Full Paper: Using Sn(II)2‐ethylhexanoate (SnOct2) in combination with 1,4‐butanediol, 1,1,1‐tris(hydroxymethyl)propane (THMP) or pentaerythritol as initiator systems telechelic polyesters or star‐shaped homopolyesters of ε‐caprolactone (ε‐CL) or L,L‐lactide (L‐LA) were prepared. These homopolymers were characterized by solution viscosities, 1H NMR and MALDI‐TOF mass spectra. Using preformed homopolymers as coinitiators star‐shaped polymers having two‐block star arms were obtained. Polymerizations of equimolar mixtures of two monomers yielded star‐shaped polymers with random copolyesters as star arms. The blockiness or randomness of the star arms was characterized by 13C NMR spectroscopy. Homo‐ and copolymerizations of trimethylene carbonate (TMC) were also studied, but they were plagued by intensive side reactions such as formation of cyclic oligomers, coinitiation with water and crosslinking via transesterification.

Telechelic random copolymers were prepared by copolymerization of ε-caprolactone (εCL) and trimethylene carbonate (TMC) in bulk using bismuth(III) hexanoate, Bi(OHex)3, as initiator. These copolymers were characterized by 1H and 13C NMR spectroscopy. A−B−A triblock copolymers were synthesized by chain extension of these random copolymers with l-lactide, whereby the chain length of the amorphous central blocks and of the crystalline poly(l-lactide) blocks were varied. Finally, the triblock copolymers were transformed into multiblock copolymers by chain extension with 1,6-hexamethylenediisocyanate. All three synthetic steps were performed in a “one-pot-procedure”. The resulting multiblock copolymers were characterized by 1H NMR spectroscopy, SEC measurements, DSC measurements, and stress−strain measurements. Typical mechanical properties of thermoplastic elastomers were detected.

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A series of A−B−A triblock copolymers was prepared by a Sn(II) 2-ethylhexanoate catalyzed chain extension of various telechelic soft segments, with l,l-lactide (LLA). Poly(ε-caprolactone)s, PεCL, poly(ethylene glycol)s, PEG, and a poly(dimethyl siloxane), PDMS, served as soft segments. The lengths of both soft and hard segments (poly-LLA, PLLA, or poly-d,d-lactide, PDLA) were varied. For the lactide blocks, average lengths of 25, 50, and 100 lactic acid units were selected. An analogous series of A−B−A triblock copolymers was prepared with DLA. Compositions and block lengths of these triblock copolymers were characterized by 1H NMR spectroscopy. Furthermore, stereocomplexes (racemates) of PLLA- and PDLA-based triblock copolymers were prepared to find out if the attractive forces between PLLA and PDLA blocks or the repulsive forces between incompatible soft segments dominate the morphology. Both neat triblock copolymers and stereocomplexes were characterized by differential scanning calorimtery measurements by wide-angle X-ray diffraction. The influence of block lengths on glass transition (T g) and melting temperatures (T m) of soft and hard segments was studied in detail. Stereocomplexes of all triblock combinations were obtained, even when the soft segments were incompatible.

Copolymerizations of epsilon-caprolactone (epsilonCL) and glycolide (GL) were conducted in bulk at 120 degrees C with variation of the reaction time. Either Sn(II) 2-ethylhexanoate (SnOct(2)) or bismuth(III)subsalicylate (BiSS) were used as initiators combined with tetra(ethylene glycol) as co-initiator. The resulting copolyesters were analyzed by (1)H and (13)C NMR spectroscopy with regard to the total molar composition and to the sequence of the comonomers. Furthermore, two series of copolymerizations (either Sn- or Bi-initiated) were performed at constant time with variation of the temperature. It was found that BiSS favors alternating sequences more than SnOct(2). Time-conversion curves and MALDI-TOF mass spectrometry of homopolymerization suggest that SnOct(2) is the more efficient transesterification catalyst. A hypothetical reaction mechanism is discussed.