Towards sustainable polymeric nano-carriers and surfactants: facile low temperature enzymatic synthesis of bio-based amphiphilic copolymers in scCO2

We demonstrate that useful bio-based amphiphilic polymers can be produced enzymatically at a mild temperature, in a solvent-free system and using renewably sourced monomers, by exploiting the unique properties of supercritical CO2 (scCO2). We present the use of a novel near-ambient temperature approach to prepare renewable amphiphilic ABA copolymers in scCO2. Bio-based commercially available monomers have been polymerised to prepare chains with targeted molecular weight. The amphiphilic materials were prepared by end-capping the synthesised polymers with methoxy poly(ethylene glycol) (MPEG) chains in a one-pot high pressure reaction utilising Candida Antarctica Lipase B (CaLB) as a catalyst at a temperature as low as 35 °C. The block copolymers are characterised by 1H-NMR, GPC and DSC in order to carefully assess their structural and thermal properties. These polymers form self-assembled aggregates in aqueous environment and these nanostructures are studied through DLS, TEM and UV-Vis. Highly hydrophobic Coumarin-6 was used as a model to prove dispersion in water of lipophilic molecules. Maximum bubble pressure tests demonstrate the reduction in surface tension of these polymers and comparisons are made directly to commercial polymeric non-ionic surfactants

Synthesis of graft copolymers by the combination of ATRP and Enzymatic ROP in scC02

A simple strategy is reported for the synthesis of well-defined graft copolymers of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate) P(MMA-co-HEMA) with poly(ε-caprolactone) (PCL) grafted chains. Using scCO2 as the only solvent, a one-step synthetic approach is adopted to prepare copolymer backbones via atom transfer radical polymerization (ATRP), and grafted chains are added via enzymatic ring-opening polymerization (eROP). Exhaustive study of the enzymatic grafting efficiency showed that only the hydroxyl groups in the backbone initiated the polymerization of ε-CL, resulting in an exceptional polymer architecture which is not accessible by conventional chemical polymerization methodology. The lower grafting density obtained (ca. 30−40%) with the enzymatic polymerization of ε-CL indicates that the system is likely limited by steric hindrance

Kinetics of Enzymatic Ring-Opening Polymerization of ε-Caprolactone in Supercritical Carbon Dioxide

The kinetics of enzymatic ring-opening polymerization (eROP) of ε-caprolactone in supercritical carbon dioxide (scCO2) was investigated using a new, high-pressure sampling autoclave. The polymerization was performed using Candida antarctica lipase B (CALB) as catalyst and was found to be approximately first order with respect to monomer up to 80% conversion. For the first time we have been able to present kinetic results on the eROP of caprolactone in scCO2. These results show that high molecular weight polymer could be obtained (up to 50 kDa) with polydispersities in the range of 2. The relatively poor molecular weight control was attributed to the large degree of enzyme-catalyzed transesterification that forms both cyclic species (intramolecular transesterification) and linear polymer (intermolecular transesterification). This effect has also been observed for eROP of ε-caprolactone in conventional solvents. The formation of cyclic oligomers of poly(caprolactone) (PCL) was investigated as a function of conversion, and comparisons were made to similar studies undertaken in conventional solvents

Simultaneous Dynamic Kinetic Resolution in Combination with Enzymatic Ring-Opening Polymerization

We report the simultaneous dynamic kinetic resolution (DKR) of a secondary alcohol in combination with lipase-catalyzed ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). (R,S)-1-Phenylethanol (PhE) was used as a model secondary alcohol and incorporated into poly(ε-caprolactone) (PCL) under DKR conditions. A total of 75% of the PhE was incorporated as (R)-PhE-PCL with over 99% enantio excess (ee) in 23 h. This methodology could provide a simple one-step approach to prepare enantiopure sustained release polymeric formulations of chiral species such as drugs or drug precursors bearing a secondary hydroxyl group