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Equimolar copolymerizations of glycolide and L-lactide were conducted in bulk in the temperature range from 100 to 1608C. The catalytic efficiency of tin(II)2-ethylhexanoate (SnOct 2 ) and bismuth(III)n-hexanoate (BiHex 3 ) were compared under identical conditions. Furthermore, four copolymerizations were conducted with bismuth subsalicylate as initiator/catalyst. The isolated copolyesters were characterized by viscosity and SEC measurements, by 1 H-NMR spectroscopy with regard to their composition and with 13 C-NMR spectroscopy with regard to their sequences. BiHex 3 proved to be nearly as efficient as initiator as SnOct 2 and the sequences were somewhat closer to randomness than those obtained from SnOct 2 . All copolyesters were amorphous materials soluble in a variety of organic solvents. Chain extension with diisocyanates raised the molecular weights by a factor of 5-7.

Pentaerythritol (PENT) was polycondensed with dimethyl sebacate (DMS) in bulk at 240 °C. With Ti(OBu)4 as the catalyst, an equimolar feed ratio resulted in gelation even when no vacuum was applied. When the PENT/DMS ratio was raised to 1.2/1.0, soluble viscous resins were obtained, but despite the excess of PENT, the conversion of methyl ester groups never exceeded 92%. With bismuth(III) n-hexanoate (BiHex3) as the catalyst and an 1.0/1.0 feed ratio, nonviscous polyesters were obtained with conversions <75%. When BiHex3 was used in the beginning and Ti(OBu)4 was added at a later stage, soluble viscous resins were obtained even at 1.0/1.0 feed ratio. Further polycondensations were conducted with higher PENT/DMS ratios. The reaction products were characterized by viscosity measurements, 1H NMR spectroscopy, and MALDI-TOF mass spectrometry. These measurements indicated the formation of hyperbranched polyesters almost free of cycles and ether groups. Furthermore, acylations with acetic or methacrylic anhydride were studied and quantitative conversions of the CH2OH groups were achieved.

A complex consisting of one Bi3+ ion and two 2‐mercaptoethanol units (BiME2) was used as initiator for the ring‐opening polymerization of ε‐caprolactone in bulk. A kinetic comparison showed that BiME2 is as reactive as initiator as Sn‐octanoate and more reactive than Bi‐hexanoate. The difference to BiHex3 decreased at higher temperatures and upon addition of an alcohol as coinitiator. When tetra(ethylene glycol) was used as coinitiator, it was completely incorporated into the poly(εCL) chain, so that telechelic polylactones having two OH‐endgroups were formed. In the absence of a coinitiator, 2‐mercaptoethanol or its disulfide were incorporated in the form of ester groups. Furthermore, it was found by MALDI‐TOF mass spectrometry that small amounts of cyclic oligolactones (detected up to a degree of polymerization of 17) were formed under all reaction conditions. Higher temperatures and longer times favored a higher content of cycles. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3175–3183, 2006