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