Grafting polymers by enzymatic ring opening polymerisation—maximising the grafting efficiency

Abstract

The synthesis of well-defined graft copolymers containing alkyl methacrylate, hydroxylalkyl methacrylate monomers and/or (poly(ethylene oxide) methacrylate macromonomers and their further graft chain extension with poly(ε-caprolactone) (PCL) was studied in both conventional solvents and supercritical carbon dioxide (scCO₂). A cascade two-step synthetic approach was adopted in which copolymer backbones were prepared in a first stage via atom transfer radical polymerisation (ATRP). These copolymers take the form of methacrylate based polymer backbones. In one case hydroxyl groups are pendant and close to the backbone (poly(MMA-co-HEMA)) whilst in the second, the hydroxyl groups are present as the terminal group of a poly(ethylene oxide) graft arm significantly separated from the main backbone (poly(MMA-co-PEGMA)). In stage two, these hydroxyl moieties were then used as the initiation centre for further chain extension via enzymatic ring opening polymerisation (eROP). These studies were designed to prepare different poly(alkyl methacrylate) copolymers by ATRP to optimise the grafting efficiency of the enzymatic polymerisation. We previously reported a limited grafting level of up to 40% for the pendant hydroxyl groups of a poly(MMA-co-HEMA) copolymer system. This study demonstrated that higher grafting efficiency (up to 80%) could be obtained by using a highly randomized MMA-co-HEMA polymeric backbone. By contrast, 100% grafting was achieved by moving the hydroxyl groups away from the backbone when using the longer hydroxyl-terminated poly(ethylene oxide) methacrylate monomer (PEGMA) as a co-monomer in poly(MMA-co-PEGMA) backbones.