To begin the task of providing critically evaluated (â€œbenchmarkâ€) data for rates of initiation in radical polymerization.
Radical polymerization (RP) has always been a process of substantial scientific and industrial importance. This importance is set to increase even further with the recent advances in reversible-deactivation RP (previously known as living/controlled RP). Hence it is of great benefit for both science and industry to be able to model the kinetics of RP systems and the sizes of the polymers they produce. Such modeling is dependent on the availability of reliable values of rate coefficients. Unfortunately RP systems have proven to be difficult subjects for investigations aimed at determining accurate rate coefficients. However, over the last two decades the IUPAC Subcommittee on Modeling of Polymerization Kinetics and Processes has made considerable progress in delivering critically evaluated propagation rate coefficients, kp, and termination rate constants, kt, for RP systems.
This project aims at pursuing progress with regard to kp and kt to be matched on the initiation front. To do this there is first of all the need to make workers aware of the subtleties involved in studying and describing initiation. These include the distinction between, and interplay of, initiator decomposition (rate coefficient kd), initiator efficiency (f) and primary radical addition (ki). Secondly, there needs to be carried out an evaluation of methods for determining these kinetic parameters and for agreement to be arrived at regarding the strengths and weaknesses of each method. Finally, we will apply our conclusions to data from the initial stages of bulk polymerizations of styrene and methyl methacrylate, two otherwise well characterized systems, so as to come up with critically evaluated kinetic parameters for these systems. We will do this for at least one azo initiator, e.g. azoisobutyronitrile (AIBN) and azoisobutyromethylester (AIBME), and at least one peroxide initiator, e.g. benzoyl peroxide (BPO) and bistrimethylhexanoyl peroxide (BTMHP). These initiators are of great commercial importance and have also been widely used in academic studies.
March 2014 update – The focus of the project has been narrowed to concentrate on azo-compound initiators. Data collection and evaluation re rate coefficients associated with initiation with azo compound initiators is complete and two publications are in preparation:
(i) Moad, G., A Critical Assessment of the Kinetics and Mechanism of Initiation of Radical Polymerization with Commercially Available Dialkyldiazene Initiators – A Review.
(ii) Moad, G.; Ahr, M.; Beuermann, S.; Buback, M.; Coote, M.; Hungenberg, K.-D.; Kajiwari, A.; Russell, G. T.; Wysong, E. B., A Critical Assessment of the Kinetics and Mechanism of Initiation of Radical Polymerization with Commercially Available Dialkyldiazene Initiators – Kinetic Data.
May 2016 update – The first mss has submitted to Progress in Polymer Science for publication and was extensively revised in Sept 2015. Following publication of that work, the intent is to publish a summary of the kinetic data in Pure and Applied Chemistry.
Sep 2019 update – G. Moad. ”A Critical Assessment of the Kinetics and Mechanism of Initiation of Radical Polymerization with Commercially Available Dialkyldiazene Initiators” Progress in Polymer Science, 88, 130-88 (2019); https://doi.org/10.1016/j.progpolymsci.2018.08.003
A technical report comprising recommended kinetic data for those initiators (about three) that allow a reliable assessment to be made is being prepared for submission to Pure and Applied Chemistry.
Last update 19 Oct 2020