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Studies of the ring-opening polymerization (ROP) of L-lactide by Ph2SnX2 reveal that X (X = NMe2 or OPri) influences not only the rate of polymerization but also the nature of the polymer formed. Gel permeation chromatography (GPC), mass spectral analysis and 1H NMR spectroscopy of the polymers from these reactions revealed that for X = NMe2, a bimodal molecular weight distribution of polylactide (PLA) is obtained. This is indicative of both cycles and long chains (24 kDa) being formed by both intra- and interchain transesterification. This extensive transesterification results in regeneration of the Ph2Sn[OCHMeC(O)NMe2]2 species as the resting state of the tin(IV) species. Conversely, for X = OPri only a unimodal molecular weight distribution is observed in addition to Ph2Sn[OCHMeC(O)OPri]2, which is indicative of only interchain transesterification. Similar observations were seen in studies of ROP of L-lactide by Ph3SnX precursors.
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Chisholm et al. (2003) studied this question.
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