Hans Rytger Kricheldorf

Polymerizations of glycolide with various metal acetylacetonates showed that Zr(acac) ₄ is the most reactive catalyst, yielding low-molecular-weight cyclic poly(glycolide). The combination of WAXS and DSC measurements confirmed a Δ H 0m of 206 J g ⁻¹ .

Ring‐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 Bu ₂ Sn were reactive enough to enable quantitative polymerization within a few hours. Further ROPs with Bu ₂ Sn(acac) ₂ and Zr(acac) ₄ showed that Bu ₂ Sn(acac) ₂ had a slightly lower reactivity than the Zr complex. The ROPs catalyzed with Zr(acac) ₄ yielded number average molecular weights ( M _n ) 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 Bu ₂ Sn(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 ¹³ C NMR spectroscopy.

Pyridine is shown to be an effective catalyst for the snthesis of low molar mass cyclic polyglcolides via zwittzerionic polymerization. The cyclic polyglycolides form extended-ring crystals in the mass range below m/z 3000.

Water-initiated ring-opening polymerizations (ROPs) of L-lactide (LA) were performed in bulk at 140 °C, whereupon 11 different salts or complexes of low-toxic metal ions such as tin (II), Zn(II), Zr(IV), and Bi(III) were used as catalysts. Only four catalysts gave satisfactory results despite a long reaction time (24 h). The influence of the temperature was studied with tin (II) 2-ethylhexanoate (SnOct₂), dibutyltin oxide (Bu₂SnO), and zinc bis-acetylacetonate (Zn(acac)₂) and zirconium acetylacetonate (Zr(acac)₄). With SnOct_2, Bu₂SnO and Zr(acac)₄ the time was varied at 140 °C to compare their reactivity. Furthermore, two series of ROPs were carried out with variation of the LA/H₂O ratio to verify the control of the molecular weight by the monomer-initiator ratio. Bi- or tri-modal molecular weight distributions (MWD) were found for most of the ROPs and bimodal melting endotherms in the differential scanning calorimetry (DSC) heating traces. The thickness and dependence of the 3D packing of the crystallites were monitored by small-angle X-ray scattering (SAXS) measurements.

Cyclic poly(L-lactide)s (PLAs) were prepared in bulk either by ring-expansion polymerization (REP) or by ring-opening polymerization (ROP) with simultaneous polycondensation (ROPPOC). In contrast to REP the latter method involves formation of linear chains and thus, may involve formation of polydisperse catenanes that affect crystallization. The reprecipitated PLAs were annealed at 120 °C and compared with regard to melting temperature (T_m) and melting enthalpy (ΔH_m). For similar molar masses the PLAs prepared by REP and ROPPOC had almost identical T_m's and crystallinities. Furthermore, the influence of REP and ROPPOC catalysts on the morphology of the virgin reaction products was compared.

Preformed polylactides with CH-OH and COOH end groups were heated with three different catalysts. Proton-catalyzed esterification favored chain growth, while tin catalysts favored cycle formation.

L‐lactide (LA) is polymerized with ethyl L‐lactate or trifluoroethanol as initiators (LA/In = 30/1), and the resulting poly(L‐lactide) esters are acetylated with acetic anhydride. The acetylated PLA esters are mixed with SnOct₂, dibutyltin bis(4‐chlorophenoxide), or dibutyltin bis(pentafluorophenoxide) in solution and crystallized at 120 °C. The doped crystals are annealed at 140 °C or at 160 °C for 7, 14, and 28 days, and the chemical modifications that occurred in the solid state are monitored by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI TOF) mass spectrometry, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC) measurements. All catalysts promoted polycondensation, while no significant formation of cycles is observed. However, in the presence of SnOct₂, disproportionation of chains occurred upon annealing at 120 or 140 °C, and crystallites consisting of extended chains with M_n values ≈3500 – 3600 and low dispersities (Ð < 1.5) are formed.

L-Lactide (LA) was polymerized with neat tin(ii) 2-ethylhexanoate (SnOct₂) in toluene at 115 °C at low concentration with variation of the LA/Cat ratio.

Ring‐opening polymerizations (ROPs) of l‐lactide (LA) were performed with ethyl l‐lactate or 11‐bromoundecanol as initiators (In) and tin(II) ethyl hexanoate (SnOct₂) as catalyst (Cat) using four different LA/In ratios (20/1, 40/1, 60/1, and 100/1). One series of ROPs was conducted in bulk at 120 °C, yielding PLAs with low dispersities (Ð ~ 1.2–1.4), and a second series was conducted in bulk at 160 °C, yielding higher dispersities (Ð ~ 1.3–1.9). Samples from both series were annealed for 1 or 14 days at 140 °C in the presence of SnOct₂. Both polycondensation and disproportionation reactions occurred, so that all four samples tended to form the same type of molar mass distribution below 10,000 Da, regardless of their initially different number average molar masses (M_n). Both initiators gave nearly identical results. The thermodynamic control of all reversible transesterification processes favored the formation of crystallites composed of chains with a M_n around 3500–3700, corresponding to a crystal thickness of 10–13 nm.

The Na salt of chloroacetic acid is condensed in suspension. Furthermore, glycolic acid is condensed in bulk or in concentrated solution by means of SnCl₂ or 4‐toluene sulfonic acid (TSA) as catalysts. The temperatures are varied from 160 to 200 °C and the time from 1 to 5 days. Low molar mass cyclic poly(glycolic acid) (PGA) is detected by means of matrix‐assisted laser desorption ionization time‐of‐flight (MALDI TOF) mass spectrometry in most PGAs. A predominance of certain cycles having an even number of repeat units is observed suggesting a thermodynamically favored formation of extended‐ring crystals. Extremely high melting temperatures (up to 237.5 °C) and high melting enthalpies are found for polycondensations with TSA in 1,2‐dichlorobenzene.