Ring‐opening polymerization of cyclic esters by pincer complexes derived from alkaline earth metals

Abstract

We report the synthesis and characterization of new Ca(II) and Mg(II) complexes of the type [Ph―PPP]MR (M = Ca, R = N(SiMe3)2 (1); M = Mg, R = nBu (2)) where Ph―PPP− is a tridentate monoanionic ligand (Ph―PPP―H = bis(2‐diphenylphosphinophenyl)phosphine). Reaction of the opportune metal precursor (Ca[N(SiMe3)2]2.THF2 or MgnBu2) with 1 equiv. of the pro‐ligand Ph―PPP―H produces the corresponding calcium amido (1) or magnesium butyl (2) complex in high yield. Solution NMR studies show monomeric and kinetically stable structures for both species. The obtained complexes efficiently mediate the ring‐opening polymerization of ɛ‐caprolactone showing a turnover frequency of 40 000 h−1. In the presence of a hexogen alcohol, up to 2000 equiv. of monomer are converted by using a low loading of catalyst (5 µmol). Kinetic studies show a first‐order reaction in monomer concentration. Copyright © 2014 John Wiley & Sons, Ltd.

Catalytic Main‐Group Metal Complexes of Phosphine‐Based Pincer Ligands

“…In 2014, Mazzeo and coworkers reported two alkaline‐earth complexes of the anionic PPP‐type pincer ligand PPP ‐ 1 (Scheme 1), namely, compounds Mg ‐ 1 and Ca ‐ 1 , one featuring magnesium(II) and the other calcium(II) [46] . These were synthesized in high yields (93–98 %) by treating the protonated proligand with either Mg( n Bu) 2 or Ca[N(SiMe 3 ) 2 ] 2 in benzene containing THF.…”

“… Synthesis of PPP‐alkaline‐earth complexes Mg ‐ 1 and Ca ‐ 1 , reported by Mazzeo and coworkers in 2014 [46] …”

Ring-opening polymerization of lactide using chiral salen aluminum complexes as initiators: high productivity and stereoselectivity

“…The stereochemical configuration of the polymer chains determines the physical and chemical properties of PLA. Generally, PLA is prepared by the ring-opening polymerization (ROP) of lactide initiated by metal complexes such as some alkoxides of tin, [6][7][8] aluminum, [9][10][11][12][13][14] zinc, [15][16][17][18][19][20][21] magnesium, [22][23][24] iron, 25,26 titanium, [27][28][29][30][31][32][33] indium, 34,35 rareearth metals, [36][37][38][39][40][41][42][43][44][45] organo-catalysts, 46 and enzymes. 47 Aluminum catalysts are regarded as competent catalysts for the ROP of LA because of their effectiveness in the control the stereoregular degrees of polymers.…”

A pragmatic calcium‐based initiator for the synthesis of polycaprolactone copolymers

“…Therefore, there has been an increasing trend towards the use of non‐toxic, biocompatible initiators and catalysts for the synthesis of biodegradable polyesters destined for tissue engineering applications. A number of initiating systems have been studied, including those based on iron, magnesium, 9,11−14 calcium 15−24 and zinc . The emergence of new initiation systems has in turn led to the need to study the mechanism of such polymerizations in depth so that the polymerization processes can be exploited to give controlled, reproducible materials.…”

“…The development of calcium‐based polymerization initiators for the synthesis of PCL and PCL copolymers has been widely studied 15−22 and continues to be a field of exploration . Many of these initiating systems are based on calcium complexes and have only been used for polymerization in dilute solution, which limits their scalability compared with bulk polymerization.…”