Five-membered ring formation solvent effect

Thus, the solvent effect on five-membered ring formation could be reasonably understood because in polar media the polarization of carbonyl groups would be higher and consequently, the electrostatic repulsion between cyclic anhydride units would be enhanced. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-te(raphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic five- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere . ... 

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