 Cyclic polymers, while possessing interesting physical properties, have presented a synthetic challenge for chemists.  Zwitterionic ring-opening polymerization (ZROP) is an efficient and reliable method of producing cyclic polymers.  The N-heterocyclic carbene (NHC)-mediated ZROP of d-valerolactone was investigated in the presence of various concentrations of LiCl.  It was found that the rate of polymerization depends sensitively on [LiCl], especially at low concentrations.  Furthermore, polymer molecular weights were well-controlled with narrow polydispersities (PDI<1.2) in the presence of LiCl.  It is hypothesized that LiCl acts to break up aggregates of the propagating zwitterionic species to speed up polymerization, but forms a highly coordinated and less reactive alkoxide at high concentrations.  MALDI-TOF characterization of polymer products after isolation from aqueous methanol workup revealed that a mixture of linear and cyclic polymers is formed at lower concentrations of LiCl, whereas linear polymers are formed at the highest [LiCl], which is likely due to less efficient cyclization in the presence of LiCl.

Ion pairing effects in the zwitterionic ring-opening polymerization of d-valerolactone

248th National Meeting (2014)

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