Polymeric chain scission

The polymeric chain scission is an elimination reaction that takes place by breaking the bonds that form the polymeric chain. When the reaction takes place as a successive removal of the monomer units from the polymeric chain, it is called a depolymerization. 

The chain scission can be seen as a pyrolytic elimination reaction. All mechanisms described in Section 2.2 may take place during chain scission. A reaction of chain scission with a cyclic transition state may take place, for example, during cellulose pyrolysis  

Some other chain scissions have a free radical mechanism [4,5]. As an example, the formation of isoprene from natural rubber probably falls in this class  

Only up to 58% of natural rubber can be practically depolymerized to isoprene during pyrolysis. A random chain scission may also take place along the polymeric chain. The result is the formation of molecules of lower molecular weight. However, in order to be volatile enough to be analyzed by typical analytical techniques associated with analytical pyrolysis, these fragments have to be relatively small. The formation of monomers as a final step in the random chain scission is not uncommon, and sometimes it is difficult to decide if a depolymerization or a random chain scission was the first step in pyrolysis. 

The free radical mechanism responsible for the polymeric chain scission is basically not different from elimination involving free radicals described in Section 2.2. However, the process can be more complicated and some particularities are described below. 

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Initiation-formation of macroradicals due to the polymeric-chain scission under the action of heating. 

Depolymerization of some natural polymers is another typical example. Milling of chitin or chitosan, at ambient temperature, leads to cleavage of the cellulose polymeric chain. Scission of 1,4-glucosidic bonds takes place, and the radicals formed recombine. Based on electron spin resonance, Sasai et al. (2004) monitored both the homolysis and the radical recombination. The recombination led to the formation of midsize polymeric chains only. Some balance was established between the homolytic depolymerization and the size-limited recombination of the radicals primarily formed. 

The pyrolysis of one molecular species may consist of one or more pyrolytic reactions occurring simultaneously or sequentially. The path of a pyrolytic process depends on the experimental conditions. Mainly for polymers, after a first decomposition reaction step, it is common to have subsequent steps. In this case, the polymeric chain scission, for example, is followed by other pyrolytic reactions of the small molecules generated from the polymer. Therefore, pyrolysis of both small and large molecules occurs in the pyrolysis of a polymer. The result is a complex sequence of chemical reactions with a variety of compounds generated. 

Eliminations and other reactions do not necessarily take place only on the polymeric chain or only on the side groups. Combined reactions may take place, either with a cyclic transition state or with free radical formation. The free radicals formed during polymeric chain scission or during the side chain reactions can certainly interact with any other part of the molecule. Particularly in the case of natural organic polymers, the products of pyrolysis and the reactions that occur can be of extreme diversity. A common result in the pyrolysis of polymers is, for example, the carbonization. The carbonization is the result of a sequence of reactions of different types. This type of process occurs frequently, mainly for natural polymers. An example of combined reactions is shown below for an idealized structure of pectin. Only three units of monosaccharide are shown for idealized pectin, two of galacturonic acid and one of methylated galacturonic acid ... 

The pyrolytic process of a repetitive polymer frequently takes place with the formation of small volatile molecules and has a polymeric chain scission mechanism, as described in Section 2.6. Considering a polymer with a degree of polymerization (DP) n, the end scission reaction can be described by the chemical equation ... 

The polymeric chain scission mechanism is terminated as shown in Section 2.6 by one or more of the following types of termination reaction ... 

Polymer degradation reactions are frequently categorized based on the site in the macromolecule structure where the reaction occurs. This leads to the following classification of scission reactions a) polymeric chain scission, b) side group reactions, c) combined reactions [5, 3]. These reactions follow one of the mechanisms described previously, but this different classification allows a better correlation of the nature of the reaction products with the structure of the polymer and provides more understanding regarding the expected pyrolysis products. 

In practice, a thin film of polymer (the "resist") is coated on the surface, typically in a thickness of 0.5 to 2 pm. Selected areas are tered by exposure to the electron beam. The most common reactions induced by the beam (Fig. 1) are chain scission, crossUnking, and polymerization. Chain scissioning increases the solubility of the exposed resist which can then be washed away leaving behind the unexposed polymer to act as a mask during etching, doping, metallizing, etc. A polymer that responds to radiation by increased solubility is termed a "positive" resist. 

Hydroperoxides determination is the key factor to measure the level and the behavior of the oxidation (Carlsson et al. 1987b, Lacoste and Carlsson 1991, Shen, Yu, and McKellop 1999). Scientific studies and the American Society for Testing and Materials (ASTM) oxidation index standard (ASTM F2102) usually give the quantity of ketones and other carboxyl species present as an index of the oxidation degree. It must be pointed out that ketones, though a product of the oxidative process, do not produce polymeric chain scissions and so they do not result in substantial reduction of tiie UHMWPE mechanical properties. Quantification of ketones is reliable only if the ratio between ketones and carboxylic acids remains constant through the entire oxidative process. 

As this takes place, scission of polymeric chains is hindered and the molecular dimensions are even growing up to the temperature of 300°C. 

Elliot [38] has reported that interfacial adhesion in the NR-PP blend can be enhanced by the addition of small amounts of HOPE. Addition of HDPE does give some improvement in the notched Izod impact strength of NR-PP blend (Fig. 7). The effect of HDPE on the impact modification of NR-PP is associated with the improved crystallinity of PP, enhanced by HDPE. During the mill mixing of NR and PP, chain scission may occur to give polymeric radicals that, on reaction with... 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

The combined results of kinetic studies on condensation polymerization reactions and on the degradation of various polymers by reactions which bring about chain scission demonstrate quite clearly that the chemical reactivity of a functional group does not ordinarily depend on the size of the molecule to which it is attached. Exceptions occur only when the chain is so short as to allow the specific effect of one end group on the reactivity of the other to be appreciable. Evidence from a third type of polymer reaction, namely, that in which the lateral substituents of the polymer chain undergo reaction without alteration in the degree of polymerization, also support this conclusion. The velocity of saponification of polyvinyl acetate, for example, is very nearly the same as that for ethyl acetate under the same conditions. ... 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

The term charring refers to the complete degradation of a polymer after which there is no longer any polymeric character to observe. Charring results from chain scission reactions that are left unchecked and is the typical process by which thermosets degrade. The resulting material is typically black and brittle. 

Other more complex linear block co-, ter- and quarterpolymers, such as ABC, ABCD, ABABA can be prepared using the previously mentioned methods. An important tool in the synthesis of block copolymers involves the use of post-polymerization chemical modification reactions. These reactions must be performed under mild conditions to avoid chain scission, crosslinking, or degradation, but facile enough to give quantitative conversions. Hydrogenation, hydrolysis, hydrosilylation and quaternization reactions are among the most important post-polymerization reactions used for the preparation of block copolymers. 

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