The mechanism of chain termination

The termination of the chains is due to the reactions of free radicals with each other, in which inactive products are formed namely, 

FUNCTIONAL GROUP CONTENT IN THERMALLY OXIDIZED POLYPROPYLENE AND POLYETHYLENE [5, 7] 

Functional group Polypropylene 20 mil sheet, 2 h at 138°C Polyethylene 20 mil sheet, 100 hat 100° C  

When polymer oxidation is initiated by additives free radicals formed by the decomposition of the initiator may also take part in termination reactions  

The mutual reaction of peroxy radicals results in the formation of inter-and intramolecular tetra-oxide intermediates [212]. The decomposition of tetra-oxide gives alkyl oxyradicals and molecular oxygen namely, 

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The mechanisms of chain termination by disproportionation of secondary and tertiary peroxyl radicals are sufficiently different (see Chapter 2). Secondary R02 disproportionate by reaction [4-6]... 

As in the preceding cases, the number of end groups depends on the mechanism of chain termination, either by coupling (two end groups) or by disproportionation (one end group). 

R. A. Gregg and M. S. Matheson Chain transfer in the polymerization of styrene. VI. Chain transfer with styrene and benzoyl peroxide the efficiency of initiation and the mechanism of chain termination. J. Am. Chem. Soc. 73, 1691 (1951). 

L Tang, H Fu, MBetlach, R McDaniel. Elucidating the mechanism of chain termination switching in the picromycin/methymycin polyketide synthase. Chem Biol 6 553-558, 1999. 

Diethylzinc is employed in several ways in polyethylene production. Its earliest application was as a chain transfer agent for molecular weight control (31, 32). Today, chain transfer in Ziegler-Natta catalyst systems is achieved chiefly by hydrogenolysis (previously discussed in connection with the mechanism of chain termination in section 3.7). Use of diethylzinc for molecular weight control for polyethylene is no longer significant. 

The mechanisms of chain termination are not known the molecules can reach molecular weights of at least 50,000 daltons. 

The mechanism of chain termination is unknown, but it is possible that it could occur if the A-sulphation/epimerisation sometimes overran polymerisation, to produce a terminal incapable of acting as a further acceptor. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

As indicated in Figure 10.2, there is a distinct change in the slope of the line at carbon numbers 8 to 12, and this has also been observed by other researchers.2-3 This change in the slope cannot be explained by the ASF model, which is based on the premise that the chain growth probability factor (a) is independent from the carbon number. Some further developments of the ASF model by Wojciechowski et al.3 made use of a number of abstract kinetic parameters for the calculation of a product spectrum. Although it still predicts a straight line for the a plot, they suggested that the break in the line is due to different mechanisms of chain termination and could be explained by the superposition of two ideal distributions. This bimodal distribution explained by two different mechanisms... 

Russell [179] proposed the following mechanism of chain termination by primary and secondary peroxyl radicals with coordinated decomposition of formed tetroxide to alcohol, ketone, and 02 ... 

In the oxidized hydrocarbon, hydroperoxides break down via three routes. First, they undergo homolytic reactions with the hydrocarbon and the products of its oxidation to form free radicals. When the oxidation of RH is chain-like, these reactions do not decrease [ROOH]. Second, the hydroperoxides interact with the radicals R , RO , and R02. In this case, ROOH is consumed by a chain mechanism. Third, hydroperoxides can heterolytically react with the products of hydrocarbon oxidation. Let us consider two of the most typical kinetic schemes of the hydroperoxide behavior in the oxidized hydrocarbon. The description of 17 different schemes of chain oxidation with different mechanisms of chain termination and intermediate product decomposition can be found in a monograph by Emanuel et al. [3]. 

Inhibition Coefficients f and Rate Constants k for the Reactions of Peroxyl Radicals with Aromatic Amines in Systems with a Cyclic Mechanism of Chain Termination (Experimental Data)... 

A new cyclic mechanism of chain termination by nitroxyl radicals, including the formation of aminyl radicals as intermediate species, has been proposed by Korcek and coworkers [42,43]. It was shown that the addition of 4,4 -dioctyldiphenylnitroxyl radical to the hexa-decane that is oxidized (T = 433 K) leads to the formation of the corresponding diphenyl-amine as an intermediate compound during its transformations. The following cyclic mechanism of chain termination was suggested ... 

Cyclic mechanisms of Chain Termination in the Liquid-Phase Oxidation of Organic Compounds... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

The induction period is measured experimentally at the constant sum of concentrations of two antioxidants, namely, Co = [S]o + [InH]0 = const. Theoretically this problem was analyzed in [9] for different mechanisms of chain termination by the peroxyl radical acceptor InH (see Chapter 14). It was supposed that antioxidant S breaks ROOH catalytically and, hence, is not consumed. The induction period was defined as t = (/[InH /v, where vV2 is the rate of InH consumption at its concentration equal to 0.5[InH]o. The results of calculations are presented in Table 18.1. 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

Therefore, such alkyl radical acceptors as quinones, nitroxyl radicals, and nitro compounds retard the oxidation of PP according to the following cyclic mechanism of chain termination ... 

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