POLYLACTONES .23. POLYMERIZATION OF RACEMIC AND MESO D,L-LACTIDE WITH VARIOUS ORGANOTIN CATALYSTS STEREOCHEMICAL ASPECTS

Racemic and meso D,L-lactide were polymerized at 90 or 120-degrees-C in xylylene or at 120, 150 and 180-degrees-C in bulk. Furthermore, copolymerizations of racemic D,L-lactide and L,L-lactide were conducted at 180-degrees-C in bulk. Tributyltin methoxide (Bu3SnOMe), dibutyltin dimethoxide (Bu2Sn(OMe)2) and Sn(II) octoate were used as initiators. Despite high yields only low molecular weights were obtained with both tin methoxides. Sn(II) octoate gave significantly higher molecular weights. The stereosequences were analysed by H-1 and C-13 n.m.r. spectroscopy on the basis of tetrad effects. The signal assignments are discussed. Bu3SnOMe and BU2Sn(OMe)2 are effective transesterification catalysts and cause 'back-biting' degradation even at 90-degrees-C. In all series of polymerizations initiated with tin methoxides two tendencies are detectable: increasing randomization of the stereosequence with increasing reaction time and with higher reaction temperatures. In contrast, Sn(II) octoate does not cause transesterification at less-than-or-equal-to 120-degrees-C and even at 180-degrees-C randomization of the stereosequences is slow.