Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AM1) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans double bond is formed and no hydrogen chloride is present to catalyze trans-cis isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn or 1,4-cis mechanisms, promoting a so-called catastrophic, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination because of a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.     

On the influence of HCl on the thermal degradation of poly(vinyl chloride)

The thermal degradation of PVC was studied at 190°C in pure nitrogen and nitrogen containing 10, 20, and 40% HCl (by volume). The rate of dehydrochlorination was determined by gravimetry. Degradations in nitrogen were followed with conductometry in addition. Changes in molecular weight distribution and degree of long-chain branching (LCB) were determined by gel permeation chromatography–viscometry and polyene sequence distribution by UV spectroscopy. The rate of dehydrochlorination increases with the HCI content of the atmosphere. The rate of molecular enlargement also increases but only as a result of the increased dehydrochlorination rate. The increase in M _w and LCB are thus related to the extent of conversion only. Changes in the UV spectra indicate that the increase in rate of dehydrochlorination is mainly due to increased propagation rate in atmospheres containing less than 10% HCI. At higher HCI contents an increase in initiation rate is noted. It is suggested that this, at least in part, is due to the fact that HCI, by forming a cyclic transition state, catalyzes the random elimination of HCI. This process, in turn, is promoted by the presence of polyene sequences.     

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.     

On the thermal degradation of poly(vinyl chloride). III. Structural changes during degradation in nitrogen

The structural changes in poly(vinyl chloride) during thermal degradation in nitrogen at 190°C have been investigated. From gel permeation chromatography analyses no chain scission, but only crosslinking reactions were observed. An increase in the molecular weight was measured even at 0.3% conversion. For longer polyene sequences and at higher conversions, a crosslinking reaction competed with the “zipper” propagation. The secondary reactions, were more extensive at longer polyene sequence lengths. The growing polyene sequences can be terminated not only by branching reactions but also at existing pendent chloromethylene groups. A decrease in the amount of short chain branching with conversion also indicated other types of secondary reactions. Such a decrease was also observed during thermomechanical degradation in a Brabender Plastograph. The average polyene sequence length was calculated to be around 10, depending somewhat on the type of analysis used. Although allylic chlorine atoms seem to be the main points of initiation, other sites cannot be excluded as the number of initiation points increases appreciably during the early stages of the degradation. Such an increase is, of course, also consistent with a radical mechanism.     

On the thermal degradation of poly(vinyl chloride). II. The effect of atmosphere

The early stages of the thermal degradation of PVC were studied. Two commercial, suspension-polymerized resins were thermally treated at different temperatures and oxygen contents. Dehydrochlorination kinetics were followed by conductometric measurements and the formation of polyene sequences by ultraviolet-visible spectroscopy. Crosslinking and chain scission were followed by gel chromatography (GPC) and viscometry. No chain scission was observed in nitrogen atmosphere and no crosslinking in oxygen. Degradation in air proceeded by both reactions. The rate of dehydrochlorination for one of the investigated polymers increased linearly with the logarithm of the oxygen pressure. In nitrogen, an increasing degradation temperature was found to give both an increasing crosslinking and less discoloration. In oxygen, chain scission reactions showed a slight temperature dependence. The temperature effect on the discolorations was similar to that in nitrogen. The main difference between the investigated resins was the amount of internal doubled bonds in the original polymers, the ratio being 2:1. The higher amount resulted in a higher rate of dehydrochlorination, a larger extent of chain scission in oxygen, and a lower extent of crosslinking in nitrogen. Both in oxygen and nitrogen, the obtained results are consistent with allylic mechanisms. In nitrogen, the polyene formation supposedly proceeds by a unimolecular reaction and crosslinking by an intermolecular nonradical dehydrochlorination. In oxygen, radical reactions are superposed and may lead to chain rupture via β-scissions of alkoxy radicals.     

