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.     

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.     

Colour formation in poly(ethylene terephthalate) during melt processing

The discolouration, that occurs in virgin poly(ethylene terephthalate) - PET during melt processing, was studied using various bulk and surface analytical techniques. Proton nuclear magnetic resonance (¹H NMR) was used to study the bulk chemical changes occurring in the polymer during thermo-oxidative degradation. Chemical derivatisation with trifluoroacetic anhydride (TFAA) was used to label the hydroxyl groups introduced on the polymer surface by thermal oxidation.From the surface analysis studies using photoacoustic Fourier transform infrared spectroscopy (PA/FT-IR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS) it was evident that colour formation starts initially with the hydroxylation of the terephthalic ring. Further, the formation of additional carbonyl functionalities and conjugated chromophoric systems complete the colour formation process.     

Photolysis of poly(ethylene terephthalate)

The photolysis of poly(ethylene terephthalate) films was studied in vacuo with light of wavelengths 2537 and 3130 A. A very stable filter system which cuts out the 3025 A. line was developed to isolate 3130 A. from a mercury spectrum. Despite the fact that the penetration of 2537 A. light was limited to a depth of a ca. 10³ A. whereas 3130 A. light was more uniformly absorbed it was possible to demonstrate that the quantum yields for CO and CO₂ formation were in agreement for the two wavelengths. Quantum yields for fractures and crosslinks were estimated by sol-gel analysis. An absorption maximum which develops near 13 μ after exposure of poly(ethylene terephthalate) to light or γ-rays was attributed to the formation of groups formed by elimination of CO and CO₂. ESR spectra for trapped radicals were tentatively assigned to the components p-C₆H₃· and ·O? CH₂? CH₂? . It is suggested that the former radicals combine to form crosslinks. Quantum yields (× 10⁴) with 3130 A. light are: CO, 6; CO₂, 2; crosslinks, 5.5; trapped radicals, 1.5; With 2537 A. light, quantum yields are: CO, 6–9; CO₂, 2–3; the network formed was not characterized as to crosslinks and fractures; trapped radicals were observed to exist but not determined.     

Unexpected efficiency of cyclic amidine catalysts in depolymerizing poly(ethylene terephthalate)

This article describes studies on the catalytic activity of several nitrogen‐based organic catalysts for the depolymerization of poly(ethylene terephthalate) (PET), in which a few cyclic amidines work more effectively than a potent, bifunctional guanidine‐based catalyst 1,5,7‐triazabicyclo‐[4,4,0]‐dec‐5‐ene (TBD) in the presence of short chain diols that play a role in activation of carbonyl groups through hydrogen bonding. Further studies prove that the catalytic efficiency at the above specific conditions depends only on the extent of activation of a hydroxyl group rather than simply the pK_a of the bases. For glycolysis with excess short‐chain alkanediols, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene is the best catalyst. In contrast, TBD shows outstanding catalytic activity in depolymerizations of PET with mono‐alcohols and longer‐chain diols. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

Cyclo-depolymerization of poly(propylene terephthalate): some ring-opening polymerizations of the cyclic oligomers produced

We report the cyclo-depolymerization of poly(propylene terephthalate) to give a mixture of cyclic oligomers in 94% yield, the characterization of the mixture by ¹H-NMR spectroscopy, matrix assisted laser desorption ionization time of flight mass spectrometry and gel permeation chromatography. The major cyclic oligomer in the mixture was shown to be the cyclic dimer. It was isolated and its X-ray crystal structure determined. Some entropically-driven ring-opening polymerizations of the cyclic oligomers were carried out. So too were some copolymerizations using mixtures of the cyclic oligomers and those derived similarly from poly(ethylene terephthalate) and poly(butylene terephthalate). ¹³C-NMR spectroscopic analysis showed that the copolymers were random. Copyright © 2003 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     

Mesophase structures in poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(ethylene naphthalate bibenzoate)

Films of poly(ethylene naphthalate) (PEN) and poly(ethylene naphthalate bibenzoate) (PENBB) have been drawn under a variety of conditions of temperature and strain rate to determine the conditions under which a nematic-like mesophase structure can be produced. In PEN the combination of low temperature and high-strain rate encourages mesophase formation, while in PENBB the mesophase was formed under all conditions where it proved possible to draw the material at all. A molecular modelling study of the mesophase in PEN and in poly(ethylene terephthalate) (PET) offers possible structures for the mesophase and showed that the mesophase structure could be stable once formed © 1997 John Wiley & Sons, Ltd.     

Kinetic aspects of catalyzed reactions in the formation of poly(ethylene terephthalate)

The influence of different types of catalysts on the polycondensation reaction has been studied in a model system. It has been found that two kinds of catalysts are to be distinguished. “Transesterification” catalysts (Mn, Pb, Zn) are very active in media having both a high and a low hydroxyl content. However, they are easily poisoned by very small amounts of carboxylic endgroups. This restricts their activity to the period of transesterification. The “polycondensation” catalyst antimony is insensitive to the presence of acidic endgroups. However, its activity is inversely proportional to the hydroxyl group concentration For this reason antimony is hardly effective during transesterification. During polycondensation it becomes gradually more active.     

