Polylactones, 56. ABA Triblock Copolymers Derived from ε‐Caprolactone or L‐Lactide and a Central Polysiloxane Block

A commercial oligosiloxane having two γ‐hydroxypropyl endgroups and a number‐average molecular weight (M?_n) around 1 000 Da was condensed with Bu₂Sn(OMe)₂ yielding tin‐containing macrocycles but no polymers. These macrocycles reacted with γ‐thiobutyrolactone by insertion of one thiolactone per Sn—O bond. When ε‐caprolactone was added a ring‐expansion polymerization was initiated, so that the M?_n could be controlled via the monomer/initiator (M/I) ratio. Triblock copolymers with free OH endgroups were obtained by removal of the Bu₂Sn group from the cyclic polymers with 1,2‐dimercaptoethane or methanol. Analogous cyclic and linear triblockcopolymers were prepared from L ‐lactide.