Thermal degradation of copolymers of styrene and acrylonitrile. III. Chain-scission reaction

The chain-scission reaction which occurs in copolymers of styrene and acrylonitrile has been studied at temperatures of 262, 252, and 240°C. Under these conditions volatilization is negligible, and chain scission can be studied in virtual isolation. At 262°C three kinds of chain scission are discernible, namely, at weak links which are associated with styrene units, “normal” scission in styrene segments of the chain and scission associated with the acrylonitrile units. The rate constants for normal scission and scission associated with acrylonitrile units are in the ratio of approximately 1 to 30. The molecular weight of the copolymer has no effect on the rates of scission. At 252°C the same general behavior is observed for the copolymers containing up to 24.9% acrylonitrile. The 33.4% acrylonitrile copolymer is anomalous, however. At 240°C the trends observed at 262°C appear to break down completely although individual experiments are quite reproducible. This behavior at the lower temperatures is believed to be associated with the fact that the melting points of the various copolymers are in this temperature range. Thus the viscosity of the medium, which should be expected to have a strong influence on the chain scission reaction, will be changing rapidly with temperature, copolymer composition, and molecular weight in this temperature range.     

Thermal degradation of copolymers of methyl methacrylate and methyl acrylate. II. Chain scission and the mechanism of the reaction

It has been established that one molecule of carbon dioxide is produced for each chain scission during degradation of methyl methacrylate–methyl acrylate copolymers with molar compositions in the ratios 112/1, 26/1, 7.7/1, and 2/1. Thus the relatively simple measurement of the production of carbon dioxide can be used to determine the extent of chain scission. In this way the relationships between chain scission and volatilization, zip length, copolymer composition, and the production of permanent gases have been established. The rate of chain scission is proportional to a power of the methyl acrylate content of the copolymer less than 0.5, from which it has been concluded that a significant proportion of the initial production of radicals and the subsequent attack of these radicals on the polymer chains is at random and not specifically associated with the methyl acrylate units. A mechanism for the overall thermal degradation process in this copolymer system is presented in the light of these observations.     

Thermal behaviour of binary and ternary copolymers containing acrylonitrile

Three types of acrylonitrile copolymers (acrylonitrile-styrene-butadiene copolymer (ABS¹), acrylonitrile-styrene random copolymer (SAN²) and acrylonitrile-butadiene random copolymer (BAN³) were studied by thermogravimetry (TG/DTG⁴) and by pyrolysis in a semi-batch process at 450 °C in order to find structure–thermal behaviour relationships. The overlapped thermo-oxidative degradation processes were separated and the corresponding kinetic parameters were calculated. The TG/DTG studies have evidenced that the styrene-acrylonitrile interactions stabilize the nitrile groups reacting by chain scission rather than cyclization and destabilize the styrene units. Also, the cyclization of the acrylonitrile units in ABS is favoured by interactions with the styrene and butadiene units. The pyrolysis behaviour evidenced that the styrene-acrylonitrile interactions in SAN and ABS lead to the formation of 4-phenylbutyronitrile as the most important decomposition compound. ABS shows similar composition of the degradation oil with SAN copolymer therefore in the ABS the styrene-butadiene interactions are less important than those between styrene and acrylonitrile units.     

Thermal degradation of copolymers of styrene and acrylonitrile. II. Reaction products

Hydrogen cyanide is a minor product of degradation of copolymers of styrene and acrylonitrile. The liquid products have been separated and identified by combined gas chromatography and mass spectrometry (GC-MS), as styrene, acrylonitrile, toluene, and benzene. The ratio of styrene to acrylonitrile monomers in the products is approximately twice that of the monomer units in the copolymers, and the ratios of styrene to toluene and benzene are the same as are obtained from pure polystyrene. These ratios were determined by using infrared spectral methods. The fraction of products volatile at the temperature of degradation but involatile at ambient temperature was also analyzed by using GC-MS. A series of four dimers and four trimers were fairly reliably identified. The residual material from copolymers containing up to 33.4% acrylonitrile is always soluble in toluene. The 50/50 copolymer and its residues are insoluble in toluene. Yellow coloration develops in the residues from high acrylonitrile copolymers at advanced stages of degradation. Infrared and ultraviolet spectra suggest that this is due to conjugated unsaturation in the polymer chain backbone which may be associated with the liberation of hydrogen cyanide from the acrylonitrile units.     

