Initial Steps in Thermal Degradation of Fractions of PVC with Different Tacticities

PVC samples were prepared in bulk by using AIBN and UV radiation as the initiator system. The polymerization temperatures were 40, 0, -25, and -50oC. The samples were fractionated by means of successive extractions with dioxane, tetrahydro-furan, and cyclohexanone, and two fractions of each sample were characterized in order to study their thermal degradation. This was carried out by conductivity measurements with the use of a differential conductimetry cell for degradations up to 0.1% and a single cell for degradations up to 0.4%. From the UV-visible spectra of equally degraded samples it is concluded that the higher the tacticity of PVC, the higher the proportion of long polyene sequences is; this behavior is independent of the conversion. The degraded samples were ozonized in order to measure the number of scissions per chain. This number was found to be low for syndiotactic samples and high for atactic samples, which is accounted for by the increased clustering of double bonds to form long polyene sequences in the more syndiotactic polymers. Previous results suggesting increased ease of propagation along syndiotactic sequences are therefore confirmed more quantitatively.     

Initiated oxidation of polyenes formed in the thermal degradation of PVC

Polyenes formed in the thermal degradation of PVC are readily oxidized in the liquid phase in the presence of a radical initiator. In pure oxygen at a constant rate of initiation, different length polyenes are consumed by a first-order reaction: the rate of consumption is proportional to the polyene length and to the square root of initiator concentration. Applying the relationships given by the theory of chain reactions, we determined the ratio of the rate constant of chain propagation calculated for one double bond k₂ to the square root of the chain-termination rate constant k₄. The value obtained, which is relatively high in comparison to other unsaturated hydrocarbons, reflects very well the high reactivity of polyenes of the degraded PVC sample towards oxidation. Intramolecular chain propagation steps are also likely to play a role in the oxidation of polyenes.     

Degradation of poly(vinyl chloride) by u.v. radiation—II: Mechanism

The mechanism of the light-induced degradation of solid poly(vinyl chloride) (PVC) has been investigated, and an overall reaction scheme has been developed, based on values of the quantum yields for the primary photoproducts. Only a very small fraction (0.2%) of the excited polyenes induces the degradation of PVC, primarily by photocleavage of the allylic CCl bond. The high instability of β-chloroalkyl radicals is responsible for the chain dehydrochlorination that leads to formation of polyenes. In the absence of O₂, chain scissions and crosslinking are postulated to originate mainly from α-chloroalkyl radicals through β-cleavage of CC bonds and radical coupling, respectively. In the presence of O₂, the chain dehydrochlorination still proceeds, together with an oxidative chain process which yields, via peroxy and alkoxy radicals, hydroperoxides, ketones and peroxide crosslinks. Cleavage of the polymer backbone results most probably from the decomposition of tertiary alkoxy radicals by a carbon-carbon β-scission process.     

Stereoselective substitution on PVC using phase transfer catalysts

The nucleophilic substitution with sodium benzenethiolate in aqueous suspension in the presence of a phase-transfer catalyst has been studied for two samples of PVC of different tacticities. The kinetics shows two well-defined periods: one period is very fast and involves low conversions that are greater as the isotactic content of PVC increases, while another steady period is slow and its slope seems to depend on physical features, e.g., molecular weight and crystallinity, which would affect the accessibility of nucleophile. The evolution of unreacted syndio, iso, and heterotactic triad contents with degree of substitution has been followed by ¹³C-NMR spectroscopy. Since the content of syndiotactic triads does remain unreacted at the end of the reaction and that of isotactic and heterotactic triad decreases, but at a different rate, the reaction is proved to proceed by a stereospecifically controlled mechanism. The influence of degree of substitution on both the thermal degradation rate and the evolution of UV–Visible spectra of equally degraded samples has been also studied; the highest stability in terms of degradation rate and content of short polyenes is obtained when a specific fraction of isotactic triads has reacted. From the correlation between the results of this work and those previously obtained in solution, some enlightening conclusions about the reaction mechanism are drawn.     

