Polylactones 21. Polymerization of L,L-lactide and rac-D,L-lactide with hematin and related porphyrin complexes

The polymerization of L,L-lactide was conducted in bulk at 120, 150 and 180°C. Hematin ( 1a ), hemin ( 1b ), 5,10,15,20-tetraphenylporphiniron(III) chloride, 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphiniron(III) chloride ( 3 ) and chlorophyllin ( 4a ) were used as initiators. Moderate yields and moderate molecular weights up to 15000 were obtained. Gradual racemization was observed with increasing reaction time and temperature. The results obtained with chlorophyllin and benzylated chlorophyllin suggest an anionic polymerization mechanism for the former initiator. When rac-D ,L -lactide was polymerized with hematin in bulk at 120, 150 and 180°C, yields up to 74% and molecular weights up to 10⁴ were found. The polymerization of meso-D ,L -lactide gave lower yields and molecular weights. Both ¹H- and ¹³C NMR spectroscopy indicate the formation of nearly random stereosequences in all polymerization of rac- and meso-D ,L -lactide.