The nonclassical kinetics of low-conversion, free-radical polymerization have been studied. The models adopted assume a termination rate constant for radicals of size m and n given by = km(nm)~a.Using the expression for radical population and the rate of production of polymer, a relationship between apparent rate constant of termination kt and average degree of polymerization, Xn, is found to be kt = (constant) (Xn)'2°. This relationship is in good agreement with the experimental data obtained in the polymerization of styrene and methyl methacrylate.

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The specific rate constant for the termination reaction between two flexible polymer molecules with active chain ends has been considered in relation to the segmental diffusion of chain ends in solution. The probability of reaction between two chain ends per unit time when the centers of gravity of two polymer molecules are at a distance of separation has been calculated by using the Smoluchowski equation and a Gaussian distribution of chain ends. The time during which two polymer molecules are in contact has also been calculated by using the diffusion equation and the potential energy function for intermolecular interaction. The rate constant may then be completely expressed as a complex function of the intramolecular linear expansion factor, molecular weight, and the frictional properties of the reacting polymers' segment. This expression predicts that the rate constant is inversely proportional to solvent viscosity, decreasing with increasing molecular weight to some extent, and is affected by the excluded volume effect and chain flexibility. The complete expression for the rate constant has been simplified and the result compared with experimental data. Close agreement is found between the calculated rate constants and those experimentally obtained.

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A novel reactor has been designed which permits the precise determination of absolute rate constants in photoinitiated free‐radical vinyl polymerization. A solution of monomer and initiator flows through a dark tubular reactor past regularly spaced slots through which light shines. The alternating dark and light regions produce spatially intermittent polymerization (SIP) and make the system analogous to the well‐known rotating‐sector technique. However, the SIP reactor has the advantage of producing large volumes of reaction product, at low conversion, suitable for analysis of both conversion and molecular weight. This supplies the necessary data, from a single set of experiments, for the simultaneous determination of the rate constants for propagation and termination. Experimental data are reported at 25°C for methyl methacrylate which indicate that kp = 315 I./mole‐sec, independent of polymer molecular weight, and kt is dependent on molecular weight especially at low molecular weight, approaching a lower value of kt = 30 × 106 I./mole‐sec at a molecular weight of 106. For styrene, measurements being made only at high molecular weight, kp = 74 ± 5 and kt = 37 ± 0.3 × 106 l./mole‐sec at 25°C.