Free radical initiators, activation volumes

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

Organic peroxides, which readily decompose into free radicals under the effect of thermal energy, are used under high pressures as initiators for radical polymerizations. The measurement of the influence of pressure on the rate of decomposition gives rise to the determination of the activation volume, which, in turn, allows conclusions to be drawn on the decomposition mechanism and the transition state. 

The reaction model assumed is one in which free-radical polymerisation is compartmentalised within a fixed number of reaction loci, all of which have similar volumes. As has been pointed out above, new radicals are generated in the external phase only. No nucleation of new reaction loci occurs as polymerisation proceeds, and the number of loci is not reduced by processes such as particle agglomeration. Radicals enter reaction loci from the external phase at a constant rate (which in certain cases may be zero), and thus the rate of acquisition of radicals by a single locus is kinetic-ally of zero order with respect to the concentration of radicals within the locus. Once a radical enters a reaction locus, it initiates a chain polymerisation reaction which continues until the activity of the radical within the locus is lost. Polymerisation is assumed to occur almost exclusively within the reaction loci, because the solubility of the monomer in the external phase is assumed to be low. The volumes of the reaction loci are presumed not to increase greatly as a consequence of polymerisation. Two classes of mechanism are in general available whereby the activity of radicals can be lost from reaction loci ... 

The over-all rates for free radical polymerizations increase with increasing pressure. This means simply that the pressure-induced retardation of the initiator decomposition rate is more than offset by the increase in the rate of chain propagation and the decrease in the rate of chain termination. This is formally stated in terms of activation volumes in Equation 4 (15)... 

Table 20-7. Activation Volume for Initiator Decomposition and Polymer Propagation in Free Radical Polymerizations...

The solution, having been made stable in the can, must now be made reactive on the coated surface. In the case of unsaturated polyesters, this is done by introducing factor (3) in relatively large quantities. This is an initiator (p. 65). In thermally cured, as opposed to radiation cured finishes, the substance chosen to decompose and produce free radicals is an organic peroxide. A solution of this material in unreactive solvent forms the activator pack of the two-pack finish. The amount of solvent is chosen to give a suitable finish/activator mixing ratio, e.g. 10 1 by volume. The ketone, diacyl and hydroperoxide types are most frequently used. 

To develop their model, Wen and McCormick adopted a number of simplifying assumptions. These are (1) initiation produces two equally reactive radicals, (2) chain transfer reactions are neglected, (3) the rate constants for radicals of different sizes are assumed identical, (4) the propagation rate constant kp, termination rate constant kp and the rate constant for radical trapping kb are all simple functions of free volume as shown below, and (5) there is no excess free volume. The material balance equations for the initiator, the functional group, the active radical, and the trapped radical concentrations... 

---