Mathematical models of the initial stage of the thermal degradation of poly(vinyl chloride). II. The thermal degradation of poly(vinyl chloride) with effective removal of HCl

The dehydrochlorination of different samples of PVC under vacuum with continuous removal of HCl by freezing, has been studied at 180–210°C. The comparison of the kinetic curves of the dehydrochlorination of various samples of PVC which were obtained by us and other investigators, with the theoretical curves for the thermal degradation of idealized PVC in the absence of HCl has been carried out. This had made it possible to evaluate the influence of unstable fragments present in the original polymer on the initial rate of PVC degradation quantitatively. It has been shown that the distinction between the stationary rates of the dehydrochlorination of various samples of PVC is determined by the difference of the values of the average length of dehydrochlorination chain, l_av. The most probable interval of the values of l_av has been ascertained to be 4–12. It is established that the most probable value of the constant of the rate of dehydrochlorination of normal links of PVC, k₀, is 2.1 × 10^?7?2.5 × 10^?7 s^?1 at 200°C. © 1993 John Wiley & Sons, Inc.     

Polymeric p-benzoquinone–tin derivatives as thermal stabilizers for rigid poly(vinyl chloride). II. Evaluation of the stabilizing efficiency

The polymeric p-benzoquinone-tin derivatives obtained from the reaction of p-benzoquinone with tin tetrachloride in the absence of solvent have been investigated as thermal stabilizers for rigid PVC at 200°C by measuring the rate of dehydrochlorination. The results reveal the greater stabilizing efficiency of the investigated products in relation to dibutyltin maleate and the basic lead stabilizers commonly used in industry. Evidence has been accumulated that the quinone and metallic elements (Sn? Sn bonds) of the stabilizer participate in the stabilization process by trapping the radical intermediates of degradation and blocking the odd electron sites formed on the polymer chains. Although stabilizers with high quinone content provide greater stabilization in the early stages of degradation, their efficiency sharply decreases in subsequent stages. On the other hand, stabilizers of high tin content effectively prohibit the dehydrochlorination reaction at all stages of degradation. On an equivalent basis of metal content, the results clearly demonstrate the greater stabilizing efficiency of tin atoms when found in direct contact in the stabilizer molecule. The mechanism of stabilization suggested to account for the results obtained may be considered as additional evidence in support of the radical nature of the dehydrochlorination reaction.     

The simplest mathematical model of the process of the thermal dehydrochlorination of poly(vinyl chloride)

The dehydrochlorination of PVC under vacuum (～ 10^?4 mm Hg), with continuous removal of volatile products by freezing out, has been studied at 180–250°. The equation has been deduced and solved to describe the thermal degradation of PVC. The rate constants of separate steps of polymer dehydrochlorination and the dependence of concentrations of polyenes on time of degradation are calculated.     

Synthesis and structural characterization of syndiotactic trans‐1,2 and cis‐1,2 polyhexadienes

The (E) isomer in mixtures of (E) and (Z) 1,3‐hexadiene was polymerized with the system CoCl₂(PⁱPrPh₂)₂‐MAO, a highly active and stereospecific catalyst for the preparation of 1,2 syndiotactic polybutadiene. A new crystalline polymer with a melting point of 109 °C was obtained. The polymer was characterized by IR, NMR (¹³C, ¹H in solution and ¹³C in the solid‐state), X‐ray diffraction, DSC, GPC and it was found to have a trans‐1,2 syndiotactic structure with a 5.18 ± 0.04 Å fiber periodicity. Since only the (E) isomer was polymerized, at the end of the reaction we were able to separate the (Z) isomer, which was ultimately polymerized with CpTiCl₃‐MAO at low temperature, obtaining a low molecular weight, stereoregular polymer that, characterized by IR and NMR methods, was found to exhibit a cis‐1,2 syndiotactic structure, never reported before. Molecular mechanics calculations were carried out on the trans‐1,2 syndiotactic polymer and structural models consistent with the X‐ray diffraction data are proposed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5339–5353, 2007     

Degradation of chlorinated polyethylene. I. Effect of antimony oxide on the rate of dehydrochlorination

