Mechanistic Comparison of Cyclic Ester Polymerizations by Novel Iron(III)−Alkoxide Complexes:  Single vs Multiple Site Catalysis

Abstract

The complexes Fe2(OCHPh2)6 and L2FeOR (R = Et or CHPh2, L = N,N'-bis(trimethylsilyl)benzamidinate) were structurally characterized, and comparative studies of the behavior of those compounds comprising the same alkoxide (Ph2HCO-) in polymerizations of -caprolactone (CL) and D,L-lactide (LA) were performed. Both Fe2(OCHPh2)6 and L2FeOCHPh2 are effective polymerization catalysts, as reflected by molecular weight control, polydispersities, and end group analysis, but the diiron complex generally exhibits greater polymerization control, particularly for CL. Kinetic investigations of the polymerization of CL revealed the same first-order dependence on [CL] for both catalysts, but different orders in [catalyst] that signified a distinct contrast in mechanism. Analysis that invoked the presence of a termination-causing impurity at low concentration yielded a first-order dependence on [Fe2(OCHPh2)6], but the order in [L2FeOCHPh2] was found to be one-half. This fractional dependence was interpreted by using a model of active chain aggregation. Comparison of the derived propagation rate constants (k(prop)) revealed a approximately 50-fold greater value for the diiron complex compared to the single site mononuclear compound. Implications of these findings for understanding cyclic ester polymerization mechanisms and catalyst design are discussed.

First‐Row Transition Metal Bis(amidinate) Complexes; Planar Four‐Coordination of FeII Enforced by Sterically Demanding Aryl Substituents

“…oligomerisation [3,4] or polymerisation [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] of olefins or of cyclic esters. [21,22] Due to the geometric constraints of the NCN ligand backbone, amidinates have small N-M-N bite angles (typically 63-65°). They have a rich coordination geometry in which both chelating and bridging coordination modes can be achieved.…”

Zwitterionic ring opening polymerization of lactide by metal free catalysts: Production of cyclic polymers

“…Such conditions induce side reactions such as transesterification and racemization, which hamper the control over the polymerization process. The scientific interest in well-defined architectures including telechelic-, block-, graft-, and star-shaped polymers has initiated major research efforts toward initiators to yield welldefined poly(lactide)s. The ROP of lactides have been investigated using various metal catalysts, including complexes of Sn, [5][6][7] Y, 8 Ln, [9][10][11][12][13] Fe, 14 Ca, 15 Ti, 16 Mg, 17 Al, [18][19][20] and Zn, [21][22][23][24][25] which generally proceeds by insertion of monomer into the metal-alkoxide bond. Some of these initiators have proved to be very attractive since they give rise to a controlled polymerization of lactide.…”

Lactide polymerization with iron alkoxide complexes

“…24 In both cases, activities were lower than those of bis(benzimidinate) iron(III) alkoxides, 23 diketiminate iron(II) alkoxides, 22 or anionic ferrous alkoxides Na2Fe(OAr)4(THF)4, 51 for which polymerization activity at 70 °C or room temperature was reported.…”

“…[21][22][23][24] The synthesis itself of these ligands offers the advantageous possibility of varying the steric properties around the imine branches of the molecule simply by using different amine reagents.…”

“…[19][20][21] For instance, we are aware of only a handful of reports on lactide polymerization with iron coordination complexes. [22][23][24][25] This paucity of data on the potential lactide polymerization activities of Fe(II)-based catalysts, combined with the great abundance and low toxicity of this metal, prompted us to explore the preparation of LFe(OR) complexes and their aptitude for rac-lactide polymerization.…”