Ethane main-chain scission

Figure 2. Adiabatic potential curves in the main chain scission of a model compound of polyethylene ethane (9) Alg (O) Alu (U) A2g (Q) A2u (A) Eg (A) Eu (-------------------------) singlet (---) triplet (4)...

Polyethylene displays good heat resistance in the absence of oxygen in vacuum or in an inert gas atmosphere, up to the temperature of 290°C. Higher temperature brings about the molecular-chain scission followed by a drop in the molecular-weight average. At temperatures in excess of 360°C the formation of volatile decomposition products can be observed. The main components are as follows ethane, propane, -butane, n-pentane, propylene, butenes and pentenes [7]. 

The results of Babanalbandi and co-workers (221), in which new aliphatic chains ends formed by cleavage of the main chain at the ester unit are observed, are in support of a mechanism in which chain scission dominates cross-linking. These authors reported G-values for the formation of chain end structures comparable with earlier study. Furthermore, the main volatile products of radiolysis of PLA and poly(glycolic acid) (GPA) are CO2 and CO, consistent with chain scission being the most important reaction. In addition, small amounts of hydrogen and ethane gas were observed on the radiolysis of PLA. Finally, Montanari and co-workers (226) have examined the effects of radiation sterilization on the stability of PLGA microparticles used for drug delivery. 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

The hydrogenolysis at low temperatures of hydrocarbons higher than ethane shows that nickel attacks selectively the ends of the chains [308] [389] by successive a-scission, in contrast to, for instance, platinum. This means that methane is the main product on nickel. 

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