Gradient Copolymers Through Selective Monomer Depletion

Significance

Bazan and co-workers report a controlled polymerization method to generate block copolymers with gradient compositions using nickel catalysts. The procedure requires both a constant pressure of ethylene and faster initiation rates relative to chain propagation. After initiation, a large percentage of 5-norbornen-2-yl acetate is incorporated, producing a polar, amorphous block. As the polymerization proceeds (and the 5-norbornen-2-yl acetate concentration decreases) ethylene incorporation becomes predominant, giving a non-polar, crystalline block. Consequently, the x/y ratio increases with reaction time and creates a gradient of composition. Not surprisingly, the polymer’s physical properties (e.g., phase separation) depend strongly on chain length and initial concentration of the polar comonomer. For example, a lamellar morphology is observed for polymers with a number average molecular weight around 63 kDa, while lower molecular weight polymers do not phase separate in the solid state.