Tuneable zinc scorpionates for CO2 fixation into sustainable materials: access to polycarbonates and tailored hybrid poly(L-lactide)s-b-polycarbonates

Abstract

The production of industrially demanded materials mediated by metal-based catalysts under greener and more eco-friendly conditions is a current challenge to aim. This is even more attractive when these materials are capable to uptake different levels of CO2 and show improved properties. In this contribution we describe the use of inexpensive, low-toxicity and robust mononuclear alkyl zinc complexes of the type [ZnMe(κ 3-NNN’)] (4–6), supported by sterically demanding scorpionate ligands, as catalysts for the selective Ring-Opening Co-Polymerisation of cyclohexene oxide and CO2. The presence of different electron-withdrawing groups in the scorpionates play a key role in catalysts selectivity. Thus, these species can act efficiently in the absence of co-catalyst to afford poly(cyclohexene carbonate) (PCHC) materials with narrow dispersity values, under mild and solvent-free conditions (TOF = 5.6 h−1, Mass-Specific Activity, MSA = 0.169 kg PCHC · g−1 Zn). Very interestingly, only complex [ZnMe(κ3-phbptamd)] 4 also exhibited high efficiency as one-component catalyst for the preparation of the terpolymer poly(L-lactide)-b-poly(cyclohexene carbonate) (PLA-b-PCHC) in the form of hybrid diblock materials via a “one-pot/one-step” procedure, in different ratios and under similar conditions. DFT calculation demonstrate that the lower Lewis acidity of catalyst 4, compared to 5 and 6, enables the full conversion of L-LA monomer by ROP as the first step in the terpolymerisation process, prior to the ROCoP of CHO and CO2. The level of CO2 incorporation in these materials can be easily modulated by simply modifying cyclohexene oxide feed, resulting in hybrid materials with different thermal properties. Assessment by Thermogravimetric Analysis and Differential Scanning Calorimetry revealed both terpolymers with higher PCHC content showed to be stable up to 240 °C, and that the glass transition temperatures for these PLA-b-PCHC hybrids exhibit a single value (Tg = 55–60 °C), corresponding to the lactide portion of the hybrid polymer 12.