A Modified Rotating-Sector Method of Measuring Kinetic Chain Lifetimes

Abstract

IN order to evaluate the individual velocity coefficients for the propagation and termination reactions involved in polymerization and other chain processes, it is necessary to measure the lifetime of the kinetic chain. This is usually done by means of the rotating-sector method¹, or one of the non-stationary state methods^2–6. The main disadvantage of the former method is that the complete determination of a kinetic chain lifetime cannot normally be made with a single filling of a dilatometer, unless the reaction is taken beyond the initial stage which may lead to gel effects and other complications. A number of dilatometers are therefore usually employed in a single determination of a lifetime, and errors often occur due to difficulty in reproducing the exact conditions, especially when the monomer involved is difficult to purify. The non-stationary state methods, on the other hand, yield a value for the lifetime in ≤30 sec.; but unfortunately they often are inaccurate when lifetimes of ≤0.5 sec. occur, since instrument lags or personal response times become particularly important for the very short lifetimes. The actual rate measurements, however, are quite accurate in this range of lifetimes, and can be measured in about 10 sec., that is, when 0.01 per cent reaction has occurred. By combining the two methods it is possible to cope with lifetimes of ±0.5 sec., and to carry out a complete determination of a lifetime of the kinetic chain before 1 per cent reaction has occurred. The method is not valid for lifetimes ≫0.5 sec., since when flashtimes of several seconds are used the measurement of the rate by non-stationary state methods becomes inaccurate.