Thermal degradation of graft copolymers of PVC prepared by mastication with styrene and methyl methacrylate,and of further PVC mixtures and vinyl chloride copolymers

Graft copolymers prepared by mastication of PVC in the presence of styrene or of a styrene/ methyl methacrylate mixture, have been studied by thermogravimetry, estimation of hydrogen chloride, thermal volatilization analysis, and flash pyrolysis/g.l.c. The degradation behaviour of PVC/ polystyrene mixtures, vinyl chloride/styrene random copolymers, a random copolymer of methyl methacrylate and styrene, and PVC/poly-α-methylstyrene mixtures has also been studied. The graft copolymers resemble the PVC/methacrylate graft copolymers previously studied in showing retardation of the dehydrochlorination reaction, but contrast with them in yielding chain fragments but no monomer during HCl production. Some stabilization of the second component at higher temperatures is also found. PVC/polystyrene mixtures behave in the same way as the corresponding graft copolymers, but vinyl chloride/styrene copolymers show reduced stability towards both dehydrochlorination and monomer production compared with the homopolymers. PVC/poly-α-methylstyrene mixtures yield some monomer concurrently with HCl loss, and display marked retardation of the latter reaction. Stabilization of the second polymer at higher temperatures is again observed. Many of these results add further strong support to the view that chlorine atoms are involved as chain carriers in the thermal dehydrochlorination of PVC.     

Mechanochemical reaction of polymers by ultrasonic irradiation. I. Mechanochemical reaction in mixtures of polystyrene,styrene, and cyclohexanone

Mechanical degradation and mechanochemical polymerization in polystyrene–styrene–cyclohexanone mixtures have been studied by ultrasonic irradiation at 60°C. The number of fresh polymer chains after the degradation is 2 × 10^?5 mole l^?1 hr^?1. The rate equations for mechanical scission and mechanochemical polymerization have been deduced. The rate equation for mechanical scission was found to be in agreement with the expression of a previous paper. In addition, the rate equation for mechanochemical polymerization is not essentially different from that for the general radical polymerization in the presence of solvents. The kinetic chain length for polymeric free radicals in the polymerization process has been calculated. The mechanochemical polymerization of styrene was initiated by only one of the two kinds of end radicals after mechanical scission of polystyrene. The molecular weight distributions of the samples after the degradation and the polymerization have been compared and discussed.     

Aspects of degradation and stability of ABS copolymers. I. Effect of β-carotene as antioxidant

The oxidative degradation of polybutadiene–styrene–acrylonitrile (ABS) copolymer was extensively investigated. Three factors were studied: the influence of ageing temperature on the rate of the oxidation of the copolymer; the efficiency of β-carotene as chain breaking antioxidant; and the effect of butadiene content on the rate of the oxidative degradation of the ABS copolymer.     

Graft copolymerization of acrylonitrile onto polystyrene

It is possible to graft vinyl monomers, such as acrylonitrile, onto polystyrene via anionic processes but not by a radical process. Both homopolymerization of the added acrylonitrile and graft copolymerization in which acrylonitrile units are added to the para position on the benzene ring in styrene occur; the conversion of acrylonitrile into polymer depends upon the time and temperature of the reaction and on the concentration of the anionic initiator, butyllithium. A constant 15–20% of the acrylonitrile is converted to graft copolymer while the remainder is homopolymerized; graft copolymer may be separated from homopolymer by selective precipitation from either N,N′-dimethylformamide or aqueous potassium thiocyanate. Treatment of the mixed graft and homopolymer with aqueous sodium hydroxide converts the nitrile into an acid salt and one may conveniently separate homopolymer from graft copolymer in this way. Each polystyrene chain is grafted with acrylonitrile units. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1275–1282, 1997     

Boundary effect on the thermal degradation of copolymers

The mechanism of thermal degradation of vinyl-type copolymers at high temperatures was investigated theoretically and experimentally. A parameter β was proposed to account for the boundary effect. Values of β for acrylonitrile–styrene and methyl methacrylate–styrene copolymers were determined experimentally. It was ascertained that the value of β was independent of the distribution of monomer sequence lengths in a copolymer, but dependent on the pyrolysis temperature and on the nature of the copolymer. The boundary effect is attributed to differences in the dissociation energies of C? C bonds connecting terminal monomer units to adjacent monomer units in copolymer chain radicals.     

‘Weak links’ in polystyrene

A first step in the thermal degradation of polystyrene prepared by radical polymerisation has been isolated by heating the polymer in the temperature range 199–280°C. In this step a chain scission process occurs without formation of volatile products, involving, on average, about one bond between structural units in every 10 000. This gives more direct evidence than hitherto of the presence of ‘weak links’ in polystyrene which are shown to be randomly distributed in the polymer chains, their scission resulting in a single break in the molecule of polystyrene to which they belong.The very low energy of activation for chain scission suggests that more than one rate determining step is involved in the process.     

