Polyene Sequence Length Research Articles

New insights into the degradation mechanism of poly(vinyl chloride), part II. Limited polyene sequence length during degradation and synergism between costabilizers

AbstractFluorescence spectroscopy has shown that the limited sequence length exhibited during the thermal degradation of poly(vinyl chloride) (PVC) was the result of an HCl readdition/splitting off equilibrium, especially in unstabilized PVC formulations. In the case of stabilized PVC, the type and mechanistic action of the stabilisers seemed to play an important role in HCl re‐addition. The possibility of a synergistic effect taking place between costabilisers for PVC, namely N‐phenyl‐3‐acetyl pyrrolidine‐2,4‐dione (P24D) and its derivative N‐isopropyl‐n‐phenyl‐3‐(1‐iminoethyl) pyrrolidine‐2,4‐dione (P24D‐d) has also been investigated by means of statistical design, degrading the samples at 170°C in air and measuring the induction time (Ti) of the degraded polymer. The results reveal that the stabilising efficiency is not only affected by the presence of the metal soap, but also by the nature of the substituents in the P24D derivatives. It may be concluded that P24D‐d on degradation operates as a “reservoir” for P24D, extending the Ti to higher values compared to their individual performances and giving rise to a synergistic combination. The ability of the costabiliser to form a keto‐enol tautomerism as well as its ability to interact with the PVC and the metal soap play a crucial role in controlling their stabilising effectiveness in the polymer. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2744–2763, 2004

Computer Simulations of Microstructure Changes Resulting from the Thermal Degradation of PVC

Recent studies of the dehydrochlorination of PVC in the solid state offer convincing evidence in support of a mechanism that involves initiation at gttg − conformations in isotactic stereosequences generally considered to be normal units. Attempts have been made to verify such a mechanism by direct measurements, using solution 13C NMR, of the changes in tacticity that result from the degradation. In this paper, computer simulations are reported that use Monte Carlo methods to simulate preferential degradation at isotactic sequences of various minimum sequence lengths. The results indicate that at the levels of degradation accessible to experimental verification by solution NMR (<2%), the predicted changes in triad probabilities are close to the limits of experimental error and are not a sensitive indicator of the mechanism. The simulations also indicate that for the degradation to result in polyene sequence lengths that correspond to experimentally determined values requires that the dehydrochlorination include some atactic and syndiotactic sequences which, in turn, reduces the changes in P mm. On the other hand, there are some indications that more specific indications can be obtained from the changes in the probabilities for pentads and longer sequences, provided that appropriate NMR data can be generated

Considerations for Raman spectroscopic determination of polyene length distribution in degraded poly(vinyl chloride)

Practical complications in the quantitative Raman spectroscopic determination of the polyene sequence length distribution in degraded PVC samples were found. If samples are measured directly, i.e. without dissolving them, the distribution of polyenes changes during the Raman experiment if power densities above 20 W/mm 2 are used. Lower power densities or short exposure times (<1 min) must be used to avoid these effects. The direct method turned out to be an appropriate method for the quantitative determination of the polyene length distribution in lightly degraded PVC (<0.05 % HCl loss)

Chemical dehydrochlorination of poly (vinyl chloride) by LiCl in dimethylformamide

The dehydrochlorination of poly (vinyl chloride) (PVC) with LiCl in dimethylformamide (DMF) is distinguished by the formation of polyenic products exhibiting an optical absorption peak at a wavelength greater than 500 nm. PVC dehydrochlorinated by other means usually absorbs below 450 nm. Since optical absorption characterizes the polyene sequence length distribution, we can assume that more polyenes with a large number of conjugated double bonds are formed. Investigating the influence of reaction conditions on the formation of long conjugated polyenes, a strong influence of reaction temperature and polymer concentration in the reaction system was found. Polyenes with an absorption peak at 630 nm could be prepared by adding an inert matrix polymer and hydrogen chloride. The mechanism of the formation of long polyene sequences is discussed by means of an ineffective Cl-Cl substitution, which changes the substituent configuration on the PVC backbone and thus creates steric opportunities for dehydrochlorination.

Resonance Raman spectroscopy in the determination of the polyene sequence length and the amount of dehydrochlorination of a poly(vinyl chloride) sample containing nickel(II) maleate

The resonance Raman spectra of a poly(vinyl chloride) sample containing nickel(II) maleate after heat treatment at 170°C are reported. The spectra were used to obtain the conjugated polyene sequence length and the amount of dehydrochlorination of the poly(vinyl chloride) sample.

