Mechanism of thermal depolymerization of trimethylsiloxy-terminated polydimethylsiloxane

“Catalyst-free” trimethylsilyl end-blocked polydimethylsiloxanes of M _n = 68,300 and 111,000 were isothermally depolymerized above 400°C. The volatilized products were a mixture of cyclic oligomers and trace quantities of Me₃Si(OSiMe₂)_nOSiMe₃ (n = 0,1). The relative degree of polymerization was followed as a function of fractional conversion. The data were consistent with the oretical curves for a mechanism involving initiation by random siloxane bond cleavage followed by a rapid and complete unzipping of the kinetically active fragments. It is suggested that catalysis may be responsible for the initiation process, thus accounting for the low activation energy (43 Kcal/mol) relative to the siloxane bond energy (108 kcal/mol).     

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.     

Damage and Repair in Informational Poly(N-substituted urethane)s

The degradation and repair of uniform sequence-defined poly(N-substituted urethane)s was studied. Polymers containing an ω-OH end-group and only ethyl carbamate main-chain repeat units rapidly degrade in NaOH solution through an ω→α depolymerization mechanism with no apparent sign of random chain cleavage. The degradation mechanism is not notably affected by the nature of the side-chain N-substituents and took place for all studied sequences. On the other hand, depolymerization is significantly influenced by the molecular structure of the main-chain repeat units. For instance, hexyl carbamate main-chain motifs block unzipping and can therefore be used to control the degradation of specific sequence sections. Interestingly, the partially degraded polymers can also be repaired; for example by using a combination of N,N′-disuccinimidyl carbonate with a secondary amine building-block. Overall, these findings open up interesting new avenues for chain-healing and sequence editing.     

Synthesis and properties of π‐conjugated dithiafulvene oligomers by addition of a monofunctionalized compound

Dithiafulvene oligomers ( 3 ) were prepared by cycloaddition polymerization of aldothioketenes with their alkynethiol tautomers derived from 1,4‐diethynylbenzene ( 2 ) with the addition of 1‐ethynyl‐4‐methylbenzene ( 1 ) as a monofunctionalized compound. Different feed ratios of 2 / 1 were used to control the molecular weights of 3 . The structures of 3 were confirmed by IR and ¹H NMR spectroscopies in comparison with those of 2‐(4‐tolylidene)‐4‐tolyl‐1,3‐dithiol ( 4 ) as a model compound, which was obtained by the treatment of lithium 2‐tolylethynethiolate with water in Et₂O. The number‐average degree of polymerization (DP) and the number‐average molecular weight were measured by gel permeation chromatographic and ¹H NMR analysis. DP increased with an increasing feed ratio of 2 / 1 . The ultraviolet–visible spectra of 3 in diluted acetonitrile showed that the absorption maxima of 3 increased with an increasing DP of 3 . These redshifts are ascribed to an effective expansion of the π‐conjugation system in 3 . The oligomers exhibited a maximum conjugation length of seven repeating units. The redox properties of 3 were examined by cyclic voltammetry. The oxidation half‐peak potentials (E_p/2) of 3 were slightly cathodically shifted with increasing DP. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 708–715, 2003     

Pyrolysis-gas-liquid-chromatography utilised for a kinetic study of the mechanisms of initiation and termination in the thermal degradation of polystyrene

