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

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), [Ca-2(DBP)(4)(DME)(4)(mu-DME)] (2), [Sr-3(mu-DBP)(6)(THF)(6)] (3), and [Sr-2(DBP)(mu-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 = kp((Ae))[2 or 4](0)[rac-LA](2)[BnNH2](0)(2.5) (kp((Ca)) approximate to 1.7 x 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.