Thermal degradation of poly(vinyl bromide), vinyl bromide-methyl methacrylate copolymers,and poly(vinyl bromide)-poly(methyl methacrylate) blends

Degradation behavior has been compared for PVB, five VB-MMA copolymers which span the composition range, PMMA, and PVC by using thermogravimetry in dynamic nitrogen and thermal volatilization analysis (TVA) under vacuum for programmed heating at 10°C/min. Volatile products have been separated by subambient TVA and identified. PVB is substantially less stable than PVC but shows inmost respects analogous degradation behavior. The introduction of VB into the PMMA chain leads to intramolecular lactonization with release of methyl bromide at temperatures a little above 100°C; after this reaction is complete, however, the polymer is more stable toward volatilization than PMMA. Copolymers with moderate and high VB contents also lose hydrogen bromide. Carbon dioxide is a significant product at intermediate compositions. The variation of product distribution with copolymer composition is discussed in relation to the several reactions involved and comparisons are made with VC-MMA copolymers. PVB-PMMA blends snow some features of degradation behavior in common with the PVC-PMMA system but also very important differences. The effect of PVB is only to stabilize the PMMA; the mechanism is discussed. The role of PVB as an additive and VB as a comonomer for fire-retardant PMMA compositions is briefly considered in relation to earlier studies.     

Changes in degree of polymerization in the thermal degradation of polystyrene

The chain scission and volatilization processes which occur in the thermal degradation of polystyrene have been simulated by Monte Carlo procedures and the results have been compared with the degrees of polymerization, determined experimentally, of polystyrene samples degraded to different extents. The results of the calculations agree with random scission of the polymer chains at temperatures at which volatilization of simple molecules does not occur and with a degradation mechanism involving extensive deactivation of radicals through hydrogen abstraction from the body of polymer chains at temperatures at which volatilization does occur. The calculations also show that the molecularity index, i.e. the ratio between the weight average and the number average degrees of polymerization, tends to 2, independently of the degradation mechanism.     

Thermal degradation of mixtures of poly(methyl methacrylate) and silver acetate

The degradation behavior of silver acetate—PMMA blends at salt/polymer ratios of 1:1, 1:5, and 1:10 has been studied by using thermal volatilization analysis (TVA) as the principal technique. Degradation of the salt has also been examined; it gives a variety of products best explained by a series of reactions resulting from an initial cleavage of CH₃COO. radicals and silver atoms. Silver acetate, when present with PMMA during degradation, results in a severe destabilization of the polymer, which breaks down to monomer at a high rate at temperatures as low as 200°C. This effect is explained by diffusion of radicals from silver acetate decomposition into the polymer phase, in which they initiate chain scission and depolymerization.     

Thermal degradation of copolymers of methyl methacrylate and methyl acrylate. I. Products and general characteristics of the reaction

The technique of thermal volatilization analysis (TVA), applied to methyl methacrylate–methyl acrylate copolymers having molar composition ratios 112/1, 26/1, 7.7/1, and 2/1, has demonstrated that the stabilization of poly(methyl methacrylate) by copolymerized methyl acrylate is due to inhibition of the depolymerization initiated at terminally unsaturated structures, probably by direct blockage by methyl acrylate units. The molecular weight of the copolymers decreases rapidly during degradation, suggesting that a random scission process is involved. The products of degradation consist of the monomers, carbon dioxide, chain fragments larger than monomer, and a permanent gas fraction which is principally hydrogen. Infrared and ultraviolet spectral measurements suggest that the residual polymer, which is colored, incorporates carbon–carbon unsaturation. The complete absence of methanol among the products is surprising in view of its abundance among the products of degradation of poly(methyl acrylate). These observations have been accounted for qualitatively in terms of acceptable polymer behavior.     

Thermal degradation of zinc polymethacrylate and a zinc methacrylate/methyl methacrylate copolymer

Methyl methacrylate and zinc methacrylate have each been polymerised in methanol solution using azodiisobutyronitrile as initiator and a copolymer of the two monomers has been prepared in the same medium.The degradation behaviour of the three materials has been studied using TG and TVA, volatile products have been investigated by infra-red and GLC analysis and infra-red spectroscopic examination of structural changes in the partially degraded polymer has been carried out for zinc polymethacrylate (ZnPMA).The breakdown of ZnPMA shows many similarities to the behaviour of the alkaline earth polymethacrylates. The effect of introducing ZnMA units into the PMMA chain by copolymerisation is to stabilise the chain considerably and to modify the degradation behaviour of the MMA units, so that methanol and carbon monoxide, resulting from side group scission, become major products in addition to MMA monomer; the ZnMA units in the copolymer behave in the same way as in ZnPMA.This study provides support for the mechanism previously proposed for the degradation of PMMA/ZnBr₂ blends, in which the ZnMA/MMA copolymer structure was regarded as an intermediate stage in the reaction.     

