Comments on the ring-opening polymerization of morpholine-2,5-dione derivatives by various metal catalysts and characterization of the products formed in the reactions involving R2SnX2, where X = OPr(i) and NMe2 and R = Bu(n), Ph and p-Me2NC6H4

(3S,6S)-3-Isopropyl-6-methyl-morpholine-2,5-dione (1), and (3S,6S)-3,6-dimethyl-morpholine-2,5-dione (2), do not enter into ring-opening polymerization reactions with metal catalyst precursors commonly employed for lactides, and with Sn(II) octanoate, only low molecular weight oligomers are obtained. Reactions with R2SnX2 compounds, where R = Ph, Bu(n) and p-Me2NC6H4 and X = OPr(i) or NMe2, reveal that ring-opening of the morpholine-2,5-diones does occur, but that polymerization is terminated by the formation of kinetically-inert products such as {Ph2Snmu,eta(3)-OCH(Me)CONCH(Pr(i))COOPr(i)]}2 (3), and {Bu(n))2Snmu,eta(3)-OCH(Me)CONCH(Me)CONMe2]}2 (4), with elimination of HX. Ph3SnOPr(i) is seen to react reversibly with morpholine-2,5-diones in toluene-d8 by 1H NMR spectroscopy while (Bu(n))3SnNMe2 reacts by ring opening to give (Bu(n))3SnOCH(Me)C(O)NHCHMeC(O)NMe2. The new organotin compounds have been characterized by 1H, 13C{1H} and 118Sn NMR spectroscopy and compounds 1, 2, 3 and 4 by single crystal X-ray crystallography.