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Eingeladener Vortrag
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At first, formation of cycles in commercial poly(Llactide)s is discussed and compared with benzyl alcoholinitiated polymerizations performed in this work. This comparison was extended to polymerizations initiated with 4-cyanophenol and pentafluorothiophenol which yielded cyclic polylactides via end-biting. The initiator/catalyst ratio and the acidity of the initiator were found to be decisive for the extent of cyclization. Further polymerizations of L-lactide were performed with various diphenols as initiators/co-catalysts. With most diphenols, cyclic polylactides were the main reaction products. Yet, only catechols yielded even-numbered cycles as main reaction products, a result which proves that their combination with SnOct2 catalyzed a ring-expansion polymerization (REP). The influence of temperature, time, co-catalyst, and catalyst concentrations was studied. Four different transesterification reactions yielding cycles were identified. For the cyclic poly(L-lactide)s weight average molecular weights (Mw’s) up to 120,000 were obtained, but 1H NMR end group analyses indicated that the extent of cyclization was slightly below 100%.
The influence of various parameters like structure of Initiator and catalyst and temperature on the formation of cyclic poly(Llactide)s has been investigated. Depending on the chosen conditions, the course of the polymerization can be varied from a process yielding exclusively linear polylactides to mainly cyclic polylactides. Three different reaction pathways for cyclization reactions have been identified.
1-Hydroxymethylnaphtalene (HMN) or 11-bromoundecanol (BUND) were used as initiators and Sn(II) 2-ethylhexanoate (SnOct2) as catalyst for ROPs of L-Lactide (LA) at 115 °C in bulk or in 4 M and 2M solutions in toluene. The LA/In ratio, the LA/Cat ratio and the time were varied. The matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectra exclusively displayed peaks of linear chains, when the ROPs were conducted in bulk. But in contrast to reports in the literature, mixtures of linear and cyclic poly(L-lactide) (PLA), were obtained, when the ROPs were performed in solution. The intensity distribution of the mass peaks of cyclic PLAs displayed a “saw-tooth pattern” after annealing in contrast to the mass peak distribution of the liner chains. This new phenomenon indicated that cyclic PLAs and linear PLAs crystallized in separate crystals from the same reaction mixture. This conclusion was confirmed by fractionated crystallization from 2 M solution, which confirmed that the cyclic PLAs nucleate and crystallize faster than the linear chains.
Ring-expansion polymerizations (REPs) catalyzed by two cyclic tin catalysts(2-stanna-1.3-dioxa-4,5,6,7-dibenzazepine [SnBiph] and
2,2-dibutyl-2-stanna-1,3-dithiolane [DSTL) are performed at 140 °C in bulk.
Small amounts (4 vol%) of chlorobenzene or other solvents are added to facilitate transesterification reactions (ring–ring equilibration) in the solid poly(l-lactide)s. In the mass range up to m/z 13 000 crystalline PLAs displaying a so-called saw-tooth pattern in the MALDI-TOF mass spectra are obtained indicating the formation of extended-ring crystals. The characteristics of extended-ring crystallites and folded-ring crystallites are discussed. Furthermore, extremely high melting temperatures (Tm’s up to 201.2 °C) and melting enthalpies (𝚫Hm’s up to 106 J g−1)) are found confirming that 𝚫Hmmax, the 𝚫Hm of a perfect crystal, is around or above 115 J g−1 in contrast to literature data.
Ring-expansion polymerizations (REPs) catalyzed by two cyclic tin catalysts (2-stanna-1.3-dioxa-4,5,6,7-dibenzazepine [SnBiph] and
2,2-dibutyl-2-stanna-1,3-dithiolane [DSTL]) are performed at 140 °C in bulk.
Small amounts (4 vol%) of chlorobenzene or other solvents are added to
facilitate transesterification reactions (ring–ring equilibration) in the solid poly(l-lactide)s. In the mass range up to m/z 13 000 crystalline PLAs displaying a so-called saw-tooth pattern in the MALDI-TOF mass spectra are obtained indicating the formation of extended-ring crystals. The characteristics of extended-ring crystallites and folded-ring crystallites are discussed. Furthermore, extremely high melting temperatures (Tm’s up to 201.2 °C) and melting enthalpies (𝚫Hm’s up to 106 J g−1)) are found confirming that 𝚫Hm max, the 𝚫Hm of a perfect crystal, is around or above 115 J g−1 in contrast to literature data.
