Mechanism of chain scission

Other additives such as waxes and polyethylene glycols melt at relatively low tem-peratnres and vaporize over a more narrow temperatnre range. Oxidation of the binder canses the temperature to increase more rapidly. For the thermolysis of polyethylene glycol (PEG) in air, decomposition occnrs by the mechanism of chain scission and oxidative degradation. 

The bands at 1795 and 1727 cm designate acid chloride and saturated aldehyde groups (29), respectively, and may arise from a mechanism of chain scission similar to BR and IR. 

After the induction time, chain scissioning became uninhibited and was manifested by loss of elongation. Mechanisms of chain scissioning and stabilization are discussed. 

Mechanism of Chain Scissioning and Stabilization. A review of the average number of chain scissions as a function of oxygen consumption shows the number of oxygen molecules consumed per chain scission increased with increasing oxygen consumption. Beyond the induction period, the mmoles of oxygen per chain scission increased rapidly with time the data are summarized in Table IV. 

Various mechanisms of chain scission have been proposed for polymethylmethacrylate. The simplest involves direct breaking of the main chain followed by disproportionation of the radicals [392], viz. 

One approach to improving the thermal stability of PFAP(II) would be to deactivate the labile moieties (III and IV) which are precursors to the phosphazane weak links (V). Improvements in the thermal stability of PFAP(II) have been realized by treatment with acid scavengers and basic compounds (2). These results are considered consonant with the proposed mechanism of chain scission at the phosphazane (V) site. 

This reaction can be blocked by acetylation of the chain ends, leading to a polymer which can survive above because there is no mechanism of chain scission with a low enough activation energy to initiate depropagation. Another approach is to copolymerize a small amoimt of a second monomer, such as styrene. The polymer is heated to initiate depropagation, which occurs imtil it encoimters the first comonomer unit, at which point further depropagation is blocked. The evolved monomer is recycled. 

As infrared characterization of solid residue and high boiling products has shown [146], carbodiimide functionalities are formed at the thermal decomposition of nylon-6 with melamine and its salts. An unusual mechanism of chain scission of nylon 6 through CH2-C(0) bonds [148] is likely to become operative in the presence of melamines (Scheme 3.4.1). The resultant isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to N- alkylisocyanurate. Carbodiimide can also trimerise to N-aikylisotiiazine. These secondary reactions increase the reactions increase the thermal stability of the solid residue and increase the yield of the char. 

More recent studies by Barnard et ai (1972) using radiochemical methods for measuring bound aldehyde and ketone groups have indicated that the major scission product is laevulinaldehyde and an alternative mechanism of chain scission has been proposed. 

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. 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

A number of workers have looked at the effect of photooxidation and photodynamic sensitizers on DNA. Rose Bengal photosensitizes strand breaks in double-stranded, supercoiled, pBR322 DNA the effect follows first-order kinetics with respect to light fluence and dye concentration. The reaction is substantially more efficient in the absence of oxygen, but the quantum yield of strand breaks in air is only 10 8. The results are consistent with the initiation of chain scission by Rose Bengal triplet, with some additional mechanism coming into play in the presence of oxygen. 

Chemical aging resulting from water absorption (i.e., hydrolysis) has not been as widely studied as physical aging. It is relatively well understood at the molecular scale (chemical mechanisms). But macromolecular (kinetics of decrease of the elastically active chain concentration) and mechanical aspects (effect of chain scissions on mechanical properties) are far from being elucidated. 

The mechanism of y-ray irradiation-induced scission of polyisobutylene was studied, based on the structural characterisation of end-groups by 13C-NMR as well as GC, GC/ MS, and SEC [77], The assignments of signals were made by comparison with those from model compounds and predictions based on empirical rules. Quantitative 13C-NMR measurements of chain-ends allowed the determination of radiation yield of products and of chain scission. 

This reaction has been put forward to explain the observed fact that the number of chain scissions corresponds to the number of carboxyl groups formed in the oxidation of polyethylene. Activation energy of both processes is 140 kj/mol. The mechanism of such an elementary fragmentation reaction remains however uncertain. The reactions of a chain scission are likely to precede the isomerization of original secondary alkyl peroxy radicals. 

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