Polymers degradation termination

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

T. Sawaguchi and M. Seno, Thermal degradation of polyisobutylene effect of end initiation from terminal double bonds, Polym. Degrad. Stab., 54(l) 33-48, October 1996. 

Successive repetition of reactions (Equation 4.26 or Equation 4.27) lead to formation of Bi°. These redox reactions act as termination steps for polymer degradation. Radical terminations by metal compounds through redox reaction are well known.35-58... 

Two steps occur in the microbial polymer degradation process, first, a depolymerisation or chain cleavage step, and second, mineralisation. The first step normally occurs outside the organism due to the size of the polymer chain and the insoluble nature of many of the polymers. Extracellular enzymes are responsible for this step, acting either endo (random cleavage of the internal linkages of the polymer chains) or exo (sequential cleavage of the terminal monomer units in the main chain). 

Silyl group transfer can also be used to functionalize chain ends. For example, allyl silanes, silyl ketene acetals, and silyl enol ethers [301-304] generate polymers with terminal allyl and methacrylate groups [Eq. (103)]. This type of transfer becomes degradative (termination) if reinitiation with silyl halides is not possible. 

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

The polymerization of methyl methacrylate initiated by the products of the thermal degradation of 9-anthryldiazomethane can serve as an example of the synthesis of a polymer vrith terminal LM using an initiator-containing luminescent group ... 

Saraf, V., Glasser, W. G., Wilkes, G. L., and McGrath, G. E. Engineering plastics from lignin IV. Structure property relationships of PEG-containing polyurethane networks, Journal Applied of Polymer Science, 30, 2207 (1985). Sarkar, S. and Adhikari, B. Thermal stability of lignin-hydroxy-terminated polybutadiene-co-polyurethanes. Polymer Degradation and Stability, 73, 169-175 (2001). 

The cured and the liquid polymers degrade essentially by the same mechanism (see Equation 6.1). The kinetic analysis of the isothermal and dynamic thermogravimetric data of the liquid polysulfide polymer cured with ammonium dichromate is explained by a kinetic model based on random initiation, followed by rapid termination by disproportionation. The average overall activation energy obtained by different methods for the decomposition is 145.3 kj/mole ... 

Thus, an increase in polymer crystallinity should generally decrease polymer degradation rates, as extensively reported for PLA matrices under alkaline hydrolytic degradation conditions.Other researchers reported accelerated hydrolysis in neutral media with increasing polymer crystallinity for PLLA and PLLA/PDLA blends. A possible explanation to this behaviour has been related to the consideration that, upon crystallization of PLLA, hydrophilic terminal groups (-OH and -COOH) can be condensed in the amorphous area between the crystalline regions (see... 

Mw reduction at 70 °C for different initial concentrations of ammonium persulfate breaker as a function of reaction time. Molecular weight decrease is most rapid within the first 10 to 20 hours. The rate of polymer Mw degradation then slows considerably. As breaker concentration increases, lower values of Mw are reached as degradation terminates. Results of degradation experiments at 80 and 90 C are similar. However, as temperature increases a lower value of Mw is reached for the same initial breaker concentration as shown in Figure 7. 

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