Cyclic polylactides were prepared in bulk at 170 °C, crystallized at 120 °C and then annealed at temperatures between 130 and 170 C with variation of catalyst, catalyst concentration and annealing time. The transformation of the initially formed low melting (LTm) crystallites, having melting temperatures (Tm) <180 °C into high melting (HTm) crystallites having Tm values > 189 °C was monitored by means of DSC measurements and characterized in selected cases by SAXS measurements. It was confirmed that the formation of HTm crystallites involves a significant growth of the thickness of the lamellar crystallites along with smoothing of their surface. Annealing at 170 °C for 1 d or longer causes thermal degradation with lowering of the molecular weights, a gradual transition of cyclic into linear chains and a moderate decrease of lamellar thickness. An unexpected result revealed by MALDI TOF mass spectrometry is a partial reorganization of the molecular weight distribution driven by a gain of crystallization enthalpy.
Alcohol-initiated ring-opening polymerizations (ROP) of L-lactide (LA) were studied in solution at 70 °C, whereupon the nature of the alcohol, the LA/initiator ratio, the LA/SnOct2 ratio and the time were varied. In contrast to literature, neat SnOct2 is catalytically active in THF and several aromatic but donor solvents, such as 1,3-dioxolane, dimethylformamide (DMF) or N-methyl pyrrolidone (NMP), strongly reduce the activity of SnOct2. In agreement with literature, no cycles were formed by neat SnOct2 at 70 °C in toluene, whereas almost complete cyclization occurs at 115 °C. This finding is attributed to strongly reduced mobility of the initially formed linear chains having one Sn-O-CH and one anhydride end group. Due to better solvation and enhanced mobility cyclization occurs in THF at 70 °C.
Alcohol-initiated ROPs of L-Lactide were performed at 140 ◦C in bulk with variation of the initiator/catalyst ratio and time. Lower ratios favor the formation of cycles which upon annealing display a change of the MALDI mass peak distribution towards a new maximum with a “saw-tooth pattern” of the mass peaks representing the cycles.
Such a pattern was not observed for the mass peak of the linear chains. The coexistence of these patterns indicate that linear and cyclic poly (L-lactide)s (PLA) crystallize in separate crystals, and that the crystallites of the cycles are made up by extended rings. High Tm and ΔHm values confirm that these extended-ring crystallites represent a thermodynamically optimized form of PLA. Experiments with preformed cyclic and linear PLAs support this interpretation.
The main objective of this work was to design new advanced sorbent phases, alternating copolymers, derived from isosorbide and 2,6-difluorpyridine, to be used for the removal of aromatic organic pollutants present in water at low concentrations. Six different monomers, dianhydrohexitols isomers and bisphenol derivatives, weresynthesized in order to make it possible to study their hydrophilic and hydrophobiceffect on the sorption efficiency of the resulting polymeric phases. Before this study, we have confirmed the chemicals structures, molecular weights, and thermal properties of the obtained polymeric phases. Sorption results show a higher adsorption efficiency of P6 co-poly(ether-pyridine) based on bisphenol substituted with pyridineunits, for all tested pollutants, hydrophobic and hydrophilic ones, due to its less compact structure. Two aromatic organic pollutants, p-hydroxybenzoic acid and toluicacid, were selected as sorbates to study the adsorption characteristic, kinetics and isotherms of the copoly(ether pyridine) P6. Langmuir model led to a better fitting of the sorption isotherms; the sorption of toluic acid is easier than of that p-hydroxybenzoic acid. Comparing 1/n values for benzoic acid was two time lower for P6 compared to that for biochar and for crosslinked methacrylate resin, showing a higher efficiency
εCL was polymerized using the triflates of lanthanum, samarium, magnesium, aluminum, scandium, and bismuth as catalysts. Bismuth triflate proved to be extraordinarily reactive, and catalyzed polymerizations of εCL even at 20 °C. Adding DTBMP reduced the polymerization rate only slightly. Furthermore, no evidence of a cationic mechanism was found by end‐group analyses. Polymerization at 20 °C either in bulk or in solution only yielded polyesters of low or medium molecular weights. Yet addition of alcohols allowed for a proper control of molecular weight and end‐groups. Additionally, low catalyst concentrations and low temperature resulted in narrow molecular weight distributions and polylactones almost free of cyclic compounds.