Studies on copolymers of vinyl chloride with diethyl fumarate,isobutylene and vinyl bromide as PVC models with chain irregularities—III The influence of the tacticity and of the copolymer composition on the thermal degradation: The influence of the tacticity and of the copolymer composition on the thermal degradation

The thermal degradation kinetics of pairs of vinyl chloride copolymers with diethyl fumarate, isobutylene and vinyl bromide, having the same composition but different tacticity, were followed by conductivity measurements. From the results for degradation up to 0·3% and up to 10%, it follows that syndiotactic sequences in PVC give rise to high degradation rates in agreement with results of previous work on homopolymers of vinyl chloride. On the basis of kinetic aspects as well as u.v.- visible spectra of degraded samples, the influence of tacticity on the thermal degradation of copolymers is discussed. This effect is compared to that of weak points introduced in the PVC chain by means of copolymerization.     

Preparation and degradation of head-to-head PVC

The course of the chlorination reaction of cis-1,4-polybutadiene is dependent on the choice of solvent. When methylene chloride is used, a pure addition reaction of chlorine leads to a polymer with the structure of head-to-head, tail-to-tail PVC. The thermal stability of the head-to-head PVC polymer has been studied by thermal volatilization analysis, thermogravimetry, and evolved gas analysis for hydrogen chloride, and the changes in the ultraviolet (UV) spectrum of the polymer during degradation have been investigated. The head-to-head polymer has a lower threshold temperature of degradation than normal PVC, but reaches its maximum rate of degradation at a higher temperature for powder samples of the polymer under programmed heating conditions. Blends of head-to-head PVC with poly(methyl methacrylate) have also been degraded, and the presence of the head-to-head polymers, like that of normal PVC, results in depolymerization of the PMMA as soon as the dehydrochlorination reaction commences. The mechanism of degradation of head-to-head PVC is discussed.     

Selective Substitution Reactions on PVC. Lability of Some “Normal” Structures

The nucleophilic substitution reaction on PVC with sodium thiophenate in cyclohexanone solution is studied. The reaction appears to be S_N2 substitution and it involves three different steps with decreasing reaction constants. By using ¹³C-NMR spectroscopy, the fast step is proposed to be due to the high reactivity of the central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads. The study of the thermal degradation of the modified samples shows that during the fast step of substitution a stabilizing effect occurs, and that as soon as the first step is over, the polymer stability decreases markedly. These effects are accompanied by changes in poly-ene distribution as proved by UV-VIS absorption spectroscopy study on degraded samples. The results, as discussed on the basis of the possible conformations of triads in PVC, suggest that some chlorine atoms in both the GTTG isotactic and the TTTG heterotactic triads should be considered as labile structures in PVC. It follows from the results obtained that on the one hand the initiation of degradation may occur by normal structures, and on the other the build-up of polyenes is favored by the presence of …TTTT… sequences. Both features contributing to a better understanding of the degradation of PVC.     

New approaches to the study of labile structures in poly(vinyl chloride) by phenolysis

Three samples of PVC having different tacticities were treated with phenol in cyclohexanone at 60° until there was no more reaction (96 hr). The incorporation of phenate into the chain was observed by u.v.-visible spectroscopy. The phenolysis extent was shown to be lower as the syndiotactic content of the polymer increased. On the other hand, the modified polymers showed enhanced thermal stability relative to the starting materials particularly for the more isotactic sample. The observed behaviour of either the starting materials in phenolysis or the treated polymers in thermal degradation appears to be very similar to that observed previously in the reaction with sodium thiophenate. On the basis of this similarity, it might be concluded that the phenolysis occurs with chlorine atoms at isotactic placements. That effect would confirm the thermal lability of some normal chlorine atoms as indicated by other results.     

