Ingrid Kreiser-Saunders

Polylactones: 31. Sn(II)octoate-initiated polymerization of L-lactide: a mechanistic study

Abstract The polymerization of L-lactide was catalysed with Sn(II)2-ethylhexanoate (SnOct 2 ) in the presence or absence of benzyl alcohol. The molecular weights parallel the lactide/benzyl alcohol ratio, but never the lactide/Sn ratio. 1 H n.m.r. spectroscopy revealed the existence of benzylester and −CH(CH 3 )OH end-groups. Polymerizations conducted at low lactide/catalyst ratios in the absence of an alcohol yield polylactide with a low content of 2-ethylhexanoate end-groups. 1 H and 119 Sn n.m.r. spectroscopy of CHCl 3 solutions also demonstrated that SnOct 2 forms strong complexes with both benzyl alcohol and ethyl lactate and weaker complexes with lactide. A similar but weaker complexation was also detected for Bu 2 SnOct 2 in combination with either benzyl alcohol or lactide. A new polymerization mechanism is discussed, assuming the reaction between lactide and OH end-groups bound to a Sn atom via two sp 3 d 2 orbitals.

Polylactones, 39. Zn lactate-catalyzed copolymerization of L-lactide with glycolide or ε-caprolactone†‡

Copolymerizations of glycolide (Glyc) and L-lactide (L-Lac) were conducted at 150°C in bulk to prepare random copolyesters. Four resorbable catalysts were used and compared: ZnCI 2 , ZnO 2 , Zn stearate (ZnSte 2 ) and Zn lactate (ZnLac 2 ). Separate time-conversion curves were recorded for glycolide and lactide by means of 1 H NMR spectroscopy. Glycolide polymerized faster than L-lactide under all circumstances, but the difference was smallest when ZnLac 2 was used as catalyst. Sequence analyses were performed using 13 C NMR spectroscopy. Completely random sequences were never obtained, but the sequences resulting from a catalysis with ZnLac 2 came closest to the desired randomness. Preparative 1:1 (mole ratio) polymerizations were conducted with ZnSte 2 and ZnLac 2 . High yields were obtained with both catalysts, but ZnLac 2 yielded far higher molecular weights. Furthermore, ZnLac 2 -catalyzed copolymerizations were conducted with variation of the Glyc/Lac ratio. It was found that small scale and large scale copolymerizations yield copolyesters having different properties, because the superheating in the early stage of the polymerizations (resulting from glycolide) favors the transesterification. Finally, 1:1 copolymerization of e-caprolactone and L-lactide were studied in bulk at 150°C. Again high yields, high molecular weights and nearly random sequences were obtained.

Polylactones: 30. Vitamins, hormones and drugs as co-initiators of AlEt3-initiated polymerizations of lactide

Abstract The polymerization of l -lactide with triethylaluminium and alkylaluminium alkoxides was studied in detail. It was found that commercial triethylaluminium is often contaminated with aluminium ethoxide groups resulting from the rapid oxidation of triethylaluminium. The ethoxide group is a far more reactive initiator than the aluminium-carbon bond. Therefore reactive initiators can be prepared in situ by the reaction of pure triethylaluminium with vitamins, hormones or drugs containing hydroxyl groups. Polymerization of l -lactide with these in situ prepared initiators yields oligo- or poly( l -lactide) with covalently bound vitamins, hormones or drugs. The bioactive co-initiators used in this study were geraniol, stigmasterol, tocopherol, testosterone, pregnenolone, ergocalciferol, cortisone and quinine. It is demonstrated by 13 C n.m.r. spectroscopy that the keto groups of steroids do not undergo redox reactions during the polymerization process.