Chain scission random thermal

High-temperature SEC finds wide application in polymerization studies, as the molecular mass distribution is an artefact of the various reactions involved in polymerization, initiation, termination, and transfer. It is diagnostic of living systems and random polymerization reactions, such as condensation and radical initiated polymerizations, for which the distributions are Poisson and normal respectively. In the polymerization of ethene and propene by Ziegler-Natta catalysts, the determination of the concentration of active centres as a function of conversion defines catalyst type. Similar studies have been made in the study of chain scission by thermal degradation or by irradiation, in defining the number of molecules produced from the inverse of the number average molecular mass and random chain scission eventually leads to a normal molecular mass distribution, with polydispersities close to 2.0. This has, of course, been widely used to produce narrow from broad molecular mass distribution samples prior to fractionation. 

Unlike the polyalkyl acrylates, which are thermally degraded by random chain scission, polyalkyl methacrylates unzip when heated, and excellent yields of the monomers are produced when the polymers of the lower homologues are heated. When higher homologues are heated, there is also some thermal degradation of the alkyl substituents. 

PEG can be severely degraded in air. Its melting point and heat of fusion are reduced by as much as 13 °C and 32 kJ kg"1, respectively [81]. The thermal degradation of PEG in air follows a random chain scission oxidation mechanism, and could be suppressed by addition of an antioxidant, 2, 2,-methylene-bis (4-methyl-6-tert-butylphenol) (MBMTBP), due to the reaction of MBMTBP with ROO radicals formed in the propagation step [79]. Low-molecular-weight esters including formic esters are produced as the main products of the thermal degradation of PEG (Scheme 3.17) [80]. 

For both polyethylene and its many copolymeric variants and polypropylene, the main thermal degradative routes follow initial random chain scission. These reactions are only slightly affected by the differences in the physical structure such as crystallinity, but are influenced by the presence of impurities. However, it is largely true that while these may influence the proces-sibility and long-term stability of respective polyolefins, they may have little or no effect on the flammability. 

Although population balance methods are relatively undeveloped for interesting polymers, the author believes they have great potential. Each of the four distinct thermal degradation mechanisms mentioned earlier will be analyzed in detail and examples involving specific polymers will be discussed wherever possible. Furthermore, some relevant combinations of mechanisms (such as combined random scission and end-chain scission) will also be discussed. 

It has already been noted above that the thermal degradation of PS is thought to involve a mixture or end-chain and random scission mechanisms. Guaita et al. have investigated this case using a mixture of experimental and Monte Carlo methods32 and we now consider the corresponding PBM. 

In cases where no additional oxygen is present, polystyrene can undergo nearly pure thermal degradation. The two prevalent mechanisms are sequential elimination of monomer units, which is called unzipping or depolymerization. In this case, styrene monomer is formed. Random chain scission can also occur. It is sometimes combined with unzipping at the reactive broken chain ends. At temperatures approaching 300 °C, up to 40 % of a polystyrene molecule can be converted to styrene monomer. 

Good coincidence of the X-ray patterns of these two kinds of oligomers suggests that the thermal chain scission in as-polymerized poly-DSP crystals does not proceed randomly at the position of cyclobutane ring in the molecular chain but is somewhat favored on the position of cyclobutane ring in the middle of the chain655. Thus, the oligomer crystal is preferentially accumulated in thermal depolymerization under the polymer crystal-lattice control. This abnormal chain scission is most plausibly explained when the... 

Thermal polymer degradation is determined by the chemical structure and length of the polymer chain, by the presence of unstable structures (such as impurities or additives) and by the temperature level inside the reactor, which must be high enough to break the weakest, primary chemical bonds. Madorsky and Straus [39] found that some polymers (such as PMMA and PTFE) mainly revert to their monomers upon heating, while others (such as PE) yield a great many decomposition products. These two types of dominant thermal polymer degradation are called end-chain scission and random-chain... 

Polypropylene (PP). Pyrolysis of PP is favored by the branched structure of the polymer the thermal degradation also proceeds in this case via a random-chain scission, but the influence of the temperature on the product spectrum is more pronounced than in the case of PE [43, 31], At temperatures as low as 515°C, Predel and Kaminsky [26] found that PP pyrolysis leads to the production of 6.8% of gases, 36.7% of oils, 21.6% of hght waxes and 34.6% of heavy waxes. At these low temperatures the main compounds in the gas fraction are propene and butenes (about 51 and 17% in [26]), but at higher temperatures these products are converted into others [43]. Ponte et al. [31] found a remarkable... 

Thermal degradation of plastics can be classified as depolymerization, random decomposition and mid chain degradation [54, 55], In the process of depolymerization, the conjunction bonds between monomers are broken up, which leads to the forming of monomers. Depolymerization type plastics mainly include a-polymethyl styrene, polymethyl methacrylate and polytetrachloroethylene. In the random decomposition process, scission of carbon chains occurs randomly, and low-molecular hydrocarbons are produced. Random-decomposition-type plastics include PP, PVC and so on. In most cases, both decompositions take place. To be more specific, the degradation of polyolefins can be classified as the following three types ... 

The model chosen to describe the degradation of polyethylene was random chain scission. Lenz (3,) in his section on degradation reactions of polymers cites work which supports the contention that polyethylene does thermally degrade in a random chain scission manner as opposed to depolymerization. For this model a statistical treatment has been developed by Montroll and Simha (). The extent of reaction may be related to the number average molecular weight by ... 

In a sense, it is unfortunate that discussions of the chemical degradation of VI improvers combine thermal and oxidative effects since the two processes are quite different. A simple thermal process is one which can take place in the absence of oxygen and would include processes such as the random thermal scission of a polymer backbone, which may be followed by depolymerization. Another possibility for PMAs or SPEs is pyrolysis of the ester side chains to form olefin and acid. The acid, in turn, can react with an adjacent ester to form a cyclic anhydride with elimination of alcohol. Adjacent acid groups can eliminate water to make an anhydride [74], There is no evidence that depolymerization and ester pyrolysis are issues in the engine oil itself, but they may be a factor if the VI improver is trapped in deposits. 

Weight loss by the thermal degradation or thermooxidative degradation of a polymer itself (as opposed to the volatilization of small molecules which might have been trapped in the polymeric structure) invariably requires the breakage of chemical bonds. Once chemical bonds start to break, reactive chain ends and other free radicals are created, and degradation can proceed either by depolymerization or by random chain scission [11,12]. 

A review and some new results on the thermal degradation of poly-p-xylylene have been presented by Jellinek and Lipovac [303]. Little volatile material is formed but appreciable amounts of dimer, trimer, tetramer and pentamer were isolated. Typical vacuum volatilization curves are given in Fig. 73. It has been proposed that the mechanism consists of random chain scission at abnormal structures in the chain, followed by a depropagation reaction resulting in low molecular weight polymer but very little monomer. 

The thermal degradation of irradiated isotactic PMMA has also provided information on the structure of the end groups formed at the site of main-chain scission [408]. The weight loss of non-irradiated PMMA at 250°C has been shown by Grassie and Melville [409] to be mainly due to depolymerization initiated at the carbon—carbon double bonds situated at the chain ends. A small proportion of randomly initiated depolymerization also occurs at this temperature. In agreement with this mechanism, the rate of volatilization has been found to be much higher for atactic than for isotactic PMMA, the latter having no double bonds at the chain ends. If 4.3 scissions per chain are produced by 7-irradiation in the isotactic sample, the rate of monomer evolution is identical to that of the initial unirradiated isotactic sample. This proves that chain ends of the type... 

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