Secondary Processes of Thermal Degradation of PVC

A brief summary of secondary processes in PVC thermal degradation is given. This includes some details of crosslinking and gel formation, cyclization, chain scission and benzene formation, Diels-Alder reaction with additives, as well as oxidation of polyenes.     

Degradation of PVCs obtained by controlled chemical dehydrochlorination

Thermal and thermo-oxidative degradation of poly(vinyl chloride)s (PVCs) containing increased concentrations of allylic chlorines, PVC(A)s, prepared by controlled chemical dehydrochlorination with potassium-t-butoxide (t-BuOK) have been studied. The introduction of small amounts of internal allylic chlorines into PVC significantly decreases the thermal and thermo-oxidative stability of the resin. A linear relationship exists between the initial rates (V_HCl)₀ of thermal and thermooxidative dehydrochlorination of solid PVC(A)s and the concentration S of internal allylic chlorines. Both the slope and the intercept of the thermo-oxidative (V_HCl)₀ vs. S plot are higher in oxygen than those obtained in nitrogen at the same temperature; this finding is attributed to fast oxidation of polyenes, and to peroxy radicals formed during polyene oxidation, which initiate subsequent HCl loss by attacking normal repeat units in PVC. The extent of HCl loss as a function of time during thermal degradation of PVC(A)s in intert solvent shows a rapid initial phase followed by a slower stationary phase. The first phase is due to dehydrochlorination involving the labile chlorines, while the stationary phase indicates random initiation of HCl loss at normal? CH₂? CHCl? repeat units. Initial rates of HCl loss increase with S, while the rates of HCl loss during the stationary phase are independent of S. The rate constant of initiation of HCl loss at internal allylic chlorines is almost four orders of magnitude higher than that of random initiation; however, the former is still orders of magnitude lower than that of chain propagation. Quantitative analysis of UV-visible spectra of PVC(A)s degraded in solution suggests geometric polyene distribution. The average length of polyenes decreases as the extent of HCl loss increases and reaches a constant value of ca. 3 at ca. 1% HCl loss for all the investigated PVC(A) samples.     

Quantitative analysis of random scission and chain-end scission in the thermal degradation of polyethylene

The thermal degradation of polyethylene includes two different kinds of pathways. These are random and chain-end scissions which include β-scission on the chain end and radical transfer scission. We conducted a quantitative analysis on these pathways by Pyrolysis-GC/MS and computer simulation. Two different distributions of scission products of polyethylene were observed at different temperatures. They are determined by the relationship between rate of reaction and that of volatilisation. Furthermore, a characteristic distribution was observed in lower molecular weight. It could be explained by direct scission and one to five-step radical transfer scissions. The pathway possibilities calculated with the accumulated schemes showed that the direct scission and one-step-radical transfer increased with the temperature. This indicates that β-scission occurs on the chain end before the radical transfer because the rate of the β-scission becomes faster as the temperature rises.     

Determination of multiple thermal degradation mechanisms of poly(3-hydroxybutyrate)

The thermal degradation of poly(3-hydroxybutyrate) (PHB) was investigated by kinetic analyses in detail to clarify its complex degradation behavior, resulting in a finding of mixed mechanisms comprising at least a thermal random degradation with subsequent auto-accelerated transesterification, and a kinetically favored chain reaction from crotonate chain ends. The thermal degradation behavior of PHB varied with changes in time and/or temperature. From the kinetic analysis of changes in molecular weight, it was found that a non-auto-catalytic random degradation proceeding in the initial period was followed by an auto-accelerated reaction in the middle period. From the kinetic analysis of weight loss behavior, it is proposed that there are some kinetically favored scissions occurring at the chain ends, where the degradation proceeded by a 0th-order weight loss process in the middle stage. The observed 0th-order weight loss process was assumed to be an unzipping reaction occurring at ester groups neighboring the crotonate end groups.     

