Thermal decomposition of poly(vinyl chloride) and chlorinated poly(vinyl chloride). II. Organic analysis

A concerted study of poly(vinyl chloride), chlorinated poly(vinyl chloride), and poly(vinylidene chloride) polymers by spectroscopy, thermal analysis, and pyrolysis-gas chromatography resulted in a proposed mechanism for their thermal degradation. Polymer structure with respect to total chlorine content and position was determined, and the influence of these polymer units on certain of the decomposition parameters is presented. Distinguishing differences were obtained for the kinetics of decomposition, reactive macroradical intermediates, and pyrolysis product distributions for these systems. It was determined that chlorinated poly(vinyl chloride) systems with long-chain ? CHCI? units were more thermally stable than the unchlorinated precursor, exhibited increasing activation energy for the dehydrochlorination, and produced chlorine-containing macroradical intermediates and chlorinated aromatic pyrolysis products. The poly(vinyl chloride) polymer was relatively less thermally stable, exhibited decreasing activation energy during dehydrochlorination, and produced polyenyl macro-radical intermediates and aromatic pyrolysis products.     

Investigations on poly(vinyl chloride). III. Effects of metal oxides upon thermal decomposition of poly(vinyl chloride)

Thermal decomposition mechanisms of poly(vinyl chloride) (PVC) and the effects of a few metal oxides on the pyrolysis of PVC were previously reported. In the present work, 33 metal oxides were investigated to determine their effects on the thermal decomposition of PVC, by using a pyrolysis gas chromatograph. Most acidic oxides accelerate the recombination of chlorine atoms with double bonds, since PVC containing these metal oxides easily release lower aliphatics, toluene, ethylbenzene, o-xylene, and chlorobenzenes. On the other hand, most basic metal oxides, such as oxides of alkaline earths or silver, inhibit the recombination. These tendencies observed in the thermal decomposition of PVC agree with the contributions of corresponding metal salts to the dehydrochlorination of PVC proposed by other workers. This means that thermal decomposition or dehydrochlorination of PVC is affected by irregularities in head-to-tail linkages formed by the recombination of chlorine atoms during heat treatment of PVC.     

Correlation of thermal degradation mechanisms: Polyacetylene and vinyl and vinylidene polymers

The thermal degradation of poly(vinyl bromide) (PVB), poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(vinyl fluoride) (PVF), poly(vinylidene chloride) (PVC₂), and poly(vinylidene fluoride) (PVF₂) has been studied by direct pyrolysis–mass spectrometry (DP-MS) and flash pyrolysis–gas chromatography–mass spectrometry techniques. Vinyl and vinylidene polymers exhibit two competitive thermal degradation processes: (1) HX elimination with formation of polyene sequences which undergo further moleculaar rearrangements, and (2) main-chain cleavage with formation of halogenated or oxigenated compounds. The overall thermal degradation process depends on the prevailing decomposition reaction in each polymer; therefore, different behaviors are observed. The thermal degradation of polyacetylene (PA) has also been studied and found important for the elucidation of the thermal decomposition mechanism of the title polymers.     

The photodegradation of poly(vinyl chloride)—V The effect of wavelength of irradiation on the dehydrochlorination reaction

For a purified sample of poly(vinyl chloride), it is found that radiation of wavelength less than 300 nm causes dehydrochlorination. It appears that polychromatic radiation yields a constant ratio of concentrations of polyenes, even from the very early stages of reaction. It is proposed that energy transfer is an important step in the photodecomposition of poly(vinyl chloride).     

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.     

Mechanism of Reaction of Polyvinyl Chloride) with Triethylaluminum

Polyvinyl chloride) was treated with triethylaluminum in 1,2-dichloroethane solution. Negligibly small amounts of hydrogen chloride are evolved from the modified polyvinyl chloride) in decomposition at 180°C for 150 min in nitrogen. Quantitative analysis of the rate of dehydrochlorination of the modified polymer gave a calculated activation energy for the alkylation of 8.3 kcal/mole in 1,2-dichloroethane solution; the concentration of the labile chlorines in the original polyvinyl chloride) was less than 0.25 mole % Furthermore, the fact that the average polyene length of the modified polymer for the thermal decomposition was much shorter than that of the starting material suggests that the labile chlorines inherent in the polymer exist not only in the chain end but also in the polymer chain.     

Automatic equipment for studying the processes of low-temperature dehydrochlorination of chlorine-containing polymers

A multichannel, highly sensitive installation for the study of the kinetics of decomposition of halogen-containing polymers under automatic conditions has been described.Hydrogen halide produced by polymer decomposition is recorded using mercury-halide ion-selective electrodes, the results being processed by computer.Low-temperature (313–403 K) degradation of poly(vinyl chloride) has been studied. At the glass transition temperature (358 K) the Arrhenius parameters for the degradation of PVC change. A dependence between the segmental mobility of PVC macromolecules and the rate of polymer dehydrochlorination is demonstrated.     

