Heat Effects during Polymerization

Each of a number of polymerization reactions may be carried out in many different ways. Each reaction carried out in a particular way is accompanied by a particular heat effect. Tabulation of all possible heat effects for all possible polymerization reactions is a herculean task. The heat effects for a desired polymerization reaction can be calcnlated from data for reactions carried out in a standard way. When heat of polymerization is available at one temperature and pressure, it can be calculated for another temperature and pressure by extrapolation. The Clapeyron equation can be derived as follows to aid this process. 

From Appendix A, Equation (A. 18) for any process that obeys the first and second law of thermodynamics  

When the monomer and polymer are in equilibrium with each other. 

Most industrial polymerization reactors are operated at constant tanperature and constant pressure, and sometimes controlled using computer data acquisition and sensors. So, Equation (12.6) may be well applicable for these systems. Combining Equation (12.6) with Equation (12.5)  

Equation (12.7) is the Clapeyron equation. It can be used to calculate the enthalpy change of reactions at a desired tanperature and pressure when the enthalpy of the reaction at a standard state is known or some temperature and pressure is known. This can be accomplished by integrating Equation (12.7) as 

---

Muller (1959) and co-workers have made many interesting and important studies of heat effects during the mechanical stretching and relaxing of polymeric fibers. However, Dole (1959) in reviewing calculations of the enthalpy differences between drawn and undrawn fibers and annealed samples as computed from specific heat-temperature data could find no differences that could not be explained on the basis of the extent of crystallinity of the samples. 

Calorimetry is a instrumental method based on the recording of thermal effects (heat evolution) during polymerization. This method makes it possible to follow continuously the course of the process with time and in a variable temperature field, and to record other phenomena (e.g. phase transitions) occurring in the reaction system. It is used both for the study of the process in the field of ionizing radiation and for the investigation of postpolymerization. 

It is generally believed that a control of the reactant mixing, and of the heat transfer during polymerization, will not only result in a reduced polydispersity of the polymer sample but also provide a degree of predictability over the polymerization reaction. Unfortunately, however, because it is very difficult to effect such control - whether in large-scale industrial reactors or small-scale laboratory equipment - this represents the main challenge to be encountered when scaling-up a reaction process. 

Chemical Reactivity - Reactivity with Water Dissolves with mild heat effect Reactivity with Common Materials Corrosive to copper and galvanized surfaces Stability During Transport Stable Neutralizing Agents for Acids and Caustics Dilute with water Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

A typical example illustrating the separation of the net heat effect AT into parts corresponding to polymerization ATP and crystallization ATC is shown in Fig. 2.25.100 These data were obtained by direct measurement of the quantities of polymeric products formed during the reaction. This gives us the value of ATP calculated from(3(t) and the known value of the maximum temperature increase dur-... 

Figure 2.26. Heat effects, observed during anionic activated polymerization of E-caprolactam at 190°C (a) and 160°C (b). Curves 1 and 2 are components related to crystallization and polymerization, respectively. Curves 3 and 4 are calculated from Eq. (2.36) and from a simple additive rule, respectively.

Homogenous lactam polymerizations usually proceed with a decrease of volume. At atmospheric pressure the value of the pAV term is negligible and the heat of polymerization can be considered as a measure of the difference in internal energy between the linear and cyclic monomer unit. At very high pressures, however, the effect of volume contraction during polymerization on the monomer-polymer equilibrium cannot be neg-... 

Polymerizations as part of liquid-phase organic reactions are also influenced by mass and heat transfer and residence time distribuhon [37, 48]. This was first shown with largely heat-releasing radical polymerizations such as for butyl acrylate (evident already at dilute concentration) [49]. Here, a clear influence of microreactor operation on the polydispersity index was determined. Issues of mass transfer and residence time distribution in particular come into play when the soluhon becomes much more viscous during the reachon. Polymerizahons change viscosities by orders of magnitude when carried out at high concentration or even in the bulk. The heat released is then even more of an issue, since tremendous hot spots may arise locally and lead to thermal runaway, known in polymer science as the Norrish-Tromsdorff effect. 

M. Vrkljan, T. M. Foster, M. EPowers, J. Henkin, W. R. Porter, H. Staack, J. F. Carpenter, andM. C. Manning, Thermal stability of low molecular weight urokinase during heat treatment, n. Effect of polymeric additives, Pharm. Res. 11, 1004-1008(1994). 

During polymerization with a CSTR, the monomer and the other components of the polymerization recipe are fed continuously into the reactor while the polymerization product mixture is continually withdrawn from the reactor. The application of the CSTR in suitable polymerization processes reduces, to some extent, the heat removal problems encountered in batch and tubular reactors due to the cooling effect from the addition of cold feed and the removal of the heat of reaction with the effluent. Even though the supporting equipment requirements may be relatively substantial, continuous stirred tank reactors are economically attractive for industrial production and consistent product quality. 

---