The degradation of two chlorinated polyethylene compounds CPE 25 (45% chlorine) and CPE 16 (36% chlorine) was studied by following their rates of dehydrochlorination at two temperatures, 150°C and 180°C in pure nitrogen and pure oxygen atmospheres. Studies on the powdered polymers showed that the dehydrochlorination rate of CPE 25 is about fourteen times faster than that of CPE 16 in nitrogen atmospheres and only three to four times faster in oxygen. The molded polymers gave a lower rate of dehydrochlorination than when in the powdered form. This effect is attributed to diffusion factors. The antimony oxide brought about an induction period in the dehydrochlorination reaction during which only a small amount of HCl is evolved, followed by a very fast rate of dehydrochlorination both in oxygen and nitrogen atmospheres. The duration of the induction period increases with increase in the Sb₂O₃ concentration, but is followed by an accelerated HCl loss which is faster when Sb₂O₃ concentration is higher. This work provides supporting evidence that SbCl₃ was formed and lost during degradation. Mechanisms of dehydrochlorination are suggested for the reaction in the case of pure chlorinated polyethylene and for the polymer containing antimony oxide.     

Characterization of diene polymers. I. Infrared and NMR studies: Nonadditive behavior of characteristic infrared bands

Extinction coefficients of the characteristic infrared bands due to isomeric structural units were measured for polybutadiene and polyisoprene in CS₂ or CCl₄ solutions and were compared with the isomer composition determined by NMR. The NMR signal assignments were made on the basis of the spectra of deutero derivatives of the polymers. In the case of polyisoprene, linear relations were obtained between the extinction coefficients and the isomer contents determined by NMR for the absorption bands at 1385 cm^?1 (characteristic of trans-1,4 units), 1376 cm^?1 (cis-1,4 units), and 889 cm^?1 (3,4 units). However, for the absorption bands at 840 cm^?1 (characteristic of cis-1,4 and trans-1,4 units), isomerized polyisoprenes did not give such a linear relationship. In polybutadiene, the extinction coefficient for the atactic 1,2 units was found to be lower than that of the syndiotactic 1,2 unit. These experimental facts lead to the conclusion that additivity of the extinction coefficients does not always hold for diene polymers. The deviation from the linear relation may be associated with regular sequences of one isomeric conformation in the chain.     

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.     

Kinetic study of the crosslinking reaction of 1,2‐polybutadiene with dicumyl peroxide in the absence and presence of vinyl acetate

The crosslinking reaction of 1,2-polybutadiene (1,2-PB) with dicumyl peroxide (DCPO) in dioxane was kinetically studied by means of Fourier transform near-infrared spectroscopy (FTNIR). The crosslinking reaction was followed in situ by the monitoring of the disappearance of the pendant vinyl group of 1,2-PB with FTNIR. The initial disappearance rate (R₀) of the vinyl group was expressed by R₀ = k[DCPO]^0.8[vinyl group]^−0.2 (120 °C). The overall activation energy of the reaction was estimated to be 38.3 kcal/mol. The unusual rate equation was explained in terms of the polymerization of the pendant vinyl group as an allyl monomer involving degradative chain transfer to the monomer. The reaction mixture involved electron spin resonance (ESR)-observable polymer radicals, of which the concentration rapidly increased with time owing to a progress of crosslinking after an induction period of 200 min. The crosslinking reaction of 1,2-PB with DCPO was also examined in the presence of vinyl acetate (VAc), which was regarded as a copolymerization of the vinyl group with VAc. The vinyl group of 1,2-PB was found to show a reactivity much higher than 1-octene and 3-methyl-1-hexene as model compounds in the copolymerization with VAc. This unexpectedly high reactivity of the vinyl group suggested that an intramolecular polymerization process proceeds between the pendant vinyl groups located on the same polymer chain, possibly leading to the formation of block-like polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4437–4447, 2004     

Structure and stability of halogenated polymers: Part 2—Chain-chlorinated polystyrene

