Absolute Rate Constants in Polymerization

By measuring the dark period time which corresponds to the time required for the radicals to decay, one obtains the average radical concentration (from the rate of loss of monomer) and finally the ratio Together with the photostationary measurements of kt lkp, this allows kt and kp to be determined separately. This method is useful only in systems in which termination and propagation have different orders with respect to radicals. Although it studies the nonstationary period of the chain reaction, it studies it in a periodic or stationary way. 

more direct methods have been devised (Burnett, Zoc. ciL) for studying the nonstationary period. Most of them involve complex electronic equipment, because the period is usually very short. However, it can be lengthened by choosing sufficiently low radical concentrations, but this means lower rates of polymerization. If sufficiently sensitive means of observing low polymerization rates are available, single radical decay periods can be studied directly. A method for doing so with simple di-latometric equipment has been described by Benson and North.  

Because of the inherently large errors involved in the measurements of radical decay rates, the agreements between different laboratories have not usually been better than a factor of 2. Table XVI.3 presents a compilation of some values obtained recently by rotating-sector and other methods. Because of the large errors in determining fce/fcp, the activation energies listed are probably not better than 1 Kcal/mole, with corresponding errors in the Arrhenius A constants. 

The Arrhenius A factors for the propagation reactions are low and of the order one would expect from any of the transition-state theories for a bimolecular reaction between two large molecules (Table XII.2). The activation energies Ep for propagation are also low and of the order observed for similar addition reactions in the gas phase of radicals to a double bond. The values of At are in the range to be expected for diffusion-controlled reactions (Sec. XV.2) except for vinyl chloride, which must certainly be in error. As pointed out earlier in discussing diffusion-controlled reactions, it is expected that the activation energies will be of the order of a 

Hate Constants (liters/mole-sec) are in Arrhenius form log k = log A —. /4.575T. The values of ki reported by British workers are usually twice those given by Americans because of the difference in definition of kt. 

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