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

• Published in: Macromolecules 2005-06, Vol.38 (12), p.5017-5024
• Main Authors: Kricheldorf, Hans R, Bornhorst, Kirstin, Hachmann-Thiessen, Heiko
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma047873o

Using bismuth(III) hexanoate (BiHex3) 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 BiHex3 was used as initiator. In contrast, parallel experiments with tin(II) 2-ethylhexanoate (SnOct2) 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 BiHex3 is less efficient as initiator and transesterification catalyst than SnOct2. 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 BiHex3 a particular useful initiator for biomedical applications of polylactones.