Nitroxyl reaction with polymers

The possibility of stabilizing polymers with hindered nitroxyl radicals is based on their reaction with polymer radicals which are engaged in propagating polymer oxidation. It was established that hindered amines, the precursors of nitroxyl radicals, are also effective photostabilizers and, as such, they are more effective than most of the common photostabilizers for polymers. 

Scheme 9.18 Reaction of nitroxyl radicals with polymers. 

Antioxidants that break chains by reactions with alkyl radicals. These are compounds, such as quinones, nitrones, iminoquinones, methylenequinones, stable nitroxyl radicals, and nitrocompounds that readily accept alkyl radicals. Such antioxidants are efficient at very low concentrations of dioxygen and in solid polymers. 

Hence, the phenomena of the low reaction rate in the polymer matrix cannot be explained by the limiting rate of reactant orientation (rotational diffusion) in the cage. This result becomes the impetus to formulate the conception of the rigid cage of polymer matrix [16-20]. In addition to the experiments with comparison of the rate constants in the liquid phase and polymer matrix, experiments on the kinetic study of radical reactions in polymers with different amounts of introduced plasticizer were carried out [7,9,15,21], A correlation between the rate constant of the reaction k and the frequency of rotation vOT of the nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-/Y-oxyI) was found. The values of the rate constants for the reaction... 

The Values of Er and Eor for Bimolecular Reactions of Nitroxyl Radicals with Phenols Calculated According to the Rigid Cage Model for Reaction in a Polymer Matrix (Equation (19.7)) [7,9,14,15,21]... 

FIGURE 19.2 The correlation of rate constants of various free radical reactions with molecular mobility of nitroxyl radical in the polymer matrix of different polymers with addition of plastificator I in IPP, II in preliminary oxidized IPP, III in PE, and IV in PS. Line 1 for the reaction of 2,6-bis(l,l-dimethy-lethyl)phenoxyl radical with hydroperoxide groups at T — 295 K line 2 for the reaction of 2,2,6, 6-tetramethyl-4-bcnzoyloxypiperidinc-/V-oxyl with 1-naphthol at T = 333 K line 3 for the reaction of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-iV-oxyl with 2,6-bis(l,l-dimethylethyl)phenol at T = 333 K line 4 for the same reaction at 7 — 303 K line 5 for the same reaction at T = 313 K and line 6 for the same reaction at T — 323 K [18]. 

Dependence of Rate Constant on the Volume of Reactants for Bimolecular Reactions of Nitroxyl Radicals with Phenols in Polymer Matrix [8,10,11]—continued... 

Acceptors of alkyl radicals are known to be very weak inhibitors of liquid-phase hydrocarbon oxidation because they compete with dioxygen, which reacts very rapidly with alkyl radicals. The situation dramatically changes in polymers where an alkyl radical acceptor effectively terminates the chains [3,49], The study of the inhibiting action of p-benzoquinone [50], nitroxyl radicals [51-53], and nitro compounds [54] in oxidizing PP showed that these alkyl radical acceptors effectively retard the oxidation of the solid polymer at concentrations ( 10-3 mol L 1) at which they have no retarding effect on liquid hydrocarbon oxidation. It was proved from experiments on initiated PP oxidation at different p02 that these inhibitors terminate chains by the reaction with alkyl macroradicals. The general scheme of such inhibitors action on chain oxidation includes the following steps ... 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

Hindered amine which formula is presented below is introduced into polymer. As a result of amine reaction with peroxide radicals formed during polymer oxidation process (Scheme 5) stable nitroxyl radicals are formed. Nitroxyl radicals may be formed only at those parts of polymers where there are peroxide radicals, i.e. process of thermo-oxidative (or photo-oxidative) destruction proceeds. Then one determines spatial distribution of nitroxyl radicals through the sample and so, it becomes possible to identify those regions of polymer in which oxidation reaction proceeds. 

A mechanism for a reaction between HALS and hydrogen peroxide has been proposed by Scott and co-workers [8], These hydroperoxides are formed in the polymer and react with the HALS to form stable nitroxyl radicals that are believed to be responsible for the stabilization [3], Figure 4.5 shows this reaction with the nitroxyl radical, which is a very effective scavenger of alkyl or macroalkyl radical as well as the substituted hydroxylamine formed by this reaction. Substituted hydroxylamines can react with peroxy radicals to regenerate nitroxyl radicals [3],... 

Nitroxyl radicals may also react with polymer radical to form hydroxylamine (Reaction 1.98), and the latter can react with peroxy radicals and hydroperoxides according to Reaction 1.99 and Reaction 1.100 ... 

Nitroxyl radicals react with polymer alkyl radicals by disproportionation or termination mechanism, and subsequently, they are regenerated in reaction with peroxyl radicals (Scheme 12.13). 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Special experiments with preliminary withdrawal of all low-molecular products formed by the reaction of nitroxyl with alkyl macroradicals from the polymer sample were performed. These... 

Most of the functional groups obtained in this work have very high absorption coefficients comparatively with the parent functions and this technique can also be usefull to improve their detection in polymer matrix. From this point of view, reactions as saturation of double bonds or oxidation of NH group into nitroxyl radical can allow to follow these groups in situ along typical reactions as the ageing. 

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