maximum ceiling temperature. 

Free radical polymerization may be carried out in various media. Bulk polymerization is the simplest, but while the reactants (monomers) are most often liquid, the product (polymer) is solid. This leads to problems when removing the polymer from the reactor. In addition, since most free radical polymerizations are highly exothermic, the high viscosity of the monomer/polymer mix inhibits the removal of the heat of reaction. Solution polymerization will reduce, to some extent, the viscosity of the polymerizing mass, but it brings with it the environmental and health issues of organic solvents. In addition, the solvent reduces the monomer concentration, and hence the rate of polymerization. Finally, recovery and recycling of the solvent can add substantially to the cost of the process. Nevertheless, solution polymerization of vinylic monomers is used in a number of commercial processes. 

Both the monomer and polymer are soluble in the solvent in these reactions. Fairly high polymer concentrations can be obtained by judicious choice of solvent. Solution processes are used in the production of c(5-polybutadiene with butyl lithium catalyst in hexane solvent (Section 9.2.7). The cationic polymerization of isobutene in methyl chloride (Section 9.4.4) is initiated as a homogeneous reaction, but the polymer precipitates as it is formed. Diluents are necessary in these reactions to control the ionic polymerizations. Their use is avoided where possible in free-radical chain growth or in step-growth polymerizations because of the added costs involved in handling and recovering the solvents. 

In the above-mentioned cases of polymerization of lower alkyf acrylates, because of the high rate of the process = 1260 dm mol sec for MA (Bagdasar yan, 1966) and the high monomer concentration in particles (Gerrens, 1964), the surface of the monomer-polymer phase increases at a rate that may exceed the rate of attainment of equilibrium adsoiption. 

Polymerization can be catalytic or noncatalytic, and can be homogeneously or heterogeneously catalyzed. Polymers that form from the liquid phase may remain dissolved in the remaining monomer or solvent, or they may precipitate. Sometimes beads are formed and remain in suspension sometimes emulsions form. In some processes solid polymers precipitate from a fluidized gas phase. Polymerization processes are also characterized by extremes in temperature, viscosity, and reaction times. For instance, many industrial polymers are mixtures with molecular weights of 104 to 107. In polymerization of styrene the viscosity increased by a factor of 106 as conversion increased from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1800 K (3240°R). Initiators of the chain reactions have concentration as low as 10-8 g-moFL, so they are highly sensitive to small concentrations of poisons and impurities. 

Chemical polymerization onto sulfonated (dopant-containing) synthetic polymers has also been described.45 Sulfonated polyethylene-polystyrene was exposed to monomer and then the oxidant. A mixture of Fe11 and Fem led to more accurate control of the E° value of solution. These same workers also described a novel chem-ical/electrochemical method, in which pyrrole was initially polymerized using a low concentration of Fe111. The reduced Fem could then be reoxidized electrochemically to regenerate the oxidant. Using this chemical/electrochemical process, composite polymers with conductivities as high at 35 S cm-1 were obtained. 

Depending on such factors as the relative nucleophilicity of heteroatoms in the monomer and in the polymer, the flexibility of the chain (leading to the larger or smaller conformational assistance-neighbouring group participation) the relative rates of both processes may be different. If the rate of cyclization is comparable to the rate of propagation (like in oxetane polymerization) considerable amounts of cyclic oligomers (compared to final equilibrium concentration) are formed within the time needed to attain complete monomer conversion (cf. Sect. 5.3.5). If, however, the rate of cyclization is low (e.g. THF), macrocycles will still form slowly after the monomer-polymer equilibrium has been established. 

Hash devolatilization is a simple and effective method to remove the majority of solvent and unreacted monomers from the polymer solution. Product from the reactor is charged to a flash vessel and throttled to vacuum conditions whereby the volatile solvent and monomers are recovered and condensed. In the process, the polymer melt cools, sometimes considerably, due to the evaporation of volatiles. The polymer product is pumped from the bottom of the flash vessel with a gear pump or other suitable pump for viscous materials. Critical to operation of the flash devolatilization unit is prevention of air back into the unit that reduces stripping ability and potentially allows oxygen into the unit that can discolor products or pose a safety hazard if low autoignition temperature solvents are used. Often one flash devolatilization unit is insufficient to reduce the residual material to a sufficient level and thus additional units can be added in series [61]. In each vessel, the equilibrium concentration of volatile material in the polymer melt, is a function of the pressure and temperature the flash unit operates at, with consideration for the polymer solvent interaction effects described by the Hory-Huggins equation. Flash devolatilization units, while simple to operate, may be prone to foam development as the superheated volatiles rapidly escape from the polymer melt. Viscous polymers or polymers with mixed functionalities... 

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