Pyrolysis-gas-liquid-chromatography (“thermocouple feedback” technique) has been used to study the thermal degradation kinetics of ionically-initiated and free-radical-initiated samples of polystyrene. Although mass-spectrometric measurements confirm that the pyrolysis products from large samples (1 mg) contain oligomers up to at least hexamer in addition to monomer, only monomer is detected when small thin samples (0.1 μg, 10²–10⁵ Å) are used. This effect is not due to a sensitivity problem in detecting oligomers, nor to the incapacity of such compounds of limited volatility to elute from the GLC apparatus. In studying the kinetics of monomer evolution from thin films, initial work was concerned with the effect of film thickness and the limits of first-order behaviour. Then the specific rate of monomer evolution (k_obs) was measured as a function of molecular weight for both types of sample at 723 K and 753 K; the results indicate that the pyrolysis mechanism involves both initiation at the chain-ends and initiation by random scission. Kinetic schemes involving mixed initiation have been proposed, and on this basis the results have been analysed to yield activation energies for scission and end-initiation for both types of sample. Comparison of the activation energies obtained with the quoted value for scission of a CC bond has shown that the depolymerization chain termination process cannot be second order and must be first order in the concentration of long chain radicals. The experimental results also indicate that the ionically-initiated polystyrenes are more stable than free-radical-initiated samples of comparable molecular weight. Possible initiation sites have been discussed with reference to the samples examined and to previous published studies. Several mechanisms leading to first order termination have been proposed; it is suggested that the most probable process involves intramolecular transfer with subsequent scission to give an oligomer radical which is small enough to diffuse readily from the system without further reaction.     

Thermal Degradation of Polyphosphazene Homopolymers and Copolymers Prepared by the Anionic Polymerization of Phosphoranimines

Abstract

The thermal degradation of poly(bis-trifluoroethoxyphosphazene), as well as random and block copolymers bearing trifluoroethoxy and alkoxyalkoxy groups synthesized by the anionically initiated polymerization of phosphoranimines, has been investigated by TGA. These studies indicate that the thermal stability of the copolymers decreases with incorporation of alkoxyalkoxy groups. There is no direct correlation between the calculated activation energies of decomposition. This has been attributed to entropy differences. Molecular weight vs conversion plots indicate that depolymerization occurs by chain end initiation followed by complete unzipping to cyclic trimer. This is in contrast to polymers prepared by the ring-opening method. This difference has been attributed to a defect-free structure. The primary product of the decomposition of these polymers is the corresponding cyclic trimer.     

Computational Study of Metathesis Degradation of Rubber, 3. Distribution of Cyclic and Linear Oligomers via Intermolecular Degradation of cis‐Poly(butadiene)

The molecular modeling of the product distributions for the intermolecular metathesis degradation of cis‐poly(butadiene) (cis‐PB) in the presence of ethylene as chain‐transfer agent (CTA) at 298.15 K using the B3LYP/6‐31G (d, p) level of theory reveals that chain–ring and chain‐chain equilibria are shifted toward the formation of 1,5‐hexadiene. The amount of cyclic oligomers at equilibrium with linear molecules is negligible. The α,ω‐vinyl‐terminated butadiene oligomers–1,5‐hexadiene equilibrium constant depends on the cis/trans isomer ratio in linear butadiene molecules. While the concentration of 1,5‐hexadiene at equilibrium with cis‐butadiene oligomers is 86 mol‐%, this value for trans‐butadiene oligomers corresponds to 50 mol‐% of 1,5‐hexadiene. The results of calculations are in reasonable agreement with recent experimental data on the intermolecular metathesis of 1,4‐cis‐PB with ethylene using a well‐defined ruthenium alkylidene catalyst. The calculations predict that cis‐butene as a CTA is more efficient in the metathesis depolymerization of cis‐PB compared with ethylene.     

Synthesis and kinetics of oxidation of some dipeptides with anodically generated manganese(III) sulphate: Mechanistic study

Dipeptides (DP) namely phenylalanyl–proline (Phe–Pro), isoleucyl–proline (Ile–Pro), and leucyl–proline (Leu–Pro) were synthesized by classical solution method and characterized. The kinetics of oxidation of these DP by Mn(III) have been studied in the presence of sulphate ions in acidic medium at 26°C. The reaction was followed spectrophotometrically at λ_max = 500 nm. A first‐order dependence of rate on both [Mn(III)]₀ and [DP]₀ was observed. The rate is independent of the concentration of reduction product, Mn(II), and hydrogen ions. The effects of varying dielectric constant of the medium and addition of anions such as sulphate, chloride, and perchlorate were studied. The activation parameters have been evaluated using Arrhenius and Eyring plots. The oxidation products were isolated and characterized. A mechanism involving the reaction of DP with Mn(III) in the rate‐limiting step is suggested. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 438–444, 2002     