Determination of residual levels of unsaturation in partially hydrogenated poly(2,3-diphenyl-1,3-butadiene) using TG

The thermal degradation characteristics of head-to-head poly(styrene) [HHPS] should provide insight with respect to the impact of head-to-head placement on the thermal stability of traditional atactic head-to-tail polymer [HTPS]. The synthesis of head-to-head poly(styrene) must be accomplished indirectly. The head-to-head polymer is most satisfactorily obtained by dissolving metal reduction of poly(2,3-diphenyl-1,3-butadiene) [PDBD] generated by radical polymerization of the corresponding diene monomer. Full saturation of the polymer mainchain requires several iterations of the reduction procedure. Since the decomposition of poly(2,3-diphenyl-1,3-butadiene) is prominent at 374°C and that for head-to-head poly(styrene) is similarly facile at 406°C, it seemed feasible that TG of partially hydrogenated PDBD might be utilized as a convenient means of monitoring the extent of hydrogenation. This has been demonstrated for various levels of unsaturation remaining - from approximately 90 to less than 10%. Within this range the peak areas from the DTG plots of the partially hydrogenated polymer provide a good reflection of the ratio of unsaturated to saturated units in the polymer. Even low levels of unsaturation in the polymer may be detected by the asymmetry of the decomposition peak for the polymer. This revised version was published online in August 2006 with corrections to the Cover Date.     

Preparation and Characterization of Head-to-Head Polymers. II. Head-to-Head Poly(methyl Acrylate)

Head-to-head poly(methyl acrylate) was prepared by esterification of the known alternating copolymer of ethylene and maleic anhydride. Some of the chemical,physical, and mechanical properties and the thermal degradation behavior of head-to-head poly(methyl acrylate) were studied and compared with those of head-to-tail poly(methyl acrylate). The T_g of the head-to-head polymer was higher than that of the head-to-tail polymer, but the solubilities of both types of polymers of comparable molecular weight were similar. Head-to-head poly(methyl acrylate) degraded thermally at approximately the same temperature and with a rate similar to head-to-tail poly(methyl acrylate). Unlike poly(methyl cinnamates) which cleanly degraded to monomers, poly(methyl acrylates), head-to-head and head-to-tail, degrade to very small molecules, such as CO₂, methanol, but also larger polymer fragments and char. Trace amounts of monomers (methyl acrylate) were also observed.     

The use of laser Raman spectroscopy to study the degradation of poly(vinyl chloride)

The results of natural weathering tests carried out on plasticised poly(vinyl chloride) in three different locations representing cool/wet, hot/wet and hot/dry climates are detailed. The value of laser-Raman spectroscopy for determining dehydrochlorination levels of naturally weathered samples is demonstrated. The results indicate that plasticisers can play a significant role in this dehydrochlorination process. Results of studies on the effects of thermal stabilisers and amines, particularly those used as catalysts for polyurethane foam formation, on the stability on poly(vinyl chloride) are also reported.     

Determination of residual levels of unsaturation in partially hydrogenated poly(2,3-diphenyl-1,3-butadiene) using thermogravimetry

The thermal degradation characteristics of head-to-head poly(styrene) (HHPS) should provide insight with respect to the impact of head-to-head placement on the thermal stability of the traditional atactic head-to-tail polymer (HTPS). The synthesis of head-to-head poly(styrene) must be accomplished indirectly. The HHPS is most satisfactorily obtained by dissolving metal reduction of poly(2,3-diphenyl-1,3-butadiene) (PDBD) generated by radical polymerization of the corresponding diene monomer. Full saturation of the polymer mainchain requires several iterations of the reduction procedure. Since the decomposition of PDBD is prominent at 374 °C and that for HHPS is similarly facile at 406 °C, it seemed feasible that TGA of partially hydrogenated PDBD might be utilized as a convenient means of monitoring the extent of hydrogenation. This has been demonstrated for various levels of unsaturation remaining—from approximately 90 to less than 10%. Within this range the peak areas from the DTG plots of the partially hydrogenated polymer provide a good reflection of the ratio of unsaturated to saturated units in the polymer. Even low levels of unsaturation in the polymer may be detected by the asymmetry of the decomposition peak for the polymer.     

Thermal stability of homo- and copolymers of vinyl fluoride

The thermal stability of poly(vinyl fluoride) (PVF) was studied by thermal gravimetry and mass spectrometry (TGA and TGA–MS). In low-molecular-weight polymers a two-step decomposition pattern was observed. It consisted of the dehydrofluorination to a polyene chain followed by decomposition of the resulting polyene at higher temperatures. Copolymers of vinyl fluoride–vinyl acetate (VF–VAc) and vinyl fluoride-vinyl chloride (VF–VCl) showed a simultaneous evolution of hydrofluoric acid and acetic acid and hydrofluoric acid and hydrochloric acid, respectively. This suggests that after the elimination of the weakest link a spontaneous elimination of neighboring HF molecules takes place.     