Zum mechanismus der thermischen abspaltung von chlorwasserstoff aus polyvinylchlorid. 5. Mitt.: Über die verteilung der polyensequenzlängen in thermisch abgebauten technischen polyvinylchloriden und in anwesenheit stabilisierend wirkender substanzen

The rates of dehydrochlorination of commercial poly (vinyl chlorides) and the distribution of the lengths of the resulting polyene sequences may vary widely depending on the origin of the polymer. – In the presence of diphenyllead dichloride the rate of dehydrochlorination is increased, while the resulting discoloration is less intense compared to that of pure poly (vinyl chloride) because of a shift of the polyene sequence lengths distribution. Genuine stabilizers, on the other hand, effect retardation in dehydrochlorination as well as a shift of the sequence lengths distribution towards shorter polyene sequences. The causes for this behavior of admixed stabilizers using lead and cadmium octoate as examples as well as of internally stabilized PVC are discussed.     

Effect of polymerization initiator on early colour of poly(vinyl chloride)

The development of early colour and the dehydrochlorination rate were compared for poly(vinyl chloride) samples obtained by suspension polymerization using two different initiators, dicetylperoxydicarbonate and butylperoxyneodecanoate. The degree of discolouration was measured on pressed plates and expressed as yellowness index (YI), the polyene sequence distribution was monitored by UV/Vis-spectroscopy, and the degradation rate was determined by measuring evolved HCI conductometrically. The PVC sample initiated by dicetylperoxydicarbonate exhibited more extensive early colour and a higher dehydrochlorination rate as compared to the sample obtained with butylperoxyneodecanoate as initiator. The UV/Vis-spectra showed that the early colour originates from polyene sequences. After extraction of the PVC resins with heptane: acetone (85 : 15) the early colour turned out to be almost the same for the two samples. From ¹³C-NMR measurements it was found that the extract of the sample polymerized with dicetylperoxydicarbonate contains unreacted initiator. We suggest that the radicals formed when the remaining initiator decomposes initiate dehydrochlorination. Our results also indicate that radicals from dicetylperoxydicarbonate may cause long-chain branches during polymerization. The radicals formed from butylperoxyneodecanoate, on the other hand, do not seem to react with the polymer molecules. © 1993 John Wiley & Sons, Inc.     

Structural changes of PVC in PVC/LDPE melt‐blends: Effects of LDPE content and number of extrusions

This paper investigates the structural changes of polyvinyl chloride (PVC) in melt‐blends of a low‐density polyethylene (LDPE) and polyvinyl chloride (PVC), and the effects of LDPE content and number of extrusion passes. These effects were examined in terms of changes in weight average molecular weight and number average molecular weight, polyene and carbonyl indices, color changes of the blend, and the variations in glass transition and decomposition temperatures. It was found that loading LDPE into PVC led to the formation of short‐chain LDPE grafted PVC (s‐LDPE‐g‐PVC) copolymers, via a macro‐radical cross‐recombination reaction, which had greater weight average molecular weight with unchanged number average molecular weight, increased decomposition temperature, lower glass transition temperature, as compared to the pure PVC sample. The dehydrochlorination reaction of PVC was suppressed by the macro‐radical cross‐recombination reaction with addition of LDPE, the effect being more pronounced at 13.0 wt% LDPE. For a given LDPE content, the macro‐radical cross‐recombination and dehydrochlorination reactions competed with one another, thus causing the increases in molecular weight average and molecular weight number up to the 4th extrusion pass. At the 5th extrusion pass, the dehydrochlorination reaction was predominant owing to a depletion of LDPE content to be grafted onto PVC molecular chains. The glass transition and decomposition temperature decreased with increasing number of extrusion passes. Polym. Eng. Sci. 44:487–495, 2004. © 2004 Society of Plastics Engineers.     

Studies of the solid-state thermal degradation of PVC. I. Autocatalysis by hydrogen chloride

The thermal degradation of virgin and HCI-treated PVC in powder form, as well as of PVC films of different thicknesses, has been studied as a function of time and temperature. The rate of dehydrochlorination was determined conductimetrically and from the polyene sequence distributions as obtained by UV spectroscopy. Increases in the rate of dehydrochlorination, ranging between 30 and 45%, were observed at all temperatures for the samples pretreated with HCI, while the corresponding activation energies were found to be lower by about 20%. For the PVC films, the rate increased with thickness, i.e., with longer residence time of evolved HCI within the sample. The results offer insight regarding the autocatalytic role of evolved HCI.     