The synthesis and characterization of poly(methylene terephthalate)

Poly(methylene terephthalate) (1GT) has been synthesized via the reaction of cesium or potassium terephthalates with dibromomethane or bromochloromethane in N-methylpyrrolidone at temperatures of 80–125°C. The polymerization was relatively slower with the latter substrate, though the potassium salt was found to be equally as efficient as its cesium counterpart with dibromomethane. The polymer is insoluble in all common polyester solvents, and its high molecular weight nature (DP _n ≥ 25) was inferred from elemental analyses and its fiber forming capacity. Thermal analyses indicated that 1GT possesses poor thermal stability and decomposes rapidly during melting, the initial process being thought to be the splitting out of formaldehyde. 1GT polymers were shown to contain a homologous series of cyclic oligomers (from dimer to decamer); the two most predominant were tentatively identified as the cyclic trimer and tetramer. No change in the cyclization efficiency was observed when the potassium counterion was substituted for cesium with Br CH₂ Br whereas a drastic reduction in the cyclic content was obtained using Br CH₂ Cl (with Cs⁺). The two most important features of the polymerization are the insensitivity of the reaction to the stoichiometric equivalence of the reactants and the production of reasonably high molecular weights at low conversions. It is suspected that the polymerization might be occurring through an interfacial mechanism.     

The effect of ammonium polyphosphate on the mechanism of thermal degradation of polyureas

The thermal decomposition of structurally related N–H and N,N′-disubstituted polyureas (Table I) and their mixtures with ammonium polyphosphate (APP) was investigated by thermogravimetry (TG) and direct pyrolysis in a mass spectrometer (MS). The N–H polyureas (IV–VI) undergo a quantitative depolymerization process with the formation of oligomers with amine and isocyanate end groups. In contrast, the thermal degradation of the N,N′-disubstituted polyureas (I–III) proceeds by a different mechanism as a function of their chemical structure. The addition of APP lowers the thermal stability of the N,N′-disubstituted polyureas, whereas that of the N–H polyureas is unaltered. However, our data show that APP does not change the nature of the pyrolytic products. The destabilizing effect of the additive can be attributed to the catalytic action of the acid species formed by its thermal decomposition.     

Novel synthesis of cyclic methylvinylsiloxanes by the reaction of methylvinyl diethoxysilane with phosphorous pentachloride

We report a novel synthesis of the cyclic oligomers [(H₂C?CH)(CH₃)SiO]_n obtained by the reaction between phosphorous pentachloride and methylvinyl diethoxysilane. According to gas chromatography/mass spectrometry data, the reaction product consisted of a mixture of cyclic oligomers in which the most important fractions were composed of cycles with n = 5, 6, or 7. The reaction product was also characterized by Fourier transform infrared and ¹H and ¹³C NMR spectroscopy. Experimental results suggested a new kind of reaction between the phosphorous pentachloride and terminal olefins directly bonded to silicon, which was probably associated with sterical effects favoring C? O? Si bond cleavage of ethoxy groups instead of the conventional addition of phosphorous pentachloride to olefinic linkages. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3182–3189, 2002     

Cyclopolymerization of (E,E) - [6.2]paracyclophane-1,5-diene

Polymerization of (E,E)-[6.2]paracyclophane-1,5-diene proceeds by an intra–intermolecular mechanism to give a polymer containing a [3.2]paracyclophane in the repeat unit. Polymerization occurs with either free radical or cationic initiation; anionic initiation was unsuccessful. Cationic polymerization is favored and appears to proceed through a stabilized carbonium ion intermediate. Spectroscopic and model compound studies are consistent with the proposed polymer structure. Thermal analyses of the polymer indicate a complex thermooxidative behavior in the presence of oxygen, while depolymerization occurs above about 400°C in an inert atmosphere.     

