Hydroperoxide groups polypropylene

On the basis of the above findings, grafting of vinyl monomers onto irradiated polypropylene has been attempted successfully by the mutual method. Upon irradiation hydroperoxide groups are introduced, which provide sites for grafting. During mutual irradiation in the presence of the monomer in aqueous medium, these hydroperoxide groups and water undergo decomposi-... 

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... 

In a similar manner isotactic polypropylene can be treated with oxygen under pressure at 70 to 80°C to yield hydroperoxide groups... 

Polymeric hydroperoxides are the major product of low temperature oxidation [Refs. 46, 113,115, 148, 189, 191, 193,195,196, 300]. Yields of hydroperoxides in oxidized polypropylene exceed 40%, and more than half that amount is contained in low molecular weight degradation products. It was shown that isolated hydroperoxide groups in thermally oxidized polypropylene constitute less than 10% of the total hydroperoxide concentration, and the majority of the polypropylene hydroperoxide groups occur in dimers, trimers, or longer sequences [148]. 

The PPO2 radical reacts inter- or intramolecularly with polypropylene to form PP (Equation 3.5), which will react with oxygen as shown in the reaction represented by Equation 3.4, so that the oxidation reaction will repeat itself. The hydroperoxide group formed in the reaction shown by Equation 3.5 reacts with UV light or heat to yield a tertiary alkoxy radical and a hydroxy radical (Equation 3.6). The tertiary alkoxide may react as shown in the reactions represented by Equation 3.7a or Equation 3.7b and the hydroxy radical can also react with polypropylene as shown in the reaction represented by Equation 3.8, in order that, in each case, the product is another polypropylene radical that can react with oxygen as presented in Equation 3.4. Equation 3.7b leads to chain scission and reduction in the molecular weight of the polymer... 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

However, the low-temperature oxidation of solid polypropylene (70-110°C) proceeds with alternating intramolecular and intermolecular chain transfer. Intramolecular kinetic of extension chains is limited to small parts of the macromolecule with a favorable set of conformations. As a result, blocks of hydroperoxide can be short. In the solid polypropylene has found about 60% of paired units and about 20% of triads, the share of units with a higher number of hydroperoxide groups is small. It should be noted that in other carbon-chain polymers increases the probability of intramolecular reaction at the high rate of conformational motions. For example, in the polymers with a saturated C-C bond (such as... 

Clementini and Spagnoli (7) patented a method in which polypropylene was first functionalized with a hydroperoxide group before extrusion with acrylic acid. Other reactive sites could also be incorporated into polypropylene during its production. Chung (8) has several patents that discuss the incorporation of borane or joflra-methylstyrene into polypropylene. These functional sites can then be reacted with unsaturated monomers. Both methods supposedly limit both the amount of homopolymerization and the level of molecular weight reduction that occurs compared with other grafting methods. 

Peroxyl radical reaction with the polymer. This is a generally much slower process, which is structure dependent. In saturated hydrocarbon polymers, e g. polyethylene (PE) and polypropylene (PP), it is exclusively a hydrogen atom abstraction. In this case. Per is a hydroperoxide group (POOH). The corresponding rate constant is very low = 10 -10 l.mor. s at ambient temperature (see Table 12.7). In polyenic elastomers, e g. polybutadiene (PBD) and polyisoprene (PIP), step 3 can also be an addition to double bonds. In this case. Per is a peroxide bridge (POOP). The corresponding rate constant is also very low typically = 10 -10 l.mor. s at ambient temperature for an intramolecular addition (see Table 12.7). 

The photolysis of hydroperoxide groups under solar irradiation is a slow process. The average lifetime of an —OOH group in 10/im polypropylene film under constant UV irradiation is 24h, equivalent to roughly 4-5 days of solar radiation [374, 387]. 

Hydroperoxide groups formed at high processing temperatures in polyethylene (160 °C) [125, 848, 1918, 2279] and polypropylene (135 °C) [443, 1879,1975,2339] are associated (adjacent) with each other, and are rapidly thermally- and/or photo-decomposed into free radicals which initiate the degradation process. 

