Facile synthesis of norbornene–ethylene–vinyl acetate/vinyl alcohol multiblock copolymers by the olefin cross-metathesis of polynorbornene with poly(5-acetoxy-1-octenylene)

Abstract

New norbornene-based functional multiblock copolymers containing ethylene–vinyl acetate/vinyl alcohol sequences are synthesized via the olefin cross-metathesis reaction of polynorbornene with poly(5-acetoxy-1-octenylene) followed by CC bond hydrogenation and acetoxy group deprotection. Macromolecular cross-metathesis, a recently developed type of interchange reaction, was carried out in the presence of Grubbs’ ruthenium catalysts and monitored by in situ¹H NMR and ex situ¹³C NMR spectroscopy. This results in copolymer formation and its gradual randomization by segment reshuffling promoted by polymer-based Ru-carbenes. The kinetic study reveals that, despite the presence of bulky acetoxy substituents in polyoctenamer chains, Grubbs’ catalyst interacts with them more easily than with polynorbornene chains, which have cyclopentane rings in the backbone. The rate of the subsequent interchange reaction is controlled by the interaction of [Ru]-poly(5-acetoxy-1-octenylene) carbenes with norbornene units. It is found that the acetoxy groups markedly slow down the interaction of [Ru]-polynorbornene carbenes with the CC bonds in octenamer units. Since these carbenes are relatively unstable, they cause a noticeable decay of the reaction active centers. Our study demonstrates that the chain structure and, therefore, physico-chemical characteristics of multiblock copolymers can be tailored by adjusting the cross-metathesis conditions, such as the polymer concentration, polymer to catalyst ratio, and reaction time. Polymer-analogous reactions of hydrogenation and deacetylation transform octenamer units into ethylene–vinyl acetate/vinyl alcohol segments and thus open up more possibilities for tuning the thermal, gas barrier, and hydrophobic/hydrophilic properties of the synthesized norbornene-based multiblock copolymers.