Lactones cyclic oligomers formation

The most effective initiators for anionic polymerization are alkali metals, alkali metal oxides (lithium, sodium and potassium alkoxides) and alkah metal naphtha-lenide complexes with crown ethers [2]. Anionic polymerizations are sometimes associated with termination and transfer reactions, while cycHc oHgomers are formed as a result of back-biting reactions. In CL polymerization initiated with potassium tertibutoxide, a huge amount of cyclic lactones was formed, yet in the presence of Hthium tertibutoxide in an apolar solvent the oligomer formation was significantly reduced. 

Lactones. Polymer supports have been tested as media for cyclizations of a>-hydroxy-alkanoic acids to lactones (Equation 30). Lactones larger tlum 7-membeied are difti-cult to prepare because of a large decrease in entropy. There is also an unfavorable enthalpy of cyclization to the strained 8-12 membered rings. The side reactions that conqrlete with unimolecular cydization are bimolecular formation of cyclic and acyclic dimers and higher oligomers (Equations 30 and 31). 

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