Spirocyclic bisphenoxides of Ge, Zr, and Sn as catalysts for ring‐expansion polymerizations of l‐ and meso‐lactide

Abstract

Spirocyclic phenoxides of germanium, zirconium, and tin were prepared from 2,2 0 -dihydroxybiphenyl and 2,2 0 -dihydroxy-1,1 0 -binaphthyl. Ring-expansion polymerizations of L-lactide are mainly studied at 160 or 180 C. The reactivity of the catalysts increases in the order: Zr < Ge < Sn. Regardless of catalyst, the weightaverage molecular weights (M w ) never exceed 50,000 g mol −1 . The resulting poly(L-lactide)s are optically pure and have a cyclic architecture. Decreasing temperature and time favor formation of evennumbered cycles, and at 102 cyclics, almost free of odd-numbered rings are obtained. Analogous polymerizations of meso-lactide give similar results >120 C, but different results at 100 or 80 C. Surprisingly, bell-shaped narrow molecular weight distributions are obtained <140 C, resembling the pattern of living polymerizations found for alcohol-initiated polymerizations. An unusual transesterification mechanism yielding narrow distributions of odd-numbered cycles is discovered too.

Cyclic poly(lactide)s via the ROPPOC method catalyzed by alkyl‐ or aryltin chlorides

“…Due to the high reactivity of Ph 2 SnCl 2 (relative to Bu 3 SnCl), it seemed to be reasonable to perform experiments at temperatures below 160 C to find out to which extent side reactions and equilibration reactions can be suppressed and the formation of cycles favored. In agreement with previous results, [28][29][30] it was expected that theses side reactions possess higher energies of activation. The experiments listed in Table 5 were conducted with L-lactide at 140, 120 and 102 C ( Table 5).…”

Stereocomplexation of cyclic polylactides with each other and with linear poly(l-lactide)s

“…In this context it was the purpose of the present work to elucidate to what extent optically pure cyclic polylactides prepared by various tin catalysts (Scheme 1) can form stereocomplexes with each other or with linear polylactides of similar molecular weight. [42][43][44][45][46][47] Four different cyclic polylactides were used for this study that varied both by their weight average molecular weight (M w ) in the range of 10 000 to 300 000 g mol −1 , and by their dispersities.…”

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

“…The tin(IV) compound of 2,2′‐dihydroxy‐1,1‐binaphthyl (SnNaph 2 , see Scheme ) was prepared from tin tetra‐t.‐butoxide and the double molar amount of the diphenol as described recently. [ 6 ] d ‐lactide was kindly supplied by Uhde Inventa‐Fischer (Berlin, Germany) and was twice recrystallized from dry toluene. Dichloromethane was distilled over P 4 O 10 .…”

“…[ 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 polymerizations (REPs) of lactides. [ 5–8 ] Syntheses of cyclic polylactides were reported by several groups. [ 5–18 ] When a zwitterionic mechanism was involved, ring–ring equilibration was also observed, but only in solution or in the melt.…”

“…[ 5–8 ] Syntheses of cyclic polylactides were reported by several groups. [ 5–18 ] When a zwitterionic mechanism was involved, ring–ring equilibration was also observed, but only in solution or in the melt. [ 11–13,16 ] 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.…”