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This article reviews and discusses information on the toxicity of numerous tin-based catalysts used in academic research and industrial production for the synthesis of various polymers, such as polyesters, polyurethanes, and polysiloxanes. Tin(IV) compounds used as additives and as PVC stabilizers are also considered. Emphasis is put on the differentiation between tin(II) salts on the one hand and tin(IV) compounds on the other hand. In the first chapter, historical facts about human contact with stannous ions are discussed, along with toxicity studies of canned foods and beverages. In the second part, toxicity tests based on cell cultures and medical applications of tin(II)-containing films or implants are described. The third part focuses on alkyl and aryl tin(IV) compounds.
Two poly(L-lactide)s (PLAs) with a degree of polymerization (DP) of 20 or 100 were prepared by trifluoroethanol-initiated ring-opening polymerization (ROP) catalyzed by tin(II) 2-ethyl hexanoate (SnOct2). These PLAs were annealed at 140 ◦C or at 160 ◦C in the presence of SnOct2, and the changes in topology and molecular weight distribution (MWD) were monitored by matrix-assisted laser desorption/ionization time-of flight (MALDI TOF) mass spectrometry and gel permeation chromatography (GPC). For the PLA with a DP 20, the main reaction was polycondensation combined with higher dispersities. In the case of the DP 100, PLA polycondensation was combined with disproportionation and the formation of a new MWD maximum around m/z 3500. In addition, extensive cyclization occurred, and the resulting cyclic PLAs crystallized separately from the linear chains in the form of extended ring crystals. These results also suggest that both extended chain and extended ring crystals posses the same crystal thickness as a result of thermodynamically controlled transesterification in the solid state.
Oligo(butylene terephthalate)s with degrees of polymerization (DP) ranging from 15 to 20 were synthesized via Ti(OEt)4-catalyzed transesterification in bulk, using either an excess of 1,4-butanediol or dimethyl terephthalate. The PBTs were annealed at 190 °C and 210 °C, either in the absence or presence of catalysts, such as tin(II) 2-ethyl hexanoate, Bu2SnO, and Zr(acac)4. The solid-state reactions were monitored by matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectrometry, which allowed differentiation between different end-group combinations. Furthermore, the reaction products were characterized using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) measurements. Solid-state polycondensation (SSP) primarily proceeded by the reaction of chains with CH2OH end groups due to the formation of loops on the crystallite surface. In contrast, a predominance of methyl ester end groups proved unfavorable for the progress of the SSP. Under optimized conditions, SSP increased the molecular weight by a factor of eight and involved formation of cycles with a strong tendency toward formation of monodisperse extended-ring crystallites.
Star-shaped poly( L-lactide)s (s-PLAs), were prepared by ring-opening polymerization of with pentaerythritol and catalyzed by tin(II) 2-ethylhyexanoate (SnOct out in bulk at either 120 ◦ C or 160 ◦ L-lactide (LA) initiated 2 ). Two small series of ROPs were carried C with variation of the LA/initiator ratio. Matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectra revealed traces of cycles in 120 ◦ C samples only at low initiator/catalyst ratios. In contrast, higher fractions of cycles were detected in samples prepared at 160 C, suggesting a "back-biting" origin. The influence of arm length and thermal history on crystallinity was investigated by differential scanning calorimetry (DSC) and wide-angle x-ray scattering (WAXS). It was shown that annealing increased the melting temperature (T ◦ ◦ m ) and crystallinity. However, annealing at 140 ◦ C for 14 d also caused significant solid-state transesterification. In addition, crystallites composed of s-PLAs with extended arms and, simultaneously, crystallites composed exclusively of extended rings were formed, identified by their typical "sawtooth" pattern in the MALDI mass spectra. Small angle x-ray scattering (SAXS) indicated that the crystal thickness was lower than that of linear PLAs annealed under identical conditions, but increased upon annealing at 140 C.
ABSTRACTRing‐opening polymerizations (ROPs) of l‐lactide (LA) were performed at 120°C, 140°C, 150°C, and 180°C in bulk catalyzed by acetylacetonate (acac) complexes of manganese(II), nickel(II), copper(II), zirconium(IV), dibutyltin(IV), and vanadyl(V). These experiments revealed that only the acac complexes of Zr and Bu2Sn were reactive enough to enable quantitative polymerization within a few hours. Further ROPs with Bu2Sn(acac)2 and Zr(acac)4 showed that Bu2Sn(acac)2 had a slightly lower reactivity than the Zr complex. The ROPs catalyzed with Zr(acac)4 yielded number average molecular weights (Mn) up to 70,000 g mol−1. The matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectra indicated an exclusive formation of cycles in the mass range of up to m/z 13,000 for Zr(acac)4 and up to m/z 18,000 for Bu2Sn(acac)2. Unusually high melting temperatures (187°C–192°C) were also achieved along with high crystallinities. Furthermore, four lactide/glycolide copolymerizations catalyzed by Zr(acac)4 were performed, and the topology and the extent of transesterification were characterized by MALDI‐TOF mass spectrometry and 13C NMR spectroscopy.