Resonance Raman spectroscopy in the determination of the polyene sequence length and the amount of dehydrochlorination of a poly(vinyl chloride) sample containing nickel(II) maleate

The resonance Raman spectra of a poly(vinyl chloride) sample containing nickel(II) maleate after heat treatment at 170°C are reported. The spectra were used to obtain the conjugated polyene sequence length and the amount of dehydrochlorination of the poly(vinyl chloride) sample.

Generation of polyconjugated sequences in polymer chains by an elimination reaction

Distribution of polyene sequence lengths generated by elimination of a low-molecular-mass compound from a vinyl chain was calculated, using a kinetic method developed previously. The concept of submolecules was used to elucidate modification to the above distribution due to conformational non-uniformity of polyconjugated sequences. The obtained results were used to interpret experimental data on chemical dehydrochlorination of PVC.

MINDO/3 study of HCl elimination from PVC model compounds

The syn-elimination of HCl from PVC model compounds has been studied by the MINDO/3 method. The applicability of the method for such a reaction has been tested on ethyl chloride. The study of the dehydrochlorination of compounds containing chlorine in the α-position to systems of 1–4 conjugated double bonds has shown that increasing number of double bonds has only a small effect on the activation energy for HCl elimination. This has led to the conclusion that the growth of polyene sequences in PVC does not terminate because of disappearance of allyl activation with increasing polyene sequence length. A new termination mechanism based on the breakdown of allyl activation because of torsional motion of links in the polymer chain has been offered and supported by MINDO/3 calculations. An attempt has been made to explain the effect of polymer structure on the dehydrochlorination rate.

Formation of conjugated polyenes by chemical and thermal degradation of vinyl chloride copolymers and other vinyl polymers

AbstractRaman and infrared spectroscopic studies on the chemical degradation products of two vinyl chloride/vinyl acetate copolymers and a vinyl chloride/vinyl acetate/vinyl alcohol terpolymer, obtained with the system potassium tert‐butoxide/N,N‐dimethylformamide, show that substantially complete dehydrochlorination occurs. Although the vinyl acetate groups are hydrolysed to the corresponding alcohol these latter largely remain intact and block the extension of the polyene sequences, whose lengths, measured by Raman spectroscopy, are in agreement with values calculated from the initial polymer composition. No degradation other than hydrolysis occurs with vinyl acetate and vinyl alcohol homopolymers or a vinyl alcohol/vinyl acetate copolymer. The thermal degradation of the copolymers containing vinyl chloride, and of two vinyl chloride homopolymers of differing syndiotacticity, at 153°C in N,N‐dimethylformamide, gives both dehydrochlorination and elimination of the comonomer units, the latter reaction occurring at least as rapidly as the former. A crosslinked structure, in which there is some steric stabilisation and, hence, conjugated polyene sequence lengths up to about 40, is formed. A maximum sequence length of 8 is found in thermally degraded poly(vinyl acetate), but the conjugated polyenes are too short to be detected in degraded poly(vinyl alcohol) and the vinyl alcohol/vinyl acetate copolymer.

Resonance Raman spectrum of degraded poly(vinyl chloride). 4. Determination of conjugated polyene sequence lengths

Resonance Raman spectrum of degraded poly(vinyl chloride). 4. Determination of conjugated polyene sequence lengths

Dehydrochlorination of Poly(vinyl Chloride) in Pyridine. II. Characterization of Dehydrochlorinated PVC

Abstract The structural changes in PVC during thermal dehydrochlorination in pyridine have been investigated. From gel permeation chromatography analyses, chain scission and cross-linking reactions were observed. Cross-linking increased with an increase in reaction time and temperature, whereas the chain scission increased with dilution of the reaction solution. Infrared spectros-copy indicated the presence of conjugated double bond sequences in polymeric chains. Polyene sequences were analyzed by ultraviolet spectroscopy. The length of the polyene sequences and the total number of conjugated double bonds increased with reaction time. An increase in dehydrochlorination temperature lowered the fraction of long polyene sequences, and thus the discoloration. Dilution of the reaction solution decreased the polyene sequence lengths as well as the total number of conjugated double bonds. Shorter polyenes were maximum and their proportion decreased with increase in their length.

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.