Isosorbide, succinyl chloride and isophthaloyl chloride are polycondensed under various reaction conditions. The heating in bulk with or without catalysts as well in an aromatic solvent without catalyst, and polycondensation with the addition of pyridine only yield low molar mass copolyesters. However, heating in chlorobenzene with addition of SnCl2 or ZnCl2 produces satisfactory molar masses. The number average molecular weights (Mn) of most copolyesters fall into the range of 7000–15,000 Da with polydispersities (PD) in the range of 3–9. The MALDI-TOF mass spectra almost exclusively displayed peaks of cyclics indicating that the chain growth was mainly limited by cyclization and not by side reactions, stoichiometric imbalance or incomplete conversion. The glass-transition temperatures increased with the content of isophthalic acid from 75 to 180 °C and the thermo-stabilities also followed this trend.
Various copolyesters of lactide, isosorbide, and terephthalic acid are prepared by a two-step process performed in a 'one-pot' procedure, beginning with an isosorbide-initiated oligomerization of L-lactide followed by polycondensation with terephthaloyl chloride using SnCl2 as a catalyst for both steps. The SEC data show the formation of random copolyesters with high polydispersities as a consequence of the relatively high fraction of cyclics proved by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The glass-transition temperatures (89–180 °C) obey an almost linear trend with the molar composition. The thermostability decreases with increasing lactide content. The successful incorporation of phenyl phosphate results in copolyesters of lower inflammability.
Cyclic and linear poly(L-lactide)s by ring-expansion polymerization with Bu2SnO, Oct2SnO, and Bu2SnS
(2023)
To elucidate the usefulness of commercial Bu2SnO as catalyst for syntheses of linear and/or cyclic polylactides L-Lactide was polymerized in bulk with variation of the LA/Cat ratio, temperature and time. Based on a ring-expansion polymerization (REP) mechanism cyclic polylactides (PLAs) with weight average molecular weights in the range of 200,000–300,000 are obtained at the highest temperature (180 °C). Polymerizations at 150 °C yielded crystalline, mainly cyclic polylactides which, after annealing, showed high melting temperatures (up to 197.5 °C) and high crystallinities (>80%). Polymerization at 120 °C confirmed the trend towards more linear chains with lower temperatures but yielded extended-ring crystals showing a “saw-tooth pattern” in the mass spectra. Oct2SnO gave similar results as Bu2SnO. Bu2SnS proved a sluggish polymerization catalyst, but a good transesterification catalyst in solid PLA.
3,5-Bis(4-fluorobenzoyl)phenol was polycondensed under various reaction conditions. Low molecular weights were obtained (Mn < 8000 Da), and MALDI-TOF mass spectrometry indicated that the chain growth was limited by cyclization. In the best mass spectra the cyclic, hyperbranched poly(ether ketone)s were detectable up to masses around 14 000 Da. When 4,4'-difluordiphenyl sulfone was added, star-shaped polymers having a diphenyl sulfone star center were formed, but cyclization was not completely suppressed. The results were compared to those obtained from "linear polycondensations" based on 4,4'-difluorobenzophenone (DFBP) and 4-tert-butylcatechol or bisphenol A. The poly(ether ketone)s of DFBP had also low molecular weights and contained high fractions of cycles. It is concluded that also in the case of hyperbranched polymers cyclization competes with propagation at any stage of the polycondensation at any concentration. Therefore, the fundamental trend of any "abn" polycondensation is the formation of star-shaped polymers having a cyclic core and hyperbranched star arms.
L-Lactides were polymerized in bulk at 120 or 160 °C with cyclic dibutyltin catalysts derived from 1,2-dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When L-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120 °C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo- or polylactides with Elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160 °C.
Starting from dibutyltin oxide, four catalysts were synthesized, namely the dibutyltin bisphenoxides of Phenol (SnPh), 4-chlorophenol (SnCP), 4-hydroxybenzonitrile (SnCN) and pentafluorophenol (SnOPF). With the first three catalysts polymerizations of L-lactide at 160 °C in bulk yielded large fraction of linear chains having phenylester end groups at short reaction times. At longer times the fraction of cycles considerably increased at the expense of the linear chains, when SnCN was used as catalyst. With SnOPF only cyclic polylactides were obtained at low Lac/Cat ratios (< 400) with weight average molecular weights (Mw) up to 90 000 Da, whereas for high Lac/Cat ratios mixtures of cyclic and linear chains were found. Polymerizations in solution enabled variation of the molecular weight. Polymerizations of meso-lactide at temperatures down to 60 °C mainly yielded even-numbered linear chains supporting the postulated ROPPOC mechanism.