Anionic copolymerization of [60]fullerene with styrene initiated by sodium naphthalene

The novel C₆₀–styrene copolymers with different C₆₀ contents were prepared in sodium naphthalene-initiated anionic polymerization reactions. Like the pure polystyrene, these copolymers exhibited the high solvency in many common organic solvents, even for the copolymer with high C₆₀ content. In the polymerization process of C₆₀ with styrene an important side reaction, i.e., reaction of C₆₀ with sodium naphthalene, would occur simultaneously, whereas crosslinking reaction may be negligible. ¹³C-NMR results provided an evidence that C₆₀ was incorporated covalently into the polystyrene backbone. In contrast to pure polystyrene, the TGA spectrum of copolymer containing ∼ 13% of C₆₀ shows two plateaus. The polystyrene chain segment in copolymer decomposed first at 300–400°C. Then the fullerene units reptured from the corresponding polystyrene fragments attached directly to the C₆₀ cores at 500–638°C. XRD evidence indicates that the degree of order of polymers increases with the fullerene content increased in terms of crystallography. Incorporation of C₆₀ into polystyrene results in the formation of new crystal gratings or crystallization phases. In addition, it was also found that [60]fullerene and its polyanion salts [C₆₀ⁿ⁻(M⁺)_n, M = Li, Na] cannot be used to initiate the anionic polymerization of some monomers such as acrylonitrile and styrene, etc.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2653–2663, 1998     

Degradation of polymer mixtures. IX. Blends of polystyrene with polybutadiene

The degradation of cis-1,4-polybutadiene, polystyrene, and blends of PB and PS has been studied by thermogravimetry, thermal volatilization analysis, and differential scanning calorimetry. Volatile products have been investigated and separated by subambient TVA and characterized spectroscopically. In the degradation of the blends, there is no change in the nature of the volatile products of degradation, but the rate of degradation of the PS component is markedly reduced. The PB component is the first to break down, and during the initial period of degradation of the PB, the PS degradation is apparently inhibited. It is suggested that some of the volatile products of decomposition of PB, most notably 4-vinylcyclohexene, may diffuse into the PS phase in the blend and act as radical inhibitors.     

Preparation and degradation of copolymers of methyl methacrylate with alkali metal methacrylates—II: Thermal analysis of the copolymers,nature of the degradation products and general characteristics of the degradation reaction

Three series of copolymers, each spanning the composition range from alkali metal methacrylate homopolymer to methyl methacrylate homopolymer, have been prepared; their degradations have been studied under programmed heating conditions, by means of simultaneous thermogravimetry and thermal volatilization analysis (TVA) in a vacuum system and by differential thermal analysis in dynamic nitrogen. Total volatile products have been characterised by infrared spectroscopy, subambient TVA and GLC. The thermal analysis data suggest that the two types of monomer unit tend to participate in degradation processes in different temperature ranges. However, in addition to those products characteristic of the degradation of each homopolymer, the copolymers give substantial amounts of methanol; this product must arise from a reaction specific to the copolymer structure.     

Photodegradation of copolymers of methyl methacrylate and methyl acrylate at elevated temperatures

Four methyl methacrylate—methyl acrylate copolymers with molar ratios, MMA/MA, of 112/1, 26/1, 7·7/1, and 2/1 have been photodegraded at 170°C by 2537 Å radiation. The changes which occur in the molecular weight of the copolymers are typical of a random scission process and from these and volatilization data the extent of chain scission during the course of the reaction has been calculated. The pattern of volatile products is the same as that previously obtained in the thermal reaction at 300°C although there are a number of differences in detail. For example, only one in ten of the methyl acrylate units is liberated as monomer compared with one in four in the thermal reaction and the ratio CO₂/chain scissions is considerably greater than the strict 1/1 ratio observed in the thermal reaction. Zip lengths are also very much greater in the photo reaction. These minor differences between the two reactions have been accounted for in terms of the mechanism previously presented to account for the thermal reaction, bearing in mind the differences in the temperature (170 and 300°C) at which the two investigations were carried out.     

Analysis of degradation products by thermal volatilization analysis at subambient temperatures

In the subambient thermal volatilization analysis (TVA) technique, degradation products initially at ?196°C are allowed to warm up to ambient temperature in a controlled manner under vacuum conditions, and volatilization from the sample tube to a trap at ?196°C is monitored by means of a Pirani gauge. The technique is discussed in relation to earlier TVA work in which volatilization from a heated polymer sample was followed. Design and operation of a subambient TVA system are described, and examples of the application of the technique to the study of the degradation products of seven polymers are considered.