Propagation polymerization

I Propagation Polymerization occurs when the carbon radical formed in the initiation step adds to another ethylene molecule to yield another radical. 

Although a free ionic end-group, —CH2CH(Ph), capable of propagating polymerization is formed on opening the ring, the resulting dipolar species does not contribute to electric conductance. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

While the latter measurements, strictly speaking, evaluate the state of dissociation of the initiator ions only, they also provide at least a guide as to the degree of dissociation of the propagating polymeric ion pairs. In the case of vinyl polymerisations, where no living cations have been observed to date, direct evaluation of the dissociation constant Kd, of the growing ion pair is not possible. However, in a number of cyclic monomer polymerisations living characteristics are observed, and direct measurements have been possible (27). 

Figure 6. Temperature and monomer profiles in the uniformly propagating polymerization wave obtained analytically in the step-function model and numerically in the full model. Adapted from [31].

As was the case with gasless combustion, the uniformly propagating polymerization wave may become unstable as parameters are varied. We perform a linear stability analysis of the uniformly propagating polymerization wave by employing the reaction front approximation. 

Frontal polymerization, the process of propagation of a polymerization wave, is important from both fundamental and applied viewpoints. In this chapter, we reviewed theoretical results on the base model of free-radical frontal polymerization. Based on the analogy of the gasless combustion model and using the methods developed in combustion theory, we determined uniformly propagating polymerization waves and discussed their linear and nonlinear stability. 

J. Szalay, I. Nagy, I. Barkai, and M. Zsuga, Conductive composites prepared via a propagating polymerization front. Die Ang. Makr. Chem., 236 (1996), pp. 97-109. 

Figure 1. Schematic diagram showing changes in properties across a propagating polymerization front. (Courtesy of Paul Ronney.)...

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