Chain-chlorinated polystyrene samples have been prepared and characterised, containing from 2·5 to 34·3% chlorine by weight (from 0·1 to 1·3 chlorine atoms per styrene unit). Chlorination has been found to involve mainly substitution of the tertiary hydrogen atoms, followed by methylene substitution at reactant concentrations of more than 1 mole Cl₂ per styrene unit. Reaction with chlorine is quantitative in CCl₄ in the absence of air and the amount of ring-chlorination as a side reaction is very small.Chlorination in the backbone destabilises the polymer, which first loses hydrogen chloride on heating. The double bonds formed provide points of weakness for chain scission, which occurs at lower temperatures than for PS. Under programmed heating, the dehydrochlorination and chain scission reactions overlap to some extent in temperature range, but highly conjugated partially degraded polymer can be made from extensively chain-chlorinated PS. At more than 1 Cl per styrene unit, HCl is almost the sole volatile product, although the conjugated polymer breaks up to chain fragments above 300°C.     

Water‐Soluble Manganese Inorganic Polymers: The Role of Carborane Clusters and Producing Large Structural Adjustments from Minor Molecular Changes

The reaction of two different carboranylcarboxylate ligands, 1‐CH₃‐2‐CO₂H‐1,2‐closo‐C₂B₁₀H₁₀ or 1‐CO₂H‐1,2‐closo‐C₂B₁₀H₁₁, with MnCO₃ in water leads to polymeric compounds 1 a and 1 b . Both compounds have been characterized by analytical and spectroscopic techniques. Additionally, electrochemical techniques have also been used for compound 1 a . X‐ray analysis revealed substantial differences between both compounds: whereas a six‐coordinated Mn^II compound with water molecules bridging two Mn^II centers has been observed for 1 a , a square pyramidal geometry around each Mn^II ion with terminal water molecules coordinated to each Mn^II center has been found for 1 b . The observed differences have been attributed to the existence of different substituents, ?CH₃ or ?H, on one of the carbon atoms of the carboranylcarboxylate ligand. The reaction of 1 a and 1 b with coordinating solvents, such as ethers or Lewis bases, leads to the formation of new compounds with low (mononuclear 4 a , 4 b ; dinuclear 3 a , 3 b ; and trinuclear 2 a ) or high nuclearity (hybrid polymer, 5 a ), due to breakage of the corresponding polymer. X‐ray analysis shows that the structural core present in the polymeric materials is not maintained in the resulting compounds, with the exception of trinuclear compound 2 a . The magnetic properties of the compounds studied show weak antiferromagnetic coupling.     

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.     

Low- and high-molecular-weight polyacetylenes: Synthesis,characterization, and thermal isomerization

Procedures for the synthesis of polyacetylene ([CH]_x) with M _n (number average molecular weight) from 400 to about 10⁶ have been developed. This probably represents the largest range of molecular weight (MW) obtainable for a given monomer by a single initiator system. The catalyst residue level in [CH]_x can be significantly reduced by acidic-methanol purification. The very low MW polymer L-[CH]_x (polyacetylenes with M _n 400–500), has the same cis crystal structure as the higher MW polymers but is less ordered along the c-axis. It is isomerized to the trans material with apparently a more compact unit cell than high MW polymers. There is annealing of crystallite which increases the longitudinal order during thermal isomerization. This process occurs more readily and with lower activation energy in L-[CH]_x than for polymers with higher MW. Isomerization of high MW polymers tends to trap cis units which can result in degradation as evidenced by the formation of sp³ carbon vibrations in IR spectra. This is true even for L-[CH]_x after prolonged heating. The results render credence to the proposal based on anamalous resonance Raman scattering profile that there can be very short trans segments in thermally isomerized trans-[CH]_x.     

Reaction of atomic oxygen with cyclopentadiene

The reaction of 1,3-cyclopentadiene (CPD) with ground-state atomic oxygen O(³P), produced by mercury photosensitized decomposition of nitrous oxide, was studied. The identified products were carbon monoxide and the following C₄H₆ isomers: 3-methylcyclopropene, 1,3-butadiene, 1,2-butadiene, and 1-butyne. The yield of carbon monoxide over oxygen atoms produced (?_CO) was equal to the sum of the yields of C₄H₆ isomers in any experiment. ?_CO was 0.43 at the total pressure of 6.5 torr and 0.20 at 500 torr. We did not succeed in detecting any addition products such as C₅H₆O isomers. It was found that 3-methylcyclopropene was produced with excess energy and was partly isomerized to other C₄H₆ isomers, especially to 1-butyne. The excess energy was estimated to be about 50 kcal/mol. The rate coefficient of the reaction was obtained relative to those for the reactions of atomic oxygen with trans-2-butene and 1-butene. The ratios k_CPD+O/k_{trans-2-butene+O}= 2.34 and k_CPD+O/k_1-butene+O = 11.3 were obtained. Probable reaction mechanisms and intermediates are suggested.     

