Polypropylene Radical

The central feature of this mechanism is, therefore, that the phenoxyl radical is reversibly reduced and re-oxidised this leads to the continuous consumption of macroalkyl radicals. The phenoxyl radical can, therefore, react with polypropylene radicals and compete with PP-MA adduct formation in the stabilised polymer (Figure 3, curve MA-S). 

Pig. 45. Decay curves of polypropylene radicals. The black circles and open circles correspond to radicals produced mechanically and by T-irradiation respectively... 

Indeed, both reactions in Schemes 13.2 and 13.3 shows that in the balances of the species involved in the solution and molten state processes, three polypropylene radicals remain active for every two grafted SA groups. These become involved in the termination of the radical step that could proceed, as it is well accepted by disproportion and recombination, as the two major processes involved in the termination step of the classical three-step radical processes. 

The overall oxidation scheme of an unstabilized polypropylene fiber has been the subject of much investigation [112]. Polypropylene, as produced, contains trace impurities which, in the presence of heat or light, permits a polypropylene radical (PP) to be formed. This radical will react with oxygen as shown in the reaction represented by Equation 3.4. 

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... 

Augier, S., Coiai, S., Gragnoli, T., Passaglia, E., Pradel, J. L., and Flat, J. J. 2006. Coagent assisted polypropylene radical functionalization Monomer grafting modulation and molecular weight conservation. Polymer 47 5243-5252. 

The initiator was injected into the second barrel where it mixed with the unmelted polypropylene in the solids conveying zone. In the melting section, a process temperature of about 200°C resulted in an initiator half-life on the order of 5 sec. This is about the residence time in the melting section, so that when the acrylic acid was injected into barrel 4, polypropylene radicals would already have been formed. The grafting reaction of acrylic acid was essentially completed within one barrel section. An atmospheric vent was used in the fifth barrel to remove unreacted acrylic acid volatiles and entrapped air. 

In an oxygen-deficient atmosphere, vinylidene and vinyl compounds may be formed from the disproportionation of polypropylene radicals [Eqs. (46) and (47)]. 

Fig. 2.1 Decay curves for polypropylene radicals generated by sawing and irradiation [105].

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

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. 

The synergistic effect of a hydroperoxide decomposer, eg, dilauryl thiodipropionate [123-28-4] (34), and a radical scavenger, eg, tetrakis[methylene(3,5-di-/ f2 butyl-4-hydroxyhydrocinnamate)]methane (9), ia protecting polypropylene duting an oxygen-uptake test at 140°C is shown ia Table 3. 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

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 ... 

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]. 

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