Indium Catalysts for Ring Opening Polymerization: Exploring the Importance of Catalyst Aggregation

Abstract

Inexorably, the environmental persistence and damage caused by polyolefins have become major drawbacks to their continued long-term use. Global shifts in thinking from fossil-fuel to renewable biobased resources have urged researchers to focus their attention on substituting fossil-fuel based polymers with renewable and biodegradable alternatives on an industrial scale. The recent development of biodegradable polyesters from ring opening polymerization (ROP) of bioderived cyclic ester monomers has emerged as a promising new avenue toward this goal. Ever increasing numbers of metal-based initiators have been reported in the literature for the controlled ROP of cyclic esters, in particular for the polymerization of lactide to produce poly(lactic acid) (PLA). PLA has several material weaknesses, which hinder its use as a replacement for commodity plastics. Despite many advances in developing highly active and controlled catalysts for lactide polymerization, no single catalyst system has emerged to replace industrially used catalysts and provide access to PLA materials with improved properties. We reported the first example of indium(III) for the ring opening polymerization of lactide. Since then, indium(III) has emerged as a useful Lewis acid in initiators for the controlled polymerization of lactide and other cyclic esters. In particular, we have developed a large family of chiral dinuclear indium complexes bearing tridentate diaminophenolate ligands and tetradentate salen and salan ligands. Complexes within our tridentate ligand family are highly active initiators for the moderately isoselective living and immortal polymerization of rac-lactide, as well as other cyclic esters. We have shown that subtle steric effects influence aggregation in these systems, with polymerization typically proceeding through a dinuclear propagating species. In addition, profound effects on polymerization activities have been observed for central tertiary versus secondary amine donors in these and other related systems. In contrast, our well-controlled and highly active chiral indium salen systems are more isoselective than the tridentate analogues and polymerize lactide via a mononuclear propagating species. Again, we have noticed that subtle steric and electronic changes to the ligand can influence both polymerization activity and stereoselectivity via aggregation phenomena. Recently, we have reported a promising new chiral indium catalyst supported by a tetradentate salan ligand. This catalyst is remarkably water and air stable and can be activated by linear and branched alcohols to provide controlled access to multiblock copolymers in air. This catalyst represents an important step forward toward generating new, commercially relevant catalysts for ROP of cyclic esters to produce novel biodegradable polymers, and highlights the unique value of indium-based catalysts in the field.

From Zn(C6F5)2 to ZnEt2‐based Lewis Pairs: Significantly Improved Catalytic Activity and Monomer Adaptability for the Ring‐opening Polymerization of Lactones

“…Generally, tailor-made or special ligand is necessary for metal-based complex to achieve high catalytic activity and broad monomer adaptability, which requires considerable synthetic procedure. [7,8] In addition, some bio-toxic metal residues have caused extensive concerns in the pharmaceutical and biomedical field. [9] Over the past decade, the rapid expansion of organocatalyzed ROPs have been witnessed by scientific researchers, since 4-dimethylaminopyridine (DMAP) catalytic ROP of LA was reported in 2001.…”

Probing the Effect of Six-Membered N-Heterocyclic Carbene—6-Mes—on the Synthesis, Structure and Reactivity of Me2MOR(NHC) (M = Ga, In) Complexes

“…However, irrespective of the origin, the presence of transesterification in the case of the ROP of rac-LA with 1 at -20 • C, which is in contrast with the essentially no transesterification observed for the ROP of rac-LA with Me 2 GaOR(NHC) (NHC = SIMes, IMes) [1,2], led to a considerable decrease of isoselectivity (P m (probability of meso linkages in PLA) = 0.66-0.69 at −20 • C, P m = 0.63-0.66 at r.t.) in comparison with the ROP of rac-LA with Me 2 GaOR(NHC) (P m = 0.76-0.78 at −20 • C, P m = 0.65-0.68 at r.t.). Importantly, the ROP of rac-LA with 1 showed the considerable effect of the structure of NHC on the catalytic properties of Me 2 GaOR(NHC), which should be of importance and is of our current interest as Me 2 GaOR(NHC) are still rare examples of highly active metal alkoxides for the isoselective ROP of rac-LA [5][6][7][8][9][10][11][12][13].…”

“…Moreover, they represent isoselective catalysts for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), which are additionally highly active at low temperatures. Therefore, they still constitute rare examples of isoselective catalysts operating at mild conditions, both among group 13 metal complexes [5][6][7][8][9], and beyond [10][11][12][13]. Importantly, both the structure and reactivity of Me 2 MOR(NHC) (M = Ga, In; NHC = SIMes, IMes) were found to be strongly dependent on the M-C NHC bond.…”

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation

“…[31][32][33][34][35][36] Group 13 complexes also show good activity for mechanistically related reactions such as in the ringopening polymerisation of lactide, lactones or cyclic carbonates. [37][38][39][40][41][42] For the target macrocyclic catalysts, it was envisaged that combining the labile Zn(II) site with Group 13 elements could allow modification of metal Lewis acidity and alkoxide lability. When targeting new catalysts, it is proposed that both metals should have sufficiently small ionic radii to enable inplane coordination in the macrocycle cavity.…”