Structural characterization of polybutadiene synthesized via cationic mechanism

The microstructure of polybutadiene synthesized via cationic polymerization using TiCl₄‐based initiating systems has been investigated using 1D (¹Н, ²Н, and ¹³С) and 2D (HSQC and HMBC) NMR spectroscopy. It was found that trans‐1,4‐unit is predominant structure of unsaturated part of polymer chain. Besides, the small amount of 1,2‐structures was also detected, while cis‐1,4‐units were totally absent. The signals of carbon atoms of three types of head groups (trans‐1,4‐, 1,2‐, and tert‐butyl) and two types of end groups (trans‐1,4‐Cl and 1,2‐Cl) were identified for the first time in macromolecules of cationic polybutadiene. It was showed that tert‐butyl head groups were formed due to the presence in monomer of admixtures of isobutylene. The new methodology for calculation of the content of different structural units in polybutadiene chain as well as the head and end groups was proposed. It was established that main part of 1,2‐units distributed randomly along the polybutadiene chain as separate units between trans‐1,4‐structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 387–398     

Pyrolysis of 1,1‐dichloro‐1‐fluoroethane in the absence and presence of added propene or CCl4: A computer‐aided kinetic study

The thermal dehydrochlorination CCl₂FCH₃ → CClFCH₂ + HCl has been studied in a static system between 610 and 700 K at pressures ranging from 14 to 120 torr. The experiments were performed in the absence and presence of an added inhibitor (0.5 to 7 torr of C₃H₆) or catalyst (2 to 8 torr of CCl₄). The evolution of the reaction was followed by measuring the pressure rise in the quartz reaction vessel and analyzing the products by gas chromatography. All the experimental results can be explained quantitatively in terms of a reaction model both radical and molecular. The molecular dehydrochlorination has an activation energy of 57.05 kcal/mol and a preexponential factor of 10^14.02 s⁻¹. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 191–197, 2001     

Synergistic mechanisms of β-diketone derivatives and zinc-calcium soaps in PVC stabilisation

It has been possible to explain the mechanisms of stabilisation and of the synergistic effects of the β-diketone derivatives claimed in a patent as new stabilisers for improving the efficiency of those recipes based upon zinc and calcium soaps in the prevention of the initial discoloration of poly(vinyl chloride).Using chlorohexene as a model compound for allylic chloride structures and benzoylacetone as a model compound for enolised β-diketone derivatives, it has been shown that the benzoylacetone can substitute allylic chlorine atoms through a C-alkylation reaction which takes place only in the presence of ZnCl₂ as catalyst. This reaction drastically changes the percentage of the enol and causes the appearance of two bands at 1720 cm^?1 and 1680 cm^?1 in the infra-red spectrum due to the ketonic structures During the processing of the PVC on a rolling mill at 180°C in the presence of zinc and calcium stearates and benzoylacetone there is grafting of the ketone derivative through a C-alkylation reaction. There is a closed parallelism between the influence of the benzoylacetone on the dehydrochlorination of the chlorohexene and on the accumulation of chloride ions in the polymer matrix in the presence of zinc and calcium stearate. The synergistic effect of the benzoylacetone in the prevention of the initial discoloration is related to the substitution reaction through a C-alkylation which takes place only in the presence of zinc stearate which generates ZnCl₂ which, in turn, acts as a catalyst for both the C-alkylation and dehydrochlorination.     