Thermal degradation of poly(vinyl chloride) in presence of additives, part II. Changes in absorption spectra

AbstractElectronic absorption spectra of PVC degraded in ethyl benzoate at 200°C in presence of maleic anhydride (MA), benzoquinone (BQ) und anthraquinone (AQ), were studied. The distribution of polyene sequence length was calculated. It was observed that the rate of formation of polyenes having 6 conjugated double bonds was reduced in presence of MA. In case of BQ and AQ, no such inhibiting effect was observed. However, formation of polyenes having a higher number of double bonds (i. e. n=8) was reduced in the presence of these additives. The ageing behaviour of the degraded polymer samples was also investigated.

Heat Degradation of PVC Stabilized by Treatment with Alkylaluminum Compounds

Abstract Thermal degradation of PVC treated with alkylaluminum compounds has been studied. Four PVC samples of different molecular weights have been treated with Me3Al, and Et3A1, and the dehydrochlorination rates of the polymers were determined at 190 and 220°C under a nitrogen atmosphere. The alkylaluminum-treated low molecular weight samples show marked increase in thermal stability, i. e., slower rate of dehydrochlorination right from the beginning of degradation, whereas with the higher molecular weight samples stabilization becomes pronounced only after a few percent of dehydrochlorination. The color of R3Al-treated samples was much lighter (yellowish) than those of controls (dark brown) at 1% HCl loss. The average polyene sequence lengths formed during the early stages of dehydrochlorination are found to be much shorter with RsAl-treated PVC than with virgin samples. It appears as though polyene sequences which arose by zipping- initiation from allylic and/or tertiary chlorine sites are longer than...

Characterization of Polyene Sequences in Poly(vinyl Chloride)

Abstract Some aspects of the formation and propagation of polyene sequences during the thermal degradation of PVC have been discussed. The average polyene sequence length occurring in a suspension polymerized PVC, degraded in nitrogen at 190°C, was determined to ca. 10 units. The results were somewhat dependent on the type of analysis employed. The formation of internal polyene sequences was characterized by three stages. After an initial period, where the number of internal sites remained constant, new sites were rapidly formed. The high rate of formation then gradually decreased with increasing conversion. Degradation mechanisms explaining this behavior were suggested.

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

AbstractThe 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.

Polyene sequence distribution in thermally degraded poly(vinyl chloride)

AbstractA kinetic model for the formation of polyene sequences during the thermal degradation of poly(vinyl chloride) in nitrogen is proposed. The model includes a propagation step of the “zipper” type and termination by crosslinking. The initiation can occur randomly or at weak links in the polymer chain. The rate of polyene growth increased with increasing polyene sequence length and passed through a flat maximum at n = 7. The average polyene sequence length was almost constant up to about 0.5% conversion, but then slowly decreased with increasing conversion. This was the result of a larger extent of termination reactions (crosslinking) at higher conversions. At 0.2–0.5% conversion an average polyene sequence length of about 6 was observed. The degradation was carried out in nitrogen at 190°C. The experimental results were obtained from ultraviolet‐visible spectrophotometric measurements.

Dehydrochlorination of poly(vinyl chloride)

AbstractPVC has been dehydrochlorinated with alcoholic alkali in soution at 7°C for different lengths of time. At early stages of dehydrochlorination the dominant reaction is intramolecular removal of HCl and this gives rise to two intense Raman bands at ∼ 1126 (ν1) and ∼ 1518 cm−1 (ν2) and following UV irradiation, to a quadruplet ESR spectrum. Increasing polyene sequence length and intermolecular removal of HCl at later stages of reaction alters the quadruplet signal to a singlet, shifts ν1 and ν2 to lower frequencies and increases the molecular weight. The presence of polyene units stiffens the chain and increases the elastic modulus. The Tg is, however, lowered slightly due to the removal of bulky chlorine atoms which relieves steric hindrance and dipole interaction between neighboring chains. The β‐transition is also rendered less distinct.

The thermal decomposition of polyvinylchloride—iii.: An investigation of discolouration

Absorption spectra of thermally degraded PVC films are recorded for a number of polymer samples submitted to various dehydrochlorination conditions and in the presence of stabilizers. Correlations are drawn between colour in degraded PVC and dehydrochlorination; the spectra are also discussed qualitatively with special reference to stabilization. The average polyene sequence length in degraded PVC is calculated using an ozonization technique and compared with values derived from peroxide data and from absorption spectra.