The Importance of Chain-Length Dependent Kinetics in Free-Radical Polymerization: A Preliminary Guide
The effect of chain-length dependent propagation at short chain lengths on the observed kinetics in low-conversion free-radical polymerization (frp) is investigated. It is shown that although the values of individual propagation rate coefficients quickly converge to the high chain length value (at chain lengths, i, of about 10), its effect on the average propagation rate coefficients, kp, in conventional frp may be noticeable in systems with an average degree of polymerization (DPn) of up to 100. Furthermore it is shown that, unless the system is significantly retarded, the chain-length dependence of the average termination rate coefficient, kt, is not affected by the presence of chain-length dependent propagation and that there exists a simple (fairly general) scaling law between kt and DPn. This latter scaling law is a good reflection of the dependence of the termination rate coefficient between two i-meric radicals, kt i,i, on i. Although simple expressions seem to exist to describe the dependence of kp on DPn, the limited data available to date does not allow the generalization of these expressions.