Hydroperoxide groups processing

If the grafting is carried out in air, the active sites on the polymeric backbone is attacked by atmospheric oxygen leading to the formation of macroperoxy radical, which might abstract the hydrogen atom from the backbone polymer by an inter- or intramolecular process to give hydroperoxide groups as shown. 

Irradiation of the polymer in air prior to grafting introduces hydroperoxide groups in the backbone polymer by the intermolecular, intramolecular, or hydrogen abstraction process. 

Thermal aging is another simple pretreatment process that can effectively improve adhesion properties of polymers. Polyethylene becomes wettable and bondable by exposing to a blast of hot ( 500°C) air [47]. Melt-extruded polyethylene gets oxidized and as a result, carbonyl, carboxyl, and hydroperoxide groups are introduced onto the surface [48]. 

Special interest has been devoted to the reaction of oxygen on carbanionic sites.- It has been established that triplet oxygen acts as an electron acceptor, and that the reaction may yield either terminal hydroperoxy or alcohol functions. Depending on the reaction conditions, the latter group which originates from a disproportionation process between a hydro-peroxidate and a carbanlon may be observed. By choosing proper conditions one can get polymers terminated quantitatively with hydroperoxide groups. The latter can be used in a later step as a radical initiator to prepare new block copolymers. 

Table 7.14 presents the computational results for the heat of some reactions involving hydroperoxide groups, which can be considered as model processes occurring at the silica surface. As compared with the decay channel the rupture of the 0-0 bond in hydroperoxide and formation of free radicals, the activation energies for the reverse reactions of water and methanol abstraction with the recovery of DOSG are even lower. The preexponential factors for these processes are also different. For the 0-0 bond rupture, its typical value is 10(16/17)sec-1, whereas for the water abstraction via the cyclic TS it is equal to 1011 sec-1. 

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... 

Scheme 1.70. Pro-oxidant activity of alkyl mercaptans (a) and both pro-oxidant and delayed antioxidant activity of diphenyl disulfides (b) during melt processing of polymer PH containing hydroperoxide groups POOH. After Scott (1993b).

Another mechanism for the removal of surface films was discovered by Wheeler (1972), who found that fatty acid films on seawater collapsed to form particles when exposed to near-ultraviolet radiation. The results indicated that there was an introduction of hydroperoxide groups into the parent fatty acid molecule with resultant polymerization of the products. Instead of polymerization, Timmons (1962) found that the constituents of plankton oil films were converted to smaller and more soluble fragments when exposed to artificial sunlight. Photochemically initiated solubilization appears to be a process common to some constituents of crude oil and fuel oil films as well, with low molecular weight acids, sulfoxides, and peroxides comprising some of the product soluble fraction (Burwood and Speers, 1974 Hansen, 1975 Larson et al., 1977). The rate of photo-oxidation of films of various fractions of crude oil spread on water was greatly increased... 

UV wavelengths which damage certain polymers. The absorbed photons raise electrons to an excited state and cause bond dissociation reactions. An example is the photodecomposition of hydroperoxide groups introduced during melt processing. 

Polyolefin products undergo photodegradation by the absorption of UV light. This is due to the unstable product formation of polyolefins during fabrication process, due to the presence of impurities like carbonyl or hydroperoxide groups [30, 31]. 

An air-blown linseed oil-bearing hydroperoxide group is used as a macroinitiator in the nitroxide mediated radical polymerisation of styrene in the presence of 2,2, 6,6 -tetramethylpiperidinyl-l-oxy (TEMPO). The hydroperoxide groups formed on oil molecules serves as a macroinitiator in this polymerisation process. The amount of hydroperoxide groups and TEMPO affect the film properties. The use of an equal amount of TEMPO and free... 

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