Small angle X-ray scattering studies of poly(ethylene terephthalate) and poly(butylene terephthalate)

Summary Small angle X-ray studies and density measurements were carried out on isotropic PET and PBT samples. PET samples were crystallized between 60 and 260 °C, and PBT between 60 and 225 °C. The aim of these studies was to investigate the dependence of the amorphous density, the degree of crystallinity and the average transmission path through the regions of the two-phase system on the crystallization temperature. It could be shown that PET and PBT crystallize with sharp phase boundaries.Since for the evaluation of the amorphous density the knowledge the exact crystal density is very important, additional measurements of the wide angle X-ray behaviour were made. Both the crystal and the amorphous densities of PET and PBT show specific differences dependent on the crystallization temperature, which can be explained by the higher mobility of the PBT chain.The degrees of crystallization, evaluated with the individual values of crystal density and amorphous density determined on each sample, are principally higher than those calculated with the usually used values of crystal and amorphous density. Investigations of the background scattering have shown that both the specific amorphous and specific crystalline scattering background are constant.For PET and PBT the average transmission path through the amporhous regions firstly decreases with increasing crystallization temperature. This can be explained by new formation of crystallites. At higher crystallization temperatures increases. The average transmission path through the crystalline regions increases over the whole range of crystallization temperature.

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Zusammenfassung An isotropen PET- und PBT-Proben, kristallisiert bei Temperaturen zwischen 60 und 260 °C bzw. 60 und 225 °C wurden Röntgenkleinwinkel- und Dichtemessungen durchgeführt, mit dem Ziel, die amorphe Dichte, die Volumenanteile und die mittleren Durchschußlängen durch die Phasen in Abhängigkeit von der Kristallisationstemperatur zu bestimmen.Da für die Bestimmung der amorphen Dichte die Kenntnis der genauen Kristalldichte sehr wichtig ist, wurden zusätzliche Röntgenweitwinkelmessungen durchgeführt.Es konnte gezeigt werden, daß sowohl PBT als auch PET mit scharfen Phasengrenzen kristallisiert.Die Kristalldichte und die amorphe Dichte von PET bzw. PBT zeigen in Abhängigkeit von der Kristallisations-temperatur spezifische Unterschiede, die durch die höhere Beweglichkeit der PBT-Kette erklärt werden können.Die Kristallisationsgrade, die mit den von uns bestimmten Kristalldichten und amorphen Dichten ermittelt wurden, liegen generell höher als die mit den bekannten Werten von _c und _a berechneten. Untersuchungen des Streuuntergrundes zeigten, daß sowohl der spezifische amorphe als auch der spezifische kristalline Streuuntergrund konstant ist.Bei PET und PBT nehmen die mittleren Durchschußlängen durch die amorphen Phasenanteile bei geringen Kristallisationstemperaturen ab, was durch die Neubildung von Kristalliten erklärt wird, und nehmen bei höheren Kristallisationstemperaturen wieder zu.Die mittleren Durchschußlängen durch die kristallinen Phasenanteile nehmen über den gesamten Temperaturbereich zu.

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With 22 figures and 3 tables     

Organocatalytic depolymerization of poly(ethylene terephthalate)

We describe the organocatalytic depolymerization of poly(ethylene terephthalate) (PET), using a commercially available guanidine catalyst, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Postconsumer PET beverage bottles were used and processed with 1.0 mol % (0.7 wt %) of TBD and excess amount of ethylene glycol (EG) at 190 °C for 3.5 hours under atmospheric pressure to give bis(2‐hydroxyethyl) terephthalate (BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/alkoxide catalysts that are commonly used for depolymerization of PET. The BHET content in the glycolysis product was subject to the reagent loading. This catalyst influenced the rate of the depolymerization as well as the effective process temperature. We also demonstrated the recycling of the catalyst and the excess EG for more than 5 cycles. Computational and experimental studies showed that both TBD and EG activate PET through hydrogen bond formation/activation to facilitate this reaction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Heterogeneous nucleation of poly(ethylene terephthalate)

The effect of various metal salts as nucleating additives for poly(ethylene terephthalate) (PET) has been investigated. In the case of sodium benzoate and probably for all other effective nucleating additives, the nucleation process can be divided into a “heterogeneous particle nucleation” performed by the unreacted salt and a “homogeneous nucleation” due to the polymer–sodium (metal) salt formed during the extrusion. This polymer–sodium (metal) salt is the major nucleating agent in these systems. We have also shown the fundamental difference between the concept of a nucleating additive and that of a nucleating agent.     

Intrinsic birefringence of poly(ethylene terephthalate)

In the existing literature various values are given for the intrinsic birefringence of the crystalline and the amorphous phases in poly(ethylene terephthalate) (PET). These values have either been calculated theoretically or obtained from experimental data on the basis of certain models. In this investigation, using the Samuels two-phase model which correlates sonic modulus with structural parameters, intrinsic birefringence values for the crystalline (Δn_c) and amorphous (Δn_a) phases have been determined by studying 30 PET samples prepared by heat setting to have a wide range of structures; the results are Δn_c = 0.29 and Δn_a = 0.20. These values are discussed along with others in the literature and it is concluded that in the light of the present work, the values used by many authors need reexamination.     

Stress-induced crystallization of poly(ethylene terephthalate)

Quenched amorphous films of poly(ethylene terephthalate) (PET) are stretched at temperatures less than T_g; changes in density, wide-angle x-ray diffraction, and small-angle light scattering are observed. The density increase upon stretching is attributed to an increase in crystallinity accompanied by an increase in the intensity of somewhat diffuse wide-angle x-ray diffraction and of both V_V and H_V small-angle light scattering patterns. The formation of oriented rodlike superstructure may be discerned from small-angle light scattering. Annealing of these samples increases the crystallinity as measured from density and leads to an increase in the perfection of crystalline and supercrystalline structure as measured by wide-angle x-ray diffraction and small-angle light scattering. The rodlike morphology changes to form spherulitelike aggregates as observed by small-angle light scattering and light micrographs. A model is proposed to explain the observations. Studies are extended to stretching films of PET above their T_g and observing changes in birefringence, density, wide-angle x-ray diffraction and small-angle light scattering as a function of elongation and stretching temperature. The formation of defomed spherulitelike superstructure may be discèrned from light micrographs. Results are compared with those obtained upon stretching films below T_g.