Synthesis and structures of calcium and strontium 2,4-di-tert-butylphenolates and their reactivity towards the amine co-initiated ring-opening polymerisation of rac-lactide

Abstract

Calcium and strontium metals react with Hg(C₆F₅)₂ and 2,4-di-tert-butylphenol (H-DBP) in tetrahydrofuran (THF) and 1,2-dimethoxyethane (DME) to give [Ca(DBP)₂(THF)₄] (1), [Ca₂(DBP)₄(DME)₄(μ-DME)] (2), [Sr₃(μ-DBP)₆(THF)₆] (3), and [Sr₂(DBP)(μ-DBP)₃(DME)₃] (4). Compound 1 is a six coordinate trans-octahedral monomer, whereas in binuclear 2 two seven-coordinate Ca centres are bridged by a DME ligand. In 3 a central Sr is connected by three bridging DBP groups to each of two terminal Sr(THF)₃ moieties, all metal atoms being six coordinate. Compound 4 has one six- and one seven-coordinate Sr, bridged by three DBP ligands, the former Sr also having a terminal DBP and a bidentate DME ligand and the latter two DME ligands. Complexes 2 and 4 act as ring-opening polymerisation (ROP) catalysts for the benzyl alcohol or benzylamine co-initiated ROP rac-lactide forming atactic alcohol- or amine-terminated polylactide H-[PLA]-XBn (X = O or NH) with reasonable control of molecular weight via an activated monomer propagation mechanism. Kinetic studies for BnNH₂ found the unusual rate expression −d[LA]/dt = k_p(Ae)[2 or 4]₀[rac-LA]²[BnNH₂]₀^2.5 (k_p(Ca) ≈ 1.7 × k_p(Sr)). Preliminary studies suggest that [Y(DBP)₃(THF)₂] also catalyses amine or alcohol co-initiated ROP by an activated monomer mechanism without loss of a phenoxide ligand.