Norrish acceleration

Studies of the copolymerization of VDC with methyl acrylate (MA) over a composition range of 0—16 wt % showed that near the intermediate composition (8 wt %), the polymerization rates nearly followed normal solution polymerization kinetics (49). However, at the two extremes (0 and 16 wt % MA), copolymerization showed significant auto acceleration. The observations are important because they show the significant complexities in these copolymerizations. The auto acceleration for the homopolymerization, ie, 0 wt % MA, is probably the result of a surface polymerization phenomenon. On the other hand, the auto acceleration for the 16 wt % MA copolymerization could be the result of Trommsdorff and Norrish-Smith effects. 

Priebe et al. [79] investigated the chemical stabiHty of iodixanol under accelerating cleavage of the central bridge under ultraviolet irradiation by a Norrish Type-II reaction. Basic conditions (pH 14) combined with heat (60 °C) initiated a cyclisation reaction. On the other hand, less than 1 % iodixanol decomposed in solution heated to 140 °C for 2 days or under both basic conditions (pH 11,20°C, 5 days) and acidic conditions (pH 0.4,80 °C, 5 days) or under an oxygen atmosphere (100°C,3 days). 

It was first noted by Spence (56), that mercury had an accelerating effect on the formaldehyde oxidation. Lewis and von Elbe (32) suggest that the absence of an induction period in the experiments of Axford and Norrish (/), compared to those of Snowdon and Style (55), was to be attributed to the destruction of peroxides by mercury vapor from heated mercury cutoffs in the Axford-Norrish experiments. This was confirmed in Schecr s work, where mercury vapor eliminated the induction period. A freshly cleaned surface had an effect similar to mercury vapor. It was found that although the induction period represents only a slow pressure rise, extensive reaction occurs, the A ft during this time being unrelated to the formaldehyde consumption. 

With growing viscosity, the diffusion rate of propagating macromolecules decreases, the probability of their effective collision is diminished, and thus also the rate of bimolecular termination. As the rate of initiation is not appreciably affected by increasing viscosity, in a certain phase some radical polymerizations pass from a stationary to a non-stationary state in which the number of radicals increases. The polymerization is appreciably accelerated. This situation is called the gel effect or the Norrish-Tromsdorff effect [55]. As the change in the rate of termination contributes considerably to the gel effect, it will be discussed in detail in Chap. 6, Sect. 1.3. 

Bengough and Norrish observed this behaviour during vinyl chloride polymerization. They explained it by transfer to polymer chains on which immobile, long-lived and propagating radicals are formed. These centres decay by transfer to monomer or by termination with untrapped radicals from the liquid phase [47], According to these two authors, the acceleration is proportional to the surface area of the solid particles. A similar acceleration of polymerization was observed by Bamford et al. [18] with acrylonitrile... 

The low temperature polymerisation of isobutene by SnCl4 in ethyl chloride is one of the classical studies of the golden era of cationic polymerisation. Norrish and Russell " found that with no added water an extremely slow reaction period was followed by a sudden acceleration. A similar phenomentm was later reported by Polton and Sigwalt for the polymerisation of indene in a dry system. It seems reasonable to suppose that the slow initial process reflects direct initiation in both systems, and that the sudden accelemtion arises from the internal production of a cocatalyst, probably hydrogen chloride formed from the dehydrochlorination of active species. 

The conclusion that Cl atoms do not react with O2 conflicts with other findings as previously discussed, including those reported subsequently by Burns and Norrish and Stedman (cited in ref. 27). Furthermore, Iredale and Edwards found that the presence of H2 accelerated the decomposition of CI2O, which they felt could be attributed only to the reaction of CIO with H2. They suggested two reactions... 

Carabine and Norrish also reported explosion limits for a 1 3 [B2H6]/[02] mixture. Furthermore, they found that a small amount of O3 accelerated the nonexplosive reaction above the second explosion limit without raising the limit. 

