To polymer chain scission

Once the transesterification step is complete, the metal salt is always deactivated by complexation with a phosphate or phosphite compound [18], preventing unwanted side reactions as the reaction temperature is increased during polycondensation. Such side reactions lead to polymer chain scission and loss of molecular weight, as well as development of unwanted discoloration of the polymer. Care has to be taken that excess phosphorous compounds are not added to the reaction, as these compounds can significantly reduce the effectiveness of the polycondensation catalyst. 

It should be noted that reaction 6 does not lead to polymer chain scission ie, there is no molar mass reduction here, in contrast to reaction 5 where there is a molar mass reduction. It should also be noted that the formation and reaction of hydroperoxide groups attached to polymer molecules (reactions 3 and 4) are much slower, ie, rate determining, than the other processes shown. It will be apparent from the free-radical products formed in reactions 1 through 4 that this whole peroxidation procedure is a branching chain reaction. Additive chemistry is required to provide polyolefins with any sort of prolonged service fife, and such chemistry is well known and entirely effective. 

The first developed a mathematical model to relate Young s modulus to polymer chain scission and is based on entropy theory for rubber elasticity. The second was an MD study of the effect of chain scission on Young s modulus in a semi-crystalline polymer. The third study developed a model to relate Young s modulus to polymer chain scission, and is based on atomic-scale simulations for glassy polymers. 

Whereas the cleavage of /S-poly(L-malate) at neutral pH is at random [2], alkaline hydrolysis reveals characteristic patterns of the cleavage products, which is due to nonrandom chain scission (Fig. 3). The phenomenon is explained by an autocatalytic ester hydrolysis. Assuming that one (or both) of the polymer ends bends... 

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 molecular weights of the polymers before and after irradiation were followed by GPC to determine changes in the molecular weights of the polyamides. It was found that photolysis of the polymer resulted in polymer chain scission, leading to the appearance of oligomers containing thymine bases at the end of the molecules. 

Polymethyl methacrylate (PMMA) degrades under irradiation and becomes more soluble due to main chain scission. The degradation can be greatly reduced by the addition of 10% of various additives, such as aniline, thiourea, or benzoquinone. PMMA is an example of a nongelling polymer it does not form a three-dimensional network structure under irradiation. ... 

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

Depending on the fate of the secondary polymer radical produced, this could lead to either chain scission or cross-linking in the solid phase. 

Measures of the sensitivity were made in two ways, (l) Loss of ketone carbonyl was determined by FTIR on the exposed samples by measuring the relative absorbance A at 1700 cm-1. The ratio (Aq/A))7oo, was adjusted for film thickness using the styrene bands at 1600, 1495, and 1455 cm-1. This value is proportional to the rates of the Norrish type I and photoreduction processes in the copolymer (2). Changes in molecular weight result from scission in the backbone of the polymer chain. A measure, Z, of the sensitivity to main-chain scission can be derived as follows. 

Rabek and Ranby (22) have shown that a free radical induced degradation of polystyrene occurs in the presence of oxygen, that leads to rapid chain scission. Benzoyl radicals derived from Type 1 initiators abstract hydrogen atoms from the polymer and thereby start a chain degradation process. Berner, Kirchmayr and Rist (6) and others have shown that when initiator I is irradiated in solution, the benzoyl radical abstracts a hydrogen from the surrounding solvent to form benzalkdehyde and a free radical. 

Pyrolysis of poly(methylacetylene) shows rather similar behaviour 528>, with mesi-tylene as the major product but substantial yields of methyl and proton-enriched products. Thermal decomposition of this polymer sets in at around 150 °C and the mechanism is postulated to involve chain scission followed by cyclization reactions and both electron-proton and electron-methyl exchanges. Pyrolysis of poly(phenyl-acetylene) has been reported to start at 270 °C in nitrogen 529). 

Ultraviolet Absorbers. These additives are included whenever prolonged outdoor exposure is a prerequisite to performance. They absorb light in the ultraviolet region and prevent polymer chain scission at points of unsaturation created by dehydrochlorination. They are not known to produce adverse effects on the vinyl compound at use concentrations. 

Thus, the strength decreases at rate of about 1.15 gf per micromole of -NH2 per gram of silk. Because of the complex relationship between polymer chain scission, molecular weight, and strength, and the presence of side-chain amino groups in silk, it is difficult to relate these parameters theoretically. 

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