The usefulness of various N- and P-based catalysts for syntheses of cyclic polyglycolide via zwitterionic polymerization of glycolide was examined. Most catalysts produced discolored, largely insoluble polyglycolides consisting of cycles and unidentified byproducts. Soluble, cyclic polyglycolides were obtained using neat pyridine as catalyst at 120 °C, 100 °C, 80 °C, and even at 60 °C. The number-average molecular weights were extremely low and depended slightly on the glycolide-to-pyridine ratio. Three different mass distributions of the cycles were detected by mass spectrometry, depending on the reaction conditions. The cyclic polyglycolides were also characterized by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) measurements. The SAXS data in combination with the mass spectra indicate that the majority of the cycles form extended-ring crystallites
Three commercial poly(butylene terephthalate) (PBT) samples - Pocan B1300 (B13), Pocan B1600 (B16), and Addigy P 1210 (AP121) - served as the starting materials for the annealing experiments, which were conducted with and without the addition of esterification and transesterification catalysts. The catalysts used were Sn(II) 2-ethylhexanoate (SnOct₂), Zr(IV) acetylacetonate, and 4-toluene sulfonic acid (TSA). Temperatures varied between 150 and 210 °C. The PBT samples were characterized using differential scanning calorimetry (DSC), gel permeation chromatography (GPC), small-angle X-ray scattering (SAXS), and matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectrometry. The MALDI mass spectra of all three samples differed greatly but displayed mass peaks of cycles. Similar results were produced by annealing with SnOct₂ or Zr(acac)₄, which favored the formation of even-numbered cycles within the mass range below m/z 5000, along with slow degradation. TSA favored a more intensive degradation without the formation of cycles or even with the destruction of cycles. PBT chains with two carboxylic (COOH) end groups were the most stable species under all circumstances. The origin of the extremely high melting point of AP121 (241–242 °C) can be explained by the smoothing of crystallite surfaces via transesterification. The results suggest that combining MALDI mass spectrometry and SAXS measurements provides a new way to better understand the solid-state chemistry of PBT and related polyesters.
The current work comprises three stages. First, the self-catalyzed polycondensation (SCP) of glycolic acid (GA) was studied in bulk or in suspension at 190°C or at 205°C. Cyclic poly(glycolic acid)s, cPGAs, with number average molecular weights (Mn´s) up 6 700 g mol-1 and dispersities below 2.0 were obtained. These cPGAs possess an unusual molecular weight distribution with a considerable predominance of cycles having degrees of polymerization of 28, 32 and 36. The cycles were formed in the solid state under thermodynamic control which favors the formation of extended-ring crystals. These cPGAs are an off-white, brittle porous mass that is easy to grind, yielding a flowable, crystalline powder suitable for 3D printing with laser sintering (SLS). In the second stage, the influence of non-toxic catalysts such as, Mg, Zn, Ti, Sn(II), Zr and Bi salts or complexes was studied. Compared to the results obtained with self-catalyzed polycondensation in bulk, the best metal catalysts increased the molecular weight by a factor of two to three. However, when compared to the results obtained with self-catalysis in 1,2-dichlorobenzene, the increase was limited to approximately 20%. The third stage examined the influence of 4-toluene sulfonic acid (TSA), which was found to favor the formation of the most perfect crystals with crystallinities up to 84% and the highest reported melting temperatures (up to 245 °C). This indicates the existence of a high Tm morphology, analogous to that of polylactide. Small-angle X-ray scattering (SAXS) measurements indicate that significant growth in crystal thickness is mainly responsible for these effects.
Linear, low-molar-mass poly(trimethylene terephthalate) (PTT) was synthesized via the polycondensation of 1,3-propanediol and dimethyl terephthalate and investigated by matrix-assisted laser desorption/ionization time of flight (MALDI TOF) mass spectrometry, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and x-ray scattering. The obtained PTT was used in crystalline plaque or powder form for studies of its solid-state polycondensation (SSP). The combination of powder and vacuum increased the number-average molecular weight (Mn) by a factor of three. Interestingly, even-numbered cycles with degrees of polymerization (DPs) between six and 16 were preferentially formed. Annealing after doping with tin catalysts produced three cyclic main reaction products (C10, C12 and C14), which suggest that thermodynamic control of transesterification processes favors the formation of three types of monodisperse extended-ring crystallites (ERC) with thicknesses of 5, 6 of 7 repeat units. Additionally, a predominantly cyclic PTT was prepared, once again demonstrating the formation of even-numbered ERC upon annealing. Crystallinities calculated from WAXS measurements showed satisfactory agreement with those determined by DSC. SAXS measurements confirmed that the crystal thickness defined by ERC is also valid for long chains and large cycles that crystallize with chain folding.
Glycolide was polymerized in bulk at 160 °C using various metal acetylacetonates as catalysts. Zirconium acetylacetonate was particularly efficient, enabling rapid polymerization even at 130 °C. The formation of cyclic poly(glycolic acid) (PGA), most likely via a ring-expansion polymerization (REP) mechanism, was proven by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Depending on the polymerization conditions, the formation of even-numbered cycles was favored to varying degrees. Number-average molecular weights (Mn) in the range of 2000–3500 g mol−1 were achieved with dispersities below 2.0. Wide-angle X-ray scattering (WAXS) powder patterns showed that the crystal lattice was the same as that of known linear PGAs, regardless of the Mn values. These patterns enabled a comparison of crystallinities with values derived from DSC measurements.