Copolymerization of vinyl chloride with 1-vinyl-4,5,6,7-tetrahydroindole and 2-methyl-5-vinylpyridine

The radical copolymerization of vinyl chloride with 2-methyl-5-vinylpyridine and 1-vinyl-4,5,6,7-tetrahydroindole is accompanied by dehydrochlorination. In the vinyl chloride-2-methyl-5-vinylpyridine system, the evolved hydrogen chloride interacts with a pyridine hydrogen atom to give charged units of a heterocycle. In the vinyl chloride-1-vinyl-4,5,6,7-tetrahydroindole system, the hydrogen chloride being formed initiates the cationic dimerization of a nitrogen-containing monomer. The synthesized copolymers based on vinyl chloride surpass the commercial poly(vinyl chloride) in terms of thermal stability and solubility in organic solvents.     

Infrared photochemistry of cyclobutyl chloride

The decomposition of cyclobutyl chloride following multiple infraredphoton excitation has been investigated. The primary photolysis products are butadiene, from elimination of HCl, and ethylene and vinyl chloride, fromring scission. The vinyl chloride undergoes secondary decomposition to acetylene and HCl. In addition to these products, known from thermal VLPP experiments, we also find 1-butene, which may arise from a higher energy C? Cl homolysis channel. Collisions with either reactant molecules oradded buffer gas lead to cooling of the laser-produced vibrational energy distributions. The average amount of energy removed per collision is 15–20 kcal/mol for self-collisions and 2–4 kcal/mol with argon.     

Poly(vinyl chloride) stabilisation with organo-tin compounds—Part II: Catalysis of the dehydrochlorination by butyl-tin chlorides

The catalytic effect of the various butyl-tin chlorides on the dehydrochlorination reaction of chlorohexene, used as a model compound for allylic chlorides in poly(vinyl chloride), has been studied in tetrahydrofuran and dichloroethane solutions. The reaction follows an E2 mechanism, the rate determining step being the formation of a delocalised allylic carbocation. The catalytic power is directly related to the Lewis acidity of the tin chlorides and, further, RSnCl₃ is comparable with ZnCl₂, although it is more sensitive to complexing with weak Lewis bases. In the presence of poly(vinyl chloride) at 180°C, these butyl-tin chlorides show a retardation effect on dehydrochlorination, superimposed on a catalytic effect which increases with the Lewis acidity; however, in these conditions, RSnCl₃ is much less efficient than ZnCl₂ in catalysing the dehydrochlorination reaction.     

Polymerization of ordered tail-to-tail vinyl stearate bilayers

This study is a continuation of previous work done in this laboratory which has demonstrated the possibility of polymerizing built-up monomer multilayers in the solid state. In the present work, the formation, structure, and solid-state polymerization of multilayers of vinyl stearate which can be built up by the method of Langmuir and Blodgett were studied. A new technique to polymerize such films under the water surface was developed. This made possible the formation of poly(vinyl stearate) multilayers with different molecular orientations through bilayer polymerization. The mechanism of deposition and the structure and properties of head-to-tail and head-to-head, tail-to-tail poly(vinyl stearate) multilayers were investigated by using Fourier-Transform infrared spectroscopy, x-ray diffraction, and electron diffraction.     

Effect of ozone on poly(vinyl chloride) degradation

The influence of ozone on the kinetics and mechanisms of poly(vinyl chloride) degradation has been studied. The rate constants for reaction of ozone with saturated and unsaturated units of macromolecules have been measured. The products of the reaction of ozone with double bonds are inactive and do not influence the subsequent thermal dehydrochlorination of the polymer. The products of reaction of ozone with saturated units greatly increase dehydrochlorination.     

Degradation of poly(terphenylenephthalide) at high temperatures

The thermal and thermooxidative degradation of poly(terphenylenephthalide) has been studied by thermal analysis, IR and UV spectroscopy, and mass spectrometry. On the basis of the spectral data and the chemical composition of degradation products, it has been shown that both the thermal and thermooxidative degradations of poly(terphenylenephthalide) are characterized by intramolecular cyclization reactions. Depending on the mode of closure of intermediates generated in the course of thermal decomposition of neighboring phthalide groups (head-to-head or head-to-tail), either phenyl-substituted anthraquinones, fluorenones, and fluorenes or symmetric and nonsymmetric dicyclic compounds containing end anthraquinone, fluorenone, and fluorene groups may be formed. The oxidation of poly(terphenylenephthalide) likewise gives rise to cyclic products—the compounds of xanthone and dibenzofuran series.     

Kinetic study of dissolution of poly(vinyl chloride) in cyclohexanone

The kinetics of dissolution of five fractions of commercial poly(vinyl chloride) in cyclohexanone was studied at temperatures from 20 to 70°C. Good agreement was observed between the experimental results and equations expressing the dependence of the induction periods and the rates of dissolution on temperature and molecular weight. It was found that the apparent activation energy for the swelling process lies in the range 9–14 kcal/mole and the apparent activation energy for the dissolution diffusion process in the range 8–12 kcal/mole. The apparent dependence of activation energies on number-average molecular weight indicates that the chain ends are more important in determining the dissolution rate than the centers of the polymer chains.