Pyrolysis–GC–MS used to study the thermal degradation of polymers containing chlorine III. Kinetics and mechanisms of polychloroprene pyrolysis. Selected ion current plots used to evaluate rate constants for the evolution of HCl and other degradation products ☆

Abstract The thermal degradation of polychloroprene [poly(2-chlorobutadiene)/Neoprene] has been studied at 387°C by pyrolysis–GC–MS techniques. First, the overall rate of production of volatile products was measured utilising total ion current (TIC) curves obtained from sequence pyrolysis experiments in which the products were not chromatographically resolved. This overall rate decreases abruptly during the course of the pyrolysis. The early stage (Stage 1) was calculated to have a rate constant k 1 =0.065 s −1 , and in Stage 2 this falls to k 2 =0.013 s −1 . Kinetic analyses of these results show that these stages are the consequence of fast and slow independent parallel degradation processes, in which the fast process is limited in some way so that after about 40 s it is virtually complete, and then only the slow process remains. The kinetics also indicate that the limiting yield (number fraction) of molecules produced by the fast process is approx. 60% of the total molecules produced by the end of the pyrolysis. Selected Ion Current (SIC) curves corresponding to the mass numbers in the MS cracking pattern of hydrogen chloride (i.e. m / z =35, 36, 37, and 38) were then extracted from the above data, and these confirmed that HCl is a pyrolysis product. Despite the fact that the HCl is unresolved in the sequence pyrograms, its yields can be calculated from the areas of its SIC peaks, and on this basis the first order rate constant for HCl evolution ( k HCl ) was calculated as 0.095 s −1 at 387°C. This high value for HCl formation shows that this process must be contributing significantly to the fast overall rate. SIC curves for a wide range of other possible degradation products were examined, and these revealed that monomeric and linear dimeric-type products from polychloroprene are also produced with fast rates ( k mon =0.090 s −1 and k dim =0.113 s −1 respectively). The formation of such products with specific rates which are identical (within experimental uncertainty) with that for HCl evolution, suggests that the loss of HCl molecules is associated with the loss of monomer and dimer molecules. The cumulative yield curves for HCl, monomeric, and linear dimeric products indicated that their total limiting yield was approx. 50% of the total products, which is close to the estimate of 60% for fast products based on the kinetic analysis of the overall yield. The overall rate constant for the formation of products by slow processes was then estimated from data obtained by subtracting the yields for the fast products from the total yield. (These slow products include a wide range of chlorohydrocarbons and some hydrocarbons.) The value was found to be k slow =0.025 s −1 , which may be compared with the overall Phase 2 (slow) value k 2 =0.013 s −1 , above. New overall rate data were then obtained from sequential pyrolyses performed in similar pyrolysis–GC equipment, but using an FID detector. On kinetic analysis a value for the rate constant was obtained (0.016 s −1 ), which compares well with the low value for Stage 2 of the overall rate as measured by MS. However, the FID results also showed that there are also some products formed by a fast rate. Although the magnitude of this could not be measured, it was possible to estimate from the FID results that the total yield of products formed by the fast rate was ca. 10%, which is of the same order of magnitude as the yield of monomeric and dimeric products detected by MS analysis. The differences between the FID kinetic results and those from MS can be understood if it is borne in mind that the FID detector is expected to be insensitive to HCl, an important fast product, but it can detect monomer and dimeric products. Resolved pyrolysis chromatograms allowed approximate values of the ratios of the yields of the different oligomeric species to be estimated. This information allowed the calculation of the ratio of the total number of molecules of HCl evolved to the total number of units of chloroprene in the sample. This ratio was found to be much greater than the fraction of 1,2 chloroprene units in the polymer, showing that although all of these sites could be yielding HCl, an important part of the dehydochlorination is occurring by an alternative mechanism. One possibility is a process involving a concerted, or in some way associated, loss of a hydrogen chloride and a monomer molecule. On the basis of the above evidence it is concluded that the fast process could be a depropagation reaction in which there are intramolecular transfer reactions producing dimer during the unzip. Associated assumptions are that there are a limited number of initiation sites for this depropagation process, and that about half of the monomer molecules release HCl at their moment of unzip. Possible evidence for this was obtained from the observation of substantial yields of dehydrochlorinated monomer. It is also proposed that the slow process could involve random scissions, which would be expected to produce a distribution of oligomers, including some monomer. Superimposed on all of this are secondary reactions, possible examples of which are the combination of monomer with itself to form cyclic dimers, and the combination of various molecules with HCl.