【专业英语】5-Process_Conditions_of_Step_Polymerization_and_interfacial_polymerization.pdf

Process Conditions of Step Polymerization and interfacial polymerization Physical Nature of Polymerization Systems Several considerations are common to all processes for step polymerizations in order to achieve high molecular weights. One needs to employ a reaction with an absence or at least a minimum of side reactions, which would limit high conversions. Polymerizations are carried out at high concentrations to minimize cyclization and maximize the reaction rate. Highpurity reactants in stoichiometric or near- stoichiometric amounts are required. The molecular weight is controlled by the presence of controlled amounts of monofunctional reagents or an excess of one of the bifunctional reagents. Equilibrium considerations are also of prime importance. Since many step polymerizations are equilibrium reactions, appropriate means must be employed to displace the equilibrium in the direction of the polymer product. Distillation of water or other small molecule products from the reaction mixture by suitable reaction temperatures and reduced pressures are often employed for this purpose. Table 2-8 shows values of some kinetic and thermodynamic characteristics of typical step polymerizatiosn. These data have implications on the temperature at which polymerization is carried out. Most step polymerizations proceed at relatively slow rates at ordinary temperatures. High temperatures in the range of 150–200 °C and higher are frequently used to obtain reasonable polymerization rates. Table 2-8 shows that the rate constants are not large even at these temperatures. Typical rate constants are of the order of 10-3 L mol-1 s-1. There are a few exceptions of step polymerizations with significantly larger k values, for e