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A detailed mathematical model has been formulated for branching due to chain transfer to polymer in reversible addition fragmentation chain transfer (RAFT) polymerization. The traditionally adopted mechanism for RAFT polymerization has been modified by the inclusion of a relaxation process. It is proposed that this step occurs during the period immediately after a chain radical is released from the RAFT transfer intermediate. While undergoing relaxation, the so-called unrelaxed radicals are assumed to be capable of propagation, intermolecular chain transfer to polymer, and bimolecular termination, but not intramolecular chain transfer to polymer which only becomes possible when the radical is relaxed. The use of these assumptions indicates that RAFT can reduce the overall rate of branching without significantly affecting other key measurements such as conversion and average molecular weights. If relaxation is neglected, RAFT would appear not to affect the rate of branching. However, by reducing the average molecular weight, RAFT polymerization would still result in more linear polymers as compared to conventional methods. † This paper is dedicated to Professor Charles Liotta who has always been a good colleague, friend, and mentor. In addition to being a walking encyclopedia of chemical knowledge, Charlie is always good for a laugh (F.J.S.

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