Polypropylene hydroperoxides

Commereuc and co-workers [44] examined the products resulting from the photo and thermal decomposition under vacuum of a pre-oxidised isotactic polypropylenes containing a known content of hydroperoxide. In contrast to the case of polyethylene (PE), few products were retained in the polymer matrix. Detailed analysis of the gas phase was performed by GC, Fourier-transform infrared (FT-IR) spectroscopy and MS. About 70% of the hydroperoxides were converted into gaseous products such as acetone, acetic acid and methanol. Mechanisms for their formation were suggested, and the consequences of such a phenomenon for the evaluation of ageing in polypropylene (PP) were discussed. 

The results in Table 6.1 show hydroperoxide levels before and after photolysis. About 70% of the PP hydroperoxides are converted into volatile products such as acetone, acetic acid and methanol. The residual 30% can be attributed to other volatile compounds (CO, CO, and so on). 

Irradiation conditions 15 honrs at 35 °C Determined by iodometry iPP isotactic polypropylene  

Reproduced with permission from S. Commereuc, D. Vaillant, J.L. Phillippart, J. Lacoste, J. Lemaire and D.J. Carlsson, Polymer Degradation and Stability,  

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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]. 

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... 

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]. 

Secondary HAS already decompose polypropylene hydroperoxide in the solid phase at ambient temperatures [200]. The process appears to be sufficiently rapid for the destruction of ROOH during the dark period, after the photo-oxidation phase. This enhances the photostability of PP in the repeated photo-oxidation. 

The compounds LXXVI to LXXX are the more true models of oxidized polyolefin than tert-butyl hydroperoxide. However, it followed from the results obtained by means of various hydroperoxides, which simulated the oxidized chains of polyolefins, that the simplest and most readily available model of polypropylene hydroperoxide — tert-butylhydroperoxide — can be used for the simulation of mechanism and transformations of phenolic antioxidants in spite of the differences in the decomposition kinetics of the polymeric hydroperoxide and its low-molecular-weight model1 ul... 

Important information about the transformation of thiobisphenols under the conditions of inhibited oxidation were obtained on the basis of product analysis of model reactions. The detailed investigation was carried out with 4,4 -thiobis(2-methyl-6-tert-butylphenol) (CLXVIIIa), which is among the most effective thiobis-phenolic antioxidants and tert-BuOOH was chosen as a simple model of polypropylene hydroperoxide. The reaction was followed at 20 °C in benzene solution and in the presence of the catalytic amount of Co(II)acetylacetonate272). Under these conditions, the phenolic antioxidant was attacked by both alkylperoxyls and hydro-... 

Preparation of Block Copolymers from Macroinitiators. In an early paper, Ceresa reported polymerization of methyl methacrylate in the presence of oxygen, and then swelling the polymer with styrene. On heating, the polymeric peroxide decomposed initiating styrene polymerization thus forming a block copolymer(89). Even earlier, macroperoxides, useful in the preparation of block copolymers, were prepared by initiating the polymerization of butadiene with m-diisopropyl benzene dihydroperoxide in an emulsion. On further heating in the presence of styrene and Fell salts, a block copolymer was formed l Recently in independent work, polypropylene hydroperoxide was prepared by ozonolysis, and with triethylenetetra-amine as an activator was used to initiate block addition of styrene(91) and other vinyl monomers(92,93). ... 

The activation energy of the decomposition of polypropylene hydroperoxide proved to be equal to 25 kcal/mole, while in the presence of the indicated antioxidants it was 13 kcal/mole. 

K. Ingold has shown that a number of antioxidants of the class of substituted phenols and aromatic amines also accelerate the decomposition of the hydroperoxides formed in the oxidation of white oil [56]. The same result was obtained by V. V. Dudorov, A. L. Samvelyan, A. F. Lukovnikov, and P. I. Levin in an investigation of the decomposition of polypropylene hydroperoxide in the presence of antioxidants. 

Dialkyl sulfides accelerate the decomposition of polypropylene hydroperoxide according to an entirely different mechanism. Data on the influence of an admixture of dihexadecyl sulfide on the decomposition of polypropylene hydroperoxide at 110°C are cited in [55]. As can be seen from Fig. 11, the concentration of the hydroperoxide in the absence of the sulfide drops extremely slowly, while in the presence of the sulfide it drops considerably more rapidly, especially at the beginning of the reaction. Correspondingly, the rate of formation of water during the decomposition of the hydroperoxide in the presence of the sulfide is characterized by a sharply pronounced maximum. 

Fig. 12. Variation of the rate w of formation of volatile products in the decomposition of polypropylene hydroperoxide at 130 C. O) Rate of formation of acetone ) water x) acetal -dehyde ) formaldehyde.

Thermal Decomposition of Polypropylene Hydroperoxides, Journal of Polymer Science Al, 6 (1968), p. 393... 

Chien JCW, Vanderberg EJ, Jabloner H. Polymer reactions. Part HI structure of polypropylene hydroperoxide. J Polym Sci Part A-1 1968 6 381-92. 

Chien JCW, Jabloner HJ. Polymer reactions. Part IV thermal decomposition of polypropylene hydroperoxides. J Polym Sci Polym Chem Ed 1968 6 393 02. 

The cage reaction explains the absence of a photoinduction period during the decomposition of secondary hydroperoxides in polyethylene [145, 778] The inductive role of polypropylene hydroperoxides is attributed to the fact that the cage reaction does not occur, and the hydroxyl (HO ) radical diffuses rapidly from the place where it was formed ... 

During each photodegradation (in vacuum or air) small amounts of several gaseous and liquid low molecular weight compounds are formed and they can only be detected by chromatographic or mass spectrometry methods (cf. section 10.6). Table 2.1 shows, for example, products formed from polypropylene hydroperoxide photolysis. 

Table 2.1 Photolysis products from polypropylene hydroperoxides [382]...

Photolysis (and/or thermolysis) of polypropylene hydroperoxides (3.10) yields polypropylene oxy radicals (PO ), which undergo ) -scission reaction [61, 365, 372, 723, 1858] ... 

The major volatile product in the photolysis of polypropylene hydroperoxide is water, which is formed as a result of hydrogen abstraction by hydroxyl (HO ) radicals ... 

During irradiation of isopropanol in the presence of Ti02, acetone is formed by a reaction analogous to that above [1045], which has been proposed to predict the behaviour of Ti02 in polymeric media. Ti02 photocatalysed the decomposition of tert-butyl hydroperoxides and atactic polypropylene hydroperoxides [1272]. 

The atactic polypropylene hydroperoxide (initial concentration 0.36 moles/kg) undergoes decompcKition its kinetics, both under vacuum and in the presence of oxygen, is described to a good approximation by a first-order equation. In the range between 25° and 50° C temperature does not influence the decomposition rate. Oxygen pressure, po, results in a decrease of specific rate values, K f, ... 

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