High T m poly(L-lactide)s via REP or ROPPOC of l -lactide

Macro Chemistry & Physics

Scheliga

2021

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1:1 Copolymerizations of glycolide (GL) and l‐lactide (LA) are performed in bulk at 100 C and at 160 C with four cyclic tin catalysts. The resulting copolyesters are characterized by size exclusion chromatography measurements, 1H and 13C NMR spectroscopy and by MALDI‐TOF mass spectrometry. At 160 °C and longer reaction time (22 h), nearly complete conversion of both monomers is achieved, and cyclic copolymers with nearly random sequences are formed. At shorter times (0.5–3.0 h, depending on catalyst) the conversion of LA is incomplete, and only cyclics having even numbers of lactyl units are obtained. At 100 C and 22 h again cycles mainly consisting of even numbered lactyl units are formed, but with even and odd numbers of glycolyl units. Copolymerization of lactide at 160 C with small amounts of GL show that formation of high Tm crystallites (Tm > 190 C) is hindered even when only >2% of GL is added. For polyglycolide containing a smaller amount of lactide complete solubility in hexafluoroisopropanol is only observed around and above 20 mol% of lactide.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Ring‐Expansion Copolymerization of l‐Lactide and Glycolide

Macro Chemistry & Physics

Scheliga

2021

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“…A first series of polymerizations was designed to study the influence of difunctional compounds as potential cocatalysts on topology and molecular weights of the resulting poly( l ‐lactide)s. In previous studies of ROP and REP of l ‐lactide with cyclic tin catalysts 22–25 or with BiSub in combination with salicylic acid 21 it was found that reaction times <4 h are beneficial to avoid thermal degradation and racemization, when reaction temperatures >160°C were used. Since BiSub‐catalyzed ROPs and REPs had shown that this catalyst system is somewhat less reactive than most cyclic tin catalysts studied before, this first series was conducted at 170°C and the reaction time was limited to 2 h. For all the polymerizations summarized in Table 1 the 1 H NMR spectra revealed that 2 h were indeed long enough to achieve conversion around 97%, the thermodynamic equilibrium of l ‐lactide.…”

Section: Resultsmentioning

confidence: 99%

High molecular weight poly(l‐lactide) via ring‐opening polymerization with bismuth subsalicylate–The role of cocatalysts

J of Applied Polymer Sci

2020

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The catalytic potential of bismuth subsalicylate (BiSub), a commercial drug, for ring‐opening polymerization of l‐lactide was explored by variation of cocatalyst and polymerization time. Various monofunctional phenols or carboxylic acids, aromatic ortho‐hydroxy acids, and diphenols were examined as potential cocatalysts. 2,2′‐Dihydroxybiphenyl proved to be the most successful cocatalyst yielding weight average molecular weights (uncorrected Mw values up to 185,000) after optimization of reaction time and temperature. Prolonged heating (>1‐2 h) depending on catalyst concentration) caused thermal degradation. In polymerization experiments with various commercial Bi(III) salts a better alternative to BiSub was not found. By means of matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry a couple of unusual and unexpected transesterification reactions were discovered. Finally, the effectiveness of several antioxidants and potential catalyst poisons was explored, and triphenylphosphine was found to be an effective catalyst poison.

“…None of the six samples displayed a reflection in the range of scattering angles relevant for the calculation of lamella thicknesses as illustrated by Figure S5 (Supporting Information). From literature [ 37 ] and from a recent publication of the authors [ 38 ] it is known that poly( l ‐lactide)s having T m < 180 °C show L‐spacings in the range of 15–23 nm. However, the SAXS curves obtained from the samples listed in Table 1 did not show any reflection corresponding to L‐spacings in the range of 10–40 nm (a reflection representing a spacing of 4 nm found in the SAXS pattern of No.…”

Section: Resultsmentioning

confidence: 99%

“…In contrast, poly( l ‐lactide)s having T m ′s between 190 and 196 °C are formed when l ‐lactide is polymerized at 160 °C with cyclic tin catalysts which are several orders of magnitude more reactive than SnNaph 2 . [ 38 ] Those catalysts cause a more perfect “smoothing” of the crystal surfaces and a considerable thickness growth of the lamellae as indicated by SAXS measurements of the L‐spacings. Therefore, the present work and the previous study illustrate two extreme scenarios of transesterification processes that may exist in solid polylactides, when the polymerization catalysts are not poisoned and remain in the reaction products.…”

Section: Resultsmentioning

confidence: 99%

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

Meyer²,

Macro Chemistry & Physics

2020

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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...