The Cutthroat Competition Between Termination and Transfer to Shape the Kinetics of Radical Polymerization

Abstract

Summary:There is a fascinating interplay between termination and transfer that shapes the kinetics of radical polymerization (RP). In one limit all dead-chain formation is by termination, in the other by transfer. Because of chain-lengthdependent termination (CLDT), the rate law for RP takes a different form in each limit. However, common behavior is observed if one instead considers how the average termination rate coefficient varies with average degree of polymerization. Examples are given of using these principles to understand trends in actual RP data, and it is also demonstrated how to extract quantitative information on CLDT from simple steady-state experiments.Keywords: chain transfer; kinetics (polym.); radical polymerization; termination Some Introductory ThoughtsThe steady-state rate of radical polymerization (RP) is given byHere c M is monomer concentration, t time, k p propagation rate coefficient, R init rate of initiation, and k t termination rate coefficient. Measurement of initiator decomposition rates, and thus specification of R init , has never been a problem. However for much of the history of RP, the disentangling of k p and k t was a problem. This was solved in 1987 when it was shown that by relatively simple analysis of the molecular weight distribution from a pulsed-laser polymerization (PLP), the value of k p could be obtained without requirement for any knowledge of k t (or R init ).[1] So enthusiastically and successfully was this method adopted by the RP community that within just a few years it was recommended by an IUPAC WorkingParty as the method of choice for k p determination; [2] recent reviews emphasize just how widely the method has been deployed. [3,4]

Effects of Chain Transfer Agent and Temperature on Branching and β‐Scission in Radical Polymerization of 2‐Ethylhexyl Acrylate

“…In experiments with AIBN, final conversion was higher in the absence of CTA (see Table ). This is consistent with termination being faster in the presence of DDM, which is a well‐known consequence of chain‐length‐dependent termination . On the other hand, CTA‐containing‐P2EHAs synthesized with a redox initiator have a higher conversion than the non‐CTA ones, opposite to what is expected.…”

Determination of the Mode of Termination in Radical Polymerization via Mass Spectrometry

“…We consider chain-length-dependent termination (CLDT) because it is the principal phenomenon that gives rise to nonclassical behavior . Instead of eqs and respectively one has ,, c R 1 = R i k p c M + 2 ∑ j = 1 ∞ k normalt 1, j c normalR j c R i = k p c M c R i − 1 k p c M + 2 ∑ j = 1 ∞ k normalt i normal, j c normalR j , i = 2 , ∞ while instead of eqs and respec...…”

Detailed analysis of termination kinetics in radical polymerization

“…As already mentioned, this is non‐physical for diffusion. However, it turns out that the trends in Eqns () and () are the same regardless of the averaging used, while the absolute values are only affected in a minor way 26,28,30 . This means that Eqns () and () can be guaranteed to yield α accurately and k t 1,1 with reasonable accuracy.…”

“…The second approach is just to use Eqn () directly and without the involvement of Eqn (). Even though this might be considered more obvious and has the added advantage of not requiring measurement of DP n , which can be tricky, it came later to the table 26,28 . The idea is just to use how Eqn () predicts k t to vary with an easily controllable kinetics parameter, e.g.…”

“…Log–log plot of termination rate coefficient k t versus concentration of the initiator 2,2′‐azoisobutyromethyl ester, c AIBME , for steady‐state, low‐conversion, bulk polymerization of methyl methacrylate at 40 °C 26 . Points, experimental values; 27 line, linear best fit, with slope as displayed.…”