Ring-growing reactions

However, for a variety of reasons it seems extremely unlikely that the same mechanism is applicable to the polymerisation of cyclic formals and acetals. One reason is that these compounds cannot be co-polymerised with cyclic ethers another is that the polymers are predominantly cyclic, with the number of end-groups far smaller than the number of growing chains. One mechanism which has been proposed and which accounts for most of the observations involves formation of an oxonium ion (X) from the initiator and the monomer, and a subsequent propagation by a ring-expansion reaction (see 13). 

A number of anhydro sugar derivatives have also been polymerized in ring opening reactions catalyzed by Lewis acids (iO, 34,35,67,122-130, 133,134). These reactions in contrast proceed by an activated chain polymerization like that of a vinyl monomer or more closely like that of tetrahydrofuran which proceeds by means of a propagating trialkyl-oxonium ion on the end of the growing chain (/35) (Fig. 6). 

The ring formation reaction is monomolecular, and so, is given by the concentration of growing molecules ... 

Ring-opening and oUier addidon polymerization in which the polymer grows in a manner other dian by a drain reaction... 

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