Influence of tacticity on thermal degradation of PVC. V. Relation between the nature of labile conformations and the polyene distribution in the degraded polymer

Four poly(vinyl chloride) (PVC) samples, 1, 2, 3, and 4, were prepared in bulk using 2,2′-azodiisobutyronitrile at 90°C and 60°C and the same initiator together with UV irradiation at 0°C and ?50°C, respectively. Fractions were obtained from samples 2,3, and 4 by extracting with acetone, sample 1 being completely soluble in this solvent. The whole PVC's as well as their acetone soluble and insoluble fractions, were characterized by determining the intrinsic viscosity, the osmometric molecular weight, and the tacticity; then they were thermally degraded up to conversion of 0.3% in powder state. The values of degradation rate for both the insoluble fractions and the whole PVC's were in agreement with some prior results on the influence of syndiotactic sequences on propagation step. The soluble fractions proved to be very unstable in comparison with the insoluble, which, despite the lower molecular weight of the former, seems to obey their higher content of isotactic conformations. The fine polyene distribution in the degraded polymers was carried out by UV–visible spectroscopy. The results reveal the occurrence of two different polyene distributions, and allow for the peculiar one of the soluble fractions to be related to polyenes consisting of two sequences of trans conjugated double bonds separated by a single cis double bond. The results clearly shows that there are two mechanisms for initiation of the PVC degradation, depending on whether it occurs by random unstable structures or by the normal GTTG isotactic or TTTG heterotactic triads. Moreover, these two initiation processes are proved to give rise to different types of polyenes, which accounts for the occurrence of two unlike mechanisms of propagation.     

TG-FTIR分析技术在PVC热降解过程研究中的应用

利用热失重-红外光谱联机分析技术(TG-FTIR)研究了聚氯乙烯(PVC)共混物在氮气气氛下、30～900℃范围内的热降解行为。结果表明:PVC共混物的热降解过程可分为3个阶段,分别在200～380℃,380～570℃和570～758℃范围内。其中,第一阶段主要为PVC脱HCl反应阶段,热降解产物主要为HCl;第二阶段主要为共轭多烯结构的裂解和环化,产物为低烃类化合物、苯及其衍生物;第三阶段为碳酸钙的分解反应,产物为CO2。     

Impact of the presence of amines on the degradation of vinylidene chloride copolymers

Because of the prominence of vinylidene chloride/methyl acrylate copolymers in barrier plastic packing applications, the degradation of these materials continues to be the focus of much interest. The prominent degradation process at moderate temperature (<200°C) is dehydrochlorination. This reaction is a typical radical chain process with distinct initiation, propagation, and termination phases. A principal defect structure responsible for the initiation of degradation is unsaturation along the polymer backbone. Although most stabilization packages for these polymers contain basic components to absorb hydrogen chloride as it is generated, materials sufficiently basic to abstract hydrogen chloride might actually enhance the rate of degradation by introducing double bonds into the polymer structure. That this is the case has been established by the thermogravimetric examination of the degradation of a methyl acrylate (5 mol%)/ vinylidene chloride copolymer in the presence of amines of varying base strengths. In general, the initiation rate constant, k_t, for the degradation process is enhanced in the presence of amine.     

Polymer degradation by elimination of small molecules

An outline of a method of deriving the kinetic equations for polymer degradation by the elimination of low molecular weight compounds is presented. The method is based on the stochastic approach. The main ideas are that the degradation occurs via the zip mechanism and that the initiating step, i.e. the formation of an olefinic site in the polymer chain, takes place at random. Three mechanisms have been investigated in this study: (i) ordinary zip mechanism; (ii) autocatalytic mechanism with immediate zip growth and catalyzed initiation; (iii) autocatalytic mechanism with gradual zip growth. The kinetic equations derived have been used in kinetic studies of the dehydrochlorination of PVC in inert, HCl, and oxidative atmospheres, and of the degradation of polyvinyl esters, polyvinyl bromide and polyvinylidene chloride. Kinetic parameters for the degradation of individual polymers have been obtained.     

