Ring opening polymerization of rac-lactide by group 4 tetracarbamato complexes: activation, propagation and role of the metal

Abstract

A series of group 4 metal tetracarbamates M(O(2)CNR(2))(4) (M = Ti, R = Et, 1a; M = Zr, R = Et, 1b; (i)Pr, 1c; M = Hf, R = Et, 1d; R = (i)Pr, 1e) were studied as catalytic precursors in the solution polymerization of rac-lactide. The titanium complex but not the zirconium and hafnium ones increase the activity by addition of (i)PrOH. The structure of the carbamato ligand markedly influences the molar mass of polymer; the complexes with isopropyl carbamato ligands produced PLA with molar masses up to 94,000 g mol(-1). The main mechanistic aspects of the initial stages of the polymerization reactions were outlined by spectroscopic and computational analyses. In the case of zirconium and hafnium complexes, an interaction between a carbamato ligand and the CH unit of one lactide molecule is established at room temperature. This interaction is followed by the high temperature proton transfer from the lactide to the carbamato O-atom, affording a catalytic active alkoxy complex with release of CO(2) and NHR(2). The polymerization mediated by Ti(O(2)CNEt(2))(4) involves the release of a radical fragment [O(2)CNEt(2)]˙, with consequent generation of a Ti(III) center. The propagating chain is an alcoholate ligand coordinated to a Ti(IV) centre and containing a radical mainly localized at the tail of the chain (DFT, EPR).

Dinuclear Zinc–N‐Heterocyclic Carbene Complexes for Either the Controlled Ring‐Opening Polymerization of Lactide or the Controlled Degradation of Polylactide Under Mild Conditions

“…However, in most studies, the calculation methods have been described briefly, which limits the reliability of the conclusions. In contrast to the present Zn systems, all modeled ROP systems have typically involved monometallic derivatives, such as Al,29 Yb,30 Ti,31 and alkaline and alkaline‐earth ROP initiators 32. A mechanism involving coordination, nucleophilic attack, and ring‐opening reactions (the so‐called coordination–insertion mechanism) was determined for the ROP of rac ‐LA initiated by β‐diketiminate Mg II species by a thorough DFT computational analysis 33.…”

Titanium pyridonates for the homo- and copolymerization of rac-lactide and ε-caprolactone

“…While not yet possessing the high activity and control demonstrated by leading initiators, 41 these complexes are 65 competitive with other reported titanium initiators. 12,16,18,20 Notably, the ability to form random copolymers with close to equal monomer incorporation makes these complexes distinguished amongst titanium complexes. Furthermore, the ability to form block copolymers comprised of equimolar 70 monomer incorporation by using sequential ε-caprolactone followed by lactide monomer addition highlights the versatility of these initiators.…”

“…7 Some examples of random co-polymer synthesis have been reported, including our initially communicated titanium pyridonate system. [12][13][14][15] The investigation of titanium complexes as initiators [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] for the ring-opening polymerization of cyclic esters has been largely driven by the minimal cost, high abundance, and low toxicity 30 of the source metal. While they are as yet less competitive than the leading initiator systems, these practical advantages have motivated vigorous research interest into a variety of ligand scaffolds in pursuit of achieving high molecular weight polymers with control of dispersity, co-monomer incorporation and tacticity.…”

Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis, Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

“…ROP processes offer excel control in terms of molecular weight ( M n ), polydispersity (MWDs) and microstructure . Several complexes have been successfully employed in the ROP of lactide, including lithium, sodium, potassium, magnesium, calcium, strontium, aluminum, tin, zinc, some transition metals, and rare‐earth metals . Since complete elimination of catalyst rests from the polymer is virtually impossible, it is highly preferable to use non‐ or low toxic metals (for instance for biomedical applications).…”