Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes

Abstract

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M < [CL]0 < 2.6 M) in toluene-d8 at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG° (associated with Keq) and ΔG(‡) (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.

Chiral (1,2)‐Diphenylethylene‐Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide

“…Kinetic analysis of the ROP of lactide catalysed by these Al and In complexes show that the rates of polymerisation increase if electron‐withdrawing groups are added to the aromatic rings in the ligand backbone; this is consistent with observations made before for related aluminium‐salen complexes by Hillmyer, Tolman and co‐workers,35 Gibson and co‐workers,5e and Nomura and co‐workers 5f. Yet, this is the opposite of the trend detected for Al‐salan complexes 23.…”

“…These considerations highlight the current difficulty in designing at will highly active and stereospecific ROP catalysts, even if some key features of ROP reactions are now better understood 20d. 29, 31a, 35, 36 Future efforts must focus on increasing the isoselectivity and catalytic activity of these ROP precatalysts through the tuning of steric and electronic properties of the chiral ancillary ligands, to prepare high molecular PLAs that will lend themselves well to the examination of their physical properties.…”

A Comparison of Gallium and Indium Alkoxide Complexes as Catalysts for Ring-Opening Polymerization of Lactide

“…However, the 19 F NMR spectrum was more informative, showing a signal at -62.4 ppm that was assigned to 1 ox as Ac FcBAr F has poor solubility in C6D6. This paramagnetic product could be reduced back to the starting material 1 red by the addition of an equivalent of CoCp2.…”

Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation

“…On the other hand, based on previous reports, [5][6][7][8][9][10][11][12][13][18][19][20][21][22][23][24][25][26][27] after the insertion of the first equivalent of other monomers (CL, VL, and TMC), the rates of polymerization will not decrease remarkably, indicating that the insertion of the first monomers will not cause such a significant chelation effect like LA. Thus, in these cases, the energy of the initiation step can be used to estimate the energy of propagation.…”