Isolation and identification of cyclic oligomers of the poly(ethylene terephthalate)–poly(ethylene isophthalate) copolymer

New cyclic oligomers of the copolymer of poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI) were isolated and identified. A condensation polymerization was carried out at a high temperature, and the solid‐state polymerization that followed yielded the high molecular weight polymer. The oligomers were extracted from the high molecular weight PET–PEI copolymer and separated with preparative high performance liquid chromatography techniques. Their chemical structures and properties were analyzed and determined by ¹H NMR, differential scanning calorimetry, and mass spectroscopy. The oligomers observed at early retention times were a cyclic dimer and cyclic trimers and consisted of [GT]₃, [GI]₂, [GI]₃, [GT]₂[GI]₁, and [GT]₁[GI]₂. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 881–889, 2003     

Kinetic studies on the formation of cyclic oligomers in poly(ethylene terephthalate)

The mechanism of cyclic oligomer formation has been kinetically studied by determining the rate of the formation of cyclic oligomers during melt of poly(ethylene terephthalate) (PET) at several levels of average molecular weight, which were obtained by fractionation and did not initially contain oligomers. The experimental rate equation of cyclic oligomer formation was introduced and then compared with the rate equation derived theoretically. The close agreement between the two equations suggested that the cyclic oligomer formation takes place according to cyclodepolymerization by the action of hydroxyl end groups in PET. The relation is represented as [C] = m·[OH]₀·t^1–n, where [C] is the concentration of cyclic oligomers, [OH]₀ is the initial concentration of hydroxyl end groups, m and n are constants, and t is melting time. A method has also been developed for separating cyclic oligomers from PET using dimethylformamide (DMF) as a solvent.     

Determination of monomers and oligomers in polyethylene terephthalate trays and bottles for food use by using high performance liquid chromatography-electrospray ionization-mass spectrometry

The types and contents of monomers and oligomers in polyethylene terephthalate (PET) food containers were analyzed using HPLC-ESI-MS after being extracted with 50% acetonitrile or dichloromethane using an accelerated solvent extraction unit. The types of cyclic oligomers were classified into first and second series. The first series represented a type of [TG]n composed of terephthalic acid (TPA; T) and monoethylene glycol (EG; G) at a ratio of 1:1. The second series showed a type of [TG]nG in which a single G unit was substituted by diethylene glycol (DEG; GG). The oligomer level extracted using dichloromethane was measured at 4024–11576 mg kg^?1. The first series cyclic oligomers, second series cyclic oligomers and linear oligomers constituted 83.0–90.6%, 7.8–14.7% and 1.3–2.8%, of the total extracted oligomers, respectively. The extracted amounts of TPA, monohydroxyethyl terephthalate and bishydroxyethyl terephthalate using 50% acetonitrile were 3.0–28.2 mg kg^?1, 16.8–118.2 mg kg^?1 and 3.9–26.7 mg kg^?1, respectively. The A2, A3, S2 and S3 groups as modified oligomers were detected as 42.9–221.4 mg kg^?1, 17.2–250.3 mg kg^?1, 1.1–48.1 mg kg^?1 and 1.0–19.8 mg kg^?1, respectively. The results of this study demonstrate an advanced analytical approach to determine the residual oligomers and monomers in PET products for food use and imply their potential migration to foodstuffs.     

Synthesis,characterization, and ring opening polymerization of poly(1,4‐cyclohexylenedimethylene terephthalate) cyclic oligomers

Cyclic oligomers of poly(1,4‐cyclohexylenedimethylene terephthalate) (PCT) were prepared by reaction of 1,4‐cyclohexanedimethanol (CHDM) with terephthaloyl chloride under diluted conditions and separated from the linear products by silica gel column at a yield of 23.7 wt %. Cyclic dimer, trimer, tetramer, pentamer, and hexamer were further separated by high performance liquid chromatography, and found to constitute 98% of the cyclics mixtures. The structures of PCT cyclics were confirmed by means of mass spectrometry, Fourier transform infrared, and ¹H NMR analysis. A series of experiments were carried out to study the effects of catalysts and cis/trans configuration of isomers of CHDM on the yield of cyclic oligomers. Ring opening polymerization of the cyclic oligomers was carried out by heating the sample mixtures at 310 °C for 30 min in the presence of antimony oxide. Polymerization was confirmed by inherent viscosity changes and infrared spectra of the resulting polyesters. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1828–1833, 2000     