Structural defects in polyvinylchloride—I. Internal unsaturation as initiation sites for dehydrochlorination

To obtain double bonds in PVC, copolymerizations of vinyl chloride (VC) with phenylacetylene (PA) and dimethylester of 2-butyne dioic acid (ADCE) were carried out. Copolymer compositions, as measured by i.r., were compared with ozonolytically evaluated chain scission numbers. Only the PA-units seemed to be incorporated at random in the PVC-chain. Phenyl groups conjugated to the double bonds led to a striking increase in thermal dehydrochlorination. Ozonolysis on degraded PVC samples provides insight into the mechanism of the dehydrochlorination, revealing the formation of additional unsaturated sites after degradation and permitting the estimation of average polyene sequence lengths.     

Analysis of nonvolatile oxidation products of polypropylene. II. Process degradation

The nonvolatile products of polypropylene that has been process degraded in the melt by a Brabender torque rheometer have been quantitatively identified by infrared analysis and chemical reactions carried out on molded sheets. The molecular weight changes with degradation have been determined by gel-permeation chromatography (GPC). It was determined that there is only one functional group per chain scission instead of the two groups previously found for thermal oxidation in the solid state. The molecular weight distribution is similar, but the functional groups and the general scheme of oxidation differ slightly from those previously found for polyolefin oxidation carried out below the melting point. The functional group distribution differs from that determined in a process-degraded polyethylene sample.     

Influence of Tacticity on Some Reactions of PVC

The influence of the tacticity on PVC reactivity is discussed on the basis of preliminary results obtained in ionic dehydrohalo, genation and chlorination reactions. From the reaction of an atactic PVC and a 70% syndiotactic PVC with LiCl in dimethyl-formamide and hexamethylphosphortriamide as solvents, it follows that both the reaction rate and the polyene sequence distribution depend markedly on the syndiotacticity content. This effect is accounted for by the fact that the isotactic parts are preferred in dimethylformamide and the syndiotactic ones in hexamethylphosphoramide. On the other hand, the chlorination of PVC appears to be easier through the heterotactic parts than through the syndiotactic sequences as shown by ¹³C-NMR.     

Study of the miscibility and the thermal degradation of PVC/PMMA blends

The aim of this paper is to study the miscibility and the thermal degradation of PVC/PMMA blends. For that purpose, blends of variable compositions from 0 to 100 wt% were prepared with and without plasticizer. Their physico-chemical characterization was carried out by differential scanning calorimetric analysis (DSC) and Fourier transform infrared spectroscopy (FTIR). Their thermal degradation under nitrogen at 185°C was studied and the HCl evolved from PVC was measured by the pH method. Degraded samples were characterized, after purification, by FTIR and UV-visible spectroscopy. The DSC analysis showed polymer miscibility up to 60 wt% of PMMA. This miscibility is due to a specific interaction of hydrogen bonding type between carbonyl groups (C=O) of PMMA and hydrogen from (CHCl) groups of PVC as evidenced by FTIR analysis. On the other hand, it was found that PMMA exerted a stabilizing effect on the thermal degradation of PVC by reducing the zip dehydrochlorination and by leading to the formation of short polyenes.     

Cyclopentadienylation of PVC: Characterization and thermal and thermooxidative degradation studies

PVC has been cyclopentadienylated by two conventional basic, LiCp and NaCp, and a new acidic, Me₂CpAl, cyclopentadienylating agent. PVCs treated with basic cyclopentadienylating agents undergo severe random dehydrochlorination and exhibit a significant decrease in thermal and thermooxidative stability. In contrast, according to ozonization and degradation experiments, Me₂CpAl does not cause dehydrochlorination during cyclopentadienylation. The thermal stability of PVC treated with relatively high concentrations of Me₂CpAl and Me₃Al at 25°C markedly increases due to substitution of labile chlorines in PVC with methyl groups. Initial thermal dehydrochlorination behavior of virgin PVC and samples treated with Me₂CpAl at ?30°C are similar. In contrast, thermooxidative stability decreases on Me₂CpAl treatment at ?30°C; this is attributed to ease of oxidation of pendant cyclopentadienyl groups; that is, the formation of peroxy radicals that may initiate dehydrochlorination by attacking unchanged repeat units in PVC. Acceleration of thermal dehydrochlorination disappears and the length of polyene sequences is reduced on Me₂CpAl and Me₃Al treatment. These observations are attributed to differences in rates of protonation-deprotonation; that is, rates of reinitation of zipping of treated and untreated PVCs during thermal degration. The effect of traces of aluminum residues on degradation of modified PVCs, however, cannot be neglected.     

