7 июля, 2015 
Malyar It is essential to remove the large quantities of heat which are liberated during polymerisation. It is therefore imperative that reaction conditions favour a uniform conversion of monomer to polymer in order that the heat of reaction is dissipated in a controlled manner.
The majority of commercial polymerisations employ a continuous addition or “drip feed” technique in which the monomer and initiator are added to the reaction vessel at a rate contiguous with the rate of conversion of monomer to polymer. In this way a build up of unreacted monomer is prevented. The quantity of unreacted monomer present is carefully monitored and corrective action taken should this deviate from an acceptable level.
The unreacted monomer can be quantified by measurement using analytical instruments (e. g. a gas-liquid chromatograph) or it may be deduced by determination of the non-volatile content. Often a reference curve is’Employed which depicts the acceptable lower limit of the non-volatile content during the polymerisation.
Examples of such theoretical curves and those typically obtained in practice are shown below.
(i) “All-in” Process
All the monomer, solvent and initiator are charged together and the mixture brought to reaction temperature at time to
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TIME ———- ► Figure 1-66 If reaction was instantaneous the curve representing the non-volatile content would be a vertical line at to. The steepest part of the curve, that between tb and t2 represents the period when conversion of polymer to monomer is at its height. This is the time period during which most of the heat of reaction has to be removed to retain a constant reaction temperature. It can be seen by comparison of the two curves that the cooling required to retain a constant temperature will be less in practice than that required if the calculation were based on the theoretical curve. |
Large quantities of free monomer are present during a considerable part of the reaction period. When the reaction mixture is composed of several different monomer species with different reactivity ratios, the composition of the copolymer formed will vary throughout the reaction.
The ratio of monomer to initiator will change during the reaction as they are consumed at different rates. Thus one would expect a wide molecular weight distribution to result from the “all-in” process.
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