Poly(hexamethylene terephthalate)-layered silicate nanocomposites

Nanocomposites of poly(hexamethylene terephthalate) (PHT) and montmorillonite organo-modified with alkylammonium cations bearing two primary hydroxyl functions, i.e., Cloisite^® 30B (CL30B) were synthesized. Organoclay incorporation was performed either by dispersion in the PHT matrix via melt blending or by in situ ring-opening polymerization of hexamethylene terephthalate cyclic oligomers c(HT). An additional procedure combining the two methods, preparation of a highly enriched inorganic “PHT-CL30B” nanohybrid masterbatch by in situ ring-opening polymerization and blending of the masterbatch with additional PHT was explored. The obtained nanocomposites contain 3% (w/w) of inorganics and displayed a mixture of intercalated morphology and exfoliated nanolayers as evidenced by X-ray diffraction and transmission electron microscopy. The nanocomposite obtained by the masterbatch technique exhibited a higher degree of exfoliation and displayed slightly higher glass transition temperatures, better mechanical properties and higher flame resistance. The improved results achieved with the “masterbatch route” are a consequence of the reactions occurring between the nanocomposite constituents allowing for the grafting of PHT chains onto the organoclay surface.     

Isolation and identification of the linear and cyclic oligomers of poly(ethylene terephthalate) and the mechanism of cyclic oligomer formation

Chromatographic techniques are described which can be used to isolate and identify the linear and the cyclic oligomers of poly(ethylene terephthalate). Extraction of the oligomers from high molecular weight polymer produces at least eight different cyclic species, some of which are isolated and identified. The cyclic dimer, the cyclic trimer, and the cyclic tetramer of poly(ethylene terephthalate) have also been prepared by acid chloride esterification and transesterification. Similar materials can be isolated from the ethylene glycol distillate obtained from the polymer melt. The mechanism of cyclic oligomer formation has been studied by determining the rate of formation of the cyclic oligomers during polymerization and during melt extrusion of polyesters which did not initially contain cyclic oligomers. The rate of formation depends upon the concentration of hydroxyl groups; hence, the cyclic oligomers are formed by transesterification from the chain ends or cyclodepolymerization. Therefore oligomers are inevitably produced during polymerization.     

Nucleation of polypropylene by cyclic oligomers present in poly(ethylene terephthalate)

It is shown that cyclic oligomers present in poly(ethylene terephthalate) are excellent nucleating agents for the crystallization of polypropylene. However, rigorous purification of poly(ethylene terephthalate) showed that these oligomers are not responsible for the ability of the polymer to induce transcrystallinity when cooled in contact with polypropylene melts.     

Synthesis and ring‐opening polymerization of macrocyclic aryl ketone oligomers

A series of macrocyclic aryl ketone oligomers were prepared by the reaction of phthaloyl dichloride or isophthaloyl dichloride with various bridge‐linking electron‐rich aromatic hydrocarbons 3a–d under pseudo‐high dilution conditions in the presence of Lewis base via Friedel–Crafts acylation reaction. Detailed structural characterization of these oligomers confirmed the cyclic nature by a combination of MALDI‐TOF‐MS, GPC, and ¹H NMR analyses. These cyclic ketone oligomers have high solubility in organic solvents and the cyclic oligomers derived from phthaloyl dichloride are amorphous. The cyclic ketone oligomers readily undergo anionic ring‐opening polymerization in the melt by using potassium 4,4′‐biphenoxide as the initiator, producing linear, high molecular weight poly(ether ketone)s. Moreover, the isothermal chemorheology of the ring‐opening polymerization of cyclic oligomers 4a and 4b was also investigated. The results show that the shear viscosity of the molten reactive mixture is lower than 10 Pa · S at a constant shear rate of 0.05 rad/sec and increases slowly in the initial stage of ring‐opening polymerization. Copyright © 2009 John Wiley & Sons, Ltd.     

Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via ¹H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004     

Syntheses of random PET‐co‐PTTs and some related copolyesters by entropically‐driven ring‐opening polymerizations and by melt blending: Thermal properties and crystallinity

A library of random poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and seven PET–PTT copolymers has been prepared in a high throughput manner by entropically‐driven ring‐opening polymerizations of the corresponding macrocyclic oligomers. The products have been investigated by differential scanning calorimetry and wide angle X‐ray diffraction. They show that the 50:50 copolymer displays a crystalline phase. The same phase can be formed by in situ transesterification when a 50:50 mixture of PET and PTT is melt blended. Poly(butylene terephthalate) (PBT)–PET and PTT–PBT 50:50 copolymers also show crystal phases. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A novel way to study the initial stages of soap-free emulsion polymerizations: The intercalation of polystyrene oligomers into hydrotalcite