A comparison of tributyltin chloride, dibutyltin dichloride,and butyltin trichloride as catalysts of ring-opening polymerizations(ROPs) of l-lactides at 160°C in bulk reveals increasing reactivity in the above order, but only the least reactive catalysts, Bu3SnCl, yield a uniform reaction product, namely cyclic poly(L-lactide)s with weight average molecular weights (Mw ́s) in the range of 40,000–80,000. A comparison of dimethyltin , dibutyltin , and diphenyltin dichlorides resulted in the following order of reactivity: Me2SnCl2<Bu2SnCl2<<Ph2SnCl2. In this series also, the most reactive catalyst yields cyclic polylactides, but the extent of cyclization varies with the molecular weight. The formation of cyclic polylactides is explained by ROP combined with simultaneous polycondensation involving end-to-end cyclization (ROPPOC method). ROP of meso-lactide at 80 or 60°C yields even-numbered linear chains as main products, a result supporting the ROPPOC mechanism.
Various catalysts and polycondensation mechanisms (ROPPOC/REP/ROP)were presented and discussed with regard to the Formation of cyclic and linear polylactides. It could be shown that depending on the structure of the catalyst even- and od numbered cyclics were selectively formed. These data were supported by various analytical techniques like MALDI-TOF mass spectrometry and Size Exclusion Chromatography.
Glycolide was polymerized in bulk with two cyclic catalysts − 2,2-dibutyl-2-stanna-1,3-dithiolane (DSTL) and 2-stanna-1,3-dioxa-4,5,6,7-dibenzepane (SnBiph). The monomer/initiator ratio, temperature (140 – 180 °C) and time (1–––4 days) were varied. The MALDI TOF mass spectra exclusively displayed peaks of cyclic polyglycolide (PGA) and revealed an unusual “saw-tooth pattern” in the mass range below m/z 2 500 suggesting formation of extended ring crystallites. The DSC measurements indicated increasing crystallinity with higher temperature and longer time, and after annealing for 4 d at 160 °C a hitherto unknown and unexpected glass transition was found in the temperature range of 170–185 °C. Linear PGAs prepared by means of metal alkoxides under identical conditions did not show the afore-mentioned features of the cyclic PGAs, neither in the mass spectra nor in the DSC measurements. All PGAs were also characterized by SAXS measurements, which revealed relatively small L-values suggesting formation of thin crystallites in all cases with little influence of the reaction conditions.
L-Lactide is polymerized in bulk at 160 8C either with dibutyltin bis(benzylmercaptide) (SnSBzl), dibutyltin bis(benzothiazole 2-mercaptide) (SnMBT), or with dibutyltin bis(pentafluorothiophenolate) (SnSPF) as catalysts. SnSBzl yields linear polylactides having benzylthio-ester end groups in addition to cyclic polylactides, whereas SnMBT and SnSPF mainly or exclusively yield cyclic polylactides. This finding, together with model reactions, indicates that the SnS catalysts promote a combined ring-opening polymerization and polycondensation process including end-to-end cyclization. SnMBT caused slight racemization (3%–5%), when used at 160 8C. With SnSPF optically pure cyclic poly(L-lactide)s with high-molecular weights can be prepared at 160 8C.
Poly(alkylene isophthalate)s were prepared by different methods, either in solution or in bulk. The SEC measurements were evaluated in such a way that all oligomers were included. In solution (monomer conc. 0.10.7 mol/L) large fractions of rings were formed and high dispersities (up to 12) were obtained, which disagree with theoretical predictions. Polycondensations in bulk did neither generate cyclics by 'back-biting' nor by end-to-end cyclization, when the maximum temperature was limited to 210 °C. The dispersities of these perfectly linear polyesters were again higher than the theoretical values. Regardless of the synthetic method monomeric cycles were never observed. Furthermore, SEC measurements performed in tetrahydrofuran and in chloroform and SEC measurements performed in three different institutes were compared. Finally, SEC measurements of five samples were performed with universal calibration and a correction factor of 0.71 ± 0.02 was found for normal calibration with polystyrene.