Chain reaction degradation

Many polymers degrade by chain reactions some of which, as with polyformaldehyde and polymethyl methacrylate, lead to depolymerization to monomer. It has been suggested that the good heat resistance of the silicones is due, not to high bond strengths, but to the absence of a favourable reaction path by which the polymer system may degrade. Chain reactions may often be reduced in number by the use of one or more of the following approaches. 

Ultraviolet protective agents fall into two categories, those operating as UV screens, and those which operate by interfering with a degradation chain reaction in the polymer that has been initiated or catalysed by the radiation. 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

Chain transfer, the reaction of a propagating radical with a non-radical substrate to produce a dead polymer chain and a new radical capable of initiating a new polymer chain, is dealt with in Chapter 6. There are also situations intermediate between chain transfer and inhibition where the radical produced is less reactive than the propagating radical but still capable of reinitiating polymerization. In this case, polymerization is slowed and the process is termed retardation or degradative chain transfer. The process is mentioned in Section 5.3 and, when relevant, in Chapter 6. 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

However, Pacansky and his coworkers77 studied the degradation of poly(2-methyl-l-pentene sulfone) by electron beams and from infrared studies of the products suggest another mechanism. They claim that S02 was exclusively produced at low doses with no concomitant formation of the olefin. The residual polymer was considered to be essentially pure poly(2-methyl-l-pentene) and this polyolefin underwent depolymerization after further irradiation. However, the high yield of S02 requires the assumption of a chain reaction and it is difficult to think of a chain reaction which will form S02 and no olefin. 

There are several schemes for the synthesis of cellulose formates (slow) reaction of the polymer with formic acid faster reaction in the presence of a mineral acid catalyst, e.g., sulfuric or phosphoric acid. The latter route is usually associated with extensive degradation of the polymer chain. Reaction of SOCI2 with DMF produces the Vilsmeier-Haack adduct (HC(Cl) = N (CH3)2C1 ) [145]. In the presence of base, cellulose reacts with this adduct to form the unstable intermediate (Cell - O - CH = N" (CH3)2C1 ) from which cellulose formate is obtained by hydrolysis. The DS ranges from 1.2 to 2.5 and the order of reactivity is 5 > C2 > C3 [140-143,146]. 

Lipid peroxidation is a radical-mediated chain reaction resulting in the degradation of polyunsaturated fatty acids (PUFAs) that contain more than two covalent carbon-carbon double bonds (reviewed by Esterbauer et al., 1992). One of the major carriers of plasma lipids is LDL, a spherical molecule with a molecular weight of 2.5x10 . A single LDL particle contains 1300 PUFA molecules (2700 total fatty-acid molecules) and is... 

The extent of the photodegradation reaction is measured by the photodegradation quantum yield, pd, which is defined as the fraction of molecules degraded in relation to how many photons were absorbed, and quantifies the light sensitivity of the molecule (Turro 1978). Usually, pd < 1 but values higher than unity can indicate more complex processes, such as radical chain reactions. 

A carbonyl chromophore in a macromolecule can participate in a variety of photochemical processes that can have as end result the degradation of the polymer via processes like the Norrish Type I or Type II reaction, the triggering of a chain reaction leading to peroxidation, the transfer of energy to another chromophore or, it can also behave as an energy sink if a suitable, non-degradative path, is available to the triplet state. 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

The process of oxidation has three stages initiation, chain reaction and termination. Initiation is produced by external stimuli as described and activates the polymer to form a reactive radical. Initiation continues only for as long as the external stimulus persists. Removal of the energy required to form the initiator, for example the heat or light, will result in the rate of degradation decaying to zero. 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

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