The dominant species in the early stages of an emulsifier-free emulsion polymerization of styrene has been found to be an oligomer of two to three monomer units using a novel trapping technique. This involved the intercalation of charged primary oligomers between the layers of a hydrotalcite, [Mg₄Al₂(OH)₁₂]²⁺[A]^2- (where A = dianion). Hydrotalcites are an important class of lamellar, inorganic compounds whose interlayer spacing can be mod-ified by anion exchange. Our approach first involved preparing a hydrotalcite precursor in which the layers were propped apart by an organic dianion (terephthalate = TA). This material was then used to capture the negatively charged polystyrene oligomers from the emulsion polymerization reaction mixture. We found that TA was rapidly ion-exchanged for the charged oligomers. The resulting pillared hydrotalcite material was characterized using XRD and SEC. We found that the interlayer spacing between the hydroxide layers increased to 23.2 Å on exposure to the emulsion reaction mixture. This represents an interlayer expansion of 18.3 Å (after subtraction of the hydroxide layer contribution), which is cnsistent with intercalation of oligomers with two to three monomer units arranged in a bilayer. This size estimate was confirmed by the results of size exclusion chromatography. © 1995 John Wiley & Sons, Inc.     

Polymerization behavior and gel properties of ethane,ethylene and acetylene-bridged polysilsesquioxanes

Soluble bridged polysilsesquioxanes with a range of molecular weight were synthesized from bis(triethoxysilyl)ethane, ethylene, and acetylene (BTES-E1, -E2, and -E3) via hydrolysis and polycondensation reaction by adjusting the water amount. Polymerization behavior of these three trialkoxysilanes was investigated by monitoring the reaction progress by GPC, and ²⁹Si NMR spectrometry of the resulting polymers, poly(BTES-E1), poly(BTES-E2), and poly(BTES-E3), showing that BTES-E1 generated cyclic oligomers at the early stage. In contrast, polymerization of BTES-E2 and BTES-E3 provided no detectable amounts of cyclic oligomers, but afforded linear polymers only. Bulk gels were also prepared by curing the polymers. The gel from poly(BTES-E3) exhibited high thermal stability derived from the rigid acetylene spacer with respect to thermogravimetric analysis. On the other hand, the polymer film of BTES-E1 showed the highest pencil hardness index among the polymers, indicating the tight siloxane network of poly(BTES-E1).     

Organocatalyzed ring‐opening polymerization of cyclic butylene terephthalate oligomers

In this study, various organic compounds, with different activation modes, have been tested as catalysts for the ring‐opening polymerization (ROP) of cyclic butylene terephthalate oligomers (CBT) in bulk at 210 °C, using tert‐butylbenzyl alcohol (tBnOH) as initiator. Among them, 1,3,5‐triazabicyclo[4.4.0]dec‐5‐ene (TBD) appeared to be the most efficient, achieving high monomer conversions in short reaction times (within minutes). Analysis by size‐exclusion chromatography (SEC) of the poly(butylene terephthalate) (PBT) synthesized using this catalyst also showed that the polymerization follows the expected theoretical M _n trend for molecular weights up to 50 kg·mol^?1. Chain‐end fidelity relatively to the alcohol initiator has been confirmed by MALDI‐TOF mass spectroscopy, which showed that all polymer chains possess the tert‐butylbenzyl moiety as chain‐end. Finally, to demonstrate the potential of this system for the synthesis of PBT‐based block copolymers, a monomethyl ether poly(ethylene glycol) (PEG) of 5000 g·mol^?1 has been employed as initiator for the ROP of CBT. A PEO‐b‐PBT block copolymer of 15,000 g·mol^?1 could thus been obtained, as confirmed by the shift of the SEC traces towards higher molecular weights and the same diffusion coefficient determined for ¹H NMR signals of the PEO block and the PBT block. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1611–1619     

Branching and cross-linking of poly(ethylene terephthalate) and its foaming properties

The branching and cross-linking of poly(ethylene terephthalate) were investigated using two chain extenders: glycidyl methacrylate-styrene copolymer (GS) and poly(butylene terephthalate)-GS (PBT-GS) in order to improve the melt viscosity and melt strength of poly(ethylene terephthalate). An obvious increase in torque evolution associated with chain extending, branching and cross-linking was observed during the process. The properties of modified poly(ethylene terephthalate) were characterized by intrinsic viscosity and insoluble content measurements, rheological and thermal analysis. The intrinsic viscosity and rheological properties of modified PET were improved significantly when using PBT-GS, indicating that PBT-GS should be a better chain extender. Good foaming of poly(ethylene terephthalate) materials were obtained using supercritical CO₂ as blowing agent. The average cell diameter and cell density were 61 μm and 1.8 × 10⁸ cells/cm³, respectively.     

