Primary chain termination

Our readsorption model shows that carbon number distributions can be accurately described using Flory kinetics as long as olefin readsorption does not occur (/3r = 0), because primary chain termination rate constants are independent of chain size (Fig. 24). The resulting constant value of the chain termination probability equals the sum of the intrinsic rates of chain termination to olefins and paraffins (j8o + Ph)- As a result, FT synthesis products become much lighter than those formed on Co catalysts at our reaction conditions (Fig. 24, jSr = 1.2), where chain termination probabilities are much lower than jS -I- Ph for most hydrocarbon chains. The product distribution for /3r = 12 corresponds to the intermediate olefin readsorption rates experimentally observed on Co/Ti02 catalysts, where intrapellet transport restrictions limit the rate of removal of larger olefins, enhance their secondary chain initiation reactions, and increase the average chain size of FT synthesis products. 

Dependence of P0, on NO, Abundance in CO Oxidation One of the key aspects of tropospheric chemistry is the dependence of ozone production on the NO, abundance. We have derived relationships for Po, for the CO system in the limits of low and high NO,. Here we examine how Po, depends on the NO, abundance over the complete range of NO, levels. To do this, we will fix the rate of HO, production, Pho,. and vary the NO concentration at a fixed N02/NO ratio. Under conditions of high H02 radical abundance relative to NO, the primary chain-terminating reaction is the HO, + HO, reaction, HO2 + H02. This condition is referred to as NOx-limited. At sufficiently high NO, levels, chain termination results from the HO, + NO, reaction, OH 4- N02. This condition is called NOx-saturated. By varying the NO, concentration, we can explore the point at which the system crosses over from NO,-limited to NO,-saturated conditions. The crossover point occurs at the NO concentration where 3Poj/3[NO] = 0. The actual value of the NO concentration at this crossover point depends on the values of Pho, and the NO2/NO ratio. 

This means that the first polymer radical ri has a different growth probability than the radicals r with a higher degree of polymerization. The generating function in the presence of primary chain termination is... 

In the introductory example, all reactivities are independent of the chain length. In the primary chain termination example in Sect. 17.1.2.9, only the first radical with a degree of polymerization of 1 differs in reactivity from the others. If every radical has a different reactivity then the mechanistic equation will look like... 

The treatment is similar to the primary chain termination effect. Configurational statistics of copolymers has been described by Gordon and Malcolm [24]. 

Two general classifications can be used to categorize antioxidants— primary (chain terminating) and secondary (peroxide decomposing). 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

Serelis and Solomon108 found that primary radical termination of oligo(MAN) radicals (16) with 15 also gives predominantly combination. The ratio kllt/klc was found to have little, if any, dependence on the oligomer chain length (n<4). As with PMMA, disproportionation involves preferential abstraction of a methyl... 

The nature of the termination reaction in MMA polymerization has been investigated by a number of groups using a wide range of techniques (Tabic 5.5), There is general agreement that there is substantial disproportionation. However, there is considerable discrepancy in the precise values of k tk. In some cases the difference has been attributed to variations in the way molecular weight data are interpreted or to the failure to allow for other modes of termination under the polymerization conditions (chain transfer, primary radical termination).154 In other eases the reasons for the discrepancies are less clear. MALDI-TOF mass... 

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

Another innovative approach to controlling amine-initiated NCA polymerizations was reported in 2003 by Schlaad and coworkers [20]. Their strategy was to avoid formation of NCA anions, which cause significant chain termination after rearranging to isocyanocarboxylates [11, 12], through use of primary amine hydrochloride salts as initiators. The reactivity of amine hydrochlorides with NCAs was first explored by the group of Knobler, who found that they could react... 

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 ... 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

The oxidation of primary and secondary alcohols in the presence of 1-naphthylamine, 2-naphthylamine, or phenyl-1-naphthylamine is characterized by the high values of the inhibition coefficient / > 10 [1-7], Alkylperoxyl, a-ketoperoxyl radicals, and (3-hydroxyperoxyl radicals, like the peroxyl radicals derived from tertiary alcohols, appeared to be incapable of reducing the aminyl radicals formed from aromatic amines. For example, when the oxidation of tert-butanol is inhibited by 1-naphthylamine, the coefficient /is equal to 2, which coincides with the value found in the inhibited oxidation of alkanes [3], However, the addition of hydrogen peroxide to the tert-butanol getting oxidized helps to perform the cyclic chain termination mechanism (1-naphthylamine as the inhibitor, T = 393 K, cumyl peroxide as initiator, p02 = 98 kPa [8]). This is due to the participation of the formed hydroperoxyl radical in the chain termination ... 

Nitroxyl radicals (AmO ) are known to react rapidly with alkyl radicals and efficiently retard the radical polymerization of hydrocarbons [7]. At the same time, only aromatic nitroxyls are capable of reacting with alkylperoxyl radicals [10,39] and in this case the chain termination in the oxidation of saturated hydrocarbons occurs stoichiometrically. However, in the processes of oxidation of alcohols, alkenes, and primary and secondary aliphatic amines in which the chain reaction involves the HOT, >C(0H)02 , and >C(NHR)02 radicals, possessing the... 

Another situation is observed when salts or transition metal complexes are added to an alcohol (primary or secondary) or alkylamine subjected to oxidation in this case, a prolonged retardation of the initiated oxidation occurs, owing to repeated chain termination. This was discovered for the first time in the study of cyclohexanol oxidation in the presence of copper salt [49]. Copper and manganese ions also exert an inhibiting effect on the initiated oxidation of 1,2-cyclohexadiene [12], aliphatic amines [19], and 1,2-disubstituted ethenes [13]. This is accounted for, first, by the dual redox nature of the peroxyl radicals H02, >C(0H)02 and >C(NHR)02 , and, second, for the ability of ions and complexes of transition metals to accept and release an electron when they are in an higher- and lower-valence state. 

If we use initiators R-R which have very high reactivities for the chain transfer reaction to the initiator and/or primary radical termination, i.e., ordinary bimolecular termination is neglected, it is expected that a polymer will be obtained with two initiator fragments at the chain ends (Eq. 7) ... 

These iniferters thermally or photochemically dissociate at the weak bonds, and then monomer molecules are inserted by propagation, followed by primary radical termination and/or chain transfer to give polymers (9-11) (Eqs. 9-11), which also contain iniferter bonds at the chain ends. These polymers may further act as the polymeric iniferters. 

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