Anomalous Condensation Polymerizations

The difficulties of devising a basis for conveniently classifying various polymerizations in an appropriate manner have been discussed earlier in this chapter and several borderline examples which offer particular difficulty have been mentioned. One of these, the polymerization of the N-carboxyanhydrides, falls within the definition of a condensation polymerization, proceeds by a mechanism resembling a vinyl addition polymerization, and yields a product which possesses the structure of a typical condensation polymer. Definitions have been 

SO adjusted as to permit the polymerization of a diisocyanate with a glycol or a diamine to be included as a condensation type. A superficially similar polymerization is exemplified by the condensation of a dithiol with a diolefin (preferably nonconjugated), e.g.  

Two further examples illustrate the limitations of the differentiation between condensation and addition polymerizations. One of these is the polymerization of a benzyl halide, e.g. 

In principle, polymers equivalent to those obtained from vinyl and divinyl monomers may be synthesized by this method. The product in the above example possesses the same chain structure as polyethylene. The polymerization process, notwithstanding the likelihood of a metal alkyl intermediate, should conform satisfactorily to stepwise condensation. However, the product, and those obtained by Friedel-Crafts condensation as well, lack the recurrent functional groups which generally characterize condensation polymers. 

The occurrence of the various atypical condensation and addition polymerizations mentioned above does not necessarily undermine the 

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The foregoing proposals by Carothers are also commonly used as the basis of a scheme of classification of polymerization reactions. In this scheme, the processes by which polymers are formed are divided into condensation polymerizations and addition polymerizations. Thus a condensation polymerization leads to a polymer in which the structural unit contains fewer atoms than the monomer whilst an addition polymerization results in a polymer having a structural unit with the same molecular formula as the monomer. A limitation of this classification is that a somewhat anomalous situation arises when polymerization through functional groups (see section 1.4.1) is considered. If there is elimination of a by-product then clearly... 

A sol of hydrous oxide can evolve along a variety of paths, as illustrated in Fig. 3 of Chapter 3 polymers can remain in solution or condense into a polymeric gel polymers can condense into particles that remain stably suspended, or aggregate into a particulate gel, or grow so large that they settle out of suspension. In the next subsection, we examine the factors that contribute to the stability of colloidal suspensions. We shall see that silica is anomalous in this, as in so many of its properties, so that the theory that accounts for most colloidal systems fails for this important material. 

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