Isolation,spectra, structure,and ring-opening polymerization of strained macrocyclic aryl(ether ketone) dimer

Macrocyclic arylene ether ketone dimer was isolated from a mixture of cyclic oligomers obtained by the nucleophilic substitution reaction of bisphenol A and 4,4′-difluorobenzophenone and easily polymerized to high molecular weight linear poly-(ether ketone). The cyclic compound was characterized by FTIR, ¹H- and ¹³C-NMR, and single-crystal x-ray diffraction. Analysis of the spectral and crystal structure reveals extreme distortions of the phenyl rings attached to the isopropylidene center and of the turning points of the molecular polygons. The release of the ring strain on ring-opening combined with entropical difference between the linear polymer chain and the more rigid macrocycle at temperatures of polymerization may be the proposed motivating factors in the polymerization of this precursor to high molecular weight poly(ether ketone). © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1753–1761, 1997     

Mechanism of dispersion polymerization of L-lactide initiated with 2,2-dibutyl-2-stanna-1,3-dioxepane

 Biodegradable polyester microspheres were synthesized directly by ring-opening polymerization of l-lactide initiated with 2,2-dibutyl-2-stanna-1,3-dioxepane. The polymerizations were carried out at 95 °C in a mixture of organic solvents (heptane/1,4–dioxane 4:1 v:v), in the presence of poly(dodecyl acrylate)-g-poly(ɛ-caprolactone) used as a surface-active agent. Under these conditions the poly(L-lactide) synthesized was shaped into microspheres. The absence of new particles in the polymerizations with multistep monomer addition indicated that after the formation of particle seeds the propagation proceeds exclusively inside the microspheres. The mean volume of these microspheres was proportional to the monomer conversion. It was found that regardless of the initiator concentration the average number of poly(L-lactide) macromolecules in one microsphere was 1.84 × 10⁸. Matrix-assisted laser desorption ionization time of flight spectroscopy of poly(L-lactide) in the microspheres indicated that the propagation in the particles was accompanied by intra- and intermolecular transesterification side reactions, resulting in reshuffling of the polymer segments and the formation of cyclic oligomers. Received: 20 December 2000 Accepted: 7 June 2001     

Configurational properties of aromatic polyesters: Dipole moments of poly(diethylene terephthalate) chains

Dielectric constants have been determined for a fraction of poly(diethylene terephthalate) in benzene at several temperatures. The data indicate that the dipole moment ratio 〈μ²〉/Nm² is somewhat higher than that of poly(ethylene oxide), and its temperature coefficient is in the vicinity of zero. Both the dipole ratio and its temperature coefficient are in very good agreement with those predicted by the rotational isomeric state theory. Using this theory, the unperturbed dimensions of poly(diethylene terephthalate) were calculated and it was found that (〈r²〉/M)_∞ = 0.80 Ų (g mol wt)^?1, a value intermediate between those of poly(ethylene oxide) (0.57) and poly(ethylene terephthalate) (1.05).     

Sequential analysis of polyesters by stepwise chemical degradation: Preparation of degradation products

Model reactions for the sequential analysis of polyesters, especially those of the poly(ethylene terephthalate) and poly(butylene terephthalate) type, by stepwise chemical degradation were performed. The cyclic degradation products, containing the reagent and an ethylene terephthalate or butylene terephthalate unit, terephthalic acid mono(2-{o{N-<N′-[4-(iminomethyl-) benzoyl-]> 2′-(imino)ethoxycarbonyl-}iminobenzoyl-} oxyalkyl) ester N″-lactams, were deliberately synthesized.     

A new approach of characterizing the hydrolytic degradation of poly(ethylene terephthalate) by MALDI-MS

The hydrolytic degradation of technical poly(ethylene terephthalate) (PET) was investigated by means of different methods such as size-exclusion chromatography (SEC), viscometry, light-scattering, thin-layer chromatography, end-group titration, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The long-term degradation was simulated by exposing PET filament yarns to aqueous neutral conditions at 90°C for up to 18 weeks. By means of MALDI-MS and thin-layer chromatography, the formation of different oligomers was obtained during polymer degradation. As expected, an ester scission process was found generating acid terminated oligomers (H-[GT]_m-OH) and T-[GT]_m-OH and ethylene glycol terminated oligomers (H-[GT]_m-G), where G is an ethylene glycol unit and T is a terephthalic acid unit. Additionally, the scission of the ester bonds during the chemical treatment led to a strong decrease in the number of cyclic oligomers ([GT]_m). The occurrence of di-acid terminated species demonstrated a high degree of degradation. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2183–2192, 1997     

Synthesis of silazanylene-vinylene oligomers via catalytic polycondensation of divinyltetramethyldisilazane

Ruthenium (e. g., RuHCl(CO)(PPh₃)₃ and [RuCl₂(CO)₃]₂) and rhodium complexes (e. g., [RhX(cod)]₂, where X = Cl, OSiMe₃) appear to be the first effective catalysts for polycondensation of divinyltetramethyldisilazane ( I ) (ADPOL) to give poly(silazanylene-vinylene)s. Ruthenium catalysts give oligomers (M_w = 2 380, M_w/M_n = 1.21) and a mixture of trans-tactic oligomers, respectively, while rhodium complexes lead to the formation of a mixture of cyclic and linear oligomers.