Active centres and polymer

When the active centre is surrounded by a layer of solid polymer, further propagation will be controlled by the rate of monomer diffusion through the polymer layer. Usually it will be retarded. With a porous polymer layer surrounding the active centres, monomer transport will be easier. These effects must be considered when highly crystalline polymers are formed, especially when the chains grow from a non-transferring monomer as, for example, with coordination polymerizations [56]. 

Another example of the interference of a propagating active centre with solid polymer is the occlusion of a centre at the surface of a solid particle [57-59]. This is treated in Chap. 6, Sect. 1.3. 

Even with reactions of a non-radical active centre, the generated polymer is not always inert. Carbanions react with —C=N and —COOR sub-stitutents, carboxonium ions produce less acid centres by reaction with an ether-type chain (see Chap. 4, Sect. 2.3), carbocations alkylate aromatic groups, etc. All these reactions affect propagation. Sometimes the physical effect of the generated insoluble polymer is combined with its ability to react chemically in a certain way. 

We have observed a reduction in the rate of trioxane polymerization initiated by the siloxonium dication (di-ion pair) 

The precipitating polyformaldehyde deactivated the centres by formation of the oxonium complex 

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