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

Abstract

transesterification with a random attack of active chain ends on ester groups in the polymer backbone. Experimental evidence for the existence of such a mechanism in the melt at 120 or 160 °C has previously been published. [2,3] For cyclic poly(l-lactide) s existence of ring-ring equilibration in the solid state has also been found in one case. [1] Ring-ring equilibration is a normal consequence of ring-chain equilibration based on "back-biting" reactions as already pointed out by Jacobson and Stockmayer in 1950. [4] Identification of ring-ring equilibration in the absence of ring-chain equilibration is a relatively new phenomenon which requires covalent cyclic catalysts, such as those recently elaborated by the authors for ring-expansion poly merizations (REPs) of lactides. [5][6][7][8] Syntheses of cyclic polylactides were reported by several groups. [5][6][7][8][9][10][11][12][13][14][15][16][17][18] When a zwitterionic mechanism was involved, ring-ring equilibration was also observed, but only in solution or in the melt. [11][12][13]16] However, it is unlikely and unproven that the zwitterionic rings are permanently closed via contact ion pairs, in as much as the steric demands of the carbene used as catalyst for the stabilization of the cationic chain end is unfavorable for the formation of contact ion pairs. Hence, the ring-equilibration is just a consequence of ring-chain equilibration.Ring-ring equilibration in the solid state is a particularly interesting aspect of chemical reactions in the solid state, because it raises the question, which part of the solid is involved. REP of lactides is particularly suited for studies of ring-ring equilibration, because a clean REP free of transesterification reactions must yield even-numbered cycles or in rare cases odd-numbered rings.In the case of linear poly(l-lactide)s it was observed that complete equilibration of even and odd-numbered chains may be achieved in the solid state upon prolonged heating at 120 °C, when the catalyst 2-stanna-1,3-dithiolane was present. [1] This observation proves that not only the amorphous phase, but also all crystallites are somehow involved in transesterification reactions. This result raises the question, if the transesterification reactions underlying the even-odd equilibration exclusively concerns the surface of the crystallites or if a complete rearrangement of the crystallites occurs. In this context, the present work served three purposes. First, it should be found out if it is possible to find a catalyst and reaction conditions favoring ringring equilibration without ring chain equilibration in the solid state. Second, it should be elucidated, if ring-ring equilibration of all cyclic species can be achieved and how the chains inside the crystallites are involved. The third purpose was to elucidate, -numbered cyclic poly(d-lactide) and poly(l-lactide) are prepared by ringexpansion polymerization. The cyclic pol(l-lactide) is annealed either at 120 or at 160 °C for several days. The progress of transesterification in the so...

About the transformation of low Tm into high Tm poly(l-lactide)s by annealing under the influence of transesterification catalysts

“…It is interesting to compare these observations with results obtained from annealing experiments with the catalyst SnNaph 2 ( Scheme 1 ). 34 These experiments were again performed at 160 °C and extended up to 4 d. Using even-numbered species (cycles and/or linear chains) as substrate allowed for monitoring the formation of odd-numbered species. Their detection proved, that the equilibration process involved all chains proceed in solid poly( l -lactide)s. As illustrated in Scheme 2 , these transesterification reactions mainly involve the loops on the surface of the crystallites, because of their higher reactivity resulting from energetically unfavourable conformations.…”

Stereocomplex crystallization in cyclic polymer blends studied using dynamic Monte Carlo simulations

“…Due to the progress of synthesis technology, cyclic PLLA and PDLA can be successfully synthesized. [34][35][36][37][38][39][40][41] Then, researchers focused on the SC formation in cyclic PLLA/PDLA blends. 34,36,38,40,41 Though cyclic PLLA/PDLA blends can form stereocomplexes similar to linear blends, 41 they exhibit some different behaviours in terms of the SC formation.…”

“…[34][35][36][37][38][39][40][41] Then, researchers focused on the SC formation in cyclic PLLA/PDLA blends. 34,36,38,40,41 Though cyclic PLLA/PDLA blends can form stereocomplexes similar to linear blends, 41 they exhibit some different behaviours in terms of the SC formation. For instance, Sugai et al reported that cyclized PLLA/PDLA blends exhibit a weaker stereocomplexation ability compared to blends with a linear chain topology.…”