Hydrogen abstraction polypropylene

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

The ESR spectra of polyisobutylene after irradiation with ultraviolet light (6) are different from those obtained after irradiation with ionizing radiation. The spectra consists mainly of two components one, a sharp quartet which has a half life of 1% hours at liquid nitrogen temperature, has been attributed to free methyl radicals (XI), in analogy with ultraviolet-irradiated polypropylene (51). The broad component is composed of many superimposed lines and was interpreted as caused by three different radicals, all stable at liquid nitrogen temperature. One of these radicals (XV) is the counterpart to the methyl radical (XI) while the others are the two radicals (XIII and XVI) which can both be formed by hydrogen abstraction. 

This complicated scheme shows that the species R, HO, and HOO are responsible for the further abstraction of hydrogen from polypropylene leading to alcohol functions by abstraction of hydrogen from a neighboring chain of polymer, but also and mainly, to the / -scissions. Moreover, the radicals obtained can react with oxygen giving hydroperoxides. On the other... 

Polypropylene (PP), on the other hand, undergoes predominantly chain scission under all processing conditions [7, 8, 9, 10, 11] with associated reduction in the molar mass and melt viscosity (see Scheme 3). The propagation reaction (Scheme 1, reaction 3) in PP is particularly facilitated by intramolecular hydrogen abstraction leading to the formation of adjacent hydroperoxides along the polymer chain that are less stable than isolated hydroperoxides and lead to an increased rate of initiation. 

The key reaction in the propagation sequences is the reaction of polymer alkyl radicals (P ) with oxygen to form polymer peroxy radicals (2). This reaction is very fast. The next propagation step, reaction (3), is the abstraction of a hydrogen atom by the polymer peroxy radical (POO-) to yield a polymer hydroperoxide (POOH) and a new polymer alkyl radical (P ). In polypropylene (Scheme 1.56), hydrogen abstraction occurs preferentially from the tertiary carbon atoms since they are the most reactive, as shown in reaction (11). 

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... 

On account of the high internal viscosity of the polymer the combination of the two macro-radicals formed in reactions of type IA probably predominates over the RO 2 formation, especially when the concentration of the dissolved oxygen, and hence its permeability, is low. Radical combination according to reaction type IB is unlikely, since CH3CO or CH3 may diffuse from the reaction site before recapture. Other isolated macro-radicals may also be formed by hydrogen abstraction by CH3 or CH3CO. Consequently the photolysis of polypropylene ketones according to type I seems to be an important factor in the photostability of polypropylene [124]. The tentative mechanism may be summarized as... 

Reaction 6 is the hydrogen atom abstraction from PVA, an important propagation reaction. Since it was shown by the spin-trapping technique using 2-methyl-2-nitrosopropane that only the tertiary hydrogen of polypropylene was abstracted by the alkoxy radical (28), it was assumed that the first attack was also exclusively on the tertiary hydrogen of PVA. The a-hydroxyperoxy radical formed by the addition of oxygen (Reaction... 

The conclusion reached is that chemical reactions are closely related to molecular motion and structures, where molecular conformation and molecular orientation are important parameters. We have discussed whether the hydrogen abstraction reaction of peroxy radicals in the inner crystalline region of polypropylene is due to then-abstraction of the hydrogen from the intra-chain segment or inter-chain one. (1) It was confirmed that the unstable and mobile radicals participated in a rotation or a rotatory vibration of the COO group around the main chain axis. (2) The conformational structure of the mobile peroxy radicals was close to a skew conformation. (3) It could be safely concluded that the mobile peroxy radicals were trapped in disordered sites in the inner crystalline region. 

Piletsky et al. [81] had found that using a coated hydrogen abstraction photoinitiator (see Scheme 4e) very thin MIP films, which were covalently anchored and covered the entire surface of the base material, could be synthesized by a photo-initiated cross-linking graft copolymerization. This approach had been first explored with benzophenone as photo-initiator and a membrane from polypropylene as support. MIP synthesis and recognition were possible in/from water, and significantly less cross-linker than with bulk preparations was necessary to obtain the highest template specificity. Both effects were explained by a contribution of the soUd polymer support to the stabilization of the imprinted sites. The approach is very flexible because no premodification is necessary. 

A better approach is work by Chung and coworkers [288] who grafted maleic anhydride to polypropylene with the use of Borane/02 initiator. This initiator is claimed to form in situ a monooxidized adduct (R-O 0-BR2). These adducts then carry out hydrogen abstractions from the pol5q)ropylene chains at ambient temperature. This results in formation of stable tertiary polymer radicals that react with maleic anhydride to form graft copolymers ... 

Hydrogen abstraction by polymer alkoxy (PO ) and hydroxyl (HO ) radicals from the photocleavage of polymer hydroperoxy groups (POOH) in polypropylene occurs with 50% probabiUty at methyl groups. This means that up to 25% of the polymer peroxy radical (POO ) pairs produced in the initiation process may be pairs of primary peroxy radicals and the remaining 75% would be pairs between secondary, tertiary and mixed peroxy radicals. Due to the high radical concentration within such pairs of immediately adjacent radicals, the termination processes would be favoured over the slow hydrogen abstraction reaction of these radicals. 

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 ... 

The main feature of the pyrolysis spectra of polyethylene, polypropylene and isoprene can be interpreted on the basis of simultaneous breakage of carbon-carbon bonds in the main polymer chain. For a more detailed interpretation of the pyrolysis spectrum it is necessary to assume a series of radical-chain reactions in which intramolecular hydrogen abstraction plays an important role. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

MMA onto cellulose was carried out by Hecker de Carvalho and Alfred using ammonium and potassium persulfates as radical initiators [30]. Radical initiators such as H2O2, BPO dicumylperoxide, TBHP, etc. have also been used successfully for grafting vinyl monomers onto hydrocarbon backbones, such as polypropylene and polyethylene. The general mechanism seems to be that when the polymer is exposed to vinyl monomers in the presence of peroxide under conditions that permit decomposition of the peroxide to free radicals, the monomer becomes attached to the backbone of the polymer and pendant chains of vinyl monomers are grown on the active sites. The basic mechanism involves abstraction of a hydrogen from the polymer to form a free radical to which monomer adds ... 

The mechanism of the decay reaction of the methyl free radicals at —196° is not known however, the y-ray irradiation of polypropylene at — 196°C. produces only methane and no ethane (36), as demonstrated by gas analysis after warming to room temperature after irradiation. It may be that the methyl free radicals abstract hydrogen atoms on warming to room temperature or that hot methyl radicals are produced during the radiolysis with sufficient excess energy to abstract hydrogen atoms at liquid nitrogen temperature. 

---