Ring–Ring Equilibration in Solid, Even‐Numbered Cyclic Poly(l‐lactide)s and their Stereocomplexes

Abstract

AbstractEven‐numbered cyclic poly(d‐lactide) and poly(l‐lactide) are prepared by ring‐expansion polymerization. The cyclic pol(l‐lactide) is annealed either at 120 or at 160 °C for several days. The progress of transesterification in the solid state is monitored by the formation of odd‐numbered cycles via matrix‐assisted laser desorption/ionization‐time of flight mass spectrometry. The changes of the crystallinity are monitored by differential scanning calorimetry, wide‐ and small‐angle x‐ray scattering (WAXS and SAXS) measurements. Despite total even‐odd equilibration at 160 °C, the crystallinity of poly(l‐lactide) is not reduced. Furthermore, the crystallinity of the stereocomplexes of both cyclic polylactides do not decrease or vanish, as expected, when a blocky or random stereosequence is formed by transesterification. This conclusion is confirmed by 13C NMR spectroscopy. These measurements demonstrate that transesterification is a ring–ring equilibration involving the loops on the surfaces of the lamellar crystallites thereby improving crystallinity and 3D packing of crystallites without significant broadening of the molecular weight distribution.