Is the addition-fragmentation step of the RAFT polymerisation process chain length dependent?

The chain length dependence of the addition-fragmentation equilibrium constant (K) for cumyl dithiobenzoate (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 center dot in: R center dot + S = C(Z)SR' -> RSC center dot(Z)SR'), the equilibrium constant varies by over three orders of magnitude on extending Re 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-azoisobutyronitrile, are also included, the variation in K extends over five orders of magnitude. Although less significant, the influence of the R' 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' positions, and all substituent effects in the R' position, may be inadequate, particularly in modelling the initial stages of the process.