Degradation mechanism scheme

On the basis of these considerations we can confidently assume a relationship between titanium residual and PBT degradation. The different effect of Ti(0- Bu)4 and the chelated Ti(acac)2(0-/Pr)2 on PBT stability could be explained with a different capability of the two metal residuals to coordinate the OOH groups and/or a different ability to catalyse hydroperoxides decomposition. Therefore Ti(0- Bu)4 and the chelated Ti(acac)2(0-/Pr)2 should affect in a different way the key step 4 of the degradation mechanism (scheme 6) and consequently the rates of the entire degradation process are very different in the two cases. 

Researches on thermal degradation of purified and well-defined (R)-hydroxyl-to-isopropyl ester PCL have proposed a two-stage degradation mechanism - Scheme 18( 1) displays the cis-elimination mechanism and Scheme 18(2) shows the nnzipping process 852663 a.l53. ... 

Studying modem approaches for such schemes, one can see that knowledge of operational conditions and potential degradation mechanisms play a prominent role. Surprisingly, the role of NDT is often limited to tlie use of conventional methods such as ultrasonic wall thickness measurements, ultrasonic inspection, radiography, and last but not least visual inspection. 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

There are two basic methods for making polymer materials photo-chemically degradable.1,2 One method is to chemically incorporate a chromo-phore into the polymer chains. One commercially successful chromophore is the carbonyl group.1,2,7 Absorption of UV radiation leads to degradation by the Norrish type I and II processes or by an atom abstraction process (Scheme 1). Note that once radicals are introduced into the system, chain degradation occurs by the autoxidation mechanism (Scheme 2). 

Monoamine oxidase, which exists in two distinct forms, referred to as MAO A and MAO B, is one of the enzymes responsible for the degradation of biologically important amines. Compounds that block the catalytic action of MAO A, which is selective for the degradation of norepinephrine and serotinin, have antidepressant effects whereas compounds that inhibit MAO B, which degrades dopamine in the brain, are useful for treating Parkinson s disease [190, 191]. Both MAO A and MAO B contain flavin co-enzyme attached at the 8-a-position to an enzyme-active cysteine residue (54). A one-electron transfer mechanism (Scheme 15) for the oxidations catalyzed by MAO was first proposed by Silverman [192] and Krantz [193,194]. 

All of these systems operate to increase the rate of the Ifee-radical generation reaction or introduce new mechanisms for radical formation not seen in the original degradation-reaction scheme for that polymer (e.g. they enhance scission in an otherwise crosslinking polymer). 

Scheme 7. Oxidative degradation mechanism of phenolic resins...

Scheme 5 Possible chain end degradation mechanisms during the NMRP of vDMTPD. Termination can occur during the polymerization of vDMTPD or when using PvDMTPD as a macroinitiator for the polymerization of the electron conducting monomer PerAcr...

SCHEME 16.7 Photocatalytic degradation mechanism of PE-(Ti02/CuPc) films [69]. 

The stability to oxidative degradation of palladium/NHC complexes has made possible the synthesis of complexes previously unknown or much less explored. (NHC)Pd(K-OAc)(OAc) shows a unique coordination of the acetate units that stabilizes a distorted square planar geometry around the metal center. These complexes are stable in acidic media and can activate C-H bonds of simple arenes by an electrophilic metalation mechanism (Scheme 32). 

Further support for the thermal degradation mechanisms suggested for Ny66 (Schemes 5.5 and 5.6) comes from the structure of thermal degradation... 

Acrylate and methacrylate co-monomers participate in the nitrile reaction [182] but styrene type monomers act as a blocking agent. Chlorinated co-monomers degrade the AN units at lower temperatures [184] by a dehydrochlorination mechanism (Scheme XXV) ... 

Another example of model formulation that simultaneously takes into account reaction and diffusion is given by Thombre and Himmelstein (1985) and Joshi and Himmelstein (1991), who characterized one-dimensional slab and disks of surfaceeroding poly(ortho esters), respectively. They considered a kinetic scheme (the poly (ortho esters) degradation mechanism is slightly different from that of aliphatic... 

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