Synthesis of star and H-shape polymersvia a combination of cobalt-mediated radical polymerization and nitrone-mediated radical coupling reactions

Abstract

Via consecutive cobalt-mediated radical polymerization (CMRP), nitrone-mediated radical coupling (NMRC) and copper catalyzed azide-alkynecycloaddition (CuAAC), polymers with mikto-arm star and H-shape architecture were synthesized. Poly(vinyl acetate)₄₀-block-poly(acrylonitrile)₇₈-Co(acac)₂polymers were synthesized viaCMRC and subsequently coupled using an alkyne functional nitrone. The coupling efficiency of the NMRC process was assessed employing N-tert-butyl α-phenyl nitrone (PBN), which is structurally very similar to the later employed coupling agent. Generally, coupling efficiencies of close to 90% or higher were observed in all cases. Since the coupling reaction yields triblock copolymers bearing an alkoxyamine functionality (and thus also an alkynegroup) in the middle of the chain, well defined PEG conjugates could be obtained viaCuAAC. Miktoarm star polymers of the structure (PVAc-b-PAN)₂-PEG were generated as well as H-shaped material of the structure (PVAc-b-PAN)₂-PEG-(PVAc-b-PAN)₂viaconjugation with bifunctional PEG. In all cases, very narrow molecular weight material was obtained. Molecular weight analysis of the intermediate and the final products reveals that the hydrodynamic volume of the miktoarm star and the H-shaped materials is not substantially increased during the final conjugation reaction despite the fact that the absolute molecular weight increases by more than a factor of two in the latter case. Success of the conjugation reactions was confirmed via composition analysis viaNMR.