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

摘要

Calcium and strontium metals react with Hg(C_6F_5)_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(μ-DME)] (2), [Sr 3_(μ-DBP)_6(THF)_6] (3), and [Sr 2_(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 BnNH_2 found the unusual rate expression -d[LA]/dt = k_(p(Ae))[2 or 4]_0[rac-LA] 2~[BnNH_2]_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.