Trevor N. Bowmer

Nature of the side chain branches in low density polyethylene: volatile products from gamma radiolysis

Measurements of the yields of low molecular weight hydrocarbons after γ-irradiation of low density polyethylene (LDPE) in the solid and liquid phases have confirmed that the main products are ethane and butane. Alkene yields were small, contrary to some previous reports. Irradiation at 150°C doubled the total yield of hydrocarbons, but the relative proportions of different hydrocarbons did not change markedly. The compatibility of these results with 13C n.m.r. determinations of the distributions of short chain branches in the same samples of LDPE is considered.

Radiation Degradation of Poly(olefin Sulfone)s: A Volatile Product Study

Abstract The predominant degradation reaction in the γ-irradiation of nine poly(olefin sulfone)s was found to be C-S bond scission with elimination of SO2 and olefin. The extent of depolymerization, measured by the yields of the two comonomers, increased over five irradiation temperatures from 0 to 150° C and could be correlated with the ceiling temperature. Thus G (total volatile products) increased from 10 to 10,000 over this temperature range. Minor radiolysis products included the alkanes corresponding to (1) loss of the side chain group and (2) scavenging of the side chain radical by monomer olefin. There was a deficiency of olefin relative to SO2, except at high temperatures, and isomerization of the product olefin in some cases. These observations are attributed to reactions of radiation-induced polymeric cations.

Thermal degradation of poly(olefin sulphone)s. Part I-The effect of olefin structure on the yields of volatile products

Abstract The rates and mechanisms of the thermal degradation of nine alternating poly(olefin sulphone)s with different olefin structures have been investigated at 150°C and 200°C by a novel technique which is particularly suitable for studying the initial steps of the degradation. Rapid degradation was initiated at the CS bond with depolymerisation to sulphur dioxide and olefin. The rate of thermal degradation showed a moderate correlation with the ceiling temperature for monomer-polymer equilibrium and also with the number of β-hydrogen atoms, but neither parameter provided an adequate measure of the sensitivity of all the poly(olefin sulphone)s to thermal degradation. Substantial isomerisation was observed in the formation of olefin from poly(3-methyl-1-butene sulphone).

Isomerization in Olefin Formation in Radiation Degradation of Poly(Olefin Sulphone)s

SummaryGamma irradiation of poly(3-methyl-1-butene sulphone) in the solid state at 20°C produced a mixture of isomers,viz. 3-methyl-1-butene (61%), 2-methyl-1-butene (2%) and 2-methyl-2-butene (37%). Similarly, poly(1-butene sulphone) yielded 1-butene (65%) and 2-butene (35%). A hydride shift reaction in a polymer cation produced by main-chain scission is proposed to account for the isomerization.

Characterization of the short-chain branches in poly(vinyl chloride)s by γ irradiation of the reduced polymers☆

The volatile products from the γ irradiation of samples of poly(vinyl chloride) prepared under different conditions and reduced to the corresponding polyethylenes have been measured quantitatively and compared with those for low- and high-density polyethylenes and copolymers of ethylene with small amounts of α-olefins. The presence of methyl branches is clearly demonstrated and there is also evidence for ethyl and butyl branches, although these had not been considered significant in previous [13C] NMR studies. The method is shown to be extremely sensitive to small quantities of residual trialkyl tin hydride reductant occluded in, and possibly also reacted with, the polymer, and to residual chlorine (<0.5%). The yields of alkanes are higher than expected from the branch frequencies determined by i.r. and [13C] NMR and results for ethylene-α-olefin copolymers. This difference is apparently due to sensitisation of the radiation degradation by residual chlorine and reductant. Quantitative determinations of branch frequencies by the radiolytic method are unlikely to be obtained until these problems are overcome.

Post-irradiation thermal degradation of poly(olefin sulphones)

Abstract Post-irradiation thermal degradation of six poly(olefin sulphones) has been studied by measuring (1) the increases in volatile product yields upon isothermal annealing at various temperatures after irradiation at 0°C, and (2) the decay of trapped free radicals, produced by γ-irradiation at −196° and 20°C, with time at various temperatures. Above the ceiling temperature for each poly(olefin sulphone) rapid depolymerization occurred, generally yielding equal amounts of sulphur dioxide and olefin. The rates of radical decay also increased greatly in this temperature region.

γ-radiolysis of dialkyl, alkyl-aryl and diaryl sulphones. A volatile product study

Dialkyl sulphones, RSO2R, have been considered as model compounds for the radiolysis of poly(olefin sulphone)s, {A figure is presented}. They show preferential C-S scission and SO2 elimination, attribute to the relatively low strengths of these bonds. Combination of the alkyl radicals, which are produced singly or in pairs according to whether one or two C-S scissions occur in one molecule, competes with hydrogen abstraction from sulphone molecules. The latter is favoured for single C-S scissions and as the size of the radical increases and hence its mobility decreases. An important degradation reaction in radiolysis is considered to be ionization to form the cation radical of the dialkyl sulphone, followed by a single C-S scission to produce the alkyl radical and the complementary alkyl sulphonyl cation, which may undergo scission of the remaining C-S bond to produce SO2. GC/MS studies of the volatile products from dimethyl sulphone have shown that radiolysis results in a complexity of fragmentation and combination reactions, involving scission of most bonds in the molecule. The variety of products has been confirmed using CD3SO2CD3. Radiation protection by aromatic substituents has been demonstrated and branched alkyls have been shown to give higher yields of alkanes and SO2 than linear alkyls.

Radiation degradation of copolymers of ethylene and α-olefins: The temperature dependence of yields of volatile products

γ-Irradiation of polyolefins produces small amounts of volatile hydrocarbons. Their yields have been measured for irradiation of homopolymers of ethylene and copolymers with propene, 1-butene and 1-hexene at temperatures from 30 to 175°C. The alkane distribution was characteristic for each type of polymer, and dependent on the frequency of the alkyl substituents. The distribution for a particular polymer was similar at all temperatures, although the yields of alkanes increased rapidly above 100°C. The yields of alkenes, which were mainly ethylene and butene, increased proportionately more rapidly from almost nil at 30°C. The ethylene yields from all the polymers fitted a common Arrhenius relationship with an activation energy of 40 kJ mol-1 and are attributed to radiation-induced thermal depolymerization. Detection of alkyl short branches in polyolefins is more sensitive at higher temperatures, but corrections for radiation-induced thermal degradation are more significant, whereas morphological differences can apparently affect the radiation sensitivities at 30°C.