Rate of depropagation

Increasing the temperature may also increase the rate of depropagation resulting in equilibrium monomer concentrations well above an acceptable residual monomer concentration (Sawada (1976)). It seems there exists an optimum temperature of polymerization that will reduce the time of polymerzation but will avoid the problems of depolymetization and of initiator depletion. 

There must therefore exist some conditions where the rate of propagation is just equal to the rate of depropagation, i. e. where AG = 0. The temperature at which the rates of reaction (2) are equal in both directions is of special importance for polymerizations. 

The rate of depropagation is substantially lower than propagation. 

Steric factors. It follows from Equation 1 and the known values of Ep and AH that 2 is smaller by about 5 kcal. per mole for poly (methyl methacrylate) than for polystyrene. This is caused by the difference in aH which reflects steric effects. At 600° K., it means a factor of about 60 in the rates of depropagation, assuming the entropies of activation to be equal. 

In what follows it is assumed that the rates of propagation depend only on the nature of the added monomer, while the rates of depropagation in which monomer X is expelled and a new. ..M+ ending is left, viz.,... 

Temperature, in terms of temperature, the first consideration in carbocationic polymerization—as in any enthalpy-driven chain reaction—is the ceiling temperature T. is the limiting temperature above which the rate of depropagation exceeds the rate of propagation at a given monomer concentration ... 

The rate of propagation and the rate of depropagation reactions are assumed to obey the first-order kinetic rate. The rate constants, and obey Xht Arrhenius relation-... 

At equilibrium, the rate of propagation is equal to the rate of depropagation. Thus ... 

Thermal degradation of poly(caprolactone) at 220 0 was recently investigated (23). It proceeds by a zipping-off mechanism yielding monomeric caprolactone without change in reduced viscosity up to a weight loss of 53%. This means that the rate of random dissociation is small as compared to the rate of depropagation and an active species once formed will disappear immediately. 

Since Ed =Ep + AH, the depolymerization increases more rapidly with increasing temperature. At the ceiling temperature Tc, the rate of depropagation becomes equal to that of propagation, regardless of the variation of [Af ] with increasing temperature... 

Since is usually about 21 kJ/mol (5 kcal/mol), the value of Ei is largely determined by that of A//p. The rate of depropagation, ki, is higher at lower Ei, that is, the lower the heat of polymerization. 

---