Normal, ICAR and photomediated butadiene-ATRP with iron complexes

The ligand (L) and halide effects of a series of iron complexes (FeX₂ or FeX₃, X = Cl, Br)/L supported by carbon (Cp₂Fe₂(I)(CO)₄ > Cp₂Fe > Fe(CO)₅ > (Ph₂PCp)₂Fe), nitrogen (phthalocyanine ≫ bpy ≥ MeO-bpy ≫ PMDETA > phen), halide (FeX_mY_4−m/Bu₄N, X, Y = Cl ≫ Br > I), oxygen (12-crown-4 ≫ 15-crown-5 ≥ dibenzo-18-crown-6) and phosphorous (P[Ph(2,4,6-OMe)₃]₃ > P(t-Bu)₃ ≫ P(n-Bu)₃, PPh₃, P[Ph(4-CF₃)]₃, P(C₆F₅)₃) ligands, as well as ligand-free FeX₃, were evaluated in the normal, ICAR, and photo-ATRP of butadiene (BD) initiated from bromoesters, α,α-dichloro-p-xylene, or FeX₃ in toluene at 110 °C. Good polymerization control was observed in many cases, and two clear trends i.e. P[Ph(OMe)₃]₃ ≫ Bu₄NX > crown ethers > amines > C-ligands and FeCl₂, FeCl₃ ≫ FeBr₂, FeBr₃ occur consistently across all polymerizations. These effects correlate with the higher stability of the allyl PBD-Cl vs. PBD-Br chain ends and with FeCl₃ likely being a better deactivator than FeBr₃. Conversely, while basic enough to reduce FeX₃, P[Ph(2,4,6-OMe)₃]₃ is not nucleophilic enough to quaternize PBD-X in the apolar toluene and successfully enables a faster activation/deactivation equilibrium than all other ligands. As such, e.g. N-ATRP with [BD]/[R–Br]/[FeCl₃]/P[Ph(2,4,6-OMe)₃]₃ = 100/1/2/3 affords a linear M_nvs. conversion profile with PDI as low as 1.15–1.2 and a halide chain end functionality (CEF) = 0.65 at up to 50% conversion. While controlled polymerizations occur in photo-ATRP even without ligand and initiator, photoirradiation of catalytic N-ATRP with BD/R–Br/FeCl₃/P[Ph(2,4,6-OMe)₃]₃ = 100/1/0.05/0.15 significantly improves the rate (×10 vs. dark), conversion (up to 70%) and X-CEF (0.9) via the additional initiation afforded by FeX₃ photolysis, albeit with a slight PDI increase to ∼1.4. Thus, Fe-mediated BD-ATRP is achievable, and the rational selection of the polymerization variables enables minimization of side reactions and the successful synthesis of well-defined PBD with a wide range of molecular weights, narrow PDI and reasonably high X-CEF, suitable for the preparation of e.g. block copolymers.