Polymerization Mechanisms

The common thread

ATRP, RAFT, and FRP all propagate through a free radical chain end (shown in red below), the same reactive, indiscriminate species in all three. What separates a controlled radical polymerization (ATRP, RAFT) from an uncontrolled one (FRP) is entirely about what happens to that radical between monomer additions: ATRP and RAFT both add a reversible step that keeps most chains "parked" in an unreactive, dormant form (shown in blue) most of the time, so they grow in small, evenly shared increments instead of a few chains racing to completion while others never even start. ROMP is the odd one out mechanistically: it never forms a free radical at all, and gets its living character from a metal (shown in green) that stays covalently bonded to the growing chain end throughout.

ATRP: Atom Transfer Radical Polymerization

A dormant alkyl halide chain end is reversibly activated by a transition metal/ligand complex, which abstracts the halogen to briefly generate a radical chain end.

The key to why this is "controlled": the equilibrium sits heavily toward the dormant P_n–X side (k_act ≪ k_deact), so the instantaneous concentration of radicals in solution is kept very low at all times. Since termination is second order in radical concentration but propagation is only first order, suppressing [P_n•] suppresses termination much more than it slows chain growth. Nearly every chain survives, grows a little bit at a time, and ends up close to the same length. ARGET/ICAR/SARA/eATRP variants use the same equilibrium but continuously regenerate the active catalyst state instead of relying on it being present stoichiometrically from the start.

RAFT: Reversible Addition Fragmentation chain Transfer

A thiocarbonylthio compound (the RAFT agent) shuttles the radical "identity" rapidly between all growing chains via degenerate chain transfer, rather than suppressing the total radical population the way ATRP does.

Unlike ATRP, RAFT doesn't lower the total radical concentration below a normal FRP level. Termination still happens at roughly the same rate it would without the RAFT agent; it's just a small loss relative to the much larger population of chains cycling through the main equilibrium. Every active chain adds a monomer, then quickly gets "capped" back to a dormant thiocarbonylthio terminated chain while a different chain takes its turn as the radical, so on average, all chains grow in lockstep even though only one is actively propagating at any instant. The Z group tunes the RAFT agent's reactivity for a given monomer class; the R group becomes the initial leaving and reinitiating fragment and ends up as the other chain end.

ROMP: Ring Opening Metathesis Polymerization

No radicals here at all. ROMP runs on metal carbene chemistry (the Chauvin mechanism): a metal alkylidene reacts with a strained cyclic olefin through a [2+2]/retro[2+2] cycloaddition sequence, opening the ring and handing the reactive carbene off to the new chain end.

Simplified metallacyclobutane intermediate: the four membered ring that forms and immediately opens on every cycle. When it opens, the two metal–carbon bonds break, regenerating a metal carbene on one side and a new alkene on the other.

Because the metal never leaves the chain end between cycles, ROMP is "living" in the same practical sense as ATRP/RAFT (predictable Mₙ, low dispersity, chain extension possible) but for a completely different mechanistic reason. There's no dormant/active equilibrium to manage, just a metal that keeps finding the next ring to open. Leaving the reaction to run too long without quenching risks secondary metathesis (chain scission/backbiting) rather than radical termination. A closely related technique, ADMET (acyclic diene metathesis), uses the same metal carbene chemistry but builds chains from linear diene monomers with loss of ethylene instead of opening a ring. It isn't one of this calculator's tabs, but it's worth knowing it's a mechanistic cousin of ROMP.

Free Radical Polymerization (FRP)

The baseline case: a radical is generated once and propagates until it dies. There's no reversible step holding chains back, which is exactly why FRP isn't a controlled/living technique.

There's no dormant state and no equilibrium bringing a chain back once it's a radical. Each chain simply propagates at rate k_p[M] until it terminates (or transfers), which typically happens in well under a second. Because initiator decomposition (k_d) is slow and continuous, new chains keep starting throughout the whole reaction while earlier chains have already terminated, so the population is a statistical mix of chain lengths (kinetic chain length set by k_p, k_t, and the steady state radical concentration) rather than the narrow distribution set by the feed ratio that you get from ATRP/RAFT/ROMP. This is exactly why the calculator's FRP tab treats Mₙ as a simplified estimate rather than a precise design target.