Degradation mechanism and mechanical properties of PVC in PVC‐PE melt blends: Effects of molecular architecture,content, and MFI of PE

Thermal degradation mechanism and mechanical properties of poly(vinyl chloride) (PVC) in PVC–polyethylene (PE) mixtures with varying types, contents, and melt flow indexes of the PE were studied. The degradation behavior was investigated in terms of decomposition temperature and glass transition temperature, polyene index, and morphology of the PVC in the mix. The results suggested that adding small amounts of PE (5 phr) in PVC‐PE mixtures could thermally stabilize the PVC, as noted by an increase in its decomposition temperature. At higher PE loading, the PVC encountered more degradation as a result of a consumption of the heat stabilizer by PE radicals and the dehydrochlorination reaction. A radical transfer reaction was proposed to explain the degradation mechanism of the PVC in the PVC‐PE melt, specifically a progressive increase of the glass transition temperature of the PVC in the mix as a result of increasing PE content. The PVC‐PE blend using HDPE with high MFI exhibited more pronounced thermal and structural changes of PVC. The mechanical properties of the PVC‐PE blend were very much dependent on the PE content, but slightly affected by the type and MFI value of the PE. The dispersion level of the PE in the PVC phase was found to play an important role in affecting the mechanical properties of the PVC‐PE mixtures. J. VINYL. ADDIT. TECHNOL. 12:115–123, 2006. © 2006 Society of Plastics Engineers     

Thermomechanical behavior of poly(vinyl chloride) in the process of degradation

The thermomechanical behavior of poly(vinyl chloride) (PVC) was investigated during its thermal degradation by using torsional braid analysis. In thermomechanical behavior as a function of temperature, the relative rigidity G_r decreased initially with increasing temperature, then began to increase passing through a minimum at about 200°C, and finally decreased at 340°C. Increase in G_r from 200°C was caused by formation of a conjugated polyene chain accompanied by dehydrochlorination and by crosslinking reaction, and decrease in G_r at 340°C was related to scission reactions of the crosslinking network by oxidation. In the change in logarithmic decrement Δ, three peaks were observed: at 90°C, coinciding with the glass transition of the polymer; at about 200°C, due to the melting transition of crystallites, and at about 300°C, due to a loss of mechanical energy in the rheological transition of the polymer from a liquid state to a glassy state passing through a viscoelastic region. The thermomechanical properties of PVC with different molecular weights were also measured, and the effect of molecular weight G_r and Δ are discussed. In isothermal measurements of the relative rigidity in air, exponentially increasing curves were observed as a function of time. These curves were analyzed kinetically as a first-order reaction, and an activation energy of 22.7 kcal/mole was obtained.     

Thermal dehydrochlorination of pure PVC polymer: Part I—thermal degradation kinetics by thermogravimetric analysis

Thermal degradation of PVC occurs in two stages, with each stage subdivided into two substages. The first refers to the dehydrochlorination, where hydrochloric acid is formed, and giving polyene structures. Hitherto, the degradation mechanism and action of hydrochloric acid as a catalyst during the dehydrochlorination stage are poorly known. Recently, the importance of the tacticity has gained attention for its influence on the dehydrochlorination mechanism. The present work focused on the dehydrochlorination stage, studying the molecular structure by FTIR analysis and the kinetic parameters by TGA analysis in Nitrogen atmosphere, based on three mathematical methods: Friedman, Kissinger, and Flynn-Wall-Ozawa. The sample was a pure homopolymer obtained by suspension polymerization. The dehydrochlorination kinetics follows a first order reaction model and occurs by nucleation and growth. The dehydrochlorination begins with the loss of very labile chlorine atoms present in defective and isotactic molecular segments. The formed HCl acts as a catalyst in the degradation. Following 40% conversion, a drop in Ea is observed. After that, chlorine atoms present in syndiotactic and atactic sequences, are released and, added to the large number of polyene chain sequences, and an increase in Ea is observed up to 60% conversion, where the dehydrochlorination stage is concluded.     

