Chain scission of poly

Washino K, Denk O, Schnabel W (1983) OH-radical-induced main-chain scission of poly(ribonucleic acids) under anoxic conditions. Z Naturforsch 38c 100-106 Washino K, Katsikas L, Schnabel W (1984) Radiation-induced strand breakage in poly(riboadenylic acid). A pulse radiolysis study on the protective action of cysteamine under anoxic conditions. IntJRadiat Biol 46 747-758... 

Lemaire DGE, Bothe E, Schulte-Frohlinde D. (1984) Yields of radiation-induced main chain scission of poly U in aqueous solution Strand break formation via base radicals. Int J Radiat Biol A5 351-358. 

Nederberg, F. Connor, E. F Glausser, T. Hedrick, J. L. Organocatalytic chain scission of poly(lactides) A general route to controlled molecular weight, functionahty and maeromolecular architecture. Chem. Commun. 2001, 2066-2067. 

FIGURE 2.21 The rate of chain scissioning of poly(vinyl acetate) (Mowilith 70) (Hoechst) under the influence of ultraviolet light (253 nm) at different temperatures. This Arrhenius plot has been drawn conventionally with temperature increasing to the left. The rate of reaction increases markedly around the Tg (30°C) on increasing the temperature. Source Geuskens etaL (1972). 

Poly(methyl methacrylate) has no tendency to form crosslinks when irradiated as a film, or in solution. The quantum yield for random chain scission of poly(methyl methacrylate) in benzene solution is independent of... 

The quantum yield for the chain scission of poly(methylphenylsilane) in solution is 0 = 0.97, whereas in a thin film, 0 = 0.17 [2135]. The lower quantum yield of chain scission in a film, in comparison to that in solution, can be explained by the cage effect (in the solid), which hinders free motion and favours recombination of reactive sites so formed. 

These results prompted us to survey a new depolymerization strategy based on a single transesterification reaction that should occur between a primary alcohol and poly(lactide) in the presence of DMAP. The primary alcohol should selectively cleave a PLA chain and produce an a-chain-end bearing the ester of the alcohol and a o -chain-end having a secondary hydroxyl, which is dormant to subsequent reactions (Scheme 3). The feasibility of the organocatalytic chain scission of poly(lactide) was demonstrated with a commercially available high molecular weight poly(L-lactides) (Mn 50,000 g/mol, Mw/Mn = 1.60. 

In a later paper, the same authors [10] reported that naphthalene inhibited chain scission by a triplet energy transfer mechanism. Golemba and Guillet [11] determined the 0 of chain scission of poly(phenyl vinyl ketone) to be 0.25 of 313 nm. They determined that the excited state lifetime of the carbonyl group on the polymer was of the same order of magnitude as that of the analogous model compound. 

FIGURE 22 Semilog plot of the in vitro rate of hydrolytic chain scission of PCL, poly glycolic acid-co-lactic acid, and a 1 1 blend of the two polymers, demonstrating the use of blends to modify degradation rates. (From Refs. 64 and 65.)... 

Blending of PCL and poly(glycolic acid-co-lactic acid) has been also used to control the rate of chain scission of the composite. 

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

Figure 1. Adiabatic potential curves in the main chain scission of a model compound of poly(isobutylene) 2,2-, 4,4-tetramethylpentane (4). AE3l(=0.61eV), aET,(—0.35eV), and AEf (=2.05eV) are the activation energies of the main chain scission in the lowest singlet excited state (S,), the lowest triplet state (T,), and the ground state, respectively.

However, there is another parameter which seems to prepare the ground for resonance effects to occur. The bulky groups linked to the terminal Qi atom of the free radical (XVI) formed by scission of a carbon-carbon bond in the polymer chain badcbone of poly(a-methylstyrene), can have more room and hence, can lessen steric interaction among themselves as well as with the other groups and atoms in the polymer chain when... 

Hexahydropyrene sensitized chain scission of polypropylene and polyisobutylene during light irradiation Q20).Polycyclio hydrocarbons have a important role in sensitized photooxidation of polyisoprene (122).polys tyrene (123) poly(methyl methacrylate) (123-126). It is quite probable that these reactions can also occur with participation of singlet oxygen. 

The main underlying chemistry of biodegradation for polyesters is the hydrolysis reaction. In the reaction, water molecules react with ester bonds of the polymer backbone or side groups to cleavage the polymer into smaller chains. An example of the chain scission in poly lactic acid (PL A) is shown in Fig. 15.6. 

Superheatability of poly(ethylene terephthalate) was produced, or in some cases restrained in the sample annealed at about 250 °C, which was not observable for the as-polymerized sample This was not an effect of crystal size, but must be caused by tie molecules between different crystals or different locations on the same lamellae. Superheating effects due to reduction in entropy of melting are calculated theoretically in a situation where each chain molecule can simultaneously belong to many different crystalline and noncrystalline regions. In the case of natural rubber, the chain scission of molecules with stress, i.e., a large deformation with an extention ratio of more than 2, occurs easily by irradiation, since the molecules are entropy restricted... 

Pyrolysis of poly(methyl methacrylate) at low temperature produces monomer, whereas other acrylics fragment with loss of side chains, scission of the chain backbone, elimination or rearrangement of the products. Knowledge of the degradation pathways for particular polymer sequences is required to interpret the fragmentation patterns obtained from pyrolysis (65-70). 

UV and y-radiation of poly(methyl isopropenyl ketone) produced random chain scission at 23 °C. The presence of air increases unexpectedly the main chain scission of the polymer under y-radiation [377]. In a series of publications [378] the radiolysis and photolysis of poly(phenyl vinyl ketone), poly(vinyl benzophenone), and poly(/-butyl vinyl ketone) [357] were described. The authors stated that photodegradation of poly (phenyl vinyl ketone) occurred by the abstraction of a hydrogen in the y-position to a carbonyl group, followed by chain scission by a Norrish type II photoelimination mechanism. 

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