Structural analysis of linear/branched ethylene block copolymers

Chain shuttling polymerization has provided new pathway for introduction of different architectures in a single chain. Unlike the commercially available ethylene/1-octene block copolymers, synthesis and microstructure of linear/branched polyethylene with blocky nature is not extensively studied. In this work, such block copolymers are synthesized based on reversible transfer of growing chains between an ansa metallocene and -diimine catalysts, forming linear and branched structures from ethylene, respectively. Investigation of thermal properties reveals that application of 550 equivalent of chain shuttling agent makes blocky structures that show the most deviation from the longstanding relationship between melting temperature and crystallinity or density, alongside with turning broad molecular distribution into unimodal. Thermal fractionation by successive self-annealing demonstrates formation of broad distribution of linear blocks, as comprehended through appearance of uniform melting peaks at lower temperatures. Corresponding dynamic mechanical properties and crystalline structures reveal soft elastomeric properties, specifically at temperatures around -50 degrees C, opposed to the purely linear chains or linear/branched blends. Correspondingly, blend samples demonstrate significant morphological change upon treatment with a suitable solvent for the branched fraction, contrary to the blocky microstructures.