Is the Addition-Fragmentation Step of the RAFT Polymerisation Process Chain Length Dependent? a Ekaterina I. Izgorodina, Michelle L. Coote* ARC Centre of Excellence in Free Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia E-mail: mcoote@rsc.anu.edu.au Received: February 21, 2006; Revised: March 29, 2006; Accepted: March 30, 2006; DOI: 10.1002/mats.200600021 Keywords: chain length effects; kinetics (polym.); quantum chemistry; reversible addition fragmentation chain transfer (RAFT) Introduction The reversible addition fragmentation chain transfer (RAFT) polymerisation process [1] is an important new method for controlling the molecular weight and chain architecture in radical polymerisation. It combines the versatility of a radical polymerisation process with the ability to generate complex macromolecular architectures such as comb, star and block copolymers for use in bioengineering and nanotechnology applications. [2] Control is achieved by protecting the majority of the propagating species from bimolecular termination processes through their reversible trapping in a dormant thiocarbonyl compound known as a RAFT agent, as shown in Scheme 1. Its success rests upon a delicate balance of the rates of several competing reactions, so as to ensure that the con- centration of the dormant species is orders of magnitude greater than that of the active species and the exchange between the two forms is rapid. The design of appropriate kinetic models, and the measurement of accurate kinetic parameters for these models, can greatly assist in optimiz- ing the process. The first step in designing an accurate kinetic model of a chemical process is to identify the minimum set of Summary: The chain length dependence of the addition- fragmentation equilibrium constant (K) for cumyl dithio- benzoate (CDB) mediated polymerisation of styrene has been studied via high level ab initio molecular orbital calculations. The results indicate that chain length and penultimate unit effects are extremely important during the early stages of the polymerisation process. In the case of the attacking radical (i.e., R. in: R. þ S C(Z)SR 0 ! RSC.(Z)SR 0 ), the equilibrium constant varies by over three orders of magnitude on extending R. from the styryl unimer to the trimer species and actually increases with chain length, further confirming that K is high in this system. When the reactions of the cumyl leaving group and cyanoisopropyl initiating species, which are also present in CDB-mediated polymerisation of styrene in the presence of the initiator 2,2 0 -azoisobutyronitrile, are also included, the variation in K extends over five orders of magnitude. Although less significant, the influence of the R 0 group should also be taken into account in a complete kinetic model of the RAFT process. However, for most practical purposes, its chain length effects beyond the unimer stage may be ignored. These results indicate that current simplified models of the RAFT process, which typically ignore all chain length effects in the R and R 0 positions, and all substituent effects in the R 0 position, may be inadequate, particularly in modelling the initial stages of the process. Macromol. Theory Simul. 2006, 15, 394–403 ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 394 DOI: 10.1002/mats.200600021 Full Paper a : Supporting information for this article is available at the bottom of the article’s abstract page, which can be accessed from the journal’s homepage at http://www.mts-journal.de, or from the author.