Bismuth(III) n-Hexanoate and Tin(II) 2-Ethylhexanoate Initiated Copolymerizations of ε-Caprolactone and l-Lactide

Using bismuth(III) hexanoate (BiHex 3 ) as initiator and tetraethylene glycol as co-initiator, ε-caprolactone (εCL) and l -lactide (LLA) were copolymerized in bulk. Whereas monomer/initiator and monomer/co-initiator ratios were kept constant, the temperature was varied from 100 to 140 °C. Amorphous copolyesters having a perfectly random sequence were found under all circumstances when BiHex 3 was used as initiator. In contrast, parallel experiments with tin(II) 2-ethylhexanoate (SnOct 2 ) as initiator yielded crystalline copolyesters with a blocky sequence. From kinetic measurements of homopolymerizations and from MALDI−TOF spectra of homopolyesters, it was learned that BiHex 3 is less efficient as initiator and transesterification catalyst than SnOct 2 . Therefore, the formation of cycles by “backbiting degradation” is reduced, and the formation of random copolymers involves a special intramolecular transesterification mechanism. These results and the extremely low toxicity of Bi salts render BiHex 3 a particular useful initiator for biomedical applications of polylactones.