Reaction independent chain initiation

Eqs. (1) to (3) indicate that conversion studies under conditions where thermal polymerization prevails can only yield e and the product nyo, whereas the photon-induced reaction provides information on the product nq. To disentangle chain initiation and chain propagation effects an independent determination of the kinetic chain length is required. 

Of particular interest are changes in the chain length occurring in connection with the autocatalytic reaction acceleration in TS-6. Numerous thermal polymerization studies showed that the activation energy is EJl = 1.00 + 0.02 eV, independent of conversion. Consequently the autocatalytic reaction enhancement cannot be the result of an increase of the Boltzmann factor. Instead, an increase in the number of monomers consumed per primary chain initiation event has been postulated. Experimentally n(X = 0.5)/n(X = 0) = 200 is found... 

Internal olefins are much less reactive than a-olefins in chain initiation and secondary hydrogenation reactions. The reactivity of added a-olefins in chain initiation reactions increased in the order ethylene > propylene s 1 -butene C5+ olefins it becomes almost independent of chain size for C5+ a-olefins. The higher reactivity of ethylene and propylene leads to C2 and C3 selectivity below those of the rest of the distribution in Flory plots (Fig. 5) and to the low termination probabilities measured for C2 and C3 surface chains (Fig. 8), as proposed previously by others (115). 

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. 

The rate constant of the reaction of cobalt(III) acetate with benzaldehyde in the absence of dioxygen was determined in independent experiments. It turned out to be virtually the same as the rate constant of the chain initiation in the oxidation reaction in the presence of O2- However, the contribution of chain initiation to the radical formation is insignificant in the developed oxidation process. The radicals are mainly formed in the reactions of the intermediates in the process of degenerate chain branching. These reactions are also catalyzed by transition metal ions. Especially well studied is the acceleration of radical decomposition of intermediately formed hydroperoxides (see, e.g., [10]). 

Tip 2 Chain stereoregularity and active sites. In free radical polymerization, polymer chain configuration and MWD are often independent of initiator type and initiation mechanism, depending strongly on reaction temperature, initiation rate, and monomer concentration. One can, therefore, often predict chain stereoregularity and MWD without a detailed knowledge of the initiation mechanism. 

Ttp 4 Chain microstructure and propagation reactions. Propagation reactions are mainly responsible for the development of polymer chain microstructure (and control chain composition and sequence length distribution in copolymerizations). In free radical polymerization, the stereoregularity of a high molecular weight homopolymer chain depends on polymerization temperature almost exclusively. It is usually independent of initiator type and monomer concentration. Calculations on stereoregularity... 

In the presented model scheme for a chain chemical process with linear steps of transformation of active reaction centers, the initiation steps act independently. In this case the rate of the formation of the P, product may be written as the sum of rates, corresponding to the different directions of the chain reaction... 

Chain propagation is one order of magnitude faster than chain initiation. The rate coefficients for the C-C coupling and readsorption reactions are chain-length independent, starting at C3, and do not depend on H2/CO ratio. The hydrocarbon chain on the catalyst surface is most likely a species. 

Chain microstructure and molecular weight distribution are independent of initiation mechanism and type and function of reaction variables such as temperature and concentration of reactants. 

Propagation Kinetics. The kinetics of propagation for styrene and diene monomers in hydrocarbon solvents with lithium as the counterion is complicated by chain-end association (49,50,53). The kinetics of propagation can be investigated independently of initiation so that complications from cross-association with the initiator are absent. The reaction order dependence of the propagation rate on active center concentration is independent of the identity of the hydrocarbon solvent, aromatic or aliphatic, although the relative propagation rates, under equivalent conditions, are faster in aromatic versus aliphatic solvents. 

This is the cationic polymerization equivalent of the Mayo-Walling Equation (2.41). Additional terms can be added to the right-hand side to take account of other chain transfer reactions, e.g. for chain transfer to solvent the additional term is A . 5-[S]/A p[M]. In each case3c is independent of initiator concentration and in the absence of chain transfer is given by... 

If in the chain initiated reaction when v,- = const the induction period is independent of the efficiency of retardation action of the inhibitor but is determined by its concentration, then during autoxidation the inhibitor is more slowly consumed when it more efficiently terminate chains because ROOM is more slowly accumulated and the retardation period increases. Then the initiated oxidation of hydrocarbons is retarded only by compounds terminating chains. Autoxidation is retarded by compounds decomposing hydroperoxides. This decomposition, if it is not accompanied by the formation of free radicals, decreases the concentration of the accumulated hydroperoxide and, hence, the autoxidation rate. Hydroperoxide decomposition is induced by compounds of sulfur, phosphorus and various metal complexes, for example, thiophosphate, thiocarbamates of zinc, nickel, and other metals. 

One of the most sensitive tests of the dependence of chemical reactivity on the size of the reacting molecules is the comparison of the rates of reaction for compounds which are members of a homologous series with different chain lengths. Studies by Flory and others on the rates of esterification and saponification of esters were the first investigations conducted to clarify the dependence of reactivity on molecular size. The rate constants for these reactions are observed to converge quite rapidly to a constant value which is independent of molecular size, after an initial dependence on molecular size for small molecules. The effect is reminiscent of the discussion on the uniqueness of end groups in connection with Example 1.1. In the esterification of carboxylic acids, for example, the rate constants are different for acetic, propionic, and butyric acids, but constant for carboxyUc acids with 4-18 carbon atoms. This observation on nonpolymeric compounds has been generalized to apply to polymerization reactions as well. The latter are subject to several complications which are not involved in the study of simple model compounds, but when these complications are properly considered, the independence of reactivity on molecular size has been repeatedly verified. 

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