Initial steps of thermal degradation of PVC prepared at various temperatures

PVC samples were obtained by bulk polymerization initiated with AIBN and ultraviolet irradiation at 40, 25, 0, ?30, and ?50°C. They were characterized by ¹³C NMR measurements, infrared spectroscopy, GPC in hexamethylphosphoramide and B.E.T. surface area measurements. Their thermal degradation was studied between 110 and 185°C by using continuous titration of HCl evolved through a conductimetric cell. The ultraviolet spectra were recorded at various steps of the degradation. At high degradation temperatures, the more syndiotactic the polymer, the longer the polyene average sequences are. The amount of HCl evolved is minimum for a polymerization temperature of 0 or 25°C, depending on the degradation temperature and on the morphology of the polymer. The results are discussed in terms of chemical factors (tacticity distribution, molecular weight) as well as of physical factors (morphology, interval viscosity).     

Kinetics of thermal degradation of poly(vinyl chloride)

Extensively studied thermal degradation of polyvinyl chloride (PVC) occurs with formation of free hydrogen chloride and conjugated double bonds absorbing light in visible region. Thermogravimetric monitoring of PVC blends degradation kinetics by the loss of HCl is often complicated by evaporation and degradation of plasticizers and additives. Spectroscopic PVC degradation kinetics monitoring by absorbance of forming conjugated polyenes is specific and should not be affected by plasticizers loss. The kinetics of isothermal degradation monitored by thermal gravimetric analysis in real time was compared with batch data obtained by UV/Visible absorption spectroscopy. Effects of plasticizer on kinetics of polyene formation were examined. Thermal degradation of PVC films plasticized with di-(2-ethylhexyl) phthalate (DEHP) and 1,2,4-benzenedicarboxylic acid, tri-(3-ethylhexyl) ester (TOTM) was monitored by conjugated double bonds light absorption at 350 nm at 160, 180, and 200 °C. Plasticizer-free PVC powder degradation kinetics and that of plasticized films were also obtained thermogravimetrically at temperatures ranging from 160 to 220 °C. Plasticizer-free PVC powder degradation and spectroscopically monitored degradation of plasticized PVC films occurred with the same apparent activation energy of ≈150 kJ mol⁻¹. No difference in degradation kinetics of films plasticized with DEHP and TOTM was detected.     

Degradation of poly(vinyl chloride) by U.V. radiation—I. Kinetics and quantum yields

The kinetics of the photodegradation of films of poly(vinyl chloride) (PVC) has been studied both in the presence and in the absence of oxygen. The value of the quantum yield of hydrogen chloride evolved, φ_HCl = 0.011, indicates that only one in every 100 photons absorbed induces the dehydrochlorination of PVC, with formation of polyenes. The independence of φ_HCl on the irradiation time and on the initial amount of unsaturation in the polymer argues in favour of an alkene-photosensitized degradation process. The low rate of degradation observed when Pyrex-filtered light is used results primarily from both low absorbance of the PVC film in the 300–400 nm region and photobleaching of the polyenes by the hydrogen chloride evolved. The decrease of φ_HCl for extended irradiation times is attributed to the formation of a highly absorbing surface layer consisting of totally degraded PVC. Competitive chain scission and crosslinking processes develop in the PVC film photolyzed either in nitrogen or in oxygen, with a limiting value of 0.5 for the gel fraction.