F. Gugumus [21] provides an alternative view of the thermal oxidation reactions in polymers. Various possibilities arising from inter- and intramolecular reactions between hydroperoxide groups, peroxy radicals, and alkoxy radicals are postulated. The author underlines the plausible over-estimation of degradation attributed to -scissions in polypropylene (PP) and offers alternative (non (3-scission) routes that result in formation of 1,2-dioxetane which can account for auto-oxidation, chain scissions and enhanced chemiluminescence of PP oxidation products. An illustration of this proposed scheme is provided in Scheme 6.4. 

In the case of the polyolefins, random chain scission is initially the dominant process. This is shown typically for polypropylene in Scheme 2. However some low molar mass oxidation products are formed via vicinal hydroperoxides in both PP and PE [20]. The alkoxyl radicals formed by decomposition of the hydroperoxides contain weak carbon-carbon bonds in the a positions to the hydroperoxide groups, which lead to the formation of low molecular weight aldehydes and alcohols that rapidly oxidise further to carboxylic acids. These are biodegradable species, similar to products formed by hydrolysis of aliphatic polyesters and, as in the case of cis-PI, they are rapidly bioassimilated to give cell biomass (see below). 

Another phenomenon specific for polymers is the cage effect in slow bimolecu-lar reactions. It is well known that the viscosity of liquids does not influence on the rate of slow bimolecular reaction, which occurs with an activation ener and is not controlled by the rate of diffusion of reactants. However, slow reactions in the polymer matrix occur more slowly than in the liquid under the same conditions. It was proved by comparison of the experimetal rate constants of the reaction of 2,4,6-tri-tert.butylphenoxyl radical with hydroperoxide groups of polypropylene (PP) and polyethylene (PE). 

A similar regularity is observed for the reaction of R02 with 2,6-di-/crt-butylphenols with different substituents in die para-position of PPI (at 353 K in the polymer k of this reaction changes from 0.6 to 8-10 l/(mol s), whereas in ethylbenzene it changes from 2.8 to 200-10 l/(mol s)) and for the reactions of several para-substituted 2,6-di-/e -butylphenoxyl radicals with hydroperoxide groups of polypropylene. 

As pointed out above, Jellinek et al studied the Cu and Cu-oxlde catalyzed oxidation of isotactic polypropylene. One part of these studies was concerned with the formation and decomposition of hydroperoxide groups in absence and presence of catalyst. These reactions were followed by specular reflectance I.R. spectroscopy. 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

The photooxidation rates of nylon polymers have been monitored using Fourier Transform Infra-red spectroscopy. Norrish type-II reactions of carbonyl groups were considered to be the most important process. Another group of workers have studied the photooxidations rates of a nylon 6- polypropylene glycol copolymerIn this case polyether sequences are the major source of free radical attack resulting in high levels of hydroperoxides. Irradiation of nylon and polyester fibres with an excimer laser resulted in... 

Hydrogen abstraction is known to occur from secondary carbon atoms in polyethylene (12) and may also occur in polypropylene, but with lower reaction rates. For polypropylene it was shown that intramolecular hydrogen abstraction in a six-ring favourable stereochemical arrangement will preferentially lead to the formation of sequences of hydroperoxides in close proximity (Scheirs et al, 1995b, Chien et al, 1968, Mayo, 1978). Infrared studies of polypropylene hydroperoxides showed that more than 90% of these groups were intramolecularly... 

Thus the formation of free radicals in the photolysis of polypropylene at —196° C can be reasonably explained by the mechanism which involves photolysis of oxidative groups such as hydroperoxides and carbonyl groups, although Kujirai et al. (50) proposed that ash residues may be responsible for light absorption. 

Finally, it can be concluded that the chemical effects of ultraviolet irradiation of polypropylene, as well as polyethylene, are due to the photolysis of impurities such as hydroperoxides and ketonic groups. 

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