Polylactones. 40. Nanopretzels by Macrocyclic Polymerization of Lactones via a Spirocyclic Tin Initiator Derived from Pentaerythritol

A spirocyclic initiator was synthesized from dibutyltin oxide and pentaerythritol. This spirocyclic distannoxane was characterized by elemental analyses, NMR spectroscopy, and a stoichiometric reaction with γ-thiobutyrolactone. ε-Caprolactone (ε-CL) was polymerized in bulk at 70 °C with variation of the monomer/initiator ratio. The molecular weights paralleled roughly the M/I ratio and high molecular weight poly(ε-CL) was obtained ( M w ’s up to 160 000). In contrast, homopolymerizations of β- d, l -butyrolactone (β- d, l -BL) in bulk at 75 °C did not give high molecular weights ( M w up to 25 000) but high yields. The spiro-macrocyclic poly(ε-CL) was selectively ring-opened with dimercapto ethane, whereby a four-around star polyester was obtained. The end groups were identified by 1 H NMR spectroscopy. Furthermore, spirocyclic block copolymers were prepared by batchwise copolymerization of β- d, l -BL and ε-CL. Furthermore, spirocyclic random copolyesters were obtained from mixtures of β- d, l -butyrolactone and ε-CL. The block or random character of all copolylactones was characterized by 13 C NMR spectroscopy and DSC measurements. Only in the case of block copolymers, crystallized blocks of ε-CL were found. The structure of the end groups and the polymerization mechanism are discussed.