Combining Orthogonal Chain‐End Deprotections and Thiol–Maleimide Michael Coupling: Engineering Discrete Oligomers by an Iterative Growth Strategy

Orthogonal maleimide and thiol deprotections were combined with thiol–maleimide coupling to synthesize discrete oligomers/macromolecules on a gram scale with molecular weights up to 27.4 kDa (128mer, 7.9 g) using an iterative exponential growth strategy with a degree of polymerization (DP) of 2ⁿ −1. Using the same chemistry, a “readable” sequence‐defined oligomer and a discrete cyclic topology were also created. Furthermore, uniform dendrons were fabricated using sequential growth (DP=2ⁿ −1) or double exponential dendrimer growth approaches (DP=2 −1) with significantly accelerated growth rates. A versatile, efficient, and metal‐free method for construction of discrete oligomers with tailored structures and a high growth rate would greatly facilitate research into the structure–property relationships of sophisticated polymeric materials.     

Thermal degradation of poly(parabanic acid)

A variety of techniques were used to follow the thermal degradation of a poly(parabanic acid) (PPA-M). The kinetic data associated with the weight loss as a function of time at elevated temperatures indicates a random initiation process followed by depolymerization. Infrared and mass spectral techniques further confirmed the kinetic data by showing the presence of isocyanate and amide groups in addition to CO, CO₂, and nitric oxide gases being evolved. Incipient gelation occurs simultaneously with the formation of the amide. A reverse Hoffman rearrangement mechanism has been proposed which appears to be consistent with the available experimental data.     

High-pressure synthesis of cyclic phenylacetylene oligomer

New conjugated oligomers were prepared by reacting phenylacetylene under high pressure of 0.11 to 0.92 GPa at 100–200°C for 0–5 h. The number-average molecular weight M?_n, the weight-average molecular weight M?_w, and the oligomer yield increased with pressure, tem-perature, and time. The average molecular weight of the oligomer showed the maximum value (M?_n: 830, M?_w: 2400) under 0.92 GPa, the maximum pressure, where phenylacetylene was oligomerized at a constant temperature. The structure of the oligomer was investigated from ESR, infrared, UV–VIS, field desorption mass (FDMS) spectra, and ¹³C NMR spec-trum. Analysis of the FDMS spectrum revealed that the molecular weight of the oligomer was multiple of the monomer. ¹³C NMR spectrum of the oligomer showed the absence of sp-carbon (? C?). We found that the oligomer had a cyclic structure. The cyclic oligomers of pentamer or more were new compounds. © 1995 John Wiley & Sons, Inc.     

Thermal Degradation of Poly(Allyl Methacrylate) by Mass Spectroscopy and TGA

Allyl methacrylate, AMA was polymerized in CCl₄ solution by α,α′‐azoisobutyronitrile at 50°C. The thermal degradation mechanism of PAMA was characterized by MS, TGA‐FT‐IR and FT‐IR‐ATR methods. The mass spectrum and TGA thermogram showed two stage degradation. The first stage of degradation was mostly linkage type degradation for the fragmentation of pendant allyl groups at 225–350°C. In the second stage, at 395–515°C, the degradation is random scission and depolymerization types. This was also supported by direct thermal pyrolysis of polymer under vacuum. The degradation fragments of MS and TGA were in agreement. In the degradation process, monomer degraded further to CO, CO₂, allyl and ether groups. No strong monomer peak was observed in mass spectrum.     

