Kinetic constants chain scissions

The factor a is a measure of the accessibility of the bonds (13), and k is the specific rate constant for the rate of breaking of normal IC-O-4 C bonds in the anhydroglucose chain (k in Equations 1-3 equals ak). The expression 100[1 /(DPn)t — 1/(DP )0] gives the percentage of the initial number of bonds that have been broken. Equation 4 is also the equation that would apply if an equal number of bonds were broken in equal periods of time, that is, at a constant rate of chain scissioning (zero-order kinetics). Conformance to Equation 4, therefore, cannot be taken as proof of first-order kinetic behavior (12). 

Since hydrolysis is a chain scission process, it always induces embrittlement. In initially ductile linear polymers, such as PET or PAll, toughness falls off by two or three decades when the weight average molar mass reaches a critical value Mp of the order of 10-20 mol.kg corresponding to a small multiple of the entanglement molar mass (Fayolle et al., 2008). In the frame of second-order kinetics (rate constant h)> it is possible to determine a lifetime value corresponding to the time to embrittlement ... 

The chain scission rate constant and the initiator decomposition efficiency are considered as the most important kinetic parameters of this process, because the chain scission is the main reaction which dominates control of MWD. By the deterministic and stochastic modelling procedures and Monte Carlo simulation it has been established that all kinetic parameters depend on the reaction time, 30 s giving the most reliable estimations [202]. 

The hydrolytic depolymerisation of PETP in stirred potassium hydroxide solution was investigated. It was found that the depolymerisation reaction rate in a KOH solution was much more rapid than that in a neutral water solution. The correlation between the yield of product and the conversion of PETP showed that the main alkaline hydrolysis of PETP linkages was through a mechanism of chain-end scission. The result of kinetic analysis showed that the reaction rate was first order with respect to the concentration of KOH and to the concentration of PETP solids, respectively. This indicated that the ester linkages in PETP were hydrolysed sequentially. The activation energy for the depolymerisation of solid PETP in a KOH solution was 69 kJ/mol and the Arrhenius constant was 419 L/min/sq cm. 21 refs. 

A new decarboxylative route to free radicals, which has proved particularly successful in preparative work, embodies the thermal (or photochemical) decomposition reaction of 1-hy-droxypyridine-2(l/f)-thione esters 23 with tributyltin hydride, /er/-butanethiol, or a similar hydrogen donor.These esters can be easily prepared from acyl halides and the sodium salt of l-hydroxypyridine-2(l//)-thione, or from the carboxylic acid, dicyclohexylcarbodiimide and l-hydroxypyridine-2(l/f)-thione. The intermediate radicals were readily reduced to the corresponding hydrocarbons 24 in efficient chain reactions with organotin hydrides or thiols as reaction partners, and the proportion of rearranged to unrearranged products could be controlled by the choice of hydrogen donor, its concentration and the temperature. This system was sufficiently quantitative and well behaved for use in kinetic studies, and the rate constants of the (S-scission reactions of the listed cyclopropylmethyl species were determined. 

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