Carabine and Norrish found results somewhat different from those of the other studies. Furthermore, they also found that a small amount of O3 accelerated the non-explosive reaction, but did not raise the upper ignition limit. They studied the flash photolysis of B2H6-O2 mixtures with radiation below 2000 A. Intermediates BH, OH, BO, and BO2 were monitored by absorption spectroscopy. Water was not a final product of the reaction unless the B2H6 was completely consumed. Almost surely it was produced, but was removed in a rapid reaction with B2H6. 

In view of Eq. (6.26) for ideal polymerization kinetics one would normally expect the reaction rate to fall with time, since the monomer and initiator concentrations decrease with conversion. However, the exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typical example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C [49], At monomer concentrations less than about 40 wt% in this case, the rate (slope of conversion vs. time) is approximately as anticipated from the ideal kinetic scheme described in this chapter, that is, the rate decreases gradually as the reaction proceeds and the concentrations of monomer and initiator are depleted. An acceleration is observed, however, at higher monomer concentrations and the curve for the pure monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel used here is different than the usage in Chapter 5 as it refers only to the sharp increase in viscosity and not to the formation of a cross-linked polymer.) The autoaccelerative gel effect is also known as the Tromsdorff effect or Norrish-Smith effect after pioneering workers in this field. It should be noted that the gel effect is observed under isothermal conditions. It should thus not be confused with the acceleration that would be observed if a polymerization reaction were carried out under nonisotherraal conditions such that the reaction temperature increased with conversion due to exothermicity of the reaction. 

Norrish et al.18 discovered the acceleration effect in the polymerization of methyl methacrylate in 1939. In order to elucidate the cause of the effect, polymerization rates for methyl methacrylate were determined in various kinds of solvents. Schulz et al.19,20 reported that acceleration occurred at about 12% and 25% conversion in the bulk polymerization of methyl methacrylate at 50 and 70 °C, respectively, while there was no effect observed up to high conversion with styrene. Tromms-dorff et al.21 showed that this phenomenon is due to the increased viscosity of the polymerization system which is not caused by the interaction between a propagating radical and solvent. In order to investigate how the variation of the viscosity of... 

In many polymerizations, a marked increase in rate is observed toward the end of the reaction instead of the expected gradual decrease caused by the depletion of the monomer and initiator. This auto-acceleration is a direct result of the increased viscosity of the medium, and the effect is most dramatic when polymerizations are carried out in the bulk phase or in concentrated solutions. The phenomenon, sometimes known as the Trommsdorff-Norrish or gel effect, is caused by the loss of the steady state in the polymerization kinetics. 

Keywords photooxidation, UV absorbers (UVA), outdoor performance, weathering, accelerated weathering, kinetic chain length, photoinitiator, Norrish reactions, peroxide decomposer, hindered amine light stabilizers (HALS), photosensitizer, transition metal complex, UV stabilizer, time-controlled stabilization, reactive antioxidants, polymeric antioxidants. 

The stabilization of poly(vinyl chloride) against light has been reviewed by Wirth and Andreas. Detailed mechanistic studies have indicated the importance of peroxides in the process of photo-oxidation. It was suggested that protection could be successfully achieved by exclusion of radiation of A < 380 nm. E.s.r. examination of irradiated samples demonstrated the intervention of peroxides in the mechanisms with the ultimate formation of carbonyl groups which caused chain scission by Norrish cleavage. Photo-oxidation of samples of poly(vinyl chloride) modified by incorporation of acrylonitrile-butadiene-styrene, methyl methacrylate-butadiene-styrene, and methyl methacrylate-acrylonitrile-butadiene-styrene copolymers has been investigated. Discolouration was accelerated by the presence of the modifiers. Thermal pre-treatment accelerated photo-induced decomposition. Mechanical properties were also examined, and scanning electron microscopy showed surface defects due to decomposition of the modifier. ... 

The photolysis of ketones leads to reactions that we will call Norrish type 11 in Chapter 16. The reaction involves carbonyl excitation followed by hydrogen atom abstraction from a y-carbon. Imbedded carbon monoxide, in the form of an ester, placed between the ketone and the y-carbon is known to accelerate the decay of the resulting 1,4-biradical by allowing a fragmentation pathway. To ascertain the lifetime of the radical intermediates formed from the photolysis of the following a-keto ester, the incorporation of a radical clock was performed. Upon photoly-... 

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