Early stages of the hydrolysis of chromium(III) in aqueous solution-XII. Kinetics of cleavage of the trimer and tetramer in acidic solution

The kinetics of cleavage of the hydrolytic trimer and tetramer of chromium(III) have been studied in acidic solution and at elevated temperatures using spectrophotometric methods. For both reactions, the variation in absorbance with time was consistent with the presence of two consecutive rate-determining processes. The two reactions were concluded to proceed according to: Product analyses and consideration of the stability constants for these oligomers indicated that their overall cleavage reactions were irreversible under the conditions studied. However, it was unclear whether the initial step in tetramer cleavage was irreversible or not. The dimer was identified as an intermediate in the cleavage of the trimer. Each of the bridge-cleavage processes (k₁ and k₂) occurred via acid-dependent pathways, involving protonation processes in addition to bridge-cleavage processes, while only some of them displayed acid independent pathways. The acid dependence of the rate constants and associated parameters at 298 K are: (i) for trimer cleavage k₁ = k_b [H⁺], where the dimer to monomer was found to be the rate-determining step at higher temperatures (≈350 K), while cleavage of the trimer to dimer and monomer becomes rate determining at lower temperature (≈ 300 K). This change in the rate-determining step is attributable to the much higher value for k₁ compared with k₂. Possible mechanisms for trimer and tetramer cleavage are discussed and their rates compared to previously studied cleavage reactions of Cr^III oligomers.     

About the depolymerization of cellulose propionate to segments with low DP

Low molecular weight cellulose propionate (CP) segments were synthesized by depolymerization with HBr under esterification conditions using either a commercial CP with residual hydroxyl groups (DS_OH ca. 0.4) or a tripropionate (CTP). The depolymerization using CTP yielded segments having a minimum DP of as low as 7, whereas the depolymerization of the commercial CP resulted in segments having a minimum DP of 50. Differences were also seen in the rate of depolymerization and the Mark-Houwink exponent, which declined from 1 to 0.6 below DP 100 and 30 for commercial CP and CTP, respectively. The results are consistent with the hypothesis that branching is induced by transglycosidation when OH-containing cellulose derivatives are depolymerized with HBr in aprotic solution.     

Lipase‐Catalyzed Degradation of Polyester in Supercritical Carbon Dioxide

Enzymatic degradation of poly(ε‐caprolactone) has been successfully carried out in supercritical carbon dioxide (scCO₂). Candida antarctica lipase smoothly catalyzed the hydrolytic degradation in scCO₂ to give oligo(ε‐caprolactone). The degradation in the presence of acetone (5 vol.‐%) produced the oligomer of smaller molecular weight (less than 500) compared to that prepared without the additive. Matrix‐assisted laser desorption/ionization‐time of flight (MALDI‐TOF) mass spectrometry analysis showed that the degradation product was of a mixture of linear and cyclic oligomers. The addition of a very small amount of water also promoted the degradation of the polyester.     

Kinetic and mechanistic studies of indigocarmine oxidation by chloramine-T and chlorine in acidic buffer media

Oxidations of indigocarmine (IC) by chloramine-T (CAT) and aqueous chlorine (HOCl) in acidic buffer media, pH 2–6, have been kinetically studied at 30°C using spectrophotometry. The CAT reaction rate shows a first-order dependence on [IC]₀ and an inverse fractional order on [p-toluenesulfonamide]. The effect of [CAT] on the rate is strongly pH dependent with a variable order of 1–2 on [CAT]₀ in the pH range 6–2. The chlorine reaction rate follows first-order in [IC]₀ and [HOCl]₀ each in the pH range 6–2. Addition of halide ions and variation of ionic strength of the medium have no influence on the reaction rate. There is a negative effect of dielectric constant of the solvent. The kinetics of the IC oxidation with CAT at pH 6 and of that with HOCl at pHs 2–6 are similar suggesting similarity in their rate determining steps. A two-pathway mechanism for the CAT reaction and a one-pathway mechanism for the HOCl reaction, consistent with the kinetic data, have been proposed. Activation parameters have been calculated using the Arrhenius and Erying plots. © 1995 John Wiley & Sons, Inc.