Depolymerisation chain

In random degradation molecular mass decreases early, while in chain degradation the molecular mass of the polymer remains almost constant. Characterisation methods for molecular mass are thus very sensitive methods to follow random degradation. In contrast, as monomer is produced in chain depolymerisation, weight loss measurement techniques are the best methods to follow this kind of degradation. (Chapters 10-12, in Section IV, of this book focus on the methods used in the molecular characterisation and analysis of polymer degradation and polymer degradation mechanisms.)... 

Figure 34 Example of propagation step in chain depolymerisation mechanism (unzipping) of polymethylmethacrylate (PMMA).

The oxidation of starch leads to the formation of carbonyl and carboxyl groups in the polymer chain. Depolymerisation also occurs and starch turns yellowish. 

Zinc carbonate species appeared to undergo slow decarboxylation [scheme (14)] and then, as zinc alcoholate species, were involved in chain depolymerisation via backbiting reaction according to the following scheme [147] ... 

The types of polymer degradation can be divided into three general categories chain depolymerisation, random scission and substituent reactions. 

As mentioned earlier, there are three types of thermal decomposition chain depolymerisation, random decomposition and degradation by substituents reactions. 

Chain depolymerisation has been extensively studied (Simha and Wall, 1952). The two factors that are important for the course of the depolymerisation are ... 

Fifty percent of the total energy needed to produce plastic materials is used in the polymerization process. Thus at first sight, it seems sensible to re-use plastic and rubber waste materials (8 ). But Re-use processes have a negative influence on the macro-molecular structure which is mainly a chain depolymerisation effect (9 ). Therefore Re-use by itself is not a satisfactory process. In addition,a large proportion of the plastic waste is polluted and mixed with other types of waste so that Re-use is impossible. 

A polymer is thermally stable untill the decomposition process starts. Two (main) types of thermal decomposition processes are usually recognised for polymers chain depolymerisation and random decomposition. Chain depolymerisation is the release of monomer units from a chain end or at a weak link and is essentially the reverse process of polymerisation. It is often called depropagation or unzipping. Random degradation occurs by chain rupture at random points along the chain, giving a disperse mixture of fragments. These two different processes may occur separately or in combination the latter case is rather normal [2]. Both processes cause sample mass losses which can be measured with a TGA. 

Thermal stability/chemical structure correlations as reported by Van Krevelen [1] and Bicerano [2] are using and predicting the relative short-time, thermal stability values measured in an inert atmosphere. The mass losses measured under these (non-isothermal) conditions are caused by the damage of chemical bonds due to chain depolymerisation and/or random decomposition (see 2.2.1). Van Krevelen [1] distinguishes five experimental indices which characterise this non-isothermal decomposition process ... 

The estimated Td,o-temperature giving an indication of the polymers thermal stability, holds only for polymers with the more or less simple thermal degradation processes of chain depolymerisation and/or random decomposition. S. Shkolnik and E.D. Weil [30] reported, however, that for non-stabilised PK copolymer a process of furan ring formation can start at temperatures of about 250 °C. Water, one of the reaction products, is coming free during this intramolecular... 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Weak links, particularly terminal weak links, can be the site of initiation of a chain unzipping reaction. A monomer or other simple molecule may be abstracted from the end of the chain in such a way that the new chain end is also unstable. The reaction repeats itself and the polymer depolymerises or otherwise degrades. This phenomenon occurs to a serious extent with polyacetals, polyfmethyl methacrylate) and, it is believed, with PVC. 

Stepwise thermal- or base-eatalysed hydrolytic depolymerisation initiated from the hemi-formal chain end with the evolution of formaldehyde. The main reasons for end-capping and copolymerisation mechanisms described above are carried out in order to minimise this reaction. 

Oxidative attack at random along the chain leading to chain scission and subsequent depolymerisation. Initial chain scission is reduced by the use of antioxidants (see Chapter 7) and in recent formulations hindered phenols seemed to be preferred. It is reported that 2,2 -methylenebis-(4-methyl-6-t-butylphenol) is present in Celcon and 4,4 -butylidene bis-(3-methyl-6-t-butylphenol) in Derlin. The copolymerisation helps to reduce the rate of depolymerisation where initiation of depolymerisation is not completely prevented. 

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

Thermal stability is largely concerned with chemical reactivity which may involve oxygen, u.v. radiation or depolymerisation reactions. The presence of weak links and the possibility of chain reactions involving polymer chains may lead to polymers having lower thermal stability than predicted from studies of low molecular weight analogues. 

The hydrolytic depolymerisation of PETP in stirred potassium hydroxide solution was investigated. It was found that the depolymerisation reaction rate in a KOH solution was much more rapid than that in a neutral water solution. The correlation between the yield of product and the conversion of PETP showed that the main alkaline hydrolysis of PETP linkages was through a mechanism of chain-end scission. The result of kinetic analysis showed that the reaction rate was first order with respect to the concentration of KOH and to the concentration of PETP solids, respectively. This indicated that the ester linkages in PETP were hydrolysed sequentially. The activation energy for the depolymerisation of solid PETP in a KOH solution was 69 kJ/mol and the Arrhenius constant was 419 L/min/sq cm. 21 refs. 

Polymers with hetero-atoms in the chain are suitable for chemical recycling of waste materials. In addition to depolymerisation (nylon 6) and solvolysis (nylon 6,6, PETP, PU) the degradation of aliphatic polyamides with dicarboxylic acids, diamines and cyclic anhydrides, especially trimellitic anhydride, becomes more and more important. The utilisation of the obtained fragments is described. 

Although polymers in-service are required to be resistant toward hydrolysis and solar degradation, for polymer deformulation purposes hydrolysis is an asset. Highly crystalline materials such as compounded polyamides are difficult to extract. For such materials hydrolysis or other forms of chemolysis render additives accessible for analysis. Polymers, which may profitably be depolymerised into their monomers by hydrolysis include PET, PBT, PC, PU, PES, POM, PA and others. Hydrolysis occurs when moisture causes chain scissions to occur within the molecule. In polyesters, chain scissions take place at the ester linkages (R-CO-O-R ), which causes a reduction in molecular weight as well as in mechanical properties. Polyesters show their susceptibility to hydrolysis with dramatic shifts in molecular weight distribution. Apart from access to the additives fraction, hydrolysis also facilitates molecular characterisation of the polymer. In this context, it is noticed that condensation polymers (polyesters, -amides, -ethers, -carbonates, -urethanes) have also been studied much... 

Table 8 Comparison between chain degardation (depolymerisation — unzipping) and step degradation (random degradation)...

It has also to be emphasised that polymers synthesized by chain polymerisation can undergo random degradation (as polypropylene for instance), while certain polymers synthesized by step polymerisation can depolymerise (e.g., polyamide 6). 

Breaking down of a long chain molecule into relatively short chain molecules of the same empirical composition as the original substance or in some cases to the original monomer. Depolymerised Rubber (DPR)... 

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