Suspension polymerization temperature-viscosity

The suspension polymerization process has a number of distinct advantages over competitive processes. It allows excellent control over the polymerization temperature and a lower viscosity reaction medium. Furthermore,... 

However, the probability for the reaction progression greatly depends on the monomer conversion. Because the viscosity of the dispersed phase, in the first stage, is fairly low and the quantity of styrene is sufficiently high, the decomposition process (Figure 9.4) occurs only up to the benzoyloxy radical, which can directly start the kinetic chain. The purely thermal start of chains with reactive dimers of styrene, as a result of Diels-Alder reaction, can be ignored at fairly low temperatures of suspension polymerization, in contrast to the conditions for the bulk styrene process [4-7]. 

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. 

A mathematical model for styrene polymerization, based on free-radical kinetics, accounts for changes in termination coefficient with increasing conversion by an empirical function of viscosity at the polymerization temperature. Solution of the differential equations results in an expression that calculates the weight fraction of polymer of selected chain lengths. Conversions, and number, weight, and Z molecular-weight averages are also predicted as a function of time. The model was tested on peroxide-initiated suspension polymerizations and also on batch and continuous thermally initiated bulk polymerizations. 

However, problems relevant to the heat removal and temperature control, as well as to the extremely high viscosity of the polymerization system, established the suspension polymerization process much more attractive for PVC production. It is indicative that only 8% of the total PVC production is obtained by the bulk polymerization process. 

It is also possible to carry out the polymerization at 0 °C under the vapor pressure of the monomer in water with methylcellulose as surfactant [533]. High monomer conversion of 95 to 98% was observed. The intrinsic viscosity of 2.26 dL/g (in DMF) corresponds to a molar mass of about 10 g/mol. By modifying the reaction conditions (temperature and pressure) the molar mass could be controlled and p varied between 0.5 and 5 dL/g. Bis(t-butyl) hyponitrite and bis(a,a-dimethylbenzyl) hyponitrite can also act as initiators however, the conversion to polymer is low [538]. Often higher melting point, and higher crystallinity was observed than for suspension-polymerized PVF2 [544]. 

The autoacceleration effect (Trommsdorf effect) is less pronounced in solution polymerization than in bulk or suspension polymerization due to lower viscosity of the polymerizing solution. To prevent a thermal runaway reaction, the reactants are often added gradually to the reactor. The polymer molecular weight is controlled through the use of a chain transfer agent and by initiator concentration and type. Monomer concentration, solvent type, and reaction temperature also affect the molecular weight. 

Suspension polymerization is a polymerization process in which dispersed monomer droplets are stabilized by surfactant and mechanical agitation in a liquid phase such as water. As in emulsion polymerization, the monomers used for suspension polymerization are usually insoluble in water. Advantages of suspension polymerization compared with other polymerization methods include easy removal of reaction heat, effective temperature control, low viscosity of the reaetion system, simple polymerization mechanism, high purity of the produet, easy separation and purifieation of the produet and shaped product in particle form. Polymers prepared by suspension polymerization inelude... 

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