The synthesis of a novel class of reversible addition-fragmentation chain transfer (RAFT) agents-based on quantum-chemical ab initio calculations-having methylsulfonyl and phenylsulfonyl moieties as Z-group is described. The resulting C-sulfonyldithioformate transfer agents [benzyl methylsulfonyldithioformate (MSDTF) and benzyl phenylsulfonyldithioformate (PSDTF)] feature extremely strong electron deficient C=S double bonds and should thus be suitable to undergo rapid hetero Diels-Alder (HDA) reactions with variable dienes under mild (i.e., ambient and catalyst free) reaction conditions. It can be demonstrated via a series of model reactions, whose outcome is monitored via electrospray ionization mass spectrometry (ESI-MS), that C-sulfonyldithioformate based RAFT agents undergo HDA reactions with a series of diene-capped macromolecules (10 min < reaction time < 24 h) at ambient temperatures (T approximate to 25 degrees C) with reaction times ranging from 24 h (for open chain dienes) to a few minutes (for cyclopentadiene) in the absence of any catalysts. Concomitantly, PSDTF is able to efficiently mediate the polymerization of isobornyl acrylate (iBA) with living characteristics (2300 < M-n (g/mol) < 16,000, 1.08 < PDI < 1.31). In a subsequent step, it is demonstrated that conjugates of poly(iBA) and polystyrene can be constructed under mild reaction conditions (reaction time < 10 min, T approximate to 25 degrees C, M-n approximate to 6000 (g/mol), PDI approximate to 1.3, no catalyst). In addition, we highlight that sulfonyldithioformate type RAFT agents are such effective dienophiles that they can undergo HDA reactions with certain monomers, including styrene. While such a strong HDA activity limits their use in polymerizations, it opens an avenue for catalyst free efficient surface modification reactions under mild conditions with variable dienes. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6053-6071, 2009