Polyethylene macroradical

At the first stage of polyethylene thermal destruction the metallizing of polyethylene macroradical by the metal radical takes place. 

In the process of inhibition polypyrocatechin borate interacts with polyethylene macroradicals to form the B—O—C bonds. This is confirmed by the fact that the absorption spectrum of polyethylene inhibited with polypyrocatechin borate revealed the bands in the region of 1350 cm" characteristic for the B—O—C bond. There is no such a band in the spectrum of pure polypyrocatechin borate after heating under the same conditions. Chemical analysis of boron in polyethylene provides support for the IR-spectroscopy data concerning the presence of chemically bonded boron in polyethylene after destruction. 

Combination vs Disproportionation. There are two modes of termination one is the direct coupling (combination) of two free macroradicals to give a dead polymer chain of chain length i +j, with the rate coefficient At,c- The other mode is the so-called disproportionation, where a hydrogen atom is transferred from one of the radical chain ends to another radical, yielding two stabilized polymer chains, of which one carries a double bond. This reaction is associated with the rate coefficient t,d. The process is illustrated in equation 44 using polyethylene macroradicals as the example. It is important to notice that—in the case of macroradicals derived from other monomers—in principle any -hydrogen may be abstracted. 

At 300°C and in the presence of KOH an increase in the molecular weight is observed, i.e., the reaction of macropolymerization is realized [38,39]. Potassium hydroxide is effectively inhibiting thermal destruction of polyethylene at temperatures from 350-375°C. The per cent change in molecular weight is half or one-third as high as that without the use of an inhibitor. At 400°C the efficiency of inhibition is insignificant. Potassium hydroxide with an ABC carrier is effective up to the temperature of 440°C due to the increased contact surface of the inhibitor with macroradicals. 

The inhibitive efficiency of alkali metal hydroxides increases with increased branching of polyethylene. This is confirmed by more pronounced effect of these hydroxides diminishing the yield of propane and propylene than in case of ethane and ethylene. The decreased yield of propane and propylene is also conditioned by more efficient inhibition of the macroradical isomerization stage by alkali metal hydroxides. Upon thermal destruction of polyethylene with the use of inhibitors the... 

High thermostabilizing efficiency of polyamine disulphides relative to chemically cross-linked polyethylene is conditioned by the ability to accept macroradicals at the disulphide bridge and imine group. Besides, the presence of paramagnetic centers causes the adherence of macroradicals providing for an extra stabilizing effect [49]. 

Grafting reactions of polybutadiene with macrazo-inimers or polyazoesters produced polyethylene gly-col-polybutadiene crossHnked graft copolymers. Macroradicals thermally formed from macroazoinimers or polyazoesters attack 1,2-linked vinyl pendant groups of polybutadiene ... 

In this case, there are too few macroradicals available for reaction because of insufficient polymer degradation. In the disk-type extruder, a higher-stress gradient is achievable, more macroradicals are generated, and intensive cross-linking between polyethylene or highly chlorinated polyethylene and maleic anhydride or methyl methacrylate can be obtained (Heinicke 1984, Zhao et al. 2002, 2003). 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

The mechanism of crosslinking in this system is well established. The UV quanta (340-360 nm) are absorbed by the BP molecules and excited to a singlet state (S) which is short-lived and rapidly reverts to the triplet state (T) by intersystem crossing. The T state is a rather long-lived biradical which abstracts hydrogen from surrounding molecules, e.g. polyethylene chains, which gives macroradicals on the chains (formula 1). 

Unsaturated bonds in polyethylene are not the product of the photofragmentation of macroradicals alone but also of their disproportionation. The disappearance of C=C bonds corresponds to addition reactions of radicals with multiple carbon-carbon bonds. Addition reactions increase the quantum yield of crosslinks when related to that of formation of free radicals. 

If the radicals of this radi< pair move away, the comlnnation of macroradicals may occur and crosslinks in polyethylene may be formed. Self-reaction within a parent radical pair remains, however, still more probable and reduces the yield of crosslinks. 

These radicals can abstract hydrogen atoms from the polyethylene chains and give rise to new polymer alkyl radicals. This contributes to the increase of crosslinking efficiency. TTie train of subsequent reactions of radicals leads, moreover, to the appearance of a new precureor of free radicals, hydrogen peroxide. On the other hand, macroradicals of polyethylene may react with oxygen directly to form peroxy radicals which will reduce the yield of carbon-carbon crosslinks. 

A number of crosslinks formed in a macromolecular system related to the decomposed peroxide depends not only on the structure of the polymer chain but also on the source of primary radicals. For crosslinking of a polymer, it is necessary that the transfer reaction of oxy radicals predominates over the competitive fragmentation. If the two macroradicals are formed at close proximity, they may give rise to a one crosslink in the mutual recombination with hi probability. The less favourable situation arises when oxyradicals undergo parallel fragmentation. Since carbon centered radicals are less reactive in the transfer reaction with polyethylene than oxy radicals, the probability of crosslinking is reduced. 

The behavior depicted in Fig. 7.2 is observed with many polymers upon exposure to sunlight, including with commercial polyalkenes such as polyethylene and polypropylene. In the latter cases, impurity chromophores act as initiators of the autoxidation process (see Scheme 7.4 in Section 7.1.3). Important elementary reactions determining the autoxidation process are described in the following. Free radicals Rx formed during the initiation phase abstract hydrogen atoms from macromolecules PH, thus forming macroradicals P [Eq. (7-11)]. 

In the context of multifunctionality, carbon black, a polycrystalline material, merits special mention. The surface layer of carbon black particles may contain quinones, phenols, carboxy phenols, lactones, etc. Therefore, apart from being a powerful UV absorber and a quencher of excited states (such as those of carbonyl groups), carbon black acts as a scavenger of free radicals in chain-breaking reactions and as a hydroperoxide decomposer [114, 115]. In polyethylene, carbon black forms a complex with macroradicals [115]. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

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