Polymers, cyclic formation reaction

A side reaction that occurs with simple dialkylsulfo-nium salts is attack by the counterion on the alkyl group, with formation of RX and a neutral sulfide group on the main chain [path (c). Scheme 12]. This can result in incomplete conversion and moderate amounts of residual sulfur left in the polymer. Cyclic sulfonium groups do not seem to suffer from this side reaction so much (or if they do it must be reversible) and have become standard in the Wessling-Zimmerman synthesis of PPV. 

Complexes in which two metal centres are linked by one or two [NSN] ligands, e.g., [Na(15-crown-5)]2[F5Mo( -NSN)MoF5] and Cp2Zr( -NSN)2ZrCp2, are known. The cyclic zirconium system is prepared by a metathetical reaction (Eq. 7.14). However, the formation of polymers in which metal centres are linked by NSN units has not been achieved. 

During the aqueous hydrolysis of dichlorosilanes there is always a very important side reaction. It is the self-condensation of silanols which are formed initially during the hydrolysis. These reactions also give rise to the formation of cyclic siloxanes together with the linear oligomers or polymers (Reaction Scheme III). The amount of cyclic products usually depends on the hydrolysis conditions and the degree of the self-condensation attained as well as concentration considerations. 

Carbon dioxide 53 55). If excess C02 and polar solvents are used the carboxylation is quantitative and free of side reactions. In nonpolar solvents association phenomena favor ketone formation 55). An alternate way to get re-carboxylic polymers is to react the living sites with a cyclic anhydride 561. 

A common theme in the speculated polymerization reactions discussed in this section is that the polymers were generally uncharacterized or were composed of small cyclic oligomers. No evidence for high molecular weight polymers from low-coordinate phosphorus compounds was obtained. Of course, multiple bond formation, not polymerization, was the focus of these studies. 

---