Friedel-Crafts acylation reactions in copolymers of methyl methacrylate and styrene and in mixtures of the homopolymers

In the presence of SnCl₄ in 1,2-dichloroethane solution, copolymers of styrene and methyl methacrylate undergo a Friedel-Crafts acylation reaction between the ester groups and the ortho position of adjacent styrene units to form a partial ladder polymer. This has been confirmed by infrared and ultraviolet spectral analysis and by observing the influence of substituted styrenes on the rate of the reaction. A similar reaction can be induced to occur between polystyrene and poly(methyl methacrylate). Thermal analysis measurements demonstrate that the degradation properties of copolymers of styrene and methyl methacrylate are profoundly changed by this treatment.     

Synthesis and characterization of a one-dimensional polymeric copper(Ⅱ)-tetrathioporphyrazine

The title polymer PCuS4Pz was synthesized by reaction of 2,3-dicyano-5,6-dihydro-1,4-dithiin,pyromellitic dianhydride and urea with cuprous salt in optimized gentle method.The structure and properties of the PCuS4Pz were characterized by elemental analysis,X-ray powder diffraction,IR,UV-Vis,fluorescence and EPR spectra and variable-temperature magnetic susceptibility.The polymer is black sublimable crystallite and the degree of polymerization has been found to be n>4.The PCuS4Pz in H2SO4 exhibits intensive absorption bands at 236,342,656 and 767 nm and intensive fluorescence band at 410 nm or 464 nm under the excitation of the ultraviolet light of a determined wavelength at room temperature.It has been found that the polymer exhibits a weaker antiferromag-netic interaction (J=-2.cm-1,εff=1.68 B.M.) with an apparent spin S<1/2 in the ground state and its conductivity 298K is 1.01×10-5 S-cm-1 at 13.73 MPa.     

Kinetics of electron transfer reactions of manganese (III) complexes of trans-cyclohexane-1,2-diamine,-NNN′N′-tetraacetic acid and 2,2′-bipyridyl with ascorbic acid in aqueous acid media

The kinetics of the reactions of manganese(III) complexes of trans-cyclohexane-1,2-diamine-NNN′N′-tetraacetic acid (H₄cydta) and 2,2′-bipyridyl (bpy) have been investigated in the acid ranges [H⁺] = 1.00 ×10⁻⁵ − 3.16 × 10⁻³ M and [H⁺] = 0.10 − 1.00 M, respectively, at different temperatures and at a constant ionic strength. Both the molecular and mono-anionic forms of ascorbic acid have been found to be reactive in the experimental acid ranges. The monoanionic species has been found to be more reactive than the molecular form. Attempts have been made to correlate the kinetic results in terms of the Marcus relationship for outer-sphere electron transfer reaction. Differences between the calculated rates (k_0.8) and experimental rates (k) by approximately four and seven orders of magnitude (k_0.8≪ k) suggest an innersphere pathway for the reaction of Mn(cydta)(H₂O)⁻. Inner-sphere reaction is also believed to be operative for the other reaction. The activation parameters have been evaluated and compared with other reported systems.     

Synthesis and Characterization of p—[Perfluoro—1—（2—fluorosulfonylethoxy）]ethylated Polystyrene

A new fluorinated polystyrene bearing a p-sulbstiuted perfluoro[1-(2-fluorosulfonylethoxy)]ethyl group was synthesized via one-electron oxidation of polystyrene by perfluoro[2-(2-fluorosulfonylethoxy)]propionyl peroxide at different peroxide to polystyrene molar ratios.The yield of perfluoroalkylation decreases with the increase of the reactant molar ratio.The modified polymer has been characterized by various techniques:the ring pefluoro[1-(2-fluorosulfonylethoxy)]ethylation has been proved by FT-IR and ^19FNMR;the X-ray photoelectron spectra(XPS) show the maximum binding energy of F18,O18,C18(two kinds of carbon atoms,namely C-H and C-F)and S2p,respectively; desulfonylation of the fluorinated polystyrene appearing at 217℃ has been found by its thermogravimetric analysis (TGA).The determinations of contact angle,refractive index and glass transition temperature of the modified polymer have disclosed that when the contact angle increases with the increase of the molar ratio,the refractive index and glass transition temperature decrease.The polydispersity values indicate that the degradation of the polymer chains did not occur during the reaction.