Mechanism of action and effectiveness of ester thiols as thermal stabilizers for poly(vinyl chloride)

Organic thiols containing at least one carboxylate ester group (ester thiols) are excellent thermal stabilizers for both rigid and plasticized poly(vinyl chloride) (PVC). Their mechanism of action is shown to involve the deactivation of unstable structural defects by nucleophilic chloride displacement, the retardation and removal of coloration through thiol additions to polyene double bonds, and the prevention of autoacceleration during thermal dehydrochlorination through polyene shortening reactions and the scavenging of free radicals formed from polyenes and HCl. An unusually facile displacement of labile chloride that is favored by thiol acidity can account, at least in part, for the relatively high effectiveness of dipentaerythritol hexakis(mercaptoacetate) as a stabilizer. J. VINYL ADDIT. TECHNOL., 13:170–175, 2007. © 2007 Society of Plastics Engineers     

Partially dehydrochlorinated PVC. I. Structure

PVC made by suspension and anionic polymerization was partially dehydrochlorinated by three different methods. In the first one, a solution in THF and in the second one, particles swollen by dioxane were treated with alcoholic KOH at low temperatures. In the third method, a solution of PVC in DMF was heated to 130°C or refluxed (153°C). The reactions were followed by UV spectrophotometry and the products analyzed by GPC. Furthermore, the products were ozonolyzed and the molecular weights of the polymeric residues determined by GPC. Treatment with KOH in THF yielded products with randomly distributed polyene sequences. At higher degrees of dehydrochlorination a slight decrease in molecular weight was observed. The polymers were shown to be built up by PVC segments with an average degree of polymerization between 70 and 80, interspaced by polyene segments each with relatively few conjugated double bonds in sequence (fewer than 15). The reaction of KOH with particles swollen in dioxane was slower but yielded products with the same molecular structure. Thermal dehydrochlorination in DMF gave rise to long polyene sequences. They were fewer in number as evidenced by the higher molecular weights of the residues after ozonolysis.     

The effect of barium and cadmium stabilizers on the thermal dehydrochlorination of poly(vinyl chloride): A resonance Raman spectroscopic study

A laser Raman spectroscopic study has been made of the effect of barium and cadmium stabilizers on the thermal dehydrochlorination of poly(vinyl chloride) (PVC). The results obtained indicate that, in terms of the total level of dehydrochlorination, cadmium stearate is not as superior to barium stearate as the visual appearance of the polymer suggests. Also, the level of dehydrochlorination produced in the presence of a mixed stabilizer is not significantly lower than that produced in the presence of barium stearate alone, while samples of the latter exhibit much stronger color. It has also been shown that cadmium stearate can reduce the length of long polyene sequences, presumably by a reaction near the center of such a sequence.     

Degradation kinetics of PVC plasticized with different plasticizers under isothermal conditions

The aim of the present work is to elucidate the degradation kinetics of polyvinyl chloride (PVC) plasticized with some phthalate and nonphthalate plasticizers. A PVC thermomat instrument was utilized to maintain the isothermal degradation conditions at 140 and 160°C, and to suppress the oxidative degradation by means of nitrogen flow. The conductivity measurements were performed to follow hydrogen chloride (HCl) gas which is released upon PVC degradation and trapped in water. Dehydrochlorination of plasticized PVC films occurred with activation energies of about 23–160 and 26–117 kJ mol^?1 and the isokinetic temperatures, at which the dehydrochlorination rate constants of all p‐PVC films would have the same value, were found to be 171 and 128°C for initial and linear regions of dehydrochlorination curve, respectively. Plasticizer incorporation contributes to the stability of the films particularly after the consumption of stabilizer due to the dehydrochlorination. Influence of temperature rise by 20°C on the degradation rate constant is the lowest for DINCH having p‐PVC films as 0.36 and 0.42% increment at the initial region and linear region, respectively. On the other hand, DOTP reveals greater stability than the others do since the compensation ratio of the PVC film having DOTP is greater than the other films. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41579.