Polymer chemistry

ATRP, RAFT, and living polymerization

Compare living anionic chain growth with controlled radical methods: ATRP uses reversible halogen-atom transfer, while RAFT uses reversible addition–fragmentation chain transfer. Learn how activation, exchange, termination, and transfer shape molecular-weight control and polymer design.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionThe living-chain idea

Imagine many chains sharing growth fairly: each grows for a short interval, rests, then grows again. Keeping chain ends capable of reactivation narrows the chain-length spread and lets a second monomer extend the first block.

‘Living’ is an ideal limit: no irreversible termination or transfer. ATRP and RAFT are controlled radical polymerizations, not perfectly living processes; practical samples still contain dead chains and imperfect end groups.

Switch between ATRP activation/deactivation and the RAFT addition–fragmentation exchange. These schemes show idealized elementary steps; termination competes with control.

SchoolThe control principle

Definition: Controlled radical polymerization

A radical chain process in which reversible exchange between a small active population and a large dormant population makes chain growth more uniform, provided initiation is efficient and irreversible side reactions remain limited.

\ce{P_n-X + Cu^I/L <=> P_n^\bullet + X-Cu^II/L};\quad \ce{P_n^\bullet + M -> P_{n+1}^\bullet}

Here X is commonly Br or Cl and L is a ligand. The transition-metal complex activates a dormant alkyl halide, then the higher-oxidation-state halide complex rapidly deactivates a radical by returning X. The radical concentration stays low, suppressing bimolecular termination.

Example: Estimate a target chain length

A well-controlled ATRP charges 100 mol monomer per mol initiator and reaches 80% conversion. Estimate the number-average degree of polymerization if initiation is quantitative and termination negligible.

Solution

$DP_n\approx [M]_0/[I]_0\,x=100(0.80)=80$ repeat units per chain. This is an ideal mass-balance estimate, not proof of narrow dispersity or intact end groups.

What each method exchanges
Method	Dormant form / mediator	Key equilibrium
ATRP	Alkyl halide; metal/ligand redox pair	Reversible X-atom transfer
RAFT	Thiocarbonylthio chain-transfer agent	Reversible addition–fragmentation

UndergraduateRAFT exchange and kinetic control

\ce{P_n^\bullet + S=C(Z)-S-R <=> [P_n-S-C^\bullet(Z)-S-R] -> P_n-S-C(=S)-Z + R^\bullet};\quad \ce{R^\bullet + M -> R-M^\bullet}

Radical addition to the thiocarbonylthio C=S produces an intermediate radical; fragmentation releases R•, which must efficiently reinitiate the chosen monomer. Subsequent exchange distributes growth among chains. Z tunes addition/fragmentation for a monomer family; R is selected for leaving ability and reinitiation. RAFT usually needs an independent radical source.

DP_n\approx ([M]_0/[CTA]_0)X,\quad M_n\approx M_{CTA}+DP_nM_{repeat},\quad Đ=M_w/M_n

Example: RAFT target and measured molar mass

Charge [M]₀/[CTA]₀ = 200, reach X = 0.75, and use a repeat unit of 100 g mol⁻¹. Estimate ideal $DP_n$ and polymer contribution to $M_n$ .

Solution

$DP_n≈200×0.75=150$ ; polymer contribution ≈ $150×100=15{,}000$ g mol⁻¹. Add CTA/end-group contributions for total $M_n$ . SEC calibration and nonideal initiation/exchange can make measured values differ.

AdvancedWhat ‘control’ actually requires

Low radical concentration alone is insufficient. Fast, frequent exchange relative to propagation lets chains experience comparable growth histories; prompt initiation limits an early-born/late-born length gap. Termination, transfer to monomer/solvent/polymer, oxygen inhibition, catalyst speciation, and poor end-group fidelity all erode control. Diagnose conversion-dependent $M_n$ , dispersity, end groups, and successful chain extension together.

ResearchFrontiers and open questions

References

The RAFT Process: A New Method for Living Free-Radical Polymerization · Chiefari, J.; Chong, Y. K.; Ercole, F.; et al., 1998
Fundamentals of Atom Transfer Radical Polymerization · Matyjaszewski, K.; Xia, J., 2001
Controlled/Living Radical Polymerization: Progress and Challenges · Matyjaszewski, K., 2012