Polylactones, 24. Polymerizations of racemic and meso‐D,L‐lactide with AlO initiators. Analyses of stereosequences

Racemic D , L ‐lactide and meso ‐ D , L ‐lactide were polymerized with four different initiators: Al(isopropoxide) 3 , AIEt 3 /neopentyl alcohol (1 : 1, mole ratio), AIEt 3 /(+)‐menthol (1 : 1, mole ratio) and methylaumoxane. Most polymerizations were conducted in xylene at 60, 90 and 120°C, but at 60°C the yields were below 10%. 1 H NMR end‐group analyses revealed the formation of alkyl ester end‐groups from all Al‐alkoxide initiators, in agreement with an insertion mechanism. The highest molecular weights were obtained with the methylalumoxane initiator. The stereosequences of the isolated poly( D , L ‐lactide)s were analyzed by 1 H and 13 C NMR spectroscopy on the basis of tetrad effects. poly( D , L ‐Iactide)s prepared from racemic D , L ‐lactide suggest a stereospecific polymerization favoring syndiotactic growing steps. In the case of meso ‐ D , L ‐lactide all polymerizations follow Bernoullian statistics. Transesterification is poor or absent at temperatures ≤ 120°C. However, random stereosequences may be obtained by bulk polymerizations at 180°C. DSC measurements revealed that the glass transition temperature mainly depends